Improved physical layer security of visible light communications with focused light emitters

A conventional secure visible light communication (VLC) system that is composed of one transmitter, one friendly jammer with multiple light-emitting-diodes, one legitimate receiver and one eavesdropper is considered. We propose the use of an optical lens at the legitimate transmitter as an additional effective and practical security measure. We investigate the secrecy rate of a Gaussian wiretap channel VLC scenario inside a room. First, we consider friendly jamming through optimal power allocation between the information and the jamming signal. Then, we investigate the improved security through the use of the optical lens into the system. The use of this collimating thin lens spatially limits the transmitted information to a confined and adjustable area that is beneath the transmitter. Through simulation results we demonstrate that the use of the optical lens can significantly improves the secrecy rate by focusing the information transmission. We also design and implement an experimental test-bed and demonstrate the applicability of the proposed approach. We experimentally validate that the system is practically realizable, and the numerical simulations are consistent with our measurement results. Although the proposed scenario necessitates the receiving unit to be located in a stationary position, we showcase that the hardware implementation is simple, practical, and significantly lower in cost as opposed to a beam-forming type VLC system that offers directional beam forming to send the information towards a dynamic receiver. Furthermore, through a careful adjustment of the distance between the transmitter and the lens, it is possible to control the size of the region where a receiver can access the data. Both experimental and simulation results show that, with the utilization of a lens at the transmitter, VLC is a promising candidate for applications where the secrecy is a critical concern.