A New LDPC coded Scheme for Two-user Multiple-Access Channels aided with Physical-layer Network Coding

In this paper, we aim to design a new low-density parity-check (LDPC) coded scheme for two-user Gaussian multiple access channel aided with physical-layer network coding (PNC). The two users have equal transmission rate and employ the same LDPC code. We allocate the two users with unequal modulation power levels. Without loss of generality, we modulate User A's codeword and User B's codeword by the higher and lower power level signals, respectively. Since the PNC coded bits are the exclusive-OR (XOR) of User A's codeword and User B's codeword, the PNC coded bits are also a codeword of the employed LDPC code. At the receiver, the decoder estimates User A's codeword and the PNC codeword with conventional message passing algorithm separately, rather than decoding User A's and User B's codewords directly with the joint decoding algorithm. Then User B's codeword is estimated by the XOR of the decoding results of User A's codeword and the PNC codeword. We optimize the power ratio between the two power levels to minimize the required SNR for a scheme with a given rate. Numerical result shows that our proposed scheme has a better bit error rate performance and a lower decoding complexity, compared to the state-of-the-art scheme.