At present, the explosive growth of information has prompted people to pay more attention to the security and privacy protection of data transmission. However, most of the existing research on wireless communication security focuses on protecting the communication content. In the smart business, medical, military and other fields of the future, protecting the content of communications is clearly not enough to meet all the challenges; We also need to strengthen the protection of the act of communication itself. As a kind of communication method, covert communication has attracted much attention in recent years, its core goal is to prevent the existence of any communication activities from being detected by the monitor, so as to ensure the safe and reliable transmission of secret information.
Visible light communication (VLC), as one of the most potential optical wireless communication methods in the future 6G era, has attracted much attention because of its unique physical characteristics and application prospects. Especially in the field of covert communication, VLC-based technologies have aroused wide interest among researchers. For example, the literature discusses the need for optimal input signal distribution and transmitter average optical power in covert VLC, and shows that the performance of covert VLC can exceed the limits of the square root rule proposed by Bash et al. In addition, the literature focuses on DRone-assisted covert VLC systems, which enhance the covert effect of the system by optimizing the hovering height of the UAV to maximize mutual information. Although the above studies have laid the theoretical foundation for covert VLC, they are mainly focused on a single user scenario. Considering the increasing demand of large-scale connection in real life, hidden VLC in multi-user communication scenario is more realistic. Non-orthogonal Multiple access (NOMA), as an advanced multi-user access technology, can not only support large-scale connections, but also significantly improve the hidden throughput of the system.
However, due to the differences in channel characteristics and signal characteristics between VLC systems and radio frequency (RF) systems, it is not suitable to directly apply the covert communication strategy in RF systems to VLC. NOMA technology allows public users and hidden users to transmit information on the same spectrum resource block through the superposition coding of the sending end. This feature makes the communication activities of public users can cover hidden users, and increases the difficulty of monitoring the hidden communication behavior. Therefore, introducing NOMA technology into covert VLC can not only promote the development of secure communication technology, but also bring new covert communication mechanisms and technical innovations.
On the other hand, in the event that a VLC access point on the ground or indoors is damaged due to force majeure, communication can be assisted by a mobile device (such as an unmanned aerial vehicle (UAV)) carrying a light emitting diode (LED) transmitter as a temporary base station. This UAV-assisted mobile VLC solution is particularly suitable for night search and rescue, emergency communications and other scenarios. According to the existing literature, most of the research on mobile VLC focuses on maximizing network throughput, minimizing transmit power and expanding user coverage by optimizing the location of UAVs. However, these studies mainly focus on the problem of covert communication in single-user scenarios, such as the content discussed in the literature. At present, there is no relevant research on UAV assisted mobile VLC covert communication in multi-user scenarios.
Based on the covert communication of mobile VLC based on NOMA, the problem of maximizing the effective covert rate of the covert user under the joint constraints of concealment, reliability and mobility is constructed. Based on the graphic method, the optimal transmission power distribution ratio of the LED transmitter and the optimal UAV position to maximize the effective covert rate of the covert user are obtained. Compared with the benchmark scheme and OMA-VLC scheme, it is verified that the proposed scheme can significantly improve the effective hiding rate of hidden users. The simulation results also show that VLC parameters such as LED half power half Angle, PD field of view, and vertical height between LED and PD affect the concealment performance of the system to some extent. In order to ensure effective concealment communication, these parameters should not be designed too large.
In the future work, the visible light covert communication and laser covert communication of multiple covert users of NOMA will be studied, and field verification will be carried out to further prove the practical value of the system.