Abstract
In recent years, the rapidly growing need for mobile multimedia data services has prompted an upsurge of research interest in cooperative wireless communications in both academia and industry. It is well known that cooperative relaying can yield significant performance improvements due to its ability to create spatial diversity. However, the theory of cooperative communications still remains immature to gain more insights into the impact on the design of future wireless networks. In this dissertation, we present some developments in cooperative relaying and networking. The emphasis is initially on geographic relaying, followed by its application to conventional routing for reliability enhancement as well as cooperative routing for radio coverage extension. Several important issues including relay selection, cooperative routing, relaying area choices, power allocation, and cross-layer design are investigated. We first propose an efficient relay selection scheme based on the use of geographic information. The best relay is chosen as the one that has the best position, providing the most reliable source-relay-destination link. We demonstrate that the proposed relay selection scheme can efficiently improve the system reliability without frequent channel-state-information feedback and is applicable to fast-varying channels. We then present a cross-layer design framework for cooperative networks, where the proposed relay selection scheme is integrated properly with geographic routing to improve the network performance. Specifically, the geographic routing scheme performs greedy forwarding with a cross-layer design between the MAC (Medium Access Control) and Network layers, whereas the relay selection scheme is a MAC-Physical cross-layer design aiming to minimize the symbol error rate (SER). Both schemes use the underlying location information to form selection criteria, with a contention-based selection protocol realized at the MAC layer. It is shown that the proposed cross-layer protocol provides a practical and attractive solution to multi-layer integration for cooperative networks. On the basis of this framework, we also conduct a quantitative study on relaying-area choices, i.e., addressing an extended problem regarding which nodes should be candidate relays to participate in the relay selection process, for further improvement on the network performance. Different from the above-mentioned conventional geographic routing scheme with relay enhancement, a novel routing paradigm is developed by examining the physical-layer cooperation in a different perspective. It is validated that the diversity gain promised by cooperation can be leveraged to radio coverage extension at the link layer. With a series of mathematical formulations and derivations, we quantitatively identify the direct and cooperative radio coverage regions based on the average SER performance requirement, elucidating how the cooperative diversity gain can be translated into radio coverage extension. We then propose a cooperative geographic routing protocol with cross-layer design, namely the Relay-Aware Cooperative Routing (RACR) protocol, that exploits the merit of radio coverage extension not existing in non-cooperative geographic routing. To further enhance the routing performance, we also address the power allocation issue of the RACR scheme. Moreover, we give an analysis of radio coverage extension for the RACR scheme using the optimal power allocation, where its superiority over that using the equal power allocation is demonstrated.