Securing STAR-FC-RIS empowered integrated sensing and multiuser communications against target eavesdropping

This paper investigates a simultaneous-transmitting-and-reflecting fully-connected reconfigurable intelligent surface (STAR-FC-RIS) empowered integrated sensing and multiuser communications (ISAMC) network, where a dual-functional radar-communication base station detects a malicious radar target nearby and communicates with multiple legitimate users on the other side of the STAR-RIS. We utilize an integrated architecture that combines the fully-connected (FC)-RIS, an emerging type of beyond-diagonal (BD)-RIS, with the time-switching (TS)-STAR-RIS to enhance both the sensing and communications at the cost of possible target intercepting and propose the simultaneous-transmitting-and-reflecting fully-connected RIS (STAR-FCR) schemes to strike a balance between sensing and communications performance. Thereafter, observing the security-reliability performance tradeoff of the downlink ISAMC, we conduct closed-form analyses to compare COPs of round-robin scheduling (RS) and multiuser scheduling (MS) with the aid of a TS-based STAR-FC-RIS. Furthermore, we derive closed-form expressions of the sensing outage probability, communications outage probability (COP), and communications intercept probability, where an average of the three probabilities is exploited to obtain an optimized time allocation (OTA) of the STAR-FC-RIS. Numerical results verify that the STAR-FCR-MS scheme outperforms the STAR-FCR-RS scheme in terms of sensing reliability and communications security. Moreover, an OTA remarkably enhances the overall performance of the STAR-FCR schemes of ISAMC systems.