Physical Layer Security Performance Analysis of RIS-Assisted Wireless Communication Systems

Abstract

This paper examines physical layer security of a secure reconfigurable intelligent surfaces (RIS) enabled wireless communication system. Particularly, we explore a wiretap communication system wherein a source node transmits its confidential information to the legitimate destination node using an RIS-based relay terminal, and at the same time, a passive eavesdropper overhears this information from both source and RIS. Under this system setup, we first derive the closed-form expressions of the cumulative distribution function and probability density function of the end-to-end signal-to-noise ratios (SNRs) at the destination and eavesdropper by adopting the method of moments. Utilizing these distributions, we further deduce the exact expression of the secrecy outage probability (SOP) under Rayleigh fading channels. Moreover, to obtain the system’s secrecy diversity order, we present the asymptotic SOP analysis under two scenarios, viz., 1) high SNR regime, and 2) when average channel gains of main link go to infinity and average channel gains pertaining to wiretap links are fixed. It is revealed that the secrecy diversity order reduces to zero under high SNR regime, whereas a secrecy diversity order of 0. 805N can be achievable for large values of average channel gains of main link, where N is the number of RIS elements. Finally, the theoretical findings are validated via numerical and simulation studies. Our results show the impact of various involved parameters on the system’s SOP performance.