Optimization of Soft Handover Parameters for UMTS Network · PDF filei Preface This Master of...

TAMPERE UNIVERSITY OF TECHNOLOGY DEPARTMENT OF INFORMATION TECHNOLOGY

Jarosław Łącki 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 and supervision 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 to formal 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 Łącki Insinöörinkatu 60 C 208 33 720 Tampere Finland [email protected]

mailto:[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 succeeding challenges 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 simultaneous connections 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: TEKES Tietotekniikan 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 käyttäjät käyttävät 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 solunvaihtoparametrien etsintää 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

θi Angle of incidence

θr Angle of reflection

θt Angle of refraction

Pang Angular power distribution

SФ Angular spread

D Average delay

hBTS Base station effective antenna height

B Breakpoint distance

P(τ) Channel power delay profile

∆fc Coherence bandwidth

hc Critical height

Sd Delay spread

Ld Diffraction loss

v Diffraction parameter

d2 Distance from knife-edge to the receiver

d1 Distance from the knife-edge to transmitter

rc Dominant signal component

Rh Horizontal reflection coefficient

Lwi Loss of walls of type i

Φ Mean angle

µ Mean deviation

σ2 Mean power

Erec Mean value of received signal amplitude

hMS Mobile station antenna height

I0 Modified Bessel function of the first kind of order zero

n Number of penetrated walls

x

Kwi Number of penetrated walls of type i

τ Propagation delay

pr Rayleigh/Ricean probability distribution function

Ec/No Received energy per chip to noise ratio

r Received signal amplitude

rs Received slow fading signal

ε1 Refraction coefficient of the first medium

ε2 Refraction coefficient of the second medium

εr Relative permittivity

KRicean Ricean K-factor

R Separation between the transmitter and the receiver

σ Standard deviation

∆T Time to trigger

PΦ_tot Total angular received power

Varamp 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 B

Iur 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 Access

TPCcmd 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

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 speech communication 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 was EDGE (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 Mobile Telecommunications 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.

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.

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.


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,


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.


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].


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 concept Multicode, multislot, and orthogonal

variable spreading factor (OVSF) Multicode and OVSF

Interleaving Inter-frame interleaving (10, 20, 40, and 80 ms) Modulation QPSK Dedicated channel power control

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

Fast closed loop; rate = 1500 Hz

Intra-frequency handover Hard Handover Inter-frequency handover Hard Handover

Soft 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


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 of service. 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


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 is needed, 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.


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.


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 size TPCcmd

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 reducing transmission power by 1 dB.


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, soft handovers (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.


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.


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.


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.

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


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 tε 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, θi is the angle between the incident wave and the line perpendicular to the boundary of two mediums, and θt 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.


Figure 3.2 Refraction of propagating wave. In Figure 3.1 and 3.2, θi indicate the angle of incidence, θr the angle of reflection, and θt 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 cossin cos

r i rv

r i r

R i

i

θ θθ θ

+−ε ε −=

ε + ε −. (3.2)

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

2

2

sin cossin cos

i rh

i r

R i

i

θ θθ θ

− ε −=

+ ε −, (3.3)

where θi is the angle of incidence, and εr is the relative permittivity of reflecting medium. The magnitudes of each Fresnel coefficients as a function of angle of incidence θi are shown in Figure 3.3.


An g le o f in c id en c e

Mag

nitu

de o

f ref

lect

ion

coef

ficie

nt

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 h o r izo n ta lv e r t ic a 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, diffraction causes 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


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 dv hd d d d

αλ λ

+= =+

, (3.4)

where d1 is the distance from transmitter to the knife-edge, d2 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],


( ) 0dL v = , (3.5) 0.8v < −

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

10( ) 20log (0.5exp( 0.95 ))dL v v= − − , (3.7) 0 v< <1

210( ) 20log (0.4 0.1184 (0.38 0.1 )dL v v= − − − − , (3.8) 1 2v< < .4

100.225( ) 20logdL v

v⎛⎜⎝ ⎠

= − ⎞⎟ . (3.9) 2.4v >

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],

8cosci

h λθ

= . (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.


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.


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, causing large 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( )0

r

rr rp rr

⎧−⎪

2σ⎨⎪⎩

≥= σ0, <

, (3.11)

where r is the received signal amplitude, and σ2 is the mean power of all multipath terms. The mean value of envelope Erec and the variance of received signal amplitude Varamp, are given consecutively in Equations 3.12 and 3.13 [2],


( ) / 2recE r π= σ , (3.12)

2( ) 22ampVar 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 c

rrrr rrp r I ⎛

⎜⎜⎞⎟⎟

σ⎝ ⎠2σ+= −

σ, (3.14)

where the I0 is modified Bessel function of the first kind of order zero and rc is the dominant LOS signal component. When dominant LOS component rc 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 ( cRicean

rK dB = )2σ

, (3.15)

where rc/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

02 210 ( )2 10 2 10( ) exp( ) ( )

KK Kc

rc c

r rr

rp r Ir r

+−=

c

r . (3.16)


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

rp rπ 2

− µσ

=σ

, (3.17)

where rs 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 Sd is calculated from the channel power delay profile P(τ) as [1],

0

0

( )

( )d

D P dS

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.


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 SФ, by using the following formula [6],

180

180

(ang

tot

PS

PΦ+

2Φ

Φ− Φ_

Φ)d= (Φ − Φ) Φ∫ . (3.19)

In Equation 3.19, Φ is the mean angle, Pang(Φ) is angular power distribution, and PΦ_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 ∆fc is calculated as a function of multipath delay spread Sd. It is defined in the following equation [6],

12c

df

Sπ∆ = , (3.20)

where Sd 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.


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 BTS MSh hB =λ

, (3.21)

where hBTS is the base station effective antenna height, and hMS 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, and rural. 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.


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

Environment type

Angular spread

(°)

Delay spread

(µs)

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 <0.01 (N)LOS 6-10 20 15.92 Indoor 90-360 <0.01 (N)LOS 3-6 20 15.92

The picocellular environments have characteristics of narrowband (NB) system even the UMTS is wideband-based system. The system is recognized as wideband (WB), when the signal band is much larger than the coherence bandwidth of the channel. On the contrary, the system is narrowband, when signal band is much smaller than the coherence bandwidth. The characteristics in Table 3.2 present, whether the GSM, UMTS, and IS-95 (Interim Standard 95) systems behave like narrowband or wideband system. Table 3.2 Narrowband or wideband behaviors of different radio propagation environments [5].

Environment Type WCDMA GSM IS-95 Bandwidth 3.84 MHz 0.27 MHz 1 MHz Macrocellular

Urban WB NB/WB WB Rural NB/WB NB NB Hilly WB WB WB

Microcellular NB/WB NB NB/WB Indoor NB NB NB


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


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 n

wi wiL log R k L n + + −= + + +∑ . (3.22)

In Equation 3.22, R is the separation between transmitter and receiver in meters, Kwi is number 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.


0 5 10 15 20 25 30 35 40 45 5020

30

40

50

60

70

80

90

100

110

120

D is tanc e betw een trans m itter and rec eiver [m ]

Path

loss

[dB]

0 floor 2 heavy walls1 floor 2 heavy walls2 floor 2 heavy walls0 floor 2 light walls1 floor 2 light walls2 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

100

120

D is tanc e betw een trans m itter and rec eiver [m ]

Path

loss

[dB]

0 f loor s eparation1 f loor s eparation2 f loor s epar tion

Figure 3.7 Path loss without internal walls path loss information.

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 (Ec/No) 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 Ec/No, RSCP, and RSSI is described in the following equation [17],

c

o

E RSCPN 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.


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 Ec/No 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 value is 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


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.


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 active set. 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 and the 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. In this case, the active set size is one.

Figure 4.2 3GPP soft handover algorithm [24].


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.

SHOWindow

SHOWindow

(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.


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 Ec/No 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


schemes, like this example ones, distance-based, perfect, and normal. The procedure of cell selection is described in [25]. The SHO gain is provided as well by different power control algorithms. There are mainly two power control schemes for UE under SHO; namely, conventional and site selection diversity transmission (SSDT). The capacity gain due to the SHO is a combination of these two algorithms. There are also various modifications of power control algorithm under SHO. As example, one of such modifications is described in [18]. The SHO gain can be also provided by optimum SHO dynamic and static parameters, which change the SHO probability. Finding these parameters was the motivation for this Master Thesis. More information about SHO gain is included in the later part of this subchapter and measurement results in Chapter 6. 4.1.4 SHO features Soft handover introduces many advantages of the network performance. One advantage of soft handover is less “ping-pong” effect. “Ping-pong” effect occurs, when a mobile moves closer and farther from a cell boundary causing frequent handovers. This is mainly happening in hard handovers. This effect in hard handovers can be mitigated by larger hysteresis margins, but in turn, this solution may introduce longer handover delays and higher interference in neighbor cells. “Ping-pong” effect is strongly decreased by soft handovers, where few simultaneous links exist at the same time. The other feature of SHOs is smoother transmission. During soft handover, there is no break point in transmission like during hard handovers. It means that there is no data loss during soft handover. It is important feature in CDMA systems, because high data rates transmissions are allowed and even short transmission brake can result in significant data loss. SHOs introduce also lower UL interference resulting in better communication quality or greater capacity for the same QoS. Soft handover can have also negative effects. There is more complexity in implementation of SHO as well as consumption of additional radio resources in DL direction during the mobile is in SHO. SHO advantages and disadvantages give the reason to consider its parameters in


the planning phase and keep reasonably SHOs probabilities providing gain and possible losses. 4.1.5 SHO optimization Optimization of SHO procedure should be following. First, the aim of optimization should be clarified, as it is for example reduction of outage probability, increasing the capacity or coverage. Then, the manners leading to SHO optimization, like SHO algorithm or parameters of SHO should be chosen. Next, results under optimized SHO should be compared with those results measured without SHO or with different SHO parameters. In this Thesis, optimum SHO parameters, adding and dropping thresholds with their time to trigger values, has been measured, compared, and optimized ones selected.

4.2 SHO optimization methods The purpose of this subchapter is to present the results of already conducted measurements and simulation related to the soft handover function in UMTS network. In already conducted studies, there is very minor amount of research related to optimization of SHO function in indoor environment. Most of SHO study and research is related to outdoor environment. As SHO is a part of radio resource management it is supposed to be a means, providing possible gain in the network power budged, increasing the capacity, coverage, or QoS. This gain is achieved mainly by defining SHO dynamic and static parameter or by changes in SHO algorithm. Very interesting, decision avoided SHO algorithm used for UMTS indoor network is proposed in [18]. This algorithm is designed especially for indoor environment. For unstable signal in picocellular environment and deep variations of the signal, decision avoided SHO algorithm helps to avoid unnecessary handovers. The role of avoided SHO algorithm is to not allow the SHO when the signal level has been rapidly decreased for very short time. Functionality of this algorithm is presented in Figure 4.4.


Figure 4.4 Idea of decision avoided SHO algorithm. In above Figure S1 and S2 are the signals from both BSs and Sm and So are additional thresholds controlling this algorithm. In publication [18] measurements were conducted in indoor environment where users were moving from the coverage area of the one BS to the coverage area another BS. The transmission rate was 256 kbit/s. The handset height was 1.65m. The measurements were taken in the corridor width of 2.9 meters. The phone quality was unacceptable if the power level of one of BSs was below S0 level. The decision avoided handover algorithm exhibits that BS power control can mitigate the interference problems. Simulations showing the SHO gain in UMTS network are included in [19]. Simulations were conducted in 25 hexagonal cells in urban and rural area. The omni-directional antennas were used. Cell radius was up to 2000 meters. The propagation model was following the Okumura-Hata formula. The BS maximum output power was 5W in urban area. MS speed in this environment was up to 65 km/h. For the rural area, the BS output power was 20W and the maximum MS speed was up to 120 km/h. The correlation between BS’s was 50%. Active set size was set up to 3 cells. Adding and dropping thresholds were 1dB with their time to trigger values of 100 ms. Simulation results showed the SHO gain that can be seen as the improvement of the coverage or capacity. In this simulation, the soft handover gain was defined as the difference between the highest transmitted power during hard and soft handover. In this simulation environment, the results showed the SHO gain in downlink and uplink


direction. SHO gain is a function of the static and dynamic parameters, consecutively SHO window and SHO delay. The higher SHO delay and SHO window the larger was the transmission power gain. The gain was achieved for the urban and rural environment. The highest gain up to 3 dB was reached in rural environment in downlink direction. The network was slightly loaded by only one user per one BS. Reference [23] proposed the SHO optimization by choosing the optimal static and dynamic SHO parameters. The simulation model was following. The amount of hexagonal cells was 19. The cell radius was 2000 meters. Duration of the call was 120 seconds and during this time, the 8 kbit/s throughput was used. If the soft handover probability increased and the users could benefit from the diversity gain, then outage probability decreased. At the same time when outage probability was decreasing, the blocking probability was increased because the mean active set size was larger. When the soft handover probability and blocking probability was reduced, then outage probability was increased. Soft handover function provides lower signal fading, because of simultaneous connections via multiple physical radio links, which provide diversity. The SHO is a simple way for providing the power gain in the network. In this Thesis, the measurements prove previous theoretical deliberations of possible SHO gain in indoor environment. The theoretical deliberation of the indoor environment and SHO function gave the reason to conduct the measurements described in details in Chapter 5 and 6.

Chapter 5. Measurements environment and setup 55

5. Measurements environment and setup The purpose of these measurements was to find out the soft handover gain, expected form the previous theoretical deliberation. The measurements were conducted in UMTS indoor test network, installed in Tampere University of Technology, IT department building. The equipment used to provide the research result was: two Node Bs, two discrete antennas, UE, and RNC/Iub simulator [43]. Additionally WCDMA radio interface analyzer was used [42]. In this chapter, indoor test network architecture and equipment used in the measurements are presented. Configuration of the base station, antennas, radiating cables, and locations on the corridors is explained. The key parameters of the network are given. In addition, measurements scenario is described.

5.1 Description of indoor test network and measurements parameters

The measurements were conducted in indoor environment having different rooms, corridors, and surfaces reflecting the wave. This environment was IT department building in Tampere University of Technology. The whole building consists of four floors and the indoor test network covered the whole building. Every floor is build of four small corridors perpendicular to the main corridor. The measurements were accomplished using UTRA FDD frequencies. In the measurements, two base stations were used. Two cells exist in this test network and each belongs to respective BS. Base stations were connected through Iub interface to RNC/Iub simulator. In whole system, five discrete antennas and two radiating cables were installed. In the measurements campaign, only two of those transmitting antennas were utilized. Coaxial cable was employed to connect transmitting antennas and radiating cables to BSs. RNC/Iub was supporting soft handovers between BSs. The most important base station parameters are listed in Table 5.1. Additionally, within these parameters, the radio link measurement period is listed. This parameter means


that DL transmission powers were captured in every second second of each base station. Table 5.1 Base station parameters.

Parameter Value Maximum transmission power 42.6 dBm

Common pilot channel (CPICH) 27 dBm Other common channels (CCCH) 24 dBm Synchronization channel (SCH) 24 dBm

Receiver sensitivity -123.7 dBm

Base Station

Radio link measurement period 2 sec 5.1.1 Antenna configuration The distributed antenna system (DAS) was employed in UMTS test network. Antennas used in measurements were standard UMTS antennas, operating at frequencies between 1920 and 2170 MHz. In indoor test network, discrete antennas, one omni-directional, and four directional antennas as well as two radiating cables were utilized. The signal coming from the BSs is distributed to antennas and radiating cables through coaxial cable, splitters, and tappers [41]. The diameter of coaxial cable was ½” with the signal attenuation 11.3 dB/100m at 2200 MHz [44]. The 2-way splitter and 2-way tapper were part of the indoor test network. Both devices work in frequency range from 800 to 2200 MHz. The signal loss of the splitter between input and both outputs was equal, 3 dB. The tapper signal loss between the input and the first output was 7 dB and between the input and the second output was 1 dB. Antennas were vertically polarized. Omni-directional antenna gain was 2 dBi. In directional antennas, the gain was 6 dBi or 7 dBi accordingly. Vertical and horizontal radiation patterns of directional antenna are introduced in Figure 5.1.


(a) Vertical

(b) Horizontal

Figure 5.1 Antenna radiation pattern: (a) vertical, (b) horizontal [41]. The vertical and horizontal radiation patterns of omni-directional antenna are illustrated in Figure 5.2.

(a) Vertical

(b) Horizontal

Figure 5.2 Antenna radiation pattern: (a) vertical, (b) horizontal [41].


Two radiating cables were used in the UMTS test network infrastructure. The coupling loss of radiating cables was 80 dB and longitude loss 12.4 dB/100m. Length of every radiating cable is approximately 20 meters. The UMTS indoor network scheme can be found in Figure 5.3. Two antennas used in the measurement campaign are highlighted in this figure. This figure contains also coaxial and radiating cable lengths, their attenuations at 2200 MHz, antennas gain, and pilot channel power transmitted by BSs. Based on this information, effective isotropic radiated power for each antenna is calculated and included in Figure 5.3. All antennas and radiating cables are assigned to particular base station and connected to it through tappers and splitters as well as coaxial cables. The STM-1 is physical interface between NetHawk ATM adapter and the Node B.

Figure 5.3 UMTS indoor network scheme. 5.1.2 Measurements equipment The input data changed manually in RNC/Iub simulator during this measurements campaign was various SHO parameters. The most important output data was downlink transmission power of two BSs, SIR target value, BER, SHO probability, and drop call rate. Each BS’s downlink transmission power was captured in every two-second periods by the RNC/Iub simulator. The radio air interface analyzer was used to capture and store the SIR target value, BER, SHO probability. This analyzer was installed on a


laptop computer. Mobile phone was connected laptop computer. The mobile equipment as well laptop computer was placed on small trolley. The height of the trolley was approximately 1 m and height of the MS was 1.6 m. This trolley was moved manually along the corridors, on which the measurements were performed. The speed of the trolley was approximately 2 km/h. Measurement trolley is depicted in Figure 5.4.

Figure 5.4 Measurement trolley. 5.1.3 Measurements campaign In order to calculate the SHO gain, measurements were conducted on the first floor because of the clear SHO area. It means that two antennas belonging to different base stations were symmetrically placed and their beams were partly overlapping on each other. Moving people did not additionally attenuated transmitted signals, because measurements were mostly conducted during weekends and afternoons. Slowly moved trolley was imitating the low mobility user behavior, typical for indoor mobile users. During the measurements, the trolley with laptop and mobile was moved from corridor G to the corridor F, through the C corridor of the building. Measurements route and direction of antennas radiation can be found in Figure 5.5.


Figure 5.5 The measurements route. The images of the measurements route, its corridors F, G, and corridor C are presented in Figure 5.6.

(a) Corridor F

(b) Corridor G


(c) Corridor C

Figure 5.6 Images of measurements route: (a) corridor F, (b) corridor G, and (c) corridor C. While the trolley was moved, the transmission powers from both base stations were recorded by RNC/Iub simulator and stored in a text file. Air interface analyzer recorded the time, when the MS was in SHO area, and when MS was connected to only one cell. This information was necessary to calculate the SHO probability. This analyzer stored also other parameter like BER and SIR target values. The measurements were conducted for the combination of various adding and dropping threshold, with different time to trigger values. During each measurement, the transmission powers were tracked separately for the mobile being in soft handover and for the same mobile being outside the soft handover area. Power levels were later averaged in such manner that first the power outside SHO area was averaged and added to the average power transmitted during SHO. Later, this sum of two values was averaged again. The difference between power transmitted in “HHO” and SHO was defined as soft handover gain. The power transmitted during “hard handover”, i.e., when the SHO window was extremely small, was the reference point for the rest of the measurements. First, the SHO window was changed by various adding and dropping thresholds. The SHO was considered as “hard handover” for the smallest adding and dropping thresholds. The second approach was to show soft handover gain with various time to trigger values. In this case, the reference point for the rest of the measurements was the measurement with the shortest time to trigger value. The soft handover gain was calculated by comparing the powers transmitted during “hard handover” and powers transmitted during SHO. Only one mobile was used for making


a call, and this call was reflected from the RNC/Iub simulator. Reflection of the call means that the call was mobile oriented and mobile terminated by the same phone. Hence, both uplink and downlink connections were present.

5.2 Setup of measurements parameters The aim of this Thesis was to present the optimum SHO parameters for UMTS network in indoor location. In framework of this Thesis, accomplished measurements present the SHO gain expressed as DL transmission power for different SHO windows compared to “hard handover”. For obtaining this gain, various sets of adding and dropping threshold with different sets of their time to trigger values were chosen and results analyzed. The smallest SHO adding and dropping thresholds were, consecutively 0 dB and 1 dB, and the highest ones were 6 dB and 9 dB. The shortest time to trigger values for adding and dropping the signal to and from active set were, consecutively 100 ms and 240 ms, and longest were 160 ms and 1280 ms. The SHO adding threshold 0 dB and dropping threshold 1 dB with the time to trigger values 100 ms and 240 ms was considered as “hard handover”. For “HHO” parameters, almost only one connection existed during the performed call while moving through the measurement route.

Chapter 6. Measurements results 63

6. Measurements results 6.1.1 Measurements results in indoor environment The parameters used in the measurements are shown in Table 6.1. The rows contain the list of adding and dropping thresholds. Adding thresholds are in the left side of the brackets and the dropping thresholds are in the right side of the brackets. Each row is marked with the number form 1 to 12. The columns contain the time to trigger values of adding and dropping thresholds. The value on left and the value on the right value are, consecutively adding and dropping time to trigger values. The columns are numbered from 1 to 5. Numbers of rows and columns are later used in the measurement analyzis. Table 6.1 List of SHO parameters used in measurements.

Measurement number 1 2 3 4 5

Measurement number

Time to triggervalues

Threshold

[40 640] [100 640] [160 640] [160 1280] [100 240]

1 [0 1] (1,1) (1,2) (1,3) (1,4) (1,5)

2 [1 2] (2,1) (2,2) (2,3) (2,4) (2,5)

3 [1 4] (3,1) (3,2) (3,3) (3,4) (3,5)

4 [2 4] (4,1) (4,2) (4,3) (4,4) (4,5)

5 [1 5] (5,1) (5,2) (5,3) (5,4) (5,5)

6 [2 5] (6,1) (6,2) (6,3) (6,4) (6,5)

7 [3 4] (7,1) (7,2) (7,3) (7,4) (7,5)

8 [3 5] (8,1) (8,2) (8,3) (8,4) (8,5)

9 [3 6] (9,1) (9,2) (9,3) (9,4) (9,5)

10 [3 8] (10,1) (10,2) (10,3) (10,4) (10,5)

11 [4 7] (11,1) (11,2) (11,3) (11,4) (11,5)

12 [6 9] (12,1) (12,2) (12,3) (12,4) (12,5)


Table 6.2 presents DL transmission power for different set of SHO parameters. In Table 6.3, SHO probability values are listed, in Table 6.4 SIR target values are given, and BER values can be found in Table 6.5. Table 6.2 DL transmission powers as dBm.

Table 6.3 SHO probabilities as percentage.

Time to trigger values

Threshold

[40 640] [100 640] [160 640] [160 1280] [100 240]

[0 1] 28.54 29.11 30.92 28.42 31.09 [1 2] 28.31 28.67 30.15 27.32 30.87 [1 4] 27.31 27.65 29.17 27.53 30.32 [2 4] 27.12 27.52 28.32 26.42 30.87 [1 5] 26.43 26.53 28.23 26.23 30.11 [2 5] 25.87 25.34 27.32 25.83 29.44 [3 4] 28.43 29.55 30.32 25.21 30.31 [3 5] 27.65 29.21 29.32 24.79 29.73 [3 6] 27.54 28.56 28.64 24.22 29.60 [3 8] 26.43 27.93 28.20 23.58 28.99 [4 7] 24.65 25.93 26.21 23.32 27.40 [6 9] 23.22 24.94 25.18 22.69 26.56


Threshold

[40 640] [100 640] [160 640] [160 1280] [100 240]

[0 1] 21 20 19 23 16 [1 2] 21 20 20 24 17 [1 4] 22 21 20 25 17 [2 4] 23 22 21 26 18 [1 5] 25 25 23 27 20 [2 5] 27 26 24 27 21 [3 4] 29 26 25 30 22 [3 5] 30 28 27 32 24 [3 6] 31 30 28 33 25 [3 8] 35 33 32 36 28 [4 7] 38 36 35 39 31 [6 9] 48 48 47 51 43


Table 6.4 SIR target values.

Table 6.5 BER values as percentage.


Threshold

[40 640] [100 640] [160 640] [160 1280] [100 240]

[0 1] 18.54 20.50 22.45 16.40 23.41 [1 2] 17.21 19.32 23.32 16.20 24.46 [1 4] 16.43 19.12 21.23 17.54 23.85 [2 4] 16.42 18.93 18.16 15.80 25.42 [1 5] 15.12 17.56 16.92 14.12 22.69 [2 5] 14.87 16.74 16.86 15.55 21.74 [3 4] 13.61 15.58 14.63 13.36 18.61 [3 5] 13.34 15.31 15.39 12.83 17.30 [3 6] 12.81 14.27 15.65 11.68 15.57 [3 8] 13.13 16.93 14.34 10.28 15.81 [4 7] 11.79 12.25 14.82 9.36 12.95 [6 9] 9.54 9.18 11.57 9.48 11.20


Threshold

[40 640] [100 640] [160 640] [160 1280] [100 240]

[0 1] 4.30. 6.85 6.51 4.54 8.24 [1 2] 3.78 6.70 6.51 4.17 6.39 [1 4] 5.91 8.82 8.19 3.75 7.36 [2 4] 4.47 5.73 6.65 3.02 6.75 [1 5] 3.38 5.28 6.28 3.44 4.58 [2 5] 3.95 2.94 4.84 3.82 5.28 [3 4] 4.49 3.18 5.39 5.63 4.25 [3 5] 3.27 2.94 3.84 2.73 4.72 [3 6] 3.48 3.57 2.94 2.59 3.25 [3 8] 2.94 3.47 3.06 1.22 2.70 [4 7] 1.17 2.81 2.43 0.98 2.25 [6 9] 1.40 2.18 1.35 0.97 1.98


6.1.2 SHO gain for various time to trigger values For each SHO parameter, the transmitted DL power was measured. The higher was the adding and dropping thresholds, and the longer time to trigger values of SHO, the higher was the SHO probability. These measurements clearly present that the larger is the SHO probability, the lower is the DL transmission power. The SHO with parameter set (12,4) provides the lowest DL transmission power. In this case, where time to trigger values are very long, 160 ms for adding and 1280 for dropping threshold, UE enters and leaves the SHO area slowly, without rapid SHO decisions that could cause large signal variations. The last column in Table 6.2 is the worst case and the column before last one is the best case situation during the measurements in context of DL transmitted power. Accomplished measurements can be also presented as in Figure 6.1 and 6.2.

20

21

22

23

24

25

26

27

28

29

30

1 2 3 4 5 6 7 8 9 10 11 12

Measurement number

[dB

m]

[160 1280][100 640][100 240]

Figure 6.1 DL transmission power for three different set of SHO time to trigger values.


1 2 3 4 5 6 7 8 9 10 11 1218

20

22

24

26

28

30

32

Measurement point

DL

trans

mis

sion

pow

er [d

Bm

]

Best case [160 1280]Typical case [100 640]Worst case [100 240]

Figure 6.2 DL transmission power for three different set of SHO time to trigger values. Figures 6.1 and 6.2 show the comparison of transmission powers between different time to trigger set of values for particular SHO windows. These measurements prove that the gain is transmission power can be achieved using various time to trigger values. The highest SHO gain, approximately 5.5 dB, is obtained for the adding 3 dB and dropping 4 dB thresholds, if the time to trigger values are changed from [100 240] to [160 1280]. If the adding and dropping time to trigger value is too short, the “ping-pong effect” occurs often and the signal from the base stations hops rapidly because of propagation characteristic. When signal does not leave or enter SHO area unnecessarily, the transmitted power is smoother in time domain and average transmitted power is lower. DL Transmission powers are compared also for a certain set of time to trigger values with different adding and dropping thresholds. In these measurements, soft handover gain can be noted at the level almost 4.5 dB. The gain is calculated between transmitted power for adding and dropping 0 dB and 1 dB, and power transmitted for 6 dB and 9 dB adding and dropping thresholds. For all sets of time to trigger values, it can be seen that soft handover gain is significant and reaches almost 6 dB. The highest transmitted power is at the level of 31 dBm and the lowest one is over 23 dBm. As a comparison, the highest possible transmission power of the BS was 8 W, which equals to approximately 39 dBm. All this results are introduced in Figure 6.3.


20

22

24

26

28

30

32

1 2 3 4 5

Measurement number

[dB

m]

[0 1][3 4][4 7][6 9]

Figure 6.3 DL transmission power comparison between 4 different set of SHO adding and dropping thresholds for certain time to trigger values. This behavior of the signal was expected form previous deliberations. Soft handover reduces the fast fading effect, which is typical for the indoor environments. The simultaneous connections via multiple physical radio links reduce the fading dips in the signal that is also visible by the probability distribution function in Figure 6.4. In addition, in this figure it is clearly illustrated how the dynamic range at DL transmission power is larger for small SHO window sizes.

15 20 25 30 35 400

10

20

30

40

50

60

70

80

90

100

[dBm]

CD

F[%

]

Best case [6dB 9dB]Wrost case [0dB 1dB]

Figure 6.4 Cumulative distribution function of DL transmission power for the best and the worst scenario.


These curves are the measure of the signal for the adding and dropping time to trigger values 160 ms and 1280 ms consecutively. The adding and dropping thresholds for red curve are 6 dB and 9 dB, and for the blue curve the 0 dB and 1 dB, consecutively. These results also present the soft handover gain along the powers transmitted during the particular measurement. 6.1.3 SHO probability, BER, DROP call values, and SIR target. As the measurements show, the highest power gain is provided for the situations, when mobile is the longest time in SHO, meaning that the SHO probability should be the highest. This is for the following measurements (12,1), (12,2), (12,3), (12,4), (12,5). Different SHO thresholds have the highest impact on SHO probabilities. Time to trigger values change only slightly the SHO probabilities and transmission powers. The BER values are also lower for the measurements, where the SHO gain was the highest. This is the other benefit from SHO. Next, reduced drop call rate can be assumed a gain. Results show that for higher time to trigger values of adding and dropping thresholds, drop call rates are smaller that is very important feature for mobile operators. This drop calls rates are varying very deeply for SHO used with different time to trigger values. Drop call rates are much more dependent on time to trigger values than on the various adding and dropping thresholds. This is the reason for analyzing drop call rates for only one of set of adding and dropping threshold. The curve in Figure 6.5 presents drop call rates as function of SHO with different time to trigger values and for the adding and dropping threshold is 3dB and 4dB, consecutively. In addition, gain from SHO was decreased SIR target value. The SIR target value was lower for the particular measurements with higher SHO probability.


1 2 3 4 5 0

5

10

15

20

25

30

35

Measurement point

Dro

p ca

ll ra

te [%

]

Figure 6.5 Drop call rates. These measurements clearly show that larger SHO areas should be used in indoor locations, in order to mitigate the fast fluctuations of the signal level. Moreover, to avoid “ping pong” effect, longer time to trigger values are desirable. Short time to trigger values resulted also in the high amount of drop calls, which were affecting smooth transmission. The drop calls can be also avoided by use of larger time to trigger values. Moreover, the time to trigger values up to 640 ms are very short and there was no significant SHO gain, if these values were used. During the measurements, the signal added to active set with the time to trigger values within mentioned range was not significantly prolonging the time of adding the signal to active set. The only marginal SHO gain for lower time to trigger values was expected before. The conclusions of SHO optimum parameters for the radio network planning are clear. The SHO thresholds providing the large SHO area should be used as well as longer time to trigger values.

Chapter 7. Conclusions 71

7. Conclusions In radio network planning it is important to estimate correctly the capacity and plan the coverage for given area. Optimum parameters, responsible for functionality of the network, are needed to satisfy above criteria. The purpose of this Master of Science Thesis was to explore experimentally the effect of soft handover on improvement of the transmission power in downlink direction in UMTS indoor network. The measurements for this Master of Science Thesis were conducted in UMTS indoor test network. Two directional antennas were radiating in such a manner that clear soft handover area was obtained. Performance of downlink direction of transmission was researched. Soft handover gain in downlink transmission power was estimated for various sizes of the soft handover windows, i.e., adding and dropping threshold and their time to trigger values. The gain was calculated by comparing transmission powers during “hard handover” and during soft handover with different parameters. Soft handover gain achieved in these measurements was varying from 3 dB to 5.5 dB, depending on the soft handover window sizes changed by SHO adding and dropping thresholds and their adding and dropping time to trigger values. These measurements proved that soft handover provides downlink transmission power gain, which can be seen as improvement of the WCDMA network performance. In addition, it was shown that BER and SIR target values, together with drop call rates were improved for higher SHO probabilities. SHO provides macro-diversity, which seems to be very crucial for indoor environment. The measurements show the gain for one user. It means that the network with optimum SHO window in real situation could support more users compared to the situation, when small SHO windows are used. As it was mentioned, in WCDMA network the improvement of the network performance can be seen as coverage and capacity extension. High bit rate services should be provided with sufficient QoS, especially for indoor users. This requires reasonable radio resources management. An attractive and simple way to improve the downlink performance in indoor environment is to use larger soft handover areas. This causes lower transmission powers in downlink direction providing more capacity to the network, but also larger overhead is introduced, reducing the network performance. Therefore, further study is required to calculate how much downlink transmission power can improve the capacity in indoor network.

References 72

References [1] J. D. Parson, “The Mobile Radio Propagation Channel”, John Wiley & Sons,

2000. [2] R. Vaughan, J. B. Andersen, “Channels, Propagation and Antennas for Mobile

Communications”, Institution of Electrical Engineering, 2003. [3] F. M. Landstorfer, “Wave Propagation Models for the Planning of Mobile

Communication Networks”, Proceedings of European Microwave Conference, Vol. 1, pp. 1-6, October 1999.

[4] Electrical and Computer Engineering Department, University of Central

Florida, “Wireless Communication Spectrum Guidelines for ITS”, November 1999.

[5] J. Lempiäinen, M. Manninen, “Radio Interface System Planning for

GSM/GRPS/UMTS”, Kluwer Academic Publishers, 2001. [6] R. Janaswamy, “Radio Propagation and Smart Antennas for Wireless

Communications”, Kluwer Academic Publisher, 2001. [7] J. Deygout, “Multiple Knife Edge Diffraction of Microwaves”, IEEE

Transition on Antennas and Propagation, Vol. 14, Issue 4, pp. 480-489, July 1966.

[8] 3GPP TR 25.951 V6.2.0 Release 6, “FDD Base Station (BS) classification”. [9] S. Harju-Jeanty, “Space Diversity in Indoor WCDMA System”, Master of

Science Thesis, Tampere University of Technology, February 2002. [10] F. Babich, G. Lombardi, “Statistical Analysis and Characterization of the

Indoor Propagation Channel”, IEEE Transactions on Communications, Vol. 48, pp. 455-464, March 2000.

References 73

[11] H. Hashemi, “The Indoor Radio Propagation Channel”, IEEE Transactions on Vehicular Technology Conference, Vol. 81, Issue 7, pp. 943-968, July 1993.

[12] K. Kurek, D. Janusek, T. Kosiło, “Characteristics of the Indoor Propagation

Channel in 1.9 GHz Band”, Proceedings of International Conference of Microwaves, pp. 383-386, 2000.

[13] E. Walker, H. J. Zepernick, T. Wysocki, “Fading Measurements at 2.4GHz for

the Indoor Radio Propagation Channel” Proceedings of International Broadband Communications Seminar, pp. 171-176, February 1998.

[14] H. Holma, A. Toskala, “WCDMA for UMTS”, John Wiley & Sons, 2004. [15] 3GPP TS 23.101 version 3.1.0, Release 1999, “General Universal Mobile

Telecommunications System (UMTS) Architecture”. [16] Web site, http://www.umtsworld.com. [17] 3GPP TS 25.215 V4.4.0, Release 4, “Physical Layer – Measurements (FDD)”. [18] K. Nakaya, H. Ohno, Y. Kinoshita, “Performance Evaluation of a New

Decision-Avoided Handover Algorithm for CMDA Indoor Picocellular System”, IEEE Proceedings of 51st Vehicular Technology Conference, Vol. 2, pp. 1503-1507, May 2000.

[19] N. Binucci, K. Hiltunen, M. Caselli, “Soft Handover Gain in WCDMA”, IEEE

Proceedings of 52nd Vehicular Technology Conference, Vol. 3, pp. 1467-1472, September 2000.

[20] C. M. Simmonds, M. A. Beach, “Parameter Optimization for Soft Handover in

the Third Generation Cellular Environment”, Vol. 2, pp. 144-148, April 1995. [21] J. Luo, M. Dillinger, E. Schulz, “Research on Timer Settings for the Soft

Handover Algorithm with Different System Loads in WCDMA”, Proceedings of International Communication Technology Conference, Vol. 1, pp.741-747, August 2000.

References 74

[22] J. A. Flanagan, T. Novosad, “WCDMA Network Cost Function Minimization for Soft Handover Optimization with Variable User Load”, IEEE Proceedings of 56th Vehicular Technology Conference, Vol. 4, pp. 2224-2228, September 2002.

[23] X. Yang, S. Ghaheri-Niri, R. Tafazolli, “UTRA Soft Handover Optimization”,

Third International Conference on 3G Mobile Communication Technologies, pp. 37-41, May 2002.

[24] 3GPP TR 25.922 V6.1.0, Release 6, “Radio Resource Management

Strategies”. [25] J. Lempiäinen, M. Manninen, “UMTS Radio Network Planning, Optimization

and QoS management”, Kluwer Academic Publishers, 2003. [26] 3GPP TR 25.942 V5.3.0, Release 5, “Radio Frequency (RF) system

scenarios”. [27] Q. Guo, “Status of 802.20 Channel Models”, IEEE 802.20 WG Session No. 6,

January 2004. [28] J. P. Castro, “The UMTS Network and Radio Access Technology: Air Interface

Techniques for Future Mobile Systems”, John Wiley & Sons, 2001. [29] 3GPP TS 24.008 V3.1.0, Release 1999, “Mobile Radio Interface Layer 3, Core

Network Protocols – Stage 3”. [30] 3GPP TS 25.433 V4.5.0, Release 4, “UTRAN Iub Interface NBAP Signaling”. [31] ITU-R Recommendation M.1225, “Guidelines for Evaluation of Radio

Transmission Technologies for IMT-2000”, 1997. [32] R. E. Schuh, R. Anderson, A. Stranne, M. Sommer, P. Karlsson, “Interference

Analysis for Dedicated Indoor WCDMA Systems”, IEEE Proceedings of 58th Vehicular Technology Conference, Vol. 2, pp. 992-994, October 2003.

References 75

[33] M. Chopra, K. Rohani, J. Douglas Reed, “Analysis of CDMA Range Extension due to Soft handoff”, IEEE Proceedings of 45th Vehicular Technology Conference, Vol. 2, pp. 917-921, July 1995.

[34] D. Wong, T. J. Lim, “Soft Handoffs in CDMA Mobile Systems”, IEEE

Transactions on Personal Communications, Vol. 4, Issue 6, pp. 6-17, December 1997.

[35] K. Hamabe, “Adjustment Loop Transmit Power Control During Soft Handover

in CDMA Cellular Systems”, IEEE Proceedings of 52ndVehicular Technology Conference, Vol. 4, pp. 1519-1523, September 2000.

[36] A. Argon, S. R. Saunders, “Measurement-based Optimization of In-building

Coverage for UMTS”, Third International Conference on 3G Mobile Communication Technologies, pp. 221-224, May 2002.

[37] C. Mihailescu, X. Lagrange, P. Godlewski, “Soft Handover Analysis in

Downlink UMTS WCDMA System”, IEEE Workshop on Mobile Multimedia Communications, pp. 279-285, November 1999.

[38] M. I. Salkola, “CDMA Capacity – Can You Supersize That?”, IEEE Wireless

Communication and Networking, Vol. 2, pp. 768-773, March 2002. [39] S. L. Su, J. Y. Che, J. H. Huang, “Performance Analysis of Soft Handoff in

CDMA Cellular Networks”, IEEE Journal on Selected Areas in Communications, Vol. 14, Issue. 9, pp. 1762-1769, December 1996.

[40] 3GPP TS 25.331 V3.2.0, Release 1999, “Radio Resource Control (RRC)

Protocol Specification”. [41] Web site, http://www.kathrein.de, Indoor Directional Antenna 824-2170 MHz. [42] Web site, http://www.nemotechnologies.com, Nemo Technologies, Nemo

Outdoor, Air Interface Analyzer for Wireless Networks. [43] Web site, http://www.nethawk.fi, NetHawk Oyj, NetHawk RNC/Iub Simulator. [44] Web site, http://www.draka.fi, Draka NK Cable Ltd, Coaxial Cable ½”.

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