  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.
