Physical Layer Security For Multiple User Radio Frequency/Visible Light Communication Heterogeneous Networks

The sixth generation(6G)mobile communication proposed the utilization of new frequency bands to meet the needs of massive access and ultra-high-rate transmission.Therefore,visible light communication(VLC)characterized by license-free spectrum is expected to become one of the key technologies supporting 6G,providing users with highspeed data communication link.However,VLC is limited to indoor applications with short distance and coverage,and is only suitable for link-of-sight transmission between transceiver devices.An effective solution to this challenge is to adopt radio frequency(RF)/VLC heterogeneous network,using RF for ubiquitous coverage while using VLC for high-speed communication so as to improve the quality of service for users.On the other hand,the broadcast characteristic of VLC and RF wireless channels makes information vulnerable to eavesdropping.Based on the differences of wireless channels,the physical layer security(PLS)technology can distinguish different receivers,and design secure transmission schemes for legitimate users to ensure information transmission security in the physical layer.So far,most studies on the PLS of RF/VLC heterogeneous networks focused on the external eavesdropping security problem of single-user scenario,and rarely considers the internal and external eavesdropping security problems of multi-user scenario,whereas in real communication scenarios such as banks,airports and hospitals,it is necessary to consider the security needs of massive users.Thus,this paper mainly conducts a study on the PLS of multi-user RF/VLC heterogeneous networks,providing a theoretical basis for enhancing the communication security of indoor heterogeneous networks.The main work contents are as follows:(1)Assuming that the user receiver is capable of aggregating RF and VLC information,the PLS of RF/VLC heterogeneous networks with multi-user and single eavesdropper is studied.Firstly,zero forcing(ZF)beamforming is used to make the legitimate users receive signal positioned in the null space of eavesdropper’s channel,and eliminate the interference between users so that messages of each user are kept confidential from other users and the external eavesdropper.Then,a sum secrecy rate(SSR)non-convex optimization problem is formulated under the ZF constraints and the constraints of the power of the access point.The optimization problem is decomposed into two convex problems,the CVX toolbox and Karush-Kuhn-Tucker(KKT)are used to obtain the optimal solution of the original problem.Simulation results show that a positive SSR can always be achieved,and the PLS of heterogeneous network is better than that of standalone RF,standalone VLC,RF/RF and VLC/VLC network.(2)Considering that the user receiver is incapable of aggregating RF and VLC information in practical applications,the PLS of multi-user single eavesdropper RF/VLC heterogeneous networks based on secure link selection is studied.Firstly,a mixed-integer nonlinear programming problem of SSR maximization is formulated by joint optimization of access point selection and power allocation.Usually,it is difficult to obtain the optimal solution directly.Thus,it is decomposed into two sub-problems: one is the power allocation problem of access point for its associated users,and the other is the problem of secure access point selection.Then,they are solved by power allocation algorithm and a sequential game of coalition formation algorithm.Finally,simulation results show that the secrecy performance of the proposed secure access point selection algorithm is always superior to the algorithms of all VLC or all RF;the secrecy performance of the proposed algorithm is directly proportional to the user number,so the proposed scheme is more suitable for multiuser dense communication scenarios.