Fundamental Limits of Low-Density Spreading NOMA with Fading

TL;DR

This paper analyzes the spectral efficiency limits of low-density spreading NOMA in fading channels, revealing performance differences between linear and optimal decoding in various load regimes.

Contribution

It provides a theoretical analysis of low-density spreading NOMA's spectral efficiency under fading, highlighting its advantages in overloaded scenarios with linear detection.

Findings

Low-density spreading underperforms dense spreading with optimal decoding across all loads.

Linear detection shows smooth spectral efficiency variation with load, outperforming dense spreading in overloaded regimes.

Spectral efficiency of low-density spreading surpasses dense spreading in overloaded conditions.

Abstract

Spectral efficiency of low-density spreading non-orthogonal multiple access channels in the presence of fading is derived for linear detection with independent decoding as well as optimum decoding. The large system limit, where both the number of users and number of signal dimensions grow with fixed ratio, called load, is considered. In the case of optimum decoding, it is found that low-density spreading underperforms dense spreading for all loads. Conversely, linear detection is characterized by different behaviors in the underloaded vs. overloaded regimes. In particular, it is shown that spectral efficiency changes smoothly as load increases. However, in the overloaded regime, the spectral efficiency of low- density spreading is higher than that of dense spreading.