A Low Complexity Detector for Downlink SCMA Systems

Sparse Code Multiple Access (SCMA) is a novel non-orthogonal multiple access scheme for 5G systems, in which the logarithm domain message passing algorithm (Log-MPA) is applied at the receiver to achieve near-optimum performance. However, the computational complexity of Log-MPA detector is still a big challenge for practical implementation, especially for energysensitive user equipments in the downlink scenario. In this paper, a Region-Restricted detector with an improved Log-MPA (RRL detector) is proposed for downlink SCMA systems, in which the complexity is reduced from two perspectives. To avoid unnecessary calculations when searching the superposition constellation exhaustively, the proposed RRL detector updates the function nodes only within a restricted search region. While constellation points outside the search region are neglected, the performance is well maintained which is verified by simulations. Besides, the original Log-MPA heavily relies on exponential operations, resulting in high computational complexity. To solve this problem, an improved Log-MPA is also put forward in this paper to make a better compromise between complexity and performance. Simulation results show that the complexity of the RRL detector is reduced considerably while the bit error rate (BER) performance degrades unnoticeably