On information-theoretic limits of codedomain NOMA for 5G

Motivated by recent theoretical challenges for 5G, this study aims to position relevant results in the literature on codedomain non-orthogonal multiple access (NOMA) from an information-theoretic perspective, given that most of the recent intuition of NOMA relies on another domain, that is, the power domain. Theoretical derivations for several code-domain NOMA schemes are reported and interpreted, adopting a unified framework that focuses on the analysis of the NOMA spreading matrix, in terms of load, sparsity, and regularity features. The comparative analysis shows that it is beneficial to adopt extreme low-dense code-domain NOMA in the large system limit, where the number of resource elements and number of users grow unboundedly while their ratio, called load, is kept constant. Particularly, when optimum receivers are used, the adoption of a regular low-dense spreading matrix is beneficial to the system achievable rates, which are higher than those obtained with either irregular low-dense or dense formats, for any value of load. For linear receivers, which are more favourable in practice due to lower complexity, the regular low-dense NOMA still has better performance in the underloaded regime (load < 1), while the irregular counterpart outperforms all the other schemes in the overloaded scenario (load > 1).