Joint Altitude and Power Optimization for Multi-UAV-Aided Covert Communication with Relay Selection

Unmanned aerial vehicles (UAVs) are pivotal for 6G ubiquity, yet their open line-of-sight channels increase vulnerability to interception, posing new challenges for covert communication. This paper proposes a joint optimization scheme for multi-UAV relay-assisted covert communication system with the maximum channel capacity relay selection (MCRS) criterion. Distinct from conventional single-UAV approaches, this scheme uniquely couples UAV geometric positions with the time-varying characteristics of the wireless channels, exploiting spatial diversity from UAV relays to mitigate small-scale fading in dense urban environment, and jointly optimizes the transmit power and UAVs’ altitude. Specifically, we first designed an optimal relay selection strategy and derived analytical expressions for detection error and outage probabilities over altitude-dependent Nakagami-m fading channels. Furthermore, we maximized the effective covert rate by jointly optimizing the UAVs’ hovering altitude and adaptive transmit power of source and relays, subject to covert constraints. Extensive numerical results demonstrate a near-perfect match between the derived theoretical expressions and Monte Carlo simulations and validate the accuracy of our theoretical model. Compared against conventional single-UAV and multi-fixed-altitude UAV benchmark schemes, simulations demonstrate that the joint optimization scheme with relay selection proposed significantly enhances the covert performance of UAV-assisted communication systems.