Comparative Study of Overloaded and Underloaded NOMA Schemes with Two Multiuser Detectors

Different novel non-orthogonal multiple access (NOMA) schemes have been put forward in order to enable support for massive connectivity of devices and ensure optimum utilization of scarce radio resources. This paper compares three prominent NOMA schemes, multi-user shared access (MUSA), pattern division multiple access (PDMA), and sparse code multiple access (SCMA) schemes, when combined with two different types of receivers, ordered successive interference (OSIC)-based multiuser detector (MUD) and message passing algorithm (MPA)- based MUD. The three NOMA schemes in combination with the two MUDs are compared during overloading, fully loaded, and under-loading scenarios. Simulation results suggest that all the schemes when combined with the two MUDs, perform better during the under-loading scenario (apparently due to low interference occurrence) in comparison with their performances during fully loaded and overloading scenarios. The results also suggest that SCMA scheme does not work with OSIC. Also, PDMA using MPA and OSIC MUDs and SCMA using MPA perform equally well and better than the other combinations, especially at higher signal to noise ratio (SNR). This is apparently, due to similarity in the designs of both SCMA and PDMA schemes in code domain.

I. Introduction

Since the emergence of non-orthogonal multiple access (NOMA) schemes as better technologies for fifth generation (5G) of wireless networks, various forms of NOMA has been proposed for 5G standard. Among these are code-domain NOMA techniques such as trellis coded multiple access (TCMA) [1], interleave division multiple access (IDMA) [2], low-density signature (LDS) sequence based CDMA [3]. Others are most recently proposed multi-user shared access (MUSA) [4], pattern division multiple access (PDMA) [5, 6], and sparse code multiple access (SCMA) schemes [7, 8]. It should be noted that both LDS and SCMA are based on similar idea in which one user’s information is spread over a number of subcarriers [7, 8, 9, 10]. Nevertheless, the number of subcarriers that are assigned to each user is smaller in comparison with the total number of subcarriers. Consequently, this type of low spreading (sparse) attribute makes sure that the number of users that are using the same subcarrier is not too large in order to ensure that the system complexity remains manageable.