Joint 3-D Trajectory and Power Optimization for Dual-UAV-Assisted Short-Packet Covert Communications

This article investigates a dual-unmanned aerial vehicle (UAV)-assisted short-packet covert communication system in the presence of a warden. Specifically, one flying UAV serves as the base station to transmit covert information to a legitimate ground user, and the other flying UAV is deployed as a cooperative jammer to transmit artificial noise against detection by a warden. Considering a more practical scenario, where only imperfect location information of the warden is known at both UAVs, we jointly optimize both UAVs' transmit powers and 3-D trajectories to maximize the average covert transmission rate under the constraints of both UAVs' mobility, transmit powers and warden's detection error probability (DEP). On the one hand, the incomplete Gamma function involved in warden's DEP makes the covertness constraint intractable for further analysis. To facilitate the design, warden's DEP is lower bounded by Pinsker's inequality. On the other hand, the formulated optimization problem is intractable to solve directly owing to the multiple highly coupled variables and the uncertainty of the warden's location. The alternating optimization algorithm combined with the successive convex approximation and $\mathcal {S}$ -procedure techniques is leveraged to solve three optimization subproblems iteratively. Numerical results reveal that superior performance can be achieved by employing the proposed joint optimization of both UAVs' transmit powers and 3-D trajectories algorithm compared with the traditional 2-D trajectory optimization algorithm and the scheme without the assistance of the cooperative UAV.