Relay Selection for Bidirectional AF Relay Network With Outdated CSI

Most previous works on relay selection (RS) of bidirectional relay networks typically assume perfect channel state information (CSI). However, outdated CSI, which is caused by the time variation of channels, cannot be ignored in the practical system, and it will deteriorate performance. In this paper, the effect of outdated CSI on the performance of bidirectional amplify-and-forward (AF) RS is investigated. The optimal single-RS scheme in minimizing the symbol error rate (SER) is revised by incorporating the outdated channels. The analytical expression of the end-to-end SER is derived in a closed form, along with the asymptotic SER expression in a high SNR. All the analytical expressions are verified by Monte Carlo simulations. The analytical and simulation results reveal that once CSI is outdated, the diversity order degrades to one from full diversity. Furthermore, a multiple-RS scheme is proposed, which is a feasible solution to compensate the diversity loss caused by outdated CSI.