Non-Orthogonal Multiple Access (NOMA) with Multiple Intelligent Reflecting Surfaces

Abstract

In this paper, non-orthogonal multiple access (NOMA) networks assisted by multiple intelligent reflecting surfaces (IRSs) with discrete phase shifts are investigated, in which each user device (UD) is served by an IRS to improve the quality of the received signal. Two scenarios are considered based on whether there is a direct link or not between the base station (BS) and each UD, and the outage performance is analyzed for each of them. Specifically, the outage probability is approximated in the high signal-to-noise ratio (SNR) regime, and the diversity order is obtained. Orthogonal multiple access (OMA) can be regarded as a special case of NOMA, and the outage performance of the multi-IRS assisted OMA system is also characterized. It is demonstrated that NOMA outperforms OMA in multi-IRS assisted networks. Furthermore, it is shown that the use of discrete phase shifts does not degrade the diversity order. More importantly, simulation results further reveal that a 3-bit resolution for discrete phase shifts is sufficient to achieve near-optimal outage performance.