UMTS Radio Network Planning, Optimization and QOS Management: For Practical Engineering Tasks

UMTS RADIO NETWORK PLANNING, OPTIMIZATION AND QOS MANAGEMENT FOR PRACTICAL ENGINEERING TASKS This page intentionally ...

Author: Jukka Lempiainen | Matti Manninen

76 downloads 1863 Views 23MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

UMTS RADIO NETWORK PLANNING, OPTIMIZATION AND QOS MANAGEMENT FOR PRACTICAL ENGINEERING TASKS

This page intentionally left blank

UMTS RADIO NETWORK PLANNING, OPTIMIZATION AND QOS MANAGEMENT FOR PRACTICAL ENGINEERING TASKS

Edited by

Jukka Lempiäinen and

Matti Manninen European Communications Engineering (ECE) Ltd.

KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

eBook ISBN: Print ISBN:

1-4020-2599-8 1-4020-7640-1

©2004 Springer Science + Business Media, Inc. Print ©2003 Kluwer Academic Publishers Dordrecht All rights reserved No part of this eBook may be reproduced or transmitted in any form or by any means, electronic, mechanical, recording, or otherwise, without written consent from the Publisher Created in the United States of America

Visit Springer's eBookstore at: and the Springer Global Website Online at:

http://www.ebooks.kluweronline.com http://www.springeronline.com

This book is dedicated to the families of all authors.


Contents

Dedication

5

Preface

11

Acknowledgments

13

PART I: UMTS CONFIGURATION PLANNING

15

INTRODUCTION TO UMTS NETWORK JUKKA LEMPIÄINEN

17

1.

18 19 20 22 30 36 42

INTRODUCTION TO UMTS NETWORK 1.1 UNIVERSAL MOBILE TELECOMMUNICATION SYSTEM 1.2 UNIVERSAL TERRESTRIAL RADIO NETWORK 1.3 WCDMA CELLULAR CONCEPT AND PLANNING STRATEGY 1.4 UMTS RADIO INTERFACE 1.5 UMTS RADIO PLANNING PROCESS 1.6 RADIO NETWORK EVOLUTION

UMTS CONFIGURATION PLANNING JARKKO ITKONEN, RISTO JURVA

45

2.

46 46 57 59

UMTS CONFIGURATION PLANNING 2.1 BASE STATION CONFIGURATION 2.2 MOBILE STATION 2.3 POWER BUDGET

8 2.4 2.5

REPEATER CONFIGURATION INDOOR CONFIGURATION

DIGITAL MAPS KIMMO KANTO 3.

DIGITAL MAPS 3.1 MAPPING PURPOSE 3.2 MAPPING METHODS 3.3 COORDINATE SYSTEMS 3.4 DIGITAL MAP LAYERS 3.5 ADDITIONAL USE OF SOURCE MATERIALS 3.6 QUALITY REQUIREMENTS

69 72 79

80 80 84 91 95 109 111

RADIO NETWORK PLANNING TOOLS HANS AHNLUND

117

4.

118 118 123 134 138 143

RADIO NETWORK PLANNING TOOLS 4.1 RADIO NETWORK PLANNING TOOL ENVIRONMENT 4.2 DESIGN TOOL 4.3 MEASUREMENT TOOLS 4.4 ANALYSIS TOOLS 4.5 MANAGEMENT AND INFORMATION PROCESSING TOOLS

PART II: UMTS TOPOLOGY PLANNING

147

UMTS TOPOLOGY PLANNING JARNO NIEMELÄ, JUKKA LEMPIÄINEN

149

5.

UMTS TOPOLOGY PLANNING 5.1 INTRODUCTION 5.2 TOPOLOGY PLANNING 5.3 UMTS SITE CONFIGURATION 5.4 UMTS ANTENNA CONFIGURATION

150 150 151 166 186

PART III: UMTS NETWORK FUNCTIONALITY

205

WCDMA RADIO INTERFACE MATTI MANNINEN

207

6.

208 209 223

WCDMA RADIO INTERFACE 6.1 PHYSICAL LAYER 6.2 SPREADING AND MODULATION

9 6.3 6.4 6.5

PROCEDURE EXAMPLES RADIO RESOURCE MANAGEMENT IN WCDMA 3GPP PARAMETERS FOR WCDMA RADIO ACCESS

227 237 243

UMTS RADIO INTERFACE FIELD MEASUREMENTS KAI OJALA, PASI NIEMI

259

7.

260 260 266 267 276 278

UMTS RADIO INTERFACE FIELD MEASUREMENTS 7.1 MEASUREMENT PROCESS 7.2 FIELD MEASUREMENT EQUIPMENT 7.3 MEASUREMENTS IN IDLE MODE 7.4 MEASUREMENTS DURING CONNECTION ESTABLISHMENT 7.5 MEASUREMENTS IN CONNECTED MODE

QUALITY-OF-SERVICE MEASUREMENTS MARKUS AHOKANGAS, TAPIO HEIKKILÄ, TAISTO NIIRANEN, TAPIO TAIPALE, JOUKO UIMONEN

305

8.

306 306 321

QUALITY-OF-SERVICE MEASUREMENTS 8.1 QUALITY-OF-SERVICE FRAMEWORK 8.2 UMTS MEASUREMENTS IN PRACTICE

Index

337


Preface

In cellular networks, a new generation of CDMA or WCDMA-based networks will start operations in most countries in the near future. The standardized WCDMA technology generates new challenges in radio network planning, optimization and QoS management because of the dynamic nature of its radio interface and various new services and different network operating modes. Moreover, new and modified radio planning phases as well as new field measurements and emphasized QoS management are needed when UMTS networks are designed and optimized. Hence, a practical UMTS planning process must be defined in detail, from dimensioning to optimization tasks. This book follows the UMTS planning process. It is organized in three parts: Part I - UMTS configuration planning; Part II - UMTS topology planning; and Part III - UMTS network functionality. The first chapter in Part I introduces the UMTS and UTRAN systems and radio network planning strategy, and defines a planning process for UMTS. In Chapter 2, the UMTS planning process is covered, and a detailed description of the UMTS power budget is given, with planning threshold examples provided. Coverage predictions are next done in a traditional cellular planning by utilizing a planning tool together with digital maps. Therefore, digital maps and their production are introduced in Chapter 3, and the planning tool environment is covered in Chapter 4. Chapter 3 gives an overview of digital map production methods and their accuracies for different purposes. Chapter 4 defines the required UMTS planning environment: a dimensioning tool, a radio planning tool for coverage predictions and topology planning, a measurement tool for field tests, a protocol analyzing tool, and database tools. Hence, Chapters 1 through 4 explain UMTS network configuration-

12 related planning phases and the whole planning environment before actual coverage predictions in a radio network planning tool. In Part II a new UMTS planning phase called topology planning is highlighted because UMTS network coverage and system capacity should be planned together. The first part of Chapter 5 introduces coverage and capacity-related network elements, site configurations, and initial and detailed topology planning. The second half of Chapter 5 includes the most relevant and critical site configuration simulation examples during detailed topology planning. These examples include variations of base station site location and antenna direction and antenna configurations as beam width, sectoring, antenna height and down tilting. Also, the most relevant UMTS network related parameters, such as coverage overlapping (site distance or site density) and load of the network, are taken into account. Chapter 5 covers the impact of major site configurations on network coverage and system capacity. In Part III (Chapters 6 - 8), the functional properties of the UMTS radio network are explained to optimize the UMTS network and manage end-toend QoS. Chapter 6 introduces the physical layer of the UMTS network and main processes in idle, connection establishment and connected mode, in order to understand how mobiles and base station communicate with each other. Next, the main Radio Resource Management (RRM) functionalities are defined, and finally the main 3GPP-related radio parameters are explained. Chapter 7 introduces measurements related to radio interface communication (Layer 3 messages) and other relevant field measurements for idle, connection establishment, and connected modes. The measurement types defined in Chapter 7 are needed for testing, planning, and optimization purposes during different network evolution phases. Chapter 8 introduces the QoS architecture in UMTS and the most relevant QoS measurement types and systems. Together with QoS (Chapter 8) and field measurements (Chapter 7), the UMTS radio interface functionality (Chapter 6) can be optimized for different services.

Jukka Lempiäinen Matti Manninen Helsinki, July 2003

Acknowledgments

The authors would like to thank European Communications Engineering Ltd, FM Kartta, Nemo Technologies, Nethawk and Tampere University of Technology for making this project possible. Also warmest thanks have to be expressed to the families and colleagues of the authors for their support during the writing and editing process.


PART I: UMTS CONFIGURATION PLANNING


Chapter 1 INTRODUCTION TO UMTS NETWORK Overview of radio network fundamentals JUKKA LEMPIÄINEN European Communications Engineering (ECE) Ltd Tampere University of Technology (TUT)

Abstract:

In this chapter, the most relevant terms and technical aspects of the Universal Mobile Telecommunication System (UMTS) and Universal Terrestrial Radio Network (UTRAN) are introduced. A short overview of the whole UMTS system and more detailed overview of UTRAN are given, including essential network elements and the interfaces between them. Moreover, a Code Division Multiple Access (CDMA)-based cellular concept and different major planning strategies of the UTRAN are shortly presented. The technical details of the radio propagation channel are highlighted in order to point out the behavior of the Wideband Code Division Multiple Access (WCDMA) scheme in different radio propagation environments. Finally, a WCDMA-based planning process is defined and multi mode (GSM/UMTS) and multi band (GSM900/GSM1800/UMTS) network evolution challenges are described.

Key words:

Cellular concept, planning process, radio propagation channel, UMTS, UTRAN, WCDMA

17 J. Lempiäinen and M. Manninen (eds.), UMTS Radio Network Planning, Optimization and QoS Management, 17-44. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

UMTS RADIO NETWORK PLANNING, OPTIMIZATION AND QOS MANAGEMENT

18

1. INTRODUCTION TO UMTS NETWORK Mobile networks, also called cellular networks, are based on different kinds of multiple access schemes in their radio interface (communication between mobile station and base station). Traditional multiple access schemes, such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA) are utilized in first-generation (1G) and second-generation (2G) systems. Analog FDMA-based networks, such as Advanced Mobile Phone Service (AMPS), Nordic Mobile Telephone (NMT), and Personal Handy phone System (PHS), are called first-generation systems. The first digital mobile networks, such as North American TDMA (also known as D-AMPS because of the same frequency band used), and cdmaOne, European Global System for Mobile communications (GSM), and Japanese Personal Digital Cellular telecommunication system (PDC) are called second-generation (2G) systems. Enhancements of 2G systems, such as packet transmission General Packet Radio System (GPRS) and Enhanced Data calls for GSM Evolution (EDGE) - are usually referred to as 2.5G. In specification work, the International Telecommunication Union (ITU) defines the common name IMT-2000 for 3G systems, and the thirdgeneration partnership project, 3GPP, takes care of standardization work for the entire mobile network family - GSM (2G), GPRS (2.5G), EDGE (2.5G) and UMTS (3G). See Figure 1-1 [1-4]. This book concentrates on analyzing how Frequency Division Duplex (FDD) WCDMA radio access technology should be designed for a third-generation UMTS system.

ITU IMT2000 3GPP GSM

GPRS

EDGE

UMTS FDD

Figure 1-1. 3GPP mobile network family.

TDD

INTRODUCTION TO UMTS NETWORK

19

1.1 Universal Mobile Telecommunication System UMTS as a system is an evolutionary step for voice and data calls of different transmission rates measured in kbps or Mbps. The key idea of UMTS is to be as dynamic as possible and to use system resources for different purposes (for example FACH or RACH channels for both signaling and low data traffic needs). Voice calls range from low quality to high quality (6-12 kbps, for example), depending on the user profile. Data services also vary (from 0 kbps to 2 Mbps), depending on the application needs. Various data applications like video streaming and games are aggressively marketed, even though the applications most expected today are email and Multimedia Messaging Service (MMS) solutions, and the major goal of UMTS is the reduction of response time in these data transmission applications. The UMTS network contains Radio Access Network (RAN), Core Network (CN), and Network Management System (NMS); the interfaces between them are depicted in Figure 1-2 [5]. BSS

CN

BTS

BSC

A

PSTN

BTS Abis

PSTN

F

PCU

PSTN/ISDN

3G GMSC

3G MSC/VLR

TC

C

D

Gb

H Gs

HLR

AuC

EIR

IuCS

UTRAN

Gr Gf

NodeB

Gc

RNC IuPS

NodeB

Gn

SGSN

Iubis

Gi

GGSN

Iur

NodeB

NodeB

Iubis

RNC

Figure 1-2. UMTS network elements and interfaces.

Internet/Intranet

20


Radio access network is also called UMTS Terrestrial RAN (UTRAN), and radio access (radio interface) is also called UMTS Terrestrial Radio Access (UTRA). Figure 1-2 also shows that GSM is part of the UMTS network; thus the UMTS system has similar types of interfaces and network elements as in GSM. Table 1-1 gathers and compares UMTS and GSM radio network element and interface names. Table 1-1. Radio network element names in UMTS and in GSM. UMTS GSM (UT)RAN BSS RNC BSC NodeB BTS Cell Cell Site Site UE MS Iur Iub Abis Uu Um

Due to similar architecture, the UMTS part of the specification is similar to the GSM part of the specification (for example, in the names and use of signaling channels or radio interface parameters); therefore, a good knowledge of the GSM is very helpful in understanding the UMTS system.

1.2 Universal terrestrial radio network UTRAN contains Radio Network Controller (RNC) and NodeB elements and the interfaces between them. In UTRAN, two or more RNC elements can be connected to each other via Iur because of soft handovers; thus, traffic data can be utilized much better than in GSM (no connections between BSC elements). In UTRAN, Iub interface is also open to all manufacturers. The functionality of a cellular radio network like UTRAN can generally be divided into three categories: the cellular concept layer, the radio interface management layer, and the radio interface technology layer. See Figure 1-3. The main cellular concept functionalities are related to Radio Resource Management (RRM) and Mobility Management (MM). For example, location areas and paging or other signaling channel needs (such as radio access channel (RACH) in call establishment) or UE tasks in idle mode with no connection (such as measurements, system info, neighbor cells, or cell selection) are enhanced and utilized in UTRAN.


21

Radio interface management is also related to RRM that is moreover managed mainly by RNC. In practice, radio interface management happens by utilizing different signaling messages between RNC and UE. RRM and signaling needs can be divided into connection-based and cell-based needs. For each connection, fast power control (PC) and soft handovers (SHO) are required by a WCDMA radio access in order to maximize system capacity in a radio network. For RRM, in each cell, more advanced Admission Control (AC), Load Control (LC) and Resource Management (RM) are required due to different services (various bit rates from 6 kbps (voice) to 2 Mbps (data) transmission rates). Moreover, several packet transmission services (for example, MMS-over-packet transmission) set higher requirements for Packet Scheduling (PS) in order to optimize system performance. Hence, RNC design is much more complicated in UMTS than in GSM, and communication between RNC and NodeB, and between NodeB and UE, requires significantly more information and signaling.

Cellular concept (network elements and tasks, RRM, MM)

Radio interface management (Iub, Iur and Uu interfaces)

Radio interface technology requirements

Figure 1-3. Cellular radio network functionality layers.

WCDMA technology causes major changes also in NodeB receiver and transmission units because of wide bandwidth. In transmission direction (downlink), the linear amplifier of NodeB is critical because high transmit power over the whole carrier bandwidth is needed. At the receiving end, a new receiver called a RAKE receiver is needed in order to utilize multi path components in a multi path propagation environment. The performance of a RAKE receiver is essential in order to receive maximal signal power and thus to maximize the power budget and path loss. Finally, all frequencydependent elements, such as antennas, cables, power amplifiers, low noise amplifiers, filters, etc., must be designed for the 2100 MHz frequency band; and therefore new equipment is needed in a UMTS implementation. It can be concluded that WCDMA has a strong impact on radio interface management and technology layers, and thus UTRAN radio planning principles and planning and measurement tools must be updated for


22

WCDMA technology needs. Moreover, modifications are needed in the planning process and detailed planning parameters for different planning phases.

1.3 WCDMA cellular concept and planning strategy Radio or mobile networks are more generally known as cellular networks when a cellular concept is used in the radio network infrastructure. In the cellular concept [6], depicted in Figure 1-4, a base station is transmitting and receiving a radio signal and providing service for a particular coverage area called a cell (in Europe) or sector (in the U.S.). Several cells are needed to cover a wide geographical area because transmit power is typically limited due to technical challenges. Moreover, several cells are needed for higher transmission capacity. When a cell structure is repeated, frequency channels can be re-used without experiencing co-channel interference, as occurs in FDMA and TDMA networks (for example, with GSM). The performance of FDMA/TDMA-based networks depends on the frequency reuse efficiency (average number of cells between co-channels), which typically varies from 10 to 20 as a function of propagation environment and software features [67].

R f1

D

f1

Figure 1-4. FDMA/TDMA cellular concept.

In CDMA-based networks, the same frequency can be used in neighbor cells because cells and users are separated by using orthogonal codes, as shown in Figure 1-5. The transmitted data (voice or data transmission) is multiplied by a spreading code, and the received data is decoded by using the same code. The name WCDMA represents Wideband CDMA radio


23

access. Wideband refers to a flat fading radio propagation channel, even if WCDMA is not flat fading in all propagation environments. (See Section 1.4.1.3.) UE1 = 0110110101011 UE2 = 1010010111011 f1

UE1

f1

f1 UE2 f1

f1 f1

f1

f1

f1 f1

Figure 1-5. CDMA/WCDMA cellular concept.

The capacity of WCDMA radio access is typically interference-limited because all mobile phones and neighbor base stations interfere with each other in the uplink and downlink directions. In practice, non-orthogonal codes cause additional interference for mobile phones in the downlink direction due to multi path propagation. Figure 1-6 depicts all the different sources of co-channel interference in WCDMA radio access. f1 UL interference UEn f1

UE2 UE1

f1

DL interference DL interference (non-orthogonal codes)

Figure 1-6. Uplink and downlink co-channel interference in WCDMA network.


24

The sources of co-channel interference can be divided into two categories: intra-cell interference (also called “own-cell-interference”) and inter-cell interference (also called “other-cell-interference”), both of which strongly depend on the locations of the mobile station. Due to both interference sources, system level (“system” meaning a certain cluster of cells) calculations (also called simulations) are required in order to estimate the final performance of the WCDMA radio access network. This new calculation requirement has a strong impact on a UMTS radio network planning strategy and process. 1.3.1 Radio network planning strategies for UMTS UMTS radio network planning strategy must be done before actual planning can be started. In this planning strategy work, the overall layout of the radio network and the evolution path are decided upon in order to reach the optimal network configuration for quality, capacity, and coverage. Strategy work starts by analyzing the existing radio network configuration and by defining the needs of a UMTS network for a certain planning area. (See Figure 1-7.) Next, the UMTS network major topology or layout (antenna heights and site density) must be decided upon in order to define the radio propagation environment and to fix planning principles. After defining the propagation environment, the major technical definitions of the radio network can be made. These include traditional (typical cellular concept) and new technologies such as adaptive antennas, Multiple Input Multiple Output (MIMO) or Opportunity Driven Multiple Access (ODMA) concepts in the future.

EDGE EDGE

GSM/GPRS900 UMTS?

EDGE

EDGE

Figure 1-7. UMTS network over a GSM/GPRS/EDGE network.


25

1.3.1.1 2G/3G interworking strategy and principle The existing 2G networks (typically GSM/GPRS and/or EDGE combinations) were originally built for voice calls, and radio network coverage is now excessive due to several years of deployment. Typically, the lowest 2G frequency band (800-900 MHz, where outdoor propagation is better) is a coverage layer, used mainly when other network layers are not working. The next 2G layer in dual band networks operates on a higher frequency band (1800-1900 MHz). Compared to the lower frequency band, this layer has worse outdoor radio wave propagation and a reduced coverage. Therefore, this layer is mainly used for capacity extensions. When GPRS/EDGE services are added to the existing 2G radio network, the capacity layer should be utilized for its capacity, but then the coverage of data services would be limited. Thus, data services must be added to the coverage layer, and voice calls must be moved to the capacity layer along as well as some data calls. The purpose of a UMTS system in a multi mode, multi band radio network is to extend radio access capacity (more frequency band together with GSM), to offer higher data rates and a larger variety of services, and to be a new step in technology evolution. Thus, UMTS coverage has to be almost equivalent to GSM/GPRS/EDGE coverage, and UMTS capacity is mostly reserved for new services and partly reserved as a capacity layer for old services. Moreover, UMTS coverage should be continuous wherever implemented in order to maintain a consistent service level. 1.3.1.2 2G/3G radio network layout An existing coverage layer of 2G network (800-900 MHz layer in single and dual band networks, or 1800-1900 MHz in single band networks) must be a reference layer when a UMTS layer is designed. In multi band TDMAbased networks co-siting and co-sectoring (like overlay-underlay structure [7]) is preferred due to significantly easier handover planning between frequency layers, as shown in Figure 1-8. If new site locations are used for the 1800 MHz capacity layer, careful parameter planning is needed also for cell selection in call establishment in order to move traffic smoothly to the capacity layer. The same co-siting and co-sectoring approach is preferable also for a GSM/UMTS multi mode network layout. Figure 1-9 shows different 2G/3G layouts when UMTS is added to a single or dual band 2G network. UMTS operates on the 2100 MHz frequency band. UMTS coverage planning is straightforward if the existing 2G radio network is a single band 1800-1900 MHz network, because the coverage areas are almost equivalent for both frequency bands. If UMTS is added to a single band 800-900 MHz network, new sites are needed to have continuous coverage for UMTS; and efficient co-siting is challenging due to


26

the optimization of new site locations. Moreover, in a multi band multi mode network, an equivalent coverage layer should be implemented for both 2G and 3G layers. Thus, UMTS coverage planning should follow as closely as possible the 2G coverage layer site locations in order to optimize handovers.

900/900

1800/1800

900/900

1800/1800

Figure 1-8. A 900/1800 MHz multi band radio network layout when co-siting and cosectoring are utilized.

UMTS

UMTS UMTS

UMTS UMTS UMTS

UMTS

UMTS

UMTS

a)

GSM/GPRS1800

GSM/GPRS900

UMTS

UMTS

b)

Figure 1-9. 2G/3G radio network layouts: (a) based on 900 MHz GSM coverage layer, (b) based on 1800 MHz GSM coverage layer.

In UMTS site selections, co-siting with a 2G network layout is preferred due to lower site rental costs. However, the final value of co-siting must be evaluated carefully because of inter-system interference related to technical limitations and because of traffic distribution in the radio network. Traffic


27

distribution is not as important an item in TDMA-based networks as it is in CDMA-based networks, so 2G site locations are often selected with a lower traffic layer priority. Hence, co-siting is preferred only if 2G site locations are appropriately chosen for traffic demands. Before the final site selection, different base station layer structures and radio propagation environments must be defined for the UMTS radio interface, as presented in Figure 1-10. The required size of the base station coverage area must be calculated. Furthermore, the average base station antenna height for the network must be decided. During the early phase of the radio network evolution, large coverage areas are needed, mainly to cover the area. This leads to a macrocellular type of network topology where base station antennas are above the average building heights in urban areas. If enormous capacity is needed for a dense urban area, several micro cells and a microcellular type of topology are required: base station antennas must be implemented below the rooftop levels in order to avoid interference. Also special indoor systems are needed for office blocks and shopping centers to provide enough capacity. In UMTS, as in all other radio networks, one main layer (macro or micro) should be used for coverage and other layers would only help the primary layer. UMTS networks are most probably implemented with the macrocellular approach due to practical considerations (number of sites, rental costs, etc.), theoretical considerations (propagation, breakpoint distance, antenna tilting, etc.), and because of an existing 2G network. f1 Macro layer = coverage layer f1

f1 f1 f1 Micro layer = capacity layer

Figure 1-10. UMTS base station layers.

Finally, the applicability of the UMTS radio network topology must be evaluated for new software features and service concepts such as location technologies. If mobile terminal location information will have a high priority in the future, UMTS radio network topology should also be


28

optimized, based on location information needs as accuracy and area to be served. Location technologies, like Observed Time Difference of Arrival (OTDOA), cause significant requirements on the radio network topology in UMTS in order to maximize accuracy and area of applicability [8-12]. Moreover, it must be analyzed whether repeaters [13], for example, can be used for improving coverage without reducing the accuracy of location information. 1.3.2 Advanced cellular concepts Traditional radio network planning principles must be modified if new technologies like adaptive or smart antennas (also called Space Division Multiple Access, SDMA), Multiple Input Multiple Output (MIMO), or Opportunity Driven Multiple Access (ODMA) are utilized in UMTS networks in the future. Adaptive antenna systems for cellular systems [14] have different levels of complexity in terms of software (digital signal processing, DSP) and hardware requirements. When complexity is increased, system intelligence and performance are also strongly increased. Figure 1-11 shows a fixed multi beam (Butler matrix) and dynamic beam forming adaptive antenna solutions for a radio network layout. The fixed multi beam is easier to implement, and it does not cause major changes to radio network planning, but the improvement in system performance is also limited. Dynamic beam forming needs strong changes in base station implementation and also lots of DSP capacity. But system performance is also significantly increased because antenna beams can be directed separately for each user. This also means strong changes in radio planning: for example, coverage areas as well as handover decisions are dynamic.

f1

f1

Figure 1-11. Fixed multi beam and beamforming SDMA system.


29

The MIMO concept is based on the existence of multiple distinguishable radio channels between transmit and receive antenna arrays in the scattering propagation environment. When different data is transmitted through each of these channels, the capacity of the system increases significantly [15-16]. The impact of the MIMO concept on traditional radio network planning is still under research.

• • •

Figure 1-12. MIMO system.

ODMA is being studied for CDMA-based systems and is proposed, for example, for WLAN (also called intelligent relaying, or ad hoc network) and UMTS systems [17-20]. In ODMA, the mobile station creates a radio link by transmitting and receiving data via other mobile stations that are closer to the base station, as shown in Figure 1-13. This helps with power consumption, and thus interference is reduced and capacity increased. If ODMA is adapted in UMTS, the power budget and cell ranges must be defined for chain structures [17-20]. C

A

UEA2 B UEA1

UE1 UEB1

Figure 1-13. ODMA system.

30


1.4 UMTS radio interface A WCDMA radio access is utilized in the UMTS radio interface (Uu). The characteristics of a WCDMA radio propagation channel have to be known well in different propagation environments because WCDMA radio network capacity depends heavily on interference and furthermore on the received signal level. Essential WCDMA parameters for UMTS system are presented in Table 1-2. Table 1-2. WCDMA radio access parameters for UMTS. Parameter Value Access technology DS-CDMA Chip rate 3.84 Mchip/s Channel bandwidth 5.0 MHz ± Nx200 kHz, N = 1, 2, 3 Duplex Frequency Division Duplex (FDD) Time Division Duplex (TDD) Frequency band (FDD) 1920-1980 MHz (UL), 2110-2170 MHz (DL) Frequency band (TDD) 1900-1920, 2020-2025 MHz (UL/DL) Frame structure 10 ms, 15 time slots

WCDMA radio access is based on direct sequence code division multiple access (DS-CDMA) technology. This means that a user data sequence is multiplied by a spreading sequence that has a symbol (also called “chip”) rate much higher than the user data rate. This spreads the user data signal to a wider frequency band. The relation between the user data rate and chip rate is called the spreading factor ( SF = Rchip Rbit ). The chip rate in WCDMA is 3.84 Mchip/s; spreading factors are in the: range of 4 to 512; and the user net bit rates supported by one code channel are in the range of 1 to 936 kbps in the downlink direction. Up to three parallel codes can be used for one user, thus giving bit rates up to 2.3 Mbps. In the uplink direction, data rates are half of these figures due to modulation differences. The nominal channel bandwidth of WCDMA signal is 5 MHz, as illustrated in Figure 1-14. The specification gives the flexibility to define the exact channel center frequency at 200 kHz raster, and thus the actual channel separation may be smaller than the nominal 5 MHz. The WCDMA standard includes two modes of operation, WCDMA/FDD and WCDMA/TDD. WCDMA/FDD is a frequency duplex division mode, in which the uplink and downlink signals are at different frequency bands (1920-1980 MHz in uplink and 2110-2170 MHz in downlink). WCDMA/TDD is a time duplex division mode, where the uplink and downlink signals are at the same frequency (1900-1920, 2020-2025 MHz for both uplink and downlink) but separated into different time periods. In this


31

book, WCDMA or UMTS refers to WCDMA/FDD because this type of air interface is first deployed and utilized. . 0

Power / dB

10

20

30

40

4

2

0

2 4 Frequency / MHz

6

8

10

Figure 1-14. Theoretical spectrum of two WCDMA carriers with 5 MHz channel spacing.

The transmission in WCDMA is split into 10 ms radio frames that consist of 15 pieces of 666 and 2/3 µs (2560 chips) time slots. The bit rate and channel coding can be changed for every frame by offering a very flexible user data rate control. Every time slot has bits reserved for the pilot signal, power control (TPC bits), transport format indication (TFCI bits), and, if needed, for closed-loop transmit diversity (FBI bits). The exact signal format and multiplexing is quite different in uplink and downlink signaling, and the dedicated and shared channels have several differences in signal format [2122]. 1.4.1 Radio propagation channel The UMTS radio interface has special characteristics for different propagation environments that can be classified into outdoor macrocellular, outdoor microcellular, and indoor propagation types. A macrocellular environment contains an urban, suburban and rural type of area, depending on the building or other obstacle density. Each propagation environment has its own propagation characteristics, which can be defined by the following parameters: – multi path propagation, – angular spread, – delay spread,


32

– fast fading and coherence bandwidth, – slow fading, and – propagation slope. Each of the above parameters has a specific value for a different environment and an impact on planning principles. Multi path propagation causes non-idealities for orthogonal codes, which furthermore causes owncell interference in the downlink direction. Angular spread and delay spread cause fast fading, which has a strong effect on the received signal levels. Slow fading causes a variation in signal levels as well, and the propagation slope illustrates a total attenuation between the base station and the mobile station antenna as a function of distance. 1.4.1.1 Multi path propagation in UMTS Multi path propagation occurs due to reflections, diffractions, and scatterings from different obstacles such as buildings in the radio path. Figure 1-15 shows the different paths d1, d2, and d3 from the base station antenna to the mobile station antenna. In Figure 1-15, transmitted signals s1, s2, and s3 have a certain amplitude and phase, depending on the transmission direction. At reception, each signal component s1’, s2’, and s3’ (also called the multi path component) may have a different amplitude, phase, and path length because of reflections, diffractions, and scattering. These multi path components cause different incident angles (angular spread), propagation time (delay spread), and a constructive or destructive sum (fast fading) at reception. s1 = a+jb s2 = c+jd s3 = e+jf d1’

A

Reflection d1’’ d2’

d3’

Diffraction

UE1 d3’’

d3’’’ d3’’’’

P

s2’

s1’ s3’

Figure 1-15. Multi path propagation.

d1’’’ s1’ = a’+jb’ s2’ = c’+jd’ s3’ = e’+jf’

t


33

Thus, the transmitted signal is spread over time and in frequency domain, and the result at reception is the sum of different signal components. This spreading, especially in the time domain, causes interference in the downlink direction in WCDMA because spreading codes are not ideally orthogonal. Interference due to non-ideal orthogonal codes increases the load of the network, and this must be taken into account in power budget calculations. Non-ideality of orthogonal codes is measured by an orthogonal factor (OF) [23]. Orthogonal factors of 60% and 85% are typically used in a power budget for urban and rural, and for microcellular and indoor propagation channels, respectively. 1.4.1.2 Angular spread in UMTS Angular spread describes the deviation of the signal incident angle. Angular spread can be calculated based on the incident angle of the received power in the horizontal and vertical planes [24-25],

SΦ =

Φ +180

³

(Φ − Φ ) 2

Φ −180

P (Φ ) dΦ PΦ _ total

(1-1)

where Φ is the mean angle, P(Φ) is the angular power distribution, and PΦ_total is the total power. Angular spread depends mainly on the environment type, and typical values are 5-10° for macrocellular environment and up to 360° in the indoor propagation channel. 1.4.1.3 Delay spread in UMTS Delay spread SĲ can be calculated from the power delay profile PĲ,

³ (τ − τ )Pτ (τ )dτ , ∞

Sτ =

0

(1-2)

Pτ _ tot

which describes the signal power as a function of delay. A power delay profile can also be presented as an impulse (power) response of the channel. Figure 1-16 presents an example of a power delay profile based on a WCDMA channel model for a typical macrocellular urban environment [26]. In Table 1-3, calculated delay spread values are presented for different propagation environments based on Equation 1-2 and a typical power delay profile of each environment.


34

Power delay profile 0.3

.

Power

0.2

0.1

0

0

0.5

1 Delay, ms

1.5

2

2.5

Figure 1-16. Channel impulse response, typical urban channel [26].

Table 1-3. Delay spread, coherence bandwidth and type of propagation channel for different radio propagation environments. Delay spread [µs] ǻfC [MHz] WCDMA Bandwidth 3.84 MHz Macrocellular Urban 0.5 0.16 Wideband Suburban 0.1 1.6 Narrowband/Wideband Rural 3 0.053 Wideband Microcellular < 0.01 > 16 Wideband Indoor < 0.01 > 16 Narrowband

1.4.1.4 Fast fading and coherence bandwidth in UMTS Power delay profile and delay spread are time domain properties of the radio channel. The effect of the multi path propagation to the radio channel can also be described by the frequency domain properties of the radio channel. In the frequency domain, multi path propagation causes frequency selective variation of signal levels called fading: signals at different frequencies have different amplitude and phase. The frequency response of the channel can be calculated as Fast Fourier Transformation (FFT) of the complex impulse response of the channel. Figure 1-17 presents the frequency response of the channel, whose power delay profile was presented in Figure 1-16.


35

Frequency responce of channel 10 . 5

Power / dB

0

5 10

15

20

0

2

4 6 Frequency / MHz

8

10

Figure 1-17. Frequency response of the multi path channel shown in Figure 1-16.

One frequency domain property of the channel is coherence bandwidth ǻfc. It can be calculated from the time domain property delay spread. ∆f c =

1 2πSτ

(1-3)

Coherence bandwidth is the minimum frequency separation of two carriers that have significantly uncorrelated fading. In Table 1-3, typical coherence bandwidths for different radio propagation environments were presented. The system is called narrowband when the radio signal bandwidth is much narrower than the coherence bandwidth of the radio channel; it is called wideband when it is much wider. Narrowband means that the fast fading of a channel is not frequency selective and the channel can be called flat fading. In a wideband environment, the fast fading of the channel is frequency selective. 1.4.1.5 Slow fading in UMTS Slow fading, or log-normal fading occurs because of obstacles such as buildings. The standard deviation of an outdoor signal is approximately 7-10 dB at 200-3000 MHz, depending on the environment [27]. At a UMTS frequency band of 1900-2125 MHz (TDD+FDD), the slow fading is on the level of 8-9 dB with reasonable accuracy. Based on the standard deviation,


36

the slow fading margin can be defined for different location probabilities in the radio network [28]. 1.4.1.6 Propagation slope in UMTS UMTS utilizes the 1900-2125 MHz frequency band, which is close to the 1800 MHz band. Thus, the propagation slope in different propagation environments follows the 1800 MHz frequency band with reasonable accuracy.

1.5 UMTS radio planning process UMTS radio interface is based on WCDMA technology, and thus the radio network planning process must be modified from the traditional GSM (TDMA-based) planning process. The major planning phases depicted in Figure 1-18 – pre-planning (dimensioning), detailed planning, and postplanning (optimization) – are still valid. But the detailed planning phases – configuration planning, coverage planning, capacity planning, frequency planning, and parameter planning – and their content must be modified.

PRE-PLANNING ”DIMENSIONING” • Network layout • Network elements • Antenna heights

”DETAILED” PLANNING

POST-PLANNING ”OPTIMIZATION”

• Configuration • Coverage-Capacity • Code • Parameter

• Verification • Parameters • Monitoring

Figure 1-18. WCDMA planning process.

1.5.1 Dimensioning In the dimensioning phase, a rough estimate of the network layout and elements is needed, such as the number of base stations to cover a certain area and to serve a certain capacity. Moreover, one critical parameter for a detailed planning phase is the base station antenna height, which must be


37

defined in order to be able to define the characteristics of the radio propagation channel and optimized planning guidelines (such as antenna tilting) for that environment. Thus, the main result of dimensioning phase is a list of required network elements and the average antenna height. Whereas dimensioning uses hypothetical data, the detailed planning phase (described next) uses the same procedures but actual data. 1.5.2 Configuration planning In configuration planning, the base station and base station antenna line equipment is defined, and the maximum loss allowed between the base station antenna and mobile station antenna is calculated in the uplink and downlink directions. In UMTS power budget calculations, gains (antenna, amplifier, etc.), losses (cable, filters, etc.), and margins (slow fading, etc.) are added to transmit and receiving power levels, as in GSM. In UMTS, the transmission requirements of common control channels and the pilot channel must be included in the power budget. In addition, the impact of intra-cell and inter-cell interference (interference margin, IM) and new WCDMA margins (for example, fast fading margin also called power control headroom) must be added to the power budget, as presented in Table 2-6 and Table 2-7 in Section 2.3. It must be remembered that the UMTS power budget is uplink-limited when maximum coverage is targeted and the load of the network is low. Moreover, the UMTS power budget is downlink-limited when the load of the network is increased to the maximum. Finally, it must be noted that the base station antenna configuration has a strong impact on the interference level in the network and on radio network capacity, and thus antenna selection must be done carefully. The power budget must be calculated for different service profiles, and cell-breathing phenomenon must be taken into account in coverage threshold settings and cell range calculations. The result of configuration planning is a detailed base station configuration and a list of antenna line elements for different network evolution phases and the maximum uplink and downlink path loss information for coverage predictions. More detailed explanations of power budget calculations are presented in Chapter 2. 1.5.3 Topology planning UMTS coverage planning can be done as in GSM by using path loss information and prediction models such as Okumura-Hata if there is no other traffic (only one mobile terminal) and thus no interference in the radio network. When traffic is included, cell-breathing occurs and cell range, as well as coverage area, is dynamic, based on the load of the network. Thus,


38

coverage areas are linked to each other as a function of mobile terminal locations. Moreover, the maximum load or capacity of the UMTS radio network depends on the coverage areas (cell overlapping, depth of propagation slope, etc.), base station locations, and base station antenna line configurations (antenna height, direction, beam width, tilting, etc.). Hence, coverage calculations and capacity estimation must be done simultaneously. This planning phase can be called topology planning, because the final coverage and capacity of the network mainly depends mainly on the base station antenna line configuration and network layout. Topology planning in UMTS begins with coverage predictions, as shown in Figure 1-19, in order to estimate coverage overlapping and dominance areas. Coverage thresholds are used at this point for estimating coverage and dominance areas, as in GSM planning.

COVERAGE PREDICTIONS • Pilot coverage • Thresholds • Overlapping

MONTE CARLO SIMULATIONS • Traffic distribution • Services • Load • Interference

NETWORK PERFORMANCE ANALYSIS • Throughput (kbit/s) • Soft handover area • Service probability

Figure 1-19. WCDMA topology planning.

Next, system level simulations are needed to estimate maximum traffic or load of the network in different cells. These system level simulations must be done for a certain cluster of cells so that all uplink and downlink interference is included. System level simulations are based, for example, on Monte Carlo type of simulations, where a certain number of mobile terminals are located over a coverage area and distributed homogenously or non-homogenously (with greater weight for indoor locations, for example). The results of Monte Carlo simulation include coverage, capacity, and interference-related information such as transmit powers of base stations, maximum number of users in each cell, and own-cell-to-other-cell


39

interference. These results finally give an estimate on whether base station sites are located correctly (throughput, service probability) and the total capacity of the radio network for a particular area. If one or more cells or sites need to be reconfigured (new site location, new antennas, new tilting, etc.), new simulations are always needed in order to get a performance estimation for the whole cluster. The detailed impact of the base station configuration and network layout on the final capacity in the UMTS radio network are introduced in Chapter 5. 1.5.4 Code and parameter planning After topology planning, only code planning and parameter planning are needed before the network can be launched because frequency planning is actually not needed in CDMA-based systems. In code planning, scrambling codes are allocated for different cells in order to separate cells in the downlink direction (Figure 1-20). Code planning is straightforward because there are enough codes in the 3GPP specification (altogether 512 primary codes); therefore, code limitations should not occur. See Chapter 6. In parameter planning, the radio interface functionality is optimized; this mainly includes signaling and radio resource management tasks. Parameters can be divided into signaling, identifier, RRM, measurement, handover and power control groups, which are all related to idle, connection establishment and connected modes. In the parameter planning phase, all parameters are grouped to these different categories, and default values are given when the network or a cell is launched. Later on, separate parameter values (such as active set parameters for handovers) can be changed, based on the needs of the radio propagation environment. The functionality of the radio interface and code and parameter planning are explained in more detail in Chapter 6. C Active sets for soft handovers

A A = 1011010010101... B f1 f7 Inter system handovers

Figure 1-20. Code and parameter planning.

B = 1110101010010...


40

1.5.5 Verification, monitoring and optimization The UMTS radio network is designed after system level simulations in topology planning and code and parameter definitions. The next planning phase is verification, monitoring and optimization when the radio network is implemented. (See Figure 1-21.) In the verification phase, call establishments (idle mode) and soft and inter-system handovers (connected mode) are tested. Also, coverage and dominance areas must be verified and analyzed due to a strong impact on radio network capacity. Verification of the radio network is mainly carried out with the use of a radio interface field measurement tool that is explained in Chapter 7.

PROBLEM INDICATION

RADIO PLAN VERIFICATION

PROBLEM SOLVING

• Verification measurements • KPI measurements • Field measurements

• NodeB configurations • Antenna line configurations • Coverage measurements • System simulations • Code planning • Parameter values • signaling • RRM (PC, HO)

• Configuration modifications • cell / site level • system level • Parameter changes • signaling needs • PC improvements • SHO areas

PROBLEM TYPE • Coverage (no signal) • Capacity (low throughput) • Quality (bad voice quality)

Figure 1-21. Verification, monitoring and optimization.

Monitoring contains Key Performance Indicator (KPI) values that are related to call success (or call establishment failures prevented by load or admission control) rates and drop call (soft, softer, or hard handover failures, cell-breathing and lack of coverage, overload, etc.) rates. More detailed monitoring is based on signaling messages between base station and mobile station measured by a radio interface field measurement tool or by a Qualityof-Service (QoS) analyzing tool, for example, from the Iub interface. The radio interface field measurement tool is based on information gathered by a UE and thus location information is also available. Accordingly, the Qualityof-Service analyzing tool measures or gathers detailed signaling and traffic information at the cell, site, and RNC levels. These QoS measurements are


41

especially needed for end-to-end QoS management and optimization purposes. QoS management is explained in more detail in Chapter 8. Finally, optimization contains different kinds of planning-related actions to solve problems found in the verification and monitoring phases. Optimization entails continuous trouble solving; it could also be called replanning because all planning phases and their results must be checked before any modifications can be done to the actual plan. The optimization process includes radio interface field measurements and QoS measurements to understand network bottlenecks at the cell, site, and RNC levels. 1.5.6 Radio planning environment Radio network planning must be supported by different software and hardware tools in different planning phases. In the dimensioning phase, only MS Excel based calculations are needed, and different kinds of function libraries can be utilized when coverage areas or capacities in Erlangs are estimated. In detailed planning, a more advanced planning tool based on highly accurate digital maps is required to get reliable coverage predictions. Figure 1-22 shows that planning tool must have interfaces to measurement systems and the network in order to import and export data, for example, in model tuning (measurement import) and parameter planning (parameter value export). Planning Planning tool

Measurements Field measurements

Statistical performance indicators

Protocol analyzer

Network plan

Network NodeB

NodeB

3G MSC/VLR

3G GMSC

SGSN

GGSN

PSTN/ISDN

RNC

Figure 1-22. Radio planning environment.

Internet/Intranet

42


The graphical user interface (GUI) of a planning tool must follow the planning process phases exactly because man-made errors can be significantly reduced if all planning parameters are automatically set in a particular order. The planning tool also must have reasonable documentation tools in order to have essential radio network information in the same database. Digital maps and planning and measurement tool requirements are explained in more detail in Chapters 3, 4, 7, and 8. 1.5.7 Documentation In UMTS, as in GSM, all data that is related to a UMTS site or UMTS cell must be documented. This data traditionally includes input data (NodeB and antenna line configuration data and parameters) and output data (measurement data as traffic - for example, kbps per busy hour, handover statistics, drop calls, etc.). In UMTS, some new items must be included in the documentation: – new power budget-related items as Eb/N0 in different environments (must be measured in practice); – new hardware elements in NodeB as code channels (software and hardware limitations); – new parameters for signaling (physical layer), load control, admission control, packet control, power control, and handovers; and – more detailed traffic monitoring. The quality of the NodeB and antenna line documentation must be strongly emphasized in UMTS. Because cells interfere with other, every mistake in documentation means changes in radio network coverage and capacity. Therefore, a reliable documentation tool that follows the planning process exactly is strongly recommended.

1.6 Radio network evolution UMTS is in full operation when all radio planning phases and implementation have been executed. The same planning process and planning phases need to be repeated whenever there is a new coverage or capacity need in the radio network. This change in the radio network is called network evolution; it includes, for example, local traffic changes such as new residential or business areas. Network evolution should be predictable with some accuracy 3 to 5 years ahead, in order to take it into account in long-term network layout design.


43

1.6.1 UMTS site configuration and topology evolution path Network evolution has a different impact on different planning phases. In configuration planning, it must be defined whether uplink or downlink is limiting the cell or site service area and which direction is used for actual planning. Traditionally, uplink and downlink directions have been balanced (equivalent path loss in both directions), and actual radio planning has been done in the downlink direction. In UMTS, uplink limits coverage when there is no traffic in the network, but downlink transmission power limits the service when the radio network is highly loaded. Thus, cell or site coverage or service areas (site locations) should be planned directly for a certain load and for both uplink and downlink directions. Another critical topic is network topology (layout) and especially base station antenna configuration. In the first phase of UMTS, three-sectored sites are preferred in order to have reasonable antenna costs, but later on, six-sectored antenna configurations offer significantly higher capacity. It is essential to have a planning guideline (site locations and antenna configurations) for changing the network from a three-sectored to a sixsectored configuration with a minimum change in radio network functionality as in soft handover areas. At this point, inter-system operability must also be kept in mind. Finally, new applications and features such as location techniques must be considered, and the radio network configuration must be adapted for different service needs during the network evolution.

REFERENCES [1] S. Faruque, Cellular Mobile Systems Engineering, Artech House Publishers, 1996. [2] R. Steele, C. Lee, P. Gould, GSM, cdmaOne and 3G Systems, John Wiley & Sons Ltd, 2001. [3] H. Holma, A. Toskala, WCDMA for UMTS, John Wiley & Sons Ltd, 2001. [4] J. Laiho, A. Wacker, T. Novosad, Radio Network Planning and Optimisation for UMTS, John Wiley & Sons Ltd, 2002. [5] Universal Mobile Telecommunications System (UMTS), UTRAN Overall Description, 3GPP TS 25.401. [6] W. Lee, Mobile Communications Design Fundamentals, John Wiley & Sons, 1993. [7] J. Lempiäinen, M. Manninen, Radio Interface System Planning for GSM/GPRS/UMTS, Kluwer Academic Publishers, 2001. [8] M. Birchler, E911 Phase II Location Technologies, IEEE Vehicular Technology Society News, November 2002. [9] J. Syrjärinne, Studies of Modern Techniques for Personal Positioning, Doctoral Thesis, Tampere University of Technology, 2001.

44


[10] S. Ahonen, J. Lähteenmäki, H. Laitinen, S. Horsmanheimo, Usage of Mobile Location Techniques for UMTS Network Planning in Urban Environment, Proceedings of IST Mobile & Wireless Telecommunications Summit, 2002. [11] K. Watanabe, M. Kuwahara, A. Ogino, K. Tsunehara, H. Suzuki, N. Doi, A Study on the Accuracy of a CDMA-Based Location Systems, IEEE 55th Vehicular Technology Conference, vol. 4, 2002. [12] D. Porcino, Performance of a OTDOA-IPDL Positioning Receiver for 3G FDD Mode, Second International Conference on 3G Mobile Communication Technologies, Conf. Publication No. 477, 2001. [13] Universal Mobile Telecommunications System (UMTS), UTRA Repeater: Planning Guidelines and System Analysis, 3GPP TR 25.956. [14] A. Boukalov, S-G. Haggman, System Aspects of Smart-Antenna Technology in Cellular Wireless Communications – An Overview, IEEE Transactions on Microwave Theory and Techniques, vol. 48, Issue 6, June 2000. [15] R. Janaswamy, Radiowave Propagation and Smart Antennas for Wireless Communications, Kluwer Academic Publishers, 2001. [16] D. Gesbert, M. Shafi, D. Shiu, P. Smith, A. Naguib, From Theory to Practice: An Overview of MIMO Space-Time Coded Wireless Systems, IEEE Journal on Selected Areas in Communications, vol. 21, no 3, April 2003. [17] T. Rouse, I. Band and S. McLaughlin, Capacity and Power Investigation of ODMA in TDD-CDMA based Systems, IEEE International Conference on Communications, vol. 5, 2002. [18] T. Rouse, S. McLaughlin and H. Haas, Coverage-Capacity Analysis of Opportunity Driven Multiple Access (ODMA) in UTRA TDD, Second International Conference on 3G Mobile Communications Technologies, Conf. Publication No. 477, 2001. [19] T. Harrold and A. Nix, Performance Analysis of Intelligent Relaying in UTRA TDD, IEEE 56th Vehicular Technology Conference, vol. 3, 2002. [20] T. Harrold, A. Nix and M. Beach, Propagation Studies for Mobile-to-Mobile Communications, IEEE 54th Vehicular Technology Conference, vol. 3, 2001. [21] Universal Mobile Telecommunications System (UMTS), Multiplexing and Channel Coding (FDD), 3GPP TS 25.212. [22] Universal Mobile Telecommunications System (UMTS), Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD), 3GPP TS 25.211. [23] K. Pedersen, P. Mogensen, The Downlink Orthogonality Factors Influence on WCDMA System Performance, IEEE 56th Vehicular Technology Conference, vol. 4, 2002. [24] J. Laiho-Steffens, A. Wacker, Experimental Evaluation of the Two-Dimensional Mobile Propagation Environment at 2 GHz, IEEE 47th Vehicular Technology Conference, vol. 3, 1997. [25] K. Kalliola, Experimental Analysis of Multidimensional Radio Channels, Doctoral Thesis, Helsinki University of Technology, 2002. [26] Universal Mobile Telecommunications System (UMTS), Deployment Aspects, 3GPP TR 25.943. [27] J. Parsons, Mobile Radio Propagation Channel (second edition), John Wiley & Sons Ltd, 2000. [28] W. Jakes, Jr., (ed.), Microwave Mobile Communications, Wiley-Interscience, 1974.

Chapter 2 UMTS CONFIGURATION PLANNING Base station configuration and power budget JARKKO ITKONEN, RISTO JURVA European Communications Engineering (ECE) Ltd

Abstract:

In configuration planning, the radio network planner selects the optimum configuration of the base station for the planned cell or part of the planning area. The planner must figure out a configuration that meets the network quality targets, takes into account the physical limitations, and requirements of the base station site and meets the given investment and operating cost limits. The main base station configuration planning options are presented in this chapter together with power budget calculations, which connect the selected options to the radio link path loss and further to cell size. Special configurations of repeaters and indoor solutions are also presented.

Key words:

Antenna, base station, cable, configuration planning, Eb/N0, Ec/N0, indoor planning, LNA, mobile station, noise figure, path loss, performance, power budget, repeater, sensitivity


46


2. UMTS CONFIGURATION PLANNING The target of UMTS configuration planning is to find the optimum configuration of the base station for each base station site in the planning area or a nominal base station configuration for different parts of the planning area. In configuration planning, several factors must be considered: – propagation environment type (macro, micro, indoor cell); – site characteristics (indoor, outdoor, wall, mast); and – required capacity and coverage. The planner must select the optimum combination among several options in order to fulfill the quality requirements. The main tool used in configuration planning is the power budget, which links the selected base station configuration to the maximum allowed attenuation of the radio signal in the radio path, i.e., maximum path loss. The maximum cell size can be calculated from the maximum path loss with propagation prediction models. The configuration planning of an indoor base station, a micro cell, and a repeater are special cases due to the special propagation environment type, site characteristics, or equipment configuration.

2.1 Base station configuration In this chapter, the base station configuration options are categorized into two categories: base station configuration and base station antenna line configuration. 2.1.1 Base station architecture The UMTS base station architecture is driven by the use of a single highpower linear power amplifier per cell and the high signal processing power requirement set by the WCDMA air interface. A typical high-level architecture for a UMTS base station with a 3-sector configuration is presented in Figure 2-1. Antenna filter (AF) unit filters out-of-band noise and interference from the received power, e.g., from other WCDMA carriers or radio networks, and amplifies the signal in the reception path. The performance of the AF unit affects the base station noise figure, and thus the sensitivity of the base station. In the transmit path, the AF unit filters out-of-band noise and spurious signals from the transmitted power. The attenuation of the AF unit to the transmit signal has to be taken into account in the base station transmit power.

UMTS CONFIGURATION PLANNING Antenna line 1

47

UMTS Base Station AF

Antenna line 2

TRU PA

AF Antenna line 3

TRU

Base band unit

Interface unit

PA AF

TRU

O&M

PA

Figure 2-1. Base station architecture.

Power amplifier (PA) amplifies the signal to the required power level. The power amplifier is characterized by amplifier linearity, output power, and bandwidth. The amplifier linearity requirement is set by the adjacent power leakage (APL) power and transmit modulation requirements, and given in the 3GPP specifications [1]. The output power depends on the product type (typically 5–40 W). The power amplifier bandwidth for a single carrier amplifier is about 5 MHz, and for a wideband amplifier, it is 15–20 MHz, which makes it possible to amplify several carriers with a single amplifier and usually cover the whole operator frequency band. Most commercial amplifiers are of the wideband type. With multiple carriers per amplifier, the output power of the amplifier is shared between the carriers. Transceiver unit (TRU) is in the base station between analog radio frequency (RF) signal processing and digital signal processing. In the reception path, the signal is typically down-converted to the inter-frequency (IF) band, filtered, and then converted from analog to digital (A/D conversion). In the transmit path, the signal is converted from digital to analog (D/A conversion), filtered, and converted to the selected carrier frequency. The Base Band Unit (BBU) simultaneously handles numerous tasks such as the following: – common signaling channel processing – random access channel (RACH) detection – common pilot channel (CPICH) generation – L1 signal processing for common and dedicated physical channels – error detection – channel coding/error correction


48

– – – –

interleaving transport channel multiplexing spreading/despreading channel estimation for phase correction and RAKE receiver configuration – RAKE receiver – modulation – filtering – fast closed-loop power control – load estimation for all carriers. The processing power requirement of the BBU can be divided into static processing (common signaling channels) and dynamic processing (common and dedicated data channels). The BBU processing capacity must be planned according to both of these. Interface unit connects the base station to the Radio Network Controller (RNC). Operation and maintenance (O&M) unit is the interface to the network management system for network supervising and configuration. A combiner solution (not shown in Figure 2-1) must be selected if the configuration supports multiple carriers per sector. The combiner solutions are vendor-specific. Base station performance is mainly related to the transmit power, processing capacity and sensitivity. Transmit power depends on the power amplifier performance, and processing capacity depends on BBU performance and the number of hardware units installed in the base station. Base station sensitivity depends on base station RF and base band performance. Sensitivity depends also on the service type, data rate and propagation environment, but these are not related to the performance of the base station itself. Section 2.3.3 presents the calculation of the base station sensitivity and Chapter 5 includes typical sensitivity values for different services. 3GPP specifications [1] set several requirements for the characteristics and performance of the base station transmitter and receiver. The main receiver performance requirements are given as Eb/N0 levels in different propagation conditions. The specification defines the Eb/N0 as

Eb E Lchip E c Rc T frame E c Rc = c = = N 0 N 0 Linf N 0 Rb T frame N 0 Rb where

(2-1)

UMTS CONFIGURATION PLANNING

49

– Eb is the energy per information bit; – N0 is the total one-sided noise power spectral density due to all noise sources; – Ec is the received total energy per PN chip per antenna from all paths; – Lchip is the number of chips per frame; – Linf is the number of information bits in dedicated traffic channel (DTCH); excluding CRC bits per frame; – Rc is the chip rate (3.84 Mcps); – Rb is the information bit rate between Layer 1 and Layer 2; and – Tframe is the duration of the radio frame (10 ms). In Table 2-1, an example from the specification for multi path Case 3 is shown to approve the performance of the base station in multi path conditions resembling a small macro cell with the mobile speed of 120 km/h. The test configuration is for a dual branch receiver (antenna diversity) and no power control. Table 2-1. Base station performance requirement for multi path Case 3 channel [1]. Measurement channel Required received Eb/N0 BLER 12.2 kbps n.a. < 10-1 7.2 dB < 10-2 8.0 dB < 10-3 64 kbps 3.4 dB < 10-1 3.8 dB < 10-2 4.1 dB < 10-3 144 kbps 2.8 dB < 10-1 3.2 dB < 10-2 3.6 dB < 10-3 384 kbps 3.2 dB < 10-1 3.6 dB < 10-2 4.2 dB < 10-3

2.1.2 Base station antenna line configuration A typical configuration of a UMTS base station antenna line is presented in Figure 2-2. The antenna line configuration directly affects the cell coverage area, cell capacity, and interference; thus it must be planned before the detailed coverage/capacity planning phase is started (see Chapter 1). First, antenna line configuration planning must consider which systems and frequencies are supported and what are the main site characteristics, such as antenna height and base station location (roof, basement, etc.). After that comes the planning of the antenna line equipment configuration (antenna type selection, feeder cable type selection, use of LNA/MHA, selection of antenna diversity method, antenna tilting, and antenna installation equipment setup).


50

I I I I

Antenna

LNA Jumpers Base station antenna line

Feeder cable Bias - T

BTS

Figure 2-2. Typical base station antenna line elements.

2.1.2.1 Base station antenna The characteristics of the base station antenna have a direct effect on the shape and size of the cell coverage area and on the amount of interference radiated outside the cell coverage area. Hence, several requirements must be considered in the selection of the base station antenna. – Antenna physical size is limited by the base station site. Large antennas can be installed on masts and rooftops, but usually smaller antennas are used in building wall installations, micro cells, etc. – Antenna frequency band: The base station antenna can be designed for single or multiple frequency bands. A single band antenna can be a socalled broadband antenna with a wide frequency band covering multiple systems (e.g., 1710–2170 MHz covering GSM1800 and WCDMA). Multiple band antennas usually work in two or three distinct frequency bands (e.g., 880–960 MHz, 1710–1880 MHz, 1920–2170 MHz) serving different systems (GSM900, GSM1800, and UMTS). – Vertical beam width is inversely proportional to the antenna vertical size. – Horizontal beam width is set mainly by the number of sectors selected for the site.


51

– Antenna polarization is typically vertical in single polarization antennas. In polarization diversity antennas, two perpendicular polarizations are used, usually at ±45 degrees. – Antenna down tilting method can be either mechanical or electrical. Mechanical tilting is implemented by the antenna installation. Electrical tilting is a feature of the antenna element. Electrical down tilt is usually in the range of 2–8 degrees, either fixed or tunable. The most important antenna specification values and typical specifications of four different antennas designed for UMTS bands are presented in Table 2-2. Table 2-2. Typical antenna specifications [2]. Ant 1 Ant 2 Site 3-sector, 3-sector, macro macro Max. dimension 1.8 1.3 Gain 18 17 Beam width Horizontal 65 65 Vertical 4.5 5.5 Electrical tilting 5, 2 – 7, fixed variable Polarization ± 45 ± 45 Frequency range 1900 – 2179 1900 – 2179

Ant 3 6-sector, macro 1.8 20

Ant 4 Micro

Unit

0.7 15

M dBi

30 4.5 5, fixed ± 45 1900 – 2179

65 13.5 0

degrees degrees degrees

± 45 1900 – 2179

degrees MHz

2.1.2.2 Feeder cable, jumpers and connectors Feeder cable is the other main part of the base station antenna line in addition to the antenna. Together with the jumpers and connectors, it connects the base station to the antenna. The main parameter of the cable is the signal attenuation or cable loss, which depends on the cable diameter and the signal frequency. Typical cable loss figures for different frequency bands and cable sizes are listed in Table 2-3. Table 2-3. Typical cable attenuation (dB/100 m) with different signal frequencies [2]. Diameter 900 MHz 1800 MHz 2200 MHz ½" 7.1 10 12 7/8" 3.7 5.4 6.1 1 5/8” 2.0 2.7 3.0 Jumper 13 19 21

Jumper loss per 100 m is much higher than the cable loss because of the small diameter, but typically the jumpers are quite short (< 1 m) and the loss of one jumper is a fraction of a dB. Connector loss is also typically quite small, e.g., 0.02 dB/connector.


52

2.1.2.3 Low noise amplifier The base station system noise figure can be improved by using a low noise amplifier (LNA, also called Mast Head Amplifier MHA) close to the antenna before connectors and antenna feeder losses. This is achieved by amplifying the received signal by an LNA before the receiver losses. The improvement of the system noise figure of a base station will increase the uplink maximum path loss (Section 2.3), thus LNA is considered an effective method to improve cell coverage in UL. The amount of improvement in system performance depends on: – LNA gain, GLNA; – LNA noise figure, FLNA; – total antenna line loss (cable, connectors, jumpers), LC; and – BS receiver noise figure, FBS. The improvement to the system noise figure can be calculated as the ratio of the noise figure without LNA and with LNA, as presented in Equation 22.

∆F = FLNA

FBS LC 1 (FBS LC − 1) + G LNA

(2-2)

The maximum improvement ∆FMAX can be solved from Equation 2-2 when the LNA gain approaches infinity. ∆FMAX =

FBS LC FLNA

(2-3)

In the dB scale, the maximum improvement is

∆FMAX = FBS − FLNA + LC .

(2-4)

The effect of the LNA gain to the noise figure improvement with two different cable loss figures is shown in Figure 2-3. The improvement exceeds the cable loss in both cases, with typical LNA gain values of 12–14 dB.


53

System noise figure improvement / dB

7 6

6

. 5 4 3

3 2 1 0

0

5

10 LNA gain / dB

15

20

L_C = 3 dB L_C = 6 dB

Figure 2-3. System noise figure improvement, FLNA = 2 dB, FBS = 3 dB, LC = 3 dB or 6 dB.

The effect of the LNA to the uplink performance will be significant in UMTS networks as the feeder cable loss is high due to the high frequency band. The effect of the cable loss on the system noise figure and on the noise figure improvement with and without LNA is shown in Figure 2-4.

.

14 13 12 11 10 9

dB

8 7 6 5 4 3 2 1 0

0

2

4

6

8

10

Cable loss / dB

System noise figure, no LNA System noise figure with LNA (gain 12 dB) System noise figure improvement

Figure 2-4. System noise figure and noise figure improvement vs. cable loss with and without LNA (FLNA = 2 dB, FBS = 3 dB).

54


The LNA causes a loss in the downlink signal path. This loss is equal to an LNA insertion loss that is typically about 0.5 dB. This should be taken into account in the downlink power budget antenna line loss (Section 2.3). Bias-T is required to provide power to the LNA via the antenna cable. Also, an external power feed directly to the LNA can be used, or Bias-T can be included in the antenna filter unit. In both cases, Bias-T is not required. 2.1.2.4 Antenna diversity configurations Base station antenna diversity is traditionally used in most configurations to enhance the uplink coverage and capacity. Space diversity and polarization diversity are the most common methods used for antenna diversity. The selection between these methods and the achievable gain to power budget depends on the propagation environment and the antenna installation. Space diversity provides gain in all environments, but it requires two antennas with a minimum separation to achieve the optimum gain. The minimum separation depends on the environment; typically, it is 15-25 Ȝ in rural environments, 10-15 Ȝ in urban/suburban environments, and 1-2 Ȝ in urban microcellular environments [3]. If the separation is less than that, the achieved gain is still at least 2.5–3 dB from the maximum ratio combining of two signals. A practical maximum gain of about 4-5 dB in receiver sensitivity has been measured in GSM with more than the minimum separation [3]. In WCDMA multi path diversity affects the achievable gain; i.e., in indoor and micro cells, the antenna diversity gain is higher than in large macro cells [4]. Polarization diversity has good performance in propagation environments with a large number of reflections and diffractions causing random polarization of the received signal. In GSM, the achievable gain in receiver sensitivity is 4-6 dB in urban, suburban, microcellular, and indoor environments. In rural environments, the gain is typically less than with space diversity [3, 5]. In WCDMA, the amount of multi path diversity also affects the achievable gain from polarization diversity. Polarization diversity is typically implemented with one polarization antenna, making it easier to install than space diversity. Typical configurations of antenna lines for space diversity and polarization diversity are presented in Figure 2-5. More complex antenna diversity methods are proposed to further improve the performance of uplink reception. For example, four branch diversity receivers with four space diversity or two polarization diversity antennas have been proposed. The additional gain compared to two branch diversity is about 1-2 dB [6].

UMTS CONFIGURATION PLANNING X X X X

RX/TX

I I I I

RX

RX/TX

55 X X X X

I I I I

RX

RX/TX

RX

I I I I

RX/TX

I I I I

RX

Figure 2-5. RX diversity configurations for polarization and space diversity, with and without LNA.

In UMTS systems, downlink performance is often considered to be the most critical, especially in high capacity (capacity-limited) cells. In order to improve downlink performance, support for base station transmit diversity was included in the WCDMA specification. The main principle of transmit diversity is to transmit the signal of one user via two different antennas. Two different methods to control the transmission of the signal are specified: socalled open-loop and closed-loop transmit diversity. Closed-loop transmit diversity has two different modes [7]. The achievable gain from transmit diversity depends strongly on channel conditions such as orthogonality and terminal speed. In the closed-loop method, the receiver performance in measuring the received signals and controlling the transmit antennas also affects the gain. Receiver Eb/N0 gains in the range of 0 to 3.5 dB has been reported have been reported for openloop and closed-loop methods [7]. 2.1.2.5 Multi system antenna configurations The antenna line must be planned to support both GSM and WCMDA systems in many sites, because support for GSM interworking is essential for WCDMA networks and installing both systems at the same site (co-siting) is often the most cost-efficient solution. Several different antenna types, such as broadband antennas and multiple band antennas, have been developed to support co-siting. The selection of the antenna type will also influence the antenna line configuration. The main selection related to the feeder configuration is whether to use separate or shared antenna lines. Figure 2-6 presents typical antenna line


56

configurations with separate polarization diversity antennas for GSM and WCDMA systems and with separate antenna lines. The same configuration with shared antenna lines is presented in Figure 2-7. Antenna line diplexer units are required to combine the signals in the same cable and to separate the signals at the antenna end when two different systems share the same antenna. Triplexers are used when there are three systems. X X X X

GSM base station

X X X X

X X X X

WCDMA base station

GSM base station

X X X X

WCDMA base station

Figure 2-6. GSM and WCDMA co-siting with separate antennas and antenna lines.

GSM antenna

X X X X

WCDMA antenna

GSM antenna

X X X X

X X X X

WCDMA antenna

X X X X

Diplexer Diplexer

GSM base station

WCDMA base station

GSM base station

Vendor specific DC feed to LNAs

WCDMA base station

Figure 2-7. GSM and WCDMA co-siting with separate antennas and shared antenna lines.


57

A diplexer or triplexer is also required when broadband antennas are used to combine the systems in the same antenna connectors. (See Figure 2-8.) From the antenna line point of view, multi band antenna configurations are similar to configurations with separate antennas because both use separate antenna connectors for the two systems. WCDMA&GSM antenna

WCDMA&GSM antenna

X X X X

X X X X

X X X X

Diplexer Diplexer

GSM base station

WCDMA base station

GSM base station

WCDMA base station

Figure 2-8. GSM and WCDMA co-siting with broadband and multi band antennas with common antenna cables.

2.2 Mobile station Transmit power and receiver sensitivity are the main mobile station parameters in the configuration planning. The 3GPP specification [8] sets requirements for the mobile station performance. These requirements can be used as the baseline for the configuration planning, although it must be considered that the performance requirements are given for different test cases that do not necessarily reflect realistic conditions in a radio network. In UMTS, a mobile station can belong to one of the four different power classes presented in Table 2-4, depending on the maximum transmit power. Most of the handheld mobiles will belong to class 4 with +21 dBm maximum transmit power. It should be noticed in power budget calculations that the tolerance for the transmit power is large (±2 dB). Products during their first years tend to prioritize current consumption over transmit power; thus, their maximum transmit power is closer to the lower limit.


58

Table 2-4. Mobile station power classes in UMTS [8]. Power Class Nominal maximum output power 1 +33 dBm 2 +27 dBm 3 +24 dBm 4 +21 dBm

Tolerance

+1/-3 dB +1/-3 dB +1/-3 dB ± 2 dB

The specification sets requirements for the mobile station receiver performance for type approval purposes. Table 2-5 shows an example of a receiver performance requirement in the multi path propagation environment Case 1, which resembles an indoor or small micro cell environment with a small delay spread. No antenna diversity or downlink power control is used. The requirements are formulated for type approval and cannot be directly applied in network planning to power budget calculations. Table 2-5. Receiver sensitivity (case 1) [8]. DPCH _ Ec Test Number (bit rate)

BLER

I or

1 (12.2 kbps) 2 (64 kbps) 3 (144 kbps) 4 (384 kbps)

-15.0 dB -13.9 dB -10.0 dB -10.6 dB -6.8 dB -6.3 dB -2.2 dB

10-2 10-1 10-2 10-1 10-2 10-1 10-2

The performance requirements are given as a ratio of the dedicated physical channel transmitted chip energy DPCH_Ec and the total transmit power spectral density Ior. This can be projected from the transmit end to the receiver antenna connection as:

° E c ½° DPCH _ Ec ½ ® ¾ =®ˆ ¾ I or ¯ ¿TX °¯ I or °¿ RX

(2-5)

where Ec is the received chip energy at the receiver and Iôr is the power spectral density of the transmitted signal at the receiver. The specified test conditions must be understood in order to use these requirements, e.g., in the power budget. The received total interference power density N0 has been derived in the following equation in order to figure out the relation of the total interference power density and the Iˆ or


59

used in the specification. N0 is the sum of the own-cell interference, othercell interference and thermal noise spectral densities. N 0 = Iôr (1 − α ) + I oc + Nth0 = Iôr (1 − α ) + Iôr i + Nth0

(2-6)

where – Îor is the total own-cell received signal level; – Į is the orthogonality factor; – Ioc is the power spectral of a band-limited white noise source (simulating interference from cells, which are not defined in a test procedure) as measured at the UE antenna connector [8]; – little i ( i = I oc Iôr ) is commonly used for describing the level of othercell interference; and – Nth0 is the receiver thermal noise power density. (In a real environment, this also includes external interference from other carriers and systems.) The equation can be reformatted to a correction factor (CF), which can be used to calculate the Ec/N0 or Eb/N0 requirement for the service. The last approximation can be done when Ioc >> N0.

CF =

Nth0 Nth0 N0 = 1−α + i + = 1−α + i + ≈ 1−α + i Î Î I oc i or or

(2-7)

The specification [8] defines that for Case 1, the external interference level Ioc is -60 dBm/3.84 MHz, and i is 9 dB. In that case, CF = -6.5 dB with an orthogonality of 0.9. The orthogonality depends on the channel’s multi path profile [9]. Ec Ec 1 = , N 0 Iôr CF

E b E c Rc = N 0 N 0 Rb

(2-8), (2-9)

For example, a -15 dB requirement in Table 2-5 corresponds to Ec/N0 level of -8.5 dB and with Eb/N0 of 16.4 dB (Test 1, 12.2 kbps).

2.3 Power budget The target of the power budget is to calculate the maximum uplink and downlink path loss and to verify that the uplink and downlink path losses are


60

at an equal level, i.e., the link is balanced (see Chapter 1). In a UMTS system, the link balance is not constant, as it is affected by the system load. In the downlink, the effect of the load is bigger because the users in the cell share the same power amplifier. Because of that, in high traffic cells, the downlink is the limiting direction (see Chapter 5). In the downlink, the maximum path loss is calculated from the base station transmit antenna to the cell edge, and in the uplink, from the mobile transmit antenna to the base station location. The transmit power at the transmit antenna (EIRP) and the minimum planned signal level on the cell edge (planning threshold) must be calculated in order to calculate the maximum path loss in the downlink, as shown in Figure 2-9. DL EIRP

Maximum Path Loss

DL Planning threshold BS Cell range Figure 2-9. Downlink power budget.

The same calculation is repeated in the uplink, and the direction with the smaller maximum path loss is selected as the limiting direction. The cell range is calculated from the maximum path loss of the limiting direction by using the selected propagation prediction model. The calculations of the downlink and uplink path loss are shown in the following equations: Maximum DL pathloss = DL EIRP − DL Planning Threshold

(2-10)


61

Maximum UL pathloss = UL EIRP − UL Planning Threshold

(2-11)

The main principle of the planning threshold calculation is shown in Figure 2-10 and covered in more detail in the next sections. Figure 2-10 shows that the planning threshold is affected by equipment performance and configuration, cell interference level, and service quality requirements. It is also affected by propagation environment, terminal mobility, etc. Average signal level at cell edge = Planning Threshold Coverage quality, margins Signal level at antenna Antenna line parameters Signal level at reveiver input C/I requirement Total interference Amount of interference Thermal noise at receiver Receiver noise figure Background noise Figure 2-10. Calculation of planning threshold.

The planning threshold (and maximum path loss) is service-specific and it depends on the service-quality targets, e.g., data rate, BER target, coverage probability, etc. As the UMTS system supports the implementation of a multiple service network offering a wide range of different services, the power budget has to be calculated separately for each service, with the selected Quality-of-Service level and the final cell range being limited by the service that allows the lowest maximum path loss in the radio link. Typical power budget tables for a downlink and uplink power budget are presented in Tables 2-6 and 2-7. The power budget calculation phases have been described in more detail in Sections 2.3.1–2.3.7. The calculations have been presented with equations in linear scale and dB values have to be converted to linear values with conversion lin = 10 dB 10 , and dB = 10 log(lin ) .

62

Table 2-6. Downlink power budget. Receiver (MS) Antenna noise temperature Receiver noise figure Receiver noise temperature System noise temperature Receiver thermal noise density Noise bandwidth Thermal noise power Cell interference margin Total interference level User bit rate Required Eb/N0 Antenna diversity gain Soft handover diversity gain Power control gain in Eb/N0 Required C/I


290 K 8 dB 1539.8 K 1829.8 K -166.0 dBm/Hz 3.84 MHz -100.1 dBm 3 dB -97.1 dBm

Total interference

12.2 7 0 4 3 -17.0

kbps dB dB dB dB dB

Receiver sensitivity

-114.1

dBm


Antenna line losses RX/TX antenna gain Required isotropic power

0 0 -114.1

dB dBi dBm

Isotopic received power

Planning threshold (DL) SHO gain Body loss Propagation slope Outdoor coverage probability Outdoor slow fading st. dev. Outdoor slow fading margin Outdoor planning threshold Indoor coverage probability Indoor slow fading st. dev. Indoor slow fading margin Building penetration loss Indoor planning threshold Transmitter (BS) Max. power per connection BS TX losses BS Output power per connection Antenna line losses RX/TX antenna gain EIRP Maximum path loss Indoor path loss Outdoor path loss

C/I requirement

3 dB 3 dB 35 dB/dec 95 % 7 4.3 -106.8

dB dB dBm

90 % 9 3.5 15 -92.6

dB dB dB dBm

29.5 0.5 0.8 4 18 43.0

dBm dB W dB dBi dBm

135.6 149.8

dB dB

Outdoor planning threshold

Indoor planning threshold

Base station EIRP

UMTS CONFIGURATION PLANNING Table 2-7. Uplink power budget. Receiver (BS) Antenna noise temperature Receiver noise figure Receiver noise temperature System noise temperature Receiver thermal noise density Noise bandwidth Thermal noise power Cell interference margin Total interference level

User bit rate Required Eb/N0 Antenna diversity gain Soft handover diversity gain Power control gain in Eb/N0 Required C/I

63

290 K 4 dB 438.4 K 728.4 K -170.0 dBm/Hz 3.84 MHz -104.1 dBm 3 dB -101.1 dBm

Total interference

12.2 3 0 2 3 -23.0

kbps dB dB dB dB dB

C/I requirement


-124.1

dBm


Antenna line losses RX/TX antenna gain LNA gain Required isotropic power

6 18 0 -136.1

dB dBi dB dBm

Isotopic received power

Planning threshold (UL) SHO gain Body loss Propagation slope

3 dB 3 dB 35 dB/dec

Outdoor coverage probability Outdoor slow fading st. dev. Outdoor slow fading margin Outdoor planning threshold

95 % 7 4.3 -128.8

dB dB dBm

Indoor coverage probability Indoor slow fading st. dev. Indoor slow fading margin Building penetration loss Indoor planning threshold

90 % 9 3.5 15 -117.6

dB dB dB dBm

Transmitter (MS) Max. power per connection RX/TX antenna gain EIRP

21.0 0 21.0

dBm dBi dBm

138.6 149.8

dB dB

Maximum path loss Indoor path loss Outdoor path loss

Outdoor planning threshold

Indoor planning threshold

Mobile station EIRP


64

2.3.1 Total interference The calculation of the power budget starts with the calculation of the total interference level at the receiver input. Antenna noise temperature (Ta) is affected mainly by the surrounding environment and antenna radiation pattern. Under a clear sky, the antenna noise temperature can be about 60 K, but when surrounded by buildings, located indoors or inside a pocket, the antenna temperature is about the same as the ambient temperature, often approximated as 17 degrees Celsius (290 K). Receiver noise figure (F) is the measure of receiver noise performance, i.e., how much thermal noise is added to the received signal in the receiver. In the base station, the reference point is usually the input of the base station. In the mobile station, the receiver noise figure is usually defined at the antenna connector, so it includes the antenna line losses, etc. Typically, the UMTS base station noise figure is around 2.5–5 dB, and the mobile station noise figure is typically about 8 dB. The noise bandwidth (BN) is the bandwidth of the receiver filters before signal despreading. The chip-matched filter noise bandwidth is the same as the chip frequency 3.84 MHz. Cell interference margin (IM) is one of the main parameters in the UMTS power budget. It is defined as the ratio of the total interference level to the thermal noise level N:

IM =

N+I N

(2-12)

In most of the references interference margin IM takes into account only the intra system and intra frequency interference (own-cell + other-cell), but interference from other carriers, systems (e.g., GSM), and other external interference are either included in the noise floor level N or left out of the power budget calculation. The total interference level Itot is calculated with Equation 2-13, where k is the Boltzman constant (1.38E-23 J/K) and Tphys is the receiver physical temperature (usually 290 K) and Tsys is the system noise temperature.

(

)

I tot = k Ta + (1 − F )T phys B N IM = kTsys B N IM

(2-13)


65

2.3.2 Required carrier-to-interference ratio The required carrier-to-interference ratio (C/I) has to be calculated (see Table 2-6 and 2-7) in order to calculate the required signal level at the receiver when the receiver interference level is known. The service type, service quality requirements and also the propagation environment, base station configuration and terminal speed will have an effect on the C/I requirement. User bit rate (Rb) is the bit rate of the service for which the power budget is calculated. Usually the user bit rate is the bit rate between the Physical and MAC layer, i.e., it includes the L2/L3 headers but not the physical layer bits (CRC, error coding, rate matching, etc.). (See Section 2.1.1 and [10].) Also the bit rate at other protocol levels can be used, but the definition has to be the same that is used in the definition of the Eb/N0 value. The Eb/N0 value is the energy per bit Eb divided by total interference power density requirement N0 (I0 also used) for the service for which the power budget is calculated. The relationship of Eb/N0 to signal quality C/I in the radio interface can be presented using chip rate Rc and user bit rate Rb or processing gain PG. E b C Rb C 1 = = N 0 I Rc I PG

(2-14)

In practice, the required Eb/N0 depends on a large number of variables, such as: – service QoS profile (bit rate, BLER target, RLC mode, etc.), – Layer 1 channel configuration (CRC, error coding, rate matching, etc.), – receiver performance (channel estimation, etc.), – propagation conditions, – receiver speed, – effect of power control, – antenna diversity reception/transmission, and – SHO conditions (number of links, relative powers of the links). Because of the complex nature of the Eb/N0 requirement, it is impossible to give one fixed value for different services, and reasonable estimates have to be used, e.g., in power budget calculations. Some compensation factors are usually take into account in order to compensate the difference between the conditions in which the Eb/N0 has been defined and the conditions in the power budget cell. Antenna diversity gain, soft handover diversity gain and power control gain in Eb/N0 are used in the presented power budget.

66


Antenna diversity gain (GAdiv) has to be included in the power budget calculation if diversity is used and if it is not included in the Eb/N0 requirement. Both receive and transmit diversity gains have to be considered when applicable (see Section 2.1.2.4). Soft handover diversity gain (GSHOdiv) is defined as the macro diversity gain against fast (Rayleigh) fading caused by multi path propagation. The soft handover diversity gain is different in the UL and DL directions due to a different macro diversity combining method. In the downlink, the signals from different base stations are combined in the mobile station RAKE receiver, which equals to maximal ratio combining. In a soft handover situation in the UL, the signals are combined in the macro diversity combiner of the radio network controller (RNC). It typically uses frame or block selection combining based on quality information of the signals from the different base stations. In a softer handover situation, the UL signals from different sectors of the same base station are combined in the base station RAKE receiver. Soft handover diversity gain depends also on the relative powers of the handover links and the level of diversity from other diversity types (RX antenna diversity, TX antenna diversity, multi path diversity and time diversity from interleaving). For the uplink, soft handover diversity gain figures of 0–1.6 dB have been simulated with two branch receive antenna diversity [11]. It was shown that the diversity gain depends on the receiver speed and relative powers of the soft handover links. Power control gain in Eb/N0 (GPC,: also called power control headroom [4]) is taken into account in the uplink power budget when the Eb/N0 has been defined with power control. It is taken into account because at the cell edge, the mobile station transmitter is transmitting continuously at full power and thus cannot follow the fading according to the uplink power control commands. The power control gain in Eb/N0 depends especially on mobile speed and level of diversity. At low speeds and at a low level of diversity, it is highest (6-8 dB). At high speeds and a higher level of diversity, it decreases to close to 0 dB [4, 7]. The required C/I is calculated according to Equation 2-15. C E b Rb 1 = G PC I N 0 Rc G Adiv G SHOdiv

(2-15)


67

2.3.3 Receiver sensitivity Receiver sensitivity Cmin, i.e., the minimum received power per service in question, can be calculated when the total interference power and C/I requirement are known. C min =

C I tot I

(2-16)

2.3.4 Isotropic received power The calculation of the required isotropic power at the receiver antenna takes into account the effect of the antenna line on the receiver performance. The mobile station antenna gain (Ga) depends on the terminal type and usage (in the hand, pocket, etc.), and often the antenna gain is approximated to be 0 dBi because it is not the same for all users. Typically, the mobile station antenna line losses (La) are included in the mobile station receiver noise figure. Equation 2-17 shows the calculation of the downlink isotropic received power ( PIDL ). PIDL =

C min G a La

(2-17)

In the uplink, all the required figures (BS antenna gain and antenna line losses) are based on equipment manufacturer specifications (Section 2.1.2). If the LNA is used, the LNA gain GLNA is also taken into account in the calculation of the isotropic power (Section 2.1.2.3). Equation 2-18 shows the calculation of the uplink isotropic received power ( PIUL ). PIUL =

C min La Ga G LNA

(2-18)

2.3.5 Planning threshold The planning threshold calculation considers mainly the effect of slow (log-normal) fading caused by shadowing in the radio path. Shadowing is caused by terrain shape, foliage, buildings, and other obstacles. The effect of


68

the user’s body (Lbody) together with building penetration loss (LBP) for indoor coverage is also taken into account in the planning threshold calculation. The calculation of slow fading margin (MSF) is presented in [12]. The slow fading margin is affected by the variation (standard deviation) of the slow fading, coverage probability requirement for the service in question (Pcov), and the propagation slope (slope) in the area. single M SF = f (σ SF , Pcov , slope )

(2-19)

In UMTS, a soft handover also affects the slow fading statistics. In a soft handover, the user is connected to several base stations simultaneously and the statistics of the slow fading seen by the receiver is a composite of the slow fading of each link. In practice, the slow fading of the radio links in the soft handover are partly correlated. The soft handover gain can be defined as the difference of the slow fading margin with one link and the slow fading margin with multiple links with a certain level of correlation [7]. M SF =

single M SF G SHO

(2-20)

At least two planning thresholds are typically calculated for both uplink UL / DL ) and the other for indoor and downlink, one for outdoor coverage ( Pthout coverage ( PthinUL / DL ). The calculation of the indoor coverage planning threshold takes into account the attenuation caused by the building structure to the signal coming from the outdoor base station, the indoor slow fading standard deviation and the indoor coverage probability target. UL / DL out Pthout = PIUL / DL Lbody M SF , in PthinUL / DL = PIUL / DL Lbody M SF L BP

where PIUL / DL is the isotropic power for the uplink or downlink.

(2-21) (2-22)


69

2.3.6 Equivalent isotropic radiated power The uplink equivalent isotropic radiated power (EIRPUL) calculation considers the terminal antenna line and terminal transmit power. Usually antenna line losses are taken into account in the mobile station TX power ( PtxMS ). Mobile station antenna gain ( GaMS ) depends much on the terminal and terminal antenna type and also the terminal position, typically an average value of 0 dBi is used in power budget calculations. The downlink equivalent isotropic radiated power (EIRPDL) calculation takes into account the effect of the base station antenna line. The base station transmit power for each service connection ( PtxBS ) is limited by the total transmit power required for all users and services in the cell, and in low traffic cells more power can be allocated for each service. The downlink power budget can be used to find out the required downlink transmit power that gives equal path loss for the uplink and downlink by selecting the required base station transmit power. EIRPUL = PtxMS GaMS , EIRPDL =

PtxBS G aBS Lbs La

(2-23), (2-24)

2.3.7 Maximum path loss Finally, the maximum path loss can be calculated as PLmax =

EIRP Pth

(2-25)

for the uplink, downlink, indoor, and outdoor cases.

2.4 Repeater configuration In residential and rural areas or on highways, where there are large topographic or morphologic obstacles attenuating the signal, repeaters are commonly used as fast and cost-effective solutions to fill coverage holes and gaps. Another significant reason to introduce a repeater is when implementing an indoor coverage, e.g., in large shopping malls, office buildings, stations, or underground premises. In such a case, the repeater is typically connected to a distributed antenna system (DAS). A repeater amplifies the signal received from a base station and transmits it further


70

towards the coverage gap. Since a repeater is sharing the capacity of a donor base station, capacity planning must be considered especially in the case of hot spots. On the other hand, repeaters are used to save capacity. In a situation where there is a group of users having a weak downlink signal, the base station is continuously sending with high power, ‘wasting’ the available capacity. If there is a repeater taking care of this hot spot of users, it enables the base station to transmit with lower power, thus releasing the capacity for other users. This is emphasized in indoor locations, where a single user can reserve a lot of capacity due to large losses caused mainly by building penetration loss. In the uplink direction, the repeater may help to reduce the interference in the neighboring cells if the mobiles can use lower transmit power. A repeater system consists of a donor antenna, amplifier, and serving antenna, as illustrated in Figure 2-11. The role of the donor antenna is to offer a link to a donor base station. Thus, a prompt alignment is critical, especially in cases where several base stations have equivalent link performance with the repeater. The strongest signal should be at least 8-10 dB better than the others to effectively avoid a continuous soft handover. Narrow beam ( -95 dBm [8] where Ec/I0 is the received chip energy relative to the total power spectral density, and CPICH RSCP is the received signal code power for the common pilot channel [5].

142


4.4.1.2 Hardware channels In addition, one very important requirement of an analysis tool is to be able to present the number of hardware channels used by the design. This is important to be able to plan for hardware unit extensions. Hardware channels are sometimes also referred to as channel elements, and a hardware unit as a channel element kit. This is important in any evaluation of a UMTS planning tool environment since the hardware channel utilization in the network differs from vendor to vendor. 4.4.2 Drive test analysis An analysis tool for drive test data analysis can, as mentioned earlier, be a separate tool or an integrated part of a family of tools. Regardless of the implementation, a mandatory requirement of an analysis or post-processing software tool is to pinpoint pilot pollution areas. This is a major contributor to poor Quality-of-Service and as a possible result, a dropped call. As pilot pollution is related to the design of the user equipment, i.e., the number of fingers in the RAKE receiver implementation in the user equipment, settings to define this are also needed. Ultimately, it is the design engineer who decides to make design changes based on the analysis results. The design engineer’s goal is to optimize the network so that the number of pilots in any given location does not exceed the design goal. A good analysis tool provides the design engineer with support in the decision process. Not only can it determine the number of pilots in a given location, but also their origin. To do this, the analysis tool must be able to combine and analyze both data captured with user equipment and data captured with scanner equipment. A further ability is to analyze soft handover functionality in relation to the corresponding threshold parameters for the active and monitored set and also the number of cells in the active and monitored set. Too many or too few cells is not good, and the tool should help in the analysis to ultimately provide the best settings for optimized soft handover performance. Also important is the ability to provide signal delay information from cells in the network. When a NodeB timing is not aligned, it causes problems to the area affected by the footprint of this unaligned cell. Quality-related analysis support is important in terms of key quality indicators like BER and FER. Drive test analysis is further described in Chapter 7.

RADIO NETWORK PLANNING TOOLS

143

4.5 Management and information processing tools The information or amount of data stored on different devices in a planning tool environment can be numerous. There is planning design data, measurements and analysis results. Furthermore, the data stored within a tool is organized in a structured manner. However, the relation between data from different sources is far from always corresponding to each other in a one-to-one relation. That is, the task of combining data, and sometimes visualizing the combined data, from one tool with another is not always trivial. This either requires that the information processing can be handled through a central data storage implementation or through a set of independent data sources. The latter will generally have a higher grade of flexibility, at the expense of increased development and maintenance efforts. This is mainly due to data compatibility issues between independent data sources. 4.5.1 Network design and implementation management A powerful report tool is mandatory. It should be able to produce reports of network design, analysis, and simulations. Furthermore, in order to simplify any post-processing of the generated information, the capability to output reports in a variety of common file formats is beneficial. The most important information that should be producible is: – throughput statistics on a service basis; – service success and failure statistics with related causes for failures; – power statistics (pilot, common, synchronization, user, and total transmitting power); – handover statistics; and – carrier level (load, noise rise, hardware channel usage, etc.). Not mandatory, but extremely useful for documentation purposes, is the capability of the tool to generate output in specifically adopted portable document format (for example, coverage plots). During the initial rollout of a radio network and throughout the evolution of the radio network, the importance of information for groups of people indirectly dependent on planning decisions is of crucial importance. Typically, this involves, for example, project management teams, construction teams, and logistics team. This requirement is important as the outcome of overall planning will always have a direct impact on the work for these organizational units. This means that the planning tool environment not only requires handling planning-specific parameters, but also parameters related to these organizational units.

144


For example, in project management, this typically includes data such as: – status of a site; e.g., has the technical team assessed the site?; – responisble persons, e.g., for planning of a site; and – contact information, e.g., to access a site. For logistics, data related to products used in the network, e.g., – manufacturer; – pricing; – product number; and – number of units. And for constructions, data related to implementation – equipment configuration, e.g., – type of equipment; – amount of equipment; and – location of equipment; – address; – contact persons; and – blue prints, floor layouts of a NodeB equipment room. This data is gathered throughout the planning process and input in the planning system. It is important that the tool also provides the possibility to make this information available for these dependent groups of people. 4.5.2 Network Management System (NMS) For continuous tuning and optimization of a radio network, it is required to have the statistical information on network performance, gathered in a NMS system, available also in the planning environment. The regular transfer and management of the huge amount of existing statistical data is commonly done via propriety solutions that vary from vendor to vendor. In multi vendor networks, it is of key importance that the tool, either via an separate information processing tool or integrated in the design tool is able to flawlessly handle and match data structures of different vendors.

REFERENCES [1] J. Lempiäinen, M. Manninen, Radio Interface System Planning for GSM/GPRS/UMTS, Kluwer Academic Publishers, 2001. [2] A. Papoulis, S. Unnikrishna Pillai, Probability, Random Variables and Stochastic Processes, Fourth Edition, International Edition, McGraw Hill, 2002. [3] Universal Mobile Telecommunications System (UMTS), UTRA (BS) FDD; Radio Transmission and Reception, 3GPP TS 25.104.

RADIO NETWORK PLANNING TOOLS

145

[4] Universal Mobile Telecommunications System (UMTS), UE Radio Transmission and Reception (FDD), 3GPP TS 25.101. [5] J. Laiho, A. Wacker, T. Novosad, Radio Network Planning and Optimisation for UMTS, John Wiley & Sons, Ltd, 2002. [6] ETSI, Selection Procedures for the Choice of Radio Transmission Technologies of the UMTS, TR 101 112. [7] A. Toskala, H. Holma, WCDMA for the UMTS, John Wiley & Sons Ltd, 2000. [8] WCDMA Training, E6474A, Agilent Technologies 2002.


PART II: UMTS TOPOLOGY PLANNING


Chapter 5 UMTS TOPOLOGY PLANNING Coverage and capacity JARNO NIEMELÄ, JUKKA LEMPIÄINEN Tampere University of Technology (TUT)

Abstract:

Coverage and capacity planning of WCDMA-based UMTS cellular networks are strongly tied together. This chapter introduces the principles of the UMTS topology planning, which means combined coverage and capacity planning. The topology planning phase contains definitions of site locations and configurations together with the base station antenna configuration. These technical elements influence the network coverage and system capacity of a UMTS network. Critical coverage and capacity topics such as signaling channel power needs, multiple services, Erlang-B capacity, and load equations are first introduced separately as they relate to the initial topology planning phase. Finally, radio network system simulations of actual topology planning are described and the impact of different base station site configurations on system coverage and capacity are presented with examples.

Key words:

Antenna tilting, base station antenna configuration, capacity planning, cell range, coverage overlapping, coverage planning, sectoring, sector orientation, site locations, topology planning, traffic distribution


150


5. UMTS TOPOLOGY PLANNING 5.1 Introduction Required network coverage and system capacity together with sufficient Quality-of-Service (QoS) and implementation costs are the most essential elements that define an operator’s site density and site configuration for a planning area. Furthermore, traffic distribution has a great impact on the site density and configuration. In urban environments, traffic requirements are much higher than in rural areas, and thus the site density and configuration is different for these environments. Moreover, the utilized implementation strategy for the site configuration defines the overall coverage and capacity of each particular site. Site locations, number of sectors and sector directions together with antenna configuration have to be considered altogether in order to provide sufficient network coverage, system capacity, service quality, and low implementation costs. In WCDMA-based UMTS networks, network coverage and system capacity are linked because the same carrier frequency is used over the radio network and other users’ signals are seen as additional interference. Thus, attention should be paid to coverage and capacity planning phases simultaneously when planning a UMTS network – hence the name topology planning. Due to the dynamic nature of a UMTS network, the overall performance cannot be determined with analytical formulas, and system or link-level simulations are typically used for defining the network performance of different scenarios. Furthermore, the uplink and downlink directions have to be considered separately due to different traffic requirements for each direction. The main goal of this chapter is to understand the link between coverage and capacity and all the technical elements that have influence on the coverage and capacity (i.e., on the topology) of WCDMA-based UMTS networks. First, theoretical coverage and capacity are explained for a UMTS network. Next, the impact of site locations and configurations (sectoring, sector direction, and sector extension) on network coverage and system capacity are presented with system level simulation examples with the help of a sophisticated radio network planning tool to support theoretical and practical considerations. Finally, the impact of the most important base station antenna configurations (height and tilting) is shown with system level simulation examples.

UMTS TOPOLOGY PLANNING

151

5.2 Topology planning The topology planning phase combines coverage and capacity planning for UMTS networks. Figure 5-1 describes the relation between coverage and capacity of UMTS networks and shows the effect of site and antenna configurations. In the topology planning phase, the site location and configuration are determined, together with the base station antenna configuration. Site / antenna configuration

User bit rates - 12.2 kbit/s - 64 kbit/s

Cell range

Maximum path loss

Link bugdet

Interference margin

Cell load

Number of users

Service types - circuit switched - packet switched

Figure 5-1. The link between coverage and capacity of a UMTS network.

The load of a cell depends on the number of users together with service types offered and their bit rates – the more users in a cell, the more the load is increased. On the contrary, the load affects the required interference margin in the power budget. With a smaller maximum allowable path loss, the cell range is also smaller. Moreover, the sensitivity of the base station and mobile station depends on the interference level in the cell and the services provided. Because site and antenna configuration affects propagation and thus interference in the network, they link coverage and capacity together. In this section the most important aspects of network coverage and system capacity are discussed separately before initial topology planning is explained. The topology planning phase can be divided into initial and detailed topology planning. Initial topology planning can be considered a dimensioning phase where an approximation of the network configuration is calculated. Due to the dynamic nature of the UMTS network, radio network system simulations are needed to verify the network performance; these simulations are done in detailed topology planning. Thus, the result of the detailed topology planning phase is the final site and antenna configuration

152


of the network. Also, the results are detailed coverage and capacity analysis of the corresponding network configuration. 5.2.1 Coverage of a UMTS network A mobile radio network should provide sufficient coverage for its service area in order to satisfy customers’ requirements. Due to mobility requirements, a cellular system has to provide coverage both for traffic channels and for signaling channels. This coverage depends on several network elements, which are mainly defined in the configuration planning phase. In the existing 2G cellular systems, such as GSM, the coverage planning phase is straightforward due to the constant signal transmit power and receiver sensitivity. With UMTS, transmit power is shared with mobiles in the downlink direction, and the required signal level for reception depends on the interference level. Thus, in UMTS networks the coverage thresholds in a cell depend on the number of users and their bit rates, i.e., on the load of the cell. Therefore, the coverage thresholds have to be determined for each cell and service separately. What makes this more complicated is that each service can have different bit rates and different QoS targets that must be met. In practice, the service with the tightest QoS target determines the cell range and, moreover, the overall site density for the planning area. 5.2.1.1 Signaling and traffic channel coverage A cellular network requires coverage for traffic and for signaling purposes. Traffic channel coverage ensures that services can be used in the coverage area and the user is able to receive or send information. The purpose of signaling is to transmit controlling messages between the network and the terminals [1]. Because the maximum transmit power of a UMTS cell is limited, signaling channels consume the power of traffic channels – the more signaling, the less traffic capacity. Hence, UMTS network coverage can roughly be divided into two parts: common pilot channel (CPICH) coverage and dedicated channel (DCH) coverage [2]. CPICH is used for channel estimation, handover measurements and cell selection/reselection procedures. Also the other signaling channels need a certain amount of transmit power. From the point of view of UMTS radio network planning, CPICH power must be minimized to leave as much power as possible for DCHs, in order to be able to serve as many customers as possible. However, enough power must be allocated to CPICH in order to ensure the required measurements and synchronization for a cell. If the transmit power of CPICH is too low,


153

the terminals in the border areas of that cell are more likely to select the neighbor cell, i.e., the cell dominance area is decreased. In the opposite situation, if the CPICH power is increased, the cell dominance area increases. Typically, 5–10% of the total base station power is reserved for CPICH [3]. The DCH coverage depends on the user data rate, on the location of the users, and thus also on the path loss between mobile and base station receiver. In the uplink (UL), the DCH coverage is limited by the transmit power of the mobile station, and in the downlink (DL), either by the maximum power allocated for one radio link or the maximum power. Both UL and DL DCH coverage depend on the load of the cell, i.e., on the interference level of the cell. In practical terms, this means that the effective range of a base station or the cell size varies depending on the number of active users within a cell and on the other-to-own-cell interference [4]. This variation in a cell size is known as cell-breathing. 5.2.1.2 Power budget analysis The coverage of a UMTS cell depends more or less on the same network elements as the coverage of a 2G cell. Base station and antenna line equipment, such as antennas, LNAs and cables, plays an important role in network coverage. Moreover, the coverage depends on the propagation environment and location probability targets set for indoor and outdoor planning as well as for different services. One common aspect for 2G and UMTS coverage planning is that both directions must be analyzed separately, and the main objective is to balance the power budget in order to achieve balanced communication in both directions. In Table 5-1, an example of a UMTS power budget is presented for symmetric speech service and for asymmetric data service. The power budget takes into account the base station equipment and the base station antenna line configuration. In this example, the maximum allowable propagation loss is calculated for speech service of 50% load in both directions, and separately for data services when a 64 kbps bit rate of 30% load is used in UL, and a 384 kbps bit rate of 75% load is used in DL. Allowed propagation loss is higher in the DL direction for speech service and for data service. Thus, in both cases, the UL direction would limit the coverage. In a hybrid traffic scenario, speech and data services must be handled together in order to find the maximum allowable path loss for both directions separately. The cell range deviation of the previous example can be determined, for example, by using the Okumura-Hata [5] propagation model with typical urban values. Base station and mobile station antennas are set to 25 m and 1.5 m, respectively. With an area correction factor of -3 dB for an urban area

154


and a slow fading margin of 10 dB, the cell range of speech service is 1.84 km in the DL direction and 0.97 km in the UL direction. The corresponding cell ranges in DL and UL for data service are 1.07 km and 0.79 km. Table 5-1: Example of UMTS power budget for speech and data services. Parameter Speech Data Downlink Uplink Downlink Uplink Bit rate 12.2 12.2 384 64 Load 50 50 75 30

Units kbps %

Thermal noise density Receiver noise figure Noise power at receiver Interference margin Total noise power at receiver Processing gain Required Eb/N0 Receiver sensitivity

-173.93 8 -100.13 3.01 -97.12 24.98 7 -115.10

-173.93 4 -104.13 3.01 -101.12 24.98 5 -121.10

-173.93 8 -100.13 6.02 -94.11 10 1.5 -102.61

-173.93 4 -104.13 1.55 -102.58 17.78 2.5 -117.86

dBm dB dBm dB dBm dB dB dBm

RX antenna gain Cable loss / body loss Soft handover diversity gain Power control headroom Required signal level

0 2 3 0 -116.10

18 5 2 3 -133.10

0 2 3 0 -103.61

18 5 2 3 -129.86

dBi dB dB dB dBm

TX power per connection Cable loss / body loss TX antenna gain Peak EIRP

33 5 18 46

21 2 0 19

37 5 18 50

21 2 0 19

dBm dB dBi dBm

Allowed propagation loss

162.1

152.10

153.61

148.86

dBm

The previous example illustrates how different services and their quality targets, together with different load targets, affect the cell range. For detailed analysis of coverage and coverage probabilities, a more exact analysis of service location probabilities is needed. In practical UMTS network planning, high indoor coverage probabilities are required; hence cell outdoor coverage areas overlap excessively. Moreover, a mobile station is able to connect to more than one serving cell due to the nature of WCDMA. 5.2.2 Capacity of a UMTS network In TDMA/FDMA networks, the Erlang traffic model has been used for capacity dimensioning. The Erlang model is based on Erlang formulas that


155

include the average duration of a call and the average time of arrival of a call. The introduction of completely different air interface access methods and packet switched services has changed the principles of capacity planning in UMTS networks. Traffic channels that exist in TDMA/FDMA networks have been changed into codes that separate different users in the network. The concept of channel has thus changed. Packet switched services with variable bit rates make capacity analysis even more complex. Because UMTS networks include dynamically variable circuit and packet switched data connections, Erlang formulas cannot be used as such for capacity dimensioning needs in UMTS networks. 5.2.2.1 UMTS network capacity in Erlangs The capacity of the radio network can be blocking limited or interference limited [7]. In the TDMA/FDMA networks, the capacity of a cell is determined according to a number of transceivers utilized in the base station site. With a known number of transceivers, the channel structure (traffic and signaling) can be determined and, moreover, the amount of traffic served by the base station can be determined using the Erlang-B formula. Therefore, the capacity of a TDMA/FDMA network is blocking-limited, which indicates that available traffic channels are running out, rather than the interference situation being the cause of blocking. In contrast to TDMA/FDMA networks, the capacity of a UMTS network is interferencelimited (soft blocked). In soft blocked networks, a rise in interference in a cell causes the blocked calls rather than a lack of available traffic channels. The term “outage” is used to indicate blocking in UMTS networks. The capacity of a UMTS network is known to be interference-limited [6], i.e., any reduction in interference can directly and linearly be converted into an increase in capacity. If the maximum capacity is limited by the amount of interference in the air interface, it is by definition a soft capacity, since there is no single fixed value for maximum capacity [1]. For a soft capacitylimited system, the traffic in Erlangs cannot be directly calculated from the Erlang-B formula, because it would give too pessimistic results. It overestimates the capacity need, because each service is handled separately in the system calculations. On the other hand, if the system is code-limited, the capacity can be estimated by using the Erlang-B model [8]. In a codelimited situation, the noise rise in the network does not cause outage, or blocking, because the communication between links is limited by the number of codes. Hence, in a code-limited situation, the behavior of a UMTS network is like that of TDMA/FDMA networks, and therefore Erlang-B formulas can be utilized. In [9], a method has been presented for evaluating the number of traffic channels needed to handle traffic with respect to the blocking rate in a multi

156


service context in UMTS networks. Verification of the capacity need of a cell was implemented using the Erlang-B or Erlang-C model, which combines joint blocking probability calculations of different services. The results show that the multi service method avoids over-dimensioning the radio resources of the network and yields a more accurate business plan. 5.2.2.2 Load equation approach The load equation is commonly used to make a semi-analytical prediction of the average capacity of a WCDMA cell, without going into system level capacity simulations [1]. There are a number of elements that influence the load on a UMTS network, which on the contrary defines the maximum number of users per cell. Moreover, the number of users and their bit rates influence the total throughput of a cell. Also, the activity factor in speech and data services affects the load and throughput. However, probably the most important contributor to the load is the required Eb/N0 value, which depends at least on service type, data rate of the service, propagation conditions, and receiver performance. If a very good quality (i.e., high Eb/N0 requirement) connection is desired, more bits are needed for the error correction in order to guarantee the quality of the connection. Thus, when more bits are used for error correction, the air interface load is increased and capacity for the traffic channels is decreased. The own-cell and other-cell interference influences the load of a cell. If the total received noise is high, high powers are needed for the transmission in order to guarantee the communication. If high transmit powers are needed, transmit power may run out, with the result that Eb/N0 requirements for the services are not met and mobiles are out of service. The noise received from other cells also depends on the environment. In urban areas, the cells are isolated better than cells in rural areas. However, the radio network plan probably has the most significant influence on interference. An accurate radio network plan guarantees low interference levels and thus enables good quality and appropriate capacity. 5.2.2.3 Uplink load equation Depending on the maximum allowed load in a cell, the number of users can be calculated by using the load equation and assuming that the load of each individual user can be estimated. In the UL direction, the load can be expressed by Equation 5-1 [10]:


N

ηUL =

¦ j =i

1+

(E b

1 W

N o ) j ⋅ R j ⋅υ j

157

(1 + i ) .

(5-1)

In Equation 5-1, N is the number of active users in the cell and W is the system bit rate (chip rate), which in UMTS equals 3.84 Mcps. Rj is a bit rate of the jth user, and (Eb/N0)j is energy per bit over the noise spectral density requirement for the jth user. The activity factor ȣj of the jth user indicates the activity of speech when discontinuous transmission (DTX) is used. For speech service, DTX is typically assumed to be from 0.4 to 0.7 and 1.0 for data services. Other-to-own-cell interference i informs the noise power received from the neighbor cells. The pole capacity is achieved when ȘUL approaches 1, but in practice the maximum allowed load must be kept clearly below 1 to ensure stability of the network [11]. The uplink load factor defines the required interference margin (IM) (Equation 5-2) in a power budget in order to take into account the effect of cell-breathing: IM UL = −10 log10 (1 − ηUL ) .

(5-2)

The interference margin (also called noise rise) informs how much noise has to be added to the noise floor of the receiver to facilitate the exact amount of interference. The interference margin takes into account noise from own cell as well as noise from other cells. The more users there are in the network, the more interference there will be, both for own cell and from surrounding (other) cells. When the load factor approaches unity (i.e., 100% load), the interference margin approaches infinity, as seen in Figure 5-2. By assuming only voice users of a 12.2 kbps bit rate, Eb/N0 requirement 5 dB, and voice activity factor 0.6, the resulting interference margin is plotted as a function of cell throughput values using the three different otherto-own-cell interference ratios 1.1, 0.9, and 0.7. In this particular case, an uplink load of 50% (interference margin of 3 dB) equals cell throughput values of 495 kbps, 550 kbps, and 610 kbps, and an uplink load of 75% (interference margin 6 dB) to 738 kbps, 815 kbps, and 910 kbps, corresponding to interference values 1.1, 0.9, and 0.7, respectively. Figure 5-3 clearly indicates the importance of controlling the interference from other cells as well as in own cell.


158

20 18 16

Interference margin [dB]

14 12 10 8 6 4 2 0 0

10

20

30

40

50

60

70

80

90

100

Load [%]

Figure 5-2. Interference margin as a function of load.

16

14

i=1.1 i=0.9 i=0.7

Interference margin [dB]

12

10

8

6

4

2

0 0

100

200

300

400 500 600 UL throughput [kbit/s]

700

800

900

1000

Figure 5-3. Uplink interference margin as a function of cell throughput.


159

5.2.2.4 Downlink load equation In UMTS networks, downlink behaves a bit differently from uplink because multiple connections share the same base station transmit power. Moreover, each mobile at a different location experiences different interference levels (a different i for mobiles at different locations). In the downlink direction, the load equation can be presented by Equation 5-3 [1]:

(E b

N

η DL =

¦υ j =1

j

N0 ) j

W Rj

[(1 − α j ) + i j ] .

(5-3)

The downlink load equation is similar to the uplink load equation, except it includes the orthogonality factor Įj (Įj = 0…1). The value of the orthogonality factor depends on the orthogonality of the codes used in the network. Without multi path propagation, codes would be fully orthogonal (Įj = 1), but multi path propagation destroys orthogonality, causes interference, and finally increases the load, as seen from Equation 5-3. The downlink load factor also defines the base station transmit power, as shown in Equation 5-4 [12]: N

PBS =

N rf L ¦υ j j =1

(E b

N0 )j

W Rj

(5-4)

1 − η DL

where L is the average path loss between the base station and mobile station, and Nrf is the noise spectral density. When the downlink load factor saturates, the system approaches its pole capacity and the required transmit power approaches infinity. In practice, the base station transmit power is limited by the specifications and thus the base station transmit power may limit the downlink capacity and coverage in highly loaded networks. From Equations 5-1 and 5-3, it can be observed that by decreasing the other-to-own-cell interference, the load of the network can be significantly decreased. A lower load in the uplink direction affects the uplink noise rise and a lower load in the downlink direction reduces the average required base station transmit power. 5.2.3 Coverage and capacity enhancements in the topology planning In order to improve network coverage and system capacity, equipment in the power budget and network topology should be enhanced or optimized. Power budget related enhancements are presented in Chapter 2. In this

160


section, network topology related coverage and capacity enhancements are introduced. 5.2.3.1 Coverage improvements Coverage of a UMTS network should be enhanced when the network is coverage-limited in the uplink or downlink direction. First, the use of power budget related equipment such as a low noise amplifier [13-15], power amplifier, and diversity have to be optimized. After power budget enhancements, the network topology can be changed by sectoring the base station site. Sectoring improves the base station coverage due to directional high gain antennas. When increasing the number of sectors from omnidirectional to 3sectored, the sector antenna configuration has more gain than the omnidirectional configuration. Typically in 3-sectored sites, a 65° horizontal beam width antenna is utilized for best performance – both from the capacity and coverage point of view [16]. The effect of sectoring is further discussed in Section 5.3.3. Repeaters can also be classified as network topology related equipment. Repeaters provide a solution to extend the coverage of an existing base station [13]. Moreover, the operational costs of repeaters are much lower than conventional sites [17]. In a UMTS, network repeaters cause additional interference for own and surrounding cells [18], so they should be included in system simulations. Repeater planning in UMTS is explained in more detail in Chapter 2. 5.2.3.2 Capacity improvements This section gives a short introduction to some network topology related capacity enhancement methods. The capacity of a UMTS network must be increased if system capacity is lacking in some locations of the network or if growing traffic demands it. The methods for network topology related system capacity improvements are the following: – additional carriers – sectoring – micro cell deployment – base station antenna configuration. The simplest and most effective way to increase system capacity is to add one or more carriers. Most UMTS operators have more than one carrier. If an operator’s frequency allocation allows, an additional carrier can be taken into use in order to increase the capacity of the network. Using macro cell and micro cell carriers efficiently can provide the highest overall system capacity and spectrum efficiency. This requires very careful radio network


161

planning to ensure adequate isolation between macro and micro layers if the same carrier frequencies are used [15]. Sectoring in cellular CDMA systems increases the capacity in proportion to the number of sectors per cell [19]. Because of sector overlapping, additional interference is generated in the uplink and downlink direction. Hence, antenna beam width plays an important role in sectoring when an increase in capacity is desired. Micro cells provide a high capacity solution that is particularly suitable for urban and dense urban environments where high site densities are required and macro cell site acquisition becomes difficult [15]. Base station antenna configuration affects the capacity of the base station. By optimizing the base station antenna configuration, interference can be controlled thereby enhancing capacity. 5.2.4 Initial topology planning By using power budget analysis and load equation approaches, estimations of site densities and configurations for a planning area can be determined. In the initial topology planning phase, the main goal is to use the information provided by power budget calculations for different services and the information provided by load equations in order to determine the average site density in the planning area. First, the maximum cell range of the limiting service is identified. Next, the load equations are used to verify whether or not the given load limits are exceeded when using the limiting cell range. If the load of a cell does not exceed the given threshold, the cell is dimensioned to be coverage-limited. On the other hand, if the load exceeds the given threshold and if the cell range defined by the capacity evaluations is smaller than the cell range defined by the coverage evaluations, the cell is capacity-limited. The initial topology planning can also be considered as system dimensioning. 5.2.4.1 Traffic distribution Traffic in a cellular network is not distributed uniformly over a planning area. It is mainly concentrated in urban areas, which are more populated than rural areas. Thus cell ranges are smaller in urban areas. In addition to that, users are mainly located in indoor locations where propagation loss and slow fading standard deviation are higher. In order to provide indoor coverage, the coverage thresholds should be set to a sufficient level. Figure 5-4 shows an example where indoor and outdoor users are served by the same sector. The required signal power level for a sufficient communication link is higher for an indoor user than for an outdoor user. Thus, the required transmit power for an indoor user is higher. Due to

162


additional transmit power, more interference is radiated in the direction of an outdoor mobile. Moreover, this causes an increased need for transmit power for an outdoor mobile. In a TDMA/FDMA network, the situation depicted in Figure 5-4 does not cause any harm, since mobiles share either a different time slot or a different frequency. However, in UMTS networks, increased interference limits capacity.

Figure 5-4. Indoor and outdoor users.

Figure 5-5 presents coverage probabilities in three different traffic scenarios. In the upper-left corner, the simulation area is depicted. Two 3sectored sites cover this example area. The light gray area is forest/park, white is open area, and dark gray/black blocks are buildings of different heights. Buildings cover a total of 8.7% of the simulation area. In the upper-right corner a homogenous traffic distribution is used (traffic scenario 1). The yellow color indicates that the coverage probability in that area is 100%, and the blue dots mean that the coverage probability is less than 100%. As seen from Figure 5-5, outage connections occur randomly over the area where there is a homogenous traffic distribution, but are concentrated in the middle of the cells where the interference level is higher. In the lower-left picture, indoor users are included in the simulations (traffic scenario 2). An additional indoor loss of 15 dB is added in the power budget if users are located indoors. Furthermore, the standard deviation of slow fading is increased from 10 dB to 15 dB in indoor locations. Now, call


163

failures occur more frequently in indoor locations, but there are still outage connections in outdoor locations. The lower-right corner depicts a traffic situation (traffic scenario 3) where the number of indoor users is 70% of all users. In this case, nearly all failed connections occur in indoor locations, and only a few failed connections occur in outdoor locations.

Figure 5-5. Coverage of three different traffic scenarios.

In Table 5-2, the coverage probabilities of traffic scenarios 1, 2, and 3 are presented. The overall coverage and indoor coverage probabilities mean that the coverage probability for the service has exceeded the 90% threshold. Indoor coverage values are weighted average values, and for traffic scenario 1, the indoor coverage probability is calculated for building locations without indoor loss. As expected, the overall coverage probability decreases when adding indoor loss and weighting the indoor users. The indoor coverage probability for traffic scenario 3 is a few percent higher than for traffic scenario 2. This could be caused by the favorable locations of the buildings in the simulation area with respect to the base station locations.

164


Table 5-2. Overall and indoor coverage probabilities for speech users. Overall coverage Indoor coverage Traffic scenario 1 92.78 90.83 Traffic scenario 2 91.62 62.57 Traffic scenario 3 89.95 64.55

In general, non-uniform traffic degrades overall performance of the system because it creates traffic hot spots where connection failures are more likely to occur. Also, dominance areas of the sectors are not evident due to the fact that the site density grows and the cells overlap more – this happens especially in urban areas. Hence, traffic distribution and especially indoor users have to be considered in UMTS network planning.

Path loss [dB]

5.2.4.2 Coverage - Capacity Identifying the limiting direction, uplink or downlink, is vital in order to balance the power budget. The network performance can be either coverage or capacity-limited in the uplink or downlink direction. Typically, uplink is limiting when the network has a low load. When the load increases, downlink starts to limit more and more due to the fact that the base station transmit power is divided among multiple users. Figure 5-6 depicts a typical coverage-capacity scenario.

Downlink Uplink

Capacity [kbit/s]

Figure 5-6. Uplink and downlink path losses as a function of capacity.

The coverage is uplink-limited due to the smaller transmit power of the mobile. The coverage is a limiting factor when the load of the network


165

remains lower than expected in cell range calculations in the dimensioning phase. Uplink coverage-limited situation is likely to occur in rural areas where users are spread out in a very large geographical area and the cells are planned with relatively a low load. Correspondingly, a downlink coveragelimited situation could occur in an urban area with asymmetric data traffic when high indoor coverage probabilities are required. In order to improve uplink and downlink coverage, diversity reception and diversity transmission, a low noise amplifier (LNA) and power amplifiers (PA) could be utilized. A capacity-limited situation can also occur in both directions. An uplink capacity-limited situation occurs when the system load exceeds the given load threshold. The downlink capacity-limited scenario occurs when the base station’s transmit power is running out. Capacity-limited scenarios are likely to occur when the network is planned for too low load scenarios. Hence, sudden traffic congestion in a low loaded cell could easily create a capacitylimited situation. Moreover, capacity could be limited in the downlink direction in dense urban areas, where there is highly asymmetric traffic demand and more uplink load is allowed. An understanding of which direction is limiting in certain situations and a deep knowledge of power budgets and load equations are required for successful UMTS network planning. 5.2.5 Detailed topology planning In the detailed topology planning phase, the network parameters and configurations of the initial topology planning phase are moved into a radio network planning tool or simulator. The most important requirements for a radio network planning tool are defined in Chapter 4. The planning tool is mainly used for coverage predictions and capacity simulations of the planning area. The power budget is also inserted in the planning tool together with base station site configurations. Since traffic distribution is very important and has a strong effect on topology planning, the importance of traffic models also must be emphasized. The more accurate the traffic models are, the more precisely the load of the network can be modeled. Finally, coverage predictions and capacity simulations are performed by utilizing a planning tool. The simulation results include a coverage and capacity analysis of a planning area when certain base station site configurations are used. If coverage or capacity is not good enough, base station site configurations have to be modified in order to achieve better coverage or capacity. New simulations are required when any coverage or capacity related network element is modified because interference levels

166


between neighbor cells have to be recalculated. Also a new coverage and capacity analysis has to be provided. In order to achieve the maximum coverage and capacity, base station site configurations (number of sectors, sector orientations, site distance) have to be optimized for different site locations and antenna configurations (antenna height, beam width, and tilting).

5.3 UMTS site configuration Network coverage and system capacity are mainly determined by the site densities, locations, and configurations. In UMTS, the radio planning site location has to be first selected with an appropriate number of sectors that must be directed in optimal directions. Each of these three selections has a certain influence on radio network system capacity, and they have a certain influence on each other. 5.3.1 UMTS site locations In UMTS network planning, the site density for a planning area is defined by the amount of traffic together with traffic requirements and distribution. When the maximum cell range for the most sensitive service is known (usually a high speed data service), the search for the candidate site location can be started. A hexagonal grid planning would be the most efficient strategy to deploy a macrocellular network if the traffic distribution and environment were homogeneous. Unfortunately, traffic congestion is created both by the environment and building distributions, and moreover the environment is not typically favorable for hexagonal grid planning strategy. Typically, topography or morphology of the terrain creates restrictions for the ideal site locations. In urban environments where base stations are often located on top of buildings, the physical space required for the hardware of the planned site solution may not be enough. In some places, authority constraint or government regulations may prevent an operator from deploying a base station to an optimal place from their network performance point of view. Also economic reasons could persuade an operator to select another place for the site. Hence, a maximum deviation in site locations should be defined that avoids a significant reduction in network coverage and system capacity. Figure 5-7 illustrates the impact of non-hexagonal base station locations. As a consequence of new base station locations, the interference level between BS2 and BS3 is rising and thus capacity is decreasing. On the other hand, the distance between base stations BS1 and BS2 is increasing and


167

reducing interference in that direction. Also, mobiles located in the area between BS1 and BS2 may have coverage problems. Hence, the effect of deviation of the site locations is mainly related to the average distance between base stations. This analysis was made by assuming a homogenous traffic distribution. In a non-homogenous traffic distribution, if the new (randomized) base station location would be farther away from a traffic hot spot such as a business building, network performance would deteriorate.

BS1

BS3

BS2 Figure 5-7. Impact of non-hexagonal site locations.

The displacement of base station locations from the ideal hexagonal grid has been found to have negligible impact on C/I values in cellular systems in a homogenous environment and traffic distribution [20–23]. These references cover the system performance for narrowband as well as wideband cellular systems. The results have great importance due to the fact that small deviations do not affect cellular system performance. On the contrary, in [24], which concentrated on a UMTS system, the base station location was concluded to have a notable impact on downlink and uplink performance with a relatively large cell range. 5.3.1.1 BTS location deviation simulation setup in topology planning The impact of UMTS base station location deviation must be simulated in the topology planning phase (site selection) by using a radio network planning tool and moreover by utilizing a high-resolution digital map. A planning example of the impact of this planning phase is given by using a

168


reference macrocellular hexagonal grid layout, depicted in Figure 5-8. The hexagonal structure is not the optimum planning in the chosen simulation environment because the terrain has variations and the dominance areas of the base stations are not equal. However, the hexagonal grid represents an independently defined reference network that is used for comparison.

Figure 5-8. Ideal hexagonal grid of 17 base stations.

The simulation area must be defined by a digital map using very small pixels (e.g., 5m x 5m) and including topography and morphological data and building rasters. The area of the digital map represents a typical suburban or light urban environment. The performance of the radio network in different scenarios is calculated with a radio network planning tool that includes a static simulator for UMTS network system simulations. In the system simulations, a cluster of cells is needed to calculate interference. In this example, 17 base stations are arranged in a hexagonal grid with sites separated by 1.5 km (the distance between sites). Base stations are configured with three sectors each having a 65° horizontal beam width antenna. Base station and mobile station antenna heights are 25 m and 1.5 m, respectively. Antenna directions are 0°, 120°, and 240°; and those are


169

kept fixed after moving the base station to its new location. The OkumuraHata propagation model is utilized at a carrier frequency of 2140 MHz. The propagation model is adjusted with average -6.7 dB area correction factors and some topographical corrections. Propagation slope is also adjusted to 35 dB/dec. Only voice users (12.2 kbps) load the network, and the traffic distribution (homogenous or non-homogenous, indoor or outdoor users) must be accurately defined. The most relevant simulation parameters are gathered in Table 5-3, and the power budget for the simulations is presented in Table 5-4. Table 5-3. General simulation parameters. Parameter BS maximum power [dBm] CPICH [dBm] CCCHs [dBm] SCH [dBm] MS dynamic range [dB] Required Ec/N0 [dB] Standard deviation of slow fading [dB] DL orthogonality HO window [dB]

Value 43 33 33 33 70 -18 10 0.6 4

Table 5-4. Power budget for the simulations. Parameter

Voice

RX noise figure Thermal noise Noise power at receiver Interference margin Total noise power at receiver Processing gain Required Eb/N0 Receiver sensitivity

Uplink 5 -108.15 -104.15 6.02 -98.13 24.08 5 -117.21

Downlink 9 -108.15 -99.15 6.02 -93.13 24.08 8 -109.21

Units dB dBm dBm dB dBm dB dB dBm

RX antenna gain Cable loss / body loss LNA gain SHO diversity gain Power control headroom Required signal level

18 2.5 4 1.5 2 -136.21

0 3 0 3 2 -107.21

dBi dB dB dB dB dBm

TX power per connection Cable loss / body loss TX antenna gain Peak EIRP

21 3 0 18

33 2.5 18 48.5

dBm dB dBi dBm

Allowed propagation loss

154.21

155.71

dBm

170


5.3.1.2 BTS location deviation simulations An error is given for each base station location by vector, thus describing its new location in an irregular grid. In practice, the maximum deviation should not exceed one-quarter of the site separation in urban areas. Two different typical urban site separations of 1.5 km and 3.0 km are used in this example. After giving the error for the site locations, the average departure is 180 m and 356 m from the hexagonal locations, corresponding to 1.5 km and 3.0 km site separations. Altogether five different base station displacement cases are simulated to show the impact of site selection. A part of the reference (hexagonal) network and irregular grid base station locations are illustrated in Figure 5-9.

Figure 5-9. Hexagonal grid locations and other base station locations.

The network performance has to be evaluated by measuring service probability, soft handover (SHO) and softer handover (SfHO) probabilities, and averaged values of sector throughput (the same in the downlink and uplink direction), uplink noise rise and load, together with downlink average transmit power. The simulation results of 1.5 km site separation without indoor users are presented in Table 5-5. Three columns of the reference and base station irregular grid (displacement cases) represent homogenous traffic amounts of 1000, 2000, and 3000 voice users, i.e., a different network load. Irregular grid results are averaged values of five different network layouts. According to the simulation results, a small deviation in hexagonal structure does not affect the overall performance of the network in a


171

practical urban network layout when indoor coverage is required (overlapping coverage). SHO and SfHO probabilities remain practically the same as in the reference case. Moreover, the difference between other parameters is very small. This simulation example shows that the hexagonal structure is not necessarily the optimal solution for the network when terrain topography and morphological information have been taken into account. In addition, only a small increase in network performance parameters (DL average transmit power and UL load) can be observed. Table 5-5. Simulation results of irregular grid, outdoor users only and 1.5 km site separation. Parameter Reference grid Irregular grid (average) Number of users 1000 2000 3000 1000 2000 3000 Service probability [%] 99.6 98.1 72.5 99.5 98.2 72.9 SHO probability [%] 24.1 24.9 31.3 24.4 25.0 31.4 SfHO probability [%] 3.7 3.9 5.5 3.8 3.9 5.5 Throughput [kbps/sector] 159 315 381 160 318 383 UL noise rise [dB] 1.09 2.53 2.89 1.09 2.55 2.94 UL load [%] 21.9 43.2 48.1 21.9 43.4 48.6 DL power [dBm] 32.7 37.2 40.1 32.8 37.3 40.2

In practice, system simulations have to be done with indoor users. For indoor users, a standard deviation of slow fading is 15 dB and an additional 15 dB indoor loss is added in the power budget. These simulation results are shown in Table 5-6. The difference between the reference network and displacement networks is still marginal, as seen from the results. Table 5-6. Simulation results of irregular grid, indoor users and 1.5 km site separation. Parameter Reference grid Irregular grid (average) Number of users 1000 2000 3000 1000 2000 3000 Service probability [%] 98.8 97.5 73.5 98.7 97.5 74.0 SHO probability [%] 23.9 24.4 30.9 24.2 24.6 30.9 SfHO probability [%] 3.7 3.8 5.4 3.8 3.8 5.4 Throughput [kbps/sector] 158 314 384 158 315 387 UL noise rise [dB] 1.07 2.49 2.92 1.07 2.50 2.98 UL load [%] 21.5 42.6 48.4 21.6 42.8 49.0 DL power [dBm] 32.7 37.2 40.1 32.8 37.2 40.1

In order to simulate practical non-homogenous traffic, indoor users are weighted 70% and outdoor users 30%. The simulation results of this nonhomogenous traffic are only slightly changing, as shown in Table 5-7. However, the network is already uplink coverage-limited because of indoor users (hot spots), and the service probability remains less than 95% although only 1000 users are included.

172


Table 5-7. Simulation results of irregular grid, non-homogenous traffic distribution and 1.5 km site separation. Parameter Reference grid Irregular grid (average) Number of users 1000 1000 Service probability [%] 94.0 94.1 Soft HO probability [%] 23.7 23.9 Softer HO probability [%] 3.4 3.5 Throughput [kbps/sector] 149 149 UL noise rise [dB] 0.99 0.98 UL load [%] 19.2 19.9 DL power [dBm] 32.6 32.6

The relation between site location and site distance can be analyzed by increasing the site separation by 3.0 km. The randomness in site locations has an affect as seen in Table 5-8. The service probability is now 2% lower than in the reference network, and the average sector throughput has slightly dropped. The uplink load has remained lower due to the decreased service probability in irregular grid networks, as in previous simulations. In the downlink direction, the averaged transmit power is approximately the same. Table 5-8. Simulation results of irregular grid, outdoor users only and 3.0 km site separation. Parameter Reference grid Irregular grid (average) Number of users 2000 2000 Service probability [%] 96.2 94.0 SHO probability [%] 17.2 18.0 SfHO probability [%] 4.6 4.9 Throughput [kbps/sector] 289 285 UL noise rise [dB] 2.10 2.07 UL load [%] 37.6 37.1 DL power [dBm] 36.3 36.3

Pilot coverage analysis is also important in topology planning for ensuring the proper functioning of soft handovers. In Table 5-9, indoor and outdoor pilot coverage probabilities of base station displacements of 1.5 km and 3.0 km site separations are compared. Table 5-9. Indoor and outdoor coverage probabilities of 1.5 km and 3.0 km site separations. Site separation 1.5 km 3.0 km Indoor Outdoor Indoor Outdoor Reference grid 87.9 99.9 54.9 96.6 Displacement 1 90.2 99.9 50.6 96.6 Displacement 2 86.4 99.6 51.6 95.5 Displacement 3 87.9 99.6 52.7 96.9 Displacement 4 84.0 98.9 53.6 95.9 Displacement 5 87.5 99.9 52.5 95.9


173

In a 1.5 km site separation, indoor coverage probability is nearly 90% in all cases and outdoor coverage probably close to 100%. Correspondingly, the indoor service probability is closer to 50% in the case of a 3.0 km site separation. 5.3.1.3 UMTS site selection The UMTS base station location and thus the site selection is robust for small deviations when traffic hot spots (shopping malls, mobile offices) are not included and when coverage overlapping is significant, as is typical in urban areas. Hence, an optimal site configuration is often a more critical task in the topology planning phase than the definition of the site location [25]. 5.3.2 UMTS sector orientation Sector orientation (also called antenna direction) is one of the key elements that define the coverage of a UMTS network, and thus it also affects the system performance. Base station antenna directions have to be defined in the detailed topology planning phase when defining the final network configuration for coverage and capacity simulations. Sometimes base station antennas are directed by having equal spacing. In urban areas, the base station antennas are typically installed on top of buildings. However, due to obstacles close to the base station site location or due to errors, e.g., in the base station antenna implementation, antenna directions may change and affect the UMTS system performance. Figure 5-10 illustrates the effect of the base station antenna direction deviation on the handover areas.

BS1

Increased number of SfHO connections

Increased number of SHO connections BS2 Figure 5-10. Impact of base station antenna direction deviation.

174


If the directions of the antennas belonging to the same site are changed like in BS1, the result is that the number of softer handover (SfHO) connections rises due to increased sector overlapping. The number of additional SfHO connections depends strongly on the horizontal beam width of the base station antennas. On the other hand, if antennas of different base stations are directed more towards each other, additional SHO connections could occur in that area. 5.3.2.1 UMTS sector orientation simulations in topology planning An example of sector orientation is given in order to show the impact of this topology planning phase when a radio network planning tool is utilized. The same reference network (hexagonal base station locations, antenna directions and simulation parameters) are used as in Section 5.3.1.1. The indoor user characteristics are also the same. The simulation results of antenna direction deviation cases of 9.1° and 18.2° average deviation from original direction under different traffic scenarios are presented. The base station antenna directions with 18.2° average deviation are depicted in Figure 5-11.

Figure 5-11. Base station antenna direction with average 18.2° deviation.

The simulation results in Table 5-10 show that a small, random deviation in base station antenna direction does not greatly affect UMTS system


175

performance if overlapping is sufficient (as in urban areas in practice) and if traffic distribution is homogenous. However, the effect of antenna direction deviation seems to be more significant than the base station location deviation. The service probability is nearly the same with different network loads. Also the SfHO probability has increased by a small amount. Altogether, when taking into account only outdoor users, the impact of antenna direction deviation is quite small. Table 5-11 introduces the results from the same simulation when the impact of indoor users is included. The service probability has deteriorated a bit, but adding indoor users in the system does not affect the differences between reference network and antenna direction deviation networks. Table 5-10. Results of 9.1° antenna direction deviation without indoor users. Parameter Reference grid Random antenna directions (average) Number of users 1000 2000 3000 1000 2000 3000 Service probability [%] 99.6 98.1 72.5 99.5 97.9 72.0 SHO probability [%] 24.1 24.9 31.3 24.2 25.1 31.4 SfHO probability [%] 3.7 3.9 5.5 4.0 4.4 5.8 Throughput [kbps/sector] 159 315 381 160 318 379 UL noise rise [dB] 1.09 2.53 2.89 1.09 2.54 2.87 UL load [%] 21.9 43.2 48.1 21.9 43.3 47.8 DL power [dBm] 32.7 37.2 40.1 32.8 37.3 40.2 Table 5-11. Results of 9.1° antenna direction deviation with indoor users. Parameter Reference grid Random antenna directions (average) Number of users 1000 2000 3000 1000 2000 3000 Service probability [%] 98.8 97.5 73.5 98.8 97.3 73.0 SHO probability [%] 23.9 24.4 30.9 24.0 24.8 31.0 SfHO probability [%] 3.7 3.8 5.4 4.0 4.1 5.7 Throughput [kbps/sector] 158 314 384 158 315 383 UL noise rise [dB] 1.07 2.49 2.92 1.07 2.79 2.91 UL load [%] 21.5 42.6 48.4 21.6 42.7 48.2 DL power [dBm] 32.7 37.1 40.1 32.8 37.2 40.1

In Table 5-12 the impact of non-homogenous traffic distribution between indoor and outdoor users is presented. The simulation results in Table 5-10, Table 5-11, and Table 5-12 show that a small antenna deviation has no significant effect on the UMTS system performance. Furthermore, traffic distribution and indoor users must be highlighted in topology planning because they have a strong influence on the UMTS system performance.

176


Table 5-12. Results of 9.1° antenna direction deviation with non-homogenous traffic distribution and 70% weight for indoor and 30% weight for outdoor users. Parameter Reference grid Random antenna directions (average) Number of users 1000 2000 3000 1000 2000 3000 Service probability [%] 94.0 91.2 71.4 93.8 91.0 71.1 SHO connections [%] 23.7 24.6 29.2 23.9 24.6 29.3 SfHO connections [%] 3.4 3.6 4.8 3.6 3.8 5.1 Throughput [kbps/sector] 149 292 363 149 292 363 UL noise rise [dB] 0.99 2.23 2.71 0.99 2.23 2.70 UL load [%] 19.2 38.8 45.5 19.9 38.8 45.4 DL power [dBm] 32.6 36.7 39.3 32.6 36.7 39.3

Another example of sector orientation is presented in Table 5-13, and 514 by having 18.2° deviation with and without indoor users. Even if the deviation of 18.2° is significant compared to the 65° horizontal beam width of the base station antenna used in the simulations, the system performance still remains almost constant. The results in Tables 5-10 – 5-14 give an idea about expected UMTS system performance deviations when sector orientations are designed in the topology planning phase using a radio planning tool. Table 5-13. Results of 18.2° antenna direction deviation without indoor users. Parameter Reference grid Random antenna directions (average) Number of users 1000 2000 3000 1000 2000 3000 Service probability [%] 99.6 98.1 72.5 99.5 98.0 70.4 SHO connections [%] 24.1 24.9 31.3 24.1 25.0 31.3 SfHO connections [%] 3.7 3.9 5.5 4.7 4.9 6.9 Throughput [kbps/sector] 159 315 381 159 320 373 UL noise rise [dB] 1.09 2.53 2.89 1.10 2.57 2.83 UL load [%] 21.9 43.2 48.1 22.1 43.6 47.2 DL power [dBm] 32.7 37.2 40.1 32.8 37.3 40.2 Table 5-14. Results of 18.2° antenna direction deviation with indoor users. Parameter Reference grid Random antenna directions (average) Number of users 1000 2000 3000 1000 2000 3000 Service probability [%] 98.8 97.5 73.5 98.8 97.3 71.5 SHO connections [%] 23.9 24.4 30.9 23.9 24.5 30.9 SfHO connections [%] 3.7 3.8 5.4 4.7 4.8 6.7 Throughput [kbps/sector] 158 314 384 159 316 377 UL noise rise [dB] 1.07 2.49 2.92 1.08 2.52 2.86 UL load [%] 21.5 42.6 48.4 21.7 43.0 47.7 DL power [dBm] 32.7 37.1 40.1 32.8 37.2 40.1


177

5.3.2.2 Selection of UMTS sector orientation In UMTS networks, as in other cellular networks, sector orientations are selected first by using common sense: UMTS networks are started with 3sectored sites by having an approximately 120° separation in antenna directions. In practice, sector directions have to be changed and reasonable changes do not cause any major reductions in UMTS network performance in urban areas where cell overlapping is significant. However, traffic distribution and indoor users as well as rural areas have to be especially concerned in sector selections. Moreover, sector orientations can be mainly selected based on dominance area needs. 5.3.3 Base station sectoring and antenna beam width in UMTS Sectoring is used to increase the system capacity as well as coverage in UMTS networks. Along with sectoring, the importance of the antenna horizontal beam width increases because sector overlapping becomes increasingly crucial. In topology planning, the number of sectors as well as antenna beam widths must be finally selected in order to optimize the network performance. 5.3.3.1 Sectoring Sectoring means an increase in the number of sectors belonging to a site in order to increase the capacity of the cellular system [26]. Directional antennas transmit only to those directions where the signal is really intended, thus cutting down co-channel interference [27] and allowing a network to serve more mobiles. Hence, sectoring offers more capacity to the network. In order to achieve this extra capacity, antenna selection is crucial to effectively control interference and soft handover overhead [28]. In existing cellular networks, 3-sectored sites are commonly used, and 1sectored base station antennas are used only in small micro cells or in indoor cells. 2-sectored base stations are used mainly in sectored micro cells or to provide roadside coverage. A standard macrocellular solution for low or average loaded networks is utilization of 3-sectored sites, and for high capacity needs in a macrocellular environment, 6-sectored sites are required to provide better system performance. A 4- or 5-sectored site is not commonly used but may be chosen to support a specific traffic scenario [15]. In Figure 5-12, two 6-sectored sites are depicted by having fixed antenna directions of 0°, 60°, 120°, 180°, 240°, and 300°. An arbitrary, narrow beam antenna radiation pattern is drawn to illustrate the main and side lobes of narrow beam sector antennas. Although the antenna radiation pattern is not real, it depicts quite accurately how a directional antenna radiates. The main beam direction of coverage is good and all the mobiles located in that


178

particular direction have good field strength. Furthermore, mobiles in adjacent sectors are not interfered with as much as with wider antenna beam widths (MS at light gray area). But in contrast, if the mobile is not in a handover with a sector belonging to cell site 2, the directional narrow beam antenna raises the interference level of the cell. The increased interference level forces mobiles to use higher transmit powers for the communication.

Cell site 2 MS

Cell site 1 Figure 5-12. 6-sectored base stations using narrow beam antennas.

On the other hand, sectoring increases the number of softer handover connections in the network if wide beam width antennas are used, thus decreasing the capacity of the network. The handover connections and interference can be controlled with proper antenna beam width selection. 5.3.3.2 Antenna beam width Base station antenna beam width plays an important role in UMTS network performance, especially in higher-order sectoring. In order to provide sufficiently high capacity in both 3-sectored and 6-sectored sites, the other-cell interference has to be low. This can be achieved by narrowing the radiation patterns of the base station antennas [28]. In Figure 5-13, a 3-sectored base station with imaginary antenna radiation patterns is depicted. The darker areas between the sectors are sector overlapping areas and hence possible SfHO areas. SfHO areas are needed in a UMTS network in order to maintain the interference level as low as possible during the sector handover procedure, but too large SfHO areas


179

consume limited radio resources of the base station. A wider base station antenna beam width also increases the interference level of the neighboring sector, thus reducing the capacity. The importance of the base station antenna beam width is emphasized in higher-order sectoring. The sectoring efficiency is highly dependent on the base station antenna beam width.

Figure 5-13. A 3-sectored base station and illustration of sector overlapping.

5.3.3.3 Sectoring and beam width simulations in topology planning A simulation example is given in order to show the impact of sectoring and antenna beam width selection on system capacity in topology planning when a radio planning tool is utilized. In the simulations, 10 base stations are placed in a typical macrocellular environment in a hexagonal grid by having a distance of 2.0 km. In the 3-sectored sites, the antenna directions are 0°, 120°, and 240°. In the 6-sectored sites, the antenna directions are 0°, 60°, 120°, 180°, 240°, and 300°. The antenna directions are kept fixed during the simulations. A homogenous traffic distribution of outdoor voice (8 kbps) users is used. The standard deviation of slow fading is 8 dB. The maximum power per connection is 40 dBm and the handover window is 3 dB. The rest of the simulation parameters are the same as in Table 5-3 (Section 5.3.1.1). The horizontal radiation patterns of the 33°, 65°, and 90° antennas used in the simulations are depicted in Figure 5-14. First, 3-sectored sites of 90° antennas and 6-sectored sites of 90° antennas are simulated. The results are shown in Table 5-15. Two columns

180


of the same sectoring correspond to lower and higher traffic scenarios, i.e., different load simulations.

Figure 5-14. Horizontal radiation patterns of antennas (33°, 65°, 90°) used in simulations.

In Table 5-15, the service probability is observed to be quite low for both sectoring examples. This is caused by the power control error introduced in the simulation parameters of the radio planning tool. Table 5-15. Results of sectoring of 90° antennas. Parameter Service probability [%] SHO probability [%] SfHO probability [%] Throughput [kbps/site] UL noise rise [dB] UL load [%] DL power [dBm]

3-sector 90° 83.1 76.3 13.9 9.1 7.7 6.3 823 1006 3.25 4.68 52.0 65.6 39.3 41.5

6-sector 90° 84.5 84.4 11.9 11.9 19.3 19.6 901 1261 1.48 2.27 28.6 40.3 34.5 37.7

In the lower load simulation, the service probability of the 3-sectored network is lower than in the 6-sectored network. After increasing the load of the network, the performance of the 3-sectored network begins to deteriorate but the increased load has no influence on the service probability of the 6sectored network. The total number of HO connections in the lower load situation is 21.60% and 31.20%, corresponding to the 3-sectored and 6sectored networks, respectively. As the results show, sectoring increases the total number of HO connections. Especially in the 6-sectored network, due to the huge sector overlapping, the number of SfHO connections is higher than the number of SHO connections. When increasing the load of the network, the number of HO connections in the 3-sectored network decreases. This is caused by the deteriorated Ec/I0 relation due to the raised interference level. In the higher load network, the 3-sectored configuration becomes very


181

highly loaded in DL because its average transmit power is almost hitting the maximum. Secondly, antennas with 65° horizontal beam width are used and the results are presented in Table 5-16. Also in this configuration, the service probability is enhanced due to sectoring. The number of SHO connections drops a little bit from 14.20% to 12.90%, but the number of SfHO connections increases from 6.20% to 16.60%. The average site throughput is increased from 815 kbps to 889 kbps due to additional SfHO connections. The performance in both downlink and uplink directions is improved after increasing the number of sectors, because less DL transmit power is used to serve the same number of users. Table 5-16. Results of sectoring of 65° antennas. Parameter Service probability [%] SHO probability [%] SfHO probability [%] Throughput [kbps/site] UL noise rise [dB] UL load [%] DL power [dBm]

3-sector 65° 83.1 76.7 14.2 9.7 6.2 5.3 815 1010 3.18 4.60 51.1 65.6 38.9 41.2

6-sector 65° 84.4 84.3 12.9 13.0 16.6 16.8 889 1245 1.45 2.22 28.2 39.7 34.2 37.3

The corresponding results of 33° antennas in the 6-sectored network are shown in Table 5-17. The service probability is marginally decreased due to lack of coverage but the overall performance is better compared to networks of 65°/90° antennas in a 6-sectored network. Table 5-17. Results of sectoring of 33° antennas. Parameter Service probability [%] SHO probability [%] SfHO probability [%] Throughput [kbps/site] UL noise rise [dB] UL load [%] DL power [dBm]

6-sector 33°

84.2 15.4 4.8 823 1.28 25.2 32.5

84.1 15.6 4.9 1153 1.97 35.5 35.2

In Figure 5-15, DL throughputs of different antenna horizontal beam widths of 3- and 6-sectored sites are plotted as a function of DL average transmit power. The lowest curves represent 65° and 90° antennas in 3sectored configurations. The capacity performance seems to be better when using 65° antennas. The upper curves represent 6-sectored configurations of 33°, 65°, and 90° beam widths. The capacity performance of 65° and 90° antennas is equal but 33° antenna beam width gives better capacity. The


182

sectoring efficiency can be nearly 70% when using 65°/90° antennas. However, almost 100% sectoring efficiency can be achieved by using 33° antennas in 6-sectored sites.

DL throughput [kbit/s/site]

2000

3-sector /90

3-sector /65

1800

6-sector /90

6-sector /65

1600

6-sector /33 1400 1200 1000 800 32

33

34

35

36

37

38

39

40

41

42

DL average transmit power [dBm]

Figure 5-15. Capacity curves of 3- and 6-sectored sites of different beam widths.

5.3.3.4 Sectoring and antenna beam width selection in UMTS Sectoring is an important element of topology planning because it has a strong impact on system capacity. A proper number of sectors have to be selected for different network evolution phases (see Section 5.3.5), and base station antenna beam widths have to be selected based on the number of sectors. The selection of antenna beam width becomes crucial in higherorder sectoring. The consequence of too wide beam antenna is increased SfHO connections but on the contrary, with a too narrow beam antenna, coverage is easily reduced and hence the performance of the network is deteriorated. 5.3.4 Coverage overlapping in UMTS In urban areas, the coverage requirements for indoor users determine the site density of a planning area. If high indoor coverage probabilities (80 – 90%) are required, the average site density grows, automatically resulting in large coverage overlapping areas. These overlapping areas cause cellbreathing, which reduces capacity because of an increased number of soft


183

handovers and increased other-to-own-cell interference. On the other hand, placing the base stations far apart is not a solution either. Too large of a site separation yields high transmit powers for the mobiles located near the cell edges. Thus, coverage overlapping is always needed, and the impact of coverage overlapping on system capacity must be understood when site selections are done in the topology planning phase. 5.3.4.1 Coverage overlapping in system simulations The effect of coverage overlapping in topology planning is presented by an example of 3-sectored sites of 65° antennas and 6-sectored sites of 33° antennas. The distance between sites (site separation) is 1.5, 2.0, and 2.5 km in a typical light urban area. These distances of 1.5–2.5 km represent the typical site separation in this type of environment when indoor coverage is required. In both configurations, a regular hexagonal grid is used with 19 base stations. The simulation parameters are the same as in Table 5-3 in Section 5.3.1.1. Antenna tilting is not utilized (more interference in the network, see Section 5.4.3). The simulation results of a 3-sectored case of 65° antennas are shown in Table 5-18. When the load is low (2000 users), the highest service probability can be achieved by a site separation of 1.5 km. When the load is increased, a site separation of 2.0 km seems to be optimum in this reference network. A smaller base station separation increases the other-cell interference and a larger separation requires higher transmit power for base stations. However, Table 5-18 shows that the deviation between different site separations is negligible. Table 5-18. Simulation results of coverage overlapping of 3-sectored sites of 65° antennas, base station antenna height is 25 m. Parameter 1.5 km 2.0 km 2.5 km Number of users 2000 4000 2000 4000 2000 4000 Service probability [%] 100.0 86.1 99.6 87.8 99.1 86.1 SHO probability [%] 18.9 19.1 16.5 16.2 17.1 17.5 SfHO probability [%] 4.8 5.0 5.1 5.2 5.0 5.3 Throughput 272 471 267 470 266 465 [kbps/sector] UL noise rise [dB] 2.14 4.81 2.00 4.52 1.98 4.43 UL load [%] 38.6 66.4 36.3 63.8 35.9 62.8 DL power [dBm] 34.6 39.7 34.3 39.2 34.2 39.2

The simulation results of the 6-sectored network are shown in Table 519. The performance of the 6-sectored network is similar to the 3-sectored network. With a lower load network of 1.5 km, the site separation is able to serve all mobiles because of good coverage. By increasing the load,

184


interference is also increased and thus the service probability begins to decrease. Also in this reference network, a 2.0 km site separation seems to be optimum from a service probability point of view. Table 5-19. Simulation results of coverage overlapping in 6-sectored sites of 33° antennas, base station antenna height is 25 m. Parameter 1.5 km 2.0 km 2.5 km Number of users 4000 6000 4000 6000 4000 6000 Service probability [%] 100.0 96.8 99.7 97.5 99.2 96.3 SHO probability [%] 23.2 23.0 19.8 19.5 17.2 17.2 SfHO probability [%] 4.0 3.9 4.6 4.5 4.7 4.6 Throughput 282 409 274 401 265 386 [kbps/sector] UL noise rise [dB] 2.27 3.89 2.08 3.57 2.00 3.40 UL load [%] 40.1 58.1 37.4 54.8 36.1 52.7 DL power [dBm] 34.9 38.2 34.5 37.6 34.0 37.0

5.3.4.2 Importance of coverage overlapping The behavior of coverage overlapping towards interference and system capacity depends on the environment and planning thresholds. In an urban environment, where large overlapping is required due to indoor coverage, other-cell-to-own-cell interference seems to be constant if the antenna configuration is not changed. Thus, overlapping in urban areas is no more sensitive than other parameters in the topology planning phase. Correspondingly, in rural areas where the network is typically coveragelimited, small overlapping areas and cell-breathing may cause more problems if there are unexpected capacity needs in the radio network. 5.3.5 UMTS site evolution: from 3-sectored to 6-sectored In 2G networks, as in GSM, mainly 3-sectored site configurations are used and thus 3-sectored sites are the typical choice in the early days of UMTS. Three-sectored sites are also cost-efficient solutions compared to 6sectored sites because significantly more hardware is needed in 6-sectored sites. However, capacity requirements will be increased in the future and the site evolution path has to be defined. Different site evolution strategies can be utilized as cell splitting (small macro cells and micro cells when needed). Sectoring is also an efficient application of cell splitting because base station antenna heights in the network are not changed. However, the change from a 3-sectored to a 6-sectored configuration has an impact on network coverage and system capacity.


185

5.3.5.1 Coverage analysis A simulation example of site evolution from 3-sectored to 6-sectored is presented in order to show the impact of site configurations on long-term topology planning. The same environment and the same parameters are used as in Section 5.3.1.1. Altogether four different 6-sectored site solutions are compared to each other and to the original 3-sectored site of 65° antennas. The first two 6-sectored configurations are traditional site solutions of 65° and 33° antennas with fixed antenna direction. The third 6-sectored configuration is a 33° antenna with a 30° shift in antenna directions in the first tier of the base stations. This solution was done in order to fill the coverage holes caused by the narrow beam antennas. The fourth configuration is a hybrid: the original 65° three-sectored site configurations are maintained and 33° antennas are added between them. The results of pilot coverage probabilities of different site configurations are presented in Table 5-20. Table 5-20. Pilot coverage probabilities of 3- and 6-sectored sites at different thresholds. Threshold [dBm] Site type -75 -84 -104 3-sec/65 46.1 87.9 99.8 6-sec/65 61.9 91.5 99.9 6-sec/33 64.9 90.7 99.9 67.8 91.0 99.9 6-sec/33 (1st tier antennas) 6-sec/65&33 65.7 91.6 99.9

The thresholds in Table 5-20 correspond to urban in-building (-75 dBm), suburban in-building (-84 dBm), and outdoor coverage (-104 dBm). The results show that the coverage performance of the 33° antennas in 6-sectored sites is generally better than the performance of 65° antennas. On the other hand, the best suburban in-building coverage is achieved with a combined usage of 65° and 33° antennas since the coverage of 65° antennas is smoother and hence only a few coverage holes are observed. Due to the high requirement of the indoor planning threshold, the site separation was only 1.5 km and thus nearly full outdoor coverage probability was achieved in all cases. 5.3.5.2 Capacity and performance analysis The results in Table 5-21 show the performance and capacity analysis of 3- and 6-sectored sites. The number of mobiles is selected based on the approximately 95% service probability in the network of 3-sectored sites. As expected, the service probability of 6-sectored sites is nearly 100%. Moreover, the mean number of mobiles in SHO is equal in all 6-sectored configurations and a couple of percent less than in a 3-sectored network. The

186


other-to-own-cell interference is increased due to 65° antennas in 6-sectored sites because of strong sector overlapping. Hence, also the number of SfHO connections is quite high. The number of SfHO connections can easily be controlled by using narrower antennas or a combined antenna solution (65° and 33°). Table 5-21. Results of performance and capacity analysis with 2300 users in the network. Parameter 3-sec 6-sec 6-sec 6-sec 6-sec Antenna [°] 65 65 33 33* 65&33 Service probability [%] 94.9 99.7 99.8 99.8 99.7 SHO probability [%] 26.7 23.0 23.8 23.8 23.5 SfHO probability [%] 4.2 22.5 3.8 3.7 10.6 Throughput [kbps/site] 1082 1252 1099 1099 1154 UL noise rise [dB] 2.53 1.73 1.45 1.45 1.57 UL load [%] 48.0 29.7 25.6 25.5 27.3 DL power [dBm] 38.4 34.5 33.4 33.4 33.8

5.3.5.3 UMTS site evolution path An optimum UMTS network or site evolution path has to be selected based first on the original configuration in the UMTS network in order to minimize network element changes. Next, coverage and capacity analysis must be done as in the previous example in Sections 5.3.5.1 and 5.3.5.2. Even if separate sectoring and beam width calculations in Section 5.3.3.3 show that the maximum capacity is achieved using 6-sectored 33° antennas, an almost equivalent result could be obtained when a hybrid 33°/65° solution is used. Thus, a hybrid 33°/65° site configuration could be more favorable because significant cost savings can be achieved in implementation.

5.4 UMTS antenna configuration Base station antenna height, beam width, and tilting properties have a strong impact on network coverage and system capacity and thus these antenna parameters are typically modified by changing antenna types in the topology planning phase. 5.4.1 Antenna height Antenna height has an enormous effect on UMTS radio network performance. A network of too low antenna positions has poor coverage probabilities and hence site density must be increased. On the contrary, higher antenna positions create additional interference and hence reduce the capacity of the network. An average antenna height in different areas in the


187

radio network must already be defined in the dimensioning phase in order to define average cell ranges. In the topology planning phase, antenna heights can be modified but the impact of these changes on propagation and on breakpoint distance must be understood. Moreover, the effect of antenna height and antenna tilting must be known exactly. 5.4.1.1 Breakpoint distance Base station antenna height affects radio propagation near the base station antenna. Until a certain distance, the propagation near the base station antenna follows a propagation slope of 20 dB/dec. The distance where the propagation slope changes is called breakpoint distance [31]: B=4

h BTS hMS

λ

(5-5)

where hBTS and hMS are the antenna heights of the base station and mobile station and Ȝ is the wavelength. After the breakpoint distance, the propagation slope is defined by the environment. Higher antenna positions yield longer breakpoint distances and therefore larger coverage areas, but on the other hand they yield higher interference levels in surrounding cells. In a macrocellular environment, propagation occurs mainly above the average rooftop level. In Figure 5-16, two different antenna positions for macrocellular propagation are depicted: the lower antenna height is close to the average rooftop level.

Figure 5-16. Macrocellular coverage of two different antenna heights.

Figure 5-16 indicates that a radio signal will experience a harsher environment when a low antenna position is used [32]. The signal is

188


propagating above the rooftops, but after the breakpoint distance, which is closer to the base station than when the antenna position is used, the propagation slope changes to correspond to the environment. Thus, in this particular example, the coverage area is not entirely exceeded. With a higher antenna position, the required coverage is achieved, but also the interference level beyond the cell coverage area is higher. 5.4.1.2 Antenna height simulations in topology planning A simulation example related to the topology planning phase shows the effect of base station antenna height on the UMTS network system performance of 3- and 6-sectored sites of 65° and 33° antennas. In addition, the function of different antenna heights and site separations must be known. The simulation area consists of 19 base stations in a hexagonal grid. The simulation parameters are the same as in Table 5-3 in Section 5.3.1.1. In Table 5-22, the simulation results of 3-sectored sites with 65° antennas are shown. These results correspond to the base station antenna heights of 25 m and 45 m. When the base station site separation is 1.5 km, the service probability of a 45 m antenna height is only 86.7%. This is caused by the raised interference level. With a 25 m antenna height, the service probability is sufficiently good with a 1.5 km site separation. Moreover, the difference in SHO connections is also quite high. In this example, a network with lower antenna positions should be used when the site separation is 1.5 km and when tilting is not utilized. The service probability of site separations of 2.0 km and 2.5 km is sufficient with lower and higher antenna positions. Moreover, the number of SHO connections is almost constant. Other-cell-to-own-cell interference is reduced when site separation is increased and thus performance of higher antenna positions is improved. Table 5-22. Base station antenna heights of 25 m and 45 m, different site separations and 3sectored sites of 65° antennas. Parameter 1.5 km 2.0 km 2.5 km Number of users 3000 3000 3000 Service probability [%] 98.7 / 86.7 98.5 / 97.1 97.1 / 96.6 SHO probability [%] 18.5 / 34.9 16.0 / 25.2 16.8 / 22.2 SfHO probability [%] 4.7 / 4.0 5.0 / 4.3 5.0 / 4.5 Throughput 401 / 413 394 / 421 390 / 408 [kbps/sector] UL noise rise [dB] 3.77 / 4.39 3.47 / 4.22 3.41 / 3.89 UL load [%] 57.3 / 62.8 53.9 / 61.4 53.2 / 58.4 DL power [dBm] 37.8 / 39.6 37.4 / 38.8 37.2 / 38.2


189

The results of the consequent simulations of 6-sectored sites and 33° antennas are given in Table 5-23. The number of mobiles is increased in order to increase the load in the network. The performance of a 6-sectored network compared to a 3-sectored network is quite similar. A smaller site separation and higher antenna position causes a higher interference level and hence decreases service probability. Table 5-23. Base station antenna heights of 25 m and 45 m, different site separations and 6sectored 33° sites. Parameter 1.5 km 2.0 km 2.5 km Number of users 6000 6000 6000 Service probability [%] 96.8 / 85.1 97.5 / 95.1 96.3 / 96.1 SHO probability [%] 23.0 / 41.9 19.5 / 29.9 17.2 / 23.8 SfHO probability [%] 3.9 / 2.3 4.5 / 3.3 4.6 / 3.7 Throughput 409 / 428 401 / 431 386 / 409 [kbps/sector] UL noise rise [dB] 3.89 / 4.26 3.57 / 4.2 3.4 / 3.88 UL load [%] 58.1 / 61.4 54.8 / 61.1 52.7 / 58.0 DL power [dBm] 38.2 / 39.7 37.6 / 39.0 37.0 / 38.1

5.4.1.3 Antenna height selection in UMTS Base station antenna height has a significant impact on the performance of a UMTS network, as can be seen in Tables 5-22 and 5-23. Especially other-cell-to-own-cell interference is heavily increased if the antenna position is too high, cells are strongly overlapping, and antenna tilting is not utilized. Therefore selecting an average antenna height should be done carefully in the dimensioning phase, and modifications in the topology planning phase should be avoided. If antenna heights are still changed, antenna tilting should be utilized (see Section 5.4.3). 5.4.2 Antenna beam width in UMTS The impact of base station antenna beam width on UMTS network coverage and system capacity in topology planning phase is explained in detail in Section 5.3.3. 5.4.3 Antenna down tilt in UMTS Base station antenna mechanical down tilt has been traditionally used in TDMA/FDMA networks (e.g., in GSM) to decrease other-cell interference in order to minimize the frequency reuse factor and moreover to increase the capacity of the network. In UMTS networks, cell-breathing affects the cell range and no fixed coverage area exists. Thus the effect of antenna down tilt

190


is not as straightforward as in TDMA/FDMA networks. By using down tilting at the base station antenna, interference leakage to other cells can be reduced and thus the capacity of a UMTS network increased. The difference between mechanical and electrical antenna down tilt has been studied in a WCDMA network in the uplink direction by using a 3sectored site scenario [33]. The results of this study show that electrical down tilt provides slightly better capacity enhancements than mechanical tilt. Correspondingly, the results in [28] indicate that electrical down tilt should be used in urban environments as a pre-optimization method. 5.4.3.1 Mechanical down tilt In Figure 5-17, the principle of antenna mechanical tilt is illustrated. When an antenna element is physically down tilted, only the main lobe is down tilted and the back lobe is up tilted. The side lobes are only partly tilted and coverage (or on the contrary, interference) is almost attained in the side lobe direction. As a consequence of a large mechanical down tilt angle, a notch can be observed in the radiation pattern. The notch becomes larger when the tilt angle increases [34]. Thus, only interference and coverage towards the main lobe are changed.

Figure 5-17. Illustration of base station antenna mechanical down tilt.

In Figure 5-18, the effect of base station antenna mechanical down tilt on signal propagation is represented in a real propagation environment defined by a digital map. A macrocellular propagation model is used to predict the coverage. The picture on the left represents coverage without down tilt and the picture on the right represents 6° mechanical down tilt. Blue color in Figure 5-18 refers a very good signal strength and light brown the weakest signal strength. The vertical beam width of the base station antenna is 6°.


191

Figure 5-18. Effect of antenna mechanical down tilt in a real propagation environment.

Figure 5-18 shows that the signal strength in the direction of back and side lobes remains approximately the same when the antenna is down tilted. Correspondingly, the coverage in the main beam direction starts to shrink, thus also decreasing the interference leakage. 5.4.3.2 Electrical down tilt Figure 5-19 represents how the main, side, and back lobes of electrically down tilted antennas behave. In electrical down tilt, the side and back lobes of the antenna radiation pattern are also tilted. Because of side and back lobe down tilting, coverage as well as interference conditions are totally different in electrical down tilting than in mechanical down tilting. Thus, interference radiation is smaller compared to a similar mechanical down tilt situation but also the coverage area becomes smaller as the electrical down tilt angle is increased. The electrical down tilt is carried out by adjusting the antenna elements and hence it only slightly changes the antenna radiation characteristics.

Figure 5-19. Illustration of base station antenna electrical down tilt.

192


In Figure 5-20, an example of base station antenna electrical down tilt is depicted. The picture on the left represents coverage without down tilt and the picture on the right represents 6° antenna down tilt. The vertical beam width of the base station antenna is 6°. Figure 5-20 shows that the coverage towards the side and back lobe directions, as well as towards the main lobe direction, is smaller.

Figure 5-20. Effect of antenna electrical down tilt in a real propagation environment.

5.4.3.3 Optimum down tilt angle Base station down tilt angle depends strongly on the site configuration and location. Base station antenna height and vertical beam width are critical parameters when an optimal down tilt angle is defined. A large cell range (as in a rural area) indicates that a smaller tilt should be utilized in order to maintain coverage. Correspondingly, in larger overlapping areas as in urban environments, the down tilt angle for interference leakage reduction can be bigger because of good coverage. The required down tilt angle also depends on the load of the network. In a highly loaded cell, the cell range is effectively smaller than in a low loaded cell and thus the required tilt angle would be smaller. Furthermore, the type of tilting is an issue when selecting the base station antenna down tilting scheme. In a coverage-limited environment, the coverage is important and mechanical down tilt could be more useful. Correspondingly, in a capacitylimited environment, minimization of the interference is more vital and hence electrical down tilt could provide better performance. 5.4.3.4 Antenna down tilt simulations in topology planning Examples of different antenna down tilting schemes are simulated in order to show the effect of tilting in different network scenarios. The


193

simulation area consists of 19 six-sectored sites, each having 33° horizontal beam width antennas with the base stations arranged in a regular hexagonal grid. All the other simulation parameters are taken from Table 5-3 in Section 5.3.1.1, except that the HO window is 3 dB, and the Eb/N0 requirements are 6 dB for downlink and 4 dB for uplink. A homogenous traffic distribution of voice users (12.2 kbps) is used. The impact of site separation, antenna height, base station antenna vertical beam width, network load, and tilting scheme (electrical / mechanical) must be defined. The effect of site separation (coverage overlapping) is simulated by using site distance of 1.5 km and 2.2 km. In order to define the effect of base station antenna height, 25 m and 40 m base station antenna heights are chosen. In mechanical down tilt simulations, two different antennas are used – one antenna with a 6° vertical beam width and a gain of 20.7 dBi and another antenna with a 12° vertical beam width and a gain of 17.9 dBi. The vertical radiation patterns of antennas used in mechanical down tilt simulations are shown in Figure 5-21.

Figure 5-21. Vertical radiation patterns of 6° and 12° antennas used in the mechanical down tilt simulations.

In Figure 5-22, the vertical radiation patterns of an electrically down tilted antenna are presented. The gain of this antenna is 19.7 dBi. The horizontal beam width of each antenna is 33°.

194


Figure 5-22. Vertical radiation patterns of 0°, 2°, 4°, and 6° electrically down tilted antenna.

First, the site separation is set at 2.2 km, the base station antenna at 25 m, and an antenna with a 6° vertical beam width is utilized. Table 5-24 represents the simulation results based on the mechanical down tilt. Table 5-24. Results of antenna mechanical down tilt, 2.2 km site separation, low load. Parameter 0° 2° 4° 6° Service probability [%] 100.0 100.0 100.0 99.9 SHO probability [%] 16.4 15.0 13.5 12.1 SfHO probability [%] 4.3 4.2 4.9 6.6 UL load [%] 36.0 35.3 35.0 35.2 Throughput [kbps/sector] 427 421 418 420 DL power [dBm] 35.8 35.4 35.1 35.3

The results in Table 5-24 are based on the same number of users in the simulation area. The performance of the network is better when using down tilt. Due to a relatively low load, the service probability is nearly 100% in all configurations. The number of SHO connections is decreased when the down tilt angle is increased. In contrast, the number of mobile phones in SfHO is increased steadily when the down tilt angle is increased. This is due to the fact that the antenna radiation pattern is getting wider because only the main lobe is down tilted.


195

In Figure 5-23, DL throughput per sector is plotted as a function of the down tilt angle. The solid line represents capacity values of 33 dBm average base station transmit power (low load). The dashed line represents capacity values of 39 dBm average base station transmit power (high load). An optimum down tilt angle can be seen in Figure 5-23. 600

33 dBm 39 dBm

DL throughput [kbit/s/sector]

550

500

450

400

350

300 0

1

2

3

4

5

6

Tilt angle [degree]

Figure 5-23. DL sector throughput as a function of antenna mechanical down tilt angle, 2.2 km site separation, 25 m antenna height and 6° antenna vertical beam width.

In the simulations, the site separation is decreased to 1.5 km in order to have larger coverage overlapping. The results in Table 5-25 correspond to the same amount of traffic as in the 2.2 km site separation. An impact similar to the 2.2 km site separation can be observed. The number of SHO connections is decreased and the number of SfHO connections is increased as a function of the down tilt angle. The increase of SfHO probability is higher than in a 2.2 km site separation. Table 5-25. Results of antenna down tilt, 1.5 km site separation, low load. Parameter 0° 2° 4° 6° Service probability [%] 100.0 100.0 100.0 100.0 SHO probability [%] 16.8 16.5 13.9 11.8 SfHO probability [%] 3.9 4.0 4.5 6.0 UL load [%] 35.8 35.7 35.0 34.8 Throughput [kbps/sector] 425 426 419 417 DL power [dBm] 35.6 35.5 35.0 34.8

8° 100.0 10.1 9.2 35.6 422 35.2


196

In Figure 5-24, sector throughputs are depicted as a function of antenna down tilt angle. By comparing the results of 2.2 km and 1.5 km site separations in Figures 5-23 and 5-24, it can be noted that an optimum down tilt angle is larger when coverage overlapping is larger. Moreover, the down tilting becomes more significant because of the higher capacity enhancement. 650

33 dBm 39 dBm


600 550 500 450 400 350 300 0

2

4

6

8

Tilt angle [degree] Figure 5-24. DL sector throughput as a function of antenna mechanical down tilt angle, 1.5 km site separation, 25 m antenna height and 6° antenna vertical beam width.

In order to see the effect of a higher antenna position, all the base station antennas are raised to 40 m. Table 5-26 represents the corresponding simulation results. In Table 5-26, the service probability is decreased with higher down tilt angles. The curves in Figure 5-25 show that the capacity of the network is lower than with lower antenna heights when the antennas are not tilted. (See Figures 5-23 and 5-24.) With a 25 m antenna height, the capacity is nearly 550 kbps/sector (see Figures 5-23 and 5-24), and in Figure 5-25 the capacity is decreased to 490 kbps/sector. This is caused by higher antenna positions and a raised interference level. However, after down tilting the antennas, the capacity of the network is raised more steeply than with the lower antenna positions and capacity is enhanced 25% when comparing 0° and optimal


197

tilting. In addition, the capacity curves also show that too much down tilt quickly reduces the coverage and hence also the capacity of the network. Table 5-26. Results of antenna mechanical down tilt, 1.5 km site height, low load. Parameter 0° 2° 4° Service probability [%] 100.0 100.0 100.0 23.5 20.5 16.5 SHO probability [%] 3.4 3.3 3.9 SfHO probability [%] 38.6 37.1 35.6 UL load [%] 445 438 426 Throughput [kbps/sector] 37.5 36.5 35.5 DL power [dBm]

separation, 40 m antenna 6° 100.0 12.6 4.9 34.5 416 34.7

650

10° 99.6 10.2 9.7 35.3 423 34.0

33 dBm 39 dBm

600


8° 99.8 12.0 5.6 34.3 410 33.5

550 500 450 400 350 300 250 0

2

4

6

8

10

Tilt angle [degree]

Figure 5-25. DL capacity as a function of antenna mechanical down tilt angle, 1.5 km site separation, 40 m antenna height and 6° antenna vertical beam width.

Finally, antennas with a 12° vertical beam width are used with mechanical down tilting. In Table 5-27, the results of this wider beam width down tilting are presented. The optimum down tilt angle is increased up to approximately 14° because of wider vertical beam width. Also the performance of the network begins to drop steeper after too strong down tilting (see Figure 5-26).


198

Table 5-27. Results of antenna mechanical down tilting, 1.5 km site separation, 40 m antenna height and 12° antenna vertical beam width. 0° 2° 4° 6° 8° Parameter Service probability [%] 100.0 100.0 100.0 100.0 100.0 SHO probability [%] 19.1 17.8 16.7 15.5 14.3 SfHO probability [%] 2.8 2.7 2.8 3.0 3.3 UL load [%] 36.1 35.6 35.2 34.8 34.5 Throughput [kbps/sector] 431 427 432 420 417 DL power [dBm] 35.9 35.5 35.3 35.0 34.7 Parameter Service probability [%] SHO probability [%] SfHO probability [%] UL load [%] Throughput [kbps/sector] DL power [dBm]

10° 100.0 13.2 3.9 34.4 415 34.6

12° 100.0 12.1 4.8 34.4 414 34.6

14° 99.7 11.7 5.6 34.5 414 34.4

16° 99.4 10.3 6.7 34.9 418 34.7

650

33 dBm 39 dBm


600 550 500 450 400 350 300 0

2

4

6

8

10

12

14

16

Tilt angle [degree] Figure 5-26. DL capacity as a function of antenna mechanical down tilt angle, 1.5 km site separation, 40 m antenna height and 12° antenna vertical beam width.

A network equipped with electrically down tilted antennas behaves differently due to different behavior of antenna radiation pattern. In next simulations, a network with electrically down tilted antennas is studied by using 2.2 km site separation and 25 m antenna height.


199

In Table 5-28 and Figure 5-27, the performance of electrically down tilted network is presented. The capacity is rising quickly due to decreased interference but it also drops very rapidly. This is caused by the degradation in coverage due to the properties of the electrical down tilt. The deteriorated coverage conditions can be seen as decreased service probability and also as decreased sector throughput. Moreover, the changes in SHO probabilities are not as dramatic as in mechanical down tilt and also the number of SfHO is nearly constant. Table 5-28. Results of the simulations of electrically down tilted antennas, 2.2 km site separation, 25 m antenna height and 6° antenna vertical beam width. Parameter 0° 2° 4° 6° Service probability [%] 99.9 99.9 98.6 95.6 SHO probability [%] 21.6 17.8 17.4 16.1 SfHO probability [%] 4.1 3.6 3.8 4.1 UL load [%] 47.5 44.8 44.4 39.1 Throughput [kbps/sector] 390 391 391 356 DL power [dBm] 34.3 33.7 33.8 34.1 650

33 dBm 39 dBm


600 550

500 450

400 350

300 0

1

2

3

4

5

6

Tit angle [degree] Figure 5-27. DL capacity as a function of antenna electrical down tilt angle, 2.2 km site separation, 25 m antenna height and 6° antenna beam width.

In the last simulation the site separation is decreased to 1.5 km. Table 529 shows that the coverage performance of down tilted antennas is improved


200

and thus the service probability is nearly 100%. The difference in SHO probabilities is also more obvious than with larger site separation. Table 5-29. Results of the simulations of electrically down tilted antennas, 1.5 km site separation, 25 m antenna height and 6° antenna vertical beam width. Parameter 0° 2° 4° 6° Service probability [%] 99.9 99.9 100.0 99.8 SHO probability [%] 24.1 19.4 17.3 15.0 SfHO probability [%] 4.3 3.8 3.9 4.0 UL load [%] 45.0 41.1 40.5 38.2 Throughput 368 350 343 338 [kbps/sector] DL power [dBm] 34.1 33.5 33.2 32.9

Figure 5-28 shows that the sector throughput is improved as a function of electrical down tilt angle because of coverage overlapping. Also the achieved capacity is a bit better than with mechanical down tilt antennas. 650

33 dBm 39 dBm


600

550

500

450

400

350

300 0

1

2

3

4

5

6

Tilt angle [degree]

Figure 5-28. DL capacity as a function of antenna electrical down tilt angle, 1.5 km site separation, 25 m antenna height and 6° antenna vertical beam width.


201

5.4.3.5 Antenna down tilt selection in UMTS Base station antenna down tilting is an efficient way to decrease interference and hence to increase the capacity in a UMTS network. An optimum down tilt angle always exists. It is a function of the site and antenna configuration: coverage overlapping (cell range), base station antenna height, and antenna vertical beam width. Sectoring and antenna horizontal beam width also have an impact on network performance due to side lobes. Moreover, electrical down tilt gives bit better throughput values compared to mechanical down tilt because of reduced interference. However, a network of electrically down tilted antennas requires more cautious planning due to the radiation properties of electrical down tilt antennas. Hence, antenna down tilting is a key planning element in the topology planning phase for ensuring optimal network performance in UMTS.

REFERENCES [1] H. Holma, A. Toskala, WCDMA for UMTS, John Wiley & Sons Ltd, 2001. [2] T. Neubauer, E. Bonek, Impact of the Variation in the Background Noise Floor on UMTS System Capacity, IEEE 53rd Vehicular Technology Conference, vol. 4, 2001. [3] J. Laiho, A. Wacker, Radio Network Planning Process and Methods for WCDMA, Annals of Telecommunications, vol. 56, no. 5-6, Mai/Juin 2001. [4] P. Gould, Radio Planning of Third Generation Networks in Urban Areas, Third International Conference on 3G Mobile Communication Technologies, 2002. [5] M. Hata, Empirical Formula for Propagation Loss in Land Mobile Radio Services, IEEE Transactions on Vehicular Technology, vol. 29, no. 3, 1980. [6] K. Gilhousen, I. Jacobs, R. Padovani, A. Viterbi, L. Weaver Jr., C. Wheatley, On the Capacity of a Cellular CDMA System, IEEE Transactions on Vehicular Technology, vol. 40, no. 2, 1991. [7] K. Ruttik, Capacity Calculation for CDMA Network Planning, Licentiate’s Thesis, Helsinki University of Technology, 1999. [8] K. Hiltunen, R. De Bernardi, WCDMA Downlink Capacity Estimation, IEEE 51st Vehicular Technology Conference, vol. 2, 2000. [9] D. Adiego, C. Cordier, Multi-Service Radio Dimensioning for UMTS Circuit Switched Services, IEEE 54th Vehicular Technology Conference, vol. 4, 2001. [10] J. Laiho, Radio Network Planning and Optimisation for WCDMA, Doctoral Thesis, Helsinki University of Technology, 2002. [11] K. Sipilä, Z. Honkasalo, J. Laiho-Steffens, A. Wacker, Estimation of Capacity and Required Transmission Power of WCDMA Downlink Based on a Downlink Pole Equation, IEEE 51st Vehicular Technology Conference, vol. 2, 2000. [12] S. Malik, D. Zeghlache, Downlink Capacity and Performance Issues in Mixed Services UMTS WCDMA Networks, IEEE 55th Vehicular Technology Conference, vol. 4, 2002. [13] J. Lempiäinen, M. Manninen, Radio Interface System Planning for GSM/GPRS/UMTS, Kluwer Academic Publishers, 2001.

202


[14] J. Laiho-Steffens, A. Wacker, P. Aikio, The Impact of the Radio Network Planning and Site Configuration on the WCDMA Network Capacity and Quality-of-Service, IEEE 51st Vehicular Technology Conference, vol. 2, 2000. [15] J. Laiho, A. Wacker, T. Novosad, Radio Network Planning and Optimisation for UMTS, John Wiley & Sons Ltd, 2002. [16] J. Niemelä, J. Lempiäinen, Impact of the Base Station Antenna Beam Width on Capacity in WCDMA Cellular Networks, IEEE 57th Vehicular Technology Conference, vol. 1, 2003. [17] M. Bavafa, H. Xia, Repeaters for CDMA Systems, IEEE 48th Vehicular Technology Conference, vol. 2, 1998. [18] S-J Park, W. Woo Kim, B. Kwon, An Analysis of Effect of Wireless Network by a Repeater in CDMA System, IEEE 53rd Vehicular Technology Conference, vol. 4, 2001. [19] R. Steele, C-C Lee, P. Gould, GSM, cdmaOne and 3G Systems, John Wiley & Sons Ltd, 2001. [20] L-C Wang, K. Chawla, L. Greenstein, Performance Studies of Narrow-Beam Trisector Cellular Systems, IEEE 48th Vehicular Technology Conference, vol. 2, 1998. [21] A. Spilling and A. Nix, Aspects of Self-Organisation in Cellular Networks, The Ninth International Symposium on Personal, Indoor and Mobile Radio Communications, 1998. [22] V. Jovanoviü, J. Gazzola, Capacity of Present Narrowband Cellular Systems: Interference-Limited or Blocking-Limited, IEEE Transactions on Personal Communications, vol. 4, issue 6, 1997. [23] T. Brown, Cellular Performance Bounds via Shotgun Cellular Systems, IEEE Journal on Selected Areas in Communications, vol. 18, no. 11, 2000. [24] M. Nawrocki, T. Wieckowski, Optimal Site and Antenna Location for UMTS – Output Results of 3G Network Simulation Software, 14th International Conference on Microwaves, Radar and Wireless Communications, 2002. [25] E. Amaldi, A. Capone, F. Malucelli, F. Signori, UMTS Radio Planning: Optimizing Base Station Configuration, IEEE 56th Vehicular Technology Conference, vol. 2, 2002. [26] T. Ojanperä, R. Prasad, Wideband CDMA for Third Generation Mobile Communications, Artech House, 1998. [27] T. Wong, V. Prabhu, Optimum Sectorization for CDMA 1900 Base Stations IEEE 47th Vehicular Technology Conference, vol. 2, 1997. [28] A. Wacker, J. Laiho-Steffens, K. Sipilä, K. Heiska, The Impact of the Base Station Sectorisation on WCDMA Radio Network Performance, IEEE 50th Vehicular Technology Conference, vol. 5, 1999. [29] B. Christer V. Johansson, S. Stefansson, Optimizing Antenna Parameters for Sectorized W-CDMA Networks, IEEE 52nd Vehicular Technology Conference, vol. 4, 2000. [30] S. Asari, A. Jalan, N. Velayudhan, Performance Analysis of the Forward Link in a Power Controlled CDMA Network, IEEE 47th Vehicular Technology Conference, vol. 3, 1997. [31] W.C.Y Lee, Mobile Communications Design Fundamentals, John Wiley & Sons Ltd, 1993. [32] E. Benner, A. Sesay, Effects of Antenna Height, Antenna Gain, and Pattern Down Tilting for Cellular Mobile Radio, IEEE Transactions on Vehicular Technology, vol. 45, no. 2, 1996. [33] I. Forkel, A. Kemper, R. Pabst, R. Hermans, The Effect of Electrical and Mechanical Antenna Down-Tilting in UMTS Networks, IEE Proceedings of 3G Mobile Communication Technologies, 2002.


203

[34] D. J. Y. Lee, C. Xu, Mechanical Antenna Down Tilt and its Impact on System Design, IEEE 47th Vehicular Technology Conference, vol. 2, 1997.


PART III: UMTS NETWORK FUNCTIONALITY


Chapter 6 WCDMA RADIO INTERFACE Radio Resource Management and 3GPP Radio Parameters MATTI MANNINEN European Communications Engineering (ECE) Ltd

Abstract:

Understanding the WCDMA radio interface is essential for engineers dealing with UMTS radio networks. A knowledge of the structure and usage of the physical channels in the downlink and uplink directions, together with a knowledge of spreading and modulation, provides a good background for understanding how the WCDMA radio network works. Various kinds of services with different quality-of-service requirements and bit rates can be transmitted through the radio interface with the help of an effective and versatile radio resource management (RRM). The RRM consists of the handover control, power control, admission and load control, as well as resource management. In WCDMA networks, power control has an especially important role. The behavior of the RRM is controlled with a set of radio parameters. Most of these radio parameters are defined by 3GPP specifications. With these parameters and good network planning practice, the behavior of a WCDMA network can be optimized to meet coverage, capacity, and quality requirements. However, in the RRM, there can be vendor-specific implementations that may provide benefits in utilizing the radio resources.

Key words:

Radio interface, radio parameters, radio resource management


208


6. WCDMA RADIO INTERFACE This chapter describes the basics of the radio interface of UTRAN FDD, radio resource management, and the key parameters defined in the 3GPP specifications for the radio interface. The TDD radio interface is not covered in this chapter because large-scale implementation of TDD may come after a few years. However, the basics of TDD radio interface are covered in [1]. In this chapter, first the structure of the radio frame is described as well as the physical and transport channels of UTRAN FDD. Second, the basics of spreading and modulation in WCDMA are given and some of the physical layer procedures related to idle mode, call establishment and connected mode are described. Third, the key functionalities of radio resource management are explained. Finally, the key 3GPP radio parameters are explained at the end of the chapter. The WCDMA radio interface is based on direct sequence code division multiple access (DS-CDMA). In DS-CDMA, the low bit rate data signal is spread by a spreading code. The benefits in spreading the signal over a wider bandwidth are, for example, tolerance to narrowband interference, better tolerance to fast fading, and flexibility in combining different data services in the radio channel. The use of CDMA in radio interface requires special arrangements at the mobile station, base station and radio network controller. The main issues are handovers and power control. When CDMA is used, it is possible to maintain connections to two or more cells at the same time. This improves the performance of the connection but at the same time it makes the equipment more complex. Typically, CDMA networks are interference-limited and the network must be able to scope the so-called near-far effect. This means that there can be users close to the center of the cell at the same time that there are users close to the cell edge. To minimize interference in the network and to be able to handle the near-far effect, fast and accurate power control is needed. This means that both mobile station and base station tune the transmit power in such a manner that the received power is high enough for decoding the transmitted data but at the same time is as low as possible. Handovers, power control, data transmission, and utilization of resources are managed with parameters used in mobile stations, NodeBs, and RNCs. The optimization of the parameters may result in valuable improvements in coverage, capacity, and quality. In this chapter, it is assumed that the reader knows the basics of CDMA technology. The fundamentals of CDMA are described in many references. One such reference is cited below [2].

WCDMA RADIO INTERFACE

209

6.1 Physical layer In this section, the physical layer of UTRAN FDD is explained. First, the radio frame structure is covered. This is essential for knowing how different physical channels are configured in the physical layer. Next, the physical and transport layer channels are introduced and their main characteristics are described. Finally, examples of the procedures provided by the physical layer in idle mode, call establishment, and connected mode are explained. When a network communicates with a mobile station, there are different steps that have to be taken before the actual communication can take place. There are also requirements that must be fulfilled during the communication. All communication between the network and mobile station takes place in different channels. In WCDMA, the channels are divided into three layers. Logical channels provide data transfer within the MAC layer and each logical channel defines a type of information that is transferred [3]. Transport channels carry data from the upper layer over the air interface. Furthermore, the transport channels are mapped to the physical channels. 6.1.1 Radio frame structure In FDD, information is spread approximately over the 5 MHz bandwidth. There are separate 5 MHz radio channels for the uplink and the downlink. Also, the structures of these channels are different. One radio frame consists of 38,400 chips and 15 slots. The duration of the radio frame is 10 ms, which results in a chip rate of 3.84 Mcps. Every slot in a radio frame consists of 2,560 chips, which corresponds to one power control period. The radio frame structure is illustrated in Figure 6-1.

Figure 6-1. Radio frame structure.

210


In the uplink and the downlink directions, each slot in a radio frame carries a set of common channels and dedicated channels. Each physical channel is defined with a code or a set of codes. The information rate of the channel varies from 7,500 symbols per-second to 960,000 symbols persecond in the downlink direction, and from 15,000 symbols to 960,000 symbols in the uplink direction. The difference between the uplink and the downlink symbol rates comes from the available spreading factors. In the uplink, the spreading factors are from 256 to 4, and in the downlink, from 512 to 4 [4]. 6.1.2 Physical layer channels in the uplink In the uplink direction, there are both dedicated and common physical channels. Dedicated physical channels are the uplink Dedicated Physical Data Channel (uplink DPDCH) and the uplink Dedicated Physical Control Channel (uplink DPCCH). Common channels in the uplink direction are the Physical Random Access Channel (PRACH) and the Physical Common Packet Channel (PCPCH). The DPDCH and the DPCCH are I/Q code-multiplexed within each radio frame. Also, in the PRACH and the PCPCH, control and data parts are codemultiplexed. 6.1.2.1 DPDCH and DPCCH For each radio link, there is only one DPCCH. It carries control information, such as pilot bits, transmit power control (TPC) commands, feedback information (FBI), and an optional transport format combination indicator (TFCI). Pilot bits are used to support channel estimation at the RAKE receiver. TPCs are used in power control. And FBI is needed with transmit diversity. Each radio link may have no DPDCHs or several DPDCHs. The DPDCH carries the Data Channel (DCH) transport channel. The structure of the frame for the DPCCH and the DPDCH is illustrated in Figure 6-2. The spreading factor for the DPDCH can be between 256 and 4. The spreading factor of the DPCCH is always 256. In each DPCCH slot, there are always 10 bits that correspond to a bit rate of 15 kbps. In a DPDCH slot, there can be from 10 to 640 bits, depending on the spreading factor, which corresponds to channel bit rates from 15 kbps to 960 kbps. FBI bits are needed in cases when the UTRAN access point (e.g., NodeB) needs feedback from the mobile station. The FBI bits are needed in


211

the closed-loop mode of transmit diversity and site selection diversity transmission (SSDT) .

Figure 6-2. The frame structure of the DPDCH/DPCCH in the uplink.

There are dedicated physical channels that include TFCI and dedicated physical channels that do not include TFCI. The usage of TFCI depends on the services carried by the physical channel. UTRAN determines whether TFCI is sent, but all the mobile stations should support the usage of TFCI in the uplink. The mobile station uses TPC bits for the downlink fast power control. (See 6.4.1.3 for details.) The mobile station sends one or two TPC bits, depending on the slot format. If the mobile station sends two TPC bits, the allowed combinations are ‘00’ (TPC command ‘0’) and ‘11’ (TPC command ‘1’). If only one TPC bit is sent, then the TPC bit directly corresponds to the TPC command. 6.1.2.2 PRACH The Physical Random Access Channel carries Random Access Channel (RACH). The channel is used by the mobile station when the physical random access procedure is initiated. The random access procedure is explained in physical layer procedures (see Section 6.3.1). The random access transmission consists of one or several preambles and the message part. The length of the preamble is 4,096 chips. The preamble can be repeated several times if needed. The length of the message part is either 10 ms or 20 ms. The structure of the physical random access transmission is illustrated in Figure 6-3.

Figure 6-3. The structure of random access transmission.

212


The message part is sent with spreading factors of 256, 128, 64, or 32. Thus, the number of bits in the message part is 10, 20, 40, or 80 bits, which corresponds to channel bit rates of 15 kbps, 30 kbps, 60 kbps, or 120 kbps. For the control part, the spreading factor is always 256. The frame structure of the PRACH message part is illustrated in Figure 6-4.

Figure 6-4. The frame structure of the PRACH.

If the length of the message part is 20 ms, then data is sent with two consecutive 10 ms radio frames, and the TFCI is repeated in the second 10 ms radio frame. 6.1.2.3 PCPCH The Physical Common Packet Channel carries the Common Packet Channel (CPCH). The CPCH transmission uses the Digital Sense Multiple Access - Collision Detection (DSMA-CD) approach with fast acquisition indication. The PCPCH access transmission consists of Access Preambles (AP), one Collision Detection Preamble (CDP), a DPCCH Power Control Preamble (PCP), and a message. The PCPCH access transmission structure is shown in Figure 6-5.

Figure 6-5. PCPCH access transmission structure.

In the PCPCH access procedure, the mobile sends access preambles with low power. If the preamble is not detected, the mobile station increases the


213

power and sends a new preamble. When the preamble is detected, the mobile station sends a collision-detection preamble. If there is no collision, the mobile station may send a power control preamble if configured by the higher layers. The inner-loop power control takes place in both the uplink and the downlink directions during the CPCH Packet Data transmission. The frame structure of the PCPCH is very similar to the frame structure of the dedicated channels, as shown in Figure 6-6.

Figure 6-6. The frame structure of the PCPCH.

The spreading factor for the data part can be between 256 and 4. The spreading factor for the control part is always 256. In a slot of the control part, there are always 10 bits. In the data part, there can be from 10 to 640 bits per-slot, depending on the spreading factor, which corresponds to channel bit rates from 15 kbps to 960 kbps. 6.1.3 Physical layer channels in the downlink The Downlink Dedicated Physical Channel (the downlink DPCH) is the only dedicated channel in the downlink direction. The control data and user data are time-multiplexed within one DPCH, contrary to the uplink direction where the dedicated control and user data were code-multiplexed. The common channels in the downlink are the Common Pilot Channel (CPICH), the Primary Common Control Physical Channel (P-CCPCH), the Secondary Common Control Physical Channel (S-CCPCH), the Synchronization Channel (SCH), the Physical Downlink Shared Channel (PDSCH), the Acquisition Indicator Channel (AICH), the CPCH Access Preamble Acquisition Indicator Channel (AP-AICH), the CPCH Collision Detection/Channel Assignment Indicator Channel (CD/CA-ICH), the Paging Indicator Channel (PICH), and the CPCH Status Indicator Channel (CSICH). The CPICH is further divided into the Primary Common Pilot Channel (P-CPICH) and the Secondary Common Pilot Channel (S-CPICH).

214


6.1.3.1 DPCH The Downlink Dedicated Physical Channel carries both user data and control data. As in the uplink, the length of the radio frame is 10 ms and the frame is divided into 15 slots. The frame structure is shown in Figure 6-7.

Figure 6-7. The frame structure of the DPCH.

Even if the DPCH is the only dedicated physical channel, it can be seen as a combination of the downlink DPDCH and the DPCCH, which are time multiplexed onto the DPCH. The number of bits in the data part of the DPCH depends on the spreading factor. The spreading factor can vary between 512 and 4, which corresponds to 10 to 1,280 bits per a slot, or the channel bit rates of 15 kbps to 1,920 kbps. TFCI bits are optional in the DPCH. TFCI bits are used, for example, to indicate the bit rate of the channel and interleaving. There are some special cases in the configuration of the DPCH such as the downlink diversity and the DPCCH for the CPCH. These cases are explained in [5]. 6.1.3.2 CPICH On the Common Pilot Channel, a pre-defined bit sequence is transmitted with a bit rate of 30 kbps. The frame structure of the CPICH is illustrated in Figure 6-8.

Figure 6-8. The frame structure of the CPICH.


215

The CPICH is divided into two pilot channels. The Primary Common Pilot Channel (P-CPICH) is a phase reference for the SCH, Primary CCPCH, AICH, PICH AP-AICH, CD/CA-ICH, CSICH, and S-CCPCH. The P-CPICH can also be a phase reference for the DPCH. Because the PCPICH is a phase reference, it must be transmitted over the entire cell. In all cells, the same channelization code is used for the P-CPICH, but the primary scrambling code is different between neighbor cells. The Secondary Common Pilot Channel (S-CPICH) can be scrambled with a primary orsecondary scrambling code. It may be also transmitted over the entire cell or only part of it. The S-CPICH can be the phase reference for the downlink DPCH. 6.1.3.3 P-CCPCH The Primary Common Control Physical Channel carries the BCH transport channel. The P-CCPCH uses a fixed bit rate of 30 kbps with a spreading factor of 256. The frame structure of the P-CCPCH is shown in Figure 6-9.

Figure 6-9. The frame structure of the P-CCPCH.

In the P-CCPCH, there are no TPC, pilot bits, or TFCI bits. Moreover, the first 256 chips of the slot are used for the Primary and Secondary Synchronization Channels. 6.1.3.4 S-CCPCH The Secondary Common Control Physical Channel is used to carry the FACH and the PCH. In the S-CCPCH, there are data bits, pilot bits, and optional TFCI bits. The bit rate of the S-CCPCH is from 30 kbps to 1,920 kbps. The S-CCPCH can be transmitted only to a smaller area of a cell. The frame structure of the S-CCPCH is illustrated in Figure 6-10. It is important to notice that the S-CCPCH does not have the inner-loop power control.

216


Figure 6-10. The frame structure of the S-CCPCH.

6.1.3.5 SCH The Synchronization Channel (SCH) is used in the cell search procedure. (See Section 6.3.1.) The SCH is divided into the Primary Synchronization Channel and the Secondary Synchronization Channel. Both the Primary and Secondary SCH consist of 15 slots with a length of 2,560 chips, as illustrated in Figure 6-11.

10 ms

Figure 6-11. The frame structure of the SCH.

The Primary Synchronization Channel carries the Primary Synchronization Code (PSC) that is transmitted in each slot of a radio frame. The code is 256 chips long. The PSC is the same for all cells in the network. The Secondary Synchronization Channel consists of the Secondary Synchronization Codes (SSC) that are transmitted at the same time as the PSC. The system selects the SSC to be transmitted in a slot, based on the scrambling code group and the slot number; i.e., the SSC indicates to which scrambling code group the scrambling code used in a cell belongs to.


217

6.1.3.6 PDSCH The Physical Downlink Shared Channel (PDSCH) is used for data transmission by one or-several simultaneous users in the downlink direction. It is allocated for a mobile station on a radio frame basis. One mobile station can have one or several PDSCHs. If multiple PDSCHs are allocated for a mobile station in the same radio frame, the spreading factor of each PDSCH must be the same. If the multiple PDSCHs are allocated from different radio frames, the spreading factors can also be different. The system can allocate several PDSCHs for different mobile stations by using code multiplexing. For the PDSCH, there is always an associated the downlink PDCH. The PDCCH part of the PDCH carries the control information. The structure of the PDSCH is shown in Figure 6-12.

Figure 6-12. The frame structure of the PDSCH.

6.1.3.7 AICH The Acquisition Indicator Channel (AICH) is used in a random access procedure by the network to indicate that the RACH preamble was detected. It consists of 15 consecutive access slots. The length of the access slot is 5,120 chips. Each access slot is further divided into 32 Acquisition Indicators and one part that has no transmission. The frame structure of the AICH is shown in Figure 6-13.

Figure 6-13. The frame structure of the AICH.

218


6.1.3.8 AP-AICH The usage and structure of the CPCH Access Preamble Acquisition Indicator Channel are very similar to the AICH. The acquisition indicators of the AP-AICH are called AP acquisition indicators (API).

Figure 6-14. The frame structure of the AP-AICH.

6.1.3.9 CD/CA-ICH The Collision Detection Channel Assignment Indicator Channel (CD/CA-ICH) is used in the CPCH Access Procedure as well as the APAICH. The frame structure of the CD/CA-ICH is similar to the AP-AICH. The acquisition indicators of the CD/CA-ICH are called CD Indicators (CDI). The frame structure of the CD/CA-ICH is shown in Figure 6-15.

Figure 6-15. The frame structure of the CD/CA-ICH.

6.1.3.10 PICH The Paging Indicator Channel (PICH) carries Paging Indictors (PI). The paging radio frame consists of 300 bits. 288 bits are used for the paging indicators and the remaining 12 bits are reserved for future use. The length of the paging radio frame is 10 ms. The frame structure of the PICH is illustrated in Figure 6-16.


219

Figure 6-16. The frame structure of the PICH.

In each paging radio frame, there can be 18, 36, 72, or 144 paging indicators. Paging indicators are used to indicate to the mobile stations that they should decode the paging channel. Otherwise, the mobile stations are not listening to the paging channel. This reduces the tasks of the mobile stations and saves the battery. 6.1.3.11 CSICH The CPCH Status Indicator Channel (CSICH) is associated with a CPCH AP-AICH. The duration of a CSICH radio frame is 20 ms. Each frame consists of 15 access slots (AS). The length of a slot is 40 bits. Each access slot has a part that is not transmitted and a status indicator part that includes 8 status indicator bits. The part that is not transmitted is used by the AICH, AP-AICH, or CD/CA-ICH. The frame structure of the CSICH is illustrated in Figure 6-17.

Figure 6-17. The frame structure of the CSICH.

6.1.4 Logical channels In UMTS, the logical channels are categorized into control channels and traffic channels. The control channels carry the control plane information, and the traffic channels carry the user plane information. The control and traffic channels are briefly described below.

220


– Broadcast Control Channel (BCCH) broadcasts system control information for all mobiles in a cell. The BCCH is used only in the downlink direction. – Paging Control Channel (PCCH) transfers paging information in the downlink direction. This channel is used when the network does not know the location of the mobile, or the mobile is in the cell-connected state. – Common Control Channel (CCCH) is used in both the uplink and the downlink directions. This channel is used to transfer control information between the network and mobiles, especially when there is no RRC connection between the network and the mobile. The CCCH is also used when the mobile is accessing a new cell after cell reselection. – Dedicated Control Channel (DCCH) is used when there is an RRC connection between the network and the mobile. In the DCCH, dedicated control information is transferred between the mobile and the network. This channel is bi-directional. – Dedicated Traffic Channel (DTCH) is used to transfer user data between the network and the mobile in both the uplink and downlink directions. – Common Traffic Channel (CTCH) is used to transfer data from one point to all mobiles or a specified group of mobiles. Logical channels are mapped to the transport channels, and the transport channels are mapped to the physical channels. This channel mapping is described in Section 6.1.6. 6.1.5 Transport channels In the 3GPP specification [3], the transport channels describe how data is transferred over the air interface while the logical channels describe what is transferred. There are both common transport channels and dedicated transport channels. In common transport channels, mobile stations use inband signaling when using the channel for data transfer. In the dedicated channels, the mobile stations are identified by the physical channels. This means that each mobile has its own frequency and code. Here is a brief description of the transport channels: – Random Access Channel (RACH) is used to send a small amount of data in the uplink direction. One of the main purposes of the RACH is for initial access. The channel is also used to send control and user data. – Common Packet Channel (CPCH) is an uplink channel used for packet data. CPCH is a shared channel, i.e., several mobile stations may use the same channel for data transfer.


221

– Forward Access Channel (FACH) is a downlink common channel used to send small amounts of control and user data. – Downlink Shared Channel (DSCH) is a common channel used to send dedicated control and user data. – Broadcast Channel (BCH) broadcasts system information in the downlink direction for all the mobile stations in a cell. – Paging Channel (PCH) is a downlink common channel used to send paging notification messages. – Dedicated Channel (DCH) is a channel that is used to send dedicated control and user data between the mobile station and the network in the uplink and downlink directions. Transport channels allow user data to be sent on common channels or on dedicated channels. Also, the mobile station can have one or several transport channels to be used in the downlink and at the same time one or more transport channels in the uplink. The mapping of the transport channels to the physical channels is described next. 6.1.6 Channel mapping In WCDMA, there are several options for transferring data over the air interface. The differentiation is achieved by using different transport and physical channels for each logical channel. This means that data can be sent through the logical layer via different channels in the transport layer. Also, one transport channel can be sent via different physical channels. The selection of the channels in the transport and physical layer depends on the data itself and the radio resource management and its parameters. Figure 6-18 illustrates the mapping of the logical channels to the transport channels in the uplink direction.


222

Logical Channels

DTCH

DCCH

CCCH

CPCH

DCH

RACH

Transport Channels Figure 6-18. Logical channel mapping to transport channels in the uplink.

The mapping of the logical channels to the transport channels in the downlink direction is illustrated in Figure 6-19. Logical Channels

BCCH

BCH

CCCH

FACH

DCCH

DTCH

DCH

CTCH

DSCH

PCCH

PCH

Transport Channels

Figure 6-19. Logical channel mapping to the transport channels in the downlink.


223

As can be seen in Figure 6-19, the FACH in the transport layer can be used by many logical channels. The usage depends on the configuration of the channels and the parameters used in the radio resource management. Also, the DCH and the DSCH do not have a one-to-one mapping with the logical channels, but they are carrying data of two logical channels.

6.2 Spreading and modulation In this chapter, the principles of spreading and modulation used for the uplink and the downlink are introduced. A more detailed description can be found in [6]. Spreading has two phases when applied to the physical channels. First, the channelization is achieved by multiplying the data by the channelization code. The selection of the channelization code depends on the data to be sent. After channelization, the data is spread in a way that the bandwidth of the signal equals 3.84 Mcps. Because the channelization code spreads the signal over a wider bandwidth, the channelization code is sometimes called the spreading code. After channelization, the scrambling of the data is done by multiplying the data by the scrambling code. 6.2.1 Spreading Data that should be sent through the physical layer is spread by a channelization code. The spreading changes the bandwidth of the data stream. Because the chip rate after the spreading is always 3.84 Mcps, the length of the channelization code must be changed to achieve the constant chip rate. By definition, the spreading factor (SF) shows how many chips are used to present one symbol, i.e., how much wider the bandwidth grows in spreading. In WCDMA, the spreading factor varies between 4 and 256 in the uplink, and 4 to 512 in the downlink. Figure 6-20 illustrates the principle of spreading and despreading. In Figure 6-20, data stream ‘1000’ is sent over the air interface. First, the spreading is done with a spreading code of ‘1010’. In this example, the spreading factor is 4 because four chips are used for each symbol. The TX data row shows the data stream that is modulated and sent over air interface. The RX data row shows the received data stream after demodulation. The despreading is done with the same spreading code (or channelization code) that was used in the spreading. The Data row at the bottom of Figure 6-20 shows the received data. Every four consecutive chips present one symbol. In the sixth chip sent over air interface, there was an error (indicated with a rectangle) because ‘0’ was sent over the air interface but it was received as ‘1’. However, the second symbol can be decoded


224

correctly because there are three ‘0’s and only one ‘1’, and thus the error in the air interface did not distort the data transmission.

Figure 6-20. Principle of spreading and despreading.

6.2.2 Channelization codes As described earlier, the channelization codes are used to spread a data stream over a wider bandwidth. At the same time, the codes separate the channels from each other. This is achieved because the codes used for the spreading are Orthogonal Variable Spreading Factor (OVSF) codes. The length of the codes can be changed in order to have different bit rates at the same time through the same radio channel. The channelization codes are normally presented as a tree of codes, as shown in Figure 6-21. When the system is allocating codes, each code reserves all the codes above and below the same branch. For example, if the code C8,3 is used (middle of Figure 6-21), the codes above it (C4,1, C2,0, and C1) and the codes below it (C16,6, C16,7, C32,12, C32,13, etc.) cannot be used. In WCDMA, the RNC allocates the channelization codes and tries to optimize the usage of the codes.


225 [11111111]

[1111]

C8,0

C4,0

[11110000]

[11]

C8,1

C2,0

[11001100] [1100]

C8,2

C4,1

[11000011]

C32,12 C16,6 C32,13

C8,3

[1]

C32,14

C1

[10101010] [1010]

C8,4

C4,2

[10100101]

[10]

C8,5

C2,1

[10011001] [1001]

C4,3

C16,7 C32,15

C8,6 [10010110]

C8,7 SF=1

SF=2

SF=4

SF=8

SF=16

SF=32

Figure 6-21. Channelization code tree.

6.2.3 Uplink spreading and modulation In the uplink, one DPCCH and up to six DPDCHs can be spread and transmitted simultaneously. Figure 6-22 illustrates the principle of the spreading of one DPCCH and one DPDCH. The DPCCH is always spread by using the same channelization code that is 256 bits in length. The DPDCH is spread with the channelization code selected by the system. The length of the code depends on the bit rate of the channel. The amplitudes of the DPDCH and DPCCH are weighted with the gain factors, and the real value bit streams are treated as a complex valued chip stream. Finally, the proper scrambling code is applied to the chip stream. In the uplink, the system selects a unique scrambling code for a


226

mobile station which separates the mobile station from other mobile stations. In UMTS, there are over 16 million scrambling codes available in the uplink. Channelization Gain factor code ȕd Data

DPDCH

Control

DPCCH

Channelization Gain factor code ȕc

j

Scrambling code

Figure 6-22. Spreading of one DPDCH and DPCCH.

6.2.4 Downlink spreading and modulation The principle of spreading in the downlink direction is illustrated in Figure 6-23. The method presented applies to all the downlink physical channels except the SCH. 0

I Scrambling code

1 0

Serial to paraller converter

1

I+jQ

Channelization code

Q j

Figure 6-23. Spreading in the downlink.


227

In the downlink, two consecutive symbols are converted from serial to parallel and then mapped to the I and Q branches. The first symbol (‘0’ in Figure 6-23) is mapped to the I branch, and the second symbol (‘1’) is mapped to the Q branch. The result is treated as a complex valued sequence of chips. Finally, the scrambling code is applied to the sequence. The channelization codes used in the downlink are the same codes described in Section 6.2.2. The scrambling codes in the uplink direction separate individual mobile stations from each other. In the downlink, the scrambling codes are used to separate cells from each other in the same frequency. There are altogether 8,192 scrambling codes used in UMTS. These 8,192 scrambling codes are divided into 512 groups of one primary scrambling code and 15 secondary scrambling codes. The primary scrambling code should be received over an entire cell. If the system is running out of channelization codes, i.e., the code tree is fully utilized, it is possible to allocate a secondary scrambling code and reuse the channelization codes. 512 primary scrambling codes are further divided into 64 primary scrambling code groups. The use of scrambling codes in a specific scrambling code group can be important, for example, close to the national frontier.

6.3 Procedure examples In this section, some of the procedures needed in RAN are described. These procedures are involved in idle mode, call establishment, and the connected mode. The states of the mobile station can be divided into idle mode and connected mode states. The connected mode state can be divided further into CELL_DCH, CELL_FACH, CELL_PCH, and URA_PCH states. The mobile station can change its state according to well-defined rules. The connection establishment, i.e., the change of state from idle mode to connected mode, is especially important from the service accessibility point of view. Some transitions are not possible between the different states in WCDMA. Also, the mobile station can alternate its state between WCDMA and GSM. The different states of the mobile station in WCDMA and the transitions are shown in Figure 6-24 [7-8]. In idle mode, the mobile station should perform the following tasks: – PLMN selection and reselection; – cell selection and reselection; and – location registration.


228

Connected Mode

URA PCH

CELL PCH GSM Connected Mode

CELL DCH Establish/Release RRC Connection

CELL FACH Establish/Release RRC Connection

Idle Mode

GSM Idle Mode

Figure 6-24. State machine.

The prerequisites for the listed tasks are, for example, that the mobile station is synchronized with the network and the synchronization is achieved by the synchronization procedures provided by the physical layer. If the mobile station has selected a cell belonging to an allowed PLMN, it can receive the system information of the PLMN and thus perform different kinds of tasks. For example, the mobile station can establish an RRC connection by using the control channels, receive paging messages from the network, and receive broadcast services [7, 9]. In connected mode, the mobile station does various tasks. Radio bearer establishment, data transmission, measurement reporting, cell update, URA update, radio link addition and remove, handovers and paging in connected mode (e.g., receiving a voice call during data transmission) are examples of tasks in connected mode [10]. The following chapters describe some of the main procedures performed by a mobile station in idle mode, connection establishment, and connected mode. Some tasks are also explained with the help of the measurement samples in Chapter 7. 6.3.1 Cell search The mobile station performs a cell search procedure to get the initial access to the network. The mobile station tries to find a cell and determine the cell’s downlink scrambling code. The scrambling code information is needed to be able to receive system information sent on the BCCH.


229

The cell search procedure can be divided into three phases. These phases are: 1. Slot synchronization. 2. Frame synchronization and scrambling code group identification. 3. Scrambling code identification. In the slot synchronization phase, the mobile uses the synchronization code of the primary SCH. (See Section 6.1.3.5.) The synchronization code is the same for all cells in the network. If the mobile station tries to decode the primary SCH by using the synchronization code and detects a peek in the RAKE receiver, the slot timing is obtained. In the frame synchronization phase, the mobile station uses the secondary SCH to find the frame synchronization and scrambling code group. The mobile station correlates all possible secondary synchronization code sequences with the received signal and tries to find the maximum value. Due to the structure of the allocation of the secondary synchronization codes to the slots, the mobile station can detect the frame synchronization and the scrambling code group. The mobile station finds the scrambling code used in a cell by correlating all the scrambling codes belonging to the scrambling code group identified in the previous step. After the scrambling code is determined, the mobile station can decode the CCPCH and start to receive the system information. If the mobile station has prior information about the scrambling code used in a cell, phases two and three can be simplified. For example, if a neighbor cell has the scrambling code belonging to the same scrambling code group as the scrambling code of the serving cell, the cell search procedure can be quicker. 6.3.2 Cell selection and reselection When the mobile station has found a suitable PLMN, it should try to find a suitable cell to camp on. It is possible that the mobile has no prior information about a cell to camp on. Then the mobile uses the Initial Cell Selection procedure. Otherwise, the mobile can use the information of the carrier frequencies, scrambling codes, etc. [11]. The criteria for cell selection for an FDD cell are given in Equation 6-1.

S qual = S qualmeas − Qqualmin S rxlev = S rxlevmeas − Qrxlevmin − PCompensation

(6-1)

230


The mobile station should select the first cell that fulfills both Squal > 0 and Srxlev > 0. According to the criteria, the selected cell should have good enough coverage (Srxlev) and also good enough quality (Squal). Quality in this context is defined by Ec/N0. In Equation 6-1, the following definitions are used. Table 6-1. Notations used in Equation 6-1. Parameter Explanation Squal Quality value for cell selection (dB). Srxlev RX level value for cell selection (dB). Squalmeas Measured Ec/N0, presents quality (dB). Srxlevmeas Measured RX level (dB). Minimum accepted signal quality. Squalmin Srxlevmin Minimum accepted signal level. PCompensation Power compensation, MS maximum power on RACH – MS maximum power, non-negative (dB).

Cell reselection takes place when the mobile station detects a potentially better cell. In this cell reselection process, the mobile station can evaluate candidates on the same frequency or on different frequencies, in same layer or in different layers, or candidates using different radio access technology. If a candidate cell belongs to a different layer, a hierarchical cell structure (HCS) is used. This means, for example, that the mobile station is camped on a macro cell and the candidate cell is a micro cell. Before the mobile station can make any decisions, it must have measurement results from the cell reselection candidates. The measurements are started if the following rules are fulfilled (assuming HCS is not used). Table 6-2. Measurement rules. Criterion Explanation Intra-frequency measurement Squal > Sintrasearch No need to start intra-frequency measurements. Squal Sintrasearch Perform intra-frequency measurements. Sintrasearch not sent Perform intra-frequency measurements. Inter-frequency measurement Squal > Sintersearch No need to start inter-frequency measurements. Squal Sintersearch Perform inter-frequency measurements. Perform inter-frequency measurements. Sintersearch not sent Measurement on different RAT Squal > SsearchRAT-m No need to start to start measuring cells of RAT m. Squal SsearchRAT-m Measure cells of RAT m. SsearchRAT not sent Measure cells of RAT m.


231

If HCS is used, a new threshold is introduced. SsearchHCS defines the minimum signal level when HCS is used. If the signal level of the serving cell is below the threshold, all intra-frequency and inter-frequency cells should be measured. If the signal level is above the threshold, HCS priorities are used in the cell selection procedure. When HCS is used, the behavior of the mobile station depends whether or not the mobile station is a fast moving mobile. The mobile station is considered a fast moving mobile if in the time period of TCRmax the number of cell selections exceeds NCR. When the number of cell selections decreases to under NCR, the mobile station is still considered a fast moving mobile for the time period of TCRmaxHyst; and after TCRmaxHyst has elapsed, the mobile station is treated as a normal mobile station. If the signal level exceeds the SsearchHCS and the mobile station is not considered a fast moving mobile, the cell selection is performed as follows. – If Squal > Sintersearch, all the intra-frequency and inter-frequency cells having that have higher HCS priority than the serving cell are measured. – If Squal Sintersearch, all the intra-frequency and inter-frequency cells that have equal or higher HCS priority than the serving cell are measured. If the signal level exceeds SsearchHCS and the mobile station is a fast moving mobile, the mobile station should measure intra-frequency and interfrequency cells that have lower or equal HCS priority than the serving cell. Also, the mobile station should prioritize the cell reselection to the cells having lower HCS priority than the serving cell. The behavior of the mobile station in the case of HCS and high mobility is needed, for example, if the mobile station is moving fast through an area that has a lot of micro cells. It may be beneficial if the slow-moving mobiles are kept in the micro cell layer, and the fast moving mobiles are directed into the macro cell layer. If the serving cell belongs to the hierarchical cell structure, a new threshold for the inter-system measurements is also specified. SHCS,RATm shows the minimum signal level of the serving cell when HCS measurement rules are applied. If the signal level of the serving cell is below SHCS,RATm, the mobile should measure all inter-RATm cells. If the signal level is above the threshold and the mobile station is not a fast moving mobile, the following rules are applied. – If Squal > Slimit,SearchRATm, the mobile station does not have to measure the cells belonging to RATm. – If Squal Slimit,SearchRATm, the mobile station should measure all neighbor cells belonging to RATm that have equal or higher HCS priority than the serving cell. If the mobile station is a fast-moving mobile, it should measure the neighbor cells in RATm that have equal or lower HCS priority than the


232

serving cell and prioritize the neighbor cells by the lower HCS priority in the cell reselection. Based on the neighbor cell measurements, the mobile station ranks the measured cells. There are several steps in ranking. If HCS is used, the quality level threshold criterion H is calculated with the formulas in Equation 6-2.

H s = Qmeas _ LEV , s − Qhcs s H n = Qmeas _ LEV ,n − Qhcs n − TOn × Ln

(6-2)

If HCS is not used the H criterion can not be used. The cell-ranking criterion R is defined in Equation 6-3.

Rs = Qmap ,s + Qhyst s Rn = Qmap,n − Qoffset s,n − TOn × ( 1 − Ln )

(6-3)

In Equation 6-2 and Equation 6-3 TOn and Ln are defined in Equation 6-4.

TOn = TempOffset n × W ( PenaltyTimen − Tn ) W ( x) = 0 if x < 0 W ( x) = 1 if x ≥ 0

(6-4)

Ln = 0, if HCS _ PRIOn = HCS _ PRIOs Ln = 1, if HCS _ PRIOn ≠ HCS _ PRIOs A temporary offset (TempOffset) in H and R criteria is applied for the period of PenaltyTime. However, the offset is applied only if the usage of HCS is indicated in the system information. The timer Tn is started: – when the HCS priority levels of the serving cell and the neighbor cell are different and Qmeas_LEV,n > Qhcsn; – for FDD cells, when the HCS priority levels of the serving cell and the neighbor cell are equal and Qmap,n > Qmap,s + Qoffset1s,n (quality measure is CPICH RSCP); – for FDD cells, when the HCS priority levels of the serving cell and the neighbor cell are equal and Qmeas_LEV,n > Qmeas_LEV,s + Qoffset2s,n (quality measure is CPICH Ec/N0); and


233

– for all other cells, when the HCS priority levels of the serving cell and the neighbor cell are equal and Qmap,n > Qmap,s + Qoffset1s,n. The definitions used above are given in Table 6-3. Table 6-3. Definitions of the parameters. Parameter Definition Qmap,n Quality of the neighbor cell after the mapping function is applied. The parameters of the mapping function are sent in the system information. For FDD cells, CPICH RSCP or Ec/N0 is used. For GSM RXLev is used. Qmap,s Quality of the serving cell after the mapping function is applied. For FDD cells, CPICH RSCP or Ec/N0 is used. For GSM, RXLev is used. Qmeas_LEV The quality value of the received signal expressed in CPICH_Ec/N0 or CPICH RSCP level. RXLev is used for GSM. Quality offset between the serving cell (s) and neighbor (n) cell, quality Qoffset1s,n measure is CPICH RSCP. Qoffset2s,n Quality offset between the serving cell (s) and neighbor (n) cell, quality measure is Ec/N0.

The mobile station ranks only the cells fulfilling the S criterion among the cells that fulfill criterion H = 0 and have the highest priority in HCS. If there are no cells fulfilling criterion H = 0, the ranking should be done among all cells without considering HCS priority levels. The ranking is done according to R criterion and the mobile station reselects the best cell if the reselection criteria are fulfilled during the time interval indicated by the parameter Treselection [9]. 6.3.3 Paging The paging procedure is used to send paging information to mobile stations in idle mode, CELL_PCH and URA_PCH states. The core network may request paging to establish the signaling connection between the mobile station and the network. UTRAN can use paging to trigger a cell update procedure for the mobile stations in the CELL_PCH and URA_PCH states. Moreover, UTRAN can page mobile stations in idle mode, CELL_PCH or URA_PCH states and trigger them to read the updated system information. UTRAN initiates paging by sending a PAGING TYPE 1 message to the mobile station(s). This message is normally repeated several times to improve the probability of receiving the message correctly. When the mobile station receives a page, it will indicate the reception of the page; and if the mobile station is in connected mode, it will perform a cell update procedure. If the information element, BCCH modification info, is included in the page, the mobile stations in idle mode or in CELL_PCH or URA_PCH states should read the new system information. The paging procedure is shown in Figure 6-25.


234

Mobile

Network

Paging Type 1

Figure 6-25. Paging message.

6.3.4 RRC connection establishment The RRC states were illustrated in Figure 6-24. When there is a need to create a connection between the mobile station and the network, a radio resource control connection (RRC connection) should be established. RRC connection is created, for example, if there is a mobile originating or terminating a voice or data call, signaling or emergency call. The RRC connection is always requested by the mobile station by sending an RRC CONNECTION REQUEST message on the uplink CCCH, as illustrated in Figure 6-26. Mobile

RNC RRC Connection Request (CCCH)

RRC Connection Setup (CCCH)

RRC Connection Setup Complete (CCCH)

Figure 6-26. RRC connection establishment.


235

The connection request message includes information about the reason for the request, the identifier of the mobile station, the protocol used, and measurement reports. The serving RNC replies to the request by sending a RRC CONNECTION SETUP message on the downlink CCCH. If the RNC has to reject the request, it may direct the mobile station to another carrier or another system. The RRC CONNECTION SETUP message informs the mobile station about transport channels in both uplink and downlink directions, radio resource parameters, and optionally frequencies to be used. The mobile station replies to the message with an RRC CONNECTION SETUP COMPLETE message and enters the CELL_DCH state. The RRC connection is always released by the RNC. 6.3.5 Location update The location update procedure is used for several purposes in WCDMA by changing the content of the information element in the location update message. In the location updating procedure, first the RRC connection is established as described in Section 6.3.4. After the RRC connection is established, the mobile station sends the request to update the location that may be used, for example, to attach IMSI, attach GPRS, or update the location. After the request, the identification process, authentication process, and security mode control processes are carried out. Finally, the RNC sends the acceptance to the mobile station. The flow of the location update procedure is shown in Figure 6-27. Mobile

BTS

RNC

RRC Connection Request Radio Link Setup Radio Link Response RRC Connection Setup L1 Synchonization

Sync Indication

RRC Connection Setup Completed Location Update Request Identification Process Authentication Process Security Mode Control Process Location Update Accept TMSI Reallocation Complete Release RRC Connection

Figure 6-27. Location update procedure.

236


6.3.6 Radio bearer establishment When the user data is sent between the mobile station and core network (either the PS or CS core network), there should be a Radio Access Bearer between the RNC and the core network, and a Radio Bearer between the mobile station and the RNC. For the circuit switched data, the DIRECT TRANSFER message carries a CALL CONTROL SETUP message, which specifies the connection type. The RNC forwards this message to the CS core network. For the packet switched traffic, the DIRECT TRANSFER message carries a SESSION MANAGEMENT ACTIVATE PDP CONTEXT REQUEST. This message also specifies the connection parameters but the actual connection parameters are different. The RNC forwards this message from the mobile station to the SGSN. Next, the RNC sends the RADIO BEARER SETUP command to the mobile station, and the radio bearer is established. The mobile station replies to the message by sending a RADIO BEARER SETUP COMPLETE message. After this message, it is possible to start the call or to transfer data. The radio bearer establishment process is shown in Figure 6-28.

Figure 6-28. Radio bearer establishment.


237

6.4 Radio resource management in WCDMA Radio resource management (RRM) controls the utilization of the air interface resources. RRM can be divided into power control (PC), handover control (HC), admission control (AC), load control (LC), and dynamic resource allocation. The aim of power control is to minimize the interference in the WCDMA radio network. This is achieved by using as low power levels as possible to maintain the requested performance. Handover control enables the mobility in WCDMA networks. HC is also used to optimize the performance of the network by enabling the use of different layers in the network, different frequencies, and different radio access techniques. Admission controls ensure that new radio bearers will not cause the existing connections to deteriorate. Load control ensures that the system does not become unstable due to changes in the air interface. Dynamic resource allocation ensures that the physical and logical resources, such as scrambling and spreading codes, are used effectively. 6.4.1 Power control In WCDMA, many users share the same frequency at the same time. This means that users will interfere with each other and thus deteriorate the performance of the system. To achieve the best possible performance in the air interface, both the mobile stations and base stations should use as low power levels as possible. This means that power should be tuned based on the services, radio channel characteristics and fading conditions. The power control is divided into open-loop, inner-loop and outer-loop power control. 6.4.1.1 Open-loop power control The open-loop power control function is located both in the mobile station and in the network. In the uplink direction, it handles the initial power for the first preamble of PRACH and for the uplink DPCCH. In the downlink, the open-loop power control sets the power for the downlink channels. The power levels are based on the mobile station measurements. Actual algorithms are vendor-specific. In the uplink direction, the power of the first preamble of PRACH is calculated as shown in Equation 6-5 [8]. Preamble_Initial_Power = Primary CPICH TX power – CPICH_RSCP + UL interference + Constant Value

(6-5)

238


In Equation 6-5, Constant Value is the required direction. The UL interference and required C/I are sent to the mobile station on BCH. The initial power for calculated in a similar manner as the power of the indicated in Equation 6-6 [8].

C/I in the uplink by the base station uplink DPCCH is first preamble, as

DPCCH_Initial_power = DPCCH_Power_offset – CPICH_RSCP

(6-6)

DPCCH_Power_offset is transmitted to the mobile station in The uplink DPCH Power Control Info by the system. CPICH_RSCP is measured by the mobile station. 6.4.1.2 Downlink common channel power levels The power levels of the common channels are usually considered as constant, even if the specifications do not prevent changing the power dynamically. This way, the power levels of the common channels are not part of the power control. However, the power levels should be set in a way that mobile stations can receive the channels well enough while power levels are as low as possible. In Table 6-4, power levels for the downlink common channels are suggested. Table 6-4. The power levels of the downlink common channels [1]. Downlink common channels Power (dBm) P-CPICH 33 P-SCH 30 S-SCH 30 P-CCPCH 28 S-CCPCH (assumed SF=256) 28 PICH (assumed 72 paging indicator per frame) 25 AICH 25

6.4.1.3 Inner-loop power control The inner-loop power control adjusts the output power of the mobile station and base station 1500 times per second. Due to the high frequency of the power control the effect of fast fading in the radio channel can be compensated for almost all mobile stations. The inner-loop power control is based on feedback information from the receiving part. Both the base station and the mobile station have a target signal-to-interference ratio (SIR). If the received SIR is less than the SIR target, the receiving end asks the transmitting end to use higher output power. If the received SIR is higher


239

than the target SIR, the receiving end asks the transmitting end to reduce the power. In the uplink, when the base station measures the SIR and compares it to the SIR target, it will generate a power control command based on either algorithm 1 or algorithm 2. In algorithm 1, power control commands are generated for each slot. The power step can be 1 dB or 2 dB. In algorithm 2, the power step is always 1 dB, and the power is increased only if five consecutive power commands are ‘UP’. Power is reduced only if five consecutive power commands are ‘DOWN’. Otherwise, the power is left unchanged. In a soft handover, the mobile station receives power commands from several cells. If algorithm 1 is used, the mobile station combines the power commands based on soft symbol decisions and adjusts the output power according to the combined power command. If algorithm 2 is used for each radio link set, a temporary power command is calculated. The temporary power command gets the value ‘UP’ if five consecutive TPC commands are ‘UP’. The temporary power command gets the value ‘DOWN’ if five consecutive TPC commands are ‘DOWN’. Otherwise the temporary power command gets the value zero. The actual power command is calculated as an average of the temporary power commands. If the DOWN command is represented by ‘-1’, and the UP command by ‘+1’, the combined power command get the value UP if the average of the temporary power commands is greater than +0.5, and it gets the value DOWN if the average is smaller than -0.5. Otherwise the power is not changed. If algorithm 2 is used, note that: – the power can be changed only with the frequency of 300 Hz because for each decision, TPC commands from five consecutive slots are needed; and – the power step is always 1 dB. In the downlink, there are two modes for inner-loop power control. In mode 0, the power of the downlink DPCH is changed in each slot. In mode 1, the mobile station repeats the same power control command three times, and the system updates the output power in every three slots. The mobile station estimates the receiver SIR based on the pilot bits. If the estimated SIR is better than the target SIR, the mobile station sends a DOWN command. Otherwise, the mobile station sends an UP command. 6.4.1.4 Outer-loop power control The RNC monitors the received quality at the mobile station and the base station. If there is a difference in the received quality and the target quality, the RNC changes the SIR target for the mobile station or for the base station. Even if the inner-loop power control can keep SIR in the SIR target, it is possible that the quality of the connections is too good or too bad. This can

240


happen because the achieved quality (i.e., BER or BLER) varies depending on the mobile speed and/or a multi path propagation environment. If the quality of any connection is too good, the SIR target for that specific connection is reduced by the RNC. If the quality is not fulfilling the requested targets, the SIR target is increased. The actual algorithms used to adjust the SIR targets for the uplink and downlink are vendor-specific [7]. 6.4.2 Handover control The most common handovers in WCDMA are softer handover, soft handover, inter-frequency handover, and inter-system handover. Handovers are needed when the mobile station moves from the coverage area of an old cell to the coverage area of a new cell. Moreover, handovers are needed to support multilayer networks as well as multi mode operations, e.g., using WCDMA and GSM networks together [11-12]. 6.4.2.1 Soft and softer handover Soft handover may be the most important handover in WCDMA because it is almost always needed when the neighbor cells are using the same frequency. When the mobile station is in connected mode, it measures the serving cell as well as all neighbor cells indicated by the RNC. The RNC sends a list of cells that the mobile station should measure. Moreover, the RNC sends a set of thresholds that are used when the mobile station measures the neighbor cells and decides whether the measurements should be reported to the RNC. When the RNC receives the measurement results, it decides whether the active set, i.e., the set of cells that are serving the mobile station, should be updated. This update can include: – adding a new cell to the active set, – removing a cell from the active set, or – removing a cell and adding a new cell to the active set. With a soft handover, the following benefits can be gained: – reduced interference because the mobile station is always connected to the best server, – reliable handover without any interruptions for both RT and NRT services, and – improved radio performance due to macro diversity. Softer handover is similar to soft handover. In a softer handover, the mobile station is connected to the cells that are in the same base station, while in a soft handover, the cells belong to different base stations. In a softer handover, the combination of received uplink signals is done in the


241

base station, while in a soft handover, it is done in the RNC. In the downlink, the combination is done in the mobile station in both cases. 6.4.2.2 Intra-frequency handover In some cases, it may be possible that the mobile station cannot make a soft handover between cells using the same frequency. This can happen, for example, if the source cell and the target cells are controlled by different RNCs and the inter-RNC handover is not possible, for example, because the IUr is not implemented. A typical example of an intra-frequency handover is a handover in a multi-vendor network when the handover is performed across the border of two vendors. 6.4.2.3 Inter-frequency handover The inter-frequency handover is a hard handover, i.e., transmission to the old cell is stopped before transmission begins to the new cell. For RT services, this means a short interruption to the transmission, but in NRT services, the handover is lossless. Inter-frequency handover is needed if the network is using two or more frequencies. If the mobile station is in a location where the inter-frequency handover is possible, the RNC can request the mobile station to start inter-frequency measurements and report the results periodically to the RNC. The decision to make an inter-frequency handover is done by the RNC. 6.4.2.4 Inter-system handover Inter-system handover in this content covers the handovers between FDD and GSM. In the WCDMA specifications, inter-system handover has had an important role because a large number of existing GSM operators are considering implementing a WCDMA network. In order to operate these two radio network layers as one network, fast and effective handovers between the layers are needed. The mobile station has to be able to measure neighbor cells of the other system before making the handover. In GSM, the mobile station uses the idle time slots to measure the WCDMA neighbors. In WCDMA, the mobile station uses compressed mode to be able to measure the GSM neighbors. In compressed mode, the RAN creates gaps in the transmission and reception of a mobile station. These gaps can be created by splitting the spreading factor, reducing the bit rate used by the higher layers, or reducing the bit rate in the physical layer. Also, it is possible that the mobile station has two receiver chains, in which case compressed mode is not needed. Typically, an inter-system handover is used to balance traffic in the GSM and WCDMA networks. It is also used to enable new services in the areas

242


where the GSM and WCDMA layers overlap. The inter-system handover can be divided into two different cases: – inter-system handover between GSM and WCDMA for CS connections, or – cell reselection between GSM and WCDMA for PS connection. The reason for an inter-system handover can be coverage (e.g., WCDMA coverage ends), capacity (e.g., capacity in GSM is running out, but in the WCDMA layer, there is unused capacity available), or services (e.g., high bit rates are supported only in the WCDMA layer). The steps in an inter-system handover can be described as: – parameter setting: the proper inter-system parameters are set in the BSS and in the WCDMA; – measurement triggering: a measurement event is triggered (see Section 6.5.2.3 for the events); – inter-system measurements and analysis; and – a handover command is sent by the RNC or BSC. The availability and support of the inter-system handover is vendorspecific. It should be noted that the support must be in both layers, i.e., in BSS and in WCDMA. 6.4.3 Admission control and load control Admission and load control optimize the quality and capacity of the WCDMA network. Admission control and load control work in parallel. The main difference is that the admission control decides whether new connections can be established while the load control tries to maintain all the existing connections at the requested quality level. The admission control handles the new connections in the RNC level. It ensures that the new connections will not deteriorate the quality of the existing connections. However, the quality classes can be taken into account when evaluating the impact of the new connection to the existing connections. The following assumptions are usually valid: – CS connections have higher priority than PS connections (note RT PS connections); – RT connections have higher priority than NRT connections; – new NRT connections (e.g., background class) can be added even if the load in the network is high, by taking into account only the impact of the signaling; – the bit rates of NRT connections can be reduced to allow new RT connections.


243

The load in the WCDMA network varies depending on the location, time and services provided. The load control measures the load in the uplink and the downlink and tries to maximize the throughput of the network without jeopardizing the quality or coverage of the network. This means that the network tries to provide as many resources as are defined by the operator. However, it may happen that the network cannot provide the requested amount of resources for all connections. Then the load control decides how the situation is handled. Usually the throughput of the low priority connections, such as background traffic, is reduced or even stopped to ensure that the high priority connections have gotten the resources needed to maintain the required quality. When the low priority traffic is stopped, it is possible to reduce the throughput of the high priority traffic, and in an extreme case. Some of the connections may need to be dropped to ensure that the network will not become unstable. When the load decreases in the network, the load control starts to allocate the free resources once again for the low priority connections. The algorithms of the admission control and the load control are proprietary for the vendors, so there can be different approaches for dealing with an overload situation. 6.4.4 Dynamic resource allocation In WCDMA, physical and logical resources should be allocated effectively. Particularly the channelization codes should be used in a such way that the code tree is optimally utilized (see Section 6.2.2). When the amount of active connections changes in the network, there can be a need for optimizing the code tree. The dynamic resource allocation function in RNC takes care of the allocation of channelization codes and various other tasks. However, the implementation of dynamic resource allocation is vendorspecific.

6.5 3GPP parameters for WCDMA radio access In this chapter, some of the parameters related to the WCDMA radio access are described. The tuning of the parameters of the radio network is one of the key tasks in optimizing the WCDMA radio network. Thus, it is important for the persons involved in optimization to know at least the basics of the parameters. The 3GPP specification leaves many issues open from the network side, and the actual implementation is vendor-specific. This means that the parameters used in the RAN can also be partly vendor-specific. This chapter focuses on the parameters defined by 3GPP.


244

6.5.1 Identities In WCDMA, different entities must be identified. In this chapter, some of the most relevant identities for the radio network planning are covered. There are identifiers that are used only within UTRAN, between UTRAN and the core networks, and between UTRAN and GSM [13-14]. 6.5.1.1 IMSI The International Mobile Subscriber Identity (IMSI) is a unique identifier for a mobile subscriber in the GSM/UMTS system.

Figure 6-29. The structure of IMSI.

The IMSI consists of the Mobile Country Code (MCC), the Mobile Network Code (MNC), and the Mobile Subscriber Identification Number (MSIN). The MCC identifies the mobile subscriber’s country of domicile. The MNC identifies the PLMN of the subscriber, and the MSIN identifies the subscriber within the PLMN. Also, the National Mobile Subscriber Identity (NMSI) is defined as a combination of the MNC and MSIN [13]. IMSI = MCC + MNC + MSIN 6.5.1.2 LAI The Location Area Identity (LAI) consists of the MCC, the MNC, and the Location Area Code (LAC). The LAC can have any value between 1 and 65535, except 0 (0000) and 65534 (FFFE) [13]. LAI = MCC + MNC + LAC 6.5.1.3 RAI The Routing Area Identity (RAI) consists of the LAI and the Routing Area Code (RAC). The RAC can have any value between 1 and 255. As can be seen, the routing area’s maximum size is the location area. However, within one location area there can be several routing areas [13].


245

RAI = LAI + RAC = MCC + MNC + LAC + RAC 6.5.1.4 CGI and CI The Cell Global Identification (CGI) consists of the MCC, the MNC, and the Cell Identity (CI). The CI can have values from 1 to 65535 [13]. CGI = MCC + MNC + CI 6.5.1.5 BSIC The Base Station Identity Code (BSIC) consists of the Network Color Code (NCC) and the Base station Color Code (BCC). Both codes have fixed length of 3 bits, i.e., the BSIC can have values from 1 to 63. However, the usage of the NCC may be limited due to regulatory issues. The BSIC is used in GSM cells [13]. BSIC = NCC + BCC 6.5.1.6 PLMN Identifier The PLMN Identifier (PLMN-Id) consists of the MCC and the MNC. The PLMN-Id specifies the PLMN globally [13]. PLMN-Id = MCC + MNC 6.5.1.7 CN Domain Identifier The Core Network Domain Identifier specifies the CN Domain Edge Node. There are identifiers for both circuit switched and packet switched core networks. The CS core network’s CN domain identifier is called CN CS Domain-Id and consists of PLMN-Id and LAC. CN PS Domain-ID is defined for the PS core network and consists of PLMN-Id, LAC, and RAC [13]. CN CS Domain-Id = PLMN-Id + LAC CN PS Domain-Id = PLMN-Id + LAC + RAC 6.5.1.8 Global RNC-Id Each RNC is identified within UTRAN with a unique RNC identifier (RNC-Id). The global RNC identifier consists of the PLMN-Id and RNC-Id. The identifier of the serving RNC (S-RNC), controlling RNC (C-RNC), and drift RNC (D-RNC) are built in the same way [13]. Global RNC-Id = PLMN-Id + RNC-Id

246


6.5.1.9 SAI The Service Area Identity (SAI) is used, for example, in location-based services. The SAI consists of PLMN-Id, LAC, and the service area code (SAC) [13]. SAI = PLMN-Id + LAC + SAC 6.5.1.10 Cell Identifier and Local Cell Identifier In Section 6.5.1.4, the CI was defined from the core network point of view. Within UTRAN, there is a Cell Identifier (C-Id) and a Local Cell Identifier. The C-Id is unique for a cell within the radio network subsystem (RNS). The C-Id combined with the identifier of the controlling RNC results in the UTRAN Cell Identity (UC-Id). The C-Id and UC-Id are used to identify the cells in the Iub and Iur interfaces. In addition to C-Id in WCDMA, a Local Cell Identifier is also defined. The Local Cell Identifier is used to identify a set of resources in a NodeB. It is used before the C-Id is defined. The Local Cell Identifier must be unique within a NodeB but it can be used, e.g., for O&M purposes if it is unique within the UTRAN. UC-Id = C-RNC-Id + C-Id 6.5.1.11 RNTI The Radio Network Temporary Identities (RNTI) are used to identify the mobile station within UTRAN. Moreover, the RNTIs are used identify the mobile station when sending signaling messages between the mobile stations and UTRAN. There are a set of different RNTIs. – s-RNTI (serving RNC RNTI) is unique within the serving RNC. It is used to identify the mobile station under the serving RNC, the drift RNC, and the mobile station. – d-RNTI (drift RNC RNTI) is allocated by the drift RNC and used by the serving RNC to identify the mobile station for the drift RNC. – c-RNTI (cell RNTI) is used by the mobile station and the controlling RNC to identify the mobile station. – u-RNTI is allocated for the mobile station. It consists of SRNC-Id and s-RNTI. 6.5.1.12 URA Identity The URA Identity specifies a URA. The URA Identity can be used to specify the URA to be used for the mobile station and the serving RNC in cases where there is more than one URA defined for a cell.


247

6.5.2 Measurement reporting parameters In WCDMA, there are several triggers for the mobile station to start various kinds of measurements. The 3GPP specifications do not give actual parameters for the thresholds, but it is very likely that equipment vendors will implement the thresholds as parameters in UTRAN. 6.5.2.1 Intra-frequency measurements events Six different events may trigger intra-frequency measurements. However, the measurements can be done periodically if the network has not been able to add a cell into the active set. Based on measurement results, the network can request the mobile station to perform a handover. The events that may trigger the measurements are: – Event 1a: a primary CPICH enters the reporting range; – Event 1b: a primary CPICH leaves the reporting range; – Event 1c: a non-active primary CPICH becomes better than an active primary CPICH; – Event 1d: change of the best cell; – Event 1e: a primary CPICH becomes better than an absolute threshold; and – Event 1f: a primary CPICH becomes worse than an absolute threshold. The cells that are involved in a soft handover with the mobile station comprise the ‘active set’. The size of the active set can be three, for example, which means that the mobile station can communicate with a maximum of three cells simultaneously. A cell can be added if the primary CPICH is within the reporting range, which means that Ec/I0 (or RSCP) of the new cell must be within the reporting range. In Figure 6-30, the reporting range is at all times 3 dB below the Ec/I0 of the best server. First, when Cell 2 enters the reporting range, Event 1a is triggered. When Cell 2 becomes better than Cell 1, Event 1d is triggered. Because Cell 1 drops below the reporting range, Event 1b is triggered. And finally, when Cell 1 goes below the absolute threshold, Event 1f is triggered. An example of the parameters for intra-frequency measurements are presented in Table 6-5. The intra-frequency measurement reporting can be modified by introducing hysteresis, cell-specific offsets, time-to-trigger, and forbidden primary CPICHs. Hysteresis can be used, for example, to avoid unnecessary events, as shown in Figure 6-31. Cell 1 is always better than Cell 2 in Figure 6-31. Cell 2 enters the reporting range but will not trigger Event 1a until the quality of Cell 2 exceeds the reporting range by ‘Hysteresis Event 1a’. Moreover, Event 1b is not triggered even if the quality of Cell 2 gets worse than the reporting range because ‘Hysteresis Event 1b’ is applied.


248

Reporting range P-CPICH #1 P-CPICH #2 Absolute threshold

Ec/N0

Reporting range

Event 1a Event 1d Event 1b Event 1f

Figure 6-30. Intra-frequency measurement events.

Table 6-5. An example of the parameters for the intra-frequency measurements. Parameters Explanation Active set size The maximum number of cells participating the soft handover.

For each event Reporting range

Absolute threshold Hysteresis Time to trigger W Offset

The measurement of a new cell must be within the reporting range before the cell can be added to the active cell; relative to the best server. Guarantees the minimum quality for a cell . A hysteresis for the reporting range of a cell. The cell has to fulfill the criteria for the time period of “Time to trigger” before the event is triggered. A weighting factor to weight the importance of the best server and all cells in the active set. Cell specific offset.

The parameter Time-to-trigger is also used to avoid unnecessary reporting events. When Time-to-trigger is applied, the criterion for the event must be fulfilled during the time period indicated by Time-to-trigger. In Figure 6-32, Cell 1 is always better than Cell 2. When Cell 2 reaches the reporting range, Event 1a is not triggered until Cell 2 has been better than the reporting range during the time period indicated by ‘time-totrigger’. In Figure 6-32, Event 1b is not triggered until the ‘time-to-trigger’ for Event 1b has elapsed, even if Cell 2 is clearly below the reporting range. In some cases, it may be beneficial to apply cell-specific offsets for certain cells. These offsets can be applied separately for each cell, which


249

means, for example, that the behavior of the mobile station can be optimized based on knowledge of the environment.

Reporting range Cell #1

Ec/N0

Cell #2

Event 1a Hysteresis event 1b

Hysteresis event 1a

Figure 6-31. The usage of the hysteresis.

Reporting range Cell #1 Cell #2

Ec/N0

Event 1a

Event 1b

Time-to-trigger event 1a

Time-to-trigger event 1b

Time

Figure 6-32. The usage of Time-to-trigger mechanism.

In Figure 6-33, cell-specific offsets are applied for Cell 2. For Event 1a, a positive offset means that the event is triggered earlier, before the cell reaches the reporting range. This can be useful if an operator knows that Cell 2 can rapidly become stronger, as it does in Figure 6-33. The cell-specific offset is also applied to Event 1b in Figure 6-33. The positive offset causes


250

Event 1b to be triggered later than it would without the offset, as shown in Figure 6-33. Reporting range Cell #1 Cell #2

Ec/N0

Cell #2 + offset

Event 1b

Offset for cell #2

Event 1a

Time Figure 6-33. The usage of the cell-specific offset.

It is possible to forbid a cell in the active set to affect the reporting range. This can be useful when an operator knows that the quality of the forbidden cell is unstable and could cause unexpected behavior in the network. In Figure 6-34, Cell 2 is added to the active set, but it does not affect the reporting range; the reporting range is defined by Cell 1. This way, other potential and stable cells can be monitored more easily.

Ec /N0

Reporting range Cell #1 Cell #2

Time

Figure 6-34. The usage of the forbidden cell mechanism.


251

If the RNC is not able to add a cell to the active set when the cell has entered the reporting range, the mobile station will continue to send the measurement reports to the RNC, based on the parameters set by the operator. There are parameters for the reporting interval and maximum number of reports sent. Normally the mobile station will periodically send the intra-frequency measurements until the number of measurement reports indicated by ‘Amount of reporting’ are sent to the RNC. 6.5.2.2 Inter-frequency measurement events There are six different events defined for the inter-frequency measurements. These events are: – Event 2a: change of the best frequency; – Event 2b: estimated quality of the currently used frequency is below a threshold, and the estimated quality of a non-used frequency is above a threshold; – Event 2c: the estimated quality of a non-used frequency is above a threshold; – Event 2d: the estimated quality of the currently-used frequency is below a threshold; – Event 2e: the estimated quality of a non-used frequency is below a threshold; and – Event 2f: the estimated quality of the currently-used frequency is above a threshold. These events are illustrated in Figure 6-35. Cell 1 is using Frequency 1 and Cell 2 is using Frequency 1 in Figure 6-35. It is assumed that Cell 1 is the serving cell. There are thresholds for both cells that are used for the measurement events. When Cell 1 exceeds the threshold defined for the cell, Event 2f is triggered. When Cell 1 falls below the threshold, Event 2d is triggered. In Figure 6-35, Event 2b is also triggered because Cell 2 is above its own threshold. For Cell 2, similar events are Event 2c and Event 2e (Event 2e is not shown in Figure 6-35). Event 2a is triggered when Cell 2 becomes better than Cell 1. For Events 2a – 2f (as with Events 1a – 1f), there are parameters (Table 6-6) that are used with the inter-frequency measurements. In the Figure 6-35, the hysteresis parameters and Time-to-trigger parameters are not applied. Table 6-6. Parameters for the inter-frequency measurements. Parameter Explanation Threshold #1 The threshold for the serving cell. Threshold #2 The threshold for the neighbor cell on the other frequency. Hysteresis A hysteresis for an event. Time to trigger The cell must fulfill the criteria for the time period of “Time to trigger” before the event is triggered.


252

Cell #1 on freq 1 Cell #2 on freq 2 Event 2b+2d Event 2f

Threshold for cell #1 Threshold for cell #2

Ec/N0

Event 2a

Event 2c

Figure 6-35. Events for inter-frequency measurements.

6.5.2.3 Inter-system measurements events In the inter-system measurements, both the WCDMA cells and GSM cells should be measured. In WCDMA, the measurement quantity can be either the downlink Ec/N0 or the downlink RSCP. In GSM, the RSSI of the carrier is measured. For the inter-system measurements, there are four measurement events defined. These events are: – Event 3a: the estimated quality of the currently used UTRAN frequency is below a certain threshold, and the estimated quality of the GSM is above a certain threshold; – Event 3b: the estimated quality of GSM is below a certain threshold; – Event 3c: the estimated quality of GSM is above a certain threshold; and – Event 3d: change of the best cell in GSM. The hysteresis and Time-to-trigger parameters can be used for Events 3a3d. Using these parameters, it is possible to avoid unnecessary inter-system measurement events and to ensure that the measurements are done when they are really needed. 6.5.2.4 Traffic volume measurements Traffic volume measurements are used to help the control of the radio bearers. The mobile station can report the buffer occupancy, the average buffer occupancy, and the variance of the buffer occupancy. The buffer occupancy indicates the amount of data for each logical channel to be read for transmission or retransmission. The average buffer occupancy and the


253

variance of the buffer occupancy are calculated from the amount of data in the buffer [15]. For traffic measurement reporting, two events are defined: – Event 4a: traffic volume exceeds an absolute threshold; and – Event 4b: traffic volume falls below an absolute threshold. Traffic volumes are calculated for the transport channels. There are also two timers that can be used to limit the number of measurement events. Time-to-trigger indicates a time period during which the criterion of the traffic volume must be fulfilled before the actual event is triggered. The parameter Pending time after trigger indicates a time period when no new measurement reports are sent to the RNC, even if the criterion of the specific event is fulfilled. This parameter limits the number of measurement reports sent to the RNC. 6.5.2.5 Quality measurements A mobile station measures the downlink transport channel BLER. If the mobile station detects more bad Cyclic Redundancy Checks (CRC) than defined by the parameter ‘BAD CRC’, the mobile station will trigger Event 5a. The number of bad CRCs is calculated as a sliding average of bad CRCs within a number of total CRCs indicated by the parameter ‘Total CRC’. The amount of reports sent to the RNC is limited by introducing the parameter ‘Pending time after trigger’. During the time period indicated by ‘Pending time after trigger’, the mobile station is not allowed to send new reports to the RNC. 6.5.2.6 Mobile station internal measurements In FDD, the mobile station should measure the transmit power, the received signal strength, and the mobile station’s Rx-Tx time difference. The following six events are defined for these measurements: – Event 6a: the mobile station Tx power exceeds becomes an absolute threshold; – Event 6b: the mobile station Tx power falls below an absolute threshold; – Event 6c: the mobile station Tx power reaches its minimum value; – Event 6d: the mobile station Tx power reaches its maximum value; – Event 6e: the mobile station RSSI reaches the mobile station’s dynamic receiver range; – Event 6f: The mobile station Rx-Tx time difference for a radio link included in the active set exceeds an absolute threshold; and – Event 6g: The mobile station Rx-Tx time difference for a radio link included in the active set falls below an absolute threshold. The power measurements (Events 6a – 6e) can be used, for example, by the handover control to ensure that calls can be maintained. Events 6f and 6g

254


are used by the network to adjust the timing of the downlink transmission [8, 16] 6.5.3 Cell selection and reselection parameters Cell selection and reselection was described in Section 6.3.2. The parameters related to cell selection and reselection are summarized in the following table. Table 6-7. Parameters for cell selection and reselection. Parameter Explanation Sintrasearch Threshold for starting intra-frequency measurements. Sintersearch Threshold for starting inter-frequency measurements. SsearchRAT m Threshold for starting inter-system measurement for the RATm. Threshold to apply HCS. If the signal strength is below this SsearchHCS threshold, HCS rules are not applied, but intra-frequency and inter-frequency cells are measured. Time period used to determine whether the mobile station is TCRmax in high-mobility state. NCR Threshold for the number of cell reselections in high-mobility state. Time hysteresis for high-mobility state. TCrmaxHyst Treselection Cell reselection timer. Threshold for applying HCS for inter-system measurements. SHCS,RATm Slimit,SearchRATm Upper threshold for inter-system measurements for RATm. If the signal quality is above this threshold, the mobile station does not have to perform inter-system measurements. TEMP_OFFSET1 Temporary offset for a neighbor cell. The offset is applied in the cell reselection during the time period PENALTY_TIME. Applied when CPICH RSCP is used as a signal quality measure. TEMP_OFFSET2 Temporary offset for a neighbor cell. The offset is applied in the cell reselection during the time period PENALTY_TIME. Applied when CPICH Ec/N0 is used as a signal quality measure. PENALTY_TIME Time period during the TEMP_OFFSET is applied in the cell reselection. HCS_PRIO Priority for hierarchical cell structure. Qoffset1 An offset applied for a neighbor cell when ranking the cells. Applied when CPICH RSCP is used as a signal quality measure. Qoffset2 An offset applied for a neighbor cell when ranking the cells. Applied when CPICH Ec/N0 is used as a signal quality measure. Qhyst1 Hysteresis value applied for FDD cells if the quality measure is CPICH RSCP.

WCDMA RADIO INTERFACE Parameter Qhyst2

Qhcs Qqualmin Qrxlevmin UE_TXPWR_MAX_RACH P_MAX

255

Explanation Hysteresis value applied for FDD cells if the quality measure is CPICH Ec/N0. Quality level when applying HCS. Minimum required quality level in a cell. Minimum required RX level in a cell. Maximum UE TX power level when accessing the cell on the RACH. Maximum RF output power of a UE.

Many of the parameters listed in Table 6-7 are cell-specific. This means that for each cell, all the parameters in Table 6-7 have to be set. Also, a cell may have several neighbor cells that require the corresponding parameters. 6.5.4 Cell information The identifiers related to a cell were described in Section 6.5.1. In addition to these identifiers, there are other parameters that describe the configuration or performance of the cell. Table 6-8. Cell information parameters. Parameter Explanation TX Diversity Indicator Indicates whether TX diversity is used in a cell. UARFCN The frequency of the cell. Calculated by using the formula UARFCN = 5 x frequency in MHz. Primary Scrambling Identifies the downlink scrambling code for the primary CPICH. Code Tcell The offset for the start of the SCH. Tcell is used to avoid overlapping between SCHs under the same NodeB.

With the parameter TX Diversity Indicator, the use of diversity is indicated. TX diversity affects how data is sent to the mobile station. Thus the mobile has to know whether TX diversity is used in a cell. The frequency and the primary scrambling codes are basic parameters that must be correctly set but otherwise do not require much work. The Tcell parameter is used to have an offset for the SCHs of the cells under the same base station. Without correct values for this parameter, the SCHs of different cells under the same NodeB would overlap in the time domain and the synchronization of a mobile station would be more difficult. 6.5.5 Power control parameters The power control was described in Section 6.4.1. There are several parameters that are used for the power control. Table 6-9 lists the most essential ones.

256


Table 6-9. Power control parameters. Parameter Explanation DL_Power_Averaging_ The length of the downlink power averaging window in number Window_Size of slots. If the power raise is limited in the downlink the power raise calculated over the averaging window will be smaller than the limit indicated by ‘Power_Raise_Limit’. Power_Raise_Limit Defines a limit for the downlink power raise during the downlink power averaging period. Gain factor ȕc Gain factor for the control part. See also Figure 6-22. Gain factor for the control data part. See also Figure 6-22. Gain factor ȕd PowerControlAlgorithm Power control algorithm for combining the downlink power commands. Algorithm 1 works well in most of the cases. Algorithm 2 would be used if the mobile speed is above 80 km/h or below 3 km/h [1]. TPC-StepSize The power-step in the mobile station. For the Algorithm 1, the possible values are 1 dB and 2 dB. For the Algorithm 2, the only possible value is 1 dB. PO1 Power offset for the TFCI bits relative to the power of the secondary CCPCH. PO2 Power offset for the TPC bits relative to the power of the secondary CCPCH. PO3 Power offset for the pilot bits relative to the power of the secondary CCPCH. Power Ramp Step The step of the power increase in the RACH procedure. Preamble Retrans Max The maximum number of retransmissions in the RACH procedure. Power offset P p-m Power offset between the power of the last transmitted preamble and the control part of the random-access message. Preamble_Initial_Power The initial power of the preamble in the RACH procedure. Constant Value The required C/I in the uplink. Used in the RACH procedure.

Even if there are several parameters listed in Table 6-9, there can be several other parameters related to the power control, depending on the implementation. Also, the equipment manufacturers may use different names than the ones used in Table 6-9.

REFERENCE [1] J. Laiho, A. Wacker, T. Novosad, Radio Network Planning and Optimisation for UMTS, John Wiley & Sons Ltd, 2002. [2] R. Peterson, R. Ziemer and D. Borth, Introduction to Spread Spectrum Communications. Prentice-Hall, 1995. [3] Universal Mobile Telecommunications System (UMTS), Radio Interface Protocol Architecture, 3GPP TS 25.301.


257

[4] Universal Mobile Telecommunications System (UMTS), Physical Layer - General Description, 3GPP TS 25.201. [5] Universal Mobile Telecommunications System (UMTS), Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD), 3GPP TS 25.211. [6] Universal Mobile Telecommunications System (UMTS), Spreading and Modulation (FDD), 3GPP TS 25.213. [7] Universal Mobile Telecommunications System (UMTS), Physical Layer Procedures (FDD), 3GPP TS 25.214. [8] Universal Mobile Telecommunications System (UMTS), RRC Protocol Specification, 3GPP TS 25.331. [9] Universal Mobile Telecommunications System (UMTS), UE Procedures in Idle Mode and Procedures for Cell Reselection in Connected Mode, 3GPP TS 25.304. [10] Universal Mobile Telecommunications System (UMTS), Interlayer Procedures in Connected Mode, 3GPP TS 25.303. [11] Universal Mobile Telecommunications System (UMTS), Requirements for Support of Radio Resource Management (FDD), 3GPP TS 25.133. [12] Universal Mobile Telecommunications System (UMTS), Radio Resource Management Strategies, 3GPP TS 25.922. [13] Universal Mobile Telecommunications System (UMTS), Numbering, Addressing and Identification, 3GPP TS 23.003. [14] Universal Mobile Telecommunications System (UMTS), UTRAN Overall Description, 3GPP TS 25.401. [15] Universal Mobile Telecommunications System (UMTS), Medium Access Control (MAC) Protocol Specification, 3GPP TS 25.321. [16] Universal Mobile Telecommunications System (UMTS), Services Provided by the Physical Layer, 3GPP TS 25.302.


Chapter 7 UMTS RADIO INTERFACE FIELD MEASUREMENTS Key WCDMA measurements and parameters KAI OJALA, PASI NIEMI Nemo Technologies

Abstract:

In this chapter, the main WCDMA field measurements are described. Measurements are divided into three categories: measurements in idle mode, in connection establishment, and in connected mode. Measured parameters in these modes are explained with examples. Once WCDMA is new technology, new features and functionalities will have to be verified. In the beginning, coverage, power control, and soft handovers are the key issues as well as verification of different connection types provided by a WCDMA system. At the start of the 3G era, WCDMA coverage is not continuous, so inter-working with GSM/GPRS has to work as well, so that customers can access cellular services continuously. WCDMA is intended for various applications besides voice services: the testing of circuit switched and packet switched data in the field is described as well as the testing of applications that run on top of the packet switched or circuit switched connection.

Key words:

Analysis of WCDMA field measurement results, data measurements, field testing, WCDMA field measurement tool



260

7. UMTS RADIO INTERFACE FIELD MEASUREMENTS This chapter provides an overview of radio interface field measurements in the downlink direction, measurement equipment, and measured parameters.

7.1 Measurement process When a technology is evolving, there are different phases and time frames for different kind of tests. Figure 7-1 illustrates the different phases. Licence & Planning Phase

Network Implementation

Propagation Model Verification

Network Extension

Operation & Maintenance

Empirical Measurements

Penetration Loss Testing

Coverage Verification (indoor/outdoor)

Analysis Autonomous measurements Benchmarking measurements Indoor verification

Propagation measurements

Predicted Data

Figure 7-1. Measurement needs in an evolving technology.

Measurements are conducted to a different extent during different phases. However, during every phase, measurements regarding idle mode, connection establishment and connected mode need to be conducted. 7.1.1 Planning phase First, in the planning phase, there is a time frame for conducting initial tests before actual verification tests are started. This period can be used for building up expertise and putting theory into practice. Observing the radio interface process during measurements and analyzing L1 parameters and L3 signaling after field measurement tours provides useful insight on how the radio interface works.

UMTS RADIO INTERFACE FIELD MEASUREMENTS

261

7.1.2 Implementation phase At the beginning of the implementation phase, different radio access network functions and their operations should be verified at the same time as coverage verifications are performed for the planned service offerings. In the radio interface, this means testing different connection types and data rates that will be used to carry the services. Once the new radio access network solution is delivered by the network infrastructure vendor, verification of the functionality of various UTRAN features has to be conducted. The most important functions to be tested are [1-2]: 1. cell selection and reselections in idle mode, 2. setting up connections, which in 3G means the setups of different connection types with different data rates, 3. power control (open loop, closed-loop, and outer-loop), 4. soft and softer handover, 5. hard handover (mainly inter-frequency handovers in the first phase), 6. GSM/GPRS-WCDMA inter-working, 7. verification of vendor-specific traffic control algorithms – admission control, – packet scheduling, – load control, and – resource management. In general, initial tests should be performed in a laboratory. After the basic functioning has been approved there, it is time to apply the new functions to a network and conduct field measurements. This phase includes testing the functionalities in real radio environments since they should work there as well. Good preparations are needed during the field tests. These include planning the test routes, cross-checking the status of the network elements, and monitoring the state of the different elements. There are a number of different connection types that have to be tested. The variety of connection types naturally depends on network functionalities and terminal capabilities, but at least voice, circuit switched, and packet switched connections with various possible data rates should be tested. For example, voice call setups or packet switched connections can be made (radio bearer setups, attaches, PDP context activations), running tests with scripts enabling the testing of multiple setups can be conducted. Furthermore, the reliability of different setups can be verified to test network accessibility. Figure 7-2 presents an example of the call setup success ratio from one test drive. In addition, parameters controlling the way the traffic is directed on certain channels and the way this works should be verified (e.g., directing the traffic to DCH/FACH in the downlink (DL) direction, or RACH usage

262


and reliability in the uplink (UL) direction). This requires modifying the settings of certain network parameters. Testing can be conducted by sending packets of different sizes and monitoring which channels are used. Voice Call Event Information

Call Disconnect Information

Type

Value

Total

27

Successful Release

24

Dropped

3

(i.e., network release)

Percentage of Successfull Calls

88,889

%

Percentage of Dropped Calls

11,111

%

Description

Figure 7-2. Example of voice call statistics.

Radio resource management (RRM) functions are a critical part of the radio network. These algorithms can be seen as the brains in the radio network. They affect the capacity of the network and control the way new service access requests are admitted, denied and handled in the radio network. First of all, if power control is not functioning correctly, the capacity of the radio network will be degraded. Uplink and downlink power control should be tested in different environments and with different mobile speeds because the radio environment affects the way the power control (PC) performs. Parameters from UL and DL should be traced simultaneously to enable analysis in both directions. Tests should be performed in all the different cell types that are deployed, for example, urban environments (micro and macro cells), suburban cells (macro cells, base stations on rooftops, poles, masts, etc.) and rural environments. This is important because a WCDMA receiver performs differently in different radio propagation environments. These tests will give parameters for more accurate coverage estimations and also rules of thumb for evaluating the coverage compared, for example, with previous technologies (GSM900/GSM1800). Having the actual measured parameters for planning will reduce the selection of unsuitable base station (BS) sites in the future and optimize network performance. The functionality of the power control with multiple users using different data rates would also be important to test. However, it is very difficult to create an actual test case for simulating reallife situations in the network. That’s because it requires moving users located with different path losses from the base station and using the available services randomly. It is difficult to create this scenario but it would reveal if power control will handle realistic situations correctly. Evaluations with multiple users in a cell could be performed when the first friendly users are allowed to use the network. At the same time, measurements can be conducted to observe how the PC performs. Another way to gain insight on


263

PC performance could be to use multiple mobiles in one place. However, in real networks, users are scattered, which is more challenging for power control. The next step in testing is RRM functions. Soft/softer handovers (SHO) and their parameter settings should be verified. First, an operator has to decide whether periodical reporting is used or whether soft handover is based on event-triggered reporting. Testing of the latter case includes parameters affecting add/drop/replacement windows and timers defining how long a certain condition should be valid before a measurement report and its triggering event is sent to the network, which could initiate an active set update [3]. Based on field measurements, the correctness of the first “default” values should be verified. Handovers between radio network controllers (RNCs) are required in larger cities, where one RNC cannot handle all the deployed base stations (This depends on the number of BSs and the selected configuration of the RNC.) In these cases, intra-frequency hard handovers should be verified unless the network implementation uses the Iur interface, enabling soft handovers between RNCs. The operator also has to evaluate inter-frequency handovers (IF HO) at an early stage in order to be prepared for growing capacity needs. Therefore, measurements evaluating IF HO settings (in other words, thresholds for initiating inter-frequency measurements and compressed mode parameters: transmission gaps, pattern lengths, etc.) have to be conducted [1, 4]. The functioning of the GSM/GPRS-UMTS inter-working (inter-system cell reselections and inter-system handovers) will be crucial for having continuous coverage for services in the first phase of the 3G network launch. There are a number of parameters for these functions that can be altered and should be verified in field measurements. The functionality of other critical RRM functions, such as admission control, packet scheduling, load control and resource management, should also be verified to make sure that the network really functions as a cellular network. These functions either allow or deny new connections. The basis for the algorithms is usually the interference level or throughput in the network in the UL direction and the use of the base station power resources or throughput in the DL direction [1]. Basic functionality tests should include laboratory tests and field tests during unloaded and loaded networks. Before tests can be started, planning and defining the tests of the different WCDMA functions must be done. Depending on the chosen strategy, the infrastructure vendor can conduct these tests and report the test results of the different features to the operator. This, however, requires that the operator still agrees with the vendor on the acceptance criteria, and that requires expertise in WCDMA and GSM/GPRS. Another possibility for the operator

264


is to plan and define tests by themselves, but this demands more expertise. Planning and defining the tests has to be started well in advance since the task is not trivial due to new WCDMA radio interface features and functionality. Reserving time for that phase is valuable because the issues to look for during measurements have to be clear before starting the tests. In planning the verifications, one should take into account what interfaces have to be traced at the same time as field tests are driven; how to collect, store, and name measurement files, and what will be included in certain test cases? In other words, define in advance the key parameters for the analysis of test cases. Also, the human resources needed for different tasks should be planned, and training should be conducted to insure adequate expertise to conduct the tasks. 7.1.3 Operational phase When the network is up and running, it is time to move to the optimization phase. There will be a number of parameters that can be tuned, and their effect should be verified with field measurements. In addition, at this phase, regular network surveys are conducted to see how well the network is performing in changing conditions. Behavior of the UE in idle mode, during connection establishments, and during connected mode is tested all together while test drives are conducted. Situations in the network change due to traffic injected by real customers and the results vary depending on the traffic in the network. For example, special events that gather a lot of people in one place have an effect on the measurement results. There is also a need to verify the concepts and configurations used in the network, such as different antenna types, tilting, and the use of masthead amplifiers. Furthermore, new parameter settings and possible new features should be verified. Survey tests give a picture of how network upgrades or configuration changes affect the network performance. Survey tests have to be well-planned in order to get comparable results, even if an operator uses automatic field measurement tools to measure network performance or if the operator uses more traditional methods for measurements, such as field measurement tools in a van operated by a network system engineer. The measurement routes have to be defined as well as the time of day for conducting the measurements. Also, the operator must decide what connection types are used in testing and what kinds of scripts are run to conduct automated tests. Below is an example of a list of details that need to be defined before measurements: – define the test routes that will be used in repetitive tests, – define the time of day for testing in order to achieve comparable results,


265

– define the connection type and its settings for testing, – define the key performance parameters for analysis, for example: – CPICH Ec/N0, – received signal level (RSSI), – quality or block error rate, – TX power, – percentage of successful/dropped calls, – percentage of successful attaches and PDP context activations, – average data throughput during UL/DL downloading. On the basis of the measurement results, it is easy to point out areas where, for example, call setups have not been functioning correctly. The following example in Figure 7-3 depicts possible problem areas for setting up calls.

Figure 7-3. Example of possible problem areas in the network.

The red phone symbols on the map point out problem areas in the network. One way to examine the problem is to first check if the pilot Ec/N0 is at the required level. In the picture above, there has not been an adequate CPICH level available, which has caused call failures during the test drive. If there is a good CPICH level available and the calls are not succeeding, it is necessary to analyze what has happened during the setups: has the RACH procedure succeeded, and what has happened in L3 signaling? Troubleshooting call setup problems usually requires analyzers connected to other network interfaces (Iub and Iu).


266

7.2 Field measurement equipment The field measurement equipment usually consists of a laptop PC, which should optimally be dedicated for measurements, just to avoid other software interfering with the measurement configurations. The PC stores the measurement data and runs the software during the measurements. The exact location of the measured parameters has to be recorded as well, and for that purpose, a GPS receiver is connected to the PC. It is possible to use a scanner, or a mobile, or both as measurement devices.

Fast Scanner (option)

Figure 7-4. Measurement equipment.

Using both a scanner and a mobile simultaneously in WCDMA measurements enables the measuring of all pilots available in the area and the comparison of the results to the view seen by the user equipment (UE). The UE reports values based on a neighbor list received through signaling and makes cell reselections and handovers based on the planned neighbor list. However, there can be pilots available that are not defined in the neighbor lists and these can be spotted with a scanner. In other words, measurements together with a scanner and a mobile would also identify missing or interfering pilots. 7.2.1 Equipment installation The placement of the test mobile and the use of external antennas affect the results. The UE placement should be defined and the same placement should be used in repetitive tests. Today, mobile phone users mainly use


267

mobiles inside cars without external antennas, or mobiles are used indoors. In other words, it might be a valuable strategy to perform measurements using the UE inside a car. The mounting of the UE should be fixed so that different measurement tours are comparable. The difference in using external/internal antennas is around 4-10 dB, depending on the placement of the mobile and the structure of the car. If external antennas are used, losses or gains produced by additional hardware should be taken into account in the results. In other words, the person performing the measurements should find out the performance figures given by the manufacturer for the following: – connectors; – cables; and – antennas. Afterwards, when analyzing the results, the effect of the items above should be noted.

7.3 Measurements in idle mode In this section, three different types of measurements are introduced. First, noise floor measurements are described. Secondly, measurements that can be done using WCDMA scanner are introduced. Thirdly, UE measurements during idle mode are presented. Because WCDMA is an interference-limited network and power needs always affect the capacity that can be achieved, it is beneficial to conduct spectrum clearance measurements in the allocated UMTS bandwidth before anything is in the air. It is better to perform these measurements with a sensitive spectrum analyzer because the sensitivity of commercial scanners is not high enough. A band-pass filter should be connected in front of the spectrum analyzer to filter out other interfering signals. In the uplink direction, the pure noise floor can be calculated by assuming a 4-6 dB noise figure in the base station [5]: Noise floor UL

= kTFB = kT [mW/Hz] + NF [dB] + B [dBHz] = -174 dB(mW/Hz) + 4…6 dB + 67 dBHz = = -104…-102 dBm.

(7-1)

where k is Boltzman’s factor, T is temperature, F is the noise factor, B is bandwidth, and NF is the noise figure. In the same way, the pure noise floor for the downlink can be calculated by assuming a 7-10 dB noise figure in the user equipment:


268

Noise floor DL

= -174 dB(mW/Hz) + 7…10 dB + 67 dBHz = = -97…100 dBm.

(7-2)

In these measurements, the planning areas should be measured by selecting the planned places where the spectrum clearance measurements are conducted, and if the levels exceed the calculated noise floors, there is interference on the frequency band that should be cleaned away. High sensitivity is needed to point out interference, because load/admission control and packet scheduling strategies can be based on the interference level [1]. If there is wideband interference in the allocated frequency band, it will be added on top of the "clear noise floor". This will waste the capacity due to higher power needs for the radio links, and it will have a negative effect on the performance of the network. If, for example, admission control is based on measuring uplink interference, the existing wideband interference seen by admission control as artificial traffic could prohibit users from getting any service. 7.3.1 Scanner measurements A scanner gets an overall image of the available pilots in the air and it can be used as an instrument that reflects the situation for a UE that is in idle mode. The difference between a UE and a scanner is that a UE measures a situation according to the network’s commands in signaling while a scanner measures all the pilots in the air. Coverage verification can be performed in the first phase using a WCDMA scanner. The main interest at the very beginning is coverage, and CPICH Ec/N0 measurements can be used to verify the coverage of selected BS sites. CPICH Ec/N0 values give a picture of how good coverage is provided from the BS sites and how large interference tails are produced. If large interference areas are generated, the problem could be minimized later by adjusting the antenna direction or height (unfortunately, this requires a new installation), or by down tilting the antenna or by slightly tuning the pilot power levels. Operators can define their own thresholds for depicting good coverage areas, but empirically, Ec/N0 values down to -10 − -12 dB should be adequate once all sites are in operation. However, there are no requirements in the specifications for the required levels, and thus the operator has to verify good values. Ec/N0 values have to be defined with some margin for fading and other cell interference. On the other hand, Ec/No values smaller than -15 dB could be interpreted as “interference tail”. Because a scanner does not decode any real user data, measurements with commercial mobiles


269

are required for finding out the acceptable Ec/N0 level where UE still performs satisfactorily.

Figure 7-5. Scanner measurements of the CPICH Ec/N0 levels.

Another important use for a scanner is setting the neighbor lists in order and, once new cells are introduced, maintaining neighbor lists and pointing out possible interferers. See Section 7.3.3 for an example of the use of scanner results for neighbor list definitions. 7.3.2 Scanner parameters After camping on a cell, a mobile station (MS) measures only cells defined by the network; in other words, cells that are shown in the neighbor list. With a pilot scanner, a wider picture of the pilot situation in the network can be obtained. In the following sections, parameters that typically can be measured with a scanner are explained. CPICH Ec/N0 CPICH Ec/N0 is the received energy per chip divided by the power density in the band. The Ec/N0 is RSCP/RSSI. Measurement is performed on the Primary CPICH. The reference point for the CPICH Ec/N0 is the antenna connector of the UE [6]. CPICH RSCP Based on the Received Signal Code Power of a CPICH (RSCP), measurements can be used to calculate coupling loss to the NodeB (CPICH power - RSCP = coupling loss, that is, loss to the BS antenna connector).

270


Coupling loss does not take into account feeder loss and antenna gain. RSCP measurements can also be used to check where other operators have their sites. Rough propagation model tuning is also possible by measuring CPICH RSCP values or by scanning continuous wave (CW) from the BS site.

Figure 7-6. RSCP measurement using a scanner.

P-SCH Ec/N0 The TX power value is set by the radio network planning engineer and it is set on a fixed offset level compared to the pilot [1]. The measurement values should show a difference between the pilot Ec/N0 and the primary synchronization channel Ec/N0. The difference should be around -3 - -6 dB according to the first settings (an operator decides on these values). For example, if the primary synchronization channel is set at 6 dB lower than CPICH, CPICH Ec/N0 -3 dB and P-SCH Ec/No -9 dB should be observed. An example is shown in Figure 7-7. The power levels of synchronization channels should be set correctly in order to be able to perform the cell search procedure by a mobile. A terminal gets the slot timing from the primary synchronization channel.


271

Figure 7-7. Example of P-SCH versus CPICH Ec/N0.

S-SCH Ec/N0 A terminal makes frame synchronization based on the secondary synchronization channel and, based on this, it knows what group of primary scrambling codes is in use; (there are 64 code groups). Secondary and primary synchronization levels should be equivalent compared to the pilot power (typically a -3 - -6 dB difference in the pilot power) [1]. P-SCH Ec/N0 measurements over time slot The timing of primary synchronization channels can be shown as vertical lines; an example is shown in Figure 7-8. They should not overlap, and thus SCH bursts should not interfere with each other in the different cells of the NodeB in question. In Figure 7-8, a unit of the x-axis is a half chip, and the y-axis shows the proportional difference in dB. Figure 7-8 shows that P-SCH bursts from the three available cells are not overlapping. This enables the synchronizing of the UE into the network. The time difference of P-SCH bursts is an RNP parameter [1].

272


Figure 7-8. Timing of the different sectors transmitting from the same site.

CPICH time of arrival This parameter tells the timing of the different scrambling codes transmitted from different base stations. Delay profile of CPICH and delay spread The delay profile reveals the delay spread of the CPICH and together these parameters define the characteristics of the radio propagation environment in the measurement area. Both delay profile and delay spread can be measured. The first planning parameters are based on simulation values where certain radio channel models are used. By studying the delay spread, correct planning values for different areas can be chosen by comparing the delay spread shapes to the channel models that are used in the simulations. In Figure 7-9, the y-axis is presented in dB and a unit of the x-axis is a half chip. It can be seen from the figures that propagation in the micro cell is close to the pedestrian A model [7, 8], and the suburban macro cell resembles the ITU vehicular model. The delay spread size is approximately 0.5 µs in Figure 7-9 (a), and approximately 1.5 µs in Figure 7-9 (b). By conducting these measurements, the planning parameters per site can be selected more realistically.


a)

273

b)

Figure 7-9. Examples of delay profiles in (a) urban micro cell and (b) suburban macro cell.

7.3.3 Mobile measurements in idle mode Cell selection and reselection are performed based on UE measurements, threshold values and hysteresis timers signaled by the network. 7.3.3.1 Initial cell selection In an initial cell selection procedure, no prior knowledge of which RF channels are UTRA carriers is required [4]. UE scans all RF channels in its own UTRA frequency band to find a suitable cell in the selected public land mobile network (PLMN). On each carrier, the UE needs only to search for the strongest cell, and for that purpose the UE uses CPICH RSCP values. Once a suitable cell is found, it will be selected. If the cell selection is performed according to stored cell selection information, the UE will use the stored information of the carrier frequencies. Furthermore, the UE can optionally use information on cell parameters, for instance, scrambling codes from previously received measurement control information elements. 7.3.3.2 Cell reselection In UTRA FDD mode, the UE uses CPICH Ec/N0 or the RSCP value to define which cell the UE is connected to during idle mode. Cell selection criteria in FDD is fulfilled if Srxlev and Squal are both greater than 0. (Squal is the measured quality value [Qqualmeas] minus the minimum quality threshold [Qqualmin], and Srxlev is the measured RX level [Qrxlevmeas ] minus the minimum RX level threshold [Qrxlevmin].) The pre-selection timer defines the hysteresis

274


time. When camped on a cell, the UE will regularly search for a better cell according to the cell reselection criteria. If a better cell is found, that cell will be selected. A change of the cell may also cause a change of radio access technology (RAT) [2-4, 8]. Furthermore, 3GPP specifications define the formulas for S, H, and R criteria, which are used in cell reselection. For details, see Section 6.3.2. This chapter will concentrate only on the S criterion. In other words, the criteria used in hierarchical cell structures (HCS) will not be discussed. First, when the UE is switched on, it tries to camp on a cell via the cell search procedure. At the beginning, the UE tries to camp on the strongest cell. After camping on a cell, the UE conducts measurements controlled by the network and measures scrambling codes sent by RRC messages. System information block 3 (SIB3) includes parameters for cell selection and reselection, and these values are used in idle mode, CELL_FACH, CELL_PCH, and URA_PCH states. SIB4 contains the same parameters and it is used during connected mode. If SIB4 is not broadcast, SIB3 is also used in connected mode [3]. A neighbor list is sent on SIB11, and it also contains handover parameters. SIB11 is used in idle mode, CELL_FACH, CELL_PCH, and URA_PCH states. SIB12 contains the same parameters, and it is used in connected mode. If SIB12 is not sent, SIB11 is also used in connected mode. Figure 7-10 illustrates cell reselection.

Figure 7-10. Cell reselection in idle mode.


275

In Figure 7-10, the Ec/N0 level of scrambling code 285 (dark green) goes down and the Ec/N0 level of scrambling code 282 goes up, and thus the UE makes cell reselection at the point of the green star. In Figure 7-11, the pilots detected by a scanner are plotted: 285, 286 and 282. The lower part of Figure 7-11 displays the views produced by the mobile: pilots 285 and 286 are measured.

Figure 7-11. Pilots measured by the scanner and terminal in idle mode.

The reason behind being able to detect different results may be the fact that pilot 282 is not defined as a neighbor for pilot 285. In addition, the measurement speed of the UE in different modes affects the results [2]. The

276


UE measures an active cell and neighbor cells with different rates. The measurement speed depends on many factors, such as the size of the neighbor list. However, the scanner measures every selected pilot with the same speed. Therefore, the scanner measures a given frequency band and its pilots objectively. Note also that receiver structures are not the same. Therefore, there might be differences in values obtained by the scanner or the UE. Finally, in idle mode, the scanner can point out pilots that are available in the area. Thus, neighbor lists can be defined more accurately with scanner measurements than based on predictions made by a radio network planning tool, because propagation models and data of a planned environment always produce some inaccuracies.

7.4 Measurements during connection establishment In connection establishment, the UE starts the random access procedure. When the UE leaves the idle mode in order to enter the connected mode, the UE will first attempt to access a current serving cell. If the access attempt to the serving cell fails, the UE will use the cell reselection evaluation procedure. The information needed for the random access procedure, that is, available signatures, sub-channels, and the spreading factor, are given in SIB5 to the mobile [3]. Based on the parameters signaled from the network and the measured values, the UE calculates the initial transmit power, which will be used by the first preamble. The power of the first preamble is calculated based on the RSCP of the CPICH and used CPICH TX power, the UL interference level, and the constant value given by the radio network planning. These values are also given in SIB5. This functionality is named “open-loop power control”. If the first preamble is not detected by the base station, the mobile sends a new preamble with higher power. The preamble step size is determined by an operator. The UE continues sending preambles for as long as it receives an acquisition indicator from the network and then proceeds to transmit the RACH message. The maximum number of preambles allowed is an RNP parameter along with the power offset between the last preamble and the RACH message. On the other hand, the network measures the UL interference level (the value signaled to UE in SIB7), and the UE measures the RSCP of a CPICH. There are a couple of issues that should be monitored from the RACH process. First, if preambles go through with the first attempts and RACH messages are sent immediately, it is likely that the UE is using more transmit power than needed and thus extra UL interference is generated. On the other


277

hand, if too conservative RACH parameters are used, it can cause the UE to fail on RACH even though the maximum number of preambles is sent. This will result in access failures and additional RACH procedures and, moreover, additional UL interference to the network. Figures 7-12 and 7-13 show an example of a RACH process. The blue dots represent the power level of the first preamble and the red dots represent the total power of the RACH message. Furthermore, on the righthand side, there is a list of the sent preamble numbers, the preamble step size [dB], and the maximum number of allowed preambles.

Figure 7-12. Initial preamble powers vs. RACH message powers.

Figure 7-13. Example of successful RACH processes.

278


From Figure 7-12, it becomes apparent that the first UE has sent preambles but has not proceeded to send a RACH message. Therefore, it has not received the acquisition indicator or the base station has not received the preambles. Later in Figure 7-13, the RACH message has been sent and afterwards the connection has been established. A script that enables several establishments of short calls could test connection establishments of voice calls or circuit switched data calls. Based on repetitions, the success rate of setups can be calculated and problem areas of call establishments can be pointed out. In addition, the uplink interference level can be monitored by making short calls while measuring, if necessary. The same RACH process is performed in L1 when setting up packet data calls. The difference is that a connection is established to the packet data core. Therefore, success of attaches and PDP context activations should also be monitored. Call setups should be verified over the entire planned service area. The connection establishments should be verified in cell edges and also in an indoor environment. In addition, the time of day affects the results because the system will work differently in loaded and unloaded situations. When evaluating the performance success rate of the connection establishment, there are several factors that should be kept in mind when analyzing the results: - admission control (which monitors traffic status in the network), - resource manager (which monitors use of channelization codes), - packet scheduling (which monitors space for packet traffic that is available on top of circuit switched traffic), - priorities (i.e., what has been requested vs. the network situation), and - SGSN and GGSN status.

7.5 Measurements in connected mode The main things to be considered during the connected mode are the verification of available coverage, the handover success rates of different handover types, and the behavior of the network and terminal during soft, inter-system, and inter-frequency handovers. These measurements are valid, regardless of the type of service used during measurements. During data transfer, there are specific key performance parameters that should be monitored. Data measurements are further discussed in Section 7.5.5.


279

7.5.1 Coverage In the first phase, coverage verification should be performed and the coverage differences for different services should be of interest. The parameters indicating the coverage level are: – in the downlink: Ec/N0, because pure RSSI does not take into account interference in the network. The required Ec/N0 level could be evaluated by determining a level where a receiver starts to perform poorly. (An example is given in Figure 7-14.) – in the uplink: UE TX power, because a UE tries to maintain a defined quality level by adjusting the transmit power. Because of this, when the UE is in the cell edge area in the UL direction, it uses close to the maximum TX power. An estimate of the UL indoor coverage can be analyzed by subtracting the indoor loss used in the power budget from the measured TX power.

Figure 7-14. Ec/N0 and raw BER and BLER.

In Figure 7-14, the average of BLER (red curve) is at an adequate level until the raw BER (blue line) starts to rise. At that point, the CPICH Ec/N0 goes below -14 dB. Note that these values may differ in different radio environments, and different UE types may perform differently.

280


Figure 7-15 displays the coverage obtained in a DL data transfer. Ec/N0 values are shown on a map and the graph displays UE TX power. Figure 715 indicates that the downlink direction limits the coverage. Data transfer has stopped at location X even if the Ec/N0 values are still adequate. Therefore, the reason for the lack of DL data coverage might be the DL resource allocation for the used data channel.

Figure 7-15. DL/UL coverage verification based on CPICH Ec/N0 and UE TX power.

When considering the evaluation of the coverage of different services, such a base station site should be selected so that a test route moves away from the site over the predicted coverage area. The test route should be measured 2 to 3 times per connection type, and the results should be averaged to draw conclusions. An example of voice coverage versus data coverage is shown in Figure 7-16. In Figure 7-16, the end of voice coverage is represented by a yellow phone label, while the end of packet data coverage is point X. (The measurement results are shown in Figure 7-15.) In survey tests, the same route will be driven at selected intervals (for example, every month). The analysis of these measurements includes some additional remarks. The key indicators are: – average Ec/N0 values to indicate CPICH coverage in the area; – map plots of the areas where Ec/N0 has been below the set threshold;


281

– average TX power values to point out how good the available UL coverage has been; – areas where TX power has been over the set threshold to pinpoint bad UL coverage areas; – areas where BLER has been over the set threshold which reveals the selected Quality-of-Service during the test drive; and – map plots of dropped/failed call situations for indicating possible problem areas. An example of UE TX power distribution is given in Figure 7-17.

Figure 7-16. Coverage comparison of voice and packet data.

voice coverage

35 30

100

25

%

%

20 15 10 5 0 -50

-40

-30

-20

-10

0

density of tx power

10

20

30

90 80 70 60 50 40 30 20 10 0

100

95.63 85.823 76.723 55.274

23.988

-50

-40

-30

-20

-10

0

10

3.999 0 20

30

Series1

100

95.63 85.82 76.72 55.27 23.99 3.999

0

0

Series2

100

95.63 85.82 76.72 55.27 23.99 3.999

0

0

tx power [dBm]

Figure 7-17. UL coverage verification based on UE TX power.

0

282


Figure 7-17 shows that the UL power has been 90% under 10 dBm during the test drive. This indicates that in the close vicinity of the test route, the UL coverage for voice should be adequate, when the test van penetration loss around 8 dB is taken into account. For voice quality approximations, available BLER values could be monitored, and places where BLER goes over 5% (for example) could be sought in order to point out possible problem areas. 7.5.2 Power control Power control is one of the most critical functions in WCDMA. It is therefore one of the key issues in testing. Power control includes UL and DL power control. In UL power control, a UE modifies TX power according to network commands. The UE’s TX power can be logged during field measurements, and also UL parameters should simultaneously be recorded with a time stamp to find out the functionality of the UL power control. In the DL, the UE handles the outer-loop power control. The UE compares the SIR to the SIRtarget and, based on that, sends power-up or power-down requests to a base station. If the used SIRtarget doesn’t provide sufficient quality (BLER), the UE raises the SIRtarget [1]. RAKE receivers are used in WCDMA; the performance then varies on different environments. In addition, the power control functions slightly differently in different circumstances. Because of this, the power control needs to be verified in different radio areas; therefore, the test routes should be planned for urban, suburban, and rural environments. Figure 7-9 illustrated examples of delay profiles in different real radio environments. Even if delay profiles and RAKE finger allocations have an effect on the receiver performance, following the individual RAKE finger allocations during measurements does not reveal the whole picture for operator needs. In addition, other critical parts in the receiver, such as power control, channel coding, and interleaving, as well as mobile-specific implementations, affect the overall performance. Therefore, it is more beneficial to look at the final result of the receiver performance, which is shown in BLER values. Furthermore, trying to plot real-time finger allocations would require a very fast interface from the UE. Normally, the interfaces that are used cannot produce data fast enough to give a realistic picture of finger allocations. Moreover, the speed of the UE has an effect on the performance of power control. Thus, the drive tests should be made with different speeds. This also applies to soft handovers because the operations of SHO and power control are tied together.


283

Figure 7-18 displays a SIR target distribution in an urban micro cell and a suburban macro cell. The figures are examples of single field test drives. The average SIR target in the urban area was 6.1 dB and in the suburban area 6.3 dB. In other words, the difference was not significant. However, during the test drive in close vicinity of the micro cell antenna, the SIR target was approximately 5.5 dB. The difference in the SIR target was not remarkable and the values were close to the same level in different areas. The whole power control success during the test drive can be analyzed by plotting the overall cumulative distribution of BLER, which is shown in Figure 7-19. 60

40

50

35 30 25 %

%

40 30

20 15

20

10 10

5

0

0 0

1

2

3

4

5

6

7

8

9

10

0

1

2

3

4

SIR target [dB]

5

6

7

8

9

10

SIR target [dB]

a)

b)

Figure 7-18. SIR target in (a) an urban micro cell and (b) a suburban macro cell environment.

100 99

%

98 97 96 95 94 93 92 91 90 20

15

10

5

0

BLER [%] Figure 7-19. Cumulative BLER distribution in a test drive in a micro cell.

The required quality depends on the application, and an operator can define the quality target. For example, for voice, an acceptable value could


284

be BLER < 5%. This is by no means the exact truth – an operator can define its own acceptance levels for different application types. Examining the BLER values in Figure 7-19, it is apparent that 99% of the BLER values are under 5%, which is acceptable. Similarly, acceptable BLER values can be set for different service types and the operator can define what is cumulative level of BLER value that will fulfill the requirement. Figure 7-20 shows the effects of UE speed on the SIR target value. In the upper left corner, there is a graph showing the SIR target while driving 50 km/h, on the upper-right corner 100 km/h, on the lower-left corner 20 km/h, and on the lower-right corner the test routes are displayed. Each graph is taken from the same location of different test drives.

Figure 7-20. Measured SIR target when driving on different speeds.

In the graphs above, the SIR targets are 7.5 dB at 20 km/h, 6.5 dB at 50 km/h, and 7.3 dB at 100 km/h. In other words, there seems to be no significant variation in the SIR targets at different speeds. Figures 7-18, 7-19, and 7-20 show examples of different kinds of measurements that can be performed in different areas. In addition, it is important to remember that PC algorithms are vendor-specific and thus different terminals may behave differently. 7.5.3 Soft and softer handover

Intra frequency handovers, meaning soft/softer handovers, are typically made based on Ec/N0 values measured by the mobile. The mobile measures


285

cells in a certain area according to a neighbor list. Measurements are controlled by the network that is broadcasting on BCCH a list of cells that should be measured. For this purpose, SIB11 or SIB12 is used. Measurement control messages modify the neighbor list being used. The network also commands a UE to use a certain reporting type, which can be periodical or event-triggered. During a soft handover measurement, the control messages tell the UE which cells it should measure. The UE reports results back to the network, which decides whether the active set (AS) should be updated. Usually, the network allows new cells into the active set because the mobile should be connected to the best servers in order to maximize the overall capacity. The reason for denying an active set update, which would add a new scrambling code into the AS, might be the non-availability of resources in the new cell. This resource management is taken care of by the admission control during soft handovers. Figure 7-21 shows the measured Ec/N0 values of the pilots according to the neighbor lists given in SIB12 and the measurement control messages [3].

Figure 7-21. Measured CPICH Ec/N0 values of active set and neighbor set.

The blue bars represent the serving cell or cells in an active set and their scrambling code number and frequency. The measurement of Ec/N0 values is defined in [9], and the reference point for the measurement is in the antenna connector of the UE. The red bars represent neighbor cells and their

286


parameters. In addition, drop and replacement window values are given in dB and calculated according to the 3GPP specification based on signaled values in SIB11, SIB12 and measurement control messages. Trigger 1A, trigger 1B, and trigger 1C give hysteresis time values. During these timers, the measurement results must fulfill add, drop or replacement window, before the UE reports results in measurement reports to the network. If periodical reporting is used, the network commands active set updates based on measurement reports. Furthermore, during a soft or softer handover, the link quality has to remain at an acceptable level. This can be observed from BLER values, which should remain at the same level as in a single-link case. The behavior of the power control together with the soft handover functionality can be tested by driving first through individual cells. Collecting BLER and TX power values requires the barring or preferably the shutting down of the surrounding cells. Next, the same area should be tested when all cells are transmitting, and the TX power values should be compared to the first measurements when there were no SHOs. TX powers should remain at the same level, or preferably at a slightly lower level, if the SHO works acceptably. The analyzer should also check whether the UL and the DL quality values (BLER) have remained at an acceptable level. Furthermore, it is worthwhile to compare the SIR and SIR target values. The main parameters to optimize for soft handover will be add, drop, and replacement windows, and the timers related to them [1-2]. Different environments may need different hysteresis timers. The sizes of the add and drop windows will affect the soft handover probability. Examples of soft handover areas are shown in Figure 7-22.

Figure 7-22. Soft handover locations and the number of pilots in the active set over the measurement area.

The simultaneous use of a scanner and a UE will point out the pilots that are not included in the active or monitored sets but that are relatively strong when compared to the active or neighbor cells. These pilots cause


287

interference that will affect the quality of the connection. In order to improve the performance level in the network, these interfering pilots should be added onto the neighbor lists. Figure 7-23 depicts the strongest pilot and all possible neighbors based on scanner measurements. Strongest scrambling code 14 1200

Cumulated distance (meters)

1000

-15 dB -14 dB -13 dB -12 dB -11 dB

800

-10 dB -9 dB -8 dB -7 dB

600

-6 dB -5 dB -4 dB -3 dB -2 dB

400

-1 dB 0 dB

200

0 14

8

13

57

51

11

60

42

67

56

Neighbor scrambling code

Figure 7-23. Distribution of CPICH Ec/N0 values compared to strongest CPICH (14).

Based on the example in Figure 7-23, an operator could define the neighbor list to contain the strongest pilots in the area. 7.5.4 Other handover cases

There are a few examples when WCDMA handovers can be classified as hard handovers. The first type of hard handover will occur if the intrafrequency cells are under different RNCs and a soft handover cannot be made. In this case, the measurement results look like those in Figure 7-21, which displays the cells in the active set and the monitored set. The difference is that the serving cell or the cells in the active set are replaced by one pilot scrambling code that is controlled by another RNC. A second example is an inter-frequency (IF) handover, for which the UE conducts measurements only when the network commands [1]. The network commands the UE into the compressed mode for IF measurements after the defined condition has been fulfilled. The UE sends measurement reports periodically or when triggered by an event, and the RNC decides IF HO executions based on the measurement reports and the parameter settings. In

288


an inter-frequency handover, the radio interface measurement tool can be used to measure periods in the compressed mode, IF HO executions versus compressed mode, SIR target and BLER behavior during the compressed mode, and signaling during an IF HO attempt or execution. One of the most critical features in the first 3G network deployments is inter-system functionality between cellular systems meaning, for instance, cell reselections and handovers between WCDMA and GSM/GPRS. In order to be able to make measurements between systems and, especially, measurements needed for cell reselection or handovers from the WCDMA side, the UE has to be commanded into compressed mode. Another option for the UE vendors is to use a second receiver. If the UE uses compressed mode for inter-system measurements, the network will control the compressed mode gap lengths and patterns. During the compressed mode, the same WCDMA parameters should be monitored and measured by the field measurement tool as in the case with IF HO. If the UE executes a cell reselection or a handover to GSM, the parameters measured from the 2G network are the well-known GSM/GPRS parameters. The most critical items in the inter-system operations or IF HO testing will be the settings for triggers and thresholds for compressed mode in vendorspecific algorithms. Examples of these can be found in [1]. 7.5.5 Data measurements and application testing

Data measurements in WCDMA can be divided into two main categories: Circuit Switched Data (CS Data) and Packet Switched Data (PS Data). Due to the nature of these different types of data connections that are implemented in the network, the measurement configuration is also different. 7.5.5.1 Circuit switched data measurements Circuit switched data have two different modes: transparent and nontransparent. The transparent data mode does not implement any error correction and thus does not guarantee data integrity for the application. The non-transparent mode has an error correction and a resending protocol. 7.5.5.1.1 Circuit switched data test setup and connection model A circuit switched data connection also has additional elements of analog or digital modems that are used to make the connection to the target modem. Figure 7-24 illustrates the data connection model that is used when a circuit switched data call takes place. In the connection model, it can be seen that with circuit switched data, the number of layers is relatively small. This


289

means, on the other hand, that there is not a great deal of overhead on the user data, and thus circuit switched data provides efficient data transfers. Uu

Iu

MT

RNC

MSC/IWF

L2R

L2R RLP

RLP

RLC

RLC

Iu UP

Iu UP

Modem Codec

MAC

MAC

AAL2

AAL2

ISDN TA

Layer1

Layer1

ATM

ATM

Non Transparent mode Figure 7-24. CS data connection model.

Circuit switched data testing is performed by dialing with a mobile to the modem pool of the server. When the data connection is enabled, the data is transferred between these two points. Figure 7-25 illustrates that circuit switched data is routed from BTS to RNC and MSC where it connects through a public landline network to the server that is used as a terminating point of the tests.

Figure 7-25. CS data measurement setup.

Furthermore, it is possible to test circuit switched data by making a dialup to the modem pool of an Internet service provider and then negotiating a TCP/IP connection over PPP. After a successful connection, it is possible to measure the performance of the TCP connection. However,


290

although this is easy, it only adds protocols to the user data. This is especially deceptive if transparent data is measured, since a TCP brings an error correction to the data flow that does not normally have it. Between Layer 1 and the user data, only the RLC and MAC layers are present. They both add headers to the user data so the actual amount of data that is transmitted over Layer 1 is somewhat more than the user data. See Figure 7-26. User Data RLC

User Data

MAC

User Data

Layer1

RLC Header MAC Header Figure 7-26. Additional header data due to RLC and MAC layers.

By looking at the different protocol layers and the additional overhead that they introduce, it becomes apparent that the overhead in circuit switched data is marginal and it is also quite predictable. Due to the rather thin protocol layers on circuit switched data, the user data rate is comparable to the actual L1 data rate. RLC Header According to the 3GPP 25.322 specification, the RLC can either be in acknowledged mode or in unacknowledged mode (Figure 7-27). The headers added to the RLC depend on the mode that is used.


D/C Sequence Number Sequence Number P Length Indicator

HE E

291

Oct1 Oct2 Oct3 (Optional) (1)

. . .

E

Length Indicator Data

PAD or a piggybacked STATUS PDU OctN a) Acknowledged mode

Sequence Number Length Indicator

E E

Oct1 (Optional) (1)

. . .

Length Indicator

E

(Optional)

Data

PAD

(Optional) Last Octet

b) Unacknowledged mode Figure 7-27. RLC in (a) acknowledged mode or in (b) unacknowledged mode.

MAC Header

According to the 3GPP 25.321 specification, the header used in MAC depends on the subchannel used and the data mapping used. Case (a) DTCH or DCCH mapped to DCH, no multiplexing of dedicated channels on MAC: Case (b) DTCH or DCCH mapped to DCH, with multiplexing of dedicated channels on MAC:


292

Case (c) DTCH or DCCH mapped to RACH/FACH: Case (d) DTCH or DCCH mapped to DSCH or USCH: Case (e) DTCH or DCCH mapped to DSCH or USCH where DTCH or DCCH are the only logical channels: Case (f) DTCH or DCCH mapped to CPCH: Case a):

MAC SDU

Case b):

C/T

MAC SDU

Case c):

TCTF

UE-Id type

UE-Id

C/T

MAC SDU

Case d):

TCTF

UE-Id type

UE-Id

C/T

MAC SDU

UE-Id type

UE-Id

C/T

MAC SDU

Case e) and f):

Figure 7-28. Headers used in the MAC layer.

7.5.5.1.2 Circuit switched data performance metrics CS data performance measurements have many similarities with voice call measurements. In fact, the call setup is so similar that all the same metrics used for analyzing voice call performance can be used in analyzing circuit switched data call access performance. However, it should be noted that when a CS data connection is made, it also requires that inter-working functions (IWF) successfully connect to the target modem via either an analog or an ISDN modem. This additional step can cause a situation where, although there is no problem with accessing the radio bearer, the connection to the target modem can fail due to a handshaking failure between modems. Circuit switched data, once initialized, provides a constant radio bearer for the user data regardless of whether there is data being transmitted through this link. However, this radio bearer can be renegotiated with a different speed during the data call. When measuring in transparent mode, the actual L1 data throughput and user data throughput are not affected by the radio link quality, but the data error rate increases instead. Thus, the key metrics for transparent mode


293

circuit switched data measurements, in addition to RF parameters, are data error rate and user data throughput. In non-transparent mode, the data throughput is affected by the radio link quality, but due to an error correction, the data errors do not exist. See Figure 7-29 for an illustration of user data throughput when BLER is present. It can be noted from Figure 7-29 that data throughput is slower when high BLER is present; and when BLER is either low or nonexistent, data throughput is at a more constant level. Thus, the key metric for nontransparent mode circuit switched data measurements is user data throughput.

Figure 7-29. Non-transparent data throughput measurements when BLER is present.

7.5.5.2 Packet switched data measurements Packet data testing differs significantly from circuit switched data testing. First, there are more network elements and protocols involved. Second, the radio bearer is not initialized as a call, but the mobile activates a connection to the network and gets an IP address that allows a connection to the Internet. 7.5.5.2.1 Packet switched data test setup and connection model Figure 7-30 illustrates the data connection model that is used when a packet switched data connection is used. Figure 7-30 shows that in packet switched data, the number of layers in the connection model is considerably higher than in circuit switched data. Consequently, there is more overhead in


294

the user data when compared to a circuit switched data connection. Thus, the data transfer rates for the user data are going to be lower than in circuit switched data when the same data transfer rate in Layer 1 is used. Uu

Iu

MT

Gn

RNS

3G SGSN

GGSN

PDCP

PDCP

GTP-U

GTP-U

GTP-U

GTP-U

RLC

RLC

IUDP/IP

UDP/IP

UDP/IP

UDP/IP

MAC

MAC

AAL5

AAL5

L2

L2

Layer1

Layer1

ATM

ATM

L1

L1

Figure 7-30. PS data connection model.

Packet switched data testing is performed by initiating an attach and a PDP context activation in the 3G network. When this is performed, an IP connection is present and the data is being transferred between the server and the mobile unit using a TCP/IP protocol. One additional consideration when doing packet data testing is the location of the server used as a test target. For instance, if the server is connected directly to a GGSN, this ensures that the internal latency and throughput of the network are measured. On the other hand, placing the server on the public Internet by using a T1, cable, or DSL connection would allow more realistic end-to-end measurements. Having the server on the public Internet would enable verification of the end-to-end connection by making sure that packets are able to pass through the firewall to the GGSN. This would also enable measurement of the latency and throughput to a public Internet server. Figure 7-31 illustrates that in a packet switched data test setup, data is routed from BTS via RNC to SGSN and to GGSN. The test server used as a target for data testing is connected to GGSN via the Internet.


295

Figure 7-31. Packet switched data measurement setup.

In addition to the protocols in a 3G network, additional packet data protocols are needed before a user application can access the Internet with the packet data connection provided by a mobile terminal. Figure 7-32 shows the additional protocols that are introduced in the PC on top of the 3GPP network protocols before the application can access the packet network. Application

TCP or UDP Uu IP

MT

PPP PDCP

RLC

MAC

Layer1

Figure 7-32. Additional layers when MT is used with PC for packet data.

There are several different protocols present (MAC, RLC, PDCP, PPP, IP, and TCP or UDP) between Layer 1 and the user data. They all add headers to the user data; thus the actual amount of data that is transmitted over Layer1 is more than the actual user data sent by the application. After a closer look at these protocols and the overhead generated by each protocol layer, it becomes clear that in order to be able to analyze the


296

expected user data throughput, one must know how the network is configured and which protocol stacks are utilized. Next, the different kinds of headers that can be present in a PS data connection are introduced: TCP Header without compression*: Byte 1 Byte 2 Source port Sequence number Acknowledgment number Offs Flags Checksum Options Source address Destination address PTCL

Byte 3 Destination port

Byte 4

Window Urgent Pointer Padding

TCP Length

*) TCP Header Compression is a method for compressing the headers of TCP/IP datagrams to improve performance. A typical 40-byte TCP/IP header can be compressed to 3 - 16 bytes. UDP Header: Byte 1 Source port Length IP Header: Byte 1 Ver IHL Identification

Byte 2

Byte 3 Destination port Checksum

Byte 2

TOS

TTL Protocol Source address Destination Address

Byte 3 Total length 0 DF MF

Byte 4

Byte 4

Frag offset Header checksum

PPP Frame:

Flag PPP header

Data

CRC

Fla g


297

A PPP header can be 1 − 4 bytes, and it is negotiated by the protocol. CRC is 16 bits. PDCP Header:

According to the 3GPP 25.323 specification, a PDCP can either have no header/data PDU or a sequence number and a PDU header. PDCP Data PDU PDU Type

PID

PDCP SqNum PDU PDU Type Sequence number Sequence number

PID

RLC Header: According to the 3GPP 25.322 specification, the RLC can either be in acknowledge mode or in unacknowledged mode. The headers added to the RLC depend on the mode used.

Acknowledge mode: D/C Sequence Number P Sequence Number HE Length Indicator E

Oct1 Oct2 Oct3 (Optional) (1)

. . .

Length Indicator

E

Data

PAD or a piggybacked STATUS PDU OctN


298

Unacknowledge mode: Sequence Number Length Indicator

E E

Oct1 (Optional) (1)

. . .

Length Indicator

(Optional)

E

Data

PAD

(Optional) Last Octet

MAC Header:

According to the 3GPP 25.321 specification, the header used in the MAC depends on which subchannel is being used and how the data has been mapped. Case (a) DTCH or DCCH mapped to DCH, no multiplexing of dedicated channels on MAC: Case (b) DTCH or DCCH mapped to DCH, with multiplexing of dedicated channels on MAC: Case (c) DTCH or DCCH mapped to RACH/FACH: Case (d) DTCH or DCCH mapped to DSCH or USCH: Case (e) DTCH or DCCH mapped to DSCH or USCH where DTCH or DCCH are the only logical channels: Case (f) DTCH or DCCH mapped to CPCH: Case a):

MAC SDU

Case b):

C/T

MAC SDU

Case c):

TCTF

UE-Id type

UE-Id

C/T

MAC SDU

Case d):

TCTF

UE-Id type

UE-Id

C/T

MAC SDU

UE-Id type

UE-Id

C/T

MAC SDU

Case e) and f):

Figure 7-33. Headers used in MAC layer.


299

7.5.5.2.2 Packet switched data performance metrics When measuring PS data performance, there are three key metrics: accessibility, retainability, and actual data performance. Accessibility can be analyzed from the attach success rate and the PDP context activation success rate. Retainability can be analyzed from the data connection success rate, where the data connection is the actual point-to-point TCP/IP data transfer session. Furthermore, the PDP context drop counts and the attach drop counts can be used when analyzing retainability. Figure 7-34 shows an example of a quality survey analysis that can be done with a modern analyzing tool for attach events. A similar analysis can be also done for PDP context activations and data connect attempts. Attach Event Info (PD) Type

Value

Attach Attempts

22

Description

Attach Attempt Failures

1

Attach Connections

21

Total Detaches

21

Attach Event Info (PD)

Attach Connection Information

Type

Value

Total Attach Connections

21

Percentage of Attach Connections

95

Attach Event Info (PD)

Attach Failure Information

Type

Value

Total

1

Timeout

1

Percentage of Attach Failures

5

Description %

Description

%

Detach Attach Event Info (PD)

Information

Type

Value

Total

21

Description

User Detach

21

Percentage of User Detaches

100

%

Percentage of Dropped Transfers

0

%

Figure 7-34. Example of quality survey report on attach events.

300


Data performance consists of three different metrics: throughput, delay, and jitter. When analyzing throughput, it has to be understood that the user data throughput and the actual L1 throughput are two different things. As can be seen in Figure 7-35, although there is a downlink data session in process and no user data is being transmitted in the uplink, there is however uplink data being sent on Layer 1. It is important to understand that if there are problems with the uplink data transmission, it will also affect the downlink even though user data is transmitted only in the downlink direction. However, user data throughput is the only thing that matters for the data performance experienced by the end user.

Figure 7-35: Example of how on a DL data transfer, UL data is transmitted on a physical layer although it is not being transmitted on a user data layer.


301

The delay can be analyzed by making ping measurements that give a round-trip delay for data packets. Although, the delay can be measured at the same time as the user data throughput measurements, it is not advisable since the user data transfer will affect the ping times. The jitter (i.e., delay variation) can be analyzed from the variation on ping measurement results. When there is a lot of variation on the round-trip delay, it can cause problems for certain applications. In Figure 7-36, it can bee seen that although most of the time ping times are at an acceptable level (

UMTS Radio Network Planning, Optimization and QOS Management: For Practical Engineering Tasks

Radio Network Planning and Optimisation for UMTS

Radio Network Planning and Optimisation for UMTS

Radio Network Planning and Optimisation for UMTS, 2nd Edition

Evolved Cellular Network Planning and Optimization for UMTS and LTE

Radio Access Network Dimensioning for 3G UMTS

Efficient Methods for WCDMA Radio Network Planning and Optimization

UMTS Network Planning, Optimization, and Inter-Operation with GSM

The UMTS network and radio access technology

WiMAX Network Planning and Optimization

WiMAX Network Planning and Optimization

WiMAX Network Planning and Optimization

Practical Radio Engineering and Telemetry for Industry

Radio network planning and resource optimization : mathematical models and algorithms for UMTS, WLANs, and ad hoc networks

Radio Interface System Planning for GSM,GPRS,UMTS

Radio interface system planning for GSM GPRS UMTS

Radio Resource Management Strategies in UMTS

Radio Resource Management Strategies in UMTS

Indoor Radio Planning: A Practical Guide for GSM, DCS, UMTS and HSPA

Understanding UMTS Radio Network Modelling, Planning And Automated Optimisation: Theory And Practice

Network Control and Engineering for QoS, Security and Mobility, IV: Fourth IFIP International Conference on Network Control and Engineering for QoS, Security ... Federation for Information Processing)

Engineering Internet QoS

Active Sensor Planning for Multiview Vision Tasks

Practical optimization. Algorithms and engineering applications

Network Optimization and Control

HSUPA for UMTS: High Speed Radio Access for Mobile Communications

LTE for UMTS - OFDMA and SC-FDMA Based Radio Access

Radio Access Networks for UMTS: Principles and Practice

Management: Tasks, Responsibilities, Practices

CAMEL Intelligent Networks for the GSM, GPRS and UMTS Network

WCDMA for UMTS: Radio Access for Third Generation Mobile Communications

UMTS Radio Network Planning, Optimization and QOS Management: For Practical Engineering Tasks

Radio Network Planning and Optimisation for UMTS

Radio Network Planning and Optimisation for UMTS

Radio Network Planning and Optimisation for UMTS, 2nd Edition

Evolved Cellular Network Planning and Optimization for UMTS and LTE

Radio Access Network Dimensioning for 3G UMTS

Efficient Methods for WCDMA Radio Network Planning and Optimization

UMTS Network Planning, Optimization, and Inter-Operation with GSM

The UMTS network and radio access technology

WiMAX Network Planning and Optimization

WiMAX Network Planning and Optimization

WiMAX Network Planning and Optimization

Practical Radio Engineering and Telemetry for Industry

Radio network planning and resource optimization : mathematical models and algorithms for UMTS, WLANs, and ad hoc networks

Radio Interface System Planning for GSM,GPRS,UMTS

Radio interface system planning for GSM GPRS UMTS

Radio Resource Management Strategies in UMTS

Radio Resource Management Strategies in UMTS

Indoor Radio Planning: A Practical Guide for GSM, DCS, UMTS and HSPA

Understanding UMTS Radio Network Modelling, Planning And Automated Optimisation: Theory And Practice

Network Control and Engineering for QoS, Security and Mobility, IV: Fourth IFIP International Conference on Network Control and Engineering for QoS, Security ... Federation for Information Processing)

Engineering Internet QoS

Active Sensor Planning for Multiview Vision Tasks

Practical optimization. Algorithms and engineering applications

Network Optimization and Control

HSUPA for UMTS: High Speed Radio Access for Mobile Communications

LTE for UMTS - OFDMA and SC-FDMA Based Radio Access

Radio Access Networks for UMTS: Principles and Practice

Management: Tasks, Responsibilities, Practices

CAMEL Intelligent Networks for the GSM, GPRS and UMTS Network

WCDMA for UMTS: Radio Access for Third Generation Mobile Communications

Recommend Documents