On the Design of Artificial Noise for Physical Layer Security in Visible Light Communication Channels With Clipping

Though visible light communication (VLC) systems are contained to a given room, improving their security is an important criterion in any practical deployment. This paper studies the design of artificial noise (AN) to enhance physical layer security in VLC systems in the context of input signals with no explicit amplitude constraint (e.g., multicarrier systems). In such systems, clipping is needed to constrain the input signals within the limited linear ranges of the LEDs. However, this clipping process gives rise to non-linear clipping distortion, which must be incorporated into the AN design. To solve the design problem, a sub-optimal approach is presented using the Charnes-Cooper transformation and the convex-concave procedure (CCP). Then, a novel AN transmission scheme is proposed to reduce the impact of clipping distortion, thus improving the secrecy performance. The proposed scheme exploits the typical structure of LED luminaries that are composed of multiple light-emitting chips. Specifically, LED chips in each luminaire are divided into two groups driven by separate driver circuits. One group is used to transmit the information-bearing signal, while the other group transmits the AN. Numerical results show that while clipping distortion can significantly reduce the secrecy level, with proper design AN significantly improves the secrecy performance using the proposed design methodology.