  In this paper, an energy-efficient precoding scheme is designed for
multi-user visible light communication (VLC) systems in the context of physical
layer security, where users' messages are kept mutually confidential. The
design problem is shown to be non-convex fractional programming, therefore
Dinkelbach algorithm and convex-concave procedure (CCCP) based on the
first-order Taylor approximation are utilized to tackle the problem. Numerical
results are performed to show the convergence behaviors and the performance of
the proposed solution for different parameter settings.
