A soft decode-compress-forward relaying scheme for cooperative wireless networks

This paper introduces a new technique for soft information relaying (SIR) which is based on a soft decode-compress-forward (DCF) relay protocol. The proposed system provides a means of using distributed low-density parity-check (LDPC) coding in conjunction with higher order (M-ary PAM) modulation, which is effective even under poor source-relay link conditions. Ordinarily, such schemes suffer from error propagation to the destination caused by incorrect decoding at the relay when the signal-to-noise ratio (SNR) on the source-relay link is low; however, our system avoids this problem by generating soft versions of the additional (parity-bearing) PAM symbols for transmission from the relay. Furthermore, we propose a new model, which we refer to as the soft scalar model, for the soft symbols generated at the relay, and we appropriately modify the log-likelihood ratio (LLR) calculation at the destination. Simulation results demonstrate that the proposed scheme can maintain very good performance under poor source-relay SNR conditions.