Effects of Repeaters on UMTS Network Performance

TAMPERE UNIVERSITY OF TECHNOLOGY

DEPARTMENT OF INFORMATION TECHNOLOGY

PANU LÄHDEKORPI

Effects of Repeaters on UMTS Network

Performance

MASTER OF SCIENCE THESIS

SUBJECT APPROVED BY THE DEPARTMENT COUNCIL ON SEPTEMBER 21st, 2005

EXAMINERS: Prof. Jukka Lempiäinen M.Sc. Jarno Niemelä

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Preface

This Master of Science Thesis has been written at the Department of Information Technology in Tampere University of Technology (TUT), Finland. The research work for this thesis has been done during my work time in the Institute of Communications Engineering at TUT in years 2004-2005.

Firstly, I would like to thank my supervisor, Jukka Lempiäinen, for the helpful guidance and support during the work time. I would also thank my colleagues, Jarno Niemelä, Jakub Borkowski, Tero Isotalo and Jaroslaw Lacki, for providing helpful technical hints for the work. I would express my appreciations also to the Advanced Techniques for Mobile Positioning (MOT) project for funding the work.

Finally, I would like to warmly thank my fiancee, Helena Kotomäki, for love and support during the work, and my parents Päivi and Harri for the encouraging attitude. Without their contribution, this would not have been possible.

Tampere, February 22nd, 2006 Panu Lähdekorpi [email protected] Ketunleivänkatu 4 C 30 33840 Tampere Finland Tel. +358407529693

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Table of Contents

1. INTRODUCTION .................................................................................................................1

2. INTRODUCTION TO UMTS ..............................................................................................3

2.1. EVOLUTION PATH............................................................................................................3

2.2. STANDARDIZATION .........................................................................................................4

2.3. UMTS ARCHITECTURE ...................................................................................................4

2.4. WCDMA AND SPREAD SPECTRUM SYSTEMS...................................................................5

2.5. CELLULAR CONCEPT AND MOBILITY IN UMTS NETWORKS.............................................8

2.5.1. The idea of cellularity................................................................................................8

2.5.2. Handovers in UMTS..................................................................................................9

2.6. RADIO PROPAGATION IN UMTS....................................................................................10

2.6.1. Propagation environment........................................................................................10

2.6.2. Okumura-Hata propagation model .........................................................................10

2.6.3. Multipath environment ............................................................................................11

2.6.4. Signal fading ...........................................................................................................13

2.6.5. Power control ..........................................................................................................14

3. RADIO NETWORK PLANNING FOR UMTS................................................................16

3.1. RADIO NETWORK PLANNING ENVIRONMENT .................................................................16

3.2. RADIO NETWORK PLANNING PROCESS FOR UMTS........................................................17

3.2.1. Dimensioning ..........................................................................................................18

3.2.2. Detailed planning....................................................................................................18

3.2.3. Optimization ............................................................................................................19

3.3. RADIO NETWORK PERFORMANCE INDICATORS ..............................................................20

3.3.1. Service probability ..................................................................................................20

3.3.2. Other-to-own cell interference ratio........................................................................20

3.3.3. Soft handover probability ........................................................................................21

4. WCDMA REPEATERS......................................................................................................22

4.1. INTRODUCTION TO WCDMA REPEATERS .....................................................................22

4.2. REPEATER EQUIPMENT..................................................................................................23

4.2.1. Overview..................................................................................................................23

4.2.2. Antennas..................................................................................................................24

4.2.3. Repeater hardware ..................................................................................................24

4.3. THERMAL NOISE IN REPEATER TRANSMISSION PATH .....................................................25

4.4. REPEATERS IN UMTS CELL ..........................................................................................27

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5. SIMULATIONS...................................................................................................................29

5.1. SIMULATIONS IN GENERAL............................................................................................29

5.2. SIMULATION TYPES.......................................................................................................29

5.2.1. Static simulations ....................................................................................................30

5.2.2. Dynamic simulations ...............................................................................................30

5.3. NPSW STATIC SIMULATOR ...........................................................................................31

5.4. NPSW REPEATER IMPLEMENTATION ............................................................................33

5.4.1. Repeater unit ...........................................................................................................33

5.4.2. Link loss and interference update............................................................................34

5.4.3. Effective noise figure update ...................................................................................36

5.4.4. Channel model update.............................................................................................36

5.4.5. Hotspot support for NPSW ......................................................................................37

5.4.6. Other NSPW updates...............................................................................................38

5.5. SIMULATION SCENARIOS ...............................................................................................39

5.5.1. Node B and repeater site configurations.................................................................39

5.5.2. Antenna configurations ...........................................................................................40

5.5.3. Radio network parameters ......................................................................................41

5.5.4. Traffic parameters...................................................................................................43

5.5.5. RRM parameters .....................................................................................................44

6. SIMULATION RESULTS..................................................................................................45

6.1. ANALYSIS OF SIMULATION OUTPUT DATA .....................................................................45

6.1.1. Service probability ..................................................................................................46

6.1.2. Uplink other-to-own cell interference ratio.............................................................47

6.1.3. UE transmit power ..................................................................................................49

6.1.4. Node B transmit power............................................................................................50

6.1.5. SHO statistics ..........................................................................................................51

6.1.6. Outage statistics ......................................................................................................54

6.1.7. Network capacity.....................................................................................................55

6.2. ERROR ANALYSIS ..........................................................................................................58

7. DISCUSSION AND CONCLUSIONS ...............................................................................59

REFERENCES..............................................................................................................................61

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Abstract

TAMPERE UNIVERSITY OF TECHNOLOGY

Degree program in Information Technology Institute of Communications Engineering Lähdekorpi, Panu: Effects of Repeaters on UMTS Network Performance Master of Science thesis, 72 p. Examiners: Professor Jukka Lempiäinen, M. Sc. Jarno Niemelä Funding: National Technology Agency of Finland (TEKES) Department of Information Technology February 2006 Performance of the mobile communication network will always be limited to technical capabilities of network equipment and properties of propagation environment. The main target of radio network planners is to create a mobile communication network that provides maximum performance with minimum implementation costs. Choosing correct tools and doing careful planning are the key methods in reaching this target.

WCDMA radio interface technique was chosen to be used in UMTS, the 3rd generation mobile communication system used in Europe. Due to the principle of CDMA systems, all users share the same frequency channel when communicating with network elements. Thus, interference levels in UMTS play a major role in defining both the system coverage and the capacity. Hence, interference levels should be kept minimum at all times to avoid network congestions.

These levels can be controlled in many ways – by careful base station site location selections or by using correct base station antenna configurations. This thesis studies air-to-air analog repeaters as a new way to increase the system capacity by reducing transmit powers, while keeping interference at acceptable level. The results show how a significant increase in UMTS capacity can be achieved by using repeaters in certain network configurations. These results also emphasize the importance of using correct repeater configurations and settings.

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Tiivistelmä

TAMPEREEN TEKNILLINEN YLIOPISTO

Tietotekniikan koulutusohjelma Tietoliikennetekniikan laitos Lähdekorpi, Panu: Toistimien vaikutus UMTS-verkon suorituskykyyn Diplomityö, 72 s. Tarkastajat: Professori Jukka Lempiäinen, DI Jarno Niemelä Rahoittaja: TEKES Tietotekniikan osasto Helmikuu 2006 Matkaviestinverkon suorituskykyä rajoittavat pääasiallisesti järjestelmän tekniset vaatimukset ja etenemisympäristön ominaisuudet. Radioverkkosuunnittelijoiden tärkein tavoite on suunnitella matkaviestinverkko, joka tarjoaa mahdollisimman hyvän suorituskyvyn pienimmillä mahdollisilla toteutuskustannuksilla. Oikeiden työkalujen valinta ja huolellinen suunnittelu takaavat hyvän tuloksen tämän tavoitteen toteuttamiseksi.

WCDMA-radiorajapintatekniikkaa käytetään UMTS:ssa, joka on valittu Euroopan kolmannen sukupolven matkaviestinjärjestelmäksi. CDMA-tekniikan perusominaisuuksista johtuen kaikki matkaviestinverkon käyttäjät käyttävät yhteistä taajuuskaistaa viestiessään verkossa. Tästä johtuen verkon kapasiteettia ja peittoa rajoittava tekijä onkin usein verkon häiriötaso. Verkon sisäiset häiriöt on pidettävä mahdollisimman pieninä, jotta vältyttäisiin matkaviestinverkon toimintahäiriöiltä.

Verkon häiriötasoihin voidaan vaikuttaa monella tavalla, kuten tukiasemapaikan valinnoilla tai antennien asetuksien valinnalla. Tämä diplomityö käsittelee ilmarajapintaa käyttäviä analogisia toistimia matkaviestinjärjestelmän kapasiteetin kasvattamisen välineenä. Diplomityössä tutkitaan toistimien kykyä pienentää lähetystehoja verkossa. Samalla tarkastellaan verkon häiriötasojen nousua. Tulokset viittaavat selvään UMTS-verkon kapasiteetin kasvuun tietyillä verkkokonfiguraatioilla toistimia käytettäessä. Nämä tulokset korostavat myös toistinasetuksien valinnan tärkeyttä verkon kokonaiskapasiteettia ajatellen.

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Abbreviations

1G First generation 2.5G Second and a half generation (2.5 generation) 2G Second generation 3G Third generation 3GPP Third Generation Partnership Project ADSL Asymmetric digital subscriber line AGC Automatic gain control AMPS Advanced mobile phone service BLER Block error rate BS Base station CDMA Code division multiple access CN Core network D-AMPS Digital AMPS DL Downlink DS-CDMA Direct sequence – CDMA EDGE Enhanced data rates for global evolution EDT Electrical downtilt EIRP Effective isotropic radiated power ETSI European Telecommunications Standards Institute FDD Frequency division duplex FDM Frequency division multiplexing FDMA Frequency division multiple access GPRS General packet radio service GPS Global positioning system GSM Global system for mobile communications HHO Hard handover HSDF Hotspot density factor HSDPA High speed downlink packet access IMT-2000 International Mobile Telecommunications 2000 ITU International Telecommunication Union LOS Line-of-sight MDT Mechanical downtilt ME Mobile equipment MRC Maximum ratio combining MSC Mobile switching center NMT Nordic mobile telephony NPSW Network planning strategies for wideband CDMA PSTN Public switched telephone network QoS Quality-of-service RAN Radio access network RNC Radio network controller RRM Radio resource management RSCP Received signal code power RX Reception SfHO Softer handover SGSN Serving GPRS support node SHO Soft handover

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SIR Signal-to-interference ratio SMS Short message service TDD Time division duplex TDM Time division multiplexing TDMA Time division multiple access TX Transmission UE User equipment UL Uplink UMTS Universal mobile telecommunications system USIM UMTS subscriber identity module UTRA Universal terrestrial radio access UTRAN UMTS terrestrial radio access network WCDMA Wideband CDMA VLR Visitor location register

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Symbols

η Cell load in uplink λ Signal wavelength

τ Mean delay

Φ Mean signal incident angle A Propagation model constant AHS Area size of a hotspot B Propagation model constant C Correction factor in propagation model C/I Carrier-to-interference ratio d Distance between transmitter and receiver D Overall user density in a network without hotspots dkm Distance in propagation model Eb/N0 Relation of bit energy to the noise spectral density EFB Effective noise figure of a Node B f1 Cell frequency FB Node B noise figure fc User signal fcarrier Carrier frequency fi Narrowband interference FR Repeater noise figure GA Node B antenna gain GBS Node B related losses GD Repeater donor antenna gain GUE UE related losses GR Repeater gain GREP Repeater related losses GRX Receiver antenna gain GS Repeater service antenna gain GT Repeater loss GTX Transmitter antenna gain hb Node B antenna height hm Mobile station antenna height HSDF Hotspot density factor Ioth Other cell interference power Iown Own cell interference power iDL Downlink other-to-own cell interference ratio iUL Uplink other-to-own cell interference ratio k Boltzmann’s constant LFS Free space loss LINT Path loss between a repeater and Node B (Okumura-Hata) Lp Path loss LP Repeater path loss LS Repeater service path loss N Noise power at Node B in case of empty cell NiB Noise power at the input of Node B NO Noise power at the output of Node B NTH Thermal noise density

x

P(Φ) Angular power distribution PΦ_TOT Total power of a signal Pconn_1 Probability of users having only single connection to network PG Processing gain PRX Received power PSHO Soft handover probability PTX Transmitted power P(τ ) Delay profile Px_TOT Total power of a signal Rchip Chip rate Ruser User data rate SΦ Angular spread Sτ Delay spread SF Spreading factor SiB Signal power at the input of Node B SO Signal power at the output of Node B SP Service probability T Noise temperature of a component TaB Antenna noise temperature at the Node B antenna TaR Antenna noise temperature at the repeater service antenna TeB Inherent noise temperature of the Node B TeR Inherent noise temperature of the repeater THS Number of users in a hotspot Ueta Estimated load of a cell Umax Maximum allowable load in a cell W Signal bandwidth Wc Bandwidth of user signal Wi Bandwidth of narrowband interference

1. Introduction 1

1. Introduction

The time of telecommunication started in the end of the 19th century when the first successful telephone call was made using only simple equipment and a piece of wire. The second major step in the history of telecommunication was taken in the late 20th century when the technological development made wireless communication possible. The improvements in the field of electrical and chemical industry enabled the production of smaller electric circuits and enhanced batteries, leading to an idea of portable mobile phone. The interest towards the wireless communication has increased ever since, and recently wireless networks have become a part of everyday life in global mobile communication and also in local area networking business.

In Europe, the step from analog to digital mobile communication was taken in 1991, when the first GSM (global system for mobile communications) network was commercially launched by a Finnish company. GSM was designed to be a globally used network technology. Fast growth and widespread use of packet switched Internet had created a situation of two parallel global networks. Therefore, some kind of interconnection between these two was needed. The first solution to this was GPRS (general packet radio service), which, however, suffered from the limitations of the GSM radio interface. The constant data bit rate of 8 kilobits per second was enough to offer adequate quality of speech connections. However, a common need for more sophisticated mobile applications lead to a problem of insufficient connection data rates. New techniques were needed to fulfill these packet data transmission capacity requirements. UMTS (universal mobile telecommunications system) in the field of digital mobile communication tries to do the same as the ADSL (asymmetric digital subscriber line) has already done in the field of wired communication – to blow the markets with new, modern, high speed, multi-purpose, and flexible network solution, which brings the Internet within everybody’s reach.

During the time when the world celebrated the change of the new millenium, UMTS was under development and finishing. CDMA (code division multiple access) was chosen to be the radio access technique of this new mobile communication system. With the help of CDMA, the data rates could be tuned to much higher than in the GSM system. However, CDMA radio access technique introduced some new challenges to network planners. In GSM, each cell of the network was separated from the others by using different carrier frequency. In UMTS, the principle of using common frequency band in all cells of the network requires radio network planners to be extra careful. The capacity and coverage of the radio access network must be planned together due to the presence of simultaneuos interference from the network’s users.

1. Introduction 2

After a successful launch of a UMTS network, an optimization phase is needed to assure the best possible network functioning. There are many ways to further optimize the network: adjusting network parameters to give better operation of the network and downtilting of antennas to increase the cell isolation, just to name a few.

The main purpose of this thesis is to study the use of WCDMA (wideband CDMA) repeaters as a way to optimize UMTS network operation in the means of increasing the network capacity. It is shown that WCDMA repeaters have a significant impact on the interference levels in UMTS network. The studies for this thesis are carried out by performing static Monte Carlo simulations by using repeaters as a part of the network topology to see what should be taken into account when designing UMTS networks with repeaters. Different repeater configurations with variable traffic cases are simulated in order to find a general rule for the design.

This thesis consists of seven chapters. General overview to the properties of UMTS network is presented in Chapter 2. Chapter 3 describes the radio network planning process for UMTS. WCDMA repeaters are introduced along with their properties and theory behind them in Chapter 4. Chapter 5 goes through the simulations and explains the motivation behind them. Moreover, the simulation scenarios for the repeater studies are described. Chapter 6 presents the simulation results and points out the key issues observed. In the last chapter, the results are concluded and discussion is made to give some ideas for additional studies within the topic.

2. Introduction to UMTS

3


2.1. Evolution path

The time of mobile communications can be separated into generations of different techniques. The era of mobile communication systems started from the time of analog data transmission. NMT (nordic mobile telephony) and AMPS (advanced mobile phone service) were the most popular 1G (first generation) systems. NMT was used widely in Europe, while the AMPS was commonly used in the United States. These techniques were used in years 1980-2000 when the digital communication was still making its way to the public markets. [1], [2]

Digital techniques in mobile communication business were taken into use in Europe at the beginning of 1990’s by introducing GSM. It was purely based on digital transmission. The GSM enabled many new beneficial properties, such as data encryption and SMS (short message service). The GSM was based on TDMA (time division multiple access) and FDMA (frequency division multiple access). In TDMA, the data for each user is multiplexed in short consecutive time slots, whereas in FDMA the data between users is divided into narrow subfrequency bands. The cellular network architecture along with TDMA and FDMA concepts lead to improved capacity of the network. D-AMPS (digital AMPS) was the corresponding digital system used in the United States by that time. These systems are often referred as the 2G (second generation) mobile communication systems and are used by billions of people in the world today. [1], [2]

The limited capacity of 2G systems and the never-ending need for higher data rates are the main reasons for developing next generation mobile communication systems. The aim is to provide multi-purpose functionality with higher data rates. Some improvements like GPRS and EDGE (enhanced data rates for global evolution) were developed to repair the bottlenecks of the existing 2G systems. These are often called as 2.5G techniques. GPRS enables the connectivity to the packet networks and the EDGE provides higher data rates for the GPRS. However, the current solutions to the problem are UMTS and CDMA2000, techniques for the 3G (third generation) mobile communication systems. Data transmission rates up to 384 kbps are provided at regular urban outdoor environments for the UMTS. CDMA is used as a radio transmission technique to improve the network spectral efficiency. The first 3G networks are already launched to public use in many areas but the transition period from 2G to 3G seems to be quite a slow process. Plans for the time after 3G are still very open and any particular 4G technique does not exist yet. However, some upgrades to 3G systems are already defined. One of these also called 3.5G techniques is HSDPA (high speed downlink packet access), which is supposed to give even higher data rates in downlink direction. [2]


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2.2. Standardization

The development of the third generation mobile communication systems was initially started by an international organisation, ITU (International Telecommunication Union). ITU started the process of defining the standard for 3G systems, referred to as IMT-2000 (International Mobile Telecommunications 2000). In Europe, ETSI (European Telecommunications Standards Institute) was responsible of UMTS standardization process. In 1998, the 3GPP (Third Generation Partnership Project) was formed to continue the technical specification work for 3G systems. 3GPP still acts as an leading organization in producing standards for UMTS. [1], [2]

Also another project was launched in the early days of 3G system development. It ended up to a very similar solution, called CDMA2000, which has gained its popularity mainly in the United States. This system has many similar properties with UMTS but they are, however, not compatible with each other. The standardization of 3G systems still continues as new additional techniques are developed to improve the performance of 3G networks. [1], [2]

2.3. UMTS architecture

The UMTS standard includes two different air interface duplexing schemes: UTRA FDD (universal terrestrial radio access frequency division duplex) and UTRA TDD (UTRA time division duplex). In UTRA FDD, uplink and downlink channels are separated in frequency-paired channels. UTRA TDD fits uplink and downlink data to the same frequency range and separates them in time domain using TDM unpaired channels. Table 2.1 presents the frequency allocation for the UMTS FDD and TDD in Europe. In this document, the word UMTS relates only to the UTRA FDD version. [3]

Table 2.1: Universal frequency allocation for UMTS. [3]

Uplink [MHz] Downlink [MHz] Total [MHz]

UMTS-FDD 1920-1980 2110-2170 2⋅60 UMTS-TDD 1900-1920 2010-2025 20+15

UMTS has three functional parts: a group of UEs (user equipments), UTRAN (UMTS terrestrial radio access network) and CN (core network). UE is the mobile user equipment that uses the services of the network. UTRAN includes all radio-related functionality and it works as a connection point between the core network and UE. Core network handles call switching and routing along with connections to external networks such as PSTN (public switched telephone network) or the Internet. Figure 2.1 presents the essential entities from each of the three parts and the interfaces between them. [3], [4]


5

USIM

MENODE B

NODE B

NODE B

NODE B

RNC

RNC

SGSN

MSC/VLR PSTN

Internet

UE UTRAN CNExternalnetworks

Uu IuIub

Iur

Figure 2.1: UMTS system architecture. [3]

UE consists of ME (mobile equipment) and USIM (UMTS subscriber identity module). ME is the module which includes most of the radio functionalities of a mobile phone. USIM is an external replaceable information chip card that includes subscriber information and the keys used in data encryption. Uu-interface is the radio air interface that connects the UE to the UTRAN.

UTRAN can also be divided into two parts. Node B unit is located in a BS (base station) site. The main function of a Node B is to perform low level data processing to prepare the data for air interface transmission via Uu-interface. Iub-interface is used for the Node Bs to connect to the RNC (radio network controller), which works as service access point from the UTRAN to the core network.

Node B handles the common tasks of a digital transmission system, such as: channel coding, interleaving and modulation. It also handles some basic radio resource control tasks such as inner-loop power control, which is described in more detail later in this chapter. RNC is responsible for the higher level tasks of radio resource management such as: load control, admission control, congestion control and code allocation.

Core network has two important entities which are similar to the ones already used in previous network evolution phases. MSC/VLR (mobile services switching center/visitor location register) is a unit that takes care of call switching issues. It also includes information database of all UEs registered in the network together with user location information. SGSN is an abbreviation for serving GPRS support node, which enables the packet access between UMTS and the Internet. SGSN have very similar properties as the MSC/VLR but is used only with packet data services. RNCs communicate with the core network via the Iu-interface. Different RNCs can communicate with each other by using the Iur-interface. [3]

2.4. WCDMA and spread spectrum systems

The second generation mobile communication systems use TDMA and FDMA to provide the multi-access scheme for the users of the network. In FDMA, the available frequencies are divided into narrow subfrequencies so that different frequency slots are allocated for each user. TDMA uses same frequencies for each


6

user, but divides the channel into time slots. Each user has its own time slot reserved for the transmission. These methods are called narrowband in sense of trying to minimize the resources allocated for one user. They suffer from limitations in capacity, because inefficiently designed multiple access system is limiting the number of simultaneous users of the common communication channel.

Completely new approach is introduced in 3G systems. A wideband DS-CDMA (direct sequence - code division multiple access) is used as multiple access method in UMTS. This subchapter will explain the most important properties of wideband CDMA, method, which is introduced in most of the third generation mobile communication systems in the world.

The key idea of WCDMA is to let all users to share the same wideband communication channel in the whole network simultaneously by using spread spectrum signals. In this approach, each user is assigned a unique code sequence that is used to spread the information signal on this common channel. Figure 2.2 presents the basic block diagram of WCDMA transmission path.

SpreadingModulation CHANNEL Despreading Demodulation

Noise & Interference

Figure 2.2: Transmission path in WCDMA spread spectrum system. [5]

The main contributor of noise in Figure 2.2 is the thermal noise, which is added to the transmitted signal in every active circuitry in the transmission path of the signal. Interference originating from other users in the network can be included under the noise term, since interference is considered as wideband in DS-CDMA systems due to the spreading operation.

In direct sequence spread spectrum systems, signal spreading is carried out by modulating (multiplying) the data-modulated signal for the second time by a wideband spreading signal as Figure 2.3 presents [5].

+1

-1

+1

-1

+1

-1

Data

Code

Data x Code

f

f

Chip

Figure 2.3: Signal spreading process in case of spreading factor 6. [5]

A parameter, spreading factor, defines how many chips (spreading code symbols) are used to represent one user data bit. Thus, it is the ratio of the chip rate Rchip and the user data rate Ruser:


7

Another way to understand the effects of spreading phenomenon is to use the definition of processing gain. The term processing gain is defined as a ratio of transmitted wideband signal bandwidth (Wc) and user information signal bandwidth (Wi) in decibel scale:

In UMTS, the chip rate is kept constant and thus the processing gain only depends on the user data rate. The higher the user data rate, the lower the processing gain and the spreading factor. Thus, it is harder for the receiver to detect the signal correctly.

Auto correlation and cross correlation properties of the spreading codes provide orthogonality. This makes it possible to separate signals from each user in the receiving end. Code orthogonality maximises the active user capacity if the received signals are in phase. The spread signal propagates through the transmission medium with possibly introduced noise and interference. In the receiving end, the signal is despread and demodulated to obtain the original information signal. The information concerning the used codes is provided to the receiving entity by the transmitting entity before successful data transfer can take place. [5]

Along with being a method for enabling the CDMA scheme, signal spreading has also other important properties. Signal spreading provides high tolerancy to narrowband interference as depicted in Figure 2.4. In the despreading process, the narrowband interference (around frequency fi) spreads while the transmitted user signal (around frequency fc) despreads due to the multiplication operation. This makes the CDMA technique a valuable solution, e.g., for military purposes due to good protection against external radio jamming attacks. [5], [6]

fc fi fifc

Despreading

ff

Wc

Wi

Transmittedsignal

Interference

Transmittedsignal

Interference

Amplitude Amplitude

Figure 2.4: Despreading process in the presence of narrowband interference. [5]

user

chip

R

RSF = . (1)

i

c

W

WPG 10log10 ⋅= . [5] (2)


8

2.5. Cellular concept and mobility in UMTS networks

2.5.1. The idea of cellularity

Mobile communication radio networks are commonly designed to be cellular, where Node Bs provide services for certain coverage areas as in Figure 2.5. Each of these coverage areas is called a cell. Cellular concept is needed to achieve adequate user capacity over the whole network with reasonable transmit power levels. A system with several Node Bs is good also in the sense of providing better coverage in challenging terrestrial conditions, such as in shadowing hilly conditions with varying terrain height. Cellularity is used in GSM to improve the network capacity by reusing available frequencies. Same frequencies can be used in several cells if the distance between these cells is sufficient to quarantee low enough interference between the cells. UMTS does not benefit from this idea due to the use of common frequency channel over the whole network. Hence, UMTS network is typically interference limited where users and neighbour Node Bs interfere with each other in uplink and downlink directions. This other cell interference is causing less trouble in CDMA-based networks (such as UMTS) due to the code orthogonality properties. [1]

f1

f1

GSM UMTS

f1

A cell

f1

f1

f1

f1

f1

f1

f1 f1

f1

f1

f1

f1

f1

Frequencyreuse

f2

f3

f4

f1

f3

f3f3

f4

f4

f2

f2

f1

Figure 2.5: Cellular concept and the difference in frequency planning between GSM and UMTS.

[1]

The idea of cellularity raises a need for users to change cell or network when moving in the network area in UMTS. These user mobility issues are controlled by network-based events called handovers. Although handover events are network-based, UE measures which cells are heard with highest signal strength at each time moment by listening so called pilot channels, broadcasted by all Node Bs with constant power. UE then informs RNC, when radio conditions are favorable to start the handover event.

While 3G CDMA systems are becoming more popular, the coexistence of two digital mobile communication systems seems to be unavoidable. BS sites usually have support for both techniques, GSM and UMTS, simultaneously, under same site. Thus, inter-system handovers are needed between GSM and UMTS for user to change the current network, along with basic intra-system handovers.


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2.5.2. Handovers in UMTS

RNC

Node B1 Node B

2

Figure 2.6: User in a soft handover.

Figure 2.6 depicts a soft handover situation. Handover event is transparent to the mobile phone user in GSM as well as in UMTS networks. It ensures continuous communication between network and user. When user moves towards the border of one cell’s coverage area, the signal level is going down and the risk of having a drop call increases due to increased link losses and the limitations of transmit power levels in uplink and in downlink. This situation is handled by letting the network to start a process for changing the serving cell to a one that offers better signal quality to the user at that certain time moment. UMTS includes support for several types of intra-system handover situations. The most important ones are: HHO (hard handover), SHO (soft handover) and SfHO (softer handover). Inter-system handovers are hard handovers. [3], [5]

User is simultaneously connected to only one cell during HHO process. When performing a HHO, a connection to a weaker cell is dropped and a connection to a stronger cell is established at certain time moment determined by RNC. This introduces a short connection break in the radio bearer. The break in user connection is usually avoided by buffering some data before the handover process. [5]

In SHO, user can be simultaneously connected to two or more cells under the same or a different RNC in the network area. SHO areas are located in the cell boundaries where multiple cells’ pilot signal levels are in small range. In uplink, the data is combined at the RNC to be sent to the core network. In downlink the data is combined at the UE. In SHO connections, the signals from the strongest cells are selected, when combining the data. In certain situations SHO introduces gain (macro diversity gain), since the same signal is sent and received via several different radio channels. The gain can be seen as reduced uplink and downlink transmit powers. However in downlink, transmit power is needed from several base stations to carry out the handover connections. This can turn into a loss in downlink capacity in system level. Typically 20 % - 40 % of all connections are SHO connections. [3], [5]

SfHO is a handover where user is connected simultaneously to two adjacent cells under same BS site. In this case, the data combining takes place at the Node B. [3], [5]


10

2.6. Radio propagation in UMTS

2.6.1. Propagation environment

In mobile communication systems, the propagation environment is defined as a space, where radio waves travel towards the target receiver antenna. Different types of propagation environment are defined to correspond certain types of terrain and network infrastructure. The definitions for different propagation environment types are shown in Figure 2.7.

OUTDOOR INDOOR

Macro-cellular

Micro-cellular

Pico-cellular

Figure 2.7: Propagation environment types.

Environment is called macrocellular if the average Node B antenna height is above the average rooftop level of the buildings in the area. Outdoor macrocellular environment can be classified to an urban, suburban and rural areas depending on the obstacle density of the area. Outdoor microcellular environment is usually deployed in dense urban areas with lots of commercial buildings and high user density, where the antennas are deployed below the average rooftop level. Mobile communication networks can also be extended to cover isolated indoor environment, such as buildings or subway halls, by installing Node Bs and antennas inside buildings to avoid the network capacity loss due to heavy attenuations from walls. [1]

When designing basic radio link systems, the free-space propagation model is used, where the signal attenuation depends only on the distance and used frequency. This model is given in the form:

where GTX is the transmitter antenna gain, GRX is the receiver antenna gain, λ is the wavelength of the signal and d is the distance between transmitter and receiver [7]. However in modern mobile communication systems, the directivity of the used antennas is varying and obstacles between communicating entities exist. Thus, more sophisticated propagation models must be used to provide better accuracy for planning the coverage.

2.6.2. Okumura-Hata propagation model

Still the most extensively used propagation model for mobile communications is so-called Okumura-Hata model, which is based on Mr. Okumura’s field measurements in Japan in 1968 [8]. Later on these measurements were fitted into

2

4

==

dGG

P

PL RXTX

TX

RXFS

π

λ, (3)


11

a mathematical form by Mr. Hata in 1980 [9]. A COST-231 program was later launched by European Union to study the radio propagation for 3G systems [10]. One result from this program was the extension for the original Okumura-Hata propagation model to support frequencies up to 2000 MHz. This COST-231 expansion gives the possibility to use the Okumura-Hata model also for UMTS operating frequencies at adequate accuracy. The results originated from Mr. Okumura’s measurements in Tokyo are widely used in radio network planning tools to model radio wave propagation. The basic path loss model equation made by Hata from Okumura’s measurements is of the following form:

where the term a(hm) is a correction factor for mobile antenna height [9].

The following equation is tuned Okumura-Hata propagation model from the COST-231 program:

which demonstrates the changes from the original model (Equation (4)). The definitions for common parameter values for Equations (4) and (5) are listed in Tables 2.2 and 2.3. [11]

Table 2.2: COST-231 propagation model parameter description.

Parameter Description

Cm Area correction factor, Cm<0 for rural, Cm>0 for urban dkm Distance fcarrier Carrier frequency hb Node B antenna height hm Mobile station antenna height

Table 2.3: Frequency dependent constant value definitions for COST-231 equation. [11]

fcarrier 150-1000 MHz 1500-2000MHz

A 69.55 46.3 B 26.16 33.9

2.6.3. Multipath environment

Environment, where the received radio signal consists of several reflected, diffracted, and attenuated components of the original signal, is called a multipath environment. An example of multipath environment is shown in Figure 2.8. Reflections, diffractions, and attenuations can happen due to obstacles, such as buildings, water surface, foliage, or mountains. When receiving the signal, each signal component (multipath component) may have different amplitude and phase due to the phenomena of the multipath environment described before. Signal components may have traveled different paths with differing path lengths and thus

( )( ) ,loglog55.69.44

log82.13

log16.2655.69

1010

10

10

kmb

mb

carrierP

dh

hah

fL

−+

−−

⋅+=

(4)

( )( ) ,loglog55.69.44

log82.13log

1010

1010

mkmb

mbcarrierP

Cdh

hahfBAL

+−+

−−⋅+= (5)


12

be delayed at the reception. Figure 2.9 presents an example impulse response of a multipath radio channel. [11]

Reflection

DirectDiffra

ction

Figure 2.8: An example of multipath environment. [11]

t

Amplitude

Figure 2.9: An example impulse response in multipath environment. [11]

Some parameters are used to define the properties of a propagation environment. Angular spread describes the deviation of the received signal incident angle. It can be calculated using equation:

where Φ is the mean incident angle, ( )ΦP is the angular power distribution, and

TOTP _Φ is the total power [11]. Delay spread describes the signal power as a

function of delay. It can be calculated using equation:

where ( )ττP is the delay profile, τ is the mean delay and TOTP _τ is the total

power. [11]

( ) ( )∫+Φ

−Φ Φ

Φ ΦΦ

Φ−Φ=180

180 _

2d

P

PS

TOT

, (6)

( ) ( )

TOTP

dP

S_

0

2

τ

τ

τ

ττττ∫∞

−

= , (7)


13

Due to the properties of multipath environment, a new receiver architecure is introduced in UMTS to further improve the efficiency of the radio access network. A RAKE receiver is a receiver, which gathers and combines information from signal components arriving at least one chip duration apart. Figure 2.10 shows the block diagram of the RAKE receiver. [12]

CodeGenerator

Modulator

1 a1

a2

a3

Multipath channel

Demodulator2

3

a1

a2

a3

c t( - t )1

c t( - t )2

c t( - t )3

RAKE receiver

Binarydata

Figure 2.10: Principle of a RAKE receiver. [12]

In radio transmission path of UMTS, signal enters the multipath channel after spreading and modulation. In RAKE receiver architecture, multipath channel is modeled by using tapped delay lines with delays τ and attenuations a. Each signal component travels through a propagation path with its own delay and attenuation value. Received sum of multipath components is then demodulated and taken to a RAKE receiver. The receiver consists of fixed number of fingers. In each finger,

the signal is despread in a correlator with corrected attenuation ( c ) and delay factors (τ) estimated from the measured tapped line delay profile. Finally, the despread signal components are combined by using a certain algorithm, e.g. MRC (maximum ratio combining). Due to changes in multipath environment, RAKE fingers must be continuously reallocated to synchronize to new delay and attenuation profile for successful receiver operation. [12]

2.6.4. Signal fading

In wideband systems, such as UMTS, multipath environment causes short-term frequency selective variation in received signal strengths due to the fact that signals at different frequencies arrive with different amplitudes and phases as mentioned before. This fast fluctuation in the signal levels is called fast fading. [11]

Long-term slow fading, or log-normal fading is slow fluctuation of average received signal levels. Slow fading is caused by obstacles, such as buildings. Figure 2.11 makes the distinction between fast and slow fading. [11]


14

fast fading

slow fading

Amplitude[dBm]

t Figure 2.11: Fast fading and slow fading. [7]

2.6.5. Power control

The use of CDMA technique as radio access method in UMTS leads to a situation in which the users in the network will share the same frequency band when communicating with the network. In downlink, interference is always applied among users due to the imperfect code orthogonality. Hence, the transmit powers of the users in uplink and Node Bs in downlink must be controlled in order to reduce interference levels. This is a very crucial issue in the sense of network operation, especially in uplink direction. Without controlling transmit powers, a user near the Node B could block another user that is located at the cell edge by using too high transmit power (near-far effect). [3]

Power control is also important in downlink direction in order to achieve optimal cell capacity due to the fact that the users in a cell will share the total transmit power capability of the Node B hardware. If one cell user is receiving data with too high transmit power, it will leave less power to the rest of the users in the cell, thereby, reducing the downlink capacity of the cell. In addition to this, users located at the cell edge would experience higher inter-cell interference levels without power control than users located near Node Bs. The target of the power control is to keep the transmit powers at minimum level while still offering adequate signal levels at the receiving end [5]. Moreover, the power control tries to help the system to correctly detect signals originating from the source (Node B or UE) in changing radio conditions. Changing radio conditions are usually caused by user movement, some instant obstacle in the radio link path or by random noise. [4], [5]

The power control concept is divided into different parts in UMTS. The major power control parts for the UMTS are: open-loop power control, inner-loop power control (also called fast closed-loop power control) and outer-loop power control. Transmit powers in both directions (uplink and downlink) are controlled separately in each part. Figure 2.12 presents fundamental parts of the power control function.


15

Inner-loop power control

Outer-loop power control

RNCNode B

Open-loop power control

Figure 2.12: UMTS power control functions.

Open-loop power control is used in UMTS to provide rough estimate of the initial transmit power setting only when user is making the first connection attempt to network. The estimation process is based on the downlink path loss estimation. It can be done by measuring the RSCP (received signal code power) of the cell pilot signal and interpreting acceptable pilot transmit powers. Downlink path loss estimate can be used also in uplink direction since the difference between uplink and downlink frequencies is relatively small and thus the correlation between average path losses is high. [3], [5]

Inner-loop power control is used between the user and the Node B to compensate fast fluctuations of the radio channel. Furthermore, it helps in eliminating the near-far effect in uplink. Due to these properties, inner-loop power control is perhaps the most important feature in UMTS operation. In uplink direction, the Node B makes frequent estimates of the received SIR (signal-to-interference ratio) and compares it to the SIR target value set by the RNC. Depending on the result of this comparison, the Node B commands user to either increase or decrease transmit power. The event is repeated at the rate of 1500 times in a second, and is operating also in downlink direction. The high estimation frequency provides complete prevention of imbalance in received signal powers at Node B. [3], [4], [5]

Problematic areas for the inner-loop power control are cell edges. Near cell edges, users are often transmitting continuously with full power. This causes problems in sense of inner-loop power control in uplink, because the UE can not anymore respond to the power-up commands due to the limitations in transmission power capacity. This should be taken into account when planning coverage, by introducing a power control headroom margin, which quarantees the continuous service for users in the cell edges. [1]

In outer-loop power control, SIR target is adjusted for each user to guarantee sufficient quality of the connection. The adjusting is done by measuring BLER (block error rate) for each user at the RNC. SIR target should be increased if the BLER increases above an acceptable level. Otherwise it should remain as low as possible to keep transmit powers at a low level. [4], [5]

3. Radio network planning for UMTS

16


Radio network planning is a continuous process. In the beginning of the radio network planning process, first plans of new radio network are made. Actual planning continues, while the network is in full operation – to further optimize and monitor the network. The planning process can be restarted if changes in network configuration must be made. This chapter introduces the environment of radio network planning. Planning tools and methods are explained to give basic understanding of the work of radio network planner. Also the actual planning process for UMTS networks is described for better understanding the evolution of an UMTS network: from planners’ papers to the actual implemented network architecture.

3.1. Radio network planning environment

Radio network planning is not only made with paper and pencil. Many supportative tools are required in order to succesfully provide a plan for a functional radio network. Sophisticated software and hardware is used for accurate planning of the network. High detail digital maps are used for coverage predictions. Network simulator software is used, e.g., for maximum capacity estimations.

Simulations are mainly done to simulate the maximum capacity of the network. In simulations, planned BS site locations, user traffic distributions, and other network specific parameters are given to a simulator as an input. After simulations, information of several parameters, such as required Node B and UE transmit powers and statistics from dropped users can be observed. Simulation results can then be analyzed and used in planning and optimizing the real network. Simulations are usually done in parallel to actual planning, i.e., when the real network does not exist yet. However, simulations can also be performed after launching the network – to simulate how to improve the network.

Field measurements are mostly performed when planned coverage and propagation channel charasteristics are examined in an already operational network. Due to the use of common frequency in UMTS, field measurements can also be applied to study soft handover areas and network interference. A digital transceiver unit and some positioning equipment are needed in order to perform accurate field measurements. Typical field measurement set includes a vehicle with GPS (global positioning system) receiver installed to track the position in the pre-defined route. Then, the measurements can be performed by driving through certain measurement routes with a mobile station (mobile phone) connected to a laptop computer with measurement software installed. Mobile phone measures the air interface and sends measurement data to the laptop. Data is gathered continuously during a measurement run.


17

Also some external software is needed for the network management system to gather statistical data from the crucial operating network elements after launching the network. The data from all these subsections are gathered to a planning tool software, which is used to process and analyze the gathered data, and thus, to make the work of radio network planner more easy. Figure 3.1 shows the planning environment.

RNC

RNC

SGSN

MSC/VLR

NODE B

NODE B

NODE B

NODE B

Planning tool + simulator

Field measurements

Data from the network

Figure 3.1: Radio network planning environment.

3.2. Radio network planning process for UMTS

Before launching a mobile communication network, careful planning must be done to ensure correct and optimized operation of such a network. Planning process of 2G systems has usually been divided into three sections: network dimensioning, detailed planning, and network optimization. The planning process for 3G networks follows the same basic rule with some exceptions described later in this chapter. Figure 3.2 shows a high layer definition of the planning process for UMTS. The configuration of the radio access network as well as of the core network must be planned together. However, this document concentrates only to the radio access network side.

DIMENSIONINGDETAILEDPLANNING

OPTIMIZATION

- Network layout- Network elements- Antenna heights

- Configuration- Topology- Code- Parameter

- Monitoring- Verification- Parameteroptimization

Figure 3.2: Radio network planning process for UMTS.


18

3.2.1. Dimensioning

When dimensioning RAN, the planner draws some rough guidelines by performing initial coverage analysis and capacity estimation and by taking into account the traffic estimates in that particular geographical area. A rough number of needed BS sites must be estimated. Antenna heights must also be decided in this phase to make propagation channel calculations in the detailed planning phase possible. Also attention must be paid to QoS (quality-of-service) issues. [1], [5]

3.2.2. Detailed planning

Detailed planning phase in UMTS is very similar to the dimensioning phase, but instead of using estimations, actual data is used in planning. Detailed planning phase for UMTS consists of configuration planning, topology planning, code planning and parameter planning sections.

Configuration planning includes designing hardware configurations, such as antennas and cables, in Node B sites. Moreover, power budgets (also called link budgets) are calculated in uplink and downlink directions for different service speeds to achieve balanced communication in both directions. Power budgets include information of different gains and losses in the communication path of a radio link. Cell ranges can be approximated from the power budget results by using a propagation model (see Equations (4) and (5)). Table 3.1 shows an example of unbalanced UMTS power budget.

Table 3.1: An example of UMTS power budget. [1]

Speech Data Parameter

DL UL DL UL Units

Bit rate 12.2 12.2 384 64 kbps Load 50 50 75 30 % Thermal noise density -173.93 -173.93 -173.93 -173.93 dBm Receiver noise figure 8 4 8 4 dB Noise power at receiver -100.13 -104.13 -100.13 -104.13 dBm Interference margin 3.01 3.01 6.02 1.55 dB Total noise power at receiver -97.12 -101.12 -94.11 -102.58 dBm Processing gain 24.98 24.98 10 17.78 dB Required Eb/N0 7 5 1.5 2.5 dB Receiver sensitivity -115.10 -121.10 -102.61 -117.86 dBm

RX antenna gain 0 18 0 18 dBi Cable loss/body loss 2 5 2 5 dB Soft handover diversity gain 3 2 3 2 dB Power control headroom 0 3 0 3 dB Required signal level -116.10 -133.10 -103.61 -129.86 dBm

TX power per connection 33 21 37 21 dBm Cable loss/body loss 5 2 5 2 dB TX antenna gain 18 0 18 0 dBi Peak EIRP 46 19 50 19 dBm Maximum allowed path loss 162.10 152.10 153.61 148.86 dBm

In the power budget (Table 3.1), a difference of ten decibels exists in both directions in maximum allowed path loss when using speech service. These


19

differences can be balanced, e.g., by using low noise amplifiers or reception diversity methods in uplink direction. Also the effects of different service speeds and network loads to the maximum allowed path loss value are seen from Table 3.1.

In normal operation state of UMTS network, cell ranges are changing along with the variations in network loading. This happens, because all users are using the same frequency to communicate. When the traffic density in the network increases, the sensitivity of a Node B decreases due to increased interference, and thus the user will need more power to get connected to that particular cell. This variation in cell ranges is called cell breathing and must be taken into account when planning the cell coverage. The cell range variation is handled in the power budget by introducing a parameter called ‘interference margin’ (see Table 3.1).

Detailed network planning in UMTS is quite different when compared to 2G systems. In UMTS, coverage and capacity planning can not be done separately due to cell breathing phenomenon, but they are combined to a planning phase called topology planning.

Topology planning consists of coverage and capacity planning as mentioned above. In coverage planning, accurate cell ranges are calculated from power budget calculations provided in configuration planning phase. The support for different service speeds must be taken into account when calculating the coverage. When planning outdoor coverage, cell coverage areas are made to overlap excessively to give high service probability to indoor users at the cell edges [1]. Moreover, the use of soft handover provides more capacity.

Field measurements can be performed to get some concrete data, thereby helping in planning the coverage and in tuning the propagation model. System level network simulations are used in capacity planning to estimate maximum network load and to tune the network parameters to optimal settings. Simulations are explained in more detail later in Chapter 5.

Before the network can be launched, code and parameter planning are needed. Certain amount of orthogonal codes are allocated for each cell to separate users in downlink direction. In addition, parameters for signaling and radio resource management tasks should be set to optimal values. Such are, e.g., handover and power control parameters. After launching the network, these parameters can be optimized based on the needs of radio propagation environment. [1]

Main tools for the detailed planning phase are: computer with simulator software and equipment for the field measurements. After detailed planning phase, the network can be launched.

3.2.3. Optimization

The radio network planning process continues with the optimization phase, where the network is monitored by gathering statistical data from the operating network management system and used together with further measurements and simulations to fine tune the network parameters when needed. Also verificative operations are performed by testing, e.g., call establishment and different handover mode


20

connections. Coverage and dominance areas are verified by performing field measurements. Main tools for the optimization phase are: simulator software and field measurement tools.

When the network is fully operational and some changes in the network area happens, new BS sites for increased coverage or new hardware installations for increased capacity usually requires a restart of the ongoing planning process. To avoid going through an additional planning process, use of WCDMA repeaters is a one choice to give both the coverage and capacity increase in an easy and cost-efficient way. The rest of the chapters in this document concentrate on WCDMA repeaters as a solution in increasing network capacity.

3.3. Radio network performance indicators

Some indicators are defined to give information about the UMTS network performance. Some of them can be achieved from the network simulations before launching the actual network, while others can be percieved from the network measurements after network launch. However, all these indicators describe the current performance of the planned network. Some important indicators for the studies in this document are described shortly to understand the analysis performed in the following chapters in this document.

3.3.1. Service probability

Service probability gives the relationship between successfully served users and dropped users in system level. Service probability is defined by the equation:

where Served_users presents the number of successfully served users in the whole network area and All_users presents the number of total users in the whole network area. Typical target values for service probabilities are close to 100 %, since user blocking must be avoided in order to provide high quality mobile network service to the customers.

3.3.2. Other-to-own cell interference ratio

A performance indicator, ‘uplink other-to-own cell interference ratio’ (also called ‘uplink little i’), describes the relation between other cell interference and own cell interference in one cell:

where Iown is the total interference power originating from own cell users and Ioth is the corresponding total other cell interference power. In case of repeaters, users in the cell’s own repeater service area are taken as own cell users.

In uplink, iUL is a Node B specific parameter as it is calculated for each Node B receiver. Thus, iUL affects all connections of one cell similarly. Other-to-own cell

usersAll

usersServedSP

_

_= , (8)

own

oth

ULI

Ii = , (9)


21

interference ratio can be calculated also for downlink connections (iDL). However in downlink, iDL is UE specific parameter as it is calculated for each UE separately. Other-to-own cell interference ratio is one of the main issues affecting the capacity of a CDMA radio network. Typical values for iUL range from 0.15 (very well isolated cells) to 1.2 (poor radio network planning). [5]

3.3.3. Soft handover probability

Several definitions exist for the soft handover probability. In this document, the soft handover probability describes how many users, compared to the number of all users, are using a soft handover connection [5]. It can be mathematically expressed by the equation:

where Pconn_1 is the probability of a user having only one connection (i.e., is not in a soft handover situation) at some particular static time moment. Equation (10) is used in this form, since the number of users with only one connection is an essential parameter as shown later.

1_1 connSHO PP −= , (10)

4. WCDMA repeaters

22

4. WCDMA repeaters

In this document, the word ‘repeater’ is defined to mean an air-to-air analog repeater unit (with all antennas and cables included), which is designed to be used only in outdoor WCDMA mobile communication networks. This chapter introduces the repeater unit, which is the key element behind the studies presented in this document. First, some motivation for the deployment of repeaters, is given. Then, a short description of repeater equipment is shown. Finally, some interesting properties of repeaters are discussed.

4.1. Introduction to WCDMA repeaters

Repeaters can be used to extend the coverage area of one cell or to increase cell capacity in sense of reduced transmit power requirement in uplink and in downlink direction. This thesis studies only analog, air-to-air repeaters, which communicate via radio links only and do not separate signals from different users. The main interests for using such repeaters, are cost-efficiency and simplicity. Analog repeaters are simple devices due to the simple nature of the task they are solving. No intelligent hardware or software implementations are needed. The only task of a repeater is to amplify the received wideband signal inside UMTS uplink and downlink frequency bands. Repeater installations can be made afterwards without any changes to network hardware or software. Due to the simplicity and cost-efficiency properties, analog repeaters are an attractive choice for temporarily increase the coverage or capacity, e.g., in fastly growing residential areas. Repeaters are also very useful in shadowed places where BS site establishment is impossible. However, repeaters could also be considered as a long-term solutions in suitable locations. [13].

Repeaters can be applied to outdoor as well as to indoor propagation environments. However for this thesis, only outdoor repeaters are studied. Traditionally, repeaters for 2G systems have been used in outdoor environments to cover valleys or tunnels, i.e., shadowed places where the overall network coverage has been insufficient. An interesting application for outdoor repeaters is also to use them in increasing network capacity in interference limited 3G networks and under different user distribution scenarios. Repeaters could be used to serve areas with increased traffic (hotspots) [14]. Examples of real life hotspot areas are football matches, office buildings, and public concerts.

Due to the presence of inter-cell interference in 3G systems, deployment of repeaters in UMTS is not a trivial task as it might have been in 2G systems. However, when correctly configurated, deployment of a repeater can lead to a significant gain in overall network capacity as can be seen in the following chapters.

4. WCDMA repeaters

23

4.2. Repeater equipment

4.2.1. Overview

Repeater is a signal amplifier unit, which is located in between the target users of one cell and the corresponding parent cell. Repeaters have usually two antennas: one for the parent cell connection and one for the service area connection. The parent cell repeater antenna is called ‘donor antenna’ and the antenna, which is pointing to the repeater service area is called ‘service antenna’. In this document, the coverage area of the repeater service antenna is called repeater service area. Donor sector is the area or direction, where the repeater donor antenna is directioned. Figure 4.1 presents a typical outdoor repeater installation and describes the basic terms used when speaking of repeaters.

Node B

Repeaterunit

Donorantenna

Serviceantenna

Repeaterservice

area

Donorsector

Figure 4.1: A typical repeater installation in a building rooftop.

WCDMA repeater is similar to analog repeater. It does not regenerate data. This means that also noise and interference are amplified. Repeater is also transparent to the surrounding network. The parent cell does not recognize whether a repeater is installed under its coverage area or not. Repeater is usually connected to the parent cell Node B via directional radio link. In cases of poor radio conditions between the repeater and the parent cell, an optical link can be used in connection. Functional repeater equipment consists of two antennas, an amplifier unit and cables connecting these three parts. Repeater has only one requirement for the installation environment: power must be supplied for the repeater unit. [5]

Also digital repeaters could be used to increase cell coverage or capacity. The clear benefit of digital repeaters is the ability to select the wanted signal component from the total received signal and to clean off the noise and interference from the amplified signal. However, the critical drawback of digital repeaters is the price. Digital repeater has to get some information from the network codes and parameters. This is expensive to build. This thesis handles only the analog versions of repeaters, where no intelligency is needed from the equipment.

4. WCDMA repeaters

24

4.2.2. Antennas

Antennas with high directivity are typically used in the donor sector to minimize the effects of multipath channel and inter-cell interference. Also higher antenna gains are easily obtained by using antennas with narrow horizontal beamwidth in the donor sector. The choice for the repeater service antenna is free. Both, omni-directional and directive antennas can be used to serve users in repeater service area. However, the installation of repeater antennas is not trivial, since sufficient isolation between these two antennas must be maintained. The antenna isolation should be at least 15 dB higher than the used repeater gain [15]. Poor antenna isolation leads to self-oscillation of the repeater equipment. This means that the repeater will receive and amplify its own signal as illustrated in Figure 4.2. If the self-oscillation phenomenon appears, the repeater installation and also possibly the whole parent cell will be blocked due to unintended massive interference power. To prevent the self-oscillation, use of antennas with high front-to-back ratios is preferred to quarantee maximum antenna gain in main antenna direction and maximum isolation in reverse direction. Some adaptive filtering methods can also be used to solve the antenna isolation problem [16].

Amplifier

SERVICE DONOR

LEAKAGE

Poor isolation

Amplifier

SERVICE DONOR

Good isolation

Figure 4.2: Repeater self-oscillation phenomenon.

4.2.3. Repeater hardware

WCDMA outdoor repeater hardware unit is a simple linear power amplifier with passbands located in UMTS uplink and downlink frequencies. The amplification ratio (i.e., the repeater gain) is adjustable typically up to 90 dB. The gain can usually be adjusted independently in the uplink and downlink directions. Repeaters are often equipped with AGC (automatic gain control) function, which will keep the repeater gain low enough to prevent the self-oscillation phenomenon described before. However, the self-oscillation is still a problem, because it will lead to a situation with unwanted repeater operation. A typical repeater includes connectors to donor and service antennas along with the serial communication link for adjusting the repeater settings. [15]

Repeater causes some delay to the signal in uplink and in downlink direction. However, this delay is around 5 µs, which is very small compared to, e.g., one UMTS slot time of 667 µs (one UMTS radio frame consists of 15 slots) [5]. Repeaters generally have low noise generating properties. Typical value for repeater noise figure is 3 dB [5]. Ideal repeater filtering and amplification properties are assumed in this document when simulating repeaters. Thus, repeater filters have ideal frequency response and perfectly operating linear power amplifier.

4. WCDMA repeaters

25

4.3. Thermal noise in repeater transmission path

Deployment of repeater creates some structural changes to the transmission path of propagated radio signal between the UE and the Node B. These changes can be seen from the Figure 4.3. The total path loss is divided into two parts: to the service path loss (LS) and to the repeater path loss (LP). Also antenna cable losses and other repeater implementation losses should be taken into account. In this implementation, these losses are included in the path loss value LP. Parameter definitions for the Figure 4.3 are presented in Table 4.1. [15], [17]

LP

GR

GDGS

FB

FR

GA

BS siteRepeaterUE

LS

Figure 4.3: A transmission path for a repeater installation.

Table 4.1: Parameter definitions for the transmission path of repeater.

Symbol Description

FB Node B noise figure FR Repeater noise figure GA Node B antenna gain GD Repeater donor antenna gain GR Repeater gain GS Repeater service antenna gain LP Path loss between repeater and Node B LS Path loss between UE and repeater

Although repeaters have low noise generating properties, they act as an additional noise source in the transmission path. Thermal noise is added to the transmitted signal in uplink and downlink from the repeater circuitry as presented in the repeater system block diagram in Figure 4.4. Parameter definitions for the Figure 4.4 are presented in Table 4.2. In situation of Figure 4.4, repeater cable losses are ignored.

Noise density of a component can be generally expressed in the form:

where k is the Boltzmann’s constant and T is the noise temperature of a component.

kTNTH = , (11)

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26

TeR

GR

Repeater

TeB

GB

NO

SO

Node B

NiB

SiB

GD GALP

TaR TaB

Figure 4.4: Repeater system block diagram. [17]

Table 4.2: Parameter definitions for the Figure 4.4.

Symbol Description

NiB Noise power at the input of Node B NO Noise power at the output of Node B

SiB Signal power at the input of Node B SO Signal power at the output of Node B TaB Antenna noise temperature at the Node B antenna TaR Antenna noise temperature at the repeater service antenna TeB Inherent noise temperature of the Node B TeR Inherent noise temperature of the repeater

By using Equation (11) and the Figures 4.3 and 4.4, the total thermal noise contribution at the Node B is thus:

where the gains and losses between the repeater and Node B are combined to a parameter:

The effective Node B noise figure (EFB) for the total noise contribution in unloaded network scenario can now be defined as:

if OiB SS = and W is the signal bandwidth. Equation (14) can be then further

simplified to a form of:

by using the definition for the noise figure. [17]

As a result from the Equation (15), the effective Node B noise figure increases with the GT. Practically, this can be observed as high noise levels at Node Bs in cases of short repeater distances or high repeater gains. It is also clear from the

( ) ( )eBaBTeRaRO TTkGTTkN +++= , (12)

APDRT GLGGG = . (13)

( ) ( )eBaBTeRaR

O

aB

iB

O

O

iB

iB

B

TTkGTTk

S

kT

S

WNS

WNS

EF

+++

== , (14)

RTBB FGFEF ⋅+= , (15)

4. WCDMA repeaters

27

Equation (15), that the total effective noise figure of the Node B will approach to the original noise figure of the Node B, when the GT approaches zero (i.e., repeater distance approaches to infinity or repeater gain approaches zero).

4.4. Repeaters in UMTS cell

Repeaters have effects on both the coverage and the capacity of an UMTS cell. Effects of repeaters on the cell coverage are quite straightforward and can be understood by using common sense. However, UMTS cell capacity is more complicated issue to analyse due to network interference, especially when repeaters are included.

A repeater decreases the total path loss between the antennas of parent Node B and UEs located near the repeater. This has direct impact on downlink capacity of the parent cell. Repeaters increase the received signal level at the UE relative to the interference and thus directly increase the downlink cell capacity by reducing required Node B transmit powers. Also in uplink, repeaters allow UEs to use lower transmit powers, and thus to decrease interference propagated to other cells. This has more indirect positive effects on the uplink capacity. However, negative side effects are still present in both directions. In uplink, thermal noise of the repeater is amplified along with interference coming from the other cell UEs. Repeaters amplify interference also in downlink direction, i.e., to the other cell UEs. When repeaters are located in the cell border area, they have the capability to increase parent cell load by stealing users from the surrounding cells. This can be seen as increased repeater cell coverage area. [14], [18]

When repeaters are planned to be used as a coverage extension method in UMTS networks, a second power budget should be made for the repeater service area connections because of the changes in the power budget parameter values. For example, the Eb/N0 requirement for repeater connections is increased due to the fact that the signal has to travel through several receiver sub-systems. Moreover, repeaters rarely support receive diversity methods affecting the Eb/N0 requirements for repeater connections. Also antenna gains along with several implementation losses differ from the regular parent Node B power budget. [5]

Due to the noise and interference amplification property of a repeater, the location and the used gain of a repeater are crucial, when planning repeater installations. The smaller the repeater path loss (i.e., the repeater distance from the parent Node B) the higher the received noise level in the parent Node B. This will have a direct impact on the receiver sensitivity of the parent Node B and on the cell range. Similar behaviour in the noise levels is observed when using high repeater gains (see Equations (13) and (15)).

Also the effects of different repeater gain values in different loading situations and under different traffic distributions must be concerned in the planning phase. Repeaters emphasize the cell breathing phenomenon due to the increased relative interference power received in the parent Node B in network scenarios with high load present. If the user density is high in the repeater service area, the achievable benefit from the use of repeaters should be relatively large due to the signal amplification and decreased transmit powers. However, if the parent cell is highly

4. WCDMA repeaters

28

loaded, extra benefit from the repeaters will stay at low level due to incapabilities of Node B equipment.

Handovers between repeaters and parent Node Bs do not exist due to the transparency of a repeater to the UMTS network: repeater users and parent Node B users are in the same logical cell. In the eyes of UE, repeater is seen only as an additional peak in the delay profile of the RAKE receiver. Signals from repeaters and parent Node Bs are combined in the UE by the RAKE receiver.

Due to the property of being in a same logical cell with the parent Node B, repeaters are suitable to extend coverage and to improve call quality near roads and highways – without causing excessive amount of handovers. Also power control in repeater connections functions similarly as if no repeaters were installed; the repeater delay of 5 µs is small enough to make the repeater transparent in the inner-loop power control function point of view. [5], [19], [20]

More than one repeater could be used in one UMTS cell. However, the received noise in parent Node B is increased correspondingly. Repeaters could also be installed to form a chain of cascaded repeaters. However, the increase in the total delay of transmitted signal will limit the chaining capability of repeaters along with the decreased signal quality due to repeater noise issues.

It would also be possible to use repeaters in decreasing the size of pilot polluted areas (i.e., areas with high strength pilot signals from several cells hearable simultaneously) in problematic network areas. Pilot pollution is usually a problem at cell borders, where the signal strengths are weak and no single dominating cell is heard. Thus, reduction in pilot pollution corresponds to increased cell dominance. Repeaters should be able to increase the dominance of a cell when correctly located at the cell border. [21], [22]

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5. Simulations

This chapter gives detailed information concerning the system level computer simulations performed for studying the effects of repeaters in 3G systems. First an overview to the simulations is given and different simulator types are explained. Then the software implementation of repeaters to a static network simulator is described to give some background information about the implemented properties of repeaters. Finally, the simulation scenarios are presented and the parameters used in the simulations are defined. Simulation results are presented in Chapter 6.

5.1. Simulations in general

Simulations are typically used in detailed planning phase of a radio network planning process to verify the capacity and the coverage of a dimesioned radio network in different network scenarios. However, simulations can also be utilized in research activities when searching new ways to improve the existing mobile communication systems. System level simulations are performed by using a network simulator software. The simulator software can be part of a bigger planning tool software. In simulators, different elements of the radio network are modeled to provide as realistic network operation as possible. Such elements are for example: propagation models for different radio propagation environments, digital maps for accurate terrain modeling, and antenna models for realistic coverage calculation. Simulators for 3G systems include also the modeling of interference, as it is in essential role in 3G system basic operation.

The simulation process is straightforward in the eyes of the user of the simulator software. Some data, such as BS site locations and UE traffic distributions, must be given as input to the simulator software. Then, parameters describing the network configuration are adjusted to correspond to the simulated scenario. After simulations, the performance of the network is analyzed by the simulation software and the results are given as the output data. Finally, indicators describing the network performance are shown in forms of figures and numerical data.

5.2. Simulation types

Simulations can be performed at different levels. System level simulations provide information about overall system performance by means of providing the modeling of some RRM (radio resource management) functions, such as handovers and power control. Link level simulations are low level simulations, which provide information concerning only physical features of a single radio link. However, link level simulations are required to obtain parameter values for the system level simulations. In addition to the division of simulations to link level and system level, simulations can be categorized into two branches: static simulations and dynamic simulations.

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5.2.1. Static simulations

In case of static simulations, the performance of the network is analyzed over various static time instances, called ‘snapshots’. Simulations, where the concept of taking several randomized snapshots is used, are called Monte Carlo simulations. In static Monte Carlo simulations, a large number of snapshots are taken and the network performance at each snapshot is analyzed separately. The number of users in the network at each snapshot is Poisson distributed and will vary between snapshots. Also the locations of the UEs are randomized. Finally, different snapshot results are combined to give an estimate of the average network performance. [23]

The number of snapshots and UEs in a static simulation event partly defines the reliability of the results. This is caused by the fact that more network locations are covered by the simulations when large number of snapshots or UEs are used, and thus more map pixels have been taken into the simulations. Covering most of the map pixels is important in sense of variations of propagation environment in the map area. However, static simulations do not consider, e.g., the effects of user movement in the network area. In addition, the functionality of all RRM functions, such as access control or packet scheduling are not considered as a function of time in static simulations. Handovers and connection status information can be observed at static time moments in static simulations, but the network operation can not be examined as a function of time. Static simulations are used widely when modeling 3G networks. The computational power requirement for static simulations is moderate and results from static simulations can be achieved in considerable time.

5.2.2. Dynamic simulations

Dynamic simulations can be used, e.g., to get some more detailed information of the performance of RRM functions in the network. In dynamic simulations, UE moves through the network in successive time steps. Actions in successive time steps depend on the results of the previous time steps. The time resolution of a 3G dynamic simulator is usually defined by the network feature that changes the interference situation most often. However, the time resolution can be defined to be any other feature in the network, such as bit period or chip period. In dynamic simulations, new UEs can join the network and existing ones can terminate their calls. This makes it possible to simulate the operation of the RRM functions in more detailed way. The performance of access control and packet scheduler entities can be simulated with dynamic simulators. [5]

The problem of dynamic simulators is the complexity. Dynamic simulators require high amount of computational power from the simulator hardware. When calculating even a short simulation period, tremendous amount of calculations must be made to model the operation of whole network. Interference must be calculated for each UE at each time moment along with the all other network parameters. Thus, dynamic simulations are still not widely used in modeling 3G systems. [5]

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5.3. NPSW static simulator

An already built simulator software, NPSW (network planning strategies for wideband CDMA) version 5.0.0 [24], was used to get the simulation results shown in Chapter 6. NPSW is a Matlab-based static WCDMA simulator. Figure 5.1 presents a screenshot from the NPSW simulator. In this example scenario, digital map of the city of Espoo is used with link loss data imported from other planning tool. If no digital map of the target area is available, simulations have to be done using plain homogenous grid, without realistic terrain and link loss data. For such cases, NPSW supports, e.g., the Okumura-Hata propagation model.

Figure 5.1: A screenshot from NPSW. [24]

NPSW simulation event consists of three main phases: general initialization phase, combined uplink and downlink iteration phase, and post processing phase. The basic structure of NPSW is shown in Figure 5.2.

In the initialization phase, all needed parameter files are read and calculations, which are needed to be done only once, are performed. If no digital map is available, the link loss data is calculated by using a propagation model and other parameters. Link losses from all Node Bs to all map pixels are calculated to simulate the propagation environment. The complexity of this phase is mainly defined by the number of total map pixels used in the simulations (i.e., the map resolution). The calculation time rapidly increases when the number of map pixels is increased. Thus, the map resolution is the key parameter in defining the time consumption, when doing simulations. The map resolution is user-defined parameter and can be adjusted before starting the simulations. [25]

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32

Initializationphase

Global initialization

Initialize iterations

CombinedUL/DL iteration

Uplink iteration step

Downlink iteration step

Postprocessing

Post processing

Graphical outputs

Coverage analyses

Figure 5.2: Basic structure of NPSW simulator. [25]

In the iteration phase, various performance parameters are calculated iteratively. Uplink iterations are needed to allocate the transmit powers of UEs in such a way that the interference levels at Node Bs converge. The uplink iterative process is repeated, until the changes in received interference values at Node Bs are smaller than specified. The target of the downlink iterative process is to allocate correct Node B transmit powers for each UE. Downlink iteration is needed, since the carrier-to-interference ratio (C/I) at each UE is dependent on the powers allocated to the other UEs. [25]

Uplink load for each cell in the network is defined in the simulator by equation:

where Iown is own cell interference power, Ioth is other cell interference power and N is the Node B noise power in case of an empty cell. The loading value is calculated from interference power levels for each cell in every iteration round and is always smaller than one. If loading of a cell exceeds the pre-defined limit value, some users of the cell are put to outage to reduce the uplink load in that particular cell. This is repeated iteration after iteration until the cell load is below the maximum cell load value. [25]

The uplink load equation is important, since the repeaters affect the noise levels (N) of the BSs and thus to the cell loading in uplink. It must be noticed how the uplink cell loading value does not directly refer to the number of active users in a certain cell but also to the number of active users in the other cells (Ioth) and to the noise levels of the BS.

In the post processing phase, previously calculated information is post-processed to produce various color plots and statistics [25]. Result saving and averaging is done in this phase as well. Before presenting the calculated data, essential simulation results are saved to text and data files for further post-processing.

NII

II

othown

othown

++

+=η , (16)

5. Simulations

33

5.4. NPSW repeater implementation

The original version of NPSW (version 5.0.0) did not have support for repeaters. Thus, repeaters were programmed as an extension to NPSW, to establish repeater simulations for UMTS. This repeater implementation includes several new issues for the NPSW:

� Repeater unit with adjustable parameters (repeater gain, antennas, etc.), � Link loss calculations for repeater connections, � Interference calculations for repeater connections in uplink and downlink, � Noise figure corrections for Node Bs, and � Channel model adjustments for repeater connections.

In the following subchapters, these new features are described and explained briefly.

5.4.1. Repeater unit

Parameters for repeater entities are given as input to the simulator in a text file. This text file includes information concerning the repeater locations, parent Node B identificators, gains, antenna types, antenna heights, etc. These parameter values are used later in many parts of the simulation process. Figure 5.3 shows a screenshot from NPSW, where repeaters are introduced in six UMTS cells. In Figure 5.3, both repeater antennas (service and donor antenna) are visible and the link between the repeater and the parent Node B is marked with solid thin line.

Figure 5.3: Basic NPSW scenario with repeaters installed.

5. Simulations

34

5.4.2. Link loss and interference update

When repeaters are introduced in the network, additional link loss calculations are needed in order to calculate, e.g., inter-cell interference and required transmit powers to users connected through repeaters. Link loss calculations in the original NPSW version were started by calculating link losses from every Node B to all map pixels by using the selected propagation model and by taking into account Node B antenna patterns and gains. Then, link losses for all user connections were found from massive link loss data matrix to form another matrix simply containing link loss information between every user and every Node B. When introducing repeaters, link losses from all users to all Node Bs via all repeaters must be calculated, taking into account all gains and losses from the repeater installation. Figure 5.4 clarifies the situation.

Site 1

Site 2

UE 1

UE 2

Rep Site 1

Site 2

UE 1

UE 2

Figure 5.4: Link loss calculation visualization. New link paths are introduced with repeaters.

The calculation of repeater interference is done by using the repeater link loss data and allocated transmit powers. Network interference situation is recalculated in every iteration round, now taking into account also repeaters’ amplified interference. When considering a repeater cell, users having the best server connection through a repeater, will belong to the own cell user group of the parent Node B of that particular repeater. All other interference is counted as other cell interference. Figure 5.5 explains the distinction between own cell interference and other cell interference in case of a repeater cell increasing the coverage.

Rep Site 1

Site 2

own cellinterference

other cellinterference

Figure 5.5: Repeater cell with uplink own cell interference and other cell interference areas visible.

Repeaters act as an additional interference source both in uplink and in downlink. Due to repeater properties, the total received power spectrum at repeater antennas is amplified – including the interference from the other cells in downlink and from all UEs in uplink direction.

5. Simulations

35

Originally, NPSW provided an interference model presented in Figure 5.6. In this model, interference from all Node Bs and from all UEs was included. Repeater interference calculation in the NPSW repeater implementation is based on the same link model depicted already in Figure 5.6. However, many new link paths were added due to the introduction of repeaters to provide more realistic network behaviour. These additions are depicted in Figures 5.7 and 5.8.

Site 1Site 2

UEown cellinterference

Site 3


Site 1Site 2

UE

Site 3

UE

UE

UE



Figure 5.6: Original NPSW interference modeling in DL (left) and UL (right).

Rep 1

Site 1Site 2

UE


Site 3

Rep 2

LOSLOS


Site 1Site 2

UEown cell

interference

Rep 2

Site 3

LOS


UE

Figure 5.7: NPSW enhanced interference model after repeater implementation in DL (left) and UL (right).

Site 1Site 2

UERep 1

Site 3

Rep 2 other cellinterference from

other cell repeaters

Site 1Site 2

UE

Rep 1

Site 3

Rep 2 other cellinterference from

other cell repeaters

Figure 5.8: Repeaters’ other-cell interference amplification in DL (left) and UL (right).

Downlink own cell interference is received from the parent Node B directly, and via the own cell repeater that has a LOS connection to the parent Node B.

5. Simulations

36

Downlink other cell interference comes from the surrounding Node Bs, as depicted in Figure 5.7. In addition, own cell repeater amplifies also interfering signals originating from non-parent cells, as Figure 5.8 presents. Both interference models described in Figures 5.7 and 5.8 are implemented in the NPSW repeater implementation to improve the simulator behaviour. Uplink functions similarly in the opposite direction. However in uplink, interference is Node B specific, and in downlink, UE specific.

Link losses for own cell repeater connections (presented in Figure 5.7) are calculated from the equation:

where GBS includes all Node B related losses and gains (antenna pattern losses, antenna gains and cable losses), LFS is the free space loss, and GREP includes all repeater related losses and gains (antenna pattern losses, antenna gains and repeater gain). Repeater cable losses are taken into account when defining the repeater path loss. LS is the repeater service path loss (from Okumura-Hata model) and GUE includes gains and losses related to UE (antenna gain and body loss).

When calculating link losses for repeater other cell interference amplification (presented in Figure 5.8), free space loss is not used between repeaters and parent Node Bs:

In Equation (18), LINT is the path loss between the repeater and the non-parent Node B, and is calculated by using the Okumura-Hata model. All other parameters remain unchanged. The introduction of LINT to Equation (18) is a simplificative assumption, which may lead to an underestimation of inteference especially if LOS exists between repeaters.

When calculating the own cell interference, LOS between the repeater and the parent Node B is assumed and the free space propagation model (Equation (3)) is used. This can be explained by using the common sense: repeater donor antenna should be placed so that the link to parent Node B is clear of obstacles (LOS).

5.4.3. Effective noise figure update

The effects of repeaters on the Node B noise levels are taken into account by using the effective Node B noise figure calculation method described in Chapter 4. The Node B noise figure in repeater cells are replaced by the effective noise figure value calculated from the Equation (15).

5.4.4. Channel model update

Due to the expected differences in the propagation environment for the repeater connected users, channel model is changed from Vehicular A to Pedestrian A for those users connecting through repeaters. Table 5.1 shows the changes in link performance table parameters due to the channel model update.

UESREPFSBSCONNREP GLGLGLinkLoss ⋅⋅⋅⋅=_ , (17)

UESREPINTBSINTREP GLGLGLinkLoss ⋅⋅⋅⋅=_ . (18)

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Table 5.1: Parameter changes for repeater connected users (UE speed 50 km/h in both cases). [24]

Parameter Vehicular A Pedestrian A

Orthogonality factor 0.50 0.77 Uplink Eb/N0 5.80 5.18 Downlink Eb/N0 8.00 9.05 Uplink voice activity factor 0.67 0.67 Downlink voice activity factor 0.67 0.67

For repeater connected users, the orthogonality factor is slightly increased (from 0.5 to 0.77) along with the downlink Eb/N0 requirements (from 8 dB to 9.05 dB). This parameter adjusting gives more realistic behaviour to the repeater simulations, since better orthogonality should be expected with repeater users due to increased LOS connection probability and low multipath effects. Also increase in the downlink Eb/N0 values is more realistic with repeater users due to increased amount of noisy equipment on the transmission path of repeater users and reduced multipath diversity. However, uplink Eb/N0 values are slightly decreased (from 5.8 dB to 5.18 dB) along with this channel model update. Voice activity factor was remained unchanged at 0.67 for both uplink and downlink connections. The original and corrected values are interpolated from the original link performance tables which came with the NPSW simulator. [24]

5.4.5. Hotspot support for NPSW

NPSW was also missing the possibility of adding traffic hotspots to the simulation map area. Hotspots are though in essential role, especially, when simulating repeater scenarios, where the target is to increase the network capacity instead of coverage. A function was created to add rectangular or circular hotspots to the map area. The hotspot data is given to NPSW as input in the form of a text file. The text file includes information about the hotspot type, size, location, and the user density. The traffic distribution for the hotspots is calculated by using the same function created for the randomized homogenous traffic distribution generation. Only the traffic density and area size for the hotspot is adjustable. The absolute user amount of each hotspot is calculated from the equation:

where HSDF (hotspot density factor) is a constant describing the user density difference between the overall network and an individual hotspot, D is the user density in the overall network area (without hotspots) and AHS is the area size of the hotspot.

If the HSDF is defined to be 1, the whole traffic distribution is homogenous, as the traffic densities inside and outside the hotspots are the same. Small HSDF values (near zero) indicate empty hotspots and large HSDF values indicate hotspots with heavy traffic.

Figure 5.9 presents a scenario with user locations visible, where repeaters and hotspots are placed in the network area. The circular hotspots are located under repeaters’ service area and are marked with thin solid line as Figure 5.9 presents.

HSHS ADHSDFT ⋅⋅= , (19)

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Figure 5.9: An example scenario with repeaters, hotspots and users visible. Users are marked as dots. Hotspots are marked with thin line. HSDF value of 20 is used in this scenario.

5.4.6. Other NSPW updates

The original version of NPSW (version 5.0.0) did not follow the idea of Monte Carlo simulations and left out the support for performing several snapshots. However, support for several snapshots was made afterwards by hand to improve the reliability of the NPSW. Also the result averaging function was added to get overall average statistics of snapshots. In the original version of NPSW, a text file was used as an input file for the UE information. As an upgrade to this method, a new function was programmed for automatic randomized traffic distribution creation process. This function creates randomized homogenous traffic distribution for desired rectangular area given as input along with the desired amount of users. When using the support for several snapshots in NPSW simulations, the user count for each snapshot is taken from the Poisson distribution randomly, where the expected value of the Poisson distribution function is the number of total users given as an input parameter.

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5.5. Simulation scenarios

5.5.1. Node B and repeater site configurations

System level repeater simulations were performed using one fixed Node B site configuration and four different repeater configurations. The effects of repeaters were simulated in all repeater scenarios by using different repeater gains and different hotspot traffic densities. Nineteen 3-sectored BS sites were located in basic hexagonal grid with site spacing of 1000 m. In scenarios 1 and 2, six repeaters were installed near cell edges under first tier Node Bs as shown in Figure 5.10. Scenarios 3 and 4 (see Figure 5.11) included 24 repeaters installed in the same way as in scenarios 1 and 2. However in scenarios 3 and 4, repeaters were spread wider to cover most of the network area. In scenario 4, repeater locations are identical when compared to scenario 3. However, repeater antenna configuration is different in scenario 4.

12

500

m

12

333

m

Figure 5.10: Two repeater scenarios in a fixed Node B configuration. Scenario 1 (left) with repeater distance 500 m, and scenario 2 (right) with repeater distance 333 m.

Figure 5.11: Repeater scenario 3 in a fixed Node B configuration. Repeater distance is 500 m.

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In scenario 1, repeaters were located 500 m apart from the parent Node B (i.e., half of the cell spacing). In scenario 2, repeaters were brought to the distance of 333 m (i.e., one-third of the cell spacing). Scenario 3 used the same repeater distance setting (500 m) as the scenario 1. Repeater service antennas were pointed to the pilot pollution areas in a way depicted in Figure 5.10. Repeater donor antennas were pointed directly towards the parent Node B antenna. In scenario 4, only the repeater service antenna downtilt was studied. Thus, the repeater site configuration in scenario 4 was same as in scenarios 1 and 3.

The repeater path loss values were manually fixed to 100 dB for the scenarios 1, 3 and 4 and to 96 dB for the scenario 2. These values are little larger when compared to the values calculated by using the Equation (3) for the free space loss. Larger repeater path loss values were chosen to include repeater implementation losses (such as cable losses and connector losses) to the simulations. The difference in the repeater path loss values between scenarios 1 and 2 (4 dB) corresponds to the change in the repeater distance mentioned above.

Propagation environment model was based on pure Okumura-Hata calculations. Area correction factor of -15 dB was used in the propagation model (see Equation (5)) to make the network scenario more capacity limited, since the purpose was to study the effects of repeaters especially on the network capacity – not on the network coverage.

Each of the repeater scenario was run with the five different HSDF values and three network load cases. Only voice users (12.2 kbps) were used for the simulations. All results were compared to the case, where repeaters were switched off. Also the repeater gain was varied and were then compared to the case without functional repeaters. Simulations were made with adequate number of snapshots. Finally, results from the snapshots were averaged and plotted in graphical form.

5.5.2. Antenna configurations

Original NPSW antenna configuration files were used in the simulations. All Node Bs were equipped with antennas having 65° horizontal beamwidth. This 65° antenna configuration was used also in service antennas for the repeaters. Narrower, 33° antennas, were chosen as repeater donor antennas, since they should propagate less interference to the surrounding network when communicating with the parent Node B. Antenna patterns for the used repeater and Node B antenna configurations are presented in Figure 5.12.

Repeater antenna isolation is assumed to be infinite in the simulations. This assumption is valid, since the only indication of antenna isolation problems in real life situation is complete blocking of the cell where repeater is installed to. If the antenna isolation is at adequate level, the cell is functioning normally and is thus not affecting the network operation.

Simulations for all four scenarios were made by using EDT of 6 degrees in all serving antennas – including in the repeater service antennas. EDT of 2° was used in repeater donor antennas. EDT angle of 0° for repeater donor antenna would have been the best choice for the simulations, because repeater antennas and Node

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B antennas had same ground height. Now, an additional loss (4.5 dB) is introduced to the repeater link path gain (GT) due to electrical downtilt of repeater donor antennas and Node B antennas. It is however taken into account in simulations.

The studies of Node B antenna downtilting in [26] and [27] showed the importance of correct service antenna downtilting in WCDMA based mobile communication networks. By following this idea of antenna mechanical downtilt presented in [27], some simulations were made to study the effect of repeater service antenna downtilt in order to further improve the network performance. These repeater antenna downtilt simulations were named as a new scenario: ‘scenario 4’. The downtilt simulations were made by using the same repeater site scenario as in the scenario 3 with 24 repeaters installed, as it most effectively emphasizes differences when changing repeater antenna configuration. Mechanical downtilt of 6 degrees was applied in all repeater service antennas to see if the other cell interference was reduced as it was in the case of simulations presented in [27].

5.5.3. Radio network parameters

All simulations were run with repeater gains in the range of 45 to 75 dB. It was noticed that the 45 dB repeater gain is not affecting much on the network

Figure 5.12: Repeater donor antenna (33°) patterns in horizontal (a) and vertical (b) directions along with base station and repeater service antenna (65°) patterns

in horizontal (c) and vertical (d) directions.

a) b)

c) d)

−40 −30 −20 −10 0

30

210

60

240

90

270

120

300

150

330

180 0

Repeater donor antenna pattern − Horizontal

−40 −30 −20 −10 0

30

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Repeater donor antenna pattern − Vertical

−40 −30 −20 −10 0

30

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Repeater service antenna pattern − Horizontal

−40 −30 −20 −10 0

30

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180 0

Repeater service antenna pattern − Vertical

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performance, and at repeater gains above 75 dB, the simulation event did not easily converge. Thus, the range was suitable for performing the simulations. Parameter values for the RAN elements are shown in Tables 5.2 and 5.3.

Table 5.2: Fixed parameter values for Node Bs and UEs.

Parameter Value

Mobile station antenna gain 1.5 dBi Mobile station antenna height 1.5 m Mobile station body loss 1.5 dB Mobile station noise figure 8 dB Node B antenna downtilt (EDT) 6° Node B antenna gain 17 dB Node B antenna height 25 m Node B antenna horizontal beamwidth 65° Node B cable losses 3 dB Node B load limit 0.75 Node B noise figure 5 dB Node B TX power limit 43 dBm Pilot TX power 30 dBm SHO ADD window 3 dB

Table 5.3: Fixed parameter values for repeaters.

Parameter Value

Repeater antenna height 25 m Repeater donor antenna downtilt (EDT) 2° Repeater donor antenna gain 19.5 dB Repeater donor antenna horizontal beamwidth 33° Repeater noise figure 3 dB Repeater service antenna downtilt (EDT) 6° Repeater service antenna gain 17 dB Repeater service antenna horizontal beamwidth 65°

Table 5.4: Hotspot-related parameter definitions.

Parameter Value

Hotspot radius 150 m Hotspot type Circular Traffic distribution inside hotspot Homogenous HSDF value for low hotspot traffic density 0.001 HSDF value for homogenous traffic (no hotspots) 1 HSDF value for medium hotspot traffic density 6 HSDF value for high hotspot traffic density 10 HSDF value for extra high hotspot traffic density 20

Circular hotspots were located under the service area of the repeaters to see the effects of repeaters on the hotspot capacities. The radius of all hotspots was set to 150 m. Different HSDF values were used to see how flexible repeaters are in providing capacity increase for the network in different hotspot traffic cases. Hotspot-related simulation parameters are presented in Table 5.4.

5. Simulations

43

5.5.4. Traffic parameters

Table 5.5 presents all simulation cases performed for the studies in this thesis. The results show how the amount of overall initial users had to be decreased at high hotspot traffic densities to prevent network congestions. Three network load cases were simulated at each hotspot traffic density case in order to provide capacity analysis for each simulation case.

Table 5.5: Simulated traffic cases with each repeater scenario (1 - 4).

Hotspot traffic density

0.001 1 6 10 20

100 1,2,3,4 500 1,2,3,4 1,2,3,4

1000 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4 2000 1,2,3,4 1,2,3,4 1,2,3,4 1,2,3,4

Amount

of initial

users

3000 1,2,3,4 1,2,3,4 1,2,3,4

Table 5.6 shows the total number of users in the whole network area in each HSDF simulation case (with highest load) – including additional hotspot users due to high HSDF values.

Table 5.6: Total amount of users in the whole network area after hotspot implementation for each HSDF scenario with highest network load.

Traffic scenario

Total amount of users after

hotspot implementation

(average)

Scenario 1 & 2 – 3000 initial users – HSDF 0.001 2925 Scenario 1 & 2 – 3000 initial users – HSDF 1 3000 Scenario 1 & 2 – 3000 initial users – HSDF 6 3400 Scenario 1 & 2 – 2000 initial users – HSDF 10 2475 Scenario 1 & 2 – 1000 initial users – HSDF 20 1500 Scenario 3 & 4 – 3000 initial users – HSDF 0.001 2700 Scenario 3 & 4 – 3000 initial users – HSDF 1 3000 Scenario 3 & 4 – 3000 initial users – HSDF 6 4600 Scenario 3 & 4 – 2000 initial users – HSDF 10 3900 Scenario 3 & 4 – 1000 initial users – HSDF 20 3000

Since the total number of users in the network is not the same between different simulation cases, the absolute results between simulations with differing hotspot traffic density are not directly comparable with each other. However, the effect of repeaters on the network performance can be seen in each simulation case by comparing the results from different repeater gain cases and the case where no repeaters were used. The large value (4600) in total amount of users in case of scenario 3 and 4 with HSDF 6 after hotspot implementation should be pointed out, since it represents very highly loaded network configuration. Thus, effects of repeaters may not be as clearly visible as in other traffic scenarios. In addition, some results shown later may not be reliable when using this particular scenario with very high repeater gains (70 dB – 75 dB) due to very high interference levels.

5. Simulations

44

5.5.5. RRM parameters

Two important RRM parameters in repeater point of view are load control and SHO control, since they are implemented at some level to the NPSW. These parameters are shortly described below.

Network load in uplink direction is controlled in these simulations by randomly disconnecting required amount of users taken from only highly loaded cells at each iteration round. Highly loaded cells are defined by the condition:

for all Node Bs, i, where Ueta is the estimated uplink load of a cell (from Equation (16)) and Umax is the uplink load limit for that particular cell. [25]

In downlink direction, load control is implemented by using similar approach as already described for the uplink direction. Load is estimated in all Node Bs by observing total Node B transmit powers. If a certain maximum Node B TX power limit value is exceeded for a cell (see Table 5.2), load of the cell will be reduced by randomly putting some connections to outage at those cells. [25]

Due to the static nature of NPSW, only a simple SHO algorithm is included to model SHO connections in the network. This rule defines, that a UE is in SHO situation only if the highest received pilot strengths from two or more cells are within a certain window called WINDOW_ADD. In these simulations, the WINDOW_ADD parameter was set to 3 dB as was shown in Table 5.2. This is a typical value for a SHO window parameter. [27]

( ) ( )maxeta iUiU > , (20)

6. Simulation results

45


Simulation results presented in this chapter are purely based on the output data of the static network simulator described in Chapter 5. In most of the following figures, the results are averages from the whole network area. The results are also averages from different snapshots, which is typical for Monte Carlo based simulations. The target of this chapter is to present and clarify, how the average network performance was changed due to the introduction of repeaters to the network. Repeater simulation results are compared to the case, when repeaters were switched off.

Results for different simulation scenarios are examined to give an image, how the performance of the network is dependent from the used repeater scenario. The effect of changing the repeater distance can be seen by comparing the results between the scenarios 1 and 2. Moreover, the network performance in case of repeaters present in the whole network area can be seen from the results for the scenario 3. Finally, the effects of repeater service antenna downtilting to the network performance are presented by using the scenario 4 simulation data.

6.1. Analysis of simulation output data

This subchapter presents the results from the simulations in forms of figures concerning following subjects: service probability, uplink other-to-own cell interference, transmit powers (of UEs and Node Bs) and soft handover coverage. The purpose of these figures is to describe the behaviour of different parts of the RAN, when using repeaters with different settings. Finally, outage statistics and the capacity analysis are included to visualize the operation of whole WCDMA network with different repeater settings.

The locations of the curves in different figures in Y-axis give actually not much comparable information, since different hotspot traffic cases are simulated by using different network loading (different number of total users) as presented in Table 5.6. However, the behaviour in direction of X-axis is very fruitful as well as the comparison to the case with no repeaters installed (shown in plain numbers in figure legends).

In Subchapters 6.1.1 – 6.1.5, only the highest network load case is studied, since the effects of repeaters on the network are mostly visible when the network is operating at full capacity.


46

6.1.1. Service probability

Figure 6.1 presents the service probabilities for all scenarios as a function of repeater gain. A clear drop is observed at high repeater gains in case of all scenarios. In scenario 1, variations in service probabilities in respect to repeater gain are rather small. However, scenarios 3 and 4 show the importance of choosing the correct repeater gain setting, especially when having high hotspot traffic situation. Figures 6.1c and 6.1d show clear improvement in overall service probability at high hotspot traffic and at suitable repeater gains (65 dB – 70 dB).

When comparing the service probability peaks between scenarios 1 and 2 (Figures 6.1a and 6.1b), a difference in optimum repeater gain is visible. The shift of 4-5 dB is caused by the difference in repeater distance, and thus in repeater path loss and GT (see Equation (13)).

In all simulated scenarios and in cases of low hotspot traffic or when the hotspot is left out, the effect of repeaters on the overall network service probability is rather insignificant. This is quite expected result, since then repeaters are amplifying more noise and less user traffic. However, when traffic in hotspot areas is increased, the importance of repeaters becomes clearly visible. This can be observed as a major increase in service probabilities at suitable repeater gain settings. Up to 17 percent unit increase can be observed in the overall service probability in case of scenario 4. Now, repeaters are amplifying mostly user traffic with lowered transmit powers and less interference from other cells.

Figure 6.1: Service probabilities for scenarios 1 (a), 2 (b), 3 (c) and 4 (d) with different hotspot traffic cases.

(c)

(a) (b)

(d)

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3000 users − HSDF 0.001 − REP OFF: 97.0 %3000 users − HSDF 1 − REP OFF: 96.9 %3000 users − HSDF 6 − REP OFF: 84.8 %2000 users − HSDF 10 − REP OFF: 89.6 %1000 users − HSDF 20 − REP OFF: 92.1 %

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Scenario 4 − 24 repeaters at 500 m + 6 deg. MDT



47

6.1.2. Uplink other-to-own cell interference ratio

Before inspecting Figure 6.2, it must be noted that two network parameters are able to change the value of uplink other-to-own cell interference ratio. For example, a decrease of own cell interference levels has similar effect as an increase of other cell interference levels. In addition, iUL is a very Node B specific parameter: value of one cell can be totally different than values in the surrounding cells. When the results from all cells are averaged, the cell specific information is lost. Thus, no direct conclusions can be drawn from Figure 6.2. However, some observations can be made.

With all repeater scenarios simulated, an optimum repeater gain setting in eyes of iUL, can be found. Results in Figure 6.2 show that when this optimum point is exeeded, iUL starts rapidly to rise. This indicates problems or inbalance in the network operation. The rise is caused either from the rise in other cell interference or fall in own cell interference. In these simulations, the rise of averaged uplink other-to-own cell interference ratio is caused by the massive other cell interference received in the cells surrounding the repeater cells.

When comparing the results of the scenarios 1 and 2 (Figure 6.2a and 6.2b), a clear growth in uplink other-to-own cell interference ratios are observed at high repeater gains. This comes from the difference in the repeater distance: larger GT value and smaller repeater path loss value emphasize the effect on uplink

Figure 6.2: System level uplink other-to-own cell interference ratio for scenarios 1 (a), 2 (b), 3 (c) and 4 (d).

(a) (b)

(c) (d)

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i UL


3000 users − HSDF 0.001 − REP OFF: 0.5813000 users − HSDF 1 − REP OFF: 0.5923000 users − HSDF 6 − REP OFF: 0.6052000 users − HSDF 10 − REP OFF: 0.6551000 users − HSDF 20 − REP OFF: 0.775

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48

interference. The shift of 4-5 dB is again visible when looking at the interference curves between these two figures. Next, the effects on uplink interference are studied in cell level.

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HSDF 0.001 (regular cells)HSDF 1 (regular cells)HSDF 6 (regular cells)HSDF 10 (regular cells)HSDF 0.001 (repeater cells)HSDF 1 (repeater cells)HSDF 6 (repeater cells)HSDF 10 (repeater cells)

Figure 6.3: Cell level other-to-own cell interference results. Scenario 1 with 2000 overall users

was used. Repeater gain was set to 75 dB.

Table 6.1: Cell definitions.

Name Definition Related cells in scenario 1

Repeater cell A cell with repeater installed BS2, BS3, BS8, BS11, BS18, BS21

Regular cell A cell with no repeater installed BS1, BS4, BS7, BS14, BS15, BS19

Two classes of cells are defined in Table 6.1: repeater cells and regular cells. Figure 6.3 clarifies the reasons behind the increased network level iUL values: increased iUL at regular cells surrounding the repeater cells, and decreased iUL at repeater cells.

The rapid increase in the regular cells is mainly caused by the high received uplink other cell interference from the repeater cell UEs and surrounding repeater donor antennas amplifying with high power at high repeater gain values. It is shown in the next subchapter, how the average transmit powers of UEs will rise, when high repeater gain values are used.

Reduced iUL values in the repeater cells can be partly explained by the increased repeater cell coverage area (observed as increased own cell interference). The rest of the reasons behind the phenomenon are explained later when inspecting UE transmit powers.

The main conclusion from the Figures 6.2 and 6.3 is that attention must be paid to uplink interference aspects in UMTS radio network planning, when WCDMA repeaters are used in the network. If repeater parameters are not set with care, the result might be a cell congestion due to massive uplink interference.


49

6.1.3. UE transmit power

UE transmit powers are presented as a function of repeater gain in Figure 6.4 for all four simulation scenarios. The target of showing these transmit powers is to visualize the effect of repeaters in different traffic cases to help hotspot UEs to use lower power and thereby to increase network capacity and to save the batteries of UEs.

In cases of all scenarios, an optimum repeater gain setting in UE transmit power sense can be found by looking at the curves in Figure 6.4. However, the ultimate optimum repeater gain setting can not be decided only based on this figure; DL direction should also be considered. Figure 6.4 shows that the effect of repeater is insignificant in cases of small hotspot traffic density (cases HSDF 0.001 and HSDF 1). However, when the traffic density of hotspot is increased, the reduction in hotspot UE transmit powers is getting larger. An overall reduction up to 2 dB is observed using high hotspot traffic density and the scenario 3 with 24 repeaters as can be seen from Figure 6.4c. It is also important to notice how the UE transmit powers actually rise above the power level of original case with no repeaters, when too high repeater gains are used. This happens due to the excess noise at Node Bs, caused by the increased level of thermal noise at repeater cell Node Bs (see Equation (15)). The rise of UE transmit powers over the original level causes the reduction of iUL at repeater cells.

Figure 6.4: UE transmit powers for scenarios 1 (a), 2 (b), 3 (c) and 4 (d).

(c)

(b) (a)

(d)

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3000 users − HSDF 0.001 − REP OFF: −12.5 dBm3000 users − HSDF 1 − REP OFF: −12.4 dBm3000 users − HSDF 6 − REP OFF: −12.3 dBm2000 users − HSDF 10 − REP OFF: −13.0 dBm1000 users − HSDF 20 − REP OFF: −13.9 dBm

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50

6.1.4. Node B transmit power

Averaged Node B transmit powers are shown in Figure 6.5 for all four scenarios. The differences in the absolute Node B transmit power values are caused by different network loading in each scenarios. Thus, the results between different scenarios are not comparable in Y-axis dimension. However, the behaviour of the Node B transmit power as a function of repeater gain should be inspected. These results simply indicate that the capacity can be increased in downlink by using higher and higher repeater gain values. This holds for all four repeater scenarios. The continuous reduction in the Node B transmit powers indicates, how other cell interference in downlink stays at low level despite the increase in the repeater gain value. This is caused by the different properties of UMTS downlink direction. The use of directional Node B antennas instead of omnidirectional UE antennas helps to keep downlink interference at the minimum.

Although not visible in the Figure 6.5, downlink interference has been observed to rise as a function of the repeater gain. However, the rise in interference is slower than the reduction in the path loss of repeater connected users, thus leading to a significant downlink capacity gain presented later in this document.

Figure 6.5: Node B transmit powers for scenarios 1 (a), 2 (b), 3 (c) and 4 (d).

(a) (b)

(c) (d)

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3000 users − HSDF 0.001 − REP OFF: 39.2 dBm3000 users − HSDF 1 − REP OFF: 39.9 dBm3000 users − HSDF 6 − REP OFF: 40.3 dBm2000 users − HSDF 10 − REP OFF: 37.1 dBm1000 users − HSDF 20 − REP OFF: 33.0 dBm

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51

6.1.5. SHO statistics

Soft handover statistics are presented for the same simulation cases as before in this chapter. When calculating the SHO probabilities, Equation (10) was used, as the data of the number of users with only one connection was given by the simulator as output. The operation of SHO algorithm was described in Subchapter 5.5.5. Figure 6.6 presents the SHO probabilities when the repeaters were switched off and on.

The message is clear: when the gain of a repeater serving a hotspot located in a cell border is increased, the probability of getting soft handover connection is decreased due to increased dominance of the repeater. The reduction in SHO probabilities in cases of high hotspot traffic is due to the fact, that most of the users are located in hotspots – i.e., in cell edges where the SHO probability is naturally high.

When comparing the results with repeaters switched off (shown in legends of Figure 6.6), clear increase in SHO probabilities is observed if low repeater gains and high hotspot traffic are used. This difference is mainly caused by the load control function. If repeaters are switched off, the service probability (especially in hotspot areas with heavy traffic) stays at low level, thus leading to lower overall SHO probabilities.

Figure 6.6: Soft handover probabilities as a function of repeater gain for the scenarios 1 (a), 2 (b), 3 (c) and 4 (d) when repeaters are switched off and on.

a) b)

c) d)

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At low hotspot traffic densities, the results of SHO probability are quite what was expected: no major differences as a function of repeater gain. This is due to the empty or almost empty hotspots. The dominance is better inside hotspots but no users exist to exploit the increased repeater dominance area. The small rise at very high repeater gains is natural, since repeater cells are now generating more pilot pollution to the surrounding cells (and stealing users from surrounding cells) and thereby increasing the number of SHO connections in the network area.

The increase in repeater dominance area size in hotspots can be seen from the SHO statistics map of Figure 6.7. It is a snapshot from simulations done with scenario 3 and repeater gain 75 dB. The purpose of Figure 6.7 is to clarify how repeaters will affect cell dominance areas in the network area. Different number of connections at each map pixel is indicated by different color shades: dark shades represent cell dominance and light shades represent SHO areas. The lighter is the color of the pixel, the more connections will exist per one user at that particular map pixel. The information in these maps is based on comparing received pilot power level differences at each map pixel. The rule defined for the SHO algorithm (see Subchapter 5.5.5) is used here. [25]

From Figure 6.7a it is clearly visible, how the dominance area of one Node B is not large enough to cover the hotspots located in the cell edges. Therefore, hotspot users are in SHO with very high probability in this case. It should also be noticed how the dominance areas of all cells are large and clearly visible in the case where repeaters were switched off.

In case of repeaters switched on (Figure 6.7b, repeater gain 75 dB used in this case), clear improvement in dominance is observed inside hotspot areas. Now, repeaters are capable of removing the SHO area (pilot pollution) from the hotspots. If most of the users are located in hotspots in a simulation case (HSDF value 20), repeaters would thereby be very effective in removing load from several surrounding Node Bs, which were earlier connected to the hotspot users via SHO connections. This clarifies the reduction of the SHO probabilities shown in Figure 6.6. This result also leads to improved downlink capacity, which is already expected from the Figure 6.5, which presented the results of averaged Node B transmit power. Finally, it must be noted how the dominance areas of the cells located near repeater cells are shrinked and the corresponding SHO areas are spread to cover wider areas of those cells. This indicates a drawback of repeater: increased pilot pollution areas in surrounding cells.


53

Figure 6.7: Effects of repeaters on the hotspot dominance areas. Scenario 3, repeaters off (a) and repeaters on with gain 75 dB (b).

a)

b)


54

6.1.6. Outage statistics

In Figure 6.8, the information from outage statistics (Y-axis, left) is shown together with the overall network service probability (Y-axis, right) to visualize the behaviour of the network. Figure 6.8 shows that the outage statistics are very similar in nature in all four scenarios.

Service probability breakdown is caused by too high uplink load at Node Bs in all four scenarios. The uplink load limit is exceeded due to increased interference power levels in the network area caused by repeaters. In case of scenario 2 (Figure 6.8b), the breakdown is more severe due to the repeater path loss reduction compared with scenario 1 (Figure 6.8a). In cases of scenario 3 (Figure 6.8c) and 4 (Figure 6.8d), the effect of uplink interference is emphasized because of increased number of repeaters in the network. Results from scenarios 3 and 4 also indicate that the Node B power capacity problem could be totally avoided by the use of repeaters with high repeater gain. However, the limitations in uplink prevents the use of repeaters with high repeater gain.

Figure 6.8: Outage statistics for scenarios 1 (a), 2 (b), 3 (c) and 4 (d) in case of 3000 initial users and HSDF value 1.

(a) (b)

(c) (d)

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55

6.1.7. Network capacity

Network capacity results were treated in the same manner as the previous simulation results: by comparing different repater gain settings and different hotspot traffic density cases, with repeater switched on and off. However, capacity values were conducted by choosing a network load point in uplink and downlink, and then by comparing corresponding network throughput values to the repeater off –case. Each scenario was simulated with three different load cases as presented in the Table 5.5. Figure 6.9 presents the network capacity gains in uplink direction related to the case when repeaters were swithced off.

Figures 6.9a and 6.9b show that when there are only 6 repeaters in the network centre, remarkable gain can not be achieved in uplink direction (results are averaged from whole network area). However, some gain in uplink capacity is visible when HSDF is set to 20 and when the repeater gain is set correctly. Finally, the real benefit of repeaters can be observed from the Figures 6.9c and 6.9d, when repeaters are serving hotspots at almost whole network area. This makes the uplink capacity gain clearly visible, which was already expected from the service probability results in Figure 6.1c and 6.1d. In uplink, 7 % gain in capacity is observed at correct repeater gain setting when no MDT is used (scenario 3). When MDT is applied, the capacity gain increases to 12 %. No significant gain in uplink capacity is observed with low hotspot traffic cases.

Figure 6.9: Relational uplink capacity gains for scenarios 1 (a), 2 (b), 3 (c) and 4 (d).

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56

Figure 6.10 presents downlink capacity gains for the same scenarios as Figure 6.9. Downlink capacity gain curves look similar to the Node B transmit power curves in Figure 6.5 as expected. Up to 50 % increase in downlink capacity is achieved in scenarios 1 and 2 when using high hotspot traffic and high repeater gains. When the number of repeaters in the network is increased (in scenarios 3 and 4), up to 160 % gains in the downlink capacity can be observed. It can also be noticed, how the MDT affects the capacity gain more in uplink than in downlink. This is expected, since interference has more significant effects in uplink than in downlink due to the principal differences between UMTS uplink and downlink directions. These differences were already explained in the Subchapter 6.1.4 when presenting the result for Node B transmit powers.

Table 6.2 presents optimum repeater gains and observed DL capacities assuming that 5 % uplink capacity loss is accepted. The margin of 5 % represents a realistic decision for a radio network planner to make, when setting network capacity targets. Optimum repeater gain and DL capacity values are presented for each hotspot traffic density case and for each repeater scenarios.

Figure 6.10: Relational downlink capacity gains for scenarios 1 (a), 2 (b), 3 (c) and 4 (d).

a) b)

c) d)

45 50 55 60 65 70 750

20

40

60

80

100

120

140

160

180

Repeater gain [dB]

DL

cap

acit

y ga

in [

%]



45 50 55 60 65 70 750

20

40

60

80

100

120

140

160

180

Repeater gain [dB]

DL

cap

acit

y ga

in [

%]



45 50 55 60 65 70 750

20

40

60

80

100

120

140

160

180

Repeater gain [dB]

DL

cap

acit

y ga

in [

%]



45 50 55 60 65 70 750

20

40

60

80

100

120

140

160

180

Repeater gain [dB]

DL

cap

acit

y ga

in [

%]




57

Table 6.2: Downlink capacities at optimum repeater gain settings for scenarios 1, 2, 3, and 4.

HSDF

0.001

HSDF

1

HSDF

6

HSDF

10

HSDF

20

Scenario 1

Opt. GR [dB] 72 72 72 73 72

DL capacity gain [%] 13 16 27 35 48

Scenario 2

Opt. GR [dB] 68 68 67 69 66

DL capacity gain [%] 13 16 22 28 39

Scenario 3

Opt. GR [dB] 63 68 72 73 74

DL capacity gain [%] 26 42 81 123 140

Scenario 4

Opt. GR [dB] 72 73 74 75 75

DL capacity gain [%] 33 51 117 148 158

From Table 6.2 it is clear that the optimum repeater gain value is quite a case sensitive parameter. When only few repeaters are used in the network, the optimum repeater gain seems to be quite stable between different hotspot traffic density cases (scenarios 1 and 2). This can be explained by the small relative amount of repeaters to amount of Node B sites in the network. However, when large amount of repeaters are installed in the network, the optimum value for the repeater gain significantly varies along with the amount of traffic in the hotspots.

In case of scenario 3, the optimum repeater gain value varies between 63 and 74 decibels. This indicates a natural behaviour of the network: low repeater gains should be used in case of empty hotspots (no potential repeater connections) to reduce interference to other cells. When large amount of traffic is located near repeaters (high HSDF value, many potential repeater connections) large repeater gain is suggested to effectively reduce transmit powers.

At this point, the results of the scenario 4 should be compared to the results of the scenario 3. Repeater antenna downtilt effectively prevents interference propagation to other cells. This is indicated by the small difference in optimum repeater gain values when using scenario 4 with different hotspot traffic cases.

When comparing the results for scenarios 3 and 4, it becomes clear, how the different repeater service antenna configurations affect the overall network capacity in case of large amount of repeaters in the network. Downlink capacity is increased by 18 percent units after tilting down the repeater service antennas by only 6 degrees, when using optimum repeater gains.


58

6.2. Error analysis

The simulations for this thesis were made by using voice users only, although UMTS is mainly designed for the packet data users. However, as already noticed in [28], introduction of packet switched (high data rate) users will affect only the absolute gain in network capacity, not the optimum repeater gain setting.

It is clear that a static network simulator always introduces some errors to the simulation results, when compared to real life network behaviour. This is mainly due to the certain assumptions that must have been made with the static simulator. The lack of support for modeling the RRM functions, such as packet scheduling or power control, makes it impossible to perfectly model the behaviour of a mobile communication network. Also, the effect of multipath diversity is not included in the simulations along with RX space diversity at Node B.

In general, realistic approximation of real life propagation environment is a hard task to complete. Radio propagation channel measurement can be run for certain discrete terrain cases (such as cities or rural areas), but the network behaviour is very dependent of the exact radio propagation environment of the area to be planned (e.g., buildings). Thus, generalized propagation models for the transmission path are one source of error, when performing simulations. Studies in [29] show great difference in simulation and measurement results, when using static network simulator. Network capacity is remarkably overestimated, according to the studies in [29], when using modified Okumura-Hata empirical propagation model for the simulations. Due to these issues, simulations should always be verified with measurements. Although earlier repeater downlink measurements (presented in [30]) provided results in line with these simulation results for the optimum repeater gain setting, they are not directly comparable due to differences in the network configuration and RRM functionality.

In these simulations, repeater interference modeling was simplified to a level, that may cause some underestimation to total interference levels in certain network scenarios, as already described in Subchapter 5.4. Also this was caused by the problem of determining the exact radio propagation environment. Another reason for ending in a simplificative solution was to decrease the time required for performing the simulations. Calculation time of the simulation runs was remarkably increased due to increased link loss and interference data, as the number of repeaters in the network increased.

Due to the nature of Monte Carlo approach, averaging causes errors to the simulation results. The amount of error is increased, when the number of performed snapshots is decreased. This is important, since the network behaviour is not simulated in all network locations if the number of randomized snapshots is inadequate. Also the number of users in the network area and the map resolution affects this issue. With great amount of communicating users simultaneously in the network area, needed number of snapshots is smaller than in case of less crowded network. In these simulations, this source of error was estimated to be insignificant due to the suitable map resolution and user conditions.

7. Discussion and conclusions

59


These simulations have shown, that repeaters tend to be very effective in increasing the overall performance of a UMTS mobile communication network, when hotspot traffic is present at repeater service areas. However, attention must pe paid to setting the repeater parameters and settings correctly. Otherwise, repeaters may cause critical performance problems to the network, as it was seen from the simulation results.

It has been seen, how the distance of repeater from the parent Node B affects the total repeater path loss and noise levels of that particular Node B. The closer is the repeater to the parent Node B, the smaller should be the repeater gain in order to avoid excess other cell interference and noise level rise at the Node B receiver. If repeaters are located far at the cell edge, higher repeater gain values can be used. However, the other cell interference issue in uplink should be noted. Repeater service antenna downtilting was shown to be a good choice for those cases with larger repeater distances.

Uplink was observed to be the limiting direction when using repeaters. Similar observations for the behaviour of the uplink direction were already presented in [31]. In the simulations, uplink interference was acting very sensitively when the repeater gain was increased over a certain threshold. The optimum repeater gain was always noted to be a value just below this threshold. For the scenarios 1, 3 and 4 (with 500 m repeater distance), the optimum repeater gain of around 72 dB (equals to GT = 4 dB) was observed. In case of the scenario 2, the optimum repeater gain value was reduced to 68 dB (also equals to GT = 4 dB due to reduced repeater path loss) due to issues described in the previous paragraph. Somehow different results were observed in [14], where the effects of repeaters on hotspot capacity were also studied with simulations by using omnidirectional repeater service antennas. In [14], GT of −15 dB was observed as an optimal value in the uplink direction. This difference could be explained by major differences in simulator configuration and parameters. Finally, some further understanding of GT is needed, since the value should not depend on the repeater service antenna configuration.

To fight against unwanted uplink interference, one option is to use automatically on-off switching implementation, already studied in [32]. In this model, repeater unit inspects the uplink received signal power levels and decides whether there is traffic coming to the parent cell Node B or not. When traffic is observed, the transmission path of the repeater is switched on and the repeater starts functioning. This reduces the additional interference amplification when repeaters are not used for useful transmission. This model requires, however, careful planning of the repeater on-off switching threshold value, since repeater users might be connecting from far away, or by using very low powers.


60

Remarkable increase in overall network capacity was observed at high hotspot traffic densities. Up to 12 percent unit increase was detected in the overall network uplink capacity, when the scenario 4 with downtilted repeater service antennas was used with the highest simulated HSDF value for the hotspots. No remarkable improvement in the capacity was noted with the homogenous overall traffic case. Repeaters had also direct impact on transmit powers as expected. Lowest UE transmit powers were simulated, when the optimum repeater gain values were used. However in downlink direction, the decrease in Node B transmit powers was monotonous. This happened due to the different antenna configurations between UEs and Node Bs, along with the different nature in the characteristics of UMTS uplink and downlink directions. Also a decrease in SHO probabilities was observed with high hotspot traffic, when comparing the results achieved from the cases, when no repeateres were used to the cases with active repeaters. This was mainly caused by the fact, that in high HSDF cases, most of the users were located in areas (cell borders) with high SHO probability. The introduction of repeaters to the area caused clear increase in the hotspot area dominance, and thus clear decrease in the SHO probability values.

Network capacity analysis showed promising results, especially for downlink direction. Up to 160 % increase in overall capacity was observed, when scenarios 3 and 4 were used with large number of repeaters. Also around 10 % increase in uplink capacity was achieved, when repeater gains were set to the optimum repeater gain values. This result encourages to consider repeaters to use larger gain in downlink than in uplink.

An interesting continuation for these repeater studies would be a case, in which multiple repeaters were installed to serve one Node B. This field is already analytically touched in [33]. Even larger downlink capacity gains could be observed with multiple repeater implementation to one cell. However, the noise rise at Node B end will also be larger due to increased number of thermal noise sources in the receiving path.

HSDPA is also very interesting in means of repeater equipped UMTS networks. It is supposed to give increased data rate capabilities to the basic UMTS downlink direction by introducing new digital modulation techniques and enhanced packet scheduling to the radio interface of basic UMTS. Due to the increased SIR requirements of HSDPA, repeaters could be a valuable tool to broaden the HSDPA coverage and capacity in UMTS networks. Some promising link level results are already observed with repeaters and HSDPA in [34].

References

61

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Effects of Repeaters on UMTS Network Performance