STAR-RIS-Empowered Monitoring Over Two-Way Suspicious Communications

This paper investigates a simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) empowered wireless surveillance system, where a pair of suspicious nodes send messages to each other in separate time phases, and the suspicious signal is simultaneously decoded by a monitor through a STAR-RIS-enhanced link for surveillance purposes. During the STAR-RIS-assisted monitoring, cooperative jamming (CJ) can also be leveraged by the monitor to disturb the receiver of the two-way suspicious communications, so as to benefit from the full-space coverage of STAR-RISs. We propose the STAR-RIS and CJ-assisted monitoring (SR-CJM) scheme, the STAR-RIS-assisted passive monitoring (SR-PM) scheme, and derive analytical expressions of their surveillance success probabilities (SSPs) for both infinite and finite phase-shift resolutions at the STAR-RIS, respectively. Since the closed-form and asymptotic analyses facilitate an optimization of the transmission-coefficient amplitudes of the STAR-RIS, we further present the SR-CJM with an optimal transmission coefficient (OTC), denoted by the SR-CJM-OTC scheme. Numerical results not only validate the closed-form SSP analysis and the robustness of proposed schemes for different phase-shift resolutions, but also verify that faced with two-way suspicious communications, the proposed SR-CJM outperforms the conventional pure-transmission/reflection RIS assisted CJ-assisted monitoring (CR-CJM) in terms of the SSPs. Besides, the SR-CJM shows its surveillance performance advantage over the SR-PM, and the performance can be further improved by exploiting more degrees of freedom provided by the transmission-coefficient amplitudes of the STAR-RIS, especially in the high jamming power region.