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 to a fixed value depending on the ratio of the path loss of the RIS-to-destination and RIS-to-eavesdropper links and the number of users at high signal-to-noise ratio. It is shown that increasing the number of RIS elements leads to an exponential decrease in the SOP. We also compare 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 SOP of all of the NOMA users is compromised, and that our proposed scheduling scheme has better performance.

翻译：本文提出了一种在多用户可重构智能表面（RIS）辅助无线系统中提升保密性的机会用户调度方案。我们推导了保密中断概率（SOP）的近似闭合表达式及其渐近形式。渐近分析表明，SOP不依赖于发射机到RIS的距离，并在高信噪比下趋于一个由RIS到目的地与RIS到窃听者路径损耗比值及用户数量决定的固定值。研究显示，增加RIS单元数量会导致SOP呈指数级下降。我们还将所提方案与非正交多址接入（NOMA）调度方案进行了比较，后者在每个时隙中选择一对用户进行调度。比较结果表明，所有NOMA用户的SOP均受到损害，而所提调度方案具有更优的性能。