Cooperative Diversity in Wireless Networks: Relay Selection and Medium Access

Helmut Leopold Adam Matr.-Nr. 9830200 Cooperative Diversity in Wireless Networks: Relay Selection and Medium Access DISSERTATION zur Erlangung des akademischen Grades Doktor der technischen Wissenschaften Alpen-Adria-Universität Klagenfurt Fakultät fur¨ Technische Wissenschaften Begutachter: Univ.-Prof. Dr.-Ing. Christian Bettstetter Institut fur¨ Vernetzte und Eingebettete Systeme Alpen-Adria-Universität Klagenfurt Univ.-Prof. Dr. Holger Karl Institut fur¨ Informatik Universität Paderborn 08/2011 Declaration of honor I hereby confirm on my honor that I personally prepared the present academic work and carried out myself the activities directly involved with it. I also confirm that I have used no resources other than those declared. All formulations and concepts adopted literally or in their essential content from printed, un- printed or Internet sources have been cited according to the rules for academic work and identified by means of footnotes or other precise indications of source. The support provided during the work, including significant assistance from my supervisor has been indicated in full. The academic work has not been submitted to any other examination authority. The work is submitted in printed and electronic form. I confirm that the content of the digital version is completely identical to that of the printed version. I am aware that a false declaration will have legal consequences. (Signature) (Place, date) Cooperative Diversity in Wireless Networks: Relay Selection and Medium Access Wireless communication systems suffer from a phenomenon called small scale fading which causes rapid and unpredictable fluctuations in the received signal strengths. Traditional methods to mitigate small scale fading effects impose re- strictions on the data exchange process and/or the used hardware which limit their applicability. Recently, information theory has proven that cooperative diversity is an ef- fective means to combat the effects of small scale fading. In such an approach common neighbors of a communication pair can act as relays for packets which would otherwise be lost due to bad channel conditions. These studies, however, have mainly focused on physical layer aspects such as link capacity, coding, and relay positioning. In such studies, source, destination, and relaying nodes are known a priori. Setting up cooperation in a network has found less attention. In this thesis, we illustrate that obtaining theoretical gains of cooperative diversity is not straightforward and we address issues regarding networking and protocol aspects of cooperation. We first elaborate on the relay selection process. We propose and evaluate relay selection methods which increase the efficiency of cooperative diversity in terms of energy, time, and success. We show how to incorporate channel and routing information into the selection process to reduce the energy consumption and time overhead of cooperation. We illustrate how the relay selection depends on the knowledge of relay candidate cardinality. Such knowledge is also of interest in numerous other fields like medium access, routing, and information dissemi- nation to name a few. To this end, we propose and evaluate methods based on probabilistic trials which aim to estimate the number of neighbors with a de- sired accuracy and minimum time overhead. As a final step, we integrate our proposed mechanisms into the design of a novel medium access protocol which facilitates cooperative diversity by handling the cooperative packet flow, selec- ting a relay node, and making the necessary resource reservations. We evaluate the performance of the resulting system in terms of throughput and delay for different network parameters such as node density, channel coherence time, and data packet size. Our findings indicate that by using the proposed simple yet efficient mechanisms that are not restricted by hardware, we can deploy cooperative diversity with substantial gains in the large-scale wireless networks of the future consisting of low-end devices. i Acknowledgments First of all I would like to thank my advisor Prof. Dr.-Ing. Christian Bettstetter who gave me the opportunity to write this thesis in a professional, international and friendly environment. His guidance, support and patience made it possible for me to achieve publications at well known conferences and to finally write this thesis. I also want to thank Prof. Dr. Holger Karl for reading and commenting on my work. Next, I thank all my colleagues at the Mobile Systems Group for their overall support, their advice and our fruitful and entertaining discussions. Their doors were always open for all my questions and concerns. In particular, I would like to mention the original members of the mobile systems group Udo Schilcher and Sérgio Crisóstomo,and our senior researchers Wilfried Elmenreich and Ev¸sen Yanmaz. Special thanks go to Sidi Mohammed Senouci who was the project leader in the cooperative diversity project with Orange Labs France. The outcome of this project forms the basis of this thesis. I appreciated our project meetings which brought up new research directions and ideas. Writing a PhD thesis feels sometimes like a Sisyphus task. In that sense, I am grateful for the patience and the support of my family and my friends. Last but definitely not least, I would like to express my deep gratitude to my girlfriend. When I was depressed I had her shoulder to lean on and she did not get tired of cheering me up, when I was occupied with research problems she gave me the time to figure out a solution without complaining, when I needed her professional opinion she was happy to provide it, and when I needed someone to proof read my thesis she had to suffer with my German-English writing style. My discussions with her and her hints heavily contributed to the quality of my thesis. Moreover, having her in my life makes me complete. iii Contents 1 Introduction1 1.1 Fading in Wireless Communication Channels..............1 1.2 Cooperative Diversity...........................6 1.2.1 Basic Idea..............................6 1.2.2 Challenges of Cooperative Diversity...............7 1.3 General Modeling Assumptions......................9 1.4 Outline and Contribution......................... 11 2 Cooperative Diversity: An Overview 15 2.1 Building Blocks............................... 15 2.2 Physical Layer Aspects........................... 16 2.2.1 Characteristics of Nodes...................... 16 2.2.2 On the Benefits of Cooperative Diversity............ 18 2.2.3 Coding at the Relay........................ 19 2.2.4 Optimum Position of the Relay.................. 22 2.2.5 Multiple Relaying Nodes...................... 22 2.2.6 Power Allocation.......................... 23 2.2.7 Modulation............................. 23 2.3 Networking Aspects............................ 24 2.3.1 Relay Selection........................... 24 2.3.2 Cooperative Diversity and Medium Access............ 28 2.3.3 Cooperative Diversity and Routing................ 30 2.3.4 Why should nodes cooperate?................... 31 2.4 Emerging Standards............................ 32 2.4.1 WiMAX - IEEE 802.16j...................... 32 2.4.2 IMT-Advanced........................... 33 2.5 Term Clarification............................. 34 v 3 Relay Selection 37 3.1 Introduction and Motivation........................ 37 3.2 Energy-Efficiency.............................. 40 3.2.1 Basic Relay Selection....................... 40 3.2.2 Relay Selection with Early Retreat................ 45 3.2.3 Relay Selection on Demand.................... 49 3.2.4 Relay Selection on Demand with Early Retreat......... 51 3.3 Time-Efficiency............................... 52 3.3.1 Multi-Hop-Aware Cooperative Relaying: Basic Idea...... 52 3.3.2 Multi-Hop-Aware Cooperative Relaying: Protocol Architecture 53 3.3.3 Cooperation Process at each Node................ 56 3.3.4 Evaluation of multi-hop-aware cooperative relaying....... 58 3.4 Success-Efficiency.............................. 64 3.4.1 Strategy 1.............................. 65 3.4.2 Strategy 2.............................. 65 3.4.3 Comparing Strategy 1 and Strategy 2.............. 66 3.4.4 Determining the Contention Window Size............ 68 3.5 Summary.................................. 69 4 Estimation of Neighbor Cardinality 71 4.1 Introduction and Motivation........................ 71 4.2 Basic Neighbor Estimator Block..................... 73 4.2.1 Design Options........................... 74 4.2.2 Neighbor Estimation Counting Empty Slots........... 75 4.2.3 Accuracy of Neighbor Estimation................. 76 4.3 Non-Adaptive Neighbor Estimator.................... 77 4.3.1 Determining a Suitable Slot Access Probability......... 77 4.3.2 Determining Suitable Subranges of Operation.......... 80 4.3.3 Combining Estimations from Subranges............. 81 4.3.4 Early Stop by Feedback from Query Node............ 82 4.3.5 Performance............................ 83 4.4 Adaptive Neighbor Estimators...................... 85 4.4.1 Single-Feedback Neighbor Estimator............... 86 4.4.2 Multi-Feedback Neighbor Estimator............... 88 4.5 Technology Aspects............................ 92 4.6 Comparison of the Estimators....................... 95 4.7 Related Work................................ 98 vi 4.8 Summary.................................. 99 5 Cooperative Medium Access 101 5.1 Introduction and Motivation........................ 101 5.2 Protocol Description............................ 104 5.2.1 Overview of Standard CSMA/CA................ 105 5.2.2 Direct Transmission

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