Results and Discussions To verify the proposed scheme, a typical office scenario is set up for simulation, where 9 LEDs are distributed on the ceiling of an indoor room. 3 LEDs are located above the office area and the rest are arranged in the public area. The average emission power of each LED is 1 W. Without loss of generalization, two legitimate users are assumed in the office area, and an unknown number of eavesdroppers randomly appear anywhere in the public area. Both legitimate users and eavesdroppers are located on a horizontal receiving plane of 0.5 m from the ground. The other parameters are given in Table 1. Simulation results show that the maximum value of the SINR of the eavesdropper in the public area is -17.22 dB when using the proposed cooperative jamming scheme (Table 2), which represents SINR of the eavesdropper in the worst case. Compared with the artificial noise-based precoding and spatial jamming schemes, the SINR of the eavesdropper decreases by 11.73 dB and 24.30 dB in the worst case, respectively (Table 2), and the SINR of the eavesdropper is lower than the traditional two schemes in the whole public area (Fig. 5). At the same time, the minimum value of the secrecy sum-rate of the system is increased by 0.39 bit·s^-1·Hz^-1 and 0.78 bit·s^-1·Hz^-1, respectively (Fig. 6). This shows that the proposed scheme outperforms the traditional two schemes when the optimization objective is to minimize the eavesdropper's SINR in the worst case. Through the joint optimization and design, the jamming signals will disturb the eavesdropper's reception to the maximum extent without affecting the legitimate user's reception.

Visible light communication (VLC) has received extensive attention in recent years owing to its numerous advantages such as abundant spectrum resources, immunity to electromagnetic interference, low cost, etc. Due to the inherent broadcast characteristics of VLC, VLC channels are inevitably susceptible to eavesdropping by potential unauthorized users who are inside the same open area illuminated by the light-emitting diode (LED) transmitters. Therefore, security of VLC systems has become an issue of critical importance and substantial efforts have been devoted to it. Among the existing security methods, physical layer security (PLS) schemes have been applied to enhance the overall system security by complementing existing cryptography-based security techniques of upper layers. PLS techniques use channel characteristics and physical-layer features (such as multi-antenna and cooperative nodes) to reduce the attained information at the eavesdroppers. Artificial noise has been emerged as a promising technique to improve the security of multi-user multiple-input multiple-output (MISO) VLC systems. Artificial noise will disturb the eavesdroppers' reception without affecting the legitimate users' signals. Most of researches on artificial noise assume a single legitimate user with an eavesdropper of unknown location, and do not consider the realistic scenarios where multiple legitimate users and multiple eavesdroppers with random locations exist. Such scenarios are common in indoor workplaces including government offices, banks, etc. To enhance the security performance under the above typical scenarios, this paper proposes an artificial noise generation scheme based on cooperative jamming to minimize the signal to interference plus noise ratio (SINR) of the eavesdropper in the worst case and improve the security performance of the VLC system.

In a MISO visible light communication system, an artificial noise generation scheme based on cooperative jamming is proposed to improve the security performance of the system when unknown number of eavesdroppers may appear anywhere in the public area. In the proposed scheme, the signal source LEDs in the legitimate user's area jointly send jamming signals with the LEDs in the public area. Through the joint design of the jammers in the two areas, the effect of the jamming signals on the legitimate user's reception can be cancelled to zero. On this basis, we formulate an optimization problem to minimize the SINR of the eavesdropper in the worst case, and use the concave-convex process (CCP) to find the optimal solution. Through the joint optimization and design of the jammers, the generated jamming signals will disturb the eavesdropper's reception to the greatest extent without affecting the legitimate users' signals, thus enhancing the secrecy sum-rate and security performance of the system.

This paper studies the physical layer security of MISO VLC systems under typical indoor office scenarios. When users and eavesdroppers are located in different areas and the number and location of eavesdroppers are random, a cooperative jamming method is proposed to generate artificial noise. On one hand, the LEDs in the office area send confidential signals required by legitimate users. On the other hand, the LEDs in the office area jointly send jamming signals with the LEDs in the public area. Through the joint optimization and design of the jammers in different areas, the jamming signals will minimize eavesdropper's SINR in the worst case without affecting the communication quality of legitimate users. Simulation results show that compared with the artificial noise-based precoding and spatial jamming schemes, the proposed cooperative jamming scheme reduces the eavesdropper's SINR in the worst case by 11.73 dB and 24.30 dB, respectively. The secrecy sum-rate has been significantly improved, thereby improving the security of the system.