Physical-Layer Security for Mobile Users in NOMA-Enabled Visible Light Communication Networks

In view of the secrecy requirements of visible light communication (VLC) and the massive connectivity demand of future communication, this work investigates physical-layer security (PLS) for a VLC network based on nonorthogonal multiple access (NOMA). To date, however, research on PLS in NOMA-enabled VLC networks considers only users in a static state. The movements of users make it imperative to dynamically allocate optical access points (OAPs) and transmit power to mobile users (MUs). Such a resource allocation problem is transformed into the problem of dynamically allocating power in this paper. Thus, joint secure communication and power allocation optimization is formulated to maximize the network secrecy performance in each time frame subject to the constraint of the maximum power of OAPs and the constraint of power allocation based on NOMA among the associated MUs at each OAP. The formulated joint optimization problem is generally nonconvex because the logarithmic subtraction operation exists in the secrecy capacity, and we cannot directly find the optimal solution. A hierarchical power allocation algorithm is naturally proposed based on an iterative security-aware water-filling approach and the optimality conditions of Karush-Kuhn-Tucker. Convergence and effectiveness are investigated for the presented power allocation algorithms through simulations. The simulation results show that the network sum secrecy capacity depends on the number of MUs, the characteristics of the optical transceiver, and the number of OAPs deployed in the room.