Cognitive Multiple Access for Cooperative Communications and Networking

Title of Dissertation: COGNITIVE MULTIPLE ACCESS FOR COOPERATIVE COMMUNICATIONS AND NETWORKING Amr A. El-Sherif, Doctor of Philosophy, 2009 Dissertation directed by: Professor K. J. Ray Liu Department of Electrical and Computer Engineering In cooperative communications different network nodes share their antennas and resources to form a virtual antenna array and improve their performance through spatial diversity. This thesis contributes to the advancement of cooperative communications by developing and analyzing new multiple access cooperation protocols that leverage the benefits of cooperation to upper network layers. For speech communications networks, we propose a cooperative multiple access protocol that exploits inherent characteristics of speech signals, namely, long periods of silence, to enable cooperation without incurring bandwidth efficiency losses. Using analytical and simulation results we show that the proposed protocol achieves significant increase in network throughput, reduction in delay, and improved perceptual speech quality. In TDMA networks, we investigate the problem of sharing idle time slots between a group of cooperative cognitive relays helping primary users, and a group of cognitive secondary users. Analytical results reveal that, despite the apparent competition between relays and secondary users, and even in case of mutual interference between the two groups, both primary and secondary users will significantly benefit in terms of maximum stable throughput from the presence of relays. For random access networks, we find a solution to the problem of achieving cooperation gains without suffering from increased collision probability due to relay transmissions. A novel cooperation protocol is developed and analyzed for that purpose. Analytical and simulation results reveal significant improvements in terms of throughput and delay performance of the network. Moreover, collision probability is decreased. Finally, in the framework of a cognitive radio network, we study the negative effects of spectrum sensing errors on the performance of both primary and secondary networks. To alleviate those negative effects, we propose a novel joint design of the spectrum sensing and channel access mechanisms. Results show significant performance improvement in the maximum stable throughput region of both networks. COGNITIVE MULTIPLE ACCESS FOR COOPERATIVE COMMUNICATIONS AND NETWORKING