Error analysis of the decode and forward protocol with selection combining

We analyze the error performance of a cooperative diversity system having two cooperating mobile units or users (designated as source and relay) and a destination. The relay cooperates with the source through the decode and forward protocol to enable the transmission of a data symbol from the source to the destination in a flat Rayleigh fading environment. A selection combining scheme is used to obtain an easily implementable receiver structure at the destination. An analytical expression for the end-to-end symbol error probability (SEP) of the system for M-ary phase shift keying is derived using a new approach, which we call a paired error approach. As a special case, we obtain, in closed form, the end-to-end SEP for binary phase shift keying. We also present a modified selection combining scheme to incorporate the effect of the interuser link, that is, the source to relay link, in the evaluation of the error performance. Results show that cooperative diversity becomes effective for a larger range of average signal-to-noise ratio (SNR) of the user to destination link as the average SNR of the interuser link increases. We also find that the modified selection combining scheme offers a substantial SNR gain over selection combining based cooperation.