A Survey of NOMA: Current Status and Open Research Challenges

TL;DR

This survey reviews the current status of NOMA technology, compares it with MU-MIMO, and proposes methods to reduce its implementation complexity and delay, highlighting its potential for beyond 5G applications.

Contribution

It provides a comprehensive review of NOMA's current status, discusses its limitations compared to MU-MIMO, and introduces techniques to enhance its efficiency and reduce complexity.

Findings

NOMA's gains over MU-MIMO are negligible in certain scenarios.

Proposed methods effectively reduce receiver complexity and delay.

Smart techniques improve energy efficiency and transmission delay.

Abstract

Non-orthogonal multiple access (NOMA) has been considered as a study-item in 3GPP for 5G new radio (NR). However, it was decided not to continue with it as a work-item, and to leave it for possible use in beyond 5G. In this paper, we first review the discussions that ended in such decision. Particularly, we present simulation comparisons between the NOMA and multi-user multiple-input-multiple-output (MU-MIMO), where the possible gain of NOMA, compared to MU-MIMO, is negligible. Then, we propose a number of methods to reduce the implementation complexity and delay of both uplink (UL) and downlink (DL) NOMA-based transmission, as different ways to improve its efficiency. Here, particular attention is paid to reducing the receiver complexity, the cost of hybrid automatic repeat request as well as the user pairing complexity. As demonstrated, different smart techniques can be applied to improve the energy efficiency and the end-to-end transmission delay of NOMA-based systems.