What Role Do Intelligent Reflecting Surfaces Play in Multi-Antenna Non-Orthogonal Multiple Access?

Massive multiple-input multiple-output (MIMO) and non-orthogonal multiple access (NOMA) are two key techniques for enabling massive connectivity in future wireless networks. A massive MIMO-NOMA system can deliver remarkable spectral improvements and low communication latency. Nevertheless, the uncontrollable stochastic behavior of the wireless channels can still degrade its performance. In this context, the idea of an intelligent reflecting surface (IRS) has emerged as a promising technology for smartly overcoming the possibly detrimental effects of the wireless environment. The disruptive IRS concept of controlling the propagation channels via software can provide attractive performance gains to the communication networks, including higher data rates, improved user fairness, and possibly higher energy efficiency. In this article, we demonstrate the main roles of IRSs in MIMO-NOMA systems. Specifically, we identify key challenges and perform a comprehensive discussion of the main performance gains that can be achieved in IRS-assisted massive MIMO-NOMA (IRS-NOMA) networks. We outline exciting futuristic use case scenarios for IRS-NOMA and expose the main related challenges and future research directions. Furthermore, throughout the article, we support our in-depth discussions with representative numerical results.

Introduction

The fifth generation (5G) of wireless cellular systems will enable the deployment of demanding applications such as autonomous cars, massive sensor networks, telemedicine, smart homes, and more. To make these applications possible, stringent requirements such as massive connectivity, improved spectrum efficiency, and low communication latency must be fulfilled. Massive multiple-input multiple-output (MIMO) is one of 5G's key technologies for accomplishing these requirements. By exploiting the spatial domain with transmit beamforming techniques, and employing a large number of antennas, massive MIMO schemes enable resource-efficient parallel transmissions to multiple users using the same frequency and time slot. Non-orthogonal multiple access (NOMA) is another important technology envisioned to be part of future wireless systems. In particular, by employing superposition coding (SC) at the base station (BS) and successive interference cancellation (SIC) at the receivers, power-domain NOMA can simultaneously serve more than one user with a single resource block. This makes NOMA capable of providing significant connectivity improvements to communication networks. If massive MIMO and NOMA are properly combined, the features of the two techniques can be exploited to reach even higher spectral gains, which can outperform conventional systems employing orthogonal multiple access (OMA) [1].