Effective Secrecy Capacity for RIS-Assisted NOMA Communication Networks

Non-orthogonal multiple access (NOMA) and reconfigurable intelligent surface (RIS) are pivotal technologies for achieving the dual objectives of high spectral efficiency and low power consumption in the future of wireless communications. In this paper, we investigate the effective secrecy capacity (ESC) of the RIS-assisted NOMA network subject to delay constraint. The ESC serves as a crucial metric for evaluating the influence of delay on the performance of secure communication. Specifically, we consider a multi-user downlink RIS-NOMA network that includes an internal eavesdropper and provide an analytical framework for the ESC. In the special case with two users, both the accurate expression and the high signal-to-noise ratio (SNR) asymptotic expression of the ESC are derived. The analytical results are corroborated by simulations, leading to several insightful conclusions: 1) the ESC tends to be a constant value in the high SNR region; 2) the RIS-NOMA network achieves higher ESC than the RIS-orthogonal multiple access counterpart; 3) strict latency constraints lead to smaller ESC, while more lenient constraints allow for greater ESC; 4) increasing the power allocation coefficient of the user and the number of RIS elements improves the ESC.