Low Complexity Receivers for Coherent Amplify-and-Forward Cooperative Systems

Low complexity receivers for coherent amplify-and-forward (AF) cooperative diversity systems are developed. According to the availability of phase information at the relays and the destination, repetition-based and spectral-efficient distributed equal gain combining (DEGC) schemes are developed. In the former scheme, relays transmit over separate orthogonal subchannels, whereas in the latter scheme all relays transmit simultaneously. Analytical expressions for evaluation of the average output signal-to-noise ratio (SNR) and symbol error probability are derived and it is demonstrated that these schemes achieve full spatial diversity. In addition, upper and lower bounds on the ergodic capacity are obtained. The upper bound is tight in small-SNR regions whereas the lower bound is accurate for sufficiently large values of SNR. It is shown that the spectral-efficient DEGC scheme outperforms the repetition-based DEGC scheme especially by increasing the number of relays. The impact of combining loss on the performance of the proposed schemes is also studied. The average output SNR expression can be used as a simple theoretical tool for choosing the best number of relays that avoids the combining loss.