Covertness Enhancement by Optical RISs for the MISO VLC System

As a high-level security technique for indoor communications, covert visible light communication (VLC) system can prevent privacy leakage by hiding the existence of communication. In this paper, we consider a covert multiple-input single-output (MISO) VLC system consisting of multiple access points (APs), a legitimate user (Bob), and a warden (Willie). Our objective is to reconfigure the optical propagation environment by leveraging optical reconfigurable intelligent surfaces (RISs) to enhance covertness for the MISO VLC system. First, for the VLC system assisted by both liquid crystal-based RIS (LC-RIS) and mirror-based RIS (mRIS), we derive the covert constraint mathematically. Further, limited by the covert constraint, we formulate an achievable rate maximization problem to jointly optimize the orientations of mirrors, the configuration of LC-RIS, and the AP-mRIS association. Then, by analyzing optical transmission features in the RIS-aided VLC system, we decouple the optimization problem into three subproblems and finally solve them by a proposed low-complexity method. Simulation results show that the achievable covert rate can be enhanced 10 times by leveraging optical RISs in the VLC system.