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