Joint Network and LDPC Coding for Bi-directional Relaying
  In this paper, we consider joint network and LDPC coding for practically implementing the denosie-and-forward protocol over bi-directional relaying. the closed-form expressions for computing the log-likelihood ratios of the network-coded codewords have been derived for both real and complex multiple-access channels. It is revealed that the equivalent channel observed at the relay is an asymmetrical channel, where the channel input is the XOR form of the two source nodes. 
Xiaofu Wu, Weijun Zeng, Chunming Zhao, Xiaohu You
cs.IT math.IT