Secure Communications in RIS-Aided NOMA Networks Joint Active and Passive Beamforming with Physical Layer Security

Abstract

In this paper, we study the physical layer security of reconfigurable intelligent surface (RIS) aided non-orthogonal multiple access (NOMA) downlink systems. A multi-antenna base station (BS) serves two NOMA users in the presence of a passive eavesdropper, and a RIS equipped with N passive phase-shifting elements is employed to assist the communication. To enhance the sum secrecy rate, we propose a joint active and passive beamforming optimization framework for secrecy maximization based on alternating optimization (AO). The active beamformer is designed with the maximum ratio transmission (MRT) beamformer towards the user with stronger channel, and the passive RIS phase shifts are optimised by an element-wise block coordinate descent method with one-dimensional grid search. Simulation results show that the proposed scheme significantly outperforms the following three benchmark schemes, i.e., random phase RIS-NOMA, orthogonal multiple access with RIS, and conventional NOMA without RIS, over a large range of operating conditions. In particular, the proposed algorithm is shown to achieve the sum secrecy rate of 16.6 bps/Hz at 40 dB SNR, convergence within six iterations, and secrecy outage probability close to zero for all the considered SNR values. In addition, the improvements brought by more RIS elements, BS antennas, and eavesdropper distance are also confirmed by extensive Monte Carlo simulations.