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

<p>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 appeal towards the next 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 user association strategy and the precoding design for each of the access points. For the former, an algorithm based on coalitional game theory is proposed. The latter leads to a non-convex fractional programming problem, which is reformulated using the constrained convex-concave procedure, thus efficiently converging to an optimal solution. Numerical results show that the proposed hybrid RF/VLC scheme provides significant gains in terms of sum secrecy rate, compared to different baseline schemes, such as the VLC or RF standalone schemes, in addition to allowing the connection of a higher number of users.</p>