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This paper is focused on code-domain non-orthogonal multiple access(CD-NOMA), which is an emerging paradigm to support massive connectivity forfuture machine-type wireless networks. We take a comparative approach to studytwo types of overloaded CD-NOMA, i.e., sparse code multiple access (SCMA) anddense code multiple access (DCMA), which are distinctive from each other interms of their codebooks having sparsity or not. By analysing their individualdiversity orders (DO) in Rayleigh fading channels, it is found that DCMA can bedesigned to enjoy full DO which is equal to the maximum number of resourcenodes in the system. This is in contrast to SCMA whose error rate suffers fromlimited DO equal to the codebook sparsity (i.e., the effective number ofresource nodes occupied by each user). We conduct theoretical analysis for thecodebook design criteria and propose to use generalized sphere decoder for DCMAdetection. We numerically evaluate two types of multiple access schemes under"4x6" (i.e., six users communicate over four subcarriers) and "5x10" NOMAsettings and reveal that DCMA gives rise to significantly improved error rateperformance in Rayleigh fading channels, whilst having decoding complexitycomparable to that of SCMA.

Sparse or Dense: A Comparative Study of Code-Domain NOMA Systems