The increasing demands for wireless multimedia and interactive internet services along with the rapid proliferation of a multitude of communications and computational gadgets are fuelling intensive research efforts on the design of novel wireless communication system architectures for high-speed, reliable, and cost-effective transmission solutions. Within the last decade, a notable development in the area of communication theory has been the introduction of MIMO (multiple-input multiple-output) communications which have led to practical schemes such as space-time coding and spatial multiplexing. Such systems provide significant improvement in link reliability and spectral efficiency through the use of multiple antennas at the transmitter and/or receiver side. Multiple-antenna techniques are very attractive for deployment in cellular applications at base stations and have already been included in the 3rd generation cellular standards. Variations of MIMO techniques are also now a part of many existing and emerging wireless standarts such as IEEE 802.11 (WiFi), IEEE 802.16 (WiMax), and IEEE 802.20 (MBWA).
Deployment of MIMO techniques, however, is not always feasible mainly due to size and power constraints such as in cellular mobile devices as well as in wireless sensor and ad-hoc networks which are gaining popularity in recent years. An innovative approach to harness the spatial diversity without deploying multiple antennas is cooperative diversity, also known as cooperative communications. The concept of cooperative communication stands out as a fundamental shift from the conventional network design based on point-to-point communications. Instead of a classical network with isolated communicating pairs, cooperative communication builds upon a network architecture in which nodes help each other in relaying information to realize spatial diversity advantages, thereby improving their own performance and that of the whole network. Cooperative communication techniques take advantage of the broadcast nature of wireless transmission, effectively creating a virtual antenna array through cooperating nodes. This new transmission paradigm promises significant performance gains in terms of link reliability, spectral efficiency, system capacity, and transmission range.
Cooperative communication can be applied to both infrastructure-based networks such as cellular systems, WLANs (wireless local area networks), WMANs (wireless metropolitan area networks), and infrastructure-less networks such as MANETs (mobile ad-hoc networks), VANETs (vehicular ad-hoc area networks), and WSNs (wireless sensor networks). With large existing target markets and indisputable advantages, cooperative communication is one of the best opportunities today for high impact research in the area of wireless communications. As such, this emerging research area has spurred tremendous excitement within the academia and industry circles and resulted in a surge of research papers over the last few years.
This book aims to provide readers a comprehensive understanding of the fundamental principles of cooperative communications with a particular emphasis on physical layer issues and further present the latest advances and open research problems in this rapidly-evolving field. It would serve as a valuable reference for graduate students, scientists, faculty members who are conducting or wishing to conduct research in this area as well as for engineers and research strategists in the relevant industries.
For the convenience of the reader, this book is organized in five intertwined thematic areas, namely as Information Theoretical Results on Cooperative Communications (Chapters 1-4), Practical Coding Schemes for Cooperative Communications (Chapters 5-6), Distributed Transmit and Receive Diversity Techniques for Cooperative Communications (Chapters 7-13), Broadband Cooperative Communications (Chapters 14-15), and Mathematical Tools for the Analysis and Design of Cooperative Networks (Chapters 16-18) followed by a chapter providing an industrial perspective on cooperative communications. The contributions of individual chapters are outlined in the following.
In Chapter 1, M. Yuksel and E. Erkip provide a comprehensive overview of the information theoretic foundations of cooperative communications. Adopting capacity, diversity, and diversity-multiplexing tradeoffs as performance metrics, they outline the performance limits of major cooperation protocols for Gaussian, fading, and multiple-access channels considering both full-duplex and half-duplex relay nodes. While Chapter 1 places a particular emphasis on decode-and-forward (DF) and compress-and-forward (CF) relaying, the focus of Chapter 2 by I. Krikidis and J. S. Thompson shifts to amplify-and-forward (AF) relaying which is particularly attractive with its low complexity. This chapter first summarizes information theoretic performance limits of AF relaying, then discusses relay selection, cross-layer coordination, and power allocation for AF cooperative networks. As briefly discussed in Chapter 2, optimum power control is a key technique to realize the full potentials of cooperative transmission. In Chapter 3, O. Kaya and S. Ulukus present an information theoretic approach to power control with a particular focus on a two-user fading multiple access channel. They derive optimal power control methods which maximize the long term average rates achievable by user cooperation, and the associated improved rate regions. Their results lead to the characterization of jointly optimum encoding, medium access and routing policies, yielding the importance of a cross-layer approach to wireless network design. In Chapter 4, S. Chatzinotas, M. A. Imran, and R. Hoshyar focus on a particular application of cooperative communications and investigate the information theoretical performance limits of a cellular system with multi-cell processing. Specifically, they study the ergodic per-cell sum-rate capacity of the MIMO cellular channel under the assumption of correlated Rayleigh fading and uniformly distributed user terminals over a planar cellular system. Their results demonstrate that sum-rate capacity is compromised by spatial correlation at the base station while it grows linearly with the number of receive antennas under independent fading sub-channels.
While information theoretic results lay the ultimate performance limits for cooperative communications, an important practical question is how to achieve performance close to capacity or the best known achievable rates. To address this question, J. M. Shea, T. F. Wong, C. W. Wong, and B. Choi provide a framework in Chapter 5 for multi-user communications including relay-assisted transmission and cooperative multiple-access as special cases. Under this integrated overview, they summarize the main design principles and tools for code construction and present a detailed survey of research progress on practical coding schemes. In Chapter 6, M. Yu, J. Li, and H. Wang consider a multi-hop relay scenario and investigate practical and efficient coding schemes for multicast data transmission based on the principles of networking coding.
In multi-relay cooperative networks, a crucial design aspect is to decide on the method how the relay nodes will participate in the cooperation. Although repetition-based cooperation schemes (in which only one relay is allowed to transmit the signals for each time slot) are able to provide the full diversity, they suffer from poor bandwidth efficiency. Cooperative beamforming, distributed space-time coding, and relay selection have been proposed as spectral-efficient methods to coordinate the transmissions from multiple relays which are, respectively, addressed in Chapters 7, 8, 9.
In Chapter 7, L. Dong, A. P. Petropulu, and H. V. Poor discuss the concept of cooperative beamforming in which a proper weight coefficient is assigned to each relay in order to make the signals from different relay nodes add up constructively at the destination. Particularly, they present two cross-layer cooperative beamforming techniques both of which involve the MAC (medium access control) layer during the information-sharing stage and the physical layer during the beamforming stage. These techniques however differ from each other on how the message signals are shared between source and cooperating nodes, and on how they are weighted and forwarded. The performance of the two techniques is compared in terms of spectral efficiency. Under the envisioned cross-layer design, a queuing analysis is further presented to provide insight into how packet delays and the stability regions of traffic rate are affected when source and relay nodes are subject to a queue.
Distributed space-time coding, which is the focus of Chapter 8, provides an open-loop alternative to cooperative beamforming avoiding the feedback requirement for weight coefficients. It is relatively easy to integrate conventional space-time codes (i.e., designed for co-located antennas) in a DF cooperative system. Particularly, orthogonal space-time block codes are attractive with their robustness in the presence of node failures. Since these codes have orthogonal structure, node failure corresponds to deletion of a column in the code matrix, but the other columns remain orthogonal. This lets the distributed scheme still exploit the residual diversity benefits from the remaining nodes. The application of space-time codes to AF cooperative systems is however much more challenging. In Chapter 8, Z. Yi and I.-M. Kim address the design of single-symbol decodable distributed orthogonal space-time block codes (DOSTBCs) which are particularly desirable from the viewpoint of low-complexity receiver design. A major problem in the construction of the DOSTBCs for AF relaying comes from the fact that the noise covariance matrix at the destination is not diagonal in general due to the presence of terms related to forwarded signals. To overcome this problem, Yi and Kim present row-monomial DOSTBCs which guarantee uncorrelated noise terms at the destination. They present an upper bound on the data-rate of the row-monomial DOSTBCs and show that these codes can achieve approximately twice higher bandwidth efficiency than the repetition-based cooperative schemes.
Unlike cooperative beamforming and distributed space-time coding which require the participation of all (active) relay nodes in relaying phase, relay selection schemes avoid multiple relay transmissions by having only a single relay to forward the information from the source. The “best” relay with favourable channel conditions is selected based on a performance metric such as end-to-end signal-to-noise ratio or mutual information. In Chapter 9, E. Beres and R. Adve outline relay selection methods for AF and DF relaying and further discuss various practical implementation aspects including feedback overhead and system requirements. Chapter 10 by Z. Zhou, J.-H. Cui, S. Zhou, and S. Cui continues the discussion on relay selection. Different from the conventional deterministic cooperative communication where relay node(s) are determined prior to the communication, this chapter investigates “random” cooperation where the number of cooperative nodes and the cooperation pattern changes based on the random nature of the channels among source, relay, and destination nodes. The authors demonstrate that such a random cooperative scheme is more robust to dynamic wireless network environments and further discuss the challenges that lie ahead in designing general random cooperative schemes.
In Chapter 11, D. S. Michalopoulos and G. K. Karagiannidis consider the multi-relay cooperative system design from the receiver design point of view. They treat inherent end-to-end paths between source and destination nodes as the multiple branches of a virtual diversity receiver and address conventional diversity combining techniques in the context of cooperative communications. Specifically, they investigate the performance of distributed implementation of MRC (Maximal Ratio Combining), SSC (Switch and Stay Combining), and SC (Selection Combining) methods along with discussions on practical implementation aspects.
A common assumption in the existing literature on cooperative communications is the availability of perfect channel state information at relay and destination nodes. Although knowledge on fading channel coefficients is required for coherent communication systems, this can be avoided through differential techniques as discussed in Chapter 12. In this chapter, for a two-user cooperative communication system, M. R. Bhatnagar and A. Hjørungnes present single-differential and double-differential coding, the latter of which also avoids the need of estimating carrier offsets besides the fading channel coefficients. Relay selection and power allocation are further discussed for potential performance improvements. In Chapter 13, J. Harshan, G. S. Rajan, and B. S. Rajan consider a multi-relay scenario and address the design, construction, and performance analysis of differential distributed space-time block codes. They demonstrate that differential codes from Clifford algebras are the best among the existing designs in terms of the encoding-decoding complexity and also in terms of error rate performance.
The existing literature on cooperative communication has heavily focused on the frequency-flat fading channel model. Such a model fails to provide an accurate modeling for broadband wireless channels which exhibit frequency-selectivity and result in intersymbol interference (ISI). An efficient approach to mitigate ISI is OFDM (orthogonal frequency division multiplexing); a multicarrier transmission system where the high-rate data stream is demultiplexed and transmitted over a number of frequency subcarriers. In Chapter 14, I. Y. Abualhaol and M. M. Matalgah consider a cooperative broadband MIMO-OFDM system with DF relaying and present adaptive bit and power loading algorithms to optimize the error rate performance. An alternative low-complexity approach to ISI mitigation is the single-carrier frequency-domain equalization (SC-FDE). Historically shadowed by OFDM, SC-FDE provides a powerful alternative air interface architecture with a similar implementation complexity and has started receiving significant attention recently. In Chapter 15, T.-W. Yune, D.-Y. Seol, D. Kim, and G.-H. Im investigate this promising technique in the context of cooperative communications. They focus on distributed space-frequency block coded (DSFBC) single carrier transmission which is particularly useful over rapidly time varying channels. They devise a pilot-assisted channel estimation technique for DSFBC SC-FDE under consideration and present spectrally efficient cooperation protocols based on the principles of iterative multiuser detection.
A common feature of Chapters 16-18 is how they smartly take advantage of some sophisticated mathematical tools for the analysis and design of cooperative networks. In Chapter 16, A. Sezgin and E. A. Jorswieck discuss how majorization theory can be applied to solve some typical design problems encountered in cooperative communications. After they provide an overview on majorization and order preserving functions (namely Schur-convex and Schur-concave functions), they use these mathematical tools to design power allocation schemes and determine optimal node distribution along with a capacity analysis. In Chapter 17, L. S. Pillutla and V. Krishnamurthy study the problem of data gathering in correlated wireless sensor networks with distributed source coding. Using monotone comparative statics borrowed from the economics literature, they analytically study the effect of data correlation on physical layer design variables such as constellation size and number of cooperating nodes. In Chapter 18, B. Sirkeci-Mergen, A. Scaglione, and M. Gastpar study cooperative broadcasting in large-area wireless networks with randomly distributed nodes. To analyze the network behaviour, they adopt the so-called continuum limit methodology which approximates random networks by their dense limits under sum relay power constraint. They derive several analytical results including coverage behaviour, power efficiency, error propagation and maximum communication rate. Although the focus of this chapter is mainly on cooperative broadcasting, the deployed mathematical tools can be useful in the analysis of other cooperative systems as well.
The last chapter of our book presents an industrial R&D perspective on cooperative communications. In this chapter, M. Dohler, D.-E. Meddour, S.-M. Senouci, and H. Moustafa first provide an overview of state-of-the-art developments from both academia and industry, then address possible deployment architectures for cooperative multihop cellular networks as a case study. They expose some motivations, including business models, to use such networks and present a comprehensive discussion on technical challenges for real-world deployment scenarios, such as routing, appropriate QoS metrics, authentication, and authorization to services’ access.
With over fifty contributors from the field, this comprehensive book would introduce students and practitioners to the diverse research on cooperative communications and expose the underlying fundamental issues in the analysis, design, and optimization of cooperative wireless communication systems. Considering the scarcity of books in this new research area, I hope it would fill a void in the current literature as a valuable reference guide.
Murat Uysal, Editor
Waterloo, Canada, 2009