Aerial Relay to Achieve Covertness and Secrecy

In this work, we investigate a delay-tolerant covert and secure communication framework relayed by an uncrewed aerial vehicle (UAV). In this framework, a legitimate UAV serves as an aerial relay to facilitate communication when the direct link between the terrestrial transmitter and receiver is blocked. Additionally, the UAV also acts as a friendly jammer and adopts a random jamming transmit power technique to assist the covert communications. In order to achieve optimal trade-offs between these dual functionalities, a phase transition mechanism has been developed, regulated through a phase-switching factor. Given the uncertainty of the malicious nodes' positions, we formulate a robust fractional programming optimization problem aimed at maximizing the covert and secure energy efficiency by jointly optimizing the UAV's trajectory, the transmitter's power, and the phase-switching factor. An alternating optimization-based algorithm is then proposed to solve the fractional programming problem. To ensure low computational complexity, we solve the phase-switching factor sub-problem with a primal-dual search-based algorithm and the others with successive convex approximation-based algorithms. The effectiveness of the proposed algorithm is validated through numerical results.