Cooperative Relay And Cognitive Spectrum-Sharing Techniques In Wireless Communications

Wireless communication today is developing towards higher efficiency, faster transmission speed, and better system reliability. However, the uncertainty of the wireless environment as well as the scarity and non-renewable nature of the wireless spectrum, are still the two major challenges. Cooperative relay, the basis to break up the single-hop cellular concept and enrich it by multi-hop functionalities via cooperation between the so called partners, is able to provide spatial diversity and effectively deal with the uncertainty of the wireless channel. Cognitive spectrum-sharing, on the other hand, is exploited as a mean to use the wireless spectrum more efficiently, which ease the tense situation caused by the scarce and non-renewable nature of the wireless spectrum. Combining the two, more spectrum access opportunities and higher spectrum-sharing efficiency can be expected. This dissertation considers designing transmission strategies that exploit the benefits of the above two techniques as well as their combination.For cooperative relay systems, the partner selection problem in multiuser unicast networks is firstly studied, followed by an enhanced cooperative source diversity scheme dedicated for multicast/broadcast service (MBS) networks. For infrastructure network with centralized control, the partner selection problem is mathematically modeled as finding a perfect matching on a bipartite graph. Three selection strategies are proposed, providing the system with different operating points between system-level efficiency and user-level fairness. For ad hoc network, the design of partner selection strategies is more focused on energy balance of network and the way of distributed implementation. Accordingly, a power-aware partner selection strategy is proposed to maximize the network lifetime. On the other hand, applying cooperative relay to MBS network is challenged by the issues of heterogenous relay coverage and inefficient feedback. An enhanced cooperative source diversity scheme is proposed to solve the above two issues and is shown to increase the spectrum efficiency of the MBS transmission. For cognitive spectrum-sharing systems, firstly, an opportunistic power control strategy is proposed, which is proven to maximize the achievable rate of the cognitive user while not jeopardizing the outage probability of the primary user. The impact of channel estimation error is analyzed and the strategy is further modified to enhance robustness. Secondly, rate control based on opportunistic interference cancelation is considered and a joint opportunistic power and rate control strategy is proposed for multi-channel cognitive spectrum-sharing scenario, which maximized the overall achievable rate of the cognitive system.For systems that combine the techniques of cooperative relay and cognitive spectrum sharing, both inter-system cooperation and intra-system cooperation are considered. Firstly, a rateless code based cooperative spectrum access scheme is proposed for inter-system cooperation case, where the cognitive users with favorable locations opportunistically relays for the primary user in the aim of getting more chances to access idle spectrum. Secondly, two relay protocols, simple decode-and-forward (SDF) and buffered decode-and-forward (BDF), are proposed for the spectrum-sharing scenario with large-coverage cognitive system and randomly distributed small-coverage primary systems. With the combination of both pathloss gain and buffer gain, the BDF protocol is shown to effectively increase spectrum access opportunities and reduce the end-to-end delay of the cognitive relay system.