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

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 according to whether there is a direct link between the base station (BS) and each UD, and the outage performance is analyzed for each of them. Specifically, the asymptotic expressions for the upper and lower bounds of the outage probability in the high signal-to-noise ratio (SNR) regime are derived. Following that, the diversity order is obtained. It is shown that the use of discrete phase shifts does not degrade diversity order. More importantly, simulation results reveal that a 3-bit resolution for discrete phase shifts is sufficient to achieve near-optimal outage performance. Simulation results also imply the superiority of IRSs over full-duplex decode-and-forward relays.

I. Introduction

Non-orthogonal multiple access (NOMA) has been proposed as a candidate technique for future wireless networks [1]. The key idea of NOMA is to allocate multiple users to an orthogonal resource block, e.g., a time slot, a frequency band, or a spreading code, but with different power levels [2], [3]. It has been demonstrated that NOMA outperforms orthogonal multiple access (OMA) from the aspects of spectral efficiency, connection density, and user fairness [4].