An improved HARQ scheme for SCMA under random access

To meet with the low-latency requirements in 5G mobile network, grant-free random access has been a hot topic as the tedious procedure of link establishment and scheduling could be removed thus shortening overall latency. Recently, Sparse Code Multiple Access (SCMA) has been presented as a promising scheme for random access since its non-orthogonality enables base station to accommodate much more users. However, retransmission is still needed to achieve desired reliability, yet inappropriate retransmission scheme could undercut the advantage and bring in long latency. In this paper, we propose an improved hybrid automatic repeat request (HARQ) scheme dedicated for SCMA under grant-free random access scenario. Simulation results show that the new scheme improves reliability while at same time reduces the latency compared with traditional HARQ scheme under random access scenario.