LDPC coded soft forwarding with network coding for the two-way relay channel

This paper investigates a low-density parity-check (LDPC) coded soft-decode-and-forward (SDF) relaying protocol for a two way relay channel (TWRC). In this SDF protocol, the relay evaluates the reliabilities, expressed as log-likelihood ratios (LLRs), of the received signals from the two sources. The relay then forwards a network-coded combination of the parity symbols of both sources, but in the “soft” domain to avoid error propagation from the relay. We introduce a model for the effective noise experienced by the soft network coded symbols, constituting the new parameters of soft scalar and soft error; this model is then used to compute the log-likelihood ratios (LLRs) at the destination. To facilitate this latter computation, we have derived an analytical expression for soft error variance, as well as a simplified expression based on a common assumption on the statistical behavior of the LLRs. This enables low-complexity computation and tracking of the soft error variance on-the-fly. The proposed system outperforms standard competing schemes reported in the literature in terms of error rate performance over Rayleigh fading links.