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TAMPERE UNIVERSITY OF TECHNOLOGY

DEPARTMENT OF I NFORMATION TECHNOLOGY

Jarosław Łą cki

Optimization of Soft Handover Parameters for

UMTS Network in Indoor Environment

Master of Science Thesis

Subject Approved by the Department

Council on May 11th, 2005

Examiners: Professor Jukka Lempiäinen

M.Sc. Jarno Niemelä


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Preface

This Master of Science Thesis entitled “Optimization of Soft Handover Parameters for

UMTS Network in Indoor Environment” has been written in the Department of

Information Technology at the Tampere University of Technology, Finland. This Thesis

has been completed based on research conducted during my work at the Institute of

Communication Engineering, Tampere University of Technology.

I would like to express my hearty acknowledgements to my supervisor, Professor Jukka

Lempiäinen and my examiner M.Sc. Jarno Niemelä for their excellent guidance andsupervision during my work. I would also say many thanks to my colleagues from

Radio Network Planning Research Group - Panu Lähdekorpi, Jakub Borkowski, and

Tero Isotalo for their help and very nice working atmosphere. I would also thank to

Advanced Techniques for Mobile Positioning (MOT) project for founding the work and

Institute of Communication Engineering for framework.

I would like to express my thanks to Professor Markku Renfors, Ulla Siltaloppi, and

Tarja Erälaukko for their kindness, help with practical matters of my work and studies.

Moreover, I would also direct my thanks to Elina Orava for her assistance related toformal and daily matters of international studies.

Finally, I would like to express my warmest thanks to my parents Iwona and Henryk

and my sister Sonia as well as to my girlfriend Katarzyna, for their love, definite

support, and help during my whole work.

Tampere, December 7th, 2005

Jarosław Łą cki

Insinöörinkatu 60 C 208

33 720 Tampere

Finland

[email protected]

mailto:[email protected]:[email protected]


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Table of contents

Preface.................................... ........................................................................... i

Table of contents .............................................................................................. ii

Abstract ............................................................................................................ v

Tiivistelmä...................................................................................................... vii

List of symbols ................................................................................................ ix

List of abbreviations ....................................................................................... xi

1. Introduction ............................................................................................ 14

2. UMTS system .......................................................................................... 16

2.1 UMTS system architecture .................................................................................... 16

2.1.1 UE..........................................................................................................................17

2.1.2 UTRAN...................................................................................................................17 2.1.3 Core network..........................................................................................................18

2.2 WCDMA radio interface ....................................................................................... 19

2.2.1 Multiple access method.................................................... ...................................... 19

2.2.2 WCDMA parameters..............................................................................................20

2.3 Radio resource management.................................................................................. 21

2.3.1 Admission and load control ...................................................... ............................. 21

2.4 Power control......................................................................................................... 22

2.4.1 Open loop power control .......................................................... ............................. 23

2.4.2 Inner loop power control .......................................................... ............................. 24

2.4.3 Outer loop power control.......................................................................................25

2.5 Handovers .............................................................................................................. 25

2.5.1 Soft handover ......................................................... ................................................ 26

2.5.2 Softer handover......................................................................................................27

2.5.3 Intra-system handover – intra-frequency............................................ ................... 28


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2.5.4 Intra-system handover – inter-frequency............................................................... 28

2.5.5 Inter-system handover.................................. .......................................................... 28

3. Characteristics of radio wave propagation in mobile channels............. 29

3.1 Basic propagation phenomenon............................................................................. 29

3.1.1 Reflection and refraction .......................................................... ............................. 29

3.1.2 Diffraction.................................................... .......................................................... 32

3.1.3 Scattering...............................................................................................................34

3.2 Mobile radio channel ............................................................................................. 35

3.2.1 Multipath propagation..................................................... ...................................... 35

3.2.2 Fast fading ................................................... .......................................................... 36

3.2.3 Slow fading ............................................................ ................................................ 38

3.2.4 Delay spread.......................................................... ................................................ 38

3.2.5 Angular spread ...................................................... ................................................ 39

3.2.6 Coherence bandwidth ...................................................... ...................................... 39

3.2.7 Propagation slope.................... ........................................................... ................... 40

3.3 Characteristics of indoor and outdoor propagation environments ......................... 40

3.4 Indoor propagation channel ................................................................................... 42

4. Soft handover function............................................................................ 45

4.1 SHO performances................................................................................................. 45

4.1.1 SHO procedure and algorithm...............................................................................45

4.1.2 SHO probability and overhead ........................................................... ................... 49

4.1.3 SHO gain ..................................................... .......................................................... 50

4.1.4 SHO features..........................................................................................................51

4.1.5 SHO optimization...................................................................................................52

4.2 SHO optimization methods.................................................................................... 52

5. Measurements environment and setup................................................... 55

5.1 Description of indoor test network and measurements parameters ....................... 55

5.1.1 Antenna configuration ..................................................... ...................................... 56

5.1.2 Measurements equipment................................................. ...................................... 58

5.1.3 Measurements campaign .......................................................... ............................. 59

5.2 Setup of measurements parameters........................................................................ 62


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6. Measurements results ............................................................................. 63

6.1.1 Measurements results in indoor environment..................................................... ...63

6.1.2 SHO gain for various time to trigger values..........................................................66 6.1.3 SHO probability, BER, DROP call values, and SIR target....................................69

7. Conclusions ............................................................................................. 71

References ...................................................................................................... 72


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Abstract

TAMPERE UNIVERSITY OF TECHNOLOGY

International Master Degree Program in Information Technology

Institute of Communication Engineering

Łącki, Jarosław: Optimization of Soft Handover Parameters for UMTS Network in

Indoor Environment

Master of Science Thesis, 75 p.

Examiners: Professor Jukka Lempiäinen, M.Sc. Jarno Niemelä

Funding: National Technology Agency of Finland (TEKES)Department of Information Technology

December 2005

The third generation networks provide high data rate digital communication. In mobile

networks based on WCDMA air access technology, multi-services are enabled and

available in real time. Mobile phone users utilize multimedia streaming with high data

transfers, mainly in indoor locations. Along with new services, the succeedingchallenges are brought for capacity and coverage planning as well as optimization of

parameters controlling the functionality of the network.

In this Master of Science Thesis, optimum parameters for soft handovers were found

based on conducted measurements. Signal propagation in wideband indoor systems has

characteristics of the signal propagating in flat fading channel. It causes fading of the

signal together with large amplitude variations. Such propagation characteristics lead to

a degradation of system performance, which is seen as reduction of capacity, coverage,

or QoS. Soft handover function provides lower signal fading, because of simultaneousconnections via multiple physical radio links, which provide diversity. Implementation

of larger soft handover areas is quite simple and attractive way to improve indoor

system performance.

The aim of this Thesis was to analyze the downlink transmission power gain provided

by soft handover. Measurements were focused on downlink direction, because usually

this direction of data transmission requires higher data rates than the transmission in


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uplink direction. Measurements were conducted in the UMTS pico-cell test network, at

Tampere University of Technology. Soft handover gain was defined as difference

between transmitted power in downlink direction, when only “hard handover” existed,

and transmitted power in downlink direction, when soft handover was enabled. The soft

handover gain was measured for various dynamic and static soft handover parameters,

but along the same measurements route and measurements scenario. Transmission

power gain, provided by soft handover, resulted in lower interference and increased

capacity of the network.


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Tiivistelmä

TAMPEREEN TEKNILLINEN YLIOPISTO

Tietotekniikan kansainvälinen koulutusohjelma

Tietoliikennetekniikan laitos

Łącki, Jarosław: Pehmeän Solunvaihdon Parametrien Optimointi UMTS-

sisätilaverkoissa.

Diplomityö, 75 s.

Tarkastajat: Professori Jukka Lempiäinen, DI Jarno Niemelä

Rahoittajat: TEKESTietotekniikan osasto

Joulukuu 2005

Kolmannen sukupolven matkaviestinverkot tuovat mukanaan nopeita digitaalisia

tiedonsiirtoyhteyksiä kuluttajien hyödynnettäväksi. WCDMA-tekniikkaan pohjautuvat

matkaviestinverkot ovat monipalveluverkkoja tarjoten samalla mahdollisuuden useiden

erilaisten multimediapalvelujen reaaliaikaiseen käyttämiseen. Matkaviestimien käyttäjätkäyttävät nopeita tiedonsiirtoyhteyksiä vaativia multimediapalveluita pääasiassa

sisätiloissa. Palvelujen monipuolistuminen nostaa esiin uusia haasteita

matkaviestinverkon suunnitteluvaiheessa. Näitä haasteita esiintyy sekä verkon peittoa

suunniteltaessa, että verkon kapasiteettia suunniteltaessa. Muutoksia esiintyy myös

matkaviestinverkon toimintaa ohjaavien verkkosuunnitteluparametrien

optimointivaiheessa.

Tämä diplomityö käsittelee WCDMA-radioverkossa tapahtuvien pehmeiden

solunvaihtojen ohjausparametrien optimointia. Optimaalisten solunvaihtoparametrienetsintää varten tehtiin radioverkkomittauksia UMTS-sisätilaverkossa. Optimaaliset

parametrit löydettiin näitä mittaustuloksia analysoimalla ja tutkimalla. Laajan

kaistanleveyden omaava matkaviestinverkko käyttäytyy sisätiloissa kapeakaistaisen

verkon tavoin. Tämä näkyy vastaanotetun signaalin tason suurina vaihteluina.

Seuraukset havaitaan matkaviestinjärjestelmän suorituskyvyn heikkenemisenä verkon

kapasiteetin, palvelun laadun tai peiton osa-alueilla. Pehmeän solunvaihdon osuutta


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kasvattamalla voidaan ehkäistä signaalin tason heittelystä aiheutunutta suorituskyvyn

alenemista sisätilaverkoissa.

Tämän diplomityön tavoitteena oli mitata pehmeän soluvaihdon käytön vaikutusta

tukiasemien lähetystehoihin. Mittausympäristönä toimi Tampereen Teknillisen

Yliopiston Tietotalo-rakennus, johon oli asennettu toimiva, testaamiseen tarkoitettu,

UMTS-sisätilaradioverkko. Mittauksia tehtiin pehmeän solunvaihdon kanssa sekä ilman

sitä. Pehmeän solunvaihdon aiheuttamaa eroa verkon suorituskyvyssä analysoitiin

tutkimalla tukiasemien keskimääräisiä lähetystehoja. Pehmeän solunvaihdon käytöstä

johtuva lähetystehojen aleneminen näkyy suoraan pienentyneinä häiriötasoina ja siten

kasvaneena verkon kapasiteettina. Tämä motivoi tutkimaan pehmeän solunvaihdon

menetelmää tarkemmin.


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List of symbols

α Angle of diffraction

θ i Angle of incidence

θ r Angle of reflection

θ t Angle of refraction

P ang Angular power distribution

S Ф Angular spread

D Average delay

h BTS Base station effective antenna height

B Breakpoint distance

P( τ ) Channel power delay profile

∆ f c Coherence bandwidth

hc Critical height

S d Delay spread

Ld Diffraction loss

v Diffraction parameter

d 2 Distance from knife-edge to the receiver

d 1 Distance from the knife-edge to transmitter

r c Dominant signal component

Rh Horizontal reflection coefficient

Lwi Loss of walls of type i

Φ Mean angle

µ Mean deviation

σ 2 Mean power

E rec Mean value of received signal amplitude

h MS Mobile station antenna height

I 0 Modified Bessel function of the first kind of order zero

n Number of penetrated walls


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K wi Number of penetrated walls of type i

τ Propagation delay

pr Rayleigh/Ricean probability distribution function

E c /N o Received energy per chip to noise ratio

r Received signal amplitude

r s Received slow fading signal

ε1 Refraction coefficient of the first medium

ε2 Refraction coefficient of the second medium

ε r Relative permittivity

K Ricean Ricean K-factor

R Separation between the transmitter and the receiver

σ

Standard deviation

∆T Time to trigger

P Φ _tot Total angular received power

Var amp Variance of received signal amplitude

Rv Vertical reflection coefficient

λ Wavelength


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List of abbreviations

1G First Generation

2G Second Generation

3G Third Generation

3GPP Third Generation Partnership Project

AC Admission Control

AMPS Advanced Mobile Phone Services

BER Bit Error Rate

BLER Block Error Rate

BS Base Station

BTS Base Transceiver Station

CDMA Code Division Multiple Access

CN Core Network

CPICH Common Pilot Channel

CRNC Controlling Radio Network Controller

CS Circuit Switched

DAS Distributed Antenna System

DL Downlink Direction

DRNC Drift Radio Network Controller

EDGE Enhanced Data Rates for Global Evolution

EIRP Effective Isotropic Radiated Power

ETSI European Telecommunications Standards Institute

FDD Frequency Division Duplex

FDMA Frequency Division Multiple Access

GGSN Gateway GPRS Support Node

GMSC Gateway MSC

GPRS General Packet Radio Services

GSM Global System for Mobile Communication


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HC Handover Control

HHO Hard Handover

HLR Home Location Register

HO Handover

HSCSD High Speed Circuit Switched Data

HSxPA High Speed Downlink/Uplink Packet Access

IS-95 Interim Standard 95

ITU International Telecommunication Union

Iu Normalized Network Interface between UTRAN and CN

Iub Interface between RNC and Node BIur Interface between RNCs

LC Load Control

LOS Line of Sight

ME Mobile Equipment

MRC Maximal Ratio Combining

MS Mobile Station

MSC Mobile Services Switching Center

NB Narrowband

NLOS Non-Line of Sight

NMT Nordic Mobile Telephony

Node B BTS in UMTS

OVSF Orthogonal Variable Spreading Factor

PC Power Control

PS Packet Switched

PSTN Public Switched Telephone Network

QoS Quality of Service

RNC Radio Network Controller

RNS Radio Network Subsystem

RRC Radio Resource Control

RRM Radio Resource Management

RSCP Received Signal Code Power


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RSSI Received Signal Strength Indicator

SC Selection Combining

SfHO Softer Handover

SGSN Serving GPRS Support Node

SHO Soft Handover

SIR Signal to Interference Ratio

SRNC Serving Radio Network Controller

SSDT Site Selection Diversity Transmission

TDD Time Division Duplex

TDMA Time Division Multiple AccessTPCcmd Transmission Power Control Command

UE User Equipment

UL Uplink Direction

UMTS Universal Mobile Telecommunications System

USIM UMTS Subscriber Identity Module

UTRA Universal Terrestrial Radio Access

UTRAN UMTS Terrestrial Radio Access Network

Uu Radio Interface between UE and UTRAN

VLR Visitor Location Register

WAP Wireless Application Protocol

WB Wideband

WCDMA Wideband Code Division Multiple Access

WLAN Wireless Local Area Network


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Chapter 1. Introduction 14

1. Introduction

In the beginning of the 20th century, telecommunication became generally accessible

technology and universal form of communication. Initially, speech communication was

enabled, which utilized wired telephony over long distances. In the beginning of 1980’s,

the first generation (1G) analogous mobile communication system NMT (Nordic

Mobile Telephony) and AMPS (Advanced Mobile Phone Services) were launched. This

started the further evolution of the mobile telephony. However, digital mobile

communication systems replaced analogous ones. The second generation (2G) GSM

(Global System for Mobile Communication) system was capable of providing speechcommunication as well as data transfer services. In the beginning, in 2G networks, the

maximum data rate was 9.6 kbit/s. During the next stage, improvements were applied

for existing 2G systems along with HSCSD (High Speed Circuit Switched Data) and

GPRS (General Packet Radio Services), supporting data rates up to 57 kbit/s. WAP

(Wireless Application Protocol) was a standard of applications and protocols introduced

within 2G networks, enabling subscribers to communicate with Internet platforms and

servers. In addition, the wireless access to Internet like WLAN (Wireless Local Area

Network) became an inseparable part of cellular networks, particularly used in the hot

spot places. Next evolution being a step toward the third generation (3G) networks wasEDGE (Enhanced Data Rates for Global Evolution) technology, which adapted the 2G

systems to faster data transfer requirements.

Standardization of 3G mobile communication networks was carried out by ITU

(International Telecommunication Union). The WCDMA (Wideband Code Division

Multiple Access) was selected as radio interface for 3G systems in Europe by the ETSI

(European Telecommunications Standards Institute) in year 1988. Afterwards,

international standardizing organization 3GPP (3G Partnership Project) was caring

standardization process and established common name UMTS (Universal MobileTelecommunications System) for the 3G cellular networks.

UMTS guarantees high data rate communication for mobile subscribers utilizing the

wideband (WB) access technology. Multi-service WCDMA offers various data rates

depending on used service. Originally, maximum data transfer was 2 Mbit/s in downlink

direction (DL). Nowadays, in UMTS networks exist extensions like HSPxA (High

Speed Downlink/Uplink Packet Access) appropriated for higher data throughputs.


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Chapter 1. Introduction 15

Planning phase of coverage and capacity requires different approach in WCDMA

networks compared to GSM networks. In GSM, the coverage and capacity are planed in

two separate phases, while in WCDMA coverage and capacity are strictly tight to each

other. Therefore, planning of coverage and capacity is carried into effect at one stage.

WCDMA calls also for new service challenges, and therefore coverage and capacity

should be carefully planned with appropriate QoS (Quality of Service). There is also

large area of research to be accomplished in order to optimize the network parameters

defined nowadays by 3GPP.

WCDMA is interference-limited network and every additional user is seen as

interference, which decreases the capacity of the network. Coverage and capacity are

mutually dependent on each other in the WCDMA cellular network. This matter should

be considered especially in indoor environment, where large throughputs and high

priority services are utilized. Signal propagation in wideband indoor systems has

characteristics of the signal propagating in flat fading channel. Multipath phenomenon

causes frequent fading of the signal, which degrades the system performance

significantly. One way to counteract is to use larger SHO (Soft Handover) windows,

which provide diversity reducing detrimental fading effect. In this way, SHO causes

lower transmission power in downlink direction, providing gain to the power budget

and reducing overall interference.

In this Master of Science Thesis, the impact of soft handovers in indoor environment in

downlink direction is measured and described. Measurements were conducted in UMTS

indoor test network at Tampere University of Technology. The SHO gain seen as lower

downlink transmission powers, bit error rate (BER), signal to interference ratio (SIR)

target values, together with lower drop call rates, are presented. The Thesis is divided

into two parts, theoretical (Chapter 2, 3, and 4) and measurements (Chapter 5 and 6).

Chapter 2 includes UMTS system architecture and basics of WCDMA radio interface.

Chapter 3 describes fundamental information related to propagation mechanisms,

highlighting the propagation in indoor environment. This knowledge is crucial in

understanding later parts of the Thesis. The algorithm procedure for SHO and all other

crucial issues related to SHO are presented in Chapter 4. Chapter 5 describes

measurements environment and measurements setup. In Chapter 6, measurement results

are presented. Conclusions are drawn in the Chapter 7.


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Chapter 2. UMTS system 16

2. UMTS system

The Universal Mobile Telecommunications System is the one of third generation

communication technologies. UMTS provides fully integrated digital communication

with maximum data throughput up to 2 Mbit/s. High data transfers and compression

methods make possible high quality video streaming and comfortable access to web

servers. UMTS became perfect tool for providing wireless video calls and

videoconferences. It was possible until now using only fixed digital connections. UMTS

uses packet switched connection, which are integrated part of this network. WCDMA

access technology was chosen for radio access technology for UMTS.

In this chapter, UMTS system architecture is presented as well as description of

WCDMA radio interface and radio resource management (RRM).

2.1 UMTS system architecture

UMTS system is divided into three main subsystems; namely, user equipment (UE),

UMTS terrestrial radio access network (UTRAN), and core network (CN). Functional

elements are grouped in UTRAN and CN. UTRAN handle radio related functions. CN

is responsible for gathering and switching the data to the external networks. As a

completion of all system, UE is the radio interface for the user. UE is connected to

UTRAN through radio interface Uu. UTRAN subsystem is connected to CN through

network interface Iu, where radio network controller (RNC) is connected to packet

switched (PS) or circuit switched (CS) part of the core network through Iu CS or Iu PS

interface. Iur interface can be found between RNCs and Iub interface between RNC and

Node B. All of the UMTS elements have logically defined function described briefly

later in this subchapter. These elements are illustrated in Figure 2.1.


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Figure 2.1 UMTS high-level system and network elements [14], [15].

2.1.1 UE

The UMTS UE consists of mobile equipment (ME) and the UMTS subscriber identity

module (USIM). ME is the radio terminal used for communication through Uu interface

directly with Node B. USIM is a smartcard, which include information of subscribers

such as identity, authentication, and other related to security.

2.1.2 UTRAN

UTRAN consist of one or more RNSs (Radio Network Sub-systems). RNS consists of

Node B and RNC. Node B is a unit for the radio transmission and reception. The main

task of Node B is to convert the data traffic between the Uu and Iu interfaces in both

directions. The Node B also takes part in the downlink transmission power control (PC)

performed in inner loop power control. The synonyms to Node B are BS (Base Station)

and BTS (Base Transceiver Station), both used interchangeably. The RNC is the part of

UTRAN, which features the most important rule. RNC is responsible for controlling

integrity of radio resources of the Node Bs connected to particular RNC. Main tasks,


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which belong to RNC, are radio resource control (RRC), admission control (AC), load

control (LC), channel allocation, power control settings, handover control (HC), macro-

diversity, broadcast signaling, and open loop power control (PC). RNC handles data

conversion between Iu, Iur, and Iub interfaces. The Iur interface may connect RNC’s.

This inter-RNC connection enables soft handover between them, otherwise only softer

handover (SfHO) is possible. There are different logical roles of the RNC, i.e., CRNC

(Controlling RNC), SRNC (Serving RNC), and DRNC (Drift RNC). CRNC is

responsible for load and admission control through Iub interface of particular Node B.

SRNC takes control through Iu and Uu interfaces and is responsible for basic radio

resource management operations, such as handover (HO) decisions and power control.

DRNC controls the cells used by the mobile and if needed performs macro-diversity

combining and splitting.

2.1.3 Core network

The core network is divided into two domains; namely, circuit switched (CS) and packet

switched (PS). Circuit switched elements are: mobile services switching centre (MSC),

visitor location register (VLR), home location register (HLR), and gateway MSC

(GMSC). Packet switched elements are: serving GPRS support node (SGSN) and

gateway GPRS support node (GGSN). MSC/VLR (Visitor Location Register) is a

switch that handles circuit switched data and VLR contains visiting user’s profile. VLR

is an integrated part of the MSC, rather than a separate entity. HLR is a database of

home service area containing the user’s profile information, for example identity of

subscribers or sort of services, to which users have accesses. GMSC is a switch, which

connects UMTS to external circuit switched networks like the PSTN (Public Switched

Telephone Network). SGSN works similarly to MCS/VLR, but is usually used for

packed switched connection. This gateway is between RNC and core network. GGSN

works similarly to GMSC, but is used for packet switched connection.

External networks are divided in two groups, circuit switched and packet switched

networks. CS networks provide circuit switched connections like in existing telephony.

PSTN is an example of CS network. PS networks provide packet data services. The

example PS network is Internet.


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2.2 WCDMA radio interface

WCDMA is radio air interface technology used in UMTS network. Although UMTS

system, which is based on WCDMA technology, is compatible with GSM system, the

access to the air interfaces is very different. The main aspects of multiple access

method, WCDMA parameters, code, and channel allocation are considered in this

subchapter.

2.2.1 Multiple access method

There are various schemes of sharing the radio interface by the simultaneously

communicating multiple users. In cellular systems, these methods are TDMA (Time

Division Multiple Access), FDMA (Frequency Division Multiple Access), and CDMA

(Code Division Multiple Access). This multiple access schemes are shown in Figure

2.2.

(a) CDMA (b) FDMA (c) TDMA

Figure 2.2 Multiple access schemes: (a) CDMA, (b) FDMA, and (c) TDMA [14].


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In CDMA technology, the simultaneous users utilize the same frequency, but they are

separated by different codes. FDMA technology divides whole band to sub-bands, and

then assigns each subscriber to unique frequency. TDMA is an air interface that allows

subscribers to use the same frequency, but separates them by time slots. In TDMA

access method, different time slot of a channel are assigned for each user.

2.2.2 WCDMA parameters

UTRA TDD (UMTS Terrestrial Radio Access Time Division Duplex) and UTRA FDD

(UMTS Terrestrial Radio Access Frequency Division Duplex) combines accordingly

time or frequency division multiple access with CDMA scheme. In this technology, user

is assigned to different time, frequency, and unique code. The physical layer parameters

are partly various in UTRA TDD and UTRA FDD modes, as presented in Table 2.1.

Table 2.1 Comparison of UTRA TDD and UTRA FDD physical layer parameters [14].

UTRA TDD UTRA FDD

Multiple access method TDMA, CMDA (inherent FDMA) CDMA (inherent FDMA)

Duplex method TDD FDD

Channel spacing 5 MHz

Carrier Chip rate 3.84 Mcps

Time slot structure 15 slots/frame

Frame length 10 ms

Multirate conceptMulticode, multislot, and orthogonal

variable spreading factor (OVSF)Multicode and OVSF

Interleaving Inter-frame interleaving (10, 20, 40, and 80 ms)

Modulation QPSK

Dedicated channel powercontrol

Uplink: open loop; 100 Hz or 200 HzDownlink: closed loop; rate ≤ 800 Hz

Fast closed loop; rate = 1500Hz

Intra-frequency handover Hard Handover

Inter-frequency handover Hard HandoverSoft Handover

Spreading factors 1 … 16 4 … 512

In UTRA FDD mode, the frequencies are allocated as following 1900-1920 MHz in

uplink direction (UL) and 2010-2015 in downlink direction. For UTRA FDD, also two


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bands are allocated in uplink and downlink direction, consecutively 1920-1980 MHz

and 2110-2170 MHz. The bandwidth of the channel is fixed to 5 MHz and chip rate to

3.84 Mcps. The central frequency of the channel is in raster of 200 kHz [16]. In the rest

of this Thesis, UMTS FDD mode is considered.

In UMTS network, channelization and scrambling codes are used. The combination of

these codes gives pseudorandom code sequence. Channelization cedes like orthogonal

variable spreading factor (OVSF) are used to separate data and control channels of a

certain user in uplink direction and separate a different users in downlink direction.

Scrambling codes are employed to distinguish different UEs in uplink direction and

distinguish cells in downlink direction. Transmission period consists of 10 ms frames,

where every frame contains 15 slots, and each slot consists of 2560 chips.

2.3 Radio resource management

RRM in UMTS network is responsible for the utilization of the air interface resources.

The following aspects of RRM should be considered: optimization of the system

capacity, maintain the planned coverage, guarantee certain level of the quality ofservice. Keeping these aspects at the most optimum level is the priority in the radio

network planning and optimization. Radio resource management can be also divided

into the following functionalities: admission control, load control power control, and

handover control. Accordingly, UE, Node B, and RNC perform these functionalities. In

this subchapter, the terms admission control, load control, power control and handover

control are explained, as it is important for the content of the later part of this Thesis.

2.3.1 Admission and load control

In UMTS, systems capacity and coverage are depended of each other. According to

capacity request, while changing the throughput of existing radio connection between

user and BS or while adding new subscriber, the cell changes its coverage, i.e., cell is

breathing. Before the new UE is added to the cell, admission control function estimates

if addition of new connection will not cause increase of interference by such amount


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that coverage or QoS of existing radio connections will decrease below planned level.

This estimation is prepared separately for UL and DL direction. According to the result

of the estimation, the admission control rejects or accepts the request of establishing

new radio access bearer in certain cell in the network. Admission control gives

permission to connect new UE if both UL and DL are admitted. Admission control is

located in RNC, where load information from couple of cells is available.

Load control has similar role to admission control, ensures that system will be not

overloaded. If admission control works correctly, then load control is used only in

exceptional situations. If the overload in certain cell occurs, then load control recovers

the system to the target load. Load control actions are following: handover to another

WCDMA carrier, handover to GSM, decrease bit rates of real time UEs, drop low

priority calls, reduce throughput of packet data traffic, reduce UL Ec/No (Received

Energy per Chip to Noise Ratio) energy to target level used by UL inner loop power

control, deny DL power “up” commands received from the UE.

2.4 Power control

The UMTS system is an interference-limited system. The main goal is to provide

appropriate signal coverage with maximum capacity and the best quality of service.

Therefore, optimization of transmission power levels is the main task. It also means that

interference introduced by additional users and introduced by high throughputs should

be minimized. The power control is responsible for keeping the power strength at

appropriate level. Approximately 200 MHz band separates transmission in uplink and

downlink direction in frequency domain. Because of frequency separation in both

directions, various path losses in UL and DL direction occur. Therefore, separated UL

and DL power control is needed. Especially in uplink direction, the power control isneeded, because near-far effect occurs only in this direction. This effect occurs, when

one mobile station (MS) near base station uses too high transmission power compared

to other mobiles, located far away from the base station. In downlink direction, power

control is necessary to reduce inter-cell interference. Three types of power control

referred to UL and DL direction are used in UMTS network: open loop power control,

inner loop power control, and outer loop power control. The power control algorithms

involve participation of different parts of the network, which are presented in Figure

2.3.


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Figure 2.3 Power control in UMTS network [14].

2.4.1 Open loop power control

The open loop power control is used for setting initial uplink and downlink transmission

powers, when the UE is attempting to access the network. Because there was no

transmission initiated by UE, open loop power control is responsible for setting the

initial output powers to certain level. This power level is set, based on estimated path

loss from MS to Node B and received information of allowed transmission power levels

in particular cell. In normal condition, the open loop power control tolerance is ± 9 dB

and in extreme condition ± 12 dB [16]. The open loop power control is used to

compensate these negative effects of multipath propagation and is crucial in reducing

near-far effect in uplink direction.


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2.4.2 Inner loop power control

In inner loop power control, in uplink direction, an UE adjusts output transmission

power, as it is required in the transmission power control command (TPCcmd). The

decision whether to increase or decrease UE transmission power, is undertaken in order

to meet signal to interference ratio target value. The estimation of the SIR target value

for each individual inner loop power control is the task of outer loop power control.

Transmission power control is executed 1500 times per second, meaning that one

command is sent in time interval of 0.666(7) ms. In every time interval, “up” (+1) or

“down” (-1) transmission power control command is send, and then UE transmission

power is changed accordingly to the power step size. The UE output power is changed

with power step size of 1, 2, and 3 dB. In addition, smaller step size can be emulated.

The transmission power control ranges are given in Table 2.2.

Table 2.2 Transmission power control ranges [16].

Transmission power control range

1 dB step size 2 dB step size 3 dB step sizeTPCcmd

Lower Upper Lower Upper Lower Upper

+ 1 + 0.5 dB + 1.5 dB + 1.0 dB + 3.0 dB + 1.5 dB + 4.5 dB

0 - 0.5 dB + 0.5 dB - 0.5 dB + 0.5 dB - 0.5 dB + 0.5 dB

- 1 - 0.5 dB - 1.5 dB - 1.0 dB - 3.0 dB - 1.5 dB - 4.5 dB

There are different inner loop power control algorithms used. Two basic ones are

presented here. According to first algorithm, the single power control command changes

the UE output power with particular power control step. In second algorithm, all five

transmission power control “up” commands results in increasing transmission power by

1 dB or all five-transmission power control “down” commands results in reducingtransmission power by 1 dB.


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2.4.3 Outer loop power control

Outer loop power control is responsible for maintaining the required quality of the

communication using the lowest possible power. In outer loop power control, RNC

calculates the SIR target value and sends it to the Node B. The SIR target value is

evaluated accordingly to the BER or BLER (Block Error Rate) value of existing radio

connection between UE and Node B. In case the quality of radio link connection is

lower than the required, higher SIR target value is sent to Node B. If the radio link

quality is too high, a lower SIR value is delivered to Node B. The information of SIR

target value sent to node B is later used by inner loop power control.

2.5 Handovers

Mobile user is allowed to access the network service while moving. Deep variations in

the signal level and interference can be observed, especially in indoor environment.

During change of a location from one cell edge to the other, the signal from serving

base station is worsening. There is need for such a user to change the serving BS and

use the radio resources of the new cell than from the old one, where signal level is

worse. This process is known as handover. Handovers provide freedom in terms of

mobility in cellular networks.

In first generation cellular systems like NMT, handovers were quite simple. In second

generation systems based on TDMA/FDMA access technique like GSM, various

handover algorithms were introduced. In these systems, only so called hard handovers

(HHOs) exist. In a hard handover, old radio link is released before new radio link is

established. WCDMA technology introduces new kind of handovers; namely, softhandovers (SHOs) and softer handovers (SfHOs). Soft and softer handovers are

supported in UTRA FDD mode only. Moreover, WCDMA utilizes sometimes hard

handovers, which can be classified as intra-frequency, inter-frequency, and inter-system

handovers. These types of handovers are supported in both UTRA TDD and UTRA

FDD mode. HHOs can introduce unnecessary high power rise peaks, which result in

high interference causing near-far effect and reducing the capacity. This is also the

reason, why SHOs are very essential in UMTS network.


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Handover procedure can result in drop calls. This could be caused by signaling errors or

lack of the radio resources. The handover failure should be minimized especially in high

performance networks like UMTS. In UMTS network pico-cells for indoor environment

are implemented, meaning that range of such cell is very small compared to the micro-

cells and macro-cells. Usage of smaller cells is beneficial, because this is the way to

boost the capacity, but smaller cells causes that handovers occur more often than in

larger cells. Thus, handovers have to be very efficient, mainly because the access to the

service have to be assured for users during the ongoing call, and when the handovers are

performed.

2.5.1 Soft handover

Soft handovers are very characteristic feature implemented within CDMA technology.

Soft handover occurs, when two or more Node Bs serve mobile station simultaneously.

In soft handover, mobile station is in cell coverage area of two or more sectors

belonging to different Node Bs. UE during soft handover is schematically depicted in

Figure 2.4.

Figure 2.4 Soft handover function.


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The reception of the signal in soft handover is similar to multipath propagation. In

uplink direction, the signal is received by Node B and routed to the RNC. In the RNC,

the signal frames are compared with each other and the best candidate frame is selected.

This process is called selection combining (SC). In downlink direction, the Node B uses

different scrambling codes to distinguish signal coming from different sectors. The rake

fingers in the MS should perform proper despreading on the signal. Later, the signal is

combined based on maximal ratio combining (MRC) principle. The wider discussion

about soft handover is presented in the Chapter 4 and measurements related to soft

handover can be found in Chapter 6.

2.5.2 Softer handover

A softer handover is a special kind of soft handover. In SfHO, the mobile station is

simultaneously connected to adjacent sectors under the same Node B (Figure2.5).

Figure 2.5 Softer handover function.


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During softer handover, the signal reception in downlink direction is similar to the

reception in soft handover. The difference exists only in uplink direction, where signal

received by the Node B is routed to the rake receiver, and then combined with MRC

method.

2.5.3 Intra-system handover – intra-frequency

In UMTS system, hard handovers are possible as well. These intra-system hard

handover are intra-frequency or inter-frequency. The intra-frequency handover occurs

between cells operated within the same WCDMA carrier. Such handover can be

performed in UMTS network, when the MS is in SHO between the cells belonging to

different radio network subsystems and Iur interface is not established.

2.5.4 Intra-system handover – inter-frequency

The inter-frequency handover occurs within the cells belonging to different WCDMA

carriers. Such handover can be completed for example between different cell classes

like pico-cell and micro-cell.

2.5.5 Inter-system handover

Inter-system handover is the one of hard handover types allowed in UMTS network.

This handover is possible between 2G and 3G systems as well as between UTRA TDD

and UTRA FDD mode. Inter-system handover allows coexistence of different network

and can be a solution for balancing the load in the network.


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Chapter 3. Characteristics of radio wave propagation in mobile channels 29

3. Characteristics of radio wave

propagation in mobile channels

In mobile radio environment, in which propagation mechanism occurs, transmitted

signal reaches the receiver through different paths. Signal between a transmitter and a

receiver is disturbed by environmental factors, causing path losses. Path loss models of

radio channel help to predict the received signal strength, and are an important aspect in

the design of radio networks. Propagation models, which characterize signal strength

over large distances between the transmitter and the receiver, are called large scale

propagation models, and they are based on reflection, refraction, diffraction, and

scattering. Small scale propagation models describe rapid changes of the signal over

short distances, i.e., fast fading and results of it. In this chapter, basic propagation

phenomenon, large scale and small scale propagation mechanisms are considered.

3.1 Basic propagation phenomenon

3.1.1 Reflection and refraction

Reflection and refraction takes place, when a propagating wave faces an obstacle of a

large surface compared to the incident wavelength. A part of the wave is reflected from

the medium and part of the wave propagates into a new medium. The part, which has

entered the new medium, is called transmitted or refracted wave. The amount of energy,

which is reflected and refracted, depends on the electrical properties of the boundary between two mediums. These properties are: permeability, conductivity, dielectric

constant, frequency, and polarization as well as the angle of incidence of the

propagating wave. The wave can be completely reflected without any loss of energy, if

it impinges a perfect conductor. Reflection and refraction phenomena change direction,

amplitude, and phase of the propagating wave.

The refraction mechanism is described as following. The wave is refracted, when it

enters a new medium and changes the direction of the propagation at the boundary of


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two mediums. In the new medium, wave also changes its speed of propagation. If the

new medium has higher index of refraction than the previous medium, then the angle

between refracted wave and the line perpendicular to the boundary of two mediums will

be smaller compared to the angle between the wave in the first medium and the line

perpendicular to boundary of two mediums (Figure 3.2). In this case, the propagation

speed of refracted wave will be lower than the speed of the reflected part. Snell’s Law

describes the angle of incidence and the angle of refracted part of propagating wave,

and is given in Equation 3.1 [1],

1 2sin sini t ε θ ε θ = . (3.1)

Here, ε1 and ε2 are the refraction coefficients of the first and the second medium,

consecutively. In Equation 3.1, θ i is the angle between the incident wave and the line

perpendicular to the boundary of two mediums, and θ t is the angle between the refracted

wave and the line perpendicular to boundary of two mediums.

The simple rule describes the behavior of the reflected part of the wave. The wave is

reflected, when impinges upon an object of larger size than the wavelength. Therefore,

the angle between the direction of incident wave and a line perpendicular to the boundary of two mediums is equal to angle between the direction of the reflected wave

and a line perpendicular to the boundary of two mediums. The reflection and refraction

is illustrated schematically in Figures 3.1 and 3.2.

Figure 3.1 Reflection of propagating wave.


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Figure 3.2 Refraction of propagating wave.

In Figure 3.1 and 3.2, θ i indicate the angle of incidence, θ r the angle of reflection, and θ t

the angle of refraction. The reflection differs for vertically and horizontally polarized

waves. The wave is vertically polarized, when its electric field vector oscillates along a

line orthogonal to the direction of propagation. Similarly, the wave is horizontally

polarized, when its electric field vector oscillates along a line parallel to the direction of

propagation. Vertically and horizontally polarized waves are described by Fresnel

coefficients. The vertically polarized wave coefficient Rv is given by [1],

2

2

sin cos

sin cos

r i r v

r i r

R i

i

θ θ

θ θ

+−ε ε −=

ε + ε −. (3.2)

The horizontally polarized wave coefficient Rh is expressed as [1],

2

2

sin cos

sin cos

i r h

i r

R i

i

θ θ

θ θ

− ε −=

+ ε −, (3.3)

where θ i is the angle of incidence, and ε r is the relative permittivity of reflecting

medium. The magnitudes of each Fresnel coefficients as a function of angle of

incidence θ i are shown in Figure 3.3.


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Angle of i nc idence

M a g n i t u d e o f r e f l e c t i o n c o e f f i c i e n t

0 10 20 30 40 50 60 70 80 900

0 .1

0 .2

0 .3

0 .4

0 .5

0 .6

0 .7

0 .8

0 .9

1

hor i zon ta lve r t i ca l

Figure 3.3 The magnitude of vertical and horizontal reflection coefficients [2].

To simplify the problem and to avoid the scattering effect due to rough surface, the

wave is assumed to reflect from a smooth surface of relative permittivity from 2.5, to -

0.025 At Brewster angle, the wave component with vertical polarization will disappear.The Brewster angle in Figure 3.3 is 32 degrees [2].

3.1.2 Diffraction

Diffraction occurs, when the electromagnetic wave impinges with obstruction of large

dimension compared to a signal wavelength. Based on Hyugen’s theory, diffractioncauses secondary waves, which are formed behind the obstructing object and later

propagate in all directions including the direction of primary propagation. This

phenomenon explains how the electromagnetic wave can be received, if there is non-

line of sight (NLOS) situation between a transmitter and a receiver. When a single

object, such as hill or building, causes the diffraction, then knife-edge diffraction model

can be used to estimate path loss due to the diffraction. To calculate the total path loss,

the diffraction loss should be estimated and added to free space propagation loss. The


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NLOS situation caused by knife-edge diffraction of the propagating wave from

transmitter to the receiver is presented in Figure 3.4.

Figure 3.4 Knife-edge diffraction model [6].

Before calculating the diffraction loss, diffraction parameter v has to be defined first as

in 3.4 [7],

( ) ( )1 2 1 2

1 2 1 2

2( ) 2d d d d v h

d d d d α

λ λ

+= =

+, (3.4)

where d 1 is the distance from transmitter to the knife-edge, d 2 is the distance from knife-

edge to the receiver, h is the height between Line of Sight (LOS) path and cross point of

diffracted waves, α is the angle of diffraction, and λ is the wavelength. In real

environment, many obstacles can occur on the way between two antennas. In such case,

the calculation of diffraction losses can be very complex. Bullington has proposed that a

single equivalent obstacle can replace a couple of obstacles. Thus, path loss due to

diffraction can be calculated using single knife-edge diffraction model. This path loss

can be calculated, but first the diffraction parameter form Equation 3.4 must be

estimated. The diffraction losses Ld as a function of diffraction parameter v, are

calculated from Lee’s approximation in Equations from 3.5 to 3.9 [7],


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( ) 0d L v = , (3.5)0.8v < −

10( ) 20log (0.5 0.62 )d L v v= − − , 0.8 0v− < < (3.6)

10( ) 20log (0.5exp( 0.95 ))d L v v= − − , (3.7)0 v<

3.1.3 Scattering

Scattering occurs, when a propagating wave faces an obstacle, which exhibits a rough

surface and the dimension of an obstacle’s surface is smaller than surface of a

propagating wavelength. This rough surface causes that wave is scattered in different

directions and propagates into the areas that would not be covered, when the wave is

diffracted or reflected from smooth surface. To estimate the roughness of the surface,

the Rayleigh criterion has to be used, to define critical height hc, given by [2],

8cosc

i

h λ θ

= . (3.10)

If the height of the obstacle is larger than critical height hc, then the surface is

categorized as rough. When the surface is smooth, then the wave is only reflected.


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3.2 Mobile radio channel

The characteristics of mobile radio channel should be known in order to plan the radio

communication system properly in different environments. The small scale propagation

models of mobile radio channel are based on multipath propagation and fading of the

signal and these in turn, are based on propagation phenomenon described in Subchapter

3.1. The following terms: multipath propagation, fast fading, slow fading, delay spread,

angular spread, coherence bandwidth and propagating slope are important factors for

characterizing the radio propagation environment. Small scale models are used to

describe the rapid fluctuation of the amplitude of a radio signal. This subchapter

explains shortly small scale propagation models in mobile radio channel and introduces

3GPP propagation model for indoor environment.

3.2.1 Multipath propagation

In real mobile radio environment, there are different obstacles, these natural like hills,

trees, mountains and human built like buildings, towers, and houses. These structures

strongly affect the propagating radio wave. The signal on the way to receiver in real

mobile radio environment is exposed for many reflections, refractions, diffractions, and

scattering due to the obstacles. Hence, the received signal consists of many components

of different phase, amplitudes, and delays. Above described phenomenon is called

multipath propagation, and illustrated in Figure 3.5.


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Figure 3.5 Multipath propagation.

3.2.2 Fast fading

In a moving receiver, the multipath components are interfering between each other.

These components are added constructively or destructively in the receiver, causinglarge fluctuations in the received signal level. This problem is called fast fading. In an

NLOS situation, when there is no dominant component received, the phases of received

signal components are uniformly distributed and amplitudes have different values. The

amplitudes and phases of the signal are independent and all components come under

Rayleigh distribution. The Rayleigh probability distribution function is given by [2],

2

2 2

( ),exp 0( )

0r

r r r

p r r

⎧−⎪

2σ⎨⎪⎩

≥= σ0, <

, (3.11)

where r is the received signal amplitude, and σ 2 is the mean power of all multipath

terms. The mean value of envelope E rec and the variance of received signal amplitude

Var amp, are given consecutively in Equations 3.12 and 3.13 [2],


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( ) / 2rec E r π = σ , (3.12)

2( ) 22

ampVar r π ⎛ ⎞⎜ ⎟⎝ ⎠

= − σ . (3.13)

In radio communication, there may exist LOS path between transmitter and receiver.

Therefore, a dominant signal component is received, and then the amplitude of the

received signal is Ricean distributed. Ricean probability distribution function is given

by [2],

2 2

02 2 2)( ) exp( c cr

rr r r r p r I ⎛ ⎜⎜

⎞⎟⎟

σ⎝ ⎠2σ+

= −σ

, (3.14)

where the I 0 is modified Bessel function of the first kind of order zero and r c is the

dominant LOS signal component. When dominant LOS component r c can be reduced to

zero, Ricean distribution becomes Rayleigh distribution. To estimate the power

magnitude of dominant term over whole received power, the Ricean K-factor is derived

and expressed in Equation 3.15 [2],

2

10 2( ) 10log ( c Ricean

r K dB = )

2σ, (3.15)

where r c /2 is the power of dominant signal term. Taking into account the Ricean K-

factor, the Ricean probability distribution function from Equation 3.14 is modified to

the following form,

/10 2 2/10 /10

02 2

10 ( )2 10 2 10( ) exp( ) ( )

K K K c

r

c c

r r

r

r p r I

r r

+−=

c

r . (3.16)


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3.2.3 Slow fading

Large obstacles like mountains and buildings cause slow fading of the received signal.

Slow fading reduces the average level of the received signal. The dynamic range

changed by slow fading is much less than the dynamic range changed by fast fading.

Shadow fading is log-normally distributed. This distribution is defined as [2],

( )

2

1( ) exp( )

2r s

r p r

π 2

− µσ

=σ

, (3.17)

where r s is received slow fading signal, µ is mean deviation, and σ is standard deviation.

The standard deviation of slow fading depends on the environment topology and used

frequency.

3.2.4 Delay spread

Delay spread is the result of the multipath components having different paths lengths,

which arrive at different time in the receiver. Delay spread S d is calculated from the

channel power delay profile P( τ ) as [1],

0

0

( )

( )d

D P d

S

P d

∞2

∞

(τ − ) τ τ∫=

τ τ∫

, (3.18)

where D is average delay, and τ is propagation delay. Delay spread depends strongly on

the environment, where the wave propagates. Delay spread is larger in macrocellular

environment than in microcellular. Maximum excess delay is defined as the time

difference between the first signal and the last signal that arrive to the receiver.


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3.2.5 Angular spread

The deviation of angle of incidence for different multipath components is described by

angular spread. In many situations, it is desired to calculate angular spread S Ф, by using

the following formula [6],

180

180

(ang

tot

P S

P

Φ+2

ΦΦ− Φ_

Φ)d = (Φ − Φ) Φ∫ . (3.19)

In Equation 3.19, Φ is the mean angle, P ang ( Φ ) is angular power distribution, and P Φ _tot is the total angular received power.

3.2.6 Coherence bandwidth

The bandwidth, over which two frequencies of a signal experience the same fading

characteristics, is called coherence bandwidth. The coherence bandwidth ∆ f c is

calculated as a function of multipath delay spread S d . It is defined in the following

equation [6],

1

2c d f

S π ∆ = , (3.20)

where S d is the delay spread. The coherence bandwidth varies, depending on the

multipath delay spread. To avoid correlated fading of two signals, the frequency

separation between them should equal or be higher than coherence bandwidth.


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3.2.7 Propagation slope

The propagation slope indicates the amount of signal attenuation and it changes

accordingly to the propagation environment. In urban environment, the more there exist

obstacles on the signal path, the higher is the signal attenuation, which results in larger

decrease of propagation slope. For free space propagation, this slope equals square of

the distance between a transmitter and a receiver. In decibel scale, propagation slope in

free space equals to 20 dB/dec. In rural environment, propagation slope is 25 dB/dec,

but in urban environment propagation slope can be 45 dB/dec. The distance, where the

propagation slope changes over network coverage area is called breakpoint distance B.

This can be calculated using the following equation [5],

4 TS MS h h

B =λ

, (3.21)

where h BTS is the base station effective antenna height, and h MS is the mobile station

antenna height. This propagation slope is necessary factor, which should be taken in to

account in mobile radio network planning phase.

3.3 Characteristics of indoor and outdoor propagation

environments

There are three major classes of propagation environments: macrocellular,

microcellular, and indoor. Macrocellular environment consist of urban, suburban, andrural. The outdoor environment is macrocellular and microcellular. Picocellular has

been defined also as a name for indoor environment. The signal is variously attenuated

in each of these environments, because of the amount and the distribution of obstacles.

Characteristics of the signal propagation in different environments highlight the major

differences between outdoor and indoor environment. As a comparison to indoor

environment, Table 3.1 contains propagation characteristics of different environments.


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Table 3.1 Characteristics of different radio propagation environment at 900 MHz [5].

Environment

type

Angular

spread

( )

Delay

spread

( s)

Fast

fading

Slow fading

standard

deviation

(dB)

Propagation

slope

(dB/dec)

Coherence

bandwidth

(MHz)

Macrocellular

Urban 5-10 0.5 NLOS 7-8 40 0.32

Suburban 5-10 NLOS 7-8 30

Rural 5 0.1 (N)LOS 7-8 25 1.6

Hilly rural 3 (N)LOS 7-8 25 0.053

Microcellular 40-90


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3.4 Indoor propagation channel

In this subchapter, the propagation characteristics of radio wave in indoor environment

are introduced. These particular characteristics are discussed, because measurements for

this Thesis were conducted in indoor environment. In addition, differences of

propagation parameters from indoor and outdoor environments, presented in previous

subchapter, are discussed.

The indoor environment has clear differences between indoor and outdoor environment.

The major differences can be drawn as in Table 3.3.

Table 3.3 Major differences between indoor and outdoor environment [11].

Indoor Outdoor

Non-stationary in time and space

Deep fluctuations in mean signal level

Not universally established path loss model

Negligible Doppler shifts

Small delay spread

Large angular spread

Lower UE power consumption

Stationary in time and non-stationary in space

Slow changes in mean signal level

Well established path loss model

Large Doppler shifts due to high UE velocity

Large delay spread

Small angular spread

Higher UE power consumption

The indoor environment is characterized by large differences of the signal strength level

over small distances. The propagation in indoor environment differs from outdoor

environment in couple of aspects, especially interference and fading rate. Interference

level in picocellular environment is often higher than in microcellular environment.

Higher interference is caused by spurious emission of electronic devices such as,computers and by different radio systems. The fading signal can fall below certain

signal to interference level and exceed bit error rate threshold, which satisfies good

quality of service. The slowly changing slow fading rate can be explained as follows.

The indoor mobile user can spend quite long time in the locations, where the signal

strength is at low level. This situation is caused by high attenuation over small distances

and low mobility of indoor mobile users. Moreover, delay spread in indoor environment

is very small and on the contrary, coherence bandwidth is high. That fact causes that


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this environment is considered as narrowband. In indoor locations, the angular spread is

much higher than in outdoor environment. This is due to the different surfaces, which

surround the antennas.

The impulse response characterizes the channel at microscopic level. Path loss describes

the channel at macroscopic level. Path loss is very necessary information, because it

allows estimation of the coverage and helps to select optimum location for base stations

and antennas. A proper location of antennas should be chosen to satisfy coverage in the

required area in building. On the other hand, the leakage of the power should be very

low from the covered structure. Building has very complicated structure (furniture,

walls, and door) and there is possibility that power will leave the building, which is an

undesirable situation. However, there do not exist any universal statement for prediction

of indoor propagation characteristics. It is highly dependent on the structural materials

and layout of the building. The radio wave is attenuated variously in different

environments, and therefore different propagation models should be introduced. For

indoor locations, it is suitable, if the prediction of the path loss would be a model based

on the distance between transmitter and receiver for given building structure. The ideal

situation when the path loss can be the most accurately estimated is, if the rooms in

building are uniformly distributed having the same sizes and are made of the same

materials. In such case, the attenuation between each room and each floor would have

the same value, but practically, this situation is impossible. A good practical solution for

estimation of the indoor path loss L is model introduced by 3GPP [8]. The model of the

form given in Equation 3.22 is expressed in dB scale and derived from COST 231

model,

(( 2) /( 1) 0.46)1037 20 ( ) 18.3

n nwi wi L log R k L n

+ + −= + + +∑ . (3.22)

In Equation 3.22, R is the separation between transmitter and receiver in meters, K wi isnumber of penetrated walls of type i, Lwi loss of walls of type i, and n is the number of

penetrated walls. There are two types of walls: light internal and regular internal. It is

assumed that light internal and regular internal type of wall attenuates the signal,

consecutively by 3.4 dB and 6.9 dB. The slow fading deviation in indoor environment is

assumed to be 6 dB. Figure 3.6 presents the path losses between transmitter and receiver

for one, two, and three floors, and rooms with light and heavy walls. The maximum

separation between transmitter and receiver is 50 meters.


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0 5 10 15 20 25 30 35 40 45 5020

30

40

50

60

70

80

90

10 0

11 0

12 0

Distance between transmitter and receiver [m]

P a t h l o s s [ d B ]

0 floor 2 heavy walls



0 floor 2 light walls



Figure 3.6 Path loss with internal walls path loss information.

If the internal walls are not modeled, Equation 3.22 is modified to the following form

[8],

1037 30 ( ) 18.3 (( 2) /( 1) 0.46) L log R n n n= + + + + − . (3.23)

Path loss without internal walls information is shown in Figure 3.7.

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

10 0

12 0

Distance between transm itter and receiver [m]

P a t h l o s s [ d B ]

0 floor s eparation1 floor s eparation2 floor s epartion

Figure 3.7 Path loss without internal walls path loss information.


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Chapter 4. Soft Handover function 45

4. Soft handover function

In this chapter, the SHO performances like SHO procedure, algorithm, gain and other

features are described. In the later part of this chapter, SHO optimization methods are

presented.

4.1 SHO performances

4.1.1 SHO procedure and algorithm

The soft handover procedure is divided into 3 phases: measurement, decision, and

execution. In the measurement phase, mainly the ratio of received energy per chip to

noise ratio ( E c /N o) is evaluated on the downlink common pilot channel (CPICH) based

on received signal code power (RSCP) and received signal strength indicator (RSSI).

The RSCP is the received power of decoded pilot channel. The RSSI is the total

received power in the channel bandwidth. MS performs the measurements of RSCP

and RSSI. The relation between E c /N o, RSCP, and RSSI is described in the following

equation [17],

c

o

E RSCP

N RSSI = . (4.1)

Later, performed measurements are sent by UE to the Node B. All these measurements

parameters are contained in the measurement report, and then passed to RNC, where

the decision phase takes place. Performed measurements are compared with defined

soft handover criteria in the decision phase. This process is carried out by RNC. After

decision phase, the execution of soft handover is accomplished, if the soft handover

criteria are fulfilled. After execution phase, mobile station enters or leaves the soft

handover area.


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The optimization of all soft handover procedure phases should be performed to achieve

the highest possible soft handover gain. Optimization of particular phase can provide a

significant gain in uplink and downlink direction. In the measurement phase, it is

important to apply appropriate filtering time of the E c /N o measurements. Better

accuracy of the measurements works against fast fading, but longer filtering period can

cause unnecessary delays in handovers. This filtering period should be chosen as a

compromise between the accuracy of the measurements and handover delay. The other

important parameter is timing information of different CPICH channels. This

parameter delivers information of time differences of arriving signal from different

cells. This information is crucial for combining received signal components and for

adjusting the power of different signals. The optimization of decision phase, which is

performed by choosing appropriate dynamic and static SHO parameters, is an

attractive way to improve system performance. The optimization should be applied to

SHO algorithm as well. The examples of optimized SHO algorithm and parameters are

described in [18-23] and in later part of Chapter 4.

In explanation of soft handover algorithm, the following terms are crucial, and have to

be defined. Active set contains the list of cells having the connection with MS.

Monitored set contains the list of cells, which CPICH channels power are not high

enough to be added to the active set or, if active set is already full.

There are different soft handover algorithms standardized as the one used in IS-95

standard. Soft handover algorithm discussed in this paragraph is taken form technical

report TR 25.922 of 3GPP specification [24], currently used in UMTS networks.

MS measures continuously the power level of CPICH pilot signals. Based on these

measurements, the RNC decides, which SHO event is triggered. These events are

mainly: radio link addition (event 1A), radio link replacement (event 1C), and radio

link removal (event 1B), which is also called drop event. These events are illustrated in

Figure 4.1 and 4.2. The reporting range is the threshold defining whether the cell

should be added to active set or removed from it. All events are executed depending on

the signal strength level as well as the time to trigger value (∆T). Time to trigger valueis the minimal time, for which the signal level has to be above or below certain

threshold, to trigger certain event. Event 1A is completed, if pilot signal from

monitored set is strong enough to be added to active set, meaning that the signal level

from certain cell is above reporting range plus hysteresis for at least the time to trigger.

The cell can be added, if active set size is not larger than predefined. The event 1B is

executed in similar way, when the signal strength level is below reporting range minus


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hysteresis for the time to trigger. A cell can be replaced (event 1C) from the active set,

if the signal strength level of the worst cell in the active set is lower than the best cell

outside the active set. The difference between these two cells should be higher than

replacement threshold over the time to trigger.

The reference point for reporting range is the best pilot signal. It means that a certain

cell is added or dropped from active set depending on the difference, defined by

reporting range, between its pilot signal power level and the power level of the best

pilot signal in active set. If this difference is smaller or larger than the difference

between best pilot signal and predefined constant value of reporting range, then event

1A or 1B is triggered, respectively. SHO algorithm is illustrated in Figure 4.2. In the

explanation of the soft handover algorithm, all events are triggered in the order

illustrated in SHO scenario in Figure 4.1. This figure presents the user with ongoing

call moving from the cell 1 through cell 2 to the third cell. During the call, all three

events are accomplished in the following order adding (1A), replacing (1B), and

removing radio link (1C).

Figure 4.1 Soft handover scenarios.


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In the beginning, in this scenario, the MS is connected to the cell of the strongest

signal pilot 1 (green line). As the MS moves onwards, the pilot 2 (blue line) reaches

the upper hysteresis boundary of reporting range for the time ∆T and is added to activeset. Now UE is connected to cell 1 and cell 2 simultaneously, meaning that MS is in

the soft handover area. Afterwards, signal strength of pilot 3 (violet line) becomes

better than decreasing power level of pilot 1. The difference between these two pilot

signals becomes larger than hysteresis for replacement, for the time to trigger ∆T andthe event 1C is accomplished, where pilot 1 is replaced with pilot 3. Now the pilot 3

and pilot 2 are in active set, and UE is still in soft handover. After that, as the MS

moves onwards, the power level of pilot 3 decreases below the lower hysteresis

boundary of the reporting range for the time to trigger ∆T, and then pilot 3 is removed

form the active set. The mobile station is again connected only to one BS in cell 2. Inthis case, the active set size is one.

Figure 4.2 3GPP soft handover algorithm [24].


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4.1.2 SHO probability and overhead

The soft handover probability is an important topic in radio network planning. SHO

probability defines the network performance, expressed by capacity or coverage. The

SHO probability can be calculated as a ratio of users in the soft handover to the total

number of users, or as the time during the mobiles are in the soft handover to the total

time of all connections. In this Thesis, the second way of calculating the SHO

probability is used. The soft handover window traces an SHO area where its criteria

are fulfilled. These criteria are mainly adding, dropping thresholds, and their time to

trigger values. Soft handover window has a direct impact on SHO probability. For low

values of adding and dropping thresholds, the SHO window is smaller than for larger

thresholds. According to described SHO algorithm in Subchapter 4.1.1, the larger the

SHO window (Figure 4.3 a), the more probable is for a mobile station to be added to

active set. A situation, where the SHO window is smaller (Figure 4.3 b), results in a

lower SHO probability.

SHO

Window

SHO

Window

(a) Large SHO probability (b) Small SHO probability

Figure 4.3 Different size of SHO window.

During the soft handover, more connections are established. Thus, there is larger use of

the radio resources in the downlink direction, which consumes transmission power and

causes higher interference. The soft handover criteria should be planned carefully and

have to be compromise between SHO gain and additional capacity consumption. The

soft handover gain is strongly dependent of the environment, where the particular

network is operating. Achieved SHO gain is different in UL and DL direction.


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4.1.3 SHO gain

The system level performance of soft handover can be expressed by the following

factors: capacity and coverage provided for certain QoS level, outage probability,

unsuccessful soft handover rates, call blocking, soft handover probability. In CDMA

networks, there is high request for the downlink capacity. One of the key functions in

UMTS network, which can provide more capacity, is the soft handover. The amount of

soft handover is always a compromise between increasing the interference by multiple

connections, and decreasing the interference by lower transmission power for each

radio link. One of the purposes of the soft handover was enabling seamless

transmission. This is important feature in high data rate transmissions, because

possible data loss during handover is eliminated.

Soft handover diversity gain produces macro-diversity and micro-diversity gain,

obtained by different diversity combining methods. Macro-diversity gain is achieved

against slow fading and micro-diversity gain is reached against fast (Rayleigh) fading,

caused by multipath propagation. The macro-diversity gain is different for UL and DL

direction, because of various combining scenarios in each direction.

The SHO gain can be achieved mainly by micro-diversity and macro-diversity,

providing lower interference. It can be explained as following. If the target BLER or

BER of certain connection is at level of 1 %, this is possible to use two links with

lower quality with target BER of 10 %. By multiplying this two different links, the

final target BER, is this required one at level of 1 %. Decrease of the quality of the

links allows lower transmission power in DL direction. Then, interference is lower in

the middle cell, but higher in the neighbor cells. This phenomenon is called soft

capacity. The achievable macro-diversity gain is from 1 dB up to 4.5 dB and its value

depends on the relative path loss. Relative path loss is the difference of path losses

between each serving Node Bs in SHO. There are also many different ways to achieve

gain from soft handover. It is briefly presented and explained in the rest of this

subchapter.

SHO gain is also provided by different cell selection schemes. Cell selection scheme is

responsible for finding the best cells, which mobile should camp on. This process is

based on the measurements of the E c /N o of downlink CPICH. The task of cell selection

scheme is to choose the cell with good enough QoS of serving base station. Cell

selection schemes are taking part in SHO process. There exist different cell selection

umts ha noicc

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