Density evolution analysis of LDPC-coded SCMA systems

Sparse code multiple access (SCMA) is a promising non-orthogonal multiple access scheme to support massive connectivity. In this paper, we propose a novel scheme to analyze the performance of Low-density Parity-check (LDPC) codes in SCMA systems based on density evolution (DE) theory. In particular, we develop the combination method including channel adapter and log-likelihood ratio (LLR) convertor in conjunction with SCMA detection by leveraging the message passing (MPA) algorithm. For the density evolution analysis of regular and irregular LDPC codes, we design a novel multiuser DE algorithm in SCMA systems with the given principle of minimizing the maximum channel threshold of all users. As the simulation results demonstrate, proposed scheme and algorithm can obtain the channel thresholds close to the system capacity in case of regular LDPC and irregular LDPC codes, and also present the optimal degree distribution for irregular LDPC codes in the SCMA systems.