Combined with reconfigurable intelligent surfaces (RISs), visible light communications (VLCs) can increase the communication performance to a great degree. However, the research into RIS-aided VLC systems has mainly focused on mirror array-based RISs deployed on the wall while neglecting liquid crystal (LC)-based RISs in VLC receivers. With the development of advanced materials, the LC RIS has been gradually attracting attention from researchers. Inspired by the current research into the LC RIS, the applications of the LC RIS in multiple-input single-output (MISO) VLC systems are investigated in this paper. We formulate an optimization problem with asymptotic capacity maximization as the objective function and the refractive index of the LC RIS as the independent variable. As for this nonconvex optimization problem, we propose the particle swarm optimization (PSO) algorithm to determine the configuration of parameters for the LC RIS. The simulation results indicate that the employment of the LC RIS in VLC receivers can raise the communication performance of the MISO-VLC systems; meanwhile, the proposed algorithm is an effective way to deal with the optimization problems for LC RIS-based MISO-VLC systems when compared with the exhaustive search method and a baseline scheme. The LC RIS is also expected to solve the dead zone problem in traditional VLC systems.

This paper studies the use of secret key agreement for single-input single-output (SISO) visible light communication (VLC) systems. Specifically, we put forward a scheme for secret key generation and analyze the secret key rate in SISO VLC systems. First, we derive the secret key capacity bounds. Then, we analyze the achievable secret key rate distribution versus the illegal receiver’s location when the legal receiver’s location is fixed. Meanwhile, we deduce the average secret key capacity using random geometry knowledge when all the receivers’ positions are unknown. We also analyze the impact of employing a protected zone on the average secret key capacity and observe that a protected zone can obviously improve the secret performance. Finally, simulations are presented to verify the theoretical analysis.