EDGE & EGPRS.pdf

Radio Resource Control Performanceof the Mobile Data Service EGPRS

Der Fakultat fur Elektrotechnik und Informationstechnik der Rheinisch-WestfalischenTechnischen Hochschule Aachen vorgelegte Dissertation zur Erlangung des akademischen

Grades eines Doktors der Ingenieurwissenschaften

von

Diplom-IngenieurUlrich Fornefeld

aus Braunschweig

256 Seiten mit 247 Abbildungen und 33 Tabellen

KURZFASSUNG

Mobilfunksysteme der dritten Generation wie sie zur Zeit installiert sind bieten erstmaligflachendeckend Datendienste akzeptabler Bandbreite an. Die wesentlichen Einzelaspektesolcher Systeme sind in den Forschungsprojekten der letzten Jahre ausfuhrlich erortertworden. Diese Arbeit leistet einen Beitrag zum Verstandniss des Zusammenspiels derin heutigen Netzen des Enhanced Datarates for GSM Evolution (EDGE) Systems einge-setzten Funkmittelverwaltungs-Algorithmen. Hierbei wird besonderes Augenmerk auf dieOptimierung der Dienstgute gelegt. Die Arbeit leistet Beitrage zur Losung einiger Pro-bleme, die bisher ungeklart sind. Hierzu zahlen Untersuchungen zur Systemleistung beiAnwendung der EDGE Ratenanpassung im Vergleich zum Incremental Redundancy (IR)genannten hybriden Automatic Repeat Request (ARQ) Verfahren. Es wird der Einsatzvon Leistungsregelung diskutiert und deren Einsatzgebiete werden abgegrenzt. Der Einfluvon Frequenzsprungverfahren und Teillast-Betrieb (Fractional Loading) auf Paketdaten-dienste wird erstmalig untersucht. Ausserdem wird der Einflu von Zellwechseln auf dieSystemleistung untersucht.

Ein wesentliches Kapitel befat sich mit der Simulationstechnik die fur die aufgefuhr-ten Untersuchungen eigens entwickelt wurde. Spezielle Zielvorgaben wie Mehrsystem-Simulationen, groflachige Szenarien und moglichst detailgetreue Modellierung auf fastallen OSI-Schichten pragen die Anforderungen an das System. Den Kernpunkt in interfe-renzbegrenzten Systemen stellt hier die moglichst exakte Modellierung der zeitaufgelostenInterferenzsituation dar.

Den Simulationsszenarien und Auswerteverfahren wurde ein eigenes Kapitel gewidmetum das Vertrauen in die beschriebenen Optimierungsverfahren zu starken. Es folgt eineausfuhrliche Darstellung und Diskussion der Simulationsergebnisse wobei die Breite desForschungsgebietes hier nur eine einfuhrende Untersuchung erlaubt. Die Arbeit schlietmit einem Ausblick auf weiterfuhrende Untersuchungsgebiete.

Der in der Arbeit entwickelte EDGE Emulator wird unter einer Lesser GNU PublicLicense (LGPL) als Quellcode verfugbar gemacht, um die Erforschung des weltweit ammeisten verbreiteten mobilen Datenfunksystems EDGE zu fordern.

ABSTRACT

Third generation mobile radio networks as presently installed for the first time presentubiquitous data services at acceptable bandwidth. The most important aspects of suchsystems have been analysed in research projects during the past couple of years. Thisthesis is a contribution to a deeper understanding of the interaction of the radio resourcecontrol algorithms in todays radio networks of the Enhanced Datarates for GSM Evolution(EDGE) system.

Special emphasis has been put on the optimisation of the Quality of Service (QoS).The thesis contributes solutions to some hitherto unsolved problems, e. g. investigationson the system performance considering the application of the EDGE Link Adaptation(LA) in comparison to the hybrid Automatic Repeat Request (ARQ) called IncrementalRedundancy (IR). The deployment of power control is discussed and the limitations ofoperational scenarios for power control are shown. The influence of Frequency Hopping(FH) and Fractional Loading (FL) on packet services is examined for the first time. Theinfluence of cell reselections on the system performance is investigated.

A central chapter deals with simulation technique developed for the mentioned inves-tigations. Special goals like multi-system simulations, large area scenarios and modellingaccuracy as detailed as possible on almost all Open Systems Interconnection (OSI) layersshape the requirements towards the system. The focus in interference-limited systemsis set on the modelling of the time-resolved interference situation with highest possibleaccuracy.

The simulation scenarios and the evaluation methods obtain an own chapter in orderto increase trust in the described optimisation methods. Next follows an elaborated pre-sentation and discussion of the simulation results. The broad research area covered by thisthesis only allows an introductionary investigation. The thesis concludes with an outlookat proceeding research areas.

The EDGE emulator developed within this work is made available as source code underthe Lesser GNU Public License (LGPL) in order to sponsor research on EDGE, the mostsuccessful mobile radio network worldwide.

CONTENTS

Kurzfassung iii

Abstract iv

1 Introduction 21.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Contribution of this Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Fundamentals on EGPRS 52.1 System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Network Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.1.1 GPRS Session Management (SM) . . . . . . . . . . . . . . 72.1.1.2 GPRS Mobility Management (GMM) . . . . . . . . . . . . 82.1.1.3 Sub-Network dependent Convergence Protocol (SNDCP) . 9

2.1.2 Data Link Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.1.2.1 Logical Link Control (LLC) . . . . . . . . . . . . . . . . . . 102.1.2.2 Radio Resource Control (RR) Sub Layer . . . . . . . . . . 112.1.2.3 Radio Link Control (RLC) . . . . . . . . . . . . . . . . . . 112.1.2.4 Medium Access Control (MAC) . . . . . . . . . . . . . . . 13

2.1.3 Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.2 Basic Functions of MAC and PHY Sub Layer . . . . . . . . . . . . . . . . . 15

2.2.1 Initial UL TBF establishment . . . . . . . . . . . . . . . . . . . . . . 152.2.2 One Phase Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.2.3 Two Phase Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.2.4 UL TBF Reestablishment . . . . . . . . . . . . . . . . . . . . . . . . 182.2.5 UL TBF (Re)Establishment during active Downlink (DL) Tempo-

rary Block Flow (TBF) . . . . . . . . . . . . . . . . . . . . . . . . . 192.2.6 Uplink Scheduling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.2.7 UL TBF start delay of scheduling . . . . . . . . . . . . . . . . . . . 212.2.8 UL TBF Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.2.9 DL TBF Establishment . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.10 DL TBF Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.11 Multistage Round Robin Scheduling . . . . . . . . . . . . . . . . . . 232.2.12 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3 Basics of Radio Resource Control for Packet Data Services 263.1 Channel Allocation and Channel Assignment . . . . . . . . . . . . . . . . . 26

3.1.1 Multi Slot Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . 283.1.1.1 UL Multi Frame Shifting . . . . . . . . . . . . . . . . . . . 29

3.1.2 Support of onDemand PDCH Concept . . . . . . . . . . . . . . . . . 303.2 Frequency Hopping and Fractional Loading . . . . . . . . . . . . . . . . . . 30

3.2.1 Frequency Hopping . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303.2.2 Fractional Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

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3.3 Link Optimisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.3.1 Adaptation Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.3.1.1 Slow Adaptation . . . . . . . . . . . . . . . . . . . . . . . . 333.3.1.2 Fast Adaptation . . . . . . . . . . . . . . . . . . . . . . . . 33

3.3.2 Averaging and Preprocessing of Measurement Samples . . . . . . . . 333.3.2.1 Averaging and Preprocessing for Power Control . . . . . . 343.3.2.2 Averaging and Preprocessing for Link Adaptation . . . . . 353.3.2.3 Filter Weights . . . . . . . . . . . . . . . . . . . . . . . . . 383.3.2.4 Preprocessing . . . . . . . . . . . . . . . . . . . . . . . . . 383.3.2.5 Measurement Reporting . . . . . . . . . . . . . . . . . . . . 39

3.3.3 Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.3.3.1 DL power control . . . . . . . . . . . . . . . . . . . . . . . 433.3.3.2 Uplink Power Control . . . . . . . . . . . . . . . . . . . . . 45

3.3.4 Link Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.3.4.1 MCS Selection . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.3.4.1.1 Delay-optimised MCS selection . . . . . . . . . . . 483.3.4.1.2 Throughput-optimised MCS selection . . . . . . . 493.3.4.1.3 LA proposal from the standard . . . . . . . . . . . 49

3.3.4.2 TBF start and end . . . . . . . . . . . . . . . . . . . . . . . 503.3.4.3 Retransmission and Resegmentation . . . . . . . . . . . . . 50

3.3.5 Incremental Redundancy / Hybrid ARQ II and Soft Combining . . . 513.3.5.1 Incremental Redundancy . . . . . . . . . . . . . . . . . . . 513.3.5.2 Soft Combining . . . . . . . . . . . . . . . . . . . . . . . . 513.3.5.3 Combined EDGE IR/SC Approach . . . . . . . . . . . . . 513.3.5.4 Resegmentation and IR/SC combined . . . . . . . . . . . . 52

3.4 Mobility Management and Cell Reselection . . . . . . . . . . . . . . . . . . 523.4.1 Mobility Management . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.4.1.1 GMM context management . . . . . . . . . . . . . . . . . . 523.4.1.2 Paging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.4.2 Cell Selection/Reselection . . . . . . . . . . . . . . . . . . . . . . . . 543.4.2.1 Measurement and Preprocessing . . . . . . . . . . . . . . . 553.4.2.2 Cell Selection Algorithm . . . . . . . . . . . . . . . . . . . 553.4.2.3 GPRS Cell Reselection Algorithm . . . . . . . . . . . . . . 56

3.4.2.3.1 Signal Strength . . . . . . . . . . . . . . . . . . . 563.4.2.3.2 Power Budget . . . . . . . . . . . . . . . . . . . . 563.4.2.3.3 HCS Support . . . . . . . . . . . . . . . . . . . . . 56

3.4.3 Side Effects and their Mitigation . . . . . . . . . . . . . . . . . . . . 57

4 The EGPRS Emulator 594.1 Software Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

4.1.1 ISO/OSI Reference Model . . . . . . . . . . . . . . . . . . . . . . . . 604.1.2 A Universal Emulation Environment . . . . . . . . . . . . . . . . . . 614.1.3 Dynamic Creation and Deletion of Network Elements . . . . . . . . 634.1.4 Dynamic Creation and Deletion of Protocol Instances . . . . . . . . 644.1.5 Example: a GPRS Protocol Stack . . . . . . . . . . . . . . . . . . . 65

4.1.5.1 Structure of a Protocol Library . . . . . . . . . . . . . . . . 664.1.5.2 Usage of a Protocol Library . . . . . . . . . . . . . . . . . . 68

4.1.6 The Interface to the Physical Layer . . . . . . . . . . . . . . . . . . . 684.1.7 Proof of Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694.1.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Contents vii

4.2 Protocol Emulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.2.1 Core Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.2.2 Transport Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.2.2.1 Transport Control Protocol (TCP) . . . . . . . . . . . . . . 734.2.2.2 User Datagram Protocol (UDP) . . . . . . . . . . . . . . . 73

4.2.3 Network Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.2.3.1 Internet Protocol (IP) Version 4 . . . . . . . . . . . . . . . 734.2.3.2 GPRS Session Management (SM) . . . . . . . . . . . . . . 734.2.3.3 GPRS Mobility Management (GMM) . . . . . . . . . . . . 744.2.3.4 Radio Resource (RR) Management . . . . . . . . . . . . . 74

4.2.4 Data Link Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744.2.4.1 Sub-Network dependent Convergence Protocol (SNDCP) . 744.2.4.2 Logical Link Control (LLC) . . . . . . . . . . . . . . . . . . 744.2.4.3 Radio Link Control (RLC) . . . . . . . . . . . . . . . . . . 744.2.4.4 Medium Access Control (MAC) . . . . . . . . . . . . . . . 74

4.2.5 Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5 Emulator Environment 755.1 Channel Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.1.1 Activity Detection & CIR Calculation . . . . . . . . . . . . . . . . . 765.1.2 Erasure Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 765.1.3 Measurement Reporting . . . . . . . . . . . . . . . . . . . . . . . . . 765.1.4 Channel Quality Calculation . . . . . . . . . . . . . . . . . . . . . . 775.1.5 Modeling of Radio Wave Propagation . . . . . . . . . . . . . . . . . 77

5.1.5.1 Morphology Types . . . . . . . . . . . . . . . . . . . . . . . 785.1.5.2 Cell Types . . . . . . . . . . . . . . . . . . . . . . . . . . . 795.1.5.3 Path Loss Models based on Experimental Investigations . . 795.1.5.4 Test Environments . . . . . . . . . . . . . . . . . . . . . . . 80

5.1.5.4.1 A Geometrical Algorithm for Efficient Cut Calcu-lation . . . . . . . . . . . . . . . . . . . . . . . . . 81

5.1.5.5 Signal Fading . . . . . . . . . . . . . . . . . . . . . . . . . . 825.1.5.5.1 Shadowing . . . . . . . . . . . . . . . . . . . . . . 825.1.5.5.2 Multi path propagation . . . . . . . . . . . . . . . 83

5.1.5.6 Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . 835.2 Mobility and Traffic Load Generation . . . . . . . . . . . . . . . . . . . . . 85

5.2.1 Introduction to Traffic Generation . . . . . . . . . . . . . . . . . . . 855.2.2 Traffic Density Generation . . . . . . . . . . . . . . . . . . . . . . . . 85

5.2.2.1 Stochastic Traffic Generation . . . . . . . . . . . . . . . . . 855.2.2.2 Deterministic Traffic Generation . . . . . . . . . . . . . . . 85

5.2.3 Session Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865.2.4 Models for User Mobility . . . . . . . . . . . . . . . . . . . . . . . . 865.2.5 Estimation of Required Runtime for Stochatic Traffic Load Generation 875.2.6 The Heisenberg Uncertainty Principle for Mobile Radio Network

System Level Emulations . . . . . . . . . . . . . . . . . . . . . . . . 885.3 Scenarios, Performance Indicators and Measurement . . . . . . . . . . . . . 89

5.3.1 Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 895.3.1.1 Outdoor Micro Cell Scenario with random User Mobility . 895.3.1.2 Outdoor Micro Cell Scenario with directed User Mobility . 905.3.1.3 Indoor Pico Cell Scenario with directed User Mobility . . . 905.3.1.4 Two Mobiles in two Cells with directed User Mobility . . . 91

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5.3.2 Performance Indicators . . . . . . . . . . . . . . . . . . . . . . . . . 925.3.2.1 Distance dependent Performance Evaluation . . . . . . . . 93

6 EGPRS Traffic Performance Results 95

6.1 An Emulator Validation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 956.2 Scenario 1: Reference Scenario . . . . . . . . . . . . . . . . . . . . . . . . . 98

6.2.1 No Frequency Hopping, Cluster Size N = 3 . . . . . . . . . . . . . . 986.2.1.1 Link adaptation for streaming services? . . . . . . . . . . . 103

6.3 Scenario 2 - 5: Frequency Hopping and Fractional Loading . . . . . . . . . . 1046.3.1 Scenario 2: Base Band Frequency Hopping, Reuse (3/3) . . . . . . . 1046.3.2 Scenario 3: Synthesiser Hopping, Reuse (3/3) . . . . . . . . . . . . . 1106.3.3 Scenario 4: Synthesiser Hopping, Reuse (1/3) . . . . . . . . . . . . . 1156.3.4 Scenario 5: Synthesiser Hopping, Reuse (1/1) . . . . . . . . . . . . . 120

6.4 Scenario 6: Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256.4.1 Control Behaviour without Influences of Fast Fading . . . . . . . . . 1256.4.2 Control Behaviour under Fast Fading . . . . . . . . . . . . . . . . . 125

6.4.2.1 DL/UL Closed Loop, Signal Strength based Control . . . . 1256.4.2.2 DL/UL Closed Loop, Quality based Control . . . . . . . . 1266.4.2.3 DL/UL Open Loop, Signal Strength based Control . . . . . 1276.4.2.4 UL Open Loop, Quality based Control . . . . . . . . . . . 1286.4.2.5 UL Open Loop, mixed Signal Strength / Quality based

Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1296.5 Scenario 7 - 9: Link Adaptation and Incremental Redundancy . . . . . . . . 132

6.5.1 Scenario 7: Link Adaptation, Throughput-optimised . . . . . . . . . 1326.5.2 Scenario 8: Link Adaptation, Delay-optimised . . . . . . . . . . . . . 138

6.5.2.1 Discussion: Is there a Link Adaptation that optimises La-tency? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

6.5.2.2 Emulation Results . . . . . . . . . . . . . . . . . . . . . . . 1396.5.3 Scenario 9: Link Adaptation Efficiency . . . . . . . . . . . . . . . . . 144

6.5.3.1 Link Adaptation Efficiency without Influences of Fast Fading1446.5.3.2 Link Adaptation Efficiency under Fast Fading . . . . . . . 1446.5.3.3 Emulation Results . . . . . . . . . . . . . . . . . . . . . . . 144

6.5.4 Incremental Redundancy . . . . . . . . . . . . . . . . . . . . . . . . 151

7 EGPRS to serve Micro- and Pico Cells 152

7.1 Scenario 10: Cell Reselection in Micro Cells . . . . . . . . . . . . . . . . . . 1527.2 Scenario 11: Local System Performance in an Indoor Pico Cell Scenario . . 154

8 Conclusions 161

A Link Level Mapping 162

A.1 Introduction to Link Level Mapping . . . . . . . . . . . . . . . . . . . . . . 162A.2 Channel Coding for Data Services . . . . . . . . . . . . . . . . . . . . . . . 163

A.2.1 Independent Mappings, independent Parameters . . . . . . . . . . . 163A.2.1.1 GPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163A.2.1.2 EGPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

A.2.2 Consecutive Mapping, independent Parameters . . . . . . . . . . . . 167A.2.2.1 GPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167A.2.2.2 EGPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

Contents ix

B Data Evaluation 170B.1 Introduction to Data Evaluation . . . . . . . . . . . . . . . . . . . . . . . . 170

B.1.1 Writing to the Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . 170B.2 Sorting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

C Link Adaptation Strategies 174C.1 Selection of optimum MCS . . . . . . . . . . . . . . . . . . . . . . . . . . . 174C.2 Throughput-optimised MCS Selection . . . . . . . . . . . . . . . . . . . . . 174C.3 Delay-optimised MCS Selection . . . . . . . . . . . . . . . . . . . . . . . . . 176C.4 Standards proposal for MCS Selection . . . . . . . . . . . . . . . . . . . . . 178

D Application Layer Protocols 180D.1 Hypertext Transfer Protocol (HTTP) . . . . . . . . . . . . . . . . . . . . . 180D.2 Post Office Protocol (POP) Version 3 . . . . . . . . . . . . . . . . . . . . . 183D.3 File Transfer Protocol (FTP) . . . . . . . . . . . . . . . . . . . . . . . . . . 184D.4 Session Initiation Protocol (SIP) . . . . . . . . . . . . . . . . . . . . . . . . 185D.5 RTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187D.6 Voice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187D.7 Audio Streaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189D.8 Video Streaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

E Reference Scenario 193E.1 Session Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194E.2 Application Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

E.2.1 HTTP settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194E.2.2 SMTP settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195E.2.3 FTP settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195E.2.4 WAP settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195E.2.5 MMS settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195E.2.6 POP3 settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195E.2.7 Email sizing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195E.2.8 Audio stream settings . . . . . . . . . . . . . . . . . . . . . . . . . . 196E.2.9 Video stream settings . . . . . . . . . . . . . . . . . . . . . . . . . . 196E.2.10 SIP settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196E.2.11 Voice settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

E.3 Transport Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196E.3.1 Session Manager settings . . . . . . . . . . . . . . . . . . . . . . . . 196E.3.2 TCP settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197E.3.3 UDP settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

E.4 Network Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198E.5 GPRS Control Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198

E.5.1 GPRS SM Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 198E.5.2 GPRS GMM Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 198

E.6 GPRS User Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198E.6.1 GPRS SNDCP Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 198E.6.2 GPRS LLC Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 199E.6.3 GPRS RLC Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 200E.6.4 GPRS MAC Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 200

E.7 GPRS RRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202E.7.1 GPRS Channel Allocation . . . . . . . . . . . . . . . . . . . . . . . . 202

Contents 1

E.7.2 GPRS PowerControl . . . . . . . . . . . . . . . . . . . . . . . . . . . 202E.8 GPRS.PHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

E.8.1 GPRS Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . 203E.8.2 GPRS Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203E.8.3 GPRS Power Limits . . . . . . . . . . . . . . . . . . . . . . . . . . . 204E.8.4 GPRS LinkLevelInterface . . . . . . . . . . . . . . . . . . . . . . . . 204

E.9 GSM RRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204E.9.1 GSM Channel Allocation . . . . . . . . . . . . . . . . . . . . . . . . 204E.9.2 GSM Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 205E.9.3 GSM Handover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

E.10 GSM PHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207E.10.1 GSM Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207E.10.2 GSM Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208E.10.3 GSM Linklevelinterface . . . . . . . . . . . . . . . . . . . . . . . . . 209E.10.4 GSM Paging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209E.10.5 PHY Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

E.11 GSM BTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210E.12 GSM MSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210E.13 Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210

E.13.1 GPRS Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210E.13.2 General GSM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210E.13.3 General GPRS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210E.13.4 GPRS ChannelModel . . . . . . . . . . . . . . . . . . . . . . . . . . 210

List of Figures 212

List of Tables 218

Glossary 219

Nomenclature 221

List of Abbreviations 222

Bibliography 225

Unpublished Work 235

Curriculum Vitae 247

CHAPTER 1

Introduction

Content1.1 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Contribution of this Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.3 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

S ince almost two decades, the Global System for Mobile Communication (GSM) is induty as the most successful mobile radio network worldwide. With more than 2 billionsubscribers it defends market leadership uncontested. While GSM speech services - theinitial development goal - have been well understood and optimised by the research commu-nity, the investigations on packet data services like General Packet Radio Service (GPRS)and Enhanced General Packet Radio Service (EGPRS) (the 2nd+ Generation (2+G)) gotstuck at a certain point in time. Research and development power was withdrawn in orderto support the rapid introduction of 3rd Generation (3G) services like Universal MobileTelecommunication System (UMTS). 3G services indeed show a slightly higher peak datarate than 2+G services like EGPRS. However, the spectrum efficiency is considered tobe remarkably lower for low and medium data rate services such as telephony [96] and sois the ability of making profit. In addition, the roll-out of 3G systems comes along withinvestment needs remarkably higher than for GSM systems. So, the additional techni-cal benefit is low while quick Return on Investment (ROI) is endangered. In fact, manynetwork operators who introduced UMTS did not reach ROI at all after years.

As UMTS did not fulfil marketing expectations so far and due to the emerging WirelessLocal Area Network (WLAN) technology, the marketing gap for 3G services has almostclosed. In addition, the 3G world is not homogenous. Concurrent systems such as CodeDivision Multiple Access (CDMA) 2000 inhibit global usability of mobile devices. So, itis now time to concentrate on the GSM data services again, especially the packet dataservices. This thesis makes a contribution to close the still existent knowledge gap in thisarea and is therefore focussed on the EGPRS packet data service.

EGPRS with a maximum gross data rate of 473 kbit/s provides sufficient capacity forservices demanding low and medium data rate. Even low quality video services or Voiceover Internet Protocol (VoIP) services might be run via EGPRS service. Not recognised bythe public, EDGE has been deployed by most UMTS operators in Europe to strengthen thecapacity of their GPRS service that they offer in areas where an UMTS roll-out appearedto be too costly. Summed up with the operators serving the Americas, EDGE might bethe dominating technique to provide mobile packet data services worldwide.

1.1 Objectives

As the GSM packet data services provide a reasonable option, it is time to start optimisingtheir usage. Therefore, an in-depth understanding of the means of optimisation, namely ofthe Radio Resource Control (RRC) algorithms is required. Proper handling of the radioresource is crucial for optimum system performance in an interference limited networksuch as EGPRS. Once the available means of RRC are well understood, the optimisation

1.2. Contribution of this Thesis 3

towards certain key performance indicators is the next step. To reach this goal, differentcontrol algorithm types are involved and the benefits of each RRC algorithm are madetransparent. The final goal is to investigate and present the mutual influence of RRCalgorithms on each other.

1.2 Contribution of this Thesis

Since the standardisation of EDGE it is an open question whether Link Adaptation (LA)or Incremental Redundancy (IR) is the better choice to dynamically adapt to the radiochannel conditions for reaching the optimum system performance. The main contributionof the thesis is to present the strengths and weaknesses of both techniques and to givea clear indication that LA is the preferable solution. A number of model based analysesexist that in some cases are in favour of either LA [64] or IR [57, 130, 136]. The reasonfor the differing results is the abstracted system representation chosen for modeling andevaluation. This thesis, therefore, has chosen to make use of a very detailed implementationof EGPRS in form of a stochastic event driven simulation that in fact is an emulation ofEGPRS. It is established that LA contributes most part of the capacity increase possiblefrom both, LA and IR. Other important contributions of the thesis are the clarification ofthe impact of Fractional Loading (FL) and Frequency Hopping (FH) to increase systemcapacity for packet based services. Further, it is shown how to apply Power Control (PC)to gain the maximum capacity of EGPRS. It is also shown how the mobility managementmust control cell allocation in typical urban and indoor scenarios to gain the maximumcapacity.

1.3 Outline

As an introduction to the subject, an overview over the EGPRS standard is given inChapter. 2. After a short functional description of the involved protocols, an in-depthdescription of Medium Access Control (MAC) and Physical Layer (PHY) functionality isgiven.

The central topic of the thesis is presented in Chapter. 3. The RRC algorithms arepresented one by one using theoretical modelling and analyses. Channel allocation andchannel assignment provide the basis for GPRS services that are based on the on demandassignment concept. The multi slot capability of the Mobile Station (MS) limits theassignment of radio resource capacity to communication links. Frequency hopping andfractional loading allow fine-tuning of the interference situation at the mobile receiver.Link optimisation comprises power control for mitigation of signal shadowing and linkadaptation for the optimum transmission rate selection towards channel quality aimed at.Different adaptation strategies are presented. Incremental redundancy as an enhancementto link adaptation is presented finally.

Mobility Management (MM) provision in GPRS is based on an extension of the GSMMM mechanism. The basis for this is the cell reselection, a replacement of the GSMhandover. Side effects and their mitigation by the Serving GPRS Support Node (SGSN)/ Base Station System (BSS) are explained.

One of the mayor contributions is the elaborated system level emulation environmentpresented in Chapter. 4. The flexible software architecture is presented and discussed.The degree of emulation of protocol functions is chosen to be extremely detailed. Mostprotocols are being implemented as specified in the standard. The abstraction of thePHY and the channel model need detailed explanation which is being given next. Finally,the involved traffic load models are introduced and environmental effects of system level

4 1. Introduction

emulation in general are discussed. This chapter also introduces the scenarios and the keyperformance indicators. The number of scenarios considered for study has been kept aslow as possible in order to ease comparability of the results.

The EGPRS traffic performance results are presented and discussed in Chapter. 6. Areference scenario with only few RRC algorithms operational is introduced. The differentmeans of control are then compared with the reference scenario. Finally, an indoor picocell scenario with all RRC algorithms operational is investigated in Chapter. 7.

Chapter. 8 concludes the investigations and provides an outlook on further prospectiveinvestigation.

CHAPTER 2

Fundamentals on EGPRS

Content2.1 System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Basic Functions of MAC and PHY Sub Layer . . . . . . . . . . . . . . . . . . . . . . . . 15

T he General Packet Radio Service (GPRS), an add-on to the Global System for MobileCommunication (GSM) is investigated concerning RRC aspects within this thesis.As the system is complex, a basic understanding of the GSM/ GPRS architecture isrequired. The functions of the different protocols and their location in the GPRS NetworkElements (NEs) is described in Section. 2.1. Apart from the Radio Resource Control(RRC) functionality described in Chapter. 3, there are basic functions the reader shouldunderstand before plunging deeper into the subject. A very brief description of the basicMAC and PHY functionality can be found in Section. 2.2. A reader experienced withEDGE details might wish to directly continue with Chapter. 3.

2.1 System Overview

GPRS has been standardised in 1998 as an enhancement to the well-established GSMstandardised in 1991. GPRS is targeting at the introduction of a packet data service intothe GSM network. The GSM Network Elements (NEs) are reused by GPRS, some areslightly enhanced and a couple of NEs have been added, Fig. 2.1.

SMS-GMSCSMS-IWMSC

SMS Center

MSC/VLR HLR

E

Gs

GdC

D

GcA Gr

GR

EIR

other PLMN

PDN

GGSN

TE MT BSS SGSN GGSN TE

SGSN

GiGnGbUmR

GpGfGn

Data and SignallingSignalling

PCU

GPRS relatedNetwork Elements

Figure 2.1: GSM and GPRS network elements and respective reference points

Basic enhancements to GSM can be found in the Base Station System (BSS) andMobile Station (MS) supporting GPRS related PHY functions for channel coding like

6 2. Fundamentals on EGPRS

Forward Error Correction (FEC) with uniform error protection for data services. Thefollowing GPRS NEs have been added to the circuit-switched GSM network architectureto enable packet data services:

The Packet Control Unit (PCU) provides access to the air interface with basic RadioLink Control (RLC)/MAC functions. Routing packets through the core network is the main task of the Serving GPRSSupport Node (SGSN). It also handles the mobility management and parts of thenetwork management. The Gateway GPRS Support Node (GGSN) is the gate to the Packet Data Network(PDN) that keeps track of the subscribed and activated services (Packet Data Protocol(PDP) contexts) and provides Internet Protocol (IP) masquerading. The GPRS Register (GR) located at the Home Location Register (HLR) keeps theGPRS subscriber data.

It is worth noting that the PCU might be located either at the Base TransceiverStation (BTS) or at the Base Station Controller (BSC) or at the SGSN, allowing a tradeoffbetween packet delay and costs, Fig. 2.2.

CCUCCU

CCUCCU

CCUCCU

PCU

PCU

PCU

BTS

BTS

BSC Site

BSC Site GSN Site

Gb

A

B

C

Gb

Abis

Um

BTS BSC Site GSN Site

GSN Site

Key:

Packetswitching Function

Circuitswitching Function (16 or 64 kbit/s)

Figure 2.2: Alternate PCU positions

Reference points for interfacing GSM and GPRS NEs are shown in Fig. 2.1, too. TheNEs in the core network are linked by wire lines. In this thesis, the study focus is onthe Um and Gb reference points. A protocol stack for the control plane and one forthe user plane is required to provide GPRS services. Fig. 2.3 shows a combined viewof both planes. The control plane comprises the reference points Um and Gb and theprotocols Logical Link Control (LLC), GPRS Mobility Management (GMM) and SessionManagement (SM) above the relay scope as well as the protocols below. The user planecomprises the reference points Um, Gb and Gn and all protocols except GMM and SM.

The PCU handles the RLC and the MAC protocol and is linked to both, the CodingControl Unit (CCU) of the BTS and the SGSN, Fig. 2.2. The Network Services (NS)protocol (frame relay) at Gb serves the Base Station System GPRS Protocol (BSSGP).On top of the BSSGP, the LLC serves for data transfer and the GMM handles mobil-ity management related functions, Fig. 2.3. The Sub-Network Dependent ConvergenceProtocol (SNDCP) cares for compression and flow control. All the protocols at Gb from

2.1. System Overview 7

RLC

LLC

IP / PPP

HigherLayer

RelayBSSGPRLC

GTPSNDCP

IP

UDP

GTP

IP

UDPLLC

SNDCP SM

GMM

SM

BSSGP

IP / PPP

GMM

GSMRF

MAC

L1bis

MAC

MS BSS/PCU SGSN GGSN

L1bis L1

L2

L1

L2

Gi

Scope of GPRS

GnUm Gb

GSMRF

NSNS

Core Network Protocols

Figure 2.3: GPRS protocol suite, user and control plane

NS up to SNDCP belong to layer 2. SM provides maintenance for PDP contexts by usageof a GPRS Mobility Management (GMM) data transfer service.

The SGSN and the GGSN are connected via the Gn reference point where the GatewayTunnelling Protocol (GTP) is served by User Datagram Protocol (UDP). The IP tunnelsthe user data through the GPRS core network.

The counterparts of RLC/MAC, LLC, SNDCP, GMM and SM that are located in theSGSN or BSS / PCU can be found in the protocol stack of the MS. The services andprotocols described in more detail in the following are the main subjects of study in thisthesis and therefore need to be introduced.

2.1.1 Network Layer

As GPRS is a wireless extension to the Internet, the upper rim of the GPRS scope islimited by well-known protocols at the user plane like Internet Protocol version 4 (IPv4).

2.1.1.1 GPRS Session Management (SM)

The Session Management (SM) protocol is a part of the Connection Management (CM) sublayer as shown in Fig. 2.4. It is responsible for the handling of sessions, called Packet DataProtocol (PDP) contexts. Each MS might use up to 11 simultaneous PDP contexts. Atthe MS side, the activation, deactivation and re-negotiation of PDP contexts is triggeredby Attention (AT) commands and requested via the SMREG Service Access Point (SAP).A PDP context activation requires the negotiation of QoS parameters [19]. In addition,an active GPRS Mobility Management (GMM) context must exist. A so-called unitdataservice of the GMM protocol is used for message transfer between peer SM entities via theGMMSM SAP [20]. SM informs SNDCP about establishment and release of PDP contextsvia the SNSM SAP so that SNDCP might set up appropriate protocol instances.


MM Sublayer

SNDCP

SM

PDP

CM SublayerSNSM-SAP

SMSM

SMREG-SAP

GMMSM-SAP

Figure 2.4: CM sub layer

2.1.1.2 GPRS Mobility Management (GMM)

The GMM protocol is used in the MM sublayer (Fig. 2.5) and keeps track of the GPRSRouting Area (RA) of each attached MS. This is done in a given GMM context. Main-

PDP

CM Sublayer

LLGMM-SAP

GMMSM-SAP GMMREG-SAP

MM Sublayer

LLC Sublayer

GMMGMMcoord

MMcoord

MM

PD/TLLI

RR Sublayer

GMMRR-SAP RR-SAP

PD

Figure 2.5: MM sub layer

taining a GMM context requires the following actions [19]:

Attach / Detach The MS must attach to the network in order to establish a GMMcontext using the GMM protocol for attaching and detaching. Routing Area Update The Network (NW) keeps track of the MS location byregistering the RA of the MS. Update is required if the MS leaves the serving RA. Identification The network asks the MS to declare its identity. Identification istypically followed by the authentication procedure.


Authentication and Ciphering The MS must be authenticated by the networkbefore activating any PDP context. A ciphering procedure might be invoked in orderto negotiate the ciphering mode. Paging If the cell a MS has camped on is not known to the network, the MS has tobe paged in order to (re)activate the GMM context.

The GSM MM sub layer that contains the GMM protocol is shown in Fig. 2.5. Attach/ Detach functions are triggered via the GMMREG SAP (for registration) while implicitattaching happens if data is transferred via the GMMSM SAP [20]. The coordinationbetween GMM and GSM MM allows combined procedures such as combined LocationArea (LoA) / RA update. The GMM messages have highest LLC priority.

2.1.1.3 Sub-Network dependent Convergence Protocol (SNDCP)

The SNDCP sub layer, Fig. 2.6, serves as an adaptation layer between network layer userplane protocols such as IPv4 and the GPRS protocols. SNDCP basically provides the

ControlEntity

5

CM SublayerSNSM-SAP

LLC SublayerUD5 UD9 UD11

SNDCPSublayer

SNDCP Management

Entity

SNDCPEntity

SAPI

NSAPI

PDPor

Relay

PDPor

Relay

SNDCPusers

6 15

UD3

Figure 2.6: SNDCP sub layer

following functions [35]:

Multiplexing of up to 11 PDP instances Compression / decompression of user data. Compression / decompression of protocol control information, e. g. Transport Con-trol Protocol (TCP) headers. Segmentation of a network protocol data unit (N-PDU) into Logical Link ControlProtocol Data Units (LL-PDUs) and re-assembly of LL-PDUs into an N-PDU.

For data compression, the ITU recommendations V.42bis [70] or V.44 [71] are used.Data compression is performed mostly on application layer, e. g. video streaming datais highly compressed. Therefore data compression has been skipped in the successor toSNDCP, namely the Packet Data Convergence Protocol (PDCP) that is used instead ofSNDCP from GPRS release 5 on.

The TCP header compression uses one of the following approaches:

Request for Comment (RFC) 1144 [75] where header synchronisation is triggered byconclusion of the TCP action derived from the TCP segment content. RFC 2507 [15] that covers compression slow start and periodic header refresh fornon-TCP packet streams allowing quick header resynchronisation after packet loss.


[15] allows compression of Real-time Transport Protocol (RTP) streams (CompressedReal-time Transport Protocol (CRTP), [10]).

Segmentation according to the QoS profile controls the packet delay, the maximumsegment size refers to that of an Ethernet segment.

2.1.2 Data Link Layer

The GPRS data link layer [39] maintains a packet flow between MS and SGSN based onthe Logical Link Control (LLC). The RLC/MAC protocols handle packet streams overthe air interface.

2.1.2.1 Logical Link Control (LLC)

The Logical Link Control (LLC) protocol, Fig. 2.7 [38] acts as an interface providing areliable, location-decoupled link for the packet-stream. This is achieved by the following

Figure 2.7: LLC sub layer

functions:

Provision of a highly reliable logical link between MS and SGSN. Decoupling from underlying radio interfaces in order to allow introduction of alter-native GPRS radio solutions. Support of variable-length information frames. Support of QoS provisioning by flow control and optional Backward Error Correction(BEC) for user data, prioritisation and intentional frame dropping. Multiplexing of several packet streams over the same physical resource. Ciphering of the packet stream (optional). Provision of user identity confidentiality.


Four groups of SAPs are distinguished. The service user GMM is served with highestpriority at the LLGMM SAP. User plane related data exchanged via LL SAPs is handledaccording to the QoS field of the requesting service primitive. The Short Message Service(SMS) might be either routed via LLCor via circuit-switched GSM network service. LLCoffers a generic, transparent data exchange service to the Tunnelling of Messages (TOM)service user.

The acknowledged LLC service is based on a High-level Data Link Control (HDLC)derivate using piggy-backing of S-Frames. It is worth noting that all GSM / GPRS cipher-ing algorithms have been cracked, therefore since 1999 data confidentiality is not providedany longer.

2.1.2.2 Radio Resource Control (RR) Sub Layer

The basic GPRS enhancement to the GSM Radio Resource (RR) sub layer, Fig. 2.8 is theintroduction of Packet Data Channels (PDCHs). Two important instances can be distin-

Figure 2.8: RR sub layer

guished: The RR management of PDCHs realises the dynamic availability of channels forGPRS, called the onDemand concept. Thereby, a GSM Traffic Channel (TCH) temporar-ily may be assigned as PDCH. The RLC / MAC protocol processes data transmissionover PDCHs. The RR sub layer provides services to both, the MM and the LLC sub layerutilising services of the GSM Data Link Layer (DLL) and the physical link layer. TheMAC protocol multiplexes the packet streams onto the available PDCHs. Reliable trans-mission and BEC - a task of the RLC protocol - is optional, an unreliable transmission isalso available.

2.1.2.3 Radio Link Control (RLC)

The RLC provides data transmission services over the air interface between the MS andthe PCU. In particular, the RLC protocol [43] takes care of the following tasks:

Provision of an (un)acknowledged data transfer service to the RR management entity.


Segmentation of upper layer Protocol Data Units (PDUs) into RLC data blocks andcorresponding reassembly. BEC for selective retransmission of erroneous radio blocks in acknowledged mode.A Hybrid ARQ type 2 using Incremental Redundancy (IR) might be applied forEGPRS. Preservation of transmission order of higher layer PDUs in acknowledged mode. Provision of link adaptation for varying channel quality.

A functional block diagram of the RLC protocol is shown in Fig. 2.9. RLC relies on

RLC

RLCData-SAP

MAC

RR Management

SegmentationReassembly

Radio Priority 1 2 3 4

Send BufferReceive BufferV_S RLC DataRLC Data

V_AV_R

RLC HeaderAssembly + RLC Header

Acknow-ledgement

Unit

MACData-SAP

Figure 2.9: RLC protocol

services provided by the MAC protocol like setup and maintenance of Temporary BlockFlow (TBF). During TBF setup, a radio priority is negotiated. Blocks with radio priorityother than the negotiated one might be transmitted but a re-negotiation - the TBF re-establishment - is required. For the normal operation, a head-of-queue scheduling servesthe RLC entry queues. Segmentation is done according to the GPRS Coding Scheme (CS)or EGPRS Modulation and Coding Scheme (MCS) suggested by the Link Adaptation (LA)algorithm. To serve the acknowledged mode, RLC data blocks are stored in a send/receivebuffer for retransmissions resp. generation of an acknowledge bitmap. The RLC headercontaining, e. g. , a Block Sequence Number (BSN) is prepended in front of the RLCdata block. If in acknowledged mode, the RLC might apply either resegmentation, e.g.,splitting of radio blocks or (for EGPRS) Incremental Redundancy (IR) in order to copewith varying channel conditions. Up to three Puncturing Schemes (PSs) per MCS areavailable in order to increase redundancy. Acknowledgements are transmitted regardlessof the RLC mode, be it acknowledged or unacknowledged. Acknowledgements are requiredin order to transfer channel quality reports on the Uplink (UL) and link adaptation / powercontrol commands on the DL.


2.1.2.4 Medium Access Control (MAC)

The functions of the MAC protocol, Fig. 2.10 are closely coupled with those of the RLCprotocol. The interface between both - the MACData SAP - is implementation-dependent

MAC

MACData-SAP

PHY

RLC

DL Scheduler

PH-SAP

Control Message Scheduler

Entity DL TBFMaintenance

UL TBFMaintenance

UL Scheduler

MAC DataMAC Data

MAC HeaderAssembly

+ MAC Header

Control Message Handler

Channel Management

PDCH

Figure 2.10: MAC protocol, NW side

and not standardised, so the figure shows an example solution. The MAC protocol mustcover at least the following basic tasks:

Stochastic multiplexing of several users on a PDCH. Provision of a multiple PDCH access to a single user. Packet Broadcast Control Channel (PBCCH) information generation on the NWside. The information varies dynamically with changing GPRS load and channelavailability. Random access on the MS side, in collaboration with the PHY. Contention resolution after a random access. Maintenance of a packetised connection, the Temporary Block Flow (TBF) for ULand DL. Dynamic allocation/release of PDCHs if commanded by the RR management, ac-cording to the onDemand concept. Capacity scheduling between users according to the negotiated QoS profiles. Support of the PC algorithm by generation and transmission of measurement reports,e.g. the Channel Quality Report. Paging of MS in case of mobile-terminated TBF setup.

Two MAC states are to be distinguished. If a TBF is established, the GPRS MACprotocol is in Packet Transfer Mode (PTM), otherwise it is in Packet Idle Mode (PIM)and listens to the assigned control channels. If present, PBCCH and Packet CommonControl Channel (PCCCH) are used as control channels, otherwise Broadcast ControlChannel (BCCH) and Common Control Channel (CCCH) substitute the packet-orientedchannels. In PTM, common control information and broadcast control information mustbe included in the Packet Associated Control Channel (PACCH) of the TBF by the PCU


as the MS might be unable to simultaneously listen to the assigned Packet Data TrafficChannels (PDTCHs) and to the control channels.

The MAC data or control block together with the MAC header form the radio block,the basic logical transmission unit of GPRS, Fig. 2.11. The MAC functions are furtherdescribed in Section. 2.2.

Sequence (BCS)Block Check

RLC/MAC Block for Control Message Transfer

RLC/MAC Control BlockMAC Header

Radio Block

CombinedRLC/MAC Header(BSN, TFI, CPS...)

EGPRS RLC/MAC Block for Data TransferBlock CheckSequence (BCS)(optional)

RLC Data Block 2RLC Data Block 1

Radio Block

Figure 2.11: RLC/MAC Block for data transfer and control message transfer

2.1.3 Physical Layer

The PHY layer, Fig. 2.12 executes tasks directly linked with the radio transmission. This

PHY

MAC

PH-SAP

Coding

PowerControl TRX

CellReselection

PDCH

Demodulation

Decoding

Modulation

InterleavingDeinterleaving

LevelMeasurement

MPH-SAP

RR Management

QualityMeasurement

Figure 2.12: PHY layer, MS side

includes the following functions:

Provision of a data transmission service. Forward Error Correction (FEC) by channel coding and interleaving. Random access capability for the MS. Acquisition of the BCCH information by the MS. Synchronisation and channel estimation. Measurement of the signal level and channel quality. Power Control Cell Reselection Frequency Hopping

2.2. Basic Functions of MAC and PHY Sub Layer 15

The Transmitter / Receiver (TRX) organises the physical channel as a Frequency DivisionDuplex (FDD) system with Time Division Multiple Access (TDMA) / Frequency Divi-sion Multiple Access (FDMA) [40]. It also controls the frequency hopping sequence [41].The TRX might operate in different frequency bands available for GSM [36]. The userdata is channel-coded and interleaved according [37]. Measurements for Power Control,Link Adaptation and Cell Reselection are collected [44] which are transferred to the RRmanagement via the MPH SAP. Direct forwarding of measurements to the PC as shownin Fig. 2.12 is done for open loop control only, for closed loop control, the measurementsfrom the opposite link are involved. Synchronisation is performed by the TRX on anquarter-symbol time basis [42]. Note that the crypto-unit for GPRS is located in the LLCprotocol. Further details of the PHY layer are provided in Section. 2.2.

2.2 Basic Functions of MAC and PHY Sub Layer

The MAC protocol [43] maintains the Temporary Block Flows (TBFs) for UL and DLpacket data transmission. This comprises the establishment, reestablishment and releaseof such flows, an appropriate scheduling strategy that incorporates fairness between dif-ferent users and the transmission of the Packet System Information (PSI) on PBCCH.This section puts the focus on a detailed understanding of the basic functions for TBFmaintenance and scheduling. Further information can be obtained from [65] and [66].

2.2.1 Initial UL TBF establishment

Sessions might be established mobile originated or NW originated. Starting with theeasier case, the mobile originated session establishment, all important MAC functionsnecessary to understand the later chapters can be explained. For the NW originated sessionestablishment, additional paging might occur which is not treated within this thesis. Forthe initial establishment of an UL TBF, an access procedure is initiated starting with arandom access on the Random Access Channel (RACH), Fig. 2.13. The random access

PaChaReq(Rand_Ref)

RACH

LLC PDU

PaChaReq(Rand_Ref)

PaUplAss (Rand_Ref)PaUplAss (Rand_Ref)

R > Access_Persist?

R > Access_Persist?

false

true

PCUMS 1 MS 2

Figure 2.13: Contention on RACH

is executed in close collaboration with the PHY layer. In order to consider the priorityof the TBF to be established in the random access, the access persistency is introduced.The MS draws a random number R [1..16]. In case R is larger than the Access Persistencybroadcasted on BCCH for the priority to be established, the access is allowed, otherwisethe access is denied and the MS will back off and retry the access. Access retries are


executed after a random number of TDMA frames drawn from a window of size [S, S +TxInt], Fig. 2.14. As the Packet Random Access Channel (PRACH) might collapse under

Figure 2.14: GPRS PRACH access repetition

heavy load, the PCU is given a means of control for the PRACH load by parameters Sand TxInt. It broadcasts the current values for S and TxInt frequently on the PBCCH foreach persistency class.

If the access is allowed, the MS will send a Packet Channel Request (PaChaReq)containing a random reference number Rand Ref. The PCU will answer this request witha Packet Uplink Assignment (PaUplAss) message containing the same value for Rand Refand the UL TDMA frame number where the PaChaReq was received. This allows a TBFestablishment without prior knowledge of the identity of the participant. In rare cases, itmight occur that a second mobile sends a PaChaReq in the same TDMA frame which getslost due to a fading hole. As the range for Rand Ref is limited to 5 - 8 bit, both mobilesmight have chosen the same random number. In this case, the PaUplAss is valid for bothand the contention has to be resolved in a contention phase.

There are several ways to establish an UL TBF. The type of establishment is providedin the PaChaReq message. Tab. 2.1 lists the available types.

type usageshort access

RLC mode ACKed and 8 radio blocks CS 1 / MCS 1to be transmitted

two phase accessRLC mode unACKed or RLC mode ACKed and> 8 radio blocks CS 1 / MCS 1 to be transmitted

cell update cell update procedureMM procedure other GMM procedures

Table 2.1: Access types

2.2.2 One Phase Access

In RLC acknowledged mode, especially for short data transfers a one phase access is usedaccording to Fig. 2.15. The assignment of resources is decided solely on the informationobtained in the PaChaReq message. E.g. multi slot allocation is not possible as the multislot class of the MS is not knows to the NW. On expiry of the access timer T3168 anabnormal release is triggered.


LLC PDU

PaData (TLLI)

PaData (TLLI)PaUplAckNack (TLLI)

LLC PDU

End of contention

PCUMS

PaData

PaUplAss (dynamic)

PaChaReq (one phase)

N3104 = 2

N3104 = 1ti3166

End of contentionresolution

ti3168

resolutionTBF Est.

ContRes

WaitAssign

ContRes

TBF Est.

Figure 2.15: UL TBF establishment, one phase access

For the one phase access, the contention resolution phase spans over the data transfer.As it is not clear that the right MS is addressed, the Temporary Logical Link Identifier(TLLI) must be included in every radio block. This reduces the payload by 4 byte. ThePCU will answer only to the TLLI it intended to address in the PaUplAss. For the PCU,the contention resolution ends on reception of the first Packet Data (PaData) with theexpected TLLI. For the MS it ends on reception of the first Packet Uplink Acknowledge/ Not Acknowledge (PaUplAckNack). Especially for large packets and window sizes, thefirst PaUplAckNack might be send very late, limiting the UL payload size for a long time.Therefore a counter has been introduced forcing an initial PaUplAckNack message afterthe Nth received PaData. Especially for high traffic load, N = 1 is a serious selection.

In a contention situation, the MS that was not addressed by the PCU will retry theaccess if timer T3166 expires or if counter N3104 reaches the broadcasted maximum valueN3104max.

2.2.3 Two Phase Access

If the TBF establishment shall be based on detailed information about the MS, a twophase access is used, Fig. 2.16. Two phase access is mandatory if a multi slot allocationis required or if the unacknowledged RLC mode shall be set. The additional overhead atTBF start is compensated by probably better TBF performance if the data fills more than8 radio blocks using CS 1. On reception of the PaChaReq, the PCU allocates a singleradio block on the UL determined by the TBF starting time. In that block, the MS sendsa Packet Resource Request (PaResReq) message containing additional information e. g.the multi slot class of the MS. Based on these information, the PCU will send a secondPaUplAss containing the final resources for the TBF in a dynamic allocation.

In case two phase access is used, the contention resolution takes place during theestablishment. A TLLI is included in the PaResReq, the PCU will include the addressedTLLI in the second PaUplAss. The contention resolution is supervised by timer T3168,no extra timer is required. In the two-phase access, no bandwidth for payload is wastedas the TLLI is not included in PaData radio blocks.


PaChaReq (two phase)LLC PDUMS PCU

PaUplAss (single block)WaitAssign

TBF time +TBF starting

PaResReq (TLLI)startingtime

transfertime

ti3168

PaUplAss (dynamic, TLLI)

PaData

TBF Est.

LLC PDU

PaUplAckNack

PaData

PaData

End of contentionresolution TBF Est.

ContRes End of contentionresolution

Figure 2.16: UL TBF establishment, two phase access

2.2.4 UL TBF Reestablishment

If an UL TBF is already established and a PDU with different QoS requirements arrivesfrom the higher layer, a change in TBF performance is required. In case changes forcrucial parameters like the RLC mode or TBF mode are demanded, the old TBF has tobe closed and a new TBF is established as described above. However, for changes forminor parameters such as radio priority or peak throughput, a reestablishment might besufficient, Fig. 2.17. During UL TBF reestablishment, only the second part of a two phase

LLC PDU

PaData

PaData

MS PCU

TBF Est.TBF Est.

WaitAssign

TBF Est.

LLC PDU

ti3168

PaUplAckNack

PaData

PaUplAss (dynamic)

PaResReq

Figure 2.17: UL TBF reestablishment

access is performed. The old TBF is continued during the reestablishment until the startingtime contained in the PaUplAss is reached. PDUs with differing radio priority mightalready be sent while the old TBF is still valid. The main advantage of the reestablishment


procedure is the avoidance of the random access and the contention phase thus providing aquick reaction to changed QoS requirements while saving channel capacity on the PRACH.

2.2.5 UL TBF (Re)Establishment during active DL TBF

For the same reason, avoidance of random access and contention phase, a different wayof UL TBF establishment is used if a DL TBF is already established, Fig. 2.18. On

RRBP

LLC PDU PaData(supp.poll)

ti3168

PaData

PaData

PaData

PaUplAss (dynamic)

PaUplAckNack

LLC PDU

PaDowAckNack(ChanReqDesc)

MS PCU

TBF Est.TBF Est.

Figure 2.18: UL TBF establishment, DL established

reception of a PDU from higher layer, the MS waits for the next poll for a Packet Down-link Acknowledge / Not Acknowledge (PaDowAckNack). It appends a Channel RequestDescription (CRQ) Information Element (IE) to the PaDowAckNack, specifying the re-quired QoS parameters. The PCU answers by sending a PaUplAss including the assignedresources. No contention phase occurs in this case. This establishment type, in conjunctionwith a delayed TBF release, is specially suited for interactive traffic such as client-serverarchitectures where UL and DL TBF activity alternates, Fig. 2.19.

RLCGRR_Data_req(ACK)GRR_Data_ind

GRR_Data_req(DATA)LLC LLC ACK

TCP

Figure 2.19: Response to GRR Data ind


2.2.6 Uplink Scheduling

Fig. 2.20 shows the four types of allocation the PCU has to execute for the UL TBF. The

R R R R R

FBIPollR R R R R

Poll PollPDA

SBFBI

SBR R R R RR

RadioBlocks

RadioBlocks

RadioBlocks

sendFBI

RRBP,default = 3 RB

R

R

PaUplAsssend next RB

starting timedefault = 0,

PDAPoll

next free RBdefault occupied:

PaDowAsssend

sendPaUplAss

SB SBD D D D D D D D DR R R R RR

RadioBlocks

F 10 R F R F 42 3 5 6 F R 0 F R 1 F

RadioBlocks

default occupied:longer starting time

2. RRBP

3. Single / Multiple Block Allocation

4. Dynamic Allocation

USF:

R = RandF = free, 7

P PPDAFBI

1, RACH, PRACH_BLKS = 3

Figure 2.20: MAC scheduling process for UL

order of the scheduling types gives the priority of the scheduling events. In the first step,the radio blocks for the PRACH are allocated. In the example, three radio blocks per52-multi frame are reserved for the PRACH.

Next, the supplementary polling is scheduled. It is used for the acknowledgement ofcontrol messages such as Packet Downlink Assignment (PaDowAss) or final PaUplAck-Nack. The default distance for the Relative Reserved Block Period (RRBP) is three radioblocks in the scheduler model, it is enhanced up to six blocks if the scheduler does not finda free radio block before. Please note that the scheduling process is continuous over time,so the mentioned priority is given for a certain radio block by the timing; no pre-schedulingis done further than six radio blocks.

In the third step the single block allocation (GPRS) or multiple block allocation (EG-PRS) is scheduled. It is used for polling the MS to send a PaResReq as an answer to anPaUplAss. As the default starting time for this allocation is set to 0 (next radio block),the priority is lower because the supplementary polling that starts further in the future (3radio blocks) is already scheduled.

In the fourth step, all radio blocks that are hitherto free are scheduled for dynamicallocation with an appropriate scheduling strategy. For the UL, this might be a non-exhaustive round-robin scheduler with depth one or the multistage round robin schedulerdescribed below.

For a non-exhaustive round-robin scheduler, the last line in Fig. 2.20 shows the assignedUplink State Flag (USF) values in case that the PDCH carries a PRACH, addressed withthe USF value 7 = Free and six TBFs are allocated with the USF values 0 - 6.


As the polling takes precedence over the the dynamic allocation, the USF value sched-uled for radio blocks containing an answer to a poll, are not available for dynamic alloca-tion. The PCU sets a random USF value denoted as Rand. Any value might be used, butthe following limitation occurs:

2.2.7 UL TBF start delay of scheduling

If a MS receives a PaUplAss, it will switch to the assigned PDCHs after the starting time.A change of the TRX might be necessary to reach the assigned PDCHs, so the MS didnot decode any information from that PDCH in advance, Fig. 2.21. From the moment

dynamic alloc.(starting time)PaUplAss

pollforeign

S/P bitforeign

tstarting time nopolling time

switch tonew PDCH(s)

RRBP

Figure 2.21: UL TBF start at MS

of switching, the MS will notice all foreign polls and will not send on the assigned radioblocks even if it receives the USF value assigned to the PDCH. However, foreign pollsthat were commanded on the new PDCH before the starting time cannot be recognisedby the MS. Collisions will occur if the PCU schedules the newly assigned USF value. Toavoid this, the PCU in the model introduces a delayed start of the scheduling. The USFvalue for the newly assigned TBF on a PDCH will not be polled until all old priority pollshave passed. The Rand value will therfore not contain an USF value of such a new TBF.

2.2.8 UL TBF Release

For the release of the UL TBF, a countdown procedure has been defined. It allows thePCU to optimise the scheduling by providing the number of outstanding blocks. As theprocedure cannot be stopped if once initiated, it is often considered as a hindrance inrespect to flexible scheduling. In this work the procedure has therefore been minimised bysetting BS CV MAX to 1. This means, for all but the final UL radio block, the transferredCountdown Value (CV) is 1, for the last block it is 0. The release procedure is shown inFig. 2.22. Upon sending a PaData with CV = 0, the MS sets timer T3182. If the PCUreceives a PaData with CV = 0, it will send a PaUplAckNack. In case all data blocks havebeen received, the Final Ack bit is set. On reception of a PaUplAckNack with the Final Ackbit set, the MS will schedule a Packet Control Acknowledge (PaCtrlAck) for the RRBPaddressed in the message. If this is sent, the MS closes the TBF, repeated PaUplAckNackswill be answered if heard.

Applications often introduce short delays that cause frequent, unnecessary TBF re-leases and establishments. A well-known example is the TCP delayed ACK (200ms)Fig. 2.19 or the interframe duration in packet streams such as in VoIP sessions [110]. TheTBF release for both TBFs might therefore be delayed for a short period of time. Exten-sive delay blocks resources at the PCU, especially the sparse address space for USFs andTemporary Flow Identitys (TFIs) gets quickly exhausted. The delay is therefore limitedby the standard to 5s, reasonable values found in simulations by the author are 300 msDL TBF end delay and 1s UL TBF end delay.


PaCtrlAck

PaUplAckNack(FinalAck)

MS PCU

PaData(CV = 0)

WaitReleaseti3182

N3103 = 0

Empty RRBP

TransferTime

PaCtrlAck

start

N3103 = 1

RRBP +TransferTime

ResourceRelease

PaUplAckNack(FinalAck)Resource

RRBP

RRBP

LLC PDU

Release start

RRBP +

Figure 2.22: UL TBF release

2.2.9 DL TBF Establishment

For a mobile-originated session, it is assumed that an UL TBF is established or is recentlyclosed if a DL TBF establishment is requested. Paging is therefore not required. Theestablishment procedure is shown in Fig. 2.23. On reception of a higher layer PDU, the

PaDowAckNack

PaDowAss

PaData

MS PCU

suspendedTBF

TBF est.

TBF est.

RRBP

LLC PDU

Figure 2.23: DL TBF establishment

PCU send a PaDowAss message. In case of reestablishment, the old TBF is suspended.The MS schedules a PaDowAckNack for the RRBP provided in the PaDowAss. This isnot mandatory according to the standard but it helps retransmitting lost assignments. Onreception of the PaDowAss, the PCU starts transmitting PaData.

2.2.10 DL TBF Release

Traffic is asymmetric in most applications with higher load on the DL TBF. A quickrelease of the DL resources is eligible in order to minimise blocking. This requires anacknowledged TBF end. The release procedure is shown in Fig. 2.24. The PCU transmitsthe last radio block with the Final Block Indicator (FBI) bit set. The MS checks forthe completeness of the received packets and schedules a PaDowAckNack in the included


ResourceRelease

MS PCU

ti3193

ti3191

PaDowAckNack(FinalAck)

PaData(FBI, S/P)LLC PDU

ti3192 RRBP

ResourceRelease

startstart

Figure 2.24: DL TBF release

RRBP. The FinalAck bit shows that this is the last PaDowAckNack message for theTBF. The release is protected with the timer T3191 on the PCU side. In order to allowretransmissions of the radio blocks necessary for DL TBF release, both sides keep theresources for a predefined time (T3192 on the MS side and T3193 on the PCU side). Thelength of the protection timers (0.01.5 s) is a tradeoff between delay caused by the timersand delay caused by a TBF release failure due to packet damages. In the latter case, theglobal watchdog T3180DL finished the connection after 5 s.

2.2.11 Multistage Round Robin Scheduling

For the UL, no examination of the RLC queue and send buffer can be done by the PCU viathe air interface. The scheduler is blind for the UL scheduling requirements. Therefore, ifpriority scheduling shall be supported a sophisticated strategy is required that must solvethe following problems:

A remarkable prioritisation shall take place according to the QoS requirements. A hard prioritisation serving always the highest priority first is not feasible. Eachmobile with an active TBF must be polled frequently in order to obtain measurementresults and to satisfy the radio link watch dogs T3180DL/UL The mobiles having no user data to transmit will spam the radio interface withPacket Uplink Dummy Control (PaUplDummyCtrl) radio blocks. This is a waste ofbandwidth and should be reduced.

In order to meet these requirements, the multistage round robin scheduling has beendeveloped, Fig. 2.25. It belongs to the class of weighted fair queueing schedulers withspecial extensions for GPRS. For each radio priority, one round robin scheduler is used. Allschedulers are polled during one multistage cycle in random order. Each time a scheduleris polled, it is allowed to send one radio block. The total number of polls of one schedulerin one multistage cycle XP1 sets the bandwidth portion assigned to the correspondingpriority class P. XP1 is larger for higher priorities. For the following investigations, thenumber of scheduled radio blocks per multistage cycle has been set to X = {X0, ..., X3} ={4, 3, 2, 1}. One possible resulting radio block stream is shown in Fig. 2.25. Each schedulerdistributes the assigned radio blocks to the registered mobiles according to the well-knownround robin strategy. Only those mobiles with an active UL TBF of the correspondingUL radio priority are registered in the scheduler. Each registered mobile is allowed to


t

Priority

0(high)

:,

(Iow)P 1

i

TLLI 1TLLI 4

X

XX

X

TLLI N

idle flag set / not set

P 1 P 1

00

rotationfast

TLLI 2

TLLI 3

TLLI N 2rotationslowTLLI N 1

TLLI N 3

Figure 2.25: Multistage round robin scheduling

send radio blocks according to the round robin depth z = 1. If Xi radio blocks have beenscheduled, or if all registered mobiles have been polled to send the maximum number ofblocks z or if no further mobile is allowed to send, the next lower priority will be served.A mobile is not allowed to send if

the UL TBF has no PDCH assigned on the scheduled time slot or the last poll was answered with a PaUplDummyCtrl message. This shows that themobile has no UL data to transmit. In this case, the idle flag is set when the messagearrives at the PCU (TLLI 2 and TLLI N - 1 in the figure). The scheduler skips theentry of the corresponding mobile in the next multistage cycle an resets the flag.

The latter behaviour reduces the bandwidth scheduled to idle mobiles by a factor oftwo. It might be further reduced by replacing the flag with a countdown mechanismallowing higher reduction factors.

The DL is scheduled in a similar way. A three-layer approach is applied. In the firstmultistage round, DL TBFs having only PEN ACK blocks in the send buffer are notallowed to send. Only if no sending instance can be determined in the first round, thesecond round is started allowing also PEN ACK radio blocks. If still no sender is found,a Packet Downlink Dummy Control (PaDowDummyCtrl) message is sent.

2.2.12 Modulation

Two modulation types are defined for an EDGE system, Fig. 2.26. The Gaussian MinimumShift Keying (GMSK), a binary modulation, was already introduced for GSM services. Anoctal modulation 8-Phase Shift Keying (8PSK) is added with EDGE [58] in order to allowhigh throughput in good receiving conditions [103]. Mapping of bits to symbols is doneaccording to Gray [63]. For erroneous swapping of neighbouring states in case of badchannel quality, the mapping guarantees to produce not more than one erroneous bit.


Figure 2.26: EGPRS modulation

CHAPTER 3

Basics of Radio Resource Control for Packet Data Services

Content3.1 Channel Allocation and Channel Assignment . . . . . . . . . . . . . . . . . . . . . . . . 263.2 Frequency Hopping and Fractional Loading . . . . . . . . . . . . . . . . . . . . . . . . . . 303.3 Link Optimisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

3.3.3 Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.3.4 Link Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483.3.5 Incremental Redundancy / Hybrid ARQ II and Soft Combining 51

3.4 Mobility Management and Cell Reselection . . . . . . . . . . . . . . . . . . . . . . . . . . 52

F or an interference-limited network, the appropriate Radio Resource Control (RRC)is the key to success. The most essential control methods are introduced within thischapter, starting with channel allocation and channel assignment that have to share theradio resource between dedicated and packet-oriented services, Section. 3.1. Frequencyhopping provides a spreading technique for equalising interference and signal fading. Themethod and its influence on packet-data services are introduced in Section. 3.2. Twomethods are commonly used to adapt the radio link to the distance between transmitterand receiver: Power control aims to keep the receive signal at an acceptable quality fora certain gross data rate, Section. 3.3.3. The second method adapts the gross data rateto the present channel quality. It is represented by Link Adaptation, Section. 3.3.4 andIncremental Redundancy Section. 3.3.5. Mobility is supported by maintaining track of theMS in a GMM context and by connecting the MS to the most suitable cell. In GPRSnetworks, this is achieved mobile-controlled by Cell Selections / Reselections Section. 3.4or network-controlled by cell change notification commands (not discussed here).

3.1 Channel Allocation and Channel Assignment

The GSM organises the physical resource in a TDMA frame comprising 8 Time Slots (TSs).The frequency division duplex is explained later in the frequency hopping section, for nowa logical TRX is referred to as the determining unit. A logical channel resource thereforeconsists of a TS, a logical TRX, a link direction (up or down) and the frequency parametersexplained below. As the frequency spectrum is limited, the available frequency channelshave to be reused in distant cells. An area where every frequency channel is used once andonly once is called a cluster. The number of cells the cluster comprises is called clustersize.

The first TRX carries control channels such as BCCH. Although not mandatory, theTRX are typically grouped into two layers. All BCCH TRX in a cluster form the BCCHlayer, typically using a large cluster size. Frequency hopping is not applied. All otherTRX form the hopping layer using a small cluster size and frequency hopping. Fig. 3.1shows typical channel allocation strategies for a cluster size n = 3.

Radio channels might be allocated permanently to one cell in a cluster (Fixed Chan-nel Allocation (FCA)) resulting in channel groups a, b and c. The strategy provides

3.1. Channel Allocation and Channel Assignment 27

0c

0a 0bA B

C

b1a1c1b c

a

c

baA B

C

A B

C

FCA DCA HCA

Figure 3.1: Channel allocation strategies

minimum co-channel interference but the behaviour for non-uniform traffic distribu-tion throughout a cluster (hot spots) is not optimum. Channels from the channel pool may be allocated to a cell on request (DynamicChannel Allocation (DCA)). Allocation flexibility is then maximised. Since withDCA there is no permanent allocation of channels to a cell anymore, the co-channelinterference might be high in adjacent cells. Hybrid Channel Allocation (HCA) is a combination of FCA and DCA where somechannels are allocated permanently and some are allocated on request , e. g. , to servea hot spot.

It is worth noting that frequency reuse is often given as a reuse pattern denoted (Num-ber of cells per cluster / Number of frequency groups per cluster), e. g. (3/3) for FCA andHCA in Fig. 3.1. Channel assignment can be done randomly or by preselection. For theGSM voice calls, two selection criteria are used in order to minimise the interference level:First, the best time slots with the lowest number of allocated logical channel resources aresearched (best scoring), Fig. 3.2.

XX

X

XX X

X

XX X

X

X X

XXX

2 3 5 70 1 4 6123TRX

TS

LayerHopping

optimum

Figure 3.2: Channel assignment example in a GSM cell

An X denotes a GSM TCH resource occupied by a CS service. Next, from the bestTSs, one is selected randomly. A sequential selection would result in higher interference onlow TS numbers as BTSs are assumed synchronised. If FH is applied, the selection of theTRX does not influence performance. Scoring might be performed considering idle channelinterference. However, own investigations have verified the statement of Mouly/Poutet[88]: There is no reasonable effect from interference-aware channel selection for a systemunder low load or full load but there is an increased Carrier to Interference Ratio (CIR)of 1.5 dB for medium load.

The GPRS is an add-on to a GSM network. Usually, GPRS gives precedence to GSMservices interrupting service to its own users when required. With the onDemand concept,Fig. 3.3, logical channels are being allocated dynamically to the GPRS by means of PDCHsthat are carried by GSM TCH resources.

Permanently allocated PDCHs, containing e. g. the PBCCH are denoted as P whileonDemand PDCH are denoted O. It is worth noting that typically - contrary to Fig. 3.3 -the BCCH layer is used to provide PDCHs since the cluster size for such channels is muchlarger than for the other channels, resulting in low interference. Investigations on optimumchannel assignment when hopping across multiple frequency channels using TRXs in the

28 3. Basics of Radio Resource Control for Packet Data Services

XX

X XX

X XX

X

XX

2 3 5 70 1 4 6123TRX

TS

P P O OO Handover

Figure 3.3: TCH handover in order to free onDemand PDCH

hopping layer will follow in Section. 6.3. All PDCHs assigned to a MS must use the sameTRX. So, it is beneficial to group all available PDCHs in line in the TDMA frame of agiven radio channel. In order to maintain a basic GPRS service, at least one fixed PDCHmust always be allocated whilest onDemand PDCHs can be allocated flexibly within theTDMA frame. If some other channel hinders consecutive PDCH TS assignment, an intracell Handover (HO) is performed as shown in Fig. 3.3.

A TCH resource allocated to the PCU as PDCH might be assigned to one or severalGPRS users. Several connections can be multiplexed to a given PDCH in any sequentialorder. This is called stochastic multiplexing.

3.1.1 Multi Slot Capabilities

Throughput of a service scales with the number of PDCHs assigned to a TBF. Timingrequirements, specified for the multi-slot class of the MS limit the degrees of freedom ofassignment for the PCU, Fig. 3.4. The maximum number of PDCHs assigned to a single

UL

DLTimeslot

07 1 2 3 4 5 6 7 0 1 2 3

07 1 2 3 4 5 6 7 0 1 2 3

TT TxRx

Meas.

Rx ...

TTA = Transceiver Turnaround

tb raTTA TTA

TRX

Used Timeslot

Figure 3.4: Limitations of multi-slot assignment

MS (UL, DL) is (8, 8). Two MS types are distinguished: type one is half-duplex andcannot receive and transmit at the same time (one TRX) while type two can do so using2 TRX and full duplex transmission. Type one is commonly used due to terminal costs.It requires a timing cycle with four phases per TDMA frame, carried by a TRX, Fig. 3.4.

Rx Time slots occupied for DL data transfer Ttb Time to get ready to transmit ( = Transceiver Turn-around (TTA)). Tx Time slots occupied for UL data transfer. Tra Time to get ready to receive. This comprises a TTA, Neighbour Cell (NC)measurements and tuning to the receive carrier.

For MS type one, not more than 6 PDCHs are assigned simultaneously, typically (1, 5),see [40]. Obviously, the PDCH used for a MS are not freely selectable. Further limitationsresult from the PCU serving multiple MSs of different multi slot classes each one demandingits own timing. In addition, the Rx and Tx time slots must not be spread for MS typeone but must be located in a window of size Rxmax resp. Txmax.

3.1. Channel Allocation and Channel Assignment 29

Multi slot operation has an influence on the RLC operation. The large window size inEGPRS mode has to be adapted according to the number of available PDCHs, Fig. 3.5.Dependent on the type of service, a certain variation in window size is allowed.

Figure 3.5: EGPRS window size over PDCH allocation [40]

3.1.1.1 UL Multi Frame Shifting

The multi slot limitations might be released by an approach known as multi-frame shifting,Fig. 3.6. It allows to assign to a TBF up to 7 PDCHs (1,6). The shift is reached by a

UL 07 1 2 3 4 5 6 7 0 1 2 3

7 0 1 2 3 4 5 6 7 0 1 2Shift

Tta (Meas. + Tx ready)

DL

Shifted Position

Original Position

Timeslot

Meas

Trb (Rx ready)

3

07 1 2 3 4 5 6 7 0 1 2 3 Used Timeslot

Figure 3.6: UL multi frame shifting

constant Timing Advance (TA) offset. It creates a gap sufficiently large for NC measure-ments and TTA time even for 7 PDCHs allocated. Shifting reduces the maximum allowedcell radius Rmax according to the required time shift tgap, the symbol duration tSym andthe TA offset TAoffs (number of symbols):

Rmax = 35 km TAoffs 544mTAoffs =

tgaptSym

; TAoffs,max = 64

30 3. Basics of Radio Resource Control for Packet Data Services

In practice, cell radius limitation from frame shifting is not a severe restriction sincemost cells are small, nowadays, and shifting can be applied per cell.

3.1.2 Support of onDemand PDCH Concept

PDCHs are allocated to the GPRS on demand if not required for GSM services. In case aGSM connection is requested, a PDCH might be withdrawn. Then, the PCU must informall MSs about this event. Two information procedures are available.

Acknowledged PDCH cancellation information is sent individually to each MSthat runs an active TBF in form of a PaUplAss, PaDowAss or Packet TimeslotReconfigure (PaTsReconfig) message on the PDCH to be cancelled. An acknowl-edgement for these messages must be received from each MS. Interference betweenGPRS and GSM is avoided and there is a certain point in time where the channelswitches to GSM operation. Immediate PDCH cancellation does not wait for acknowledgement. It is typicallyused if an urgent GSM handover must be accepted by the serving cell. Then, a PacketPDCH Release (PaPDCHRel) message is broadcast. As the message might be lost,some MSs might continue using this PDCH causing high interference with the GSMsystem for several seconds.

3.2 Frequency Hopping and Fractional Loading

3.2.1 Frequency Hopping

GSM might operate in different frequency bands between 400 and 2100 MHz. It is anFDD system using a duplex distance of 45 to 90 MHz, depending on the frequency band.FDMA splits each frequency band into equally spaced radio channels. The channel spacingis 200 kHz.

With FCA each cell is allocated a fixed amount of radio channels, the Cell Allocation(CA), Fig. 3.7. The CA is dependent on the cluster size, here n = 4. The channels are

FrequencyBand

ARFCN

12345

121122123124

BCCH

MA

CA

HS4

1

23

MAIO

MAI

(start)

Figure 3.7: Frequency channel arrangement for frequency hopping

addressed by frequency indices, the Absolute Radio Frequency Channel Numbers (AR-FCNs). The allocation must be planned taking frequency reuse by other cells into accountresulting in co-channel interference. The target CIR value at full load for the worst caseis [60]:

3.2. Frequency Hopping and Fractional Loading 31

CIR 16

(D RR

)=

(q 1)6

=(3n 1)6

R cell radiusD distance between co-channel BTS path loss exponent, typically 3..5q co-channel interference reduction factor q = D / R

Adjacent channel interference, resulting from transmit power on channels with adjacentARFCNs, further limits the CIR. From the CA, the mobile station is allocated a logicalTRX that comprises of a set of radio channels from the CA, the Mobile Allocation (MA).Further, the MS needs to know how and if to perform frequency hopping. The mobileis allocated a starting index into the MA, the Mobile Allocation Index Offset (MAIO).The MAIO points to the first radio channel to be used. This index is used to initialisethe Hopping Sequence (HS). The HS is a pseudo-random sequence of indices pointinginto the MA. The current index is the Mobile Allocation Index (MAI). In order to breakcorrelation between the cells, each cell uses a HS different from the co-channel cells. Thecell specific sequence is broadcast via BCCH as Hopping Sequence Number (HSN).

If the size of the MA is one, no frequency hopping is performed, else, the MAI changesonce per TDMA frame according to the HS. Fig. 3.8 shows an example HS. The MA

MS 1MS 2MS 11

23

4

HS

MS 1

MS 1

MS 1MS 1MS 2

MS 2

MS 2

MS 2

MS 2

FrameNumber

FrequencyChannel

0 1 432

MS 3

freeMS 4

free

InterfererHS

5

1

23

4

StepNumber

StepNumber

1

4

23

MAIOAssignment

Interferer

Figure 3.8: Hopping sequence

of size four is stridden with a random HS (left). MS 1 uses MAIO 0 and directly followsthe HS using frequency channel 4 during the first TDMA frame, as the initial MAI pointsto the first step in the sequence. The MS using MAIO n uses the MAI n = (MAI 0 +n)modMAsize. On the right hand side, the HS of an interfering cell is displayed. Forsimplicity, the interferer is assumed to perform cyclic hopping, so the first interfering MS3 using MAIO 0 starts transmission at the frequency channel 4 thus interfering with MS1. The second interfering M

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