A soft information delivery scheme in two-way relay channels with network coding

In this paper, we propose a practical 1-bit soft forwarding protocol for a network-coded two-way relay channel. Different from the conventional estimate-and-forward (EF) protocol, the proposed protocol forwards 1-bit soft information at the relay. We employ the joint trellis coded quantization/modulation (TCQ/M) to implement 1-bit transmission of the soft information. Also, the codebooks in the TCQ are designed to be adaptive to the source-to-relay channel conditions so that the system can achieve the full diversity gain over fading channels. Specifically, in the low source-to-relay channel SNR region, we apply the TCQ/M to the soft information based on the codebook generated by the LloydMax quantizer. In the high source-to-relay channel SNR region, where the soft information is equivalent to its hard decision, we design the codebook by repeating the soft information. It has been shown that the proposed protocol outperforms both the amplify-and-forward (AF) and the decode-and-forward (DF) protocols over fading channels.