Cooperative demodulation by link adaptive relaying

In cooperative communication systems, the decode-and-forward (DF) relaying is a practically attractive strategy. Existing techniques for mitigating the error propagation in this scheme can be classified into two groups: the first of these uses link SNRs to evaluate the reliability of the data received by the relay and the second approach is to develop better combining schemes at the destination. In this paper, a combination of both methods has been proposed. The purpose of this work is obtaining performance gain by the degree of freedom in the relay. So the power scaling coefficients in the relay are proposed that give a better performance in comparison with Cooperative MRC (C-MRC). Also a suitable combiner is used in the destination that is a generalization of C-MRC and is an approximation of ML detector. We show the advantage of the new proposed scheme through simulations and analytical analysis in which the coding gain is utilized as a performance metric to compare cooperative detectors. A power scaling coefficient in the relay is proposed that is independent of instantaneous SNR at source-relay link and has a surprising gain when the relay is located close to the source. It is demonstrated that in high SNRs, the average BER of the new relaying scheme with this coefficient almost achieves a lower bound.