Relay Selection for Bi-Directional Amplify-and-Forward Wireless Networks

In this paper, we propose a relay selection amplify and-forward (RS-AF) protocol in general bi-directional relay networks with two sources and N relays. In the proposed scheme, the two sources first transmit to all the relays simultaneously, and then a single relay will be selected to broadcast the received signals back to both sources to achieve a minimum sum symbol error rate (SER). To facilitate the selection process, we propose a sub-optimal Max-Min criterion, where a single relay which minimizes the maximum SER of two source nodes will be selected. Simulation results show that the proposed Max-Min selection approaches can obtain comparable performance as the optimal selection with a reduced-complexity. We also present simple asymptotic SER expressions and make comparison with the conventional all-participate amplify-and-forward (AP-AF) relaying scheme. The analytical results are verified through simulations. To improve the system performance, optimum power allocation (OPA) between the sources and the relay is determined based on the analytical results. Simulations indicate that the pro posed RS-AF scheme with OPA yields considerable performance improvement over an equal power allocation (EPA) scheme.