STAR‐RIS‐assisted scheme for enhancing physical layer security in NOMA systems

Abstract

Abstract Simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR‐RIS) is capable of achieving full coverage area compared with conventional reflection‐only reconfigurable intelligent surface (RIS). Motivated by this observation, the secrecy performance of STAR‐RIS‐assisted non‐orthogonal multiple access (NOMA) systems over Rayleigh fading channels is investigated, where the incident signals sent by base station are reflected and transmitted to a pair of NOMA users in the presence of two eavesdroppers. To evaluate the secrecy performance of STAR‐RIS‐NOMA systems, the exact and asymptotic expressions of secrecy outage probability (SOP) and secrecy diversity order are derived, in which the correlation of the channels between the elements of each column on STAR‐RIS is taken into consideration. As a benchmark, the secrecy performance of the STAR‐RIS strategy for orthogonal multiple access (OMA) system is also analysed. Numerical results are provided to verify the effectiveness of the analytical results and demonstrate that: The SOP of STAR‐RIS‐NOMA outperforms that of STAR‐RIS‐assisted OMA and conventional cooperative communication systems. Furthermore, the scheme proposed here can obtain full order of secrecy diversity.