SEP of a scaled selection combining scheme with decode and forward relaying for MPSK signaling

The performance analysis of a cooperative diversity system with decode and forward relaying has drawn considerable attention recently. The use of conventional maximal-ratio combiners does not suit well for a cooperative diversity scenario and the assumption of no detection errors at the relay is also practically hard to achieve. Hence, we propose a scaled selection combining scheme, where we scale the relay-to-destination links by a deterministic positive scale factor to incorporate the effect of the source-to-relay links. At the destination, we choose a link which has maximum scaled instantaneous signal-to-noise ratio (SNR), from a set which consists of instantaneous SNR of the source-to-destination link and the scaled instantaneous SNRs of the relay-to-destination links. For a flat Rayleigh fading environment with statistically independent links, the end-to-end symbol error probability (SEP) of this scheme for M-ary phase-shift keying is derived in closed form. We also give a method of obtaining the optimum scale factor that minimizes the end-to-end SEP. The analysis is validated through extensive simulations.