  Sparse code multiple access (SCMA) is one of the most promising methods among
all the non-orthogonal multiple access techniques in the future 5G
communication. Compared with some other non-orthogonal multiple access
techniques such as low density signature (LDS), SCMA can achieve better
performance due to the shaping gain of the SCMA codewords. However, despite of
the sparsity of the codewords, the decoding complexity of the current message
passing algorithm (MPA) utilized by SCMA is still prohibitively high. In this
paper, by exploring the lattice structure of SCMA codewords, we propose a low
complexity decoding algorithm based on list sphere decoding (LSD). The LSD
avoids the exhaustive search for all possible hypotheses and only considers
signal within a hypersphere. As LSD can be viewed a depth-first tree search
algorithm, we further propose several methods to prune the redundancy visited
nodes in order to reduce the size of the search tree. Simulation results show
that the proposed algorithm can reduce the decoding complexity substantially
while the performance loss compared with the existing algorithm is negligible.
