Log Likelihood Ratio-Based Relaying for Distributed Turbo Codes

A distributed turbo codes(DTC) scheme with log likelihood ratio (LLR)-based threshold at the relay for a two-hop relay networks is proposed. Different from traditional DTC schemes, the retransmission scheme at the relay, where imperfect decoding occurs, is considered in the proposed scheme. By employing a LLR-based threshold at the relay in the proposed scheme, the reliability of decoder-LLRs can be measured. As a result, only reliable symbols will be forwarded to the destination and a maximum ratio combiner (MRC) is used to combine signals received from both the source and the relay. In order to obtain the optimal threshold at the relay, an equivalent model of decoder-LLRs is investigated, so as to derive the expression of the bit error probability (BEP) of the proposed scheme under binary phase shift keying (BPSK) modulation. Simulation results demonstrate that the proposed scheme can effectively mitigate error propagation at the relay and also outperforms other existing methods.