Improving primary throughput by cognitive relay beamforming with best-relay selection

In this paper, we propose a cognitive relay beamforming scheme to improve the throughput of primary user (PU), in which if a primary signal is not correctly received by the intended destination, a secondly user (SU) will be selected to retransmit it invisibly. Three issues are investigated in our work, i.e. beam weight design at SU, best-relay selection, and performance analyses. Firstly, by optimizing the beam weight, SU can keep transparent to PU, and signals retransmitted from primary transmitter (PT) and SU can constructively combine at the primary receiver (PR). Secondly, the SU leading to the largest primary throughput is selected as the best cognitive relay. Lastly, we discuss the performance improvements in the primary outage performance and throughput. Theoretical analyses and numerical results demonstrate the efficiency of the proposed scheme.