Optimal Distributed Relay Selection in Underlay Cognitive Radio Networks: An Energy-Efficient Design Approach

Most previous work in CR relay networks concentrates on maximizing physical layer QoS (e. g., capacity, spectral efficiency and achievable data rate) as relay selection criteria. However, the energy state of relay nodes is largely ignored, which has significant effects on the network lifetime. This paper proposes a distributed relay selection scheme for cooperative transmissions over underlay CR networks, while considering adaptive modulation and coding (AMC) strategy and energy state of relay nodes. The objective is to increase spectral efficiency as well as prolong the average network lifetime (i.e., achieve a good tradeoff between them). With the stochastic optimization formulation, the optimal relay selection policy has indexability property that dramatically reduces the computation and implementation complexity. Simulation results are presented to show the effectiveness of the proposed scheme.