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Though visible light communication (VLC) systems are contained to a givenroom, improving their security is an important criterion in any practicaldeployment. In this paper, the design of artificial noise (AN) to enhancephysical layer security in VLC systems is studied in the context of inputsignals with no explicit amplitude constraint (e.g., multicarrier systems). Insuch systems, clipping is needed to constrain the input signals within thelimited linear ranges of the LEDs. However, this clipping process gives rise tonon-linear clipping distortion, which must be incorporated into the AN design.To facilitate the solution of this problem, a sub-optimal design approach ispresented using the Charnes-Cooper transformation and the convex-concaveprocedure (CCP). Then, a novel AN transmission scheme is proposed to reduce theimpact of clipping distortion, thus improving the secrecy performance. Theproposed scheme exploits the common structure of LED luminaries that they areoften composed of several light-emitting chips. Capitalizing on this property,LED chips in each luminaire are divided into two groups driven by separatedriver circuits. One group is used to transmit the information-bearing signal,while the other group transmits the AN. Numerical results show that theclipping distortion significantly reduces the secrecy level, and using AN isadvantageous over the no-AN scheme in improving the secrecy performance.Moreover, the proposed AN transmission scheme is shown to achieve considerablesecrecy improvements compared with the traditional transmission approaches(e.g., about 1 bit/s/Hz improvement in the achievable secrecy rate when thestandard deviation of the LEDs' modulating current is 0.25 A and thesignal-to-interference-plus-noise ratio of the eavesdropper's received signalis limited to $0$ dB).

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