Bidirectional Air–Ground Covert Communications for Low-Altitude Networks: A Joint Optimization Approach

The development of the low-altitude networks (LANets) is highly dependent on the autonomous aerial vehicle (AAV)-related industries. However, the openness of the signal propagation environment and the high possibility of line-of-sight (LOS) in air-ground communications may lead to illegal eavesdropping and surveillance by wardens. Covert communication is a rising technology that hides the presence of information transmission, which provides more secure protection than physical layer security. In contrast to the existing studies which only consider one-way covert communication between AAV and users, this paper studies a bidirectional air-ground covert communication system. Specifically, the warden would detect both the uplink and downlink of air-ground communication. To enhance the covertness of the bidirectional communication between AAV and ground users, a joint user association, the transmit power allocation of users and AAV, as well as AAV trajectory optimization scheme is proposed subject to the bidirectional covertness constraints. A weighted average covert transmission rate (WACTR) is maximized as the goal, which is the weighted average of uplink and downlink transmission rates. To tackle with this mixed-integer non-convex problem, we design a block coordinate descent (BCD) method based iterative algorithm to optimize four sub-problems alternately. Extensive experimental results verify the convergence of the designed algorithm and the superiority of the proposed scheme over other benchmarks in terms of WACRT.