Relay selection and power allocation in amplify-and-forward cognitive radio systems

In this paper, we consider the problem of joint relay selection and power allocation at the source and relay the nodes in a cognitive radio (CR) system in which nodes are allowed to cooperate with each other. In particular, we consider an amplify-and-forward (AF) relaying secondary system that operates in the presence of a primary system. Joint relay selection and power allocation are aimed to achieve the maximum possible throughput for the secondary system subject to power constraints at the secondary source and relay nodes and by considering interference constraints from the secondary to the primary system. Using constrained optimization techniques, we derive an optimal joint power allocation and relay selection scheme for the secondary system. The proposed optimal scheme requires exhaustive search over all the candidate relay nodes and to reduce its complexity, a non-exhaustive sub-optimal search method is also proposed. By simulation, we show that both optimal and sub-optimal schemes provide a significant throughput gain when compared to random relay selection with optimal power allocation (OPA) and equal power allocations (EPA).