Performance evaluation of low-density spreading multiple access

Low-density spreading multiple access (LDSMA) was proposed for supporting massive connectivity and low latency scenarios in 5th generation systems owing to its low implementation complexity. In this study, the authors analyse the performance loss of conventional LDSMA systems from two aspects: the one from low-density spreading constellations, and the other from the mismatch between multi-user detector and channel decoders. For the first aspect, they propose a new multi-dimensional constellation designed via simple symbol remapping with the same constellation as in LDSMA in each dimension. And for the second aspect, newly designed Raptor-like low-density parity-check codes are proposed to help retrieve the performance loss. Simulation results show that the performance of the proposed enhanced system is 1.4dB and above superior to the conventional LDSMA system and less than 1.2dB away from Shannon limit of Gaussian multiple access channel with symmetric users at sum rate of 1.5 and 2.25 bits per channel use.