Weighted Bidirectional Relay Selection for Outdated Channel State Information

Most researches on relay selection (RS) in bidirectional relay network typically assume perfect channel state information (CSI). However, outdated CSI, which is caused by the the time-variation of channel, cannot be ignored in the practical system, and the performance of the conventional bidirectional RS scheme degrades greatly with outdated CSI. In this paper, to improve the performance of RS with outdated CSI, we propose a weighted bidirectional RS scheme, in which a deterministic weight factor decided by the correlation coefficient of outdated CSI, is introduced in the selection process. The outage probability bound of the weighted bidirectional RS is derived and verified, along with the asymptotic expression in high signal-to-noise ratio (SNR). Based on the analytical expressions, the optimal weight factor and the optimal power allocation scheme in minimizing the outage probability are obtained. Simulation results reveal that when the CSI is outdated, the diversity order reduces from full diversity to one. Furthermore, the weighted bidirectional RS scheme with the optimal weight factor yields a significant performance gain over the conventional bidirectional RS scheme, especially in high SNR.