Exploiting STAR-RIS for Physical Layer Security in Integrated Sensing and Communication Networks

Reconfigurable intelligence surface (RIS)-enabled integrated sensing and communication (ISAC) is vulnerable to eavesdropping, and thus its secure transmission is a crucial research field. In this paper, we integrate a promising simultaneous transmitting and reflecting RIS (STAR-RIS) into an ISAC network to improve the physical layer security, where with the presence of an eavesdropper, a base station (BS) sends the multicast communication signal combined with the dedicated sensing signal to perform both multi-user communication and single-target detection. Thereinto, the STAR-RIS can reconfigure signal propagation to ensure the sensing and communication quality while preventing the eavesdropping, and also elevate the flexibility of network deployments relying on its full-space coverage. Our goal is to maximize the secrecy rate of the whole network by jointly optimizing the transmit beamforming at the BS and the transmission&reflection beamforming at the STAR-RIS, while satisfying the sensing signal-to-noise ratio (SNR) requirement. To settle the non-convex problem, an overall iterative algorithm is developed by invoking the successive convex approximation and sequential rank-one constraint relaxation methods. Simulations verify the efficiency of the proposed algorithm, and reveal that i) the proposed STAR-RIS-enabled ISAC scheme significantly outperforms other benchmark schemes; ii) the sensing signal can enhance the secrecy rate at the high sensing SNR regions, whereas at the lower sensing SNR regions, it has no extra performance gain and can thus be removed to simplify algorithm design.