  Though visible light communication (VLC) systems are contained to a given
room, ensuring their security amongst users in a room is essential. In this
paper, the design of artificial noise (AN) to enhance physical layer security
in VLC systems is studied in the context of input signals with no explicit
amplitude constraint (such as 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 facilitate the
solution of this problem, a sub-optimal design approach is presented using the
Charnes-Cooper transformation and the convex-concave procedure (CCP). Numerical
results show that the clipping distortion significantly reduces the secrecy
level, and using AN is advantageous over the no-AN scheme in improving the
secrecy performance.
