Studies on the Viability of Cellular Multihop Networks with Fixed Relays

Studies on the Viability of Cellular Multihop Networks with Fixed Relays BOGDAN TIMUS¸ Doctoral Dissertation in Telecommunications Stockholm, Sweden 2009 Studies on the Viability of Cellular Multihop Networks with Fixed Relays BOGDAN TIMUS¸ Doctoral Dissertation in Telecommunications Stockholm, Sweden 2009 TRITA–ICT–COS–0903 KTH Communication Systems ISSN 1653-6347 SE-100 44 Stockholm ISRN KTH/COS/R–09/03–SE SWEDEN Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan framlägges till offentlig granskning för avläggande av teknologie doktorsexam i telekommunikation fre- dag den 12 juni 2009 klockan 14:00 i rum C1, Electrum 1, Kungliga Tekniska Högskolan, Isafjordsgatan 26, Kista. © Bogdan Timus,¸ May 2009 Tryck: Kista Snabbtryck Abstract The use of low cost fixed wireless relays has been proposed as a way to deploy high data-rate networks at an affordable cost. During the last decade, significant academic and industrial research has been dedicated to relays. Pro- tocol architectures for cellular-relaying networks are currently considered for standardization as part of both IEEE 802.16 and 3GPP. Various relaying techniques have successfully been commercialized over the years. This dissertation concentrates on the particular case of large scale use of low cost relays, for which focus is put on signal processing and radio resource allocation, rather than on antenna and radio frequency (RF) design, or on network planning. A key question is how low relay cost is low enough for a relaying architecture to be viable from an economic point of view? We develop a framework for evaluating the viability of relaying solutions. The framework is based on a comparison between the relaying architectures and traditional single-hop cellular architectures. This comparative analysis is done from an operator perspective, and is formulated as a network-dimensioning problem. The associated investment decisions are based on financial measures (cost or profit) and taken under technical constraints (throughput, coverage, etc.). First, we consider a large number of traditional dimensioning scenarios, in which the radio network is design for a predefined traffic demand and target quality of service level. We show that the use of low cost relays can indeed be viable, but that the cost savings vary strongly from case to case and often are only modest. Due to the half-duplex nature of the low cost relays, these relays are best suited for providing coverage to guaranteed data-rates, at low end-to- end spectral efficiency, and in environments with strong shadow fading. The type of environment and the placement of relays are more important than the specific protocols and algorithms used in the network. Therefore, traditional network planning remains an essential and challenging task, which is unlikely to be replaced by large-scale (unplanned) use of relays. Second, we suggest a new direction of research in which the viability of relays is judged considering the entire life cycle of a radio network. We give several examples in which the temporary use of relays is economically viable, especially if the service uptake is slow or the uncertainty about the future demand is high. This is particularly relevant if the last-mile cost of a network is dominated by the backhaul transmission cost, and if relaying is implemented as a feature of an access point, rather than as a new device type. Contents Preface vii List of Figures ix List of Tables xi List of Abbreviations xiii 1 Introduction: pros and cons of relaying 1 1.1 What is a “relay”? . 2 1.2 Motivation . 8 1.3 The scope of the dissertation . 13 1.4 The methodological approach – the dissertation outline . 15 1.5 Contributions . 18 I Theory, Models, and Algorithmic Tools 23 2 Relaying Techniques 25 2.1 Related literature on relays . 25 2.2 Half-duplex: The fundamental problem of low cost relays . 29 2.3 Connection Oriented Relaying Schemes . 31 2.4 Resource Allocation in Mesh Networks . 39 3 Basic economic concepts for networks with fixed relays 51 3.1 The iso-quant curve . 52 3.2 Valuating investments over many years . 54 3.3 Cost structure examples . 58 3.4 Valuating investments under uncertainty . 63 3.5 Summary . 67 iv CONTENTS v II Relay Viability Analysis 69 4 Cost Analysis of a Network Snapshot 71 4.1 Specific question and approach . 72 4.2 Basic trends . 75 4.3 Sensitivity to the choice of relaying techniques . 79 4.4 Sensitivity to the service type . 88 4.5 Summary . 91 5 How to deploy relays? 95 5.1 Sensitivity to propagation models . 96 5.2 Sensitivity to relay antenna elevation . 101 5.3 Sensitivity to planning the relay positions . 102 5.4 Summary . 105 6 Long term dynamics of network deployment 107 6.1 Cost and capacity granularity . 108 6.2 Related work . 109 6.3 Incremental network deployment with relays . 110 6.4 Incremental deployment with budget constraint . 118 6.5 Relay deployment under demand uncertainty . 128 6.6 Summary . 141 7 Discussion 143 7.1 Limitations and further studies . 144 7.2 Potential applications . 148 7.3 Pros and cons of relaying network . 151 8 Conclusions 153 8.1 Lessons learned . 154 8.2 Further work . 155 III Appendices 157 A A historic outlook at the use of relays 159 B Capacity models for amplify-and-forward relays 165 C Greedy link scheduling algorithms 171 D Properties of the cross layer optimization algorithm 175 E How to obtained the iso-service curves? 181 vi CONTENTS F Propagation models 183 G Topology models for incremental deployment 191 List of publications 195 Bibliography 197 Preface I started the work on this subject in 2003, within the Low Cost Infrastructure (LCI) project, which was part of the Affordable Wireless Services & Infrastructure (AWSI) program. The starting point in the project was the observation that, in order for future networks to provide at an affordable price services whose data-rates are order of magnitudes larger than those provided by 2-3G networks, the cost of delivering a bit must be orders of magnitude lower than in the old networks. Based on the reference literature available at that time, the low cost relays were a promising solution for achieving an affordable wireless infrastructure for high data rates. I had high hopes to easily show high economic gains, but the initial results were rather frustrating. While the interest for relays within the research community was ever increas- ing, I had hard times in answering questions from the “dis-believers”: “Why should a relay be so much cheaper than a base station if they implement so complex algorithms and trans- mit with high power? Why are operators not using them if they are so good in reducing costs?” I got doubts that I was searching in the right place. This dissertation summarizes the experience I have gathered meanwhile. Although the economic benefits are not as high as I first expected, relays and relaying techniques are useful and can be used to reduce the infrastructure cost, as they have in fact done so far. Perhaps the most important discovery is that the gains do not come from where I first expected. For instance, the use of advanced transmission techniques together with a “smart dust” deployment of relays can not replace the traditional work of planning and designing a radio network, because the main advantage of using relays is that they avoid shadowing objects. Moreover, the key to understanding the benefits of relays might be the understanding of the changes of technical requirements and the challenges a radio network experiences from the initial roll-out phase until the network reaches maturity and the market for which it was designed saturates. Acknowledgments I am grateful to my adviser Prof. Jens Zander for supporting me in this search, and for giving me the opportunity to grow in the scientific and working environment at the Radio Communications Department. The kind of questions and approach he encourages are sel- dom leading to the easy way out from the doctoral studies. But at the end of a meaningful road, this makes the completition of a dissertation into a great reward. vii viii PREFACE This work could not have been completed without my life partner Irina. With her patience, understanding, and love she has provided me with a vital support from the first day of my studies to the defense day. I am thankful to my parents for their support during these years, and to my sister for a refreshing academic competition. The discussion with my colleagues Klas Johansson and Johan Hultell gave me insights into many economic and business aspects, and helped me focus my work. A lot of important material in this dissertation is the result of the prolific and friendly collaboration with Pablo Soldati. Many insights about relaying techniques and network I obtained during the fruitfull discussions I had with Prof. S. Ben Slimane, Prof. Dongwoo Kim, and with my experienced colleagues Peter Larsson, Per Skillermark, Per Moberg, and Niklas Johansson from Ericsson. An important moment in my studies was the short visit I made at RWTH in Aachen and the discussions I had there, particularly with Daniel Schultz, Ralf Pabst, Rainer Schoenen, and Ralf Jennen; I am grateful to Prof. Dr.-Ing. Bernhard Walke and the people at ComNets for making this possible. I would also like to acknowledge the help I received from the project managers Mats Nilson and Jan Markendahl, and the input received from Ericsson Research, Telia Sonera, Avitec, Power Wave, and Mic Nordic; they have greatly contributed to refining the scenarios and the scope of this dissertation. Several others have contributed to my work with ideas and comments. I would especially like to thank Prof.

Load more