Distributed Cooperative Communications in Wireless Networks
relay; information theory; coding; capacity; user cooperation; cooperative communication; LDPC; cut-set theorem; wireless communication
The primary challenge in communication over wireless networks, unlike wireline networks, is the existence of interference and channel variations (fading). Having more users at higher data rates means that current point-to-point networks will not scale. To engineer a scalable network, we introduce a new paradigm that exploits different network characteristics. We show that cooperation between users in the net- work, network coding, not only reduces existing (destructive) interferences from other users but it can also generate constructive interference, transforming the destructive interference into useful information. In this thesis, we explore the problem of source and channel coding over wireless networks, ranging from information theoretical analysis to code design and practical implementation issues. We show that significant gains in throughput can be achieved through network coding. Despite the importance of the problem and the work done on wireless networks, little is known about network coding and the effective use of the relaying function and cooperative strategy at the intermediate nodes. A notable example is the lack of an optimal coding scheme over the relay channel, the simplest form of a network, which has remained an outstanding open question for the last three decades. We propose new approaches to network coding that improve upon the best known coding schemes by many decibels. Specifically, we develop two main coding tech- niques, one for the multi-state relay channel and the other for the multiple access with generalized feedback (MAC-GF). We show that by using the new coding tech- niques, higher transmission rates than those previously known are achievable. The first technique achieves the ultimate transmission rate (capacity) for both half-duplex and the original relay channel under certain conditions. These improved capacity re- sults for the relay channel are the only known results since Cover's in 1979 and El Gamal's in 1982. The second coding technique improves the best known achievable transmission rate for the MAC-GF by Willems in 1983. This latter result also im- proves the achievable transmission rate for the Gaussian relay channel over all other known schemes for some channel conditions. We also present a practical code design technique for the relay channel. The design gains more than 4dB over direct transmission and closes the gap to the relay channel Shannon limit to less than 1dB with a code length of only 2 x 10^4 bits. The new coding techniques and transmission strategies developed in this thesis provide important steps toward overcoming the challenges of wireless network coding.