Secure Precoding and User Association for Multiuser Hybrid RF/VLC Systems

To overcome the limitations of visible light communications (VLC) systems while still benefiting from the high data rates and inherently secure transmissions, hybrid systems combining radio-frequency (RF) and VLC have gained appealing towards future generation of wireless networks. In this paper, the potential of hybrid systems from the security perspective is investigated by proposing a joint precoding and user association (JPUA) scheme to maximize the sum secrecy rate of a multiuser hybrid RF/VLC system. Specifically, it is assumed that the RF access point is equipped with multiple antennas while the VLC access point is equipped with multiple light emitting diodes. The optimization problem is divided into two sub-problems, the precoding design and the user association strategy. The former leads to a non-convex fractional programming problem, which is reformulated using the constrained convex-concave procedure, thus efficiently converging to an optimal solution. For solving the latter, an algorithm based on coalitional game theory is proposed. Numerical results show that the JPUA scheme provides significant gains in terms of the sum secrecy rate in comparison to different baseline schemes.