Secure Opportunistic User Scheduling in RIS-Aided Networks: A Comparison with NOMA-Based Scheduling

In this paper, we propose an opportunistic user scheduling scheme in a multi-user reconfigurable intelligent surface (RIS) aided wireless system to improve secrecy. We derive the secrecy outage probability (SOP) and its asymptotic expression in approximate closed form. The asymptotic analysis shows that the SOP does not depend on the transmitter-to-RIS distance and saturates at a high signal-to-noise ratio to a fixed value that depends on the ratio of the path-loss of the RIS-to-destination and RIS-to-eavesdropper links and the number of users. It is shown that the asymptotic SOP decreases exponentially with the number of RIS elements. We also compare the secrecy performance of our scheme with that of a non-orthogonal multiple access (NOMA) scheduling scheme, which chooses a pair of users to schedule in each time slot. The comparison shows that the secrecy performance of the NOMA scheme is compromised and that our proposed scheduling scheme has better performance.