Institutionen för systemteknikDepartment of Electrical Engineering

Examensarbete

Uplink TDMA Potential in WCDMA Systems

Examensarbete utfört i Reglerteknikvid Tekniska högskolan i Linköping

av

Markus Persson

LITH-ISY-EX--08/4085--SE

Linköping 2008

Department of Electrical Engineering Linköpings tekniska högskolaLinköpings universitet Linköpings universitetSE-581 83 Linköping, Sweden 581 83 Linköping

Uplink TDMA Potential in WCDMA Systems

Examensarbete utfört i Reglerteknikvid Tekniska högskolan i Linköping

av

Markus Persson

LITH-ISY-EX--08/4085--SE

Handledare: Rikard Falkebornisy, Linköpings universitet

Erik Geijer LundinEricsson AB

Examinator: Fredrik Gunnarssonisy, Linköpings universitet

Linköping, 31 January, 2008

Avdelning, InstitutionDivision, Department

Division of Automatic ControlDepartment of Electrical EngineeringLinköpings universitetSE-581 83 Linköping, Sweden

DatumDate

2008-01-31

SpråkLanguage

� Svenska/Swedish� Engelska/English

�

�

RapporttypReport category

� Licentiatavhandling� Examensarbete� C-uppsats� D-uppsats� Övrig rapport�

�

URL för elektronisk versionhttp://www.control.isy.liu.se

http://www.ep.liu.se

ISBN—

ISRNLITH-ISY-EX--08/4085--SE

Serietitel och serienummerTitle of series, numbering

ISSN—

TitelTitle

Potential med tidsdelad uplänk i WCDMA systemUplink TDMA Potential in WCDMA Systems

FörfattareAuthor

Markus Persson

SammanfattningAbstract

The evolvement of the uplink in the third generation mobile telecommunicationsystem is an ongoing process. The Enhanced Uplink (EUL) concept is beingdeveloped to meet the expected need from more advanced services, like videostreaming and mobile broadband. One idea for further improvement in the EULconcept is to introduce Time Division Multiple Access (TDMA), which is studiedin this master thesis.

The master thesis assignment is to study the consequences of introducingTDMA in EUL. The goal has been to identify the gains and problems, and howthey can be handled. A derived theoretical framework and system simulations,using a radio network simulator, are used.

The overall conclusion is that there is a potentially large gain with anintroduction of TDMA in EUL. Simulations in favorable conditions have shownthat the system throughput can increase by 100% when there are only UserEquipment (UE) that are using EUL in the system and by 50% when there is amix of speech and EUL UE’s. When using TDMA the uplink load also showsimprovements, the mean is generally higher but the variance is generally smaller.

Due to major differences in experienced interference between passive and activeUE’s, the signal quality will very a lot. The big variation in signal quality isidentified as the main problem with introducing TDMA in EUL. It is shown thatthis problem can generate extreme high uplink load, which have a negative impactboth on the resource efficiency and the coverage.

NyckelordKeywords 3G, UMTS, WCDMA, TDMA, EUL, Enhanced Uplink

AbstractThe evolvement of the uplink in the third generation mobile telecommunicationsystem is an ongoing process. The Enhanced Uplink (EUL) concept is being devel-oped to meet the expected need from more advanced services, like video streamingand mobile broadband. One idea for further improvement in the EUL conceptis to introduce Time Division Multiple Access (TDMA), which is studied in thismaster thesis.

The master thesis assignment is to study the consequences of introducingTDMA in EUL. The goal has been to identify the gains and problems, and howthey can be handled. A derived theoretical framework and system simulations,using a radio network simulator, are used.

The overall conclusion is that there is a potentially large gain with an intro-duction of TDMA in EUL. Simulations in favorable conditions have shown thatthe system throughput can increase by 100% when there are only User Equipment(UE) that are using EUL in the system and by 50% when there is a mix of speechand EUL UE’s. When using TDMA the uplink load also shows improvements, themean is generally higher but the variance is generally smaller.

Due to major differences in experienced interference between passive and activeUE’s, the signal quality will very a lot. The big variation in signal quality isidentified as the main problem with introducing TDMA in EUL. It is shown thatthis problem can generate extreme high uplink load, which have a negative impactboth on the resource efficiency and the coverage.

v

Acknowledgments

I would like to thank all at the Radio Resource Management department at Erics-son for a great master thesis experience. I especially want to thank my supervisorat Ericsson AB Erik Geijer Lundin for always take the time to answer all myquestions, discuss my work or give me good advice. I would also like to thank myexaminer Fredrik Gunnarsson and my university supervisor Rikard Falkeborn forshowing interest in my work and making useful comments on my report.

Finally a big thank to my family and friends for all the support given duringthe autumn.

Stockholm, January 2008Markus Persson

vii

Abbreviations

3G 3rd Generation mobile communication system3GPP 3rd-Generation Partnership ProjectACK AcknowledgementARQ Automatic Repeat RequestBLER Block Error RateCDMA Code Division Multiple AccessCIR Carrier-to-Interference RatioCDF Cumulative Distribution FunctionCN Core NetworkCQI Channel Quality InformationdB DecibeldBi Decibels over IsotropicDCH Dedicated ChannelDPCCH Dedicated Physical Control ChannelDPDCH Dedicated Physical Data ChannelE-DCH Enhanced Dedicated ChannelE-DPCCH Enhanced Dedicated Physical Control ChannelE-DPDCH Enhanced Dedicated Physical Data ChannelEUL Enhanced UplinkFDD Frequency Division DuplexFDMA Frequency Division Multiple AccessHARQ Automatic Repeat RequestHSDPA High-Speed Downlink Packet AccessHSPA High-Speed Packet AccessHSUPA High-Speed Uplink Packet AccessIC Interference CancellationILPC Inner Loop Power Controlkbps Kilobit per secondMbps Megabit per secondNACK Negative AcknowledgementNBI Narrow Band InterferenceNMT Nordic Mobile TelephonyOLPC Outer Loop Power ControlOVSF Orthogonal Variable Spreading Factor

ix

x

PIC Parallel Interference CancellationPSTN Public Switched Telephone NetworkRBS Radio Base StationRNC Radio Network ControllerRoT Rise over ThermalRTT Round-Trip TimeSF Spreading FactorSIC Successive Interference CancellationSIR Signal to Interference RatioTDD Time Division DuplexTPC Transmit Power ControlTTI Transmission Time IntervalTDMA Time Division Multiple AccessUE User EquipmentUMTS Universal Mobile Telecommunication SystemUTRAN UMTS Terrestrial Radio Access NetworkWCDMA Wideband Code Division Multiple AccessWBI Wide Band Interference

Contents

1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Related Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Problem Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.4 Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.5 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Cellular Radio Systems 52.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.1 Distance Path Gain . . . . . . . . . . . . . . . . . . . . . . 62.2.2 Shadowing Fading . . . . . . . . . . . . . . . . . . . . . . . 62.2.3 Fast Fading . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2.4 Antenna Gain . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 Receiver Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3.1 Rake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.3.2 G-Rake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4 Multiple Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.4.1 FDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.4.2 TDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.4.3 CDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.5 Cellular System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.5.1 Uplink and Downlink . . . . . . . . . . . . . . . . . . . . . 112.5.2 Power Control . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.6 UMTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.6.1 WCDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.6.2 Network Architecture . . . . . . . . . . . . . . . . . . . . . 122.6.3 Uplink Power Control . . . . . . . . . . . . . . . . . . . . . 122.6.4 Interference Cancellation . . . . . . . . . . . . . . . . . . . 13

2.7 EUL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.7.1 EUL Channels . . . . . . . . . . . . . . . . . . . . . . . . . 142.7.2 Short TTI . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.7.3 Scheduling . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.7.4 Hybrid ARQ . . . . . . . . . . . . . . . . . . . . . . . . . . 16

xi

xii Contents

3 Simulator Model 173.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.1.1 Shannon’s Theorem . . . . . . . . . . . . . . . . . . . . . . 193.1.2 Noise Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.1.3 Interference Cancellation . . . . . . . . . . . . . . . . . . . 20

3.2 Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.3 Receiver Antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.4 Network Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.5 Traffic Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.5.1 Upload with EUL . . . . . . . . . . . . . . . . . . . . . . . 213.5.2 Upload without EUL . . . . . . . . . . . . . . . . . . . . . . 223.5.3 Speech . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

3.6 Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.7 Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.8 UE Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.9 Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.10 Hybrid ARQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.11 TDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.12 Interference Cancellation . . . . . . . . . . . . . . . . . . . . . . . . 253.13 EUL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.13.1 Scheduling . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.14 Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.15 System Logging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4 Results 274.1 CIR Overshoot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.1.1 Theoretical . . . . . . . . . . . . . . . . . . . . . . . . . . . 274.1.2 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.2 Performance Improvement Evaluation . . . . . . . . . . . . . . . . 304.2.1 Theoretical . . . . . . . . . . . . . . . . . . . . . . . . . . . 304.2.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.3 Power Rushes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324.4 Interference Compensation . . . . . . . . . . . . . . . . . . . . . . . 34

4.4.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 354.5 Minimum Timeslot Length . . . . . . . . . . . . . . . . . . . . . . 35

4.5.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 384.6 Interference Cancellation . . . . . . . . . . . . . . . . . . . . . . . . 38

4.6.1 Minimum Timeslot Length with SIC . . . . . . . . . . . . . 394.6.2 System Analysis . . . . . . . . . . . . . . . . . . . . . . . . 42

4.7 TDMA Combined with CDMA . . . . . . . . . . . . . . . . . . . . 45

5 Conclusions 475.1 Further Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Bibliography 49

A CDF 51

Contents xiii

B Decibel 52B.1 dB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52B.2 dBW and dBm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52B.3 dBi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

C E-DCH UE Categories 53

Chapter 1

Introduction

During the past years, there has been a tremendous growth in both the numberof users and amount of traffic in cellular systems. Today, the most widespreadmobile communication is digital cellular, with more than 3 billion users, which isalmost half of the world’s population. The ordinary voice service is still the mostimportant service that mobile users rely on and expect high quality from, but newservices such as mobile broadband are growing stronger, with new demands oncapacity.

1.1 BackgroundSince the first release, Rel 99, of the third generations’ mobile telecommunicationsystems were introduced, the evolvement has continued. WCDMA Release 5 in-troduced the improved downlink, High-Speed Downlink Packet Access (HSDPA),with better resource utilization thus higher bit rates. The natural step with Re-lease 6 was the introduction of the improved uplink, High-Speed Uplink PacketAccess (HSUPA) or Enhanced Uplink (EUL).

The algorithms used in EUL have since the introduction been improved. Theyhave been improved to the extent that it is hard to gain much more with the useof the same technique. To further increase the capacity in the uplink a radicalchange has to be done.

One promising idea to increase the capacity and improve EUL further is tochange the access scheme, from Code Division Multiple Access (CDMA) to TimeDivision Multiple Access (TDMA)

1.2 Related WorkSince TDMA in EUL is a fairly new concept there are not yet many studies pub-lished. In most studies where TDMA are compared to CDMA, there is only low

1

2 Introduction

bit rate users, such as speech users, instead of high bit rate users which is morerelevant in this study. Those studies often compare delays and the maximum ex-pected number of users that can be admitted.

In study [10], there is shown that a system with high packet load which usesTDMA have the same or higher throughput as if the system would use CDMA.But the main focus in this is study is more on the number of admitted users anddelays, so it differs from this study and not directly comparable

In study [8], the effects of discontinuous transmission in WCDMA systems isconsidered, which is similar to TDMA. It is shown that when users switch fromtransmitting to not transmitting and vice versa the channel conditions for theother users in the cell changes, which cause variation in the signal quality. Furthermore it is shown that this problem is getting worse with increasing bit rates.

Another study in [7], a changed power control algorithm in presented. The al-gorithm is capable of compensate for large power variations faster than the presentone. This might be an interesting approach to solve the problems that can occurwhen introducing TDMA in EUL.

1.3 Problem StatementUMTS networks use WCDMA as its air interface. If more than one User Equip-ment (UE) uses a high bit rate in the same cell, the interference increases andthus the coverage decreases. This forces the radio base station to reduce the bitrate for the UE’s to maintain coverage. One idea to handle this problem and alsoimproving the radio resource utilization is to change the access scheme to TDMA.The idea with TDMA is that the UE’s should take turn in transmitting and henceexperience less interference from other UE’s.

The master thesis assignment is to study the consequences of introducingTDMA in UMTS networks. What are the main problems and how can they besolved or the effects of them minimized.

The goal is to show that there are potentially significant gains with an in-troduction of TDMA under favorable conditions. Moreover, to identify the mainproblems and limitations that the existing systems bring. All results shall bothbe explained theoretically and supported by simulations.

1.4 ApproachThe following steps are taken to answer the problems stated in Section 1.3.

• Create a reference case with CDMA

• Adjust the simulator for the new access scheme, TDMA

1.5 Thesis Outline 3

• Develop and use a framework for a theoretical analysis

• Run simulations to identify the main problems

• Solve or minimize the effects of the discovered problem, verify with simula-tions.

1.5 Thesis OutlineChapter 2 gives an introduction to cellular system in general, followed by a partmore focused on the third generation’s mobile systems and particular the evolu-tion on the Enhanced Uplink. In Chapter 3 there is a system model to give anunderstanding for the system. The first part contains an introduction of the mainnotations followed by how the dynamic system is modeled.

The results are presented in the fourth chapter; the intention is to show whatthe main problems and possible solutions are. This is followed by the overallconclusions in Chapter 5.

Chapter 2

Cellular Radio Systems

This chapter gives an introduction to cellular systems. Most focus is on WidebandCode Division Multiple Access (WCDMA), which is the air interface, and theconcept of the Enhanced Uplink (EUL). A more detailed description of WCDMAcan be found in [5, 6]

2.1 History

The first generations (1G) mobile systems were introduced in the early 1980’s;they were analog networks and design for speech services. Several standards wereused around the world; one of them was the Nordic Mobile Telephony (NMT)which was introduced in the Nordic countries in 1981.

During the 1980’s the second generation (2G) of mobile networks were devel-oped, which used digital communication. With digital technology the capacityand quality were increased and it was possible to develop more interesting ser-vices. Europe launched the GSM project which presented a standard in the mid1980’s. In the mid 1990’s GSM was developed to support packet data, which oftenare referred to as 2.5G.

Demands for more advanced services e.g., video telephony, streaming videoand web browsing pushed the development further. The result was the thirdgeneration mobile systems (3G) with the first standard settled in 1998. In Europethe standard 3G system was named Universal Mobile Telecommunications Services(UMTS) with the WCDMA as its air interface. These systems use the radioresources more efficient and can therefore offer higher data rates and increasedcapacity compared to 2G.

5

6 Cellular Radio Systems

2.2 PropagationWhen radio signals are transmitted and propagate in air or close to obstaclessome basic propagation mechanisms will occur, such as reflection, diffraction anddistance attenuation. Those mechanisms result in a varying and unpredictablesignal which will cause attenuation of the received power. The total attenuationor power gain, g, is the ratio between the received power, Prx, and the transmittedpower, Ptx, and can be expressed as,

Prx

Ptx= g = gpgsgfga < 1 (2.1)

where, gp is the distance path gain, gs is the shadowing fading, gf is the fast fadingand ga is the antenna gain. All factors of the total power gain are described inmore detail below.

2.2.1 Distance Path GainThe attenuation due to the distance between the transmitter and the receiver isdescribed by the path gain. The path gain is often modeled as,

gp =C

Rn< 1 (2.2)

where C, n are constants and R is the distance in meters. In the simplest of allpropagation models, free space propagation, n = 2. Here all obstacles that mayaffect the field is disregarded. In more realistic models for telecommunication,often derived from the Okumura-Hata propagation model, n often varies between3 and 5. [2]

2.2.2 Shadowing FadingShadowing fading occurs due to large objects or terrain obstacles e.g., hills andlarge buildings between the transmitter and receiver. As a transmitter, or receiver,moves through an environment it will be shadowed from the receiver, or transmit-ter, by different obstacles. This cause the received signal power to fluctuate. Sincethe objects are on a large scale it will take some time for the transmitter to moveout of a shadow region, this makes the shadowing fading quite slow. A commonmodel for these variations is a log-normal distribution, an example on how it canlook is in Figure 2.1. [2]

2.2.3 Fast FadingFast fading, or multipath fading, is an effect of propagation that results in radiosignals reaching the receiving antenna by two or more paths. Those paths havedifferent length and therefore the received signal components will vary in phase.This may generate constructive or destructive interference, resulting in rapid vari-ations in received signal power. The standard statistical model for fast fading is adistribution called Rayleigh distribution, an example is shown in Figure 2.1. [2]

2.3 Receiver Antenna 7

Figure 2.1. A typical attenuation of the received signal in a mobile radio link wherethe fast and shadowing fading is illustrated. [2]

2.2.4 Antenna GainAntenna gain is the ratio of the power density of an antennas radiation pattern inthe direction of strongest radiation to that of a reference antenna. The referenceantenna is often an isotropic antenna and the gain is expressed in dBi (decibelsover isotropic). The gain of an antenna is a passive phenomenon; no power is addedby the antenna but redistributed to provide more power in a certain direction andless power in others. [2]

2.3 Receiver AntennaTo efficient collect signal energy that has been dispersed in time by multipathfading in a cellular radio system, an efficient receiver antenna has to be used.Today’s radio systems employ Rake receivers, which is described in the followingsection.

2.3.1 RakeDue to multipath fading the received signal may consists of more than one copyof the transmitted signal. The difference between the signal components are delayand attenuation. A Rake receiver consist of multiple correlation receivers calledfingers, those finger can be adapted to the different delays of the signal compo-nents. Each finger creates an image of a delayed component of the transmittedsignal. The Rake receiver combines all images to minimize the affects of multi-path propagation which reduce the own interference. The problem with a Rakereceiver is that it assumes that the interference is white noise, but with multipathpropagation the interference is colored. A Rake receiver is therefore not the bestsolution in a CDMA system. [4, 9]

2.3.2 G-RakeThe Generalized Rake (G-Rake) is an improved Rake which can handle coloredinterference better. One improvement in the G-Rake receiver is that there are

8 Cellular Radio Systems

extra interference fingers. Those fingers are used to capture information of theinterference and make a better approximation. [4, 9]

2.4 Multiple AccessMultiple access allows several users to transmit over the same radio resource orcarrier frequency. There are a lot of different schemes that are well suited forwireless communication. This section will briefly describe the most common intelecommunication with focus on Time Division Multiple Access and Code DivisionMultiple Access.

2.4.1 FDMAFrequency Division Multiple Access (FDMA) is a scheme which allows severalusers to share the radio spectrum. The available bandwidth is divided into anumber of band-pass channels. Between the channels there is often a certain gapor guard interval to ensure that channels do not interfere with each other, thoseguard interval leave some of the bandwidth unused. Users are assigned one ormore channels and transmit on those frequencies throughout time, illustrated inFigure 2.2(a).

2.4.2 TDMATime Division Multiple Access (TDMA) is another scheme which allows usersto share the same frequency channel. By dividing the frequency channel into anumber of timeslots, the users can be separated in time. Users are assigned oneor many time slots in which they are allowed to transmit using the full systembandwidth, illustrated in Figure 2.2(b). To achieve non-overlapping signals guardinterval, like in FDMA, and synchronization in time is needed. [5]

(a) FDMA (b) TDMA

Figure 2.2. FDMA and TDMA.

2.4 Multiple Access 9

2.4.3 CDMACode Division Multiple Access (CDMA) is a multiple access scheme where usersare separated by codes. Each users is assigned a unique code sequence (spreadingcode) and can therefore use the entire frequency domain the whole time, thus thereis no physical separation in frequency nor in time for different users. This resultsin mutual interference between the users.

On the transmitting side the user data is multiplied by the assigned spreadingcode. The spreading code consists of chips and there are chip rate, Rc, chips persecond. The spreading results in a spread signal in the frequency domain, there-fore the name spreading code. The ratio between the chip rate and the data rate,n, is referred to as the spreading factor (SF) or processing gain.

The spreading codes are so-called Orthogonal Variable Spreading Factor (OVSF).Those codes are arranged so that they are perfectly orthogonal to each other, evenif the data rates are different. This holds as long as the assigned codes for theusers are selected from different branches in the code tree. The code tree is definedby the tree structure in Figure 2.3.On the receiver side the spread signal is multiplied by the same spreading code

Figure 2.3. Code tree that is generating OVSF codes of varying length.

and the original data sequence can be retrieved. If another orthogonal spreadingcode is used to decode the same spread signal, the result will be interpreted asnoise or interference. The spreading and despreading procedure is illustrated inFigure 2.4.

Since all other users signals will be interpreted as interference, care must betaken so the interference caused by other users does not exceed the own signalpower. If that happens the own signal will drown in interference and may notbe retrievable from the despreading. As the wanted signal is despreaded the de-spreading gives a gain compared to other spread signals. This gain is called theprocessing gain, Gp, and can be calculated from Equation 2.3. Here, Rc is the

10 Cellular Radio Systems

Figure 2.4. Spreading and despreading procedure. The data is retrieved when theoriginal spreading code is used in the despreading procedure, using another code resultsin noise.

chip rate and the Rb is the bit rate.

Gp = 10 · log (Rc

Rb) , [dB] (2.3)

The spreading and despreading procedure in the frequency domain is illustratedin Figure 2.5.

Figure 2.5. Spreading and despreading in the frequency domain. 1. The originalSignal; 2. Spreading of the Original signal; 3. Transmitting and experience interference;4. Adding Wide Band Interference (WBI) and Narrow Band Interference (NBI); 5.Despreading; 6. Filtering

2.5 Cellular System 11

2.5 Cellular System

A cellular system is a radio network made up of a number of cells each served by abase station. These cells are used to cover different geographical areas to providenetwork access for many mobile users.

2.5.1 Uplink and Downlink

In a cellular system there is transmission in two directions, from the user to thebase station and vice versa. The transmission from the user to the base stationis called the uplink and in the other way is called the downlink. There is severalways to implement communication in both directions, Frequency Division Duplex(FDD) is the one commonly used in CDMA systems. FDD means that the uplinkand downlink are separated in frequency, one carrier frequency for the uplink andone for the downlink. Time Division Duplex (TDD) is another method to separatethe uplink from the downlink. Here one carrier frequency is used and the uplinkand downlink take turn in transmitting.

2.5.2 Power Control

Power control is one of the most central and important functions in a cellularsystem. The main purpose is to control the transmitted power of simultaneouslytransmitting users so the interference in the system is minimized and in the sametime provide sufficient signal quality at the receiver side. The signal quality is of-ten described by the Signal to Interference Ration (SIR). SIR is the ratio betweenthe wanted signal power and the total interference power. The interference powercan be signals from other users or from background noise.

In CDMA systems there are many users transmitting at the same time on thesame carrier frequency. These users can be far away, close to the edge of the cell,to very near the base station when they transmitting. Users transmitting withvery high power at a close distance would make signals from users far away withtoo little power undetectable. This is referred to the near-far problem.

2.6 UMTS

The Universal Mobile Telecommunication System (UMTS) is often called thirdgeneration mobile radio system (3G), and is the leading 3G technology today.The specification for UMTS has been specified by third-generation partnershipproject (3GPP) which is a joint standardization project. 3GPP is continuouslydeveloping UMTS to get higher bit rates and more efficient usage of the radioresources. Members of the 3GPP are companies such as mobile telephony systemmanufactures and operators.

12 Cellular Radio Systems

2.6.1 WCDMATo transfer information over the air, UMTS uses the Wideband Code DivisionMultiple Access (WCDMA) as the air interface, which is based on CDMA describedin Section 2.4.3. A chip rate of 3.84 Mcps is used on the original signal and thenspread out and transmitted on the 5 MHz carrier. The 5 MHz bandwidth isconsidered wide, compared to many other schemes which operate at 1.25 MHz.

2.6.2 Network ArchitectureUMTS networks are at a high-level system point of view divided in three subsys-tems, namely the core network (CN), UMTS Terrestrial Radio Access Network(UTRAN) and User Equipment (UE). An overview of the architecture can be seenin Figure 2.6. The core network routes traffic to external networks like the In-ternet and the public switched telephone network (PSTN). In the UTRAN eachRadio Network Controller (RNC) controls several Radio Base Stations (RBS), alsoknown as Node-B, and communicates with the CN. The RBS handles a numbersof cells (sectors) which each covers a geographical area. The main task for theRBS is to handle the radio signaling and traffic to the UE’s. Resources allocationlike UE-service requests is shared by the RNC and RBS.

Figure 2.6. UMTS network architecture.

2.6.3 Uplink Power ControlThe shared resource in the uplink is the interference headroom. In WCDMAsystems power control becomes crucial for efficiency and stability when the inter-ference headroom must be managed. Without it, a single overpowered UE couldblock a whole cell. The power control is divided into two parts, the inner loop andouter loop. The control mechanism is illustrated in Figure 2.7.

The goal is to keep the SIR just high enough to provide the required signalquality for the service in use. By keeping the SIR as low a possible the UE is not us-

2.6 UMTS 13

ing too much power and therefore does not create more interference then necessary.

The SIR target is steered by the Outer Loop Power Control (OLPC), which islocated in the RNC, and is regulated to achieve a specific block error rate (BLER).BLER is the ratio between erroneous received data blocks to the total number ofreceived data blocks. If the quality is better than wanted the outer loop decreasesthe SIR-target and if it is too low it will increase the target. The outer loop isupdated every Transmission Time Interval (TTI) which is the time frame in whicha UE may transmit over the air interface, e.g., every 2 ms.

The Inner Loop Power Control (ILPC) is located in the RBS and is updatedwith a frequency of 1.5 kHz; this is to be able to react to fast variation in thechannel condition, like fast fading. It makes a rapid estimation of the SIR andregulates it towards the SIR-target. Depending on if the measured SIR is higheror lower than the SIR-target, the RBS sends a Transmit Power Control (TPC)command to the UE to either raise or lower its transmission power by a fix stepsize.

Figure 2.7. Uplink power control.

2.6.4 Interference CancellationIn CDMA systems UE’s create interference to each other that affects the perfor-mance; Interference Cancellation (IC) is a method to improve the performance.The principle with IC is first to detect the information of an interfering UE, thenreconstruct the interfering signal and cancel it from the total received signal. De-tection of the desired UE’s is finally performed.

IC can be performed in two ways, Parallel Interference Cancellation (PIC) andSuccessive Interference Cancellation (SIC). PIC is when the contribution of allinterfering UE’s is cancelled simultaneously in a parallel manner while SIC cancel

14 Cellular Radio Systems

the contribution of the strongest remaining interferer iteratively. In general SICtakes more time while saving hardware compared to PIC. On the other hand PICis preferable when the amount of interference from each UE is similar. Conversely,SIC is preferable in the case when UE’s interferer with different signal strengths.[3, 6]

2.7 EULThe evolution of the WCDMA-standard is an ongoing process, as a summary,the evolution of WCDMA is illustrated in Figure 2.8. The first step was theintroduction of High-Speed downlink Packet Access (HSDPA) in Release 5 or justRel 5. The next step, in Rel 6, was to complement the improved downlink withthe Enhanced Uplink (EUL). HSPDA and EUL are often jointly referred to asHigh-Speed Packet Access (HSPA). The main goals with HSPA are:

• Reduced delays

• Increased data rates

• Increased capacity

• Increased high data rate availability

To achieve this, a lot of functionality is moved from the RNC to the RBS, e.g.,decision making functions is moved to the RBS, resulting in less signaling and re-duced delays but also reduced knowledge of the surrounding when the informationin the RNC no longer are available. With Rel 6, WCDMA-systems support datarates up to 5.7 Mbps in the uplink, compared to 2 Mbps in the first Release, R99.The general principles behind the most important features introduced with EULare described below.

Figure 2.8. The evolution of WCDMA

2.7.1 EUL ChannelsEUL introduces a new set of channels, the Enhanced Dedicated Channel (E-DCH),which is only used by EUL UE’s. This channel can co-exist with the existing Ded-icated Channel (DCH), which consist of the Dedicated Physical Control Channel(DPCCH) and Dedicated Physical Data Channel (DPDCH), from earlier releases.The E-DCH consist of two channels, the Enhanced Dedicated Physical Control

2.7 EUL 15

Channel (E-DPCCH) which carries uplink control signaling while the EnhancedDedicated Physical Data Channel (E-DPDCH) carries the data.

The E-DPCCH is only used when data is transmitted with EUL from theUE, while the DPCCH is always transmitted. The DPCCH contain e.g., controlinformation such as Channel Quality Information (CQI) for the downlink andthe RBS uses this channel for SIR measurement. The power control, describedin 2.6.3, regulates the UE power based on the measured SIR. When the E-DCHchannel is used, the power level is regulated together with the DPCCH. Whendata is transmitted using EUL the power level for the E-DPCCH and E-DPCCHare set with two offsets relative to the DPCCH power. The power for E-DPDCHis, ∆E−DPDCH · pDPCCH , illustrated in Figure 2.9.

Figure 2.9. Offsets for E-DPCCH and E-DPDCH

The offsets are of great importance, especially the one for the E-DPDCH whichis based on bit rate and the efficiency of the channel coding that the UE uses. Ingeneral higher bit rates demand higher power which leads to a higher offset forhigher rates.

2.7.2 Short TTI

The available radio resources are allocated between the UE’s accessing the systemat certain intervals, the length of those intervals is referred to Transmission TimeInterval (TTI). A short TTI, 2 ms instead of 10 ms, results in shorter delays dueto a reduced Round-Trip Time (RTT). A 2 ms long TTI consist of 3 slots, e.g.,TPC are transmitted every slot.

16 Cellular Radio Systems

2.7.3 SchedulingScheduling is another important feature, in the EUL concept it is moved from theRNC to the RBS for shorter delays. The scheduler controls when a UE are allowedto transmit and with which maximum rate. The maximum rate for each UE ischosen so that the total interference at the RBS does not exceed a certain thresh-old, a fast scheduling can respond to changes in the system faster and thereforeminimize the risk for the system to overstep the threshold.

2.7.4 Hybrid ARQIf it is possible to decode the received data an acknowledgement (ACK) is repliedto the UE, if the decoding fails there is erroneous packet and a negative acknowl-edgement (NACK) is replied instead. All erroneous packets have to be retransmit-ted by the UE. One approach to handle those retransmissions is to use AutomaticRepeat Request (ARQ). Instead of ignoring the erroneous data completely, the Hy-brid ARQ (HARQ) scheme combines the information from the earlier transmissionwith the new information from the retransmission. This procedure is called softcombining and increases the chance of a successful decoding, simply because anerroneous packet still provides useful information together with the retransmission.

As long as the UE receive a NACK, the packet will be retransmitted. In orderto be able to transmit the whole time, no need to wait for an ACK or NACK dueto the processing delay, multiple HARQ processes can be used. The retransmissionwill always take place in the same process in the next cycle. A HARQ scheme isillustrated in Figure 2.10.

Figure 2.10. Operation of 4 parallel HARQ-processes.

Chapter 3

Simulator Model

To be able to evaluate the advantages and the disadvantage with the introductionof a TDMA access scheme in EUL, a series of system simulations were done. Allthe simulations were done in a WCDMA radio network simulator with supportfor HSPA; it is developed by Ericsson AB and written entirely in Matlab. Thesimulator models everything from fast and slow fading, traffic models to differenthigh level network functionality such as load control and scheduling.

First in this chapter a number of notations and definitions will be introduced,followed by a more detailed description of the system model.

3.1 DefinitionsThe introduced equations in this section will not include the effects of delays inthe system; they will rather describe an average stationary point or the dynamicswhere the mean values of the different distributions are used.

Consider a network with one base station serving M UE’s. As discussed inSection 2.2, all signals transmitted over an air interface experience a basic pathgain. The basic path gain between each UE to the base station is defined inDefinition 3.1.

Definition 3.1 Path Gain

GM=

(g1, . . . , gM

)< 1

where gi is the total uplink path gain between UE i and the base station.

The total received power at the base station from UE is expressed as

Ctoti = pigi (3.1)

where pi is the power used by UE i. The power received from the DPCCH channelis denoted CDPCCH

i , the remaining channels are given by the DPCCH power times

17

18 Simulator Model

a power offset. To simplify, the E-DPCCH and E-DPDCH channels are groupedinto one. Ctot

i can then be divided into two parts,

Ctoti = CDPCCH

i (1 + ∆EULi ) = CDPCCH

i ∆toti (3.2)

where ∆EULi is the power offset for the E-DCH channels for UE i.

The sum of thermal noise and other non power controlled interference will bereferred to as background noise, denoted N.

The total interference power at the base station, Itot, is the background noiseplus the received signal power from all UE’s. The total interference power will bereferred to as interference or total interference.

Definition 3.2 Total Interference is defined as

Itot M=M∑i=1

Ctoti + N =

M∑i=1

pigi + N

The signal quality for each UE in a radio system is often measured in Carrier-to-Interference Ratio (CIR). CIR is the ratio between the received carrier power andthe experienced interference before the despreading in the receiver. In a CDMAradio system SIR is measured after despreading. SIR and CIR are related asSIR = SF · CIR, where SF is the spreading factor.

In UMTS networks the signal quality is measured on the DPCCH channel.The power control makes the decisions based on that measurement, so only theCDPCCH

i part of Ctoti is included in the CIR measurement.

Definition 3.3 Carrier-to-Interference Ratio is defined as

γiM=

CDPCCHi

Itot − Ctoti

where γi is the CIR for UE i.

Due to multipath propagation the signal components reach the receiver at differ-ent times, described closer in Section 2.2.3. This may cause the signal to interferewith itself. The orthogonality factor, α, describes how well the receiver can han-dle those delays. Here α is constant but in reality it varies between UE’s and time.

The diversity gain, d, describes the signal gain due to the use of several receiv-ing antennas. Also d varies with time and mobility, but it is modeled as a constant.

Both α and d affect the signal quality and therefore also CIR. Definition 3.3 isan ideal situation but can be rewritten with regard to α and d as

3.1 Definitions 19

Definition 3.4 CIR is defined as

γi =CDPCCH

i d

Itot − (1− α)Ctoti

Definition 3.4 is the definition for CIR that is going to be used throughout thisstudy.

3.1.1 Shannon’s TheoremIn the late 40s Shannon published his Communication Theory, which included theformulation of Shannon’s Theorem. Shannon’s Theorem define a theoretical upperlimit for the bit rate based on bandwidth and CIR. [11]

Theorem 3.1 Shannon’s Theorem

R = W log2(1 + γ) [Mbits],

where R is the maximum attainable bit rate in Mbps, W is the bandwidth in MHzand γ is experienced CIR

This maximum bit rate is a theoretical restriction and can never be achieved in areal system. The theorem will however be used as a model to map CIR-values toa corresponding rate that the UE’s are allowed to use.

Figure 3.1. Maximum theoretical bit rate as a function of CIR.

Notice that in this study W is constant so higher rates demand higher CIRand the increased demand on CIR is not linear to the bit rate, illustrated inFigure 3.1. Higher bit rates inevitable result in a nonlinear increasing interferencein the system.

20 Simulator Model

3.1.2 Noise RiseA common way to express the interference at the base station is to describe it asrise over thermal noise (RoT), or just noise rise, Λ. Noise rise is the ratio betweenthe total interference, Itot, and the background noise, N .

Definition 3.5 Noise rise, Λ is defined as

Λ M=Itot

N=

∑Mi=1 Ctot

i + N

N= 1 +

∑Mi=1 CDPCCH

i ∆toti

N≥ 1

Noise rise is the limited resource in the base station. If the noise rise is high theUE has to use more power to maintain its CIR. UE’s have an upper limit on thepower that they are allowed to use, so if the maximum power is used and the signalquality is too low the UE’s are power limited. If the noise rise is too high in thebase station, UE’s far away from the base station can not make themselves heard.To maintain coverage and stability in the base station a upper limit is set on thenoise rise,

Λ ≤ Λmax (3.3)

Definition 3.4 combined with Equation 3.2 can be rewritten as

CDPCCHi =

γi

d + (1− α)γi∆toti

Itot (3.4)

By combining Equation 3.4 with Definition 3.5, Λ can be expressed as

Λ =Itot

N=

1

1−∑M

i=1γi∆tot

i

d+(1−α)γi∆toti

≥ 1 (3.5)

Equation 3.3 gives a upper limit on the sum in Equation 3.5,

0 ≤M∑i=1

γi∆toti

d + (1− α)γi∆toti

< 1 (3.6)

Equation 3.6 leads to a limit on how high rates that the individually UE’s areallowed to use.

3.1.3 Interference CancellationThe following model is an SIC, which is described closer in Section 2.6.4. Thismodel will only have one iteration, so only the most interfering UE, ik, will becanceled. UE ik is experience the total interference, Itot and the other UE’sexperience a canceled interference, Iic, modeled as

Iic = N + Ctotik

(1− η) +∑i 6=ik

Ctoti ≤ Itot (3.7)

where η is the efficiency for the SIC. Since Iic is lower than Itot the UE’s that notare the ik can use lower power to maintain their signal quality. A system using

3.2 Propagation 21

SIC will therefore have lower total interference compared to a system without SIC.

To have the same amount of interference in a system with a implemented SICthat are using one iteration compared to a system without an SIC, UE ik shallfulfill the following condition

Ctotik

= Ctoticik

(1− η) (3.8)

With an SIC UE ik can create 11−η times more interference without any change in

performance. This means that UE ik can use a higher bit rate.

3.2 PropagationSection 2.2 described in more detail what fading is. In the simulator the distanceattenuation, or path gain, is derived from the Okumura-Hata propagation model.The slow fading is derived in the simulator with a log-normal distribution.

The simulation is in an urban environment and the fast fading is modeledthrough the standardized model named 3GPP Typical Urban. The model is amap consisting of precomputed fast fading values. The antenna gain is derivedfrom implemented Omni-antenna diagrams in the simulator.

3.3 Receiver AntennaThe G-Rake receiver makes a better approximation of the interference than a Rakereceiver. The orthogonality factor, α, is thus lower when a G-Rake is implemented.Therefore a G-Rake antenna is used in this study.

3.4 Network LayoutThe network used in all simulations is a single site with a single cell. This meansone RBS that handle all UE’s in the simulation.

3.5 Traffic ModelsIt is possible to use many types of traffic models in the simulator, i.e. speech,interactive, download etc. This study will cover two kinds of models for uploadinga large file at different rates; one using EUL and one not using EUL, the third oneis pure speech. All three are further described in this section.

3.5.1 Upload with EULThe standard UE is using a model for uploading a large file over the E-DPDCHchannel, which means EUL is used. The file is so large so the UE’s will be active

22 Simulator Model

during the whole simulation, regardless of bit rate. The number of UE’s will beset for each simulation individually and all of them are supporting category 61,which means they are supporting bit rates up to 5.76 Mbit. This one is used ifnothing else is stated.

3.5.2 Upload without EULThe second model is also a large file that shall be uploaded, but this model is notsupporting EUL and therefore use the DPDCH channel. All these UE’s will get abit rate of 64 kbps. As in the previous model the number of UE’s will be set foreach simulation and they stay active during the whole simulation.

3.5.3 SpeechThe data in the speech model is transmitted over the DPDCH channel and de-mands a bit rate of 12.2 kbps. In this model the UE’s are not active the wholetime. Each call lasts for a certain time which is exponentially distributed with amean time of 90 seconds. As in the other models, the number of UE’s will be setfor each simulation.

3.6 MobilityAll UE’s have the same mobility; their speed is 3 km/h, which is a rather lowmobility. When new UE’s enter the system they are initially placed randomly inthe cell according to an uniform distribution.

3.7 SynchronizationAs mentioned in Section 2.4.2, TDMA needs a guard interval too minimize therisk that two or more UE’s are transmitting at the same time and causes extrainterference for each other. In this study the system and all UE’s are perfectlysynchronized, so no guard interval is needed and therefore not further considered.

3.8 UE PowerWithin the standard UE’s are not allowed to use more than 21 2dBm when trans-mitting. If a UE is far away from the serving RBS, it can not use the highestbit rates. This is because the received power in the RBS is too low to maintainthe signal quality that is required for the higher bit rates. This study focuses onUE’s with good channel condition and that are capable of using high bit rates, soinstead of place the UE’s closer to the RBS, the maximum available power in theUE’s are increased to 50 dBm.

1See appendix C for more information about UE categories2See Appendix B

3.9 Power Control 23

3.9 Power ControlThe power control, described in Section 2.6.3, is implemented in the simulator.The step size in power that the UE take when order from the RBS is to go up ordown is 1 dB.

3.10 Hybrid ARQHARQ with soft combining are used as described in Section 2.7.4, 8 parallel HARQqueues, or processes, are used and each is one TTI long. The minimum delay isdefined by the TTI length; this is the time during which data is transmitted. If aretransmission occurs an additional delay is introduced which is specified by theHARQ Round-Trip Time, which depends on the number of processes, RTT =NHARQ · TTI. In the case with 8 processes which are 2 ms each, the extra delayfor each retransmission is 2 · 8 = 16 ms.

3.11 TDMAWithin the standard for UMTS it is possible to allocate the uplink resources foreach TTI, or for each HARQ-process. The idea is to give a group of UE’s one ormore HARQ-processes each for a certain time. During a HARQ process only oneUE is allowed to transmit. This is a way to implement a TDMA access schemeand it is used in this study. The transmission time, T , is the number of HARQprocesses in row multiplied by the TTI length, illustrated in the two followingexamples.

Example 3.1In both figures in Figure 3.2 two UE’s have four HARQ-processes each. Thefour assigned processes do not have to be in a row, they can be scattered inany way. The difference between the two figures is that in Figure 3.2(a), T isNHARQ ·TTIlength = 4 · 2 = 8 ms, and in Figure 3.2(b) T is NHARQ ·TTIlength =1 · 2 = 2 ms. As described in Section 2.7.4 the retransmission is done in the sameprocess as the first transmission, in this example it is no problem because the UE’shave the same processes during many cycles. There are just a few combinationsof the number of UE’s and transmission time that fulfill these conditions.

24 Simulator Model

(a) Four HARQ processes are in a row, T is 8 ms.

(b) One HARQ process in a row, T is 2 ms.

Figure 3.2. Two UE’s with four HARQ processes each.

Example 3.2In Figure 3.2 there is three UE’s with T = 6 ms. This combination does not fulfillthe condition that they can have the same HARQ process during more than 1cycle. If there is a retransmission, the scheduler must give that process back tothe UE that have to do the retransmission, illustrated in Figure 3.3(b).

(a) T is 6 ms, notice that each UE have different HARQprocesses each cycle.

(b) An error in HARQ process 4. In next cycle UE 2 stillhave HARQ process 4.

Figure 3.3. Three UE’s with three HARQ processes each.

3.12 Interference Cancellation 25

3.12 Interference CancellationThe SIC described in Section 3.1.3 is implemented in the simulator. In this studythe effects of SIC is only studied with a TDMA access scheme, the UE’s take turnin transmitting so there is only one active UE’s at the same time. That extrainterference that UE’s can create are translated to a higher bit rate.

The SIC is implemented in two ways to make a comparison; one way is to dothe IC before the channel decoding, pre-decoding SIC, and the other way is to doit after the channel decoding, post-decoding SIC. The benefit with pre-decodingSIC is that the delay to get the data, the most important data in this study is theTPC, is increased by only 1 slot, the disadvantage is that the efficiency depends onthe coding rate. With post-decoding SIC the delay increases with 3 slots instead,but the efficiency is high and is independent of the coding rate.

3.13 EULEUL is simulated with all the functionality described in Section 2.7. The shortTTI, 2 ms, is used throughout this study.

3.13.1 SchedulingThe scheduler controls which UE’s and when they are allowed to transmit, andwith which rate. The assigned rate is based on which rate is needed for the presentservice used by the UE and the amount of interference they are allowed to create.

Speech UE’s have the highest priority and are granted a transmission rate first.All speech UE’s are granted resources corresponding to a bit rate of 12.2 kbps.Upload UE’s without EUL have the next highest priority and are next to getresources. If there is enough resources, they get grants to use a bit rate of 64 kbps,otherwise they share the available resources equally by a scheduling algorithm.Speech UE’s and upload UE’s without EUL are put on a CDMA access scheme,which is described in Section 2.4.3. The remaining resources are distributed tothe EUL UE’s. Those UE’s are set to use a TDMA access scheme, describedin Section 2.4.2, since they take turn in transmitting one UE is granted all theremaining resources. In most simulations that are carried out in this study thereis only EUL UE’s, in those cases the active UE gets all available resources, exceptthat part that is needed for the DPCCH channels.

3.14 DelaysIt takes some time for the commands from the RBS to reach and be decoded bythe UE’s. Those delays are implemented in the simulator and give dynamics to

26 Simulator Model

the simulated network.

The two delays that affect this study is the transmit power command, TPC,and the rate grant command. The time it takes both for the algorithm in the RBSto calculate the values and the time over the air interface are listed below,

• TPC - 2 slots - 4 ms

• Rate Grant - 15 slots - 30 ms

3.15 System LoggingAll simulations are run with a simulation time of more than 100 seconds, which islong enough to get reliable statistics. The initial 10 seconds of the simulations aredisregarded in order to let the system stabilize. UE performance measures such asbit rate; power-usage etc is logged for each TTI or slot depending on the updatefrequency. System data e.g., noise rise and system throughput are logged for eachTTI, some UE data e.g., TPC are logged for each slot.

Chapter 4

Results

The results will be presented and analyzed in this chapter. All simulations arecarried out with the simulator described in Section 3. The analysis will be from asystem perspective, and thus little or no concern is taken to the fairness betweenthe UE’s.

4.1 CIR OvershootThe main problem with introducing a TDMA access scheme in EUL is the largevariation in experienced interference for the UE’s. The large variation in interfer-ence is due to large difference in transmission power when the UE’s take turn intransmitting. A transmitting, or active, UE experiences only interference from theDPCCH channels from the not transmitting, or passive, UE’s, which is a rathersmall interference. A passive UE experiences interference both from the DPCCHfrom the other UE’s and the E-DPDCH channel from the active UE, which issignificant. This major change in channel conditions affects the signal quality.

4.1.1 TheoreticalDefinition 3.4 gives an expression for the signal quality, γ. When TDMA is usedCtot varies a lot for each UE depending on if the UE is passive or active. Thetotal received carrier power, Ctot described in Equation 3.2, for an active UE is

Ctot = CDPCCH(1 + ∆EUL) (4.1)

and CIR for the same UE is expressed in Equation 4.2.

γ =CDPCCHd

Itot − (1− α)CDPCCH(1 + ∆EUL)(4.2)

The total received carrier power for a passive UE is

Ctot = CDPCCH (4.3)

27

28 Results

and the CIR for the same UE is expressed in Equation 4.4

γ =CDPCCHd

Itot − (1− α)CDPCCH(4.4)

Since ∆EUL >> 1, CDPCCH for the active UE can be reduced and still maintainthe same CIR, provided that Itot is constant. The power control strives towards aCIR-target which is independent of if the UE is passive or active. When switchingfrom passive to active the CIR gets too high, this creates a CIR overshoot. Con-versely when switching from active to passive the CIR gets too low and generatea CIR undershoot. It takes a while for the UE to adjust the power so the CIRreaches the CIR-target, the time is depending on how large ∆EUL is and the stepsize in the power control. The principle appearance of CIR when two UE’s taketurn in transmitting is illustrated in Figure 4.1.

Figure 4.1. Two UE’s take turn in transmitting. When ∆EUL is high the UE is active.The resulting over- and undershoot for each UE is shown.

4.1 CIR Overshoot 29

If the transmission time is short, the height of the CIR overshoot can be sohigh that the power control has not enough time to adjust the power so that theCIR reaches the CIR-target. The principle appearance of CIR when two UE’s taketurn in transmitting with a too short transmission time is illustrated in Figure 4.2.

Figure 4.2. Two UE’s take turn in transmitting with a short transmission time. CIRnever reaches the CIR-target.

Compared to a CDMA system where the UE’s are active the whole time, thereis no CIR under- or overshoot. Hence, CIR will follow the CIR-target the wholetime.

4.1.2 Simulations

The affects of introducing TDMA in EUL can also be seen in the simulator. InFigure 4.3, measured CIR and the CIR-target is plotted for different bit rates. Asshown in Section 4.1.1, it takes a while for the UE to adjust the power so the CIRreaches the CIR-target after switching from transmitting to not transmitting andvice versa. It takes longer time to adjust the power when higher bit rates are used.For the highest bit rate, there is not enough time to reach the CIR-target, all thisis illustrated in Figure 4.3.

As illustrated in Figure 4.3, for the lowest bit rate there is basically no over- orundershoot, CIR is on target the whole time. With a bit rate of 3 Mbps, the over-and undershoot is clearly visible but there is enough time for the UE to reach theCIR target. With the highest bit rate, the over- and undershoot is even larger.There is not always enough time to reach the CIR target. The principles from thetheoretical part are observed in the simulator as well.

30 Results

Figure 4.3. Simulations of CIR and CIR-target for varying bit rates when two UE’stake turn in transmitting. The over- and undershoots are larger for higher bit rates.

4.2 Performance Improvement Evaluation

The idea with introducing TDMA in EUL is to be able to offer higher bit ratesto many UE’s simultaneously. This leads to a higher cell throughput which is thesum of all UE’s bit rate in the cell.

In the following simulations the UE’s are not power limited, they all use thetraffic model upload with EUL, which is described closer in Section 3.5.1.

4.2.1 Theoretical

With TDMA the UE’s take turn in transmitting, therefore they do not creatingany interference for each other. As there is just one active UE at a time, the RBSshall give the active UE all the available uplink resources. When the RBS haveto serve a lot of UE’s the control channels, mainly the DPCCH, take more andmore of the uplink resources. This results in that the maximum available bit ratedecreases with increasing number of UE’s in the system.

In a CDMA system when the UE’s transmit simultaneously, the bit rate will de-pend more on the number of UE’s. More UE’s increase the amount of interferencein the system, and lower bit rates have to be used to not create too much noise rise.

4.2 Performance Improvement Evaluation 31

4.2.2 ResultsThe cell throughput is plotted against the number of UE’s in the cell in Figure 4.4.There is both throughput for a system using CDMA, which is the reference case,and a system using TDMA with different transmission times, T .

Figure 4.4. System throughput for a CDMA system and a TDMA system with differenttransmission times. For short transmission times, the system throughput is not as highexpected.

As seen in Figure 4.4, TDMA with a transmission time longer than 3 HARQprocesses, or 3 TTI which is 6 ms, behaves as expected. The throughput is al-most constant for a low number of UE’s then start to decrease when the DPCCHtake more and more resources. The gain with introducing TDMA compared toCDMA is clearly shown here. The system throughput is more than 100% higherfor the TDMA system compared to the same system using CDMA for two or moreUE’s. Note that if there is only one UE, the active UE in both systems are aloneand gets all the resources, therefore there is no difference between the two systems.

For transmission times shorter than 3 TTI, the system does not behave asexpected. The rate is so high that the transmission time is too short for the CIRto reaches the CIR-target. When this occurs the system can be unstable. InFigure 4.5, the CDF1 of the noise rise are plotted. The curves are for three UE’sin the system, one curve is the CDMA system and the two others are the TDMAsystem with a transmission time of 1 and 4 TTI. The threshold for the noise riseis set to 12 dB. This means that the system tries to keep the noise rise below thisvalue to maintain coverage.

1Se appendix A

32 Results

Figure 4.5. CDF of the noise rise for three UE’s. A CDMA system and a TDMAsystem with a transmission time of 1 and 4 TTI. The TDMA system with a transmissiontime of 1 TTI have very high and many noise rise peaks.

As seen in Figure 4.5, the TDMA system with a long transmission time, 4 TTI,has very few values above 12 dB, which is what the system aiming at. This meansthat the UE’s are using the available resources efficiently. The reference case,CDMA, has some high peaks but nothing extreme. The TDMA case with a shorttransmission time, 1 TTI, which corresponds to the lowest system throughput inFigure 4.4, has an extremely bad curve. More than 30% of the values are abovethe threshold. This system uses most resources and still has the lowest throughput.

The problems seen in this section are

• Power rushes when a short transmission time is used

• Dramatically decreased system throughput when a short transmission timeis used.

4.3 Power RushesThe TDMA systems with a short transmission time in Section 4.2.2 had very highnoise rise peaks and used a lot of resources. What happens in the system is thatthe UE’s trigger each other to use more and more power. This increase in powerusage is to maintain the signal quality due to more interference in the system. InFigure 4.6 the cycle is illustrated.

During the first interval, all UE’s use a rather low bit rate. Therefore there isa lot of available resources, the RBS then allow the UE’s to use higher bit rates.

4.3 Power Rushes 33

Figure 4.6. Noise rise and used bit rate in the system.

The granted bit rate is set so when the rate is used the noise rise in the systemshall be below 12 dB. It is a delay of 30 ms for UE to receive the grant and beable to use the higher bit rate.

During the second interval, all UE’s start to use the higher allowed bit rate.Since more power is used the interference, or noise rise, in the system increases.

During the third interval the UE’s use the granted bit rate. When the UE’s us-ing those bit rates, the noise rise is supposed to be below 12 dB. Seen in Figure 4.6,the noise rise passes the 12 dB threshold. The RBS reacts to the increased noiserise, but it takes 30 ms before the UE’s start to use the lower bit rate assign by theRBS for lowering the noise rise. During those 30 ms the noise rise building up andhave peaks around 30 dB. The next question is why the granted bit rates, which arebased on that the noise rise shall be below 12 dB, generate so high noise rise peaks.

TDMA is used, which generates CIR overshoot as described in Section 4.1. Ittakes a certain time to compensate for the CIR overshoot. If the transmission timeis not long enough, there is no enough time to decrease the power to reach thetarget. What happens then is that the passive UE’s have to increase their powerto maintain their CIR.

In the following discussion, Equation 4.4 is used for expressing CIR for thepassive UE’s and Equation 4.2 is used for the active UE. In Figure 4.7 three UE’stake turn in transmitting, UE 1 starts to transmit and creates extra interferencefor UE 2 and UE 3. UE 1 gets an overshoot in CIR and start to decrease its powerwhile UE 2 and UE 3 gets an undershoot and start to increase their power. The

34 Results

transmission time is 1 TTI, which is 3 slots, so the UE’s going up or down 3 dB.In the second TTI, UE 2 starts to transmit and gets a CIR overshoot and startsto decrease the power. UE 1 experiences the extra interference that UE 2 createswhich results in a too low signal quality and start to increase the power. SinceUE 2 increased the power during the first TTI even more interference was createdfor UE 3 that gets a new undershoot and have to increase the power even more.During the third TTI UE 3, which has increased the power with 6 dB, starts totransmit. UE 1 and UE 2 gets even more interference, and have to increase theirpower. During this cycle, all UE’s goes up 6 dB and goes down 3 dB which resultsin a 3 dB increase during 3 TTI. More and more power is used, which leads topower rushes and increasing noise rise in system. To avoid this, the transmissiontime have to be so long that CIR reaches the CIR target.

Figure 4.7. A system with three UE’s and a transmission time of 1 TTI. The UE’strigger each other to use more and more power, which leads to noise rise peaks.

In the rest of this chapter a stable system means that transmission time is longenough so the power control has time to compensate for the CIR overshoot.

4.4 Interference CompensationOne way to compensate for the large change in interference is to scale the power,by a factor k, that the active UE is transmitting with. If k is correctly calculatedthen CIR is the same when a UE goes from being passive to active, and thereforedoes not create any CIR overshoot which results in a more constant power usage.The CIR shall be same regardless if the UE is active or passive, hence Equation 4.2and Equation 4.4 shall be equal. Furthermore Equation 3.1, 4.1 and 4.3 is alsoused. Thus k is a function of bit rate, ∆, and expressed in Equation 4.5

pactive = ppassive · k = ppassived + (1− α)γ

d + (1− α)γ(1 + ∆)(4.5)

4.5 Minimum Timeslot Length 35

4.4.1 SimulationsIn Figure 4.8 the system throughput and the CDF of the noise rise is plotted forthe TDMA system with the scaling factor implemented. The transmission time is1 TTI, which is the worst case from Section 4.2.

Seen in Figure 4.8(a), if the power for the active UE’s is scaled by k, the systemthroughput increases. It does not reach the system throughput as systems withlonger transmission times in Section 4.2. That is because α and d in Equation 4.5is modeled as constant but in reality and in the simulator they are not constant.The system throughput can be improved by optimizing k. This optimization isnot done in this study, but to show that the throughput can be increased k arescaled by 1.2. This leads to higher throughput, seen in Figure 4.8.

Shown in Figure 4.8(b), the scaling factor, if properly calculated, can improvethe resource efficiency. It can even make an unstable system stable.

Since the UE’s do not have the information needed to calculate k, this is notpossible to do within the standard today. The idea with this is to show that it ispossible to compensate for the CIR overshoot and minimize the risk for gettingan unstable system.

4.5 Minimum Timeslot LengthSeen in Section 4.2, a TDMA system need a certain transmission time to behaveproperly, and Section 4.1 showed that the CIR overshoot is larger for higher bitrates. Hence it will take longer time for a system where the UE’s are using highbit rates to stabilize compared to a system where the UE’s are using lower bit rates.

The time that is needed is depending on how long it takes for the power controlto increase or decrease the power so CIR is the same after the switch from nottransmitting to transmitting, or vice versa. This gives the condition

γpassive = γactive = γ (4.6)

The path gain in Equation 3.1 is assumed to be equal when transmitting and not.

gpassive = gactive (4.7)

Since the power control work in dB and the difference in used power by the UE’sis expressed in that scale by using the following logarithmic rule

(ppassive)dB − (pactive)dB =(

ppassive

pactive

)dB

(4.8)

To get an expression for the height of the CIR overshoot for different bit rates,Equation 4.2, 4.4, 4.6, 4.7 and 4.8 are combined. The result is expressed in Equa-tion 4.9.

36 Results

(a) With the scaling factor implemented the system throughput isimproved.

(b) CDF of the noise rise, with three UE’s in each system. With acorrectly calculated k, the noise rise distribution is improved.

Figure 4.8. The TDMA system with the scaling factor implemented.

4.5 Minimum Timeslot Length 37

(pit)dB − (pi)dB =

d + (1− α)(1 + ∆EULit

)d + (1− α)γ

(4.9)

To get γ(∆) system simulations with a constant bit rate was carried out for aset of data rates. A link simulation is a simulation with only one UE that using onefixed bit rate. Mean CIR, γ, and offset, ∆, are measured during the simulations.

The results from the system simulations are used in Equation 4.9 to get theheight of the CIR overshoot which is then translated to the time it takes to recover,or converge, the CIR before the transition to or from being active for different datarates. The result is plotted in Figure 4.9. The difference is expressed in dB. Thepower control step size is 1 dB, so a CIR overshoot with a height of 1 dB takes 1slot to compensate for assuming no power control errors.

Figure 4.9. The number of slots it takes to converge for different data rates.

The +2 dB line in Figure 4.9 is due to the delay for the TPC. It takes 2 slotsfor the UE to get the information, if the power shall increase or decrease, from theRBS. This results in that the UE can take 2 steps in wrong direction. This makesthe used power by the UE can differ up to 2 dB from ideal power. To be sure thatthe CIR overshoot is compensated for, 2 extra slots are needed.

Figure 4.9 strengthen the discussion in Section 4.2. When bit rates around3-4 Mbps are used it takes around 7 slots to be sure that the power control hascompensated for the CIR overshoot. Every TTI consist of 3 slots, so the time ittakes corresponds to 3 TTI.

38 Results

4.5.1 SimulationsA set of simulations with two UE’s take turn in transmitting was carried out tosee if the theoretical part holds. The UE’s used a transmission time of 5 TTI,which is long, and that is to be sure that the system has time to converge. Likethe system simulations, a fixed bit rate was used for each simulation. The time ittook for the UE’s to reach the CIR-target, or compensate for the CIR overshoot,was measured and the result is plotted in Figure 4.10.

Figure 4.10. The simulated result of the number of slots it takes to converge for differentdata rates. The simulations are in line with the theoretical assessments.

In the simulations, effects from all kinds of fading are included. Therefore thesimulated values represent the time it takes for the power control to compensatefor 95% of all CIR overshoots. The results from the simulations are in line withtheoretical part, the values are on the upper limit, the +2 dB line. Also theseresults strengthen the discussion in Section 4.2.

4.6 Interference CancellationWith interference cancellation, the interference created by the transmitting UEfor the passive UE’s depends on the efficiency, η, in the SIC. As described in Sec-tion 3.12, the extra interference that the active UE can create is first translated toa higher bit rate. If the efficiency is high enough, the active UE’s will be grantedthe highest bit rate and still the experienced interference for the passive UE’s willbe lower. The disadvantage with introducing an SIC is the extra delay for theTPC. Two kinds of SIC is implemented for comparison, the pre-decoding SIC hasan extra delay of 1 slot and the post-decoding SIC has an extra delay of 3 slots.Those extra delays make the used power, due to extra delay for TPC, to oscillate

4.6 Interference Cancellation 39

with higher amplitude.

The decrease in experienced interference will affect height the time it takesto converge, or the height of the CIR overshoot will be lower, hence the passiveUE’s can use a lower power and still maintain their CIR. This affects how shorttransmission times that can be used and still maintain a stable system.

4.6.1 Minimum Timeslot Length with SIC

The time it takes for the system with an SIC to converge can be calculated in thesame way as in Section 4.5. The only difference is Equation 4.4, the passive UE’sexperience a CIR depending on the efficiency, η, in the SIC as

γ =CDPCCH

passive d

Itot − η(1− α)(1 + ∆EUL)CDPCCHactive − (1− α)CDPCCH

passive

(4.10)

By combining Equation 4.2, 4.6, 4.7, 4.8 and 4.10, an expression of the time ittakes for converge with an SIC is received and expressed in Equation 4.11

(ppassive)dB − (pactive)dB =d + γ(1− η)(1− α)(1 + ∆EUL)

d + (1− α)γ(4.11)

To get the time it takes to converge for different bit rates, the same system simu-lations as in Section 4.5 are used in Equation 4.11. As described in Section 3.12, apost-decoding SIC has a high efficiency, here modeled as 95%, and a pre-decodingSIC a efficiency depending on the rate, here modeled by both an efficiency of 50%and 95%. The extra delay is illustrated with higher safety margin, +3 and +5 dBrespectively. The result is illustrated in Figure 4.11.

Even if the efficiency in the pre-decoding SIC is only 50%, the time it takesto guarantee that the CIR has converged is comparable to the post-decoding SICwhich has an efficiency of 95%. This is because the post-decoding SIC has a longerdelay for retrieving the data. If the efficiency in the pre-decoding SIC is increased,the time to converge will be shorter then the post-decoding SIC.

Similar simulations are done as in Section 4.5.1, the only difference is that thepre-decoding SIC and post-decoding SIC are implemented. The result is illustratedin Figure 4.12.

The values from the simulations follow the theoretical assessment; they arebelow the upper limit. The result for the post-decoding SIC are well below theupper limit. The best results are obtained with an efficient pre-decoding SIC, butthe efficiency in the pre-decoding SIC depends on the bit rate. If the efficiency isnot high enough the post-decoding SIC can perform better.

40 Results

(a) Pre-decoding SIC with an efficiency of both 50% and 95%.

(b) Post-decoding SIC

Figure 4.11. The number of slots it takes to converge for different data rates when anSIC is used.

4.6 Interference Cancellation 41

(a) With pre-decoding SIC and an efficiency of 95%, the results arebellow the upper limit. With 50% the result are close to the upperlimit.

(b) With post-decoding SIC, the simulated result are bellow the up-per limit.

Figure 4.12. The simulated result of the number of slots it takes to converge for differentdata rates

42 Results

4.6.2 System AnalysisIn this section, basically the same study as in Section 4.2 will be carried out. Thesystem throughput will be compared between the TDMA system without an SICand the two systems with the pre-decoding and post-decoding SIC implemented.The conditions are the same as in Section 4.2, except that the TPC delay is in-creased for the two systems with an SIC implemented.

In Section 4.2 the system throughput for systems with a very short transmis-sion time performed much lower then those with longer transmission time. InFigure 4.13 the system throughput and the CDF of the noise rise is plotted for aTDMA system with a transmission time of 2 TTI, but with a pre-decoding SICand a post-decoding SIC implemented respectively. The noise rise curves representsystems with three UE’s.

Shown in Section 4.6.1, with an SIC it would be possible to converge the CIRin a system that are using a transmission time of 2 TTI. As seen in Figure 4.13(a),the system does not perform as good as those with longer transmission times, andone reason for that is the longer delays that creates more oscillations in the powerused by the UE’s. But still, there is a significant improvement when comparing thenoise rise in Figure 4.13(b). This shows that an efficient SIC can improve resourceutilization dramatically. The pre-decoding SIC with an efficiency of 95% showsthe best result, thus much better noise rise distribution with similar throughputas the system without an SIC.

Next step is at look on a system with longer transmission times. The trans-mission time here is 4 TTI, which guarantees a stable system. In Figure 4.14(a)the system throughput is plotted.

In Figure 4.14(b) it is shown that the four systems perform almost identicalwhen looking at the distribution of the noise rise. The main difference is in Fig-ure 4.14(a), the system without an SIC and the system with the pre-decodingSIC show basically the same curves. The efficiency in the SIC when using a pre-decoding SIC matters in some extend, but not so much. The most interesting inthat figure is the large difference in throughput between post and pre-decoding.This is due to the larger oscillations in power caused by the longer delays.

4.6 Interference Cancellation 43

(a) System throughput. Note the low efficiency for the post-decodingSIC

(b) CDF of the noise rise, three UE’s in each system. In the systemwith a 50% efficient pre-decoding SIC, the CIR overshoots are sohigh that there is not enough time to converge.

Figure 4.13. A comparison between a system without an SIC and two systems with anSIC. The transmission time is 2 TTI.

44 Results

(a) System throughput. Note the low efficiency for the post-decodingSIC

(b) CDF of the noise rise, three UE’s in each system. All systemsbehave similar.

Figure 4.14. A comparison between a system without an SIC and two systems with anSIC. The transmission time is 4 TTI.

4.7 TDMA Combined with CDMA 45

4.7 TDMA Combined with CDMATo use the resources more efficient, not all UE’s are using a TDMA access scheme,only the high bit rate UE’s. If a low bit rate UE, such as a speech UE, would usea time slot alone the system throughput would decrease a lot. All low bit rateUE’s are instead using a CDMA access scheme. This section will briefly look athow the system performs when the two access schemes are used simultaneously.

In the following simulations it is 15 speech UE’s, using the speech model de-scribed in Section 3.5.3, 1-10 upload UE’s that are using the upload without EULmodel, described in Section 3.5.2 and 3 EUL UE’s. All speech UE’s and uploadUE’s without EUL are put on the CDMA access scheme and the EUL UE’s areusing, as before, a TDMA access scheme. This system is compared to a systemwhere all UE’s use a CDMA access scheme, as the 3G networks are implementedtoday. Notice that there is no SIC implemented in neither of the systems. Thesystem throughput is plotted in Figure 4.15(a), with varying transmission timesfor the EUL UE’s. The CDF of the noise rise, with three upload users withoutEUL, is plotted in Figure 4.15(b).

As seen in Figure 4.15, when the 3 EUL UE’s are using TDMA instead ofCDMA the system throughput is around 50% higher. The three CDF are verysimilar, so there is no significant difference in the resource utilization. Also in-teresting is that the TDMA system with a transmission time of 2 TTI, which inSection 4.2 had a bad noise rise distribution, now has a much better distribution.The reason for this is that the EUL UE’s use a lower bit rate, which leads to alower CIR overshoot and less time is needed to compensate for it.

46 Results

(a) TDMA improve the system throughput by more than 50%.

(b) CDF of the noise rise for the systems with three upload UE’swithout EUL. The three systems behave similar.

Figure 4.15. Combining CDMA and TDMA in one system

Chapter 5

Conclusions

The aim of this master thesis has been to evaluate an introduction of Time Divi-sion Multiple Access (TDMA) in Enhanced Uplink (EUL). The goal has to beento identify the gains and problems with an introduction of TDMA, and how theycan be handled.

The overall conclusion is that under favorable conditions there is a potentiallylarge gain with an introduction of TDMA in EUL. Simulations in favorable condi-tions have shown that the system throughput can increase by 100% when there areonly EUL UE’s in the system and by 50% when there is a mix of speech and EULUE’s. When using TDMA the noise rise distribution also shows improvements,the peaks are generally lower but the average is generally higher. But still, theaverage is well below the maximum allowed limit, so the increased average is dueto better resource utilization.

Due to major differences in experienced interference between passive and activeUE’s, CIR will vary a lot and create CIR overshoots. Those CIR overshoots areidentified as the main problem. It is shown that the CIR overshoots can generatelarge variations in CIR and even power rushes. Power rushes occur only when thetransmission time is very short, but they cause extremely high noise rise whichhave a negative impact on the coverage.

To minimize the effects of the CIR overshoot a long transmission time can beused. This solution has its disadvantages. The CIR overshoots are still presentwhich affects the power consumption and create some extra unwanted interference,both in the own cell but also for neighboring cells. Two ways to get rid of theCIR overshoot have been investigated. The first method is to scale the power inthe UE’s when they are transmitting. This method shows promising results butrequires changes in the standard. The second method is to implement interferencecancellation. With efficient interference cancellation, the system throughput canbe maintained with very short transmission times as well. In this study SuccessiveInterference Cancellation (SIC) has been used. Two kinds of implementations

47

48 Conclusions

have been studied; pre coding SIC and post coding SIC. The pre-decoding SIChas shown the highest system throughput, even if the efficiency is much lowercompared to the post-decoding SIC.

5.1 Further WorkIn this study, the conditions have been very favorable. All UE’s have had a lotof power to use and data to upload. The network consists of only one single cell,which leads to no interference from UE’s in neighboring cell.

The next step is to investigate a more realistic scenario. The most interestingchanges, that will have a major affect on system performance for a system usingTDMA, are stated below.

• Power limited UE’s, which can lead to that UE’s can not use all the resourcesthey are allowed to use during their transmission interval.

• Multi-cell scenario.

• A more realistic traffic model, which include bursty traffic.

All these changes will probably lead to a decrease in system throughput; especiallyfor system with TDMA, but also for system with CDMA. It would be interestingto see how large the gain is with TDMA compared to CDMA with these conditionsinstead.

Bibliography

[1] Ericsson AB. Basic Concept of HSPA. Available online:http://www.ericsson.com/technology/whitepapers/3087_basic_conc_hspa_a.pdf,February 2007.

[2] Lars Ahlin, Jens Zander, and Ben Slimane. Principles of Wireless Communi-cation. Studentlitteratur, 2006. ISBN 91-44-03080-0.

[3] Nevio Benvenuto and Paola Bisahlia. Parallel and successive interference can-cellation for MC-CDMA and their near-far resistance. Vehicular TechnologyConference, IEEE 58:th, volume 2, pages 1045 - 1049, October 2003.

[4] Gregory E. Bottomley, Douglas A. Cairns, Carmela Cozzo, Tracy L. Fulghum,Ali S. Khayrallah, Per Lindel, Magnus Sundelin, and Y.-P. Eric Wang. Ad-vanced receivers for WCDMA terminal platforms and base stations. EricssonReview No. 2, 2006.

[5] Erik Dahlman, Stefan Parkvall, Johan Sköld, and Per Beming. 3G EvolutionHSPA and LTE for Mobile Broadband. Elsevier, 1st edition, 2007. ISBN9-780-12372533-2.

[6] Harri Holma and Antti Toskala. WCDMA for UMTS. John Wiley Sons Ltd,1st edition, 2000. ISBN 0-471-72051-8.

[7] Philippe Kauffmann. Fast Power Control for Third Generation DS-CDMAMobile Systems. Broadband Communications, 2000. Proceedings. Pages: 9 -13, February 2000.

[8] Roberto Perillo, Velio Tralli, and Michele Zorzi. Effects of DiscontinuousTransmission on the Performance of a WCDMA Cellular System. VehicularTechnology, IEEE Transactions on volume 53, Pages:1387 - 1399, September2004.

[9] Jay Kumar Sundararajan, Vaibhav Maheshwar, and David R.Koilpillai. Throughput Enhancement in WCDMA us-ing the Generalized RAKE Receiver. Available online:http://web.mit.edu/jaykumar/www/papers/iete.pdf.

49

50 Bibliography

[10] Srikanth V. Knshnamurthyt, Anthony S. Acampora, and Michele Zorzi. Onthe Capacity of TDMA and CDMA for Broadband Wireless Packet Access. Personal, Indoor and Mobile Radio Communications. The Ninth IEEEInternational Symposium on Vol 1, Pages:167 - 172, September 1998.

[11] Jacobs Wozencraft. Principles of Communication Engineering. ISBN 0-88133-554-1, 1965.

Appendix A

CDF

(a) Normally distributed samples (b) Cumulative histogram of the values in Fig-ure A.1(a)

(c) Resulting CDF

Figure A.1. Illustration of how the Cumulative Distribution Function (CDF) is created.

51

Appendix B

Decibel

B.1 dBDecibel (dB) is a logarithmic unit to describe a ratio between two values, the ratiois therefore unit less. Decibel is defined as,

x with respect to y in dB = 10 log10(x

y) dB (B.1)

B.2 dBW and dBmThe dBW notation has the reference value 1 W. With the notation dBm, thereference value is instead 1 mW.

1 dBW = 10 log10(11) = 10 log10(

10.001

) = 30 dBm (B.2)

B.3 dBiThe gain offered by an antenna relative an isotropic antenna is often measured indB over Isotropic (dBi). An isotropic antenna has a uniform power distribution inall direction, thus 0 dBi in all directions.

52

Appendix C

E-DCH UE Categories

The different HSUPA categories available today are stated in Table C.

E-DCH Maximum Minimum Support Support MaximumUE category number of codes SF 10 ms TTI 2 ms TTI bit rate

1 1 SF4 X 0.73 Mbps2 2 SF4 X X 1.46 Mbps3 2 SF4 X 1.46 Mbps4 2 SF2 X X 2.92 Mbps5 2 SF2 X 2.00 Mbps6 4 SF2 X X 5.76 Mbps

Table C.1. UE categories [1].

53

54 E-DCH UE Categories

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