Analysis and Optimization of STAR-RIS-Assisted Proactive Eavesdropping With Statistical CSI.

In wireless surveillance, the monitor (E) should be far away from the suspicious system to maintain concealment, leading to very poor eavesdropping performance. To tackle this challenge, we propose in this paper a novel simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted proactive eavesdropping strategy. Specifically, a STAR-RIS is deployed between E and the suspicious system to implement reflection (R) and transmission (T) for the suspicious signal of the suspicious transmitter and the jamming signal of E, to concurrently vary the channel power gains of the suspicious transmission link and the legitimate eavesdropping link and then facilitate E's eavesdropping. Assuming that E can only access the global statistical channel state information, we aim to maximize E's average (ergodic) eavesdropping rate, by jointly optimizing the energy splitting ratio of the STAR-RIS and E's jamming power. With rigorous analysis, we reveal that only two cases should be considered to achieve the optimal eavesdropping rate: i) the STAR-RIS works in T&R mode, and E does not conduct jamming; ii) the STAR-RIS only operates in T mode, and E interferes with the suspicious destination. In both cases, the optimal energy splitting ratio or jamming power is derived in (semi) closed form. Also, the eavesdropping energy (cost) efficiency is analyzed. Since our proposed strategy is adaptive, it can achieve significant eavesdropping performance gain compared to competitive benchmarks, as verified by numerical results.