Exploiting Intelligent Reflecting Surfaces in NOMA Networks: Joint Beamforming Optimization

This paper investigates a downlink multiple-input single-output intelligent reflecting surface (IRS) aided non-orthogonal multiple access (NOMA) system, where a base station (BS) serves multiple users with the aid of IRSs. Our goal is to maximize the sum rate of all users by jointly optimizing the active beamforming at the BS and the passive beamforming at the IRS, subject to successive interference cancellation decoding rate conditions and IRS reflecting elements constraints. In term of the characteristics of reflection amplitudes and phase shifts, we consider ideal and non-ideal IRS assumptions. To tackle the formulated non-convex problems, we propose efficient algorithms by invoking alternating optimization, which design the active beamforming and passive beamforming alternately. For the ideal IRS scenario, the two subproblems are solved by invoking the successive convex approximation technique. For the non-ideal IRS scenario, constant modulus IRS elements are further divided into continuous phase shifts and discrete phase shifts. To tackle the passive beamforming problem with continuous phase shifts, a novel algorithm is developed by utilizing the sequential rank-one constraint relaxation approach, which is guaranteed to find a locally optimal rank-one solution. Then, a quantization-based scheme is proposed for discrete phase shifts. Finally, numerical results illustrate that: i) the system sum rate can be significantly improved by deploying the IRS with the proposed algorithms; ii) 3-bit phase shifters are capable of achieving almost the same performance as the ideal IRS; iii) the proposed IRS-aided NOMA systems achieve higher system sum rate than the IRS-aided orthogonal multiple access system.

I. Introduction

Recently, intelligent reflecting surface (IRS) aided transmission has drawn significant attention due to its superior performance in enhancing spectrum efficiency (SE) and energy efficiency (EE) for wireless communication networks [2], [3]. An IRS is a planar array comprising a large number of low cost passive reflecting elements, which can passively reflect the incident electromagnetic wave and simultaneously change its amplitude and phase shift [4]. With this new degree-of-freedom (DoF), an IRS can be deployed to create an additional reflection link when the transmitter and receiver are blocked by obstacles. Compared with conventional relaying technologies (e.g amplify-and-forward (AF) and decode-and-forward (DF) relays), IRSs require much less energy consumption due to the nearly passive characteristics [5]. Therefore, IRS technology has drawn tremendous attention from both academia and industry, and has been regarded as a promising solution in future 6G networks [5], [6].