Unmanned aerial vehicle (UAV) radio frequency (RF) communication is vulnerable to attacks such as interference and eavesdropping due to its open channel and broadcast nature, which poses significant threats to communication reliability and security. The introduction of artificial interference can conceal communication activity by confusing malicious users on the legitimate communication process, thus ensuring communication security. However, both artificial interference and legitimate signals operate within the RF band, which can reduce the communication rate of legitimate signals. Free-space optical (FSO) communication offers high transmission rates, resistance to RF interference, and greater security against eavesdropping. However, it requires precise alignment and is sensitive to channel conditions. We propose a UAV covert communication strategy based on a hybrid RF/FSO link, where FSO is used to compensate for RF communication to mitigate the self-interference effects of artificial interference on RF signals.

Under the condition that the hostile user implements communication monitoring, we focus on the UAV relay communication scenario, specifically by adding a relay UAV to assist the reconnaissance UAV in realizing information transmission to the ground receiving station. Communication between the UAVs is realized through RF and FSO links, with the relay UAV generating artificial interference to enable covert communication. The maximum ratio combination (MRC) scheme is adopted to process the received RF/FSO signals. The information is transmitted to the receiving station over the RF link using the decode-and-forward protocol. In this paper, we first derive the minimum total detection error probability of the hostile user, then construct an optimization model with covert communication requirements, transmit power, and deployment location as constraints, and link transmission rate as the objective. An iterative solution algorithm based on block coordinate descent (BCD) and successive convex approximation (SCA) is proposed to obtain a suboptimal solution through the joint optimization of UAV relay deployment location, interference, and RF/FSO transmit power.

With the increase in the covert communication requirement, the transmit power of both RF and FSO signals shows a decreasing trend. However, as the maximum transmit power of the UAV increases, the transmit power of RF and FSO signals shows an increasing trend. The trend in the transmit power of RF signals is mainly affected by the transmit power of artificial interference, and it has a linear relationship with both variables. The transmit power of FSO signals varies with the transmit power of RF signals, which exhibits a linear relationship between the square of FSO transmit power and RF transmit power (Figs. 2 and 3). As the covert communication requirement rises, the transmit power of artificial interference decreases, though its rate of change increases. To satisfy heightened covert communication needs, both the transmit power of artificial interference and RF signals can be appropriately reduced. When the maximum transmit power increases linearly, artificial jamming power also rises linearly, as the fixed demand for covert communication requires only the jamming and RF signals to adjust linearly (Fig. 4). The proposed strategy leverages the fast transmission rate of the FSO signal, which maintains around 4 Mbit/s under various conditions, while the baseline scheme struggles to maintain approximately 0.03 Mbit/s due to interference on the RF link. Furthermore, increasing the maximum transmit power enhances the FSO signal’s transmission rate by allowing a corresponding rise in RF signal power. In contrast, the baseline scheme’s transmission rate remains largely constant due to the linear increase in both interference and RF transmit power (Figs. 5 and 6).

We propose a covert communication strategy for UAV communication based on a hybrid RF/FSO link to achieve both covertness and effectiveness. We construct an optimization model with constraints on covert communication requirements, transmit power, and deployment position while aiming to maximize the link transmission rate. An iterative solution algorithm based on BCD and SCA is introduced for the joint optimization of deployment location, artificial interference, and RF/FSO power. Simulation results indicate that a transmission rate of 4 Mbit/s can be achieved with the hybrid RF/FSO link, which significantly outperforms schemes that rely solely on RF links. Furthermore, increasing the UAV’s maximum transmit power can further enhance the transmission rate. The proposed algorithm reformulates the problem as a convex optimization task, which allows for polynomial-time solutions and convergence to approximate optimal solutions, in contrast to traditional heuristic methods. Future research will focus on the flexible allocation of RF/FSO power under complex channel conditions and on extending RF/FSO cooperative covert communication to multi-user or network scenarios, thereby improving the technology’s adaptability to various communication environments.