Andreas Könsgen Design and Simulation of Spectrum Management Methods for Wireless Local Area Networks
VIEWEG+TEUBNER RESEARCH Advanced Studies Mobile Research Center Bremen Herausgeber Editors: Prof. Dr. Otthein Herzog Prof. Dr. Carmelita Görg Prof. Dr.-Ing. Bernd Scholz-Reiter Das Mobile Research Center Bremen (MRC) erforscht, entwickelt und erprobt in enger Zusammenarbeit mit der Wirtschaft mobile Informatik-, Informations- und Kommunikationstechnologien. Als Forschungs- und Transfer institut des Landes Bremen vernetzt und koordiniert das MRC hochschul übergreifend eine Vielzahl von Arbeitsgruppen, die sich mit der Entwicklung und Anwendung mobiler Lösungen beschäftigen. Die Reihe Advanced Studies präsentiert ausgewählte hervorragende Arbeits - ergebnisse aus der Forschungstätigkeit der Mitglieder des MRC. In close collaboration with the industry, the Mobile Research Center Bremen (MRC) investigates, develops and tests mobile computing, information and communication technologies. This research association from the state of Bremen links together and coordinates a multiplicity of research teams from different universities and institutions, which are concerned with the development and application of mobile solutions. The series Advanced Studies presents a selection of outstanding results of MRC s research projects.
Andreas Könsgen Design and Simulation of Spectrum Management Methods for Wireless Local Area Networks VIEWEG+TEUBNER RESEARCH
Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available in the Internet at http://dnb.d-nb.de. Dissertation Universität Bremen, 2009 Gedruckt mit freundlicher Unterstützung des MRC Mobile Research Center der Universität Bremen Printed with friendly support of MRC Mobile Research Center, Universität Bremen 1st Edition 2010 All rights reserved Vieweg+Teubner Verlag Springer Fachmedien Wiesbaden GmbH 2010 Editorial Office: Ute Wrasmann Anita Wilke Vieweg+Teubner Verlag is a brand of Springer Fachmedien. Springer Fachmedien is part of Springer Science+Business Media. www.viewegteubner.de No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photo copying, recording, or otherwise, without the prior written permission of the copyright holder. Registered and/or industrial names, trade names, trade descriptions etc. cited in this publication are part of the law for trade-mark protection and may not be used free in any form or by any means even if this is not specifically marked. Cover design: KünkelLopka Medienentwicklung, Heidelberg Printing company: STRAUSS GMBH, Mörlenbach Printed on acid-free paper Printed in Germany ISBN 978-3-8348-1244-5
Für meine Eltern For my parents For Ira
Preface Wireless communication has become an integral part of daily life which results in an increasing number of wireless LAN devices deployed both in business, educational and residential environments and thus in increasing mutual interference between these devices. In addition, the requirements for the transmission platforms are increasing: voice-over-ip or video telephony rely on quality-of-service guarantees which have to be maintained even in case of concurrent access of multiple users. This work discusses possible solutions for the above-mentioned problems based on research work which I did in two projects funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG): In CoCoNet, automated spectrum management between neighbouring networks to reduce mutual interference was investigated while in XLayer, the focus was the resource allocation of a wireless LAN base station which serves a number of mobile terminals with data flows. I thank Prof. Carmelita Görg for giving the opportunity to perform this research work under her supervision, and to Prof. Bernhard Walke, Stefan Mangold and Guido Hiertz, Aachen University of Technology, for the cooperation in the Co- CoNet research project and for providing the WARP2 simulator which was used as a working basis in this analysis. Further thanks to Prof. Karl-Dirk Kammeyer, University of Bremen, who co-supervised the XLayer project and was also a reviewer of this work. Furthermore, I thank Ronald Böhnke and Andreas Timm-Giel for the cooperation in the XLayer project. I also thank the development community of Linux and various software tools, in particular the TEX/L A TEX typesetting system, which were used to run the simulations and edit the manuscript. Finally, I wish to say thank you to my wife Irangani for her endless patience and mental support during the years that I worked on this research project. Andreas Könsgen
Abstract Wireless local area networks (WLANs) according to the IEEE 802.11 standards have rapidly emerged in recent years. Increasing demands for quality of service require an efficient usage of spectrum resources. Optimising the working principles of WLANs is possible in different ways. The interference between neighbouring networks can be reduced by methods defined in the 802.11h standard: Dynamic Frequency Selection (DFS) can change the frequency channel during an ongoing connection, whereas Transmit Power Control (TPC) reduces the transmission power to a minimum which is required to transmit with a given data rate. For the data traffic inside a network, further optimisations are possible by centralised allocation of airtime by the access point so that each user is served at an optimum time; this is defined in 802.11e. Improvements can also be achieved by enhancing the radio transmission by multiple-antenna systems (MIMO), which is considered in 802.11n. However, the standards only describe the signalling, for example to initiate a measurement or the assignment of airtime to a certain station. The decision methods to access radio channel resources such as allocating transmit power or airtime are not treated in the standard. These decision methods for the spectrum management are the topic of this work. The theoretical basics for the design of spectrum management methods are discussed. Based on this, different spectrum assignment methods for decentralised and centralised networks are developed. In case of centralised channel access, a cross-layer approach between the media access layer and the physical layer is introduced which allows the assignment of channel resources both considering the quality-of-service requirements of the applications and the conditions of the MIMO radio channel. For the investigation of networks compliant to the IEEE 802.11 standard, a simulator called WARP2 is used which is extended by the spectrum management functions mentioned above. The effects of the different spectrum management algorithms on the interference and the quality-of-service parameters of the throughput and latency are evaluated. It is shown that by spectrum management these parameters can be significantly improved; the best results are achieved if the different spectrum management methods are combined. An increase of the throughput also reduces the latency due to smaller queueing delays and less channel congestion. In case of centralised access
X Abstract control, it is possible to match QoS requirements of time-critical data flows according to the user requirements. Non-time-critical flows are served in a best-effort manner; the QoS for these flows is significantly enhanced by providing parallelised transmission based on OFDMA or SDMA in combination with cross-layer communication between the MAC and the PHY layer.
Contents Preface Abstract Contents List of Tables List of Figures List of Abbreviations List of Symbols VII IX XI XV XVII XXI XXIII 1 Introduction 1 2 The IEEE 802.11 Standard Series 5 2.1 The ISO/OSI Reference Model... 8 2.2 IEEE 802.11 Architecture... 12 2.3 IEEE 802.11 Protocol Stack... 14 2.3.1 IEEE 802.11 PHY Layer... 15 2.3.1.1 OFDM Transmission... 17 2.3.1.2 Structure of the 802.11a PHY PDU... 21 2.3.1.3 Clear Channel Assessment... 22 2.3.2 IEEE 802.11 MAC Layer... 22 2.3.2.1 Contention Window Size Control... 28 2.3.2.2 Interframe Spacing... 29 2.3.2.3 Timing sequence of a 802.11 DCF packet transmission... 30 2.3.3 RTS/CTS Extension... 32 2.3.4 Power Management... 34 2.3.5 Format of the MAC Frame... 34 2.4 IEEE 802.11h Spectrum Management Extensions... 35 2.4.1 Dynamic Frequency Selection... 36
XII Contents 2.4.2 Transmit Power Control... 39 2.4.3 Negotiation of Spectrum Management Capabilities... 41 2.5 Centralised Channel Access... 42 2.6 Enhancing Transmission Speed by MIMO... 45 2.7 Summary... 48 3 Theoretical Aspects of Wireless LAN Performance 49 3.1 Related Work... 49 3.1.1 Frequency Channel Selection... 49 3.1.2 Power Control... 53 3.1.3 Link Adaptation... 57 3.1.4 Cross-Layer Design... 58 3.2 Performance of IEEE 802.11 Wireless LANs... 60 3.2.1 Throughput... 63 3.2.1.1 Network with Two Stations... 63 3.2.1.2 General Case: Network with n Stations... 65 3.2.2 Delay in Case of Ideal Channel... 73 3.3 Performance in Case of Limited Transmission Ranges... 74 3.3.1 Concurrent Transmissions... 75 3.3.2 Effect of the CCA Threshold... 80 3.3.3 Stations Outside Each Other s CCA Range... 82 3.3.4 Hidden Stations... 84 3.4 Effects of Spectrum Management on the Capacity... 86 3.4.1 Power Control... 86 3.4.2 Dynamic Frequency Selection... 87 3.4.2.1 Effects on the Performance... 87 3.4.2.2 DFS Convergence Characteristics... 89 3.5 Assigning Airtime in Centralised Operation... 96 3.5.1 Sequential Transmission... 96 3.5.2 Parallelised Transmission... 101 3.6 Summary... 102 4 Spectrum Management Algorithms 103 4.1 Dynamic Frequency Selection... 103 4.1.1 Acquisition of Measurements... 104 4.1.1.1 Measuring Sequence... 104 4.1.1.2 Measurement Values... 105 4.1.2 Principle of Controllers... 106 4.1.3 Decision Algorithms... 107
Contents XIII 4.1.3.1 Least Interfered... 109 4.1.3.2 Fuzzy Logic... 109 4.1.3.3 Genetic Algorithm... 113 4.1.3.4 Introduction of Randomness... 117 4.1.4 Simulated Annealing... 118 4.1.5 Error Recovery... 119 4.2 Transmit Power Control... 120 4.2.1 Infrastructure Networks... 121 4.2.2 Ad-Hoc Networks... 125 4.2.2.1 Initialisation State... 126 4.2.2.2 Testing State... 127 4.2.2.3 Controlling State... 127 4.2.2.4 Monitoring State... 128 4.3 Signalling Issues for Power Control... 129 4.4 Link Adaptation... 131 4.5 Summary... 132 5 Cross-Layer Architecture 133 5.1 Introduction... 133 5.2 Two-Stage Cross-Layer Scheduler... 134 5.2.1 QoS Aware Scheduler... 139 5.2.2 MIMO-TDMA... 143 5.3 Scheduling for Parallelised Transmissions... 146 5.3.1 MIMO-OFDMA... 147 5.3.2 MIMO-SDMA... 149 5.4 Summary... 150 6 Simulation Environment 153 6.1 Discrete Event Simulators... 153 6.2 The WARP2 Simulator... 154 6.2.1 Specification and Description Language... 154 6.2.2 Structure of the Simulator... 155 6.3 Simulator Extension by Spectrum Management... 158 6.4 Simulator Extension by Cross-Layer Scheduling... 158 6.5 Summary... 160 7 Simulation Results 163 7.1 Spectrum Management Results... 163 7.1.1 Network Performance without Spectrum Management.. 163
XIV Contents 7.1.2 Dynamic Frequency Selection (DFS)... 165 7.1.2.1 Static Scenario... 165 7.1.2.2 Convergence Characteristics... 168 7.1.2.3 Mobility Scenario... 172 7.1.3 Transmit Power Control (TPC)... 175 7.1.3.1 Validation of the Theoretical Model... 175 7.1.3.2 CCA Effect... 177 7.1.3.3 TPC Convergence: Infrastructure... 180 7.1.3.4 TPC Convergence: Ad-Hoc... 183 7.1.3.5 TPC with Variable Load and Number of Stations 183 7.1.4 DFS and TPC... 189 7.1.4.1 Regular Scenario... 189 7.1.4.2 Irregular Scenario... 192 7.1.5 Integration of DFS, TPC and Link Adaptation (LA)... 194 7.2 Performance of the Cross-Layer Architecture... 197 7.2.1 Cross-Layer Scheduler... 198 7.2.2 Quality-of-Service Scheduling on the MAC Layer... 206 7.2.3 Parallel Transmission of Data... 210 7.3 Summary... 214 8 Conclusions and Outlook 217 8.1 Conclusions... 217 8.2 Outlook... 219 Bibliography 223
List of Tables 2.1 IEEE 802.11 task groups... 7 2.2 Modulation schemes used for the OFDM subcarriers... 19 2.3 PHY modes used for IEEE 802.11a/g... 20 2.4 Minimum receive power levels for the different PHY modes... 40 3.1 Mapping between detailed and simplified diagram for 3 networks. 92 3.2 Mean transmission time and number of services for n data flows. 100 4.1 Possible combinations when assigning 3 networks to 2 channels. 108 4.2 Distribution of 3 networks to 2 particular channels... 108 4.3 Distribution of three networks to two channels independent of channel number... 108 4.4 Number of monitoring state cycles for different PHY modes... 129 5.1 Priority assignment for modified Round Robin scheduling... 138 5.2 Parameters for the longest queue/longest lifetime scheduler... 139 7.1 Average number of channel switches for different DFS algorithms in scenario given in fig. 7.3... 168 7.2 Average number μ of frequency changes for different algorithms and numbers of networks... 169 7.3 Number of packets transmitted in 4 station scenario in fig. 7.9, W =15... 177 7.4 Number of packets transmitted in 4 station scenario in fig. 7.9, W = 15... 1023...... 177 7.5 Number of packets transmitted in 6 station scenario in fig. 7.12, contention window size W = 15 and attenuation factor γ... 179 7.6 Number of packets transmitted in 6 station scenario in fig. 7.12, contention window size W =15... 1023, attenuation factor γ... 179 7.7 Per-route throughput in Mbit/s for the 6 station scenario in fig. 7.12, W = 15... 1023... 180 7.8 Traffic models and loads used for investigations in subsection 7.2.1 199
List of Figures 2.1 OSI reference model... 8 2.2 Interface between the protocol layers... 9 2.3 Encapsulation of a packet by protocol layers... 9 2.4 Infrastructure (BSS) and ad-hoc network (IBSS)... 13 2.5 Infrastructure networks forming an ESS... 13 2.6 IEEE 802.11 protocol stack... 15 2.7 Frequency spectrum S(f) of an OFDM signal with 5 subcarriers. 18 2.8 Hidden station problem... 23 2.9 Illustration of problem with finite signal propagation speed... 24 2.10 Unslotted and slotted channel access... 26 2.11 IEEE 802.11 backoff timings... 28 2.12 Timing for IEEE 802.11... 30 2.13 Structure of the 802.11 IFS/backoff period... 31 2.14 Suspending the backoff when the wireless medium becomes busy 32 2.15 Timing Chart for RTS/CTS... 33 2.16 Generic MAC packet... 34 2.17 General structure of a spectrum management frame... 36 2.18 Measurement Request/Report action field format... 38 2.19 Signalling inside a DFS enabled network... 39 2.20 Structure of a TPC request field... 40 2.21 Structure of a TPC report field... 41 2.22 Structure of a beacon frame body... 43 2.23 M N MIMO transmission system... 46 3.1 BER as a function of the C/I for different modulation schemes [96] 61 3.2 Hidden station problem... 63 3.3 Markov chain to model the 802.11a backoff states [8]... 66 3.4 τ and p for W min = 16 and m =7... 70 3.5 Simple scenario with simultaneous transmission... 76 3.6 Scenario to demonstrate the variable CCA threshold effect... 81 3.7 Example arrangement of stations to demonstrate minimum CCA threshold effect... 83 3.8 Diagram with channel allocations for a 2-network scenario... 90
XVIII List of Figures 3.9 Markov chain for change of the allocated channels... 94 3.10 Comparison between calculated and simulated results... 96 4.1 General structure of a controller... 106 4.2 Example for a membership function... 110 4.3 Fuzzy Associative Memory (FAM) matrix... 112 4.4 Principle of the genetic algorithm... 116 4.5 Infrastructure TPC example scenario... 124 4.6 Results for TPC example scenario in fig. 4.5... 125 5.1 Design of the cross-layer scheduler... 135 5.2 Integration of the cross-layer enhancements into the IEEE 802.11 protocol stack... 136 5.3 Priority calculation in the QoS aware scheduler... 140 5.4 Adaptation of the maxddel value... 143 5.5 Channel capacities for two data flows as a function of the time.. 145 5.6 Waterfilling method for single-carrier MIMO system... 145 5.7 Design of the parallelised cross-layer scheduler... 146 5.8 Subcarrier assignment for TDMA and OFDMA... 148 5.9 Pseudo waterfilling method for MIMO-OFDMA system... 149 6.1 Structure of the WARP2 simulator... 156 6.2 Structure of the WARP2 Simulator for Spectrum Management Simulation... 159 6.3 Structure of the WARP2 simulator for cross-layer simulation... 161 7.1 Static 3 3 matrix scenario for throughput measurement without spectrum management... 164 7.2 Total throughput vs. no of stations with γ as parameter for scenario infig.7.1... 164 7.3 Static 3 3 matrix scenario for DFS measurements... 166 7.4 Throughput, delay and C/I for the 3 3 matrix scenario in figure 7.3167 7.5 Convergence characteristics for the DFS Genetic Algorithm in case of 2, 3, 4 and 5 networks/channels... 171 7.6 Convergence characteristics for the DFS Least Interfered algorithm in case of 2 and 4 networks... 171 7.7 DFS mobility scenario... 172 7.8 Results of mobility measurements... 174 7.9 Measurement of QoS parameters with variable attenuation factor. 175
List of Figures XIX 7.10 Results for 4 station scenario in fig. 7.9, W =15... 176 7.11 Results for 4 station scenario in fig. 7.9, W = 15... 1023... 176 7.12 6 station scenario with full coverage... 178 7.13 Results for scenario according to fig. 7.12, W =15... 178 7.14 Scenario for infrastructure TPC convergence investigations... 181 7.15 Results for 6 station static infrastructure TPC scenario in figure 7.14181 7.16 Scenario for infrastructure TPC convergence investigations... 182 7.17 Results for 6 station mobility infrastructure TPC scenario according to fig. 7.16... 182 7.18 Scenario for ad-hoc TPC convergence investigations... 183 7.19 Results for 6 station static ad-hoc scenario in fig. 7.18... 184 7.20 Results for 6 station mobility ad-hoc scenario in fig. 7.18... 185 7.21 Test scenario for power control with variable load/stations... 186 7.22 Per-flow throughput for varying load in scenario of fig. 7.21, averaged over all data flows... 187 7.23 Delay in scenario in fig. 7.21 for varying load, averaged over all flows... 187 7.24 Power for scenario in fig. 7.21 for varying load, averaged over all flows... 187 7.25 Per-flow throughput for varying number of stations in scenario of fig. 7.21, averaged over all data flows... 188 7.26 Delay for varying number of stations in scenario of fig. 7.21, averaged over all data flows... 188 7.27 TX power for varying number of stations in scenario of fig. 7.21 averaged over all data flows... 188 7.28 Combination of DFS and TPC: Regular scenario... 190 7.29 Throughput for regular scenario in fig. 7.28... 190 7.30 Total delay for regular scenario in fig. 7.28... 191 7.31 Transmission delay for regular scenario in fig. 7.28... 191 7.32 Combination of DFS and TPC: irregular scenario... 192 7.33 Throughput for irregular scenario in fig. 7.32... 193 7.34 Delay for irregular scenario in fig. 7.32... 193 7.35 Transmission delay for irregular scenario in fig. 7.32... 194 7.36 Test scenario with a matrix of networks arranged in 3 rows and 3 columns for different combinations of DFS/TPC/LA... 195 7.37 Throughput for the scenario in fig. 7.36, infrastructure TPC... 195 7.38 Throughput for the scenario in fig. 7.36, ad-hoc TPC... 196 7.39 Scenario with 7 stations... 197
XX List of Figures 7.40 Per-flow throughput with the combination of DFS, infrastructure/adhoctpcandla... 197 7.41 Arrangement of stations for investigations of the cross-layer scheduler... 198 7.42 Throughput for each flow using the Round Robin scheduler... 200 7.43 Throughput for each flow using the queue length/lifetime scheduler 201 7.44 Mean delay for each flow using the Round Robin scheduler... 202 7.45 Mean delay for each flow using the queue length/lifetime scheduler 203 7.46 Delay in case of disabled PHY scheduling... 204 7.47 Delay in case of processing one entry from the list.... 204 7.48 Delay in case of processing the entire list... 205 7.49 Delay in case of processing entries for 2.5 ms... 205 7.50 Throughput with packet ageing... 207 7.51 Delay with packet ageing... 208 7.52 Throughput without packet ageing... 209 7.53 Delay without packet ageing... 210 7.54 Per-flow throughput assuming 1, 2 or 3 independent transmission channels... 211 7.55 Per-flow throughput for TDMA, OFDMA and SDMA, combined with Round Robin and QoS scheduler... 212 7.56 Total throughput for TDMA/OFDMA/SDMA using RR and QoS scheduling... 213 7.57 Per-flow delay for TDMA, OFDMA and SDMA, combined with RR and QoS scheduler... 214
List of Abbreviations ACK AP ARQ BER BSS CCA Acknowledge Access Point Automatic Repeat request Bit Error Rate Basic Service Set Clear Channel Assessment CCDF Complementary Cumulative Distribution Function CFP Contention Free Period CNCL Communication Networks Class Library CPU Central Processing Unit CRC Cyclic Redundancy Check CSMA/CA Carrier Sense Multiple Access with Collision Avoidance CTS Clear To Send DARPA Defense Advance Researach Project Agency db Decibels dbm Decibels related to 1 mw DBPSK Differential Binary Phase Shift Keying DCF Distributed Coordination Function DFS Dynamic Frequency Selection DIFS Distributed Coordination Function Interframe Spacing DQPSK Differential Quaternary Phase Shift Keying DS DSSS EDCA ESS FCS FEC FDM GHz GSM HCCA HCF HLA IBSS IEEE IFS IP ISO khz LAN LLC ms μs Mbit/s MAC Distribution System Direct Sequence Spread Spectrum Enhanced Distributed Channel Access Extended Service Set Frame Check Sequence Forward Error Correction Frequency Division Multiplex Gigahertz Global System for Mobile Communication HCF Controlled Channel Access Hybrid Coordination Function High Level Architecture Independent Basic Service Set Institute of Electrical and Electronics Enigneers Interframe Spacing Internet Protocl International Organization for Standardization Kilohertz Local Area Network Logical Link Control Milliseconds Microseconds Megabit per Second Media Access Control
XXII List of Abbreviations MHz Megahertz RA Receiver Address MIMO MLME OFDM OFDMA OSI PCF PDF PDU PHY PIFS PLCP PLME PMD PPDU QAM Multiple Input Multiple Output MAC Layer Management Entity Orthogonal Frequency Division Multiplex Orthogonal Frequency Division Multiple Access Open Systems Interconnection Point Coordination Function Probability Distribution Function Protocol Data Unit Physical Point Coordination Function Interframe Spacing Physical Layer Convergence Protocol Physical Layer Management Entity Physical Media Dependent Physical layer Protocol Data Unit Quadrature Amplitude Modulation RF RTI RTS RR RX SA SAP SDMA SDU SIFS SME STA TA TCP TDMA TPC TX WLAN WPAN Radio Frequency Real Time Infrastructure Request To Send Round Robin Receiver Sender Address Service Access Point Space Division Multiple Access Service Data Unit Short Interframe Spacing Station Management Entity Station Transmitter Address Transport Control Protocol Time Division Multiple Access Transmit Power Control Transmitter Wireless Local Area Network Wireless Personal Area Network QoS Quality of Service WM Wireless Media
List of Symbols Symbol Area See Page Meaning α SpecMan 115 weighting factor for genetic algorithm β MAC 74 used in calculation of WLAN capacity γ PHY 62 attenuation factor Δf PHY 17 frequency spacing between OFDM subcarriers θ PHY 144 waterfilling level λ PHY 144 eigenvalue of channel matrix μ SpecMan 92 mean value ψ PHY 147 weighting factor for OFDMA/SDMA algorithm σ MAC 64 length of a backoff time slot σ SpecMan 169 standard deviation τ MAC 65 probability that a station transmits within a particular time slot ρ SpecMan 115 vigilance for DFS algorithm A recv PHY 62 surface of the sphere across which the power of a station is distributed avgtxlen(i) XLayer 141 average transmission length for flow i a(i) SpecMan 112 fuzzy AND input value b(i, k) MAC 67 stationary distribution for the backoff counter b(j) SpecMan 112 fuzzy AND input value C PHY 144 transmission capacity C i XLayer 141 weighting factor for flow i C/I PHY 105 carrier to interference c(i, j) SpecMan 112 fuzzy AND output value D MAC 64 minimum contention window size d SpecMan 114 Euclidean distance E[D] MAC 71 expected value of user payload H PHY 47 channel matrix
XXIV List of Symbols Symbol Area See Page Meaning H T PHY 47 transposed channel matrix I PHY 47 identity matrix i MAC 65 backoff stage counter K PHY 147 number of users in MIMO system k MAC 66 backoff counter k SpecMan 118 Boltzmann constant k PHY 147 user index in MIMO system L PHY 147 number of OFDM subcarriers l PHY 144 subcarrier index in OFDM transmission M PHY 147 number of transmit antennas m PHY 147 transmit antenna index m(x) SpecMan 111 fuzzy membership function maxdel(i) XLayer 141 maximum delay for user i maxddel(i) XLayer 141 maximum delay for user related to transmission timei N PHY 147 number of receive antennas n MAC 71 number of stations n MAC 64 slot number within a backoff period n MAC 72 number of items available for choice PktAge XLayer 141 packet age P R PHY 62 received power P T PHY 62 transmit power p 1256 MAC 81 successful transmission probability in a 6-station scenario p 34 MAC 82 successful transmission prob. in a 6- station scenario p{i, k l, m} MAC 67 transition probability from backoff slot i and backoff stage k to backoff slot l and backoff stage m p s MAC 70 conditional probability for a successful transmission p c MAC 77 collision probability p SpecMan 115 probability for prototype selection Q PHY 150 covariance matrix r PHY 61 carrier-sense range r PHY 62 minimum of M and N
List of Symbols XXV Symbol Area See Page Meaning S SpecMan 64 fuzzy associative memory matrix T B MAC 73 average time spent in backoff T s MAC 71 time which elapses for a successful 802.11 transmission T c MAC 71 time which elapses for a collision T early XLayer 141 timer for cross-layer scheduler T late XLayer 141 timer for cross-layer scheduler t ACK MAC 64 time to send an ACK packet t backoff MAC 64 backoff time t DATA MAC 64 time to send user data t DIFS MAC 64 DIFS time t SIFS MAC 64 SIFS time W MAC 64 contention window size W PHY 144 radio channel bandwidth W i MAC 67 contention window size in backoff stage i W min MAC 64 minimum contention window size