Leveraging RIS to Assist Communication and Against CSI-Based Passive Sensing

This letter explores the use of reconfigurable intelligent surface (RIS) to assist user equipment (UE) communication meanwhile against passive sensing by the UE based on channel state information (CSI). The RIS-based system studied is termed RIS-CAPS. To comprehend the passive sensing ability of UE, we establish a novel model that characterizes how a UE, with zero prior knowledge of the physical environment, can tell different environmental states via estimating and analyzing CSI. We derive closed-form probability metrics of the UE's passive sensing as functions of the CSI estimates and RIS configuration. Further, we address the optimal RIS configuration in RIS-CAPS to improve the communication rate of UE while limiting the passive sensing performance. We analyze the communication rate upper bound, showing the tension between anti sensing and the rate of UE in different channel conditions. A greedy algorithm is developed to tackle the optimization and return RIS configuration solutions.