STAR-RIS-Empowered Integrated Sensing and Covert Communication System With Movable Elements: Joint Robust Beamforming and Element Deployment Design

This paper investigates the integrated sensing and covert communication (ISCC) system, which is empowered by a simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS) with movable elements (MEs). The base station (BS) serves legitimate users (including covert inside users and overt outside users) with the assistance of a STAR-RIS and senses several malicious targets simultaneously. To protect the information of covert users, it is necessary to ensure that the communications between the BS and covert users are hidden from the sensing targets. The STAR-RIS is equipped with MEs that enable a reconfigurable array response and signal control (reflection/transmission), i.e., flexible and passive beamforming (FAPB). Based on the imperfect channel state information (CSI) assumption, we aim to maximize both the communication and the sensing performance by jointly designing the active beamforming of BS and the FAPB of STAR-RIS. Both STAR-RISs with independent and coupled phase shift are considered. For the case of independent phase shift, we develop an iteration-based joint active and flexible passive beamforming (JAFPB) algorithm to obtain a high-quality solution. For the case of coupled phase shift, we develop a double-layer penalty-based iteration (DLPBI) algorithm to efficiently solve this problem. The numerical results validate the effectiveness and robustness of the proposed algorithm and further reveal that element mobility can improve the performance of the STAR-RIS-aided system.