Air-to-Ground Two-Phase Covert Transmission Aided by a Full-Duplex UAV Relay

Abstract

This paper investigates an air-to-ground two phase covert transmission (AtG-TPCT) system, which is associated with an unmanned aerial vehicle transmitter Alice (UAV-A), a fullduplex UAV relay (UAV-R) and multiple uncertainly located wardens. Accordingly, given the bounded location uncertainty of wardens, we formulate a robust average covert rate (ACR) maximization problem by jointly optimizing the 3D trajectory of UAV-R, and the transmit powers of UAV-A and UAV-R, with the worst-case covertness constraint. The problem is explicitly non-convex due to factors that, the covert end-to-end achievable rate is coupled with the two adjacent legitimate transmission phases, while the coupled dependence is observed among the relay self-interference, transmit powers, and the UAV-R trajectory. To tackle this issue, an iterative optimization algorithm is developed based on block coordinate descent and successive convex approximation. Numerical results demonstrate that, regarding the performance metric of ACR, the proposed AtG-TPCT is capable of achieving rapid convergence to its optimal covertness, while can distinctively outperform in performance with its benchmark counterpart.