Joint Multiuser LDPC Coding Scheme for Error Control in DS-CDMA Systems

In this paper, we introduce the application of low density parity check (LDPC) codes to multiuser detection through additive white gaussian noise (AWGN) channels. An ensemble of binary LDPC codes with an irregular degree distribution is used. We introduce Joint Multiuser LDPC (JML) Tanner graph for joint decoding of information bits of all users. The decoding of information sources is done by a modified iterative belief propagation (BP) algorithm, which deals with vector messages rather than scalar ones. Good degree distribution pairs are extracted by maximizing the threshold of the decoder. Next, performance of the generated degree distribution pairs is evaluated over AWGN channels. Furthermore, some performance analysis criteria of the proposed method such as multiuser efficiency and bit error rate (BER) are compared with optimum and minimum mean square error (MMSE) receivers and also with receivers, which use convolutional codes. Finally simulation results demonstrate that the new proposed method outperforms these receivers for some good degree distribution pairs.