Cooperative communications is a new communication paradigm in which different terminals in the wireless network share their antennas and resources for distributed transmission and processing. Recent studies have shown that cooperative communications can yield significant performance improvement due to spatial diversity gains. The theory of cooperative communications is however still immature to fully understand its broader impacts on the design of future wireless networks. This thesis contributes to the advancement of cooperative communications by developing and analyzing cooperation protocols at different network levels, with the goal to provide significant improvements in signal reliability, coverage area, network throughput, and energy efficiency with respect to other existing alternatives.

We first propose a family of cooperative protocols for multi-node cooperative communications. We demonstrate that full diversity gains is achieved, which yields a significant improvement in the error performance. Based on the derived symbol-error-rate expressions, we characterize the optimal power allocation strategy among the relays and the source to further improve the performance of the system.

We develop distributed relay assignment protocols, and analyze their outage performance. We derive lower bounds on any relay-assignment scheme to benchmark the performance of our proposed schemes. We study the impact of our proposed protocols on increasing the coverage area of cellular networks without increasing the transmit power or adding extra base-stations.

We demonstrate that the gains promised by cooperation can be leveraged to the multiple-access layer. We propose the deployment of cognitive relays to utilize the periods of silence of the terminals to enable cooperation. This alleviates the spectral inefficiency problems inherent in conventional cooperation protocols. Our results reveal significant improvements in the maximum stable throughput region and delay performance of the network.

Finally, an analytical framework for studying the energy efficiency of cooperation in wireless networks is presented. This framework considers the overhead in the processing and receiving powers introduced by cooperation. The results characterize the regions where cooperation is more energy efficient than direct transmission. The results also provide guidelines for the design of power allocation strategies, relay-assignment algorithms and the selection of the optimal number of relays to help the source.