A Novel HARQ Scheme for SCMA Systems.

This letter considers the hybrid automatic repeat request (HARQ) scheme for sparse code multiple access (SCMA) systems, where the message passing algorithm (MPA) is adopted in the multiuser detection. In MPA, the reliability can be improved when parts of messages are pre-known. Motivated by this fact, a novel HARQ scheme for SCMA systems is proposed, where some correctly decoded codewords are still retransmitted in the next round of transmission. Analysis and simulation results show that, compared with the traditional HARQ scheme, the reliability of the proposed scheme is enhanced obviously, while the latency increases slightly. Hence, in the proposed scheme, an affordable tradeoff between the reliability and latency can be achieved.