Performance of decode-and-forward opportunistic cooperation with the Nth best relay selected

In this paper, we study the outage performance of decode-and-forward (DF) opportunistic communication systems with the Nth best relay selection scheme under independent but not necessarily identical Rayleigh fading channels. In traditional opportunistic cooperation, only the best relay helps forward the source's messages to the destination. However, in practical systems, the best relay may be unavailable due to scheduling and overload problems. In this case, another suboptimal but available relay may be selected to participate in cooperation. Without loss of generality, we analyze the performance when the Nth best relay is selected to cooperate and take the availability of the direct link between the source and the destination into consideration. In particular, we derive the exact expression of outage probability for arbitrary signal-to-noise (SNR), then we obtain the closed-form expression in high SNR region. Through asymptotic analysis, the diversity order which reflects the effect of the direct link is obtained and it is linearly related with the number of relays and the selection criterion N. Simulation results are presented to verify the analysis.