Physical layer security of STAR-RIS-aided RSMA systems

In this paper, we investigate the physical layer security (PLS) of a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided rate-splitting multiple access (RSMA) system in the presence of an eavesdropper. The STAR-RIS is deployed to aid the communications from the base-station to two legitimate users, while preventing the eavesdropper from wiretapping the legitimate communications. We derive closed-form expressions for channel statistics of the combined and transmitted links by taking into account Nakagami-m fading. The impacts of the STAR-RIS and Nakagami-m fading on the scerecy performance are investigated. Specifically, we derive the closed-form expressions for the legitimate users’ secrecy outage probability (SOP) and investigate the asymptotic behaviors of users’ SOP in the high signal-to-noise ratio (SNR) region. Additionally, the impacts of transmit power, target secrecy rate, power allocation parameters, and the number of reflection and transmission elements of STAR-RIS on the system secrecy performance are studied. Numerical results demonstrate the superiority of the STAR-RIS-aided RSMA scheme over other STAR-RIS-aided multiple access schemes in terms of the system secrecy performance.