Distributed cooperative communications in wireless networks
Khojastepour, Mohammad Ali
Doctor of Philosophy thesis
The primary challenge in 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, a new paradigm is needed to exploit different network characteristics. We show that cooperation between users in the network can effectively exchange possible interferences from other users in favor of 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 in this area, little is known about network coding and the optimal relaying function at the intermediate nodes. A notable example is the communication over the relay channel, the simplest form of a network, that has been an outstanding question in the last three decades. We propose new approaches to network coding that improve upon the best known coding schemes by many dB. Specifically, we develop two main coding techniques, one for the multi-state relay channel and the other for the multiple access channel with generalized feedback (MAC-GF). These coding techniques enable us to achieve higher rates than those previously known for the relay and MAC-GF channels. The first technique provides some capacity results for both half-duplex and the original relay channel. The new capacity results for the relay channel are the only known results besides Cover's in 1979 and El Gamal's in 1982. The second coding technique improves the best known achievable rate for the MAC-GF by Willems in 1983. This results also provides a new achievable rate for the Gaussian relay channel which improves 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 then 4dB over direct transmission and close the gap to the relay channel Shannon limit to less than 1dB with a code length of only 2 x 104 bits.
Electronics; Electrical engineering