# What is the Benefit of Code-domain NOMA in Massive MIMO?

## TL;DR

This paper investigates how code-domain NOMA can enhance spectral efficiency in Massive MIMO systems, especially in scenarios with poor propagation conditions and spatially close users, expanding its benefits beyond overloaded regimes.

## Contribution

It identifies specific cases where code-domain NOMA improves Massive MIMO performance in non-overloaded regimes with challenging propagation conditions.

## Key findings

- NOMA benefits in practical scenarios with spatially close UEs.
- Performance gains in systems with poor propagation conditions.
- Applicability of NOMA beyond overloaded regimes.

## Abstract

In overloaded Massive MIMO systems, wherein the number K of user equipments (UEs) exceeds the number of base station antennas M, it has recently been shown that non-orthogonal multiple access (NOMA) can increase performance. This paper aims at identifying cases of the classical operating regime K < M, where code-domain NOMA can also improve the spectral efficiency of Massive MIMO. Particular attention is given to use cases in which poor favorable propagation conditions are experienced. Numerical results show that Massive MIMO with planar antenna arrays can benefit from NOMA in practical scenarios where the UEs are spatially close to each other.