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This paper investigates the covert communication in an air-to-ground (A2G)system, where a UAV (Alice) can adopt the omnidirectional microwave (OM) ordirectional mmWave (DM) transmission mode to transmit covert data to a grounduser (Bob) while suffering from the detection of an adversary (Willie). Forboth the OM and DM modes, we first conduct theoretical analysis to reveal theinherent relationship between the transmit rate/transmit power and basic covertperformance metrics in terms of detection error probability (DEP), effectivecovert rate (ECR), and covert Shannon capacity (CSC). To facilitate thetransmission mode selection at Alice, we then explore the optimization oftransmit rate and transmit power for ECR/CSC maximization under the OM and DMmodes, and further propose a hybrid OM/DM transmission mode which allows theUAV to adaptively select between the OM and DM modes to achieve the maximum ECRand CSC at a given location of UAV. Finally, extensive numerical results areprovided to illustrate the covert performances of the concerned A2G systemunder different transmission modes, and demonstrate that the hybrid OM/DMtransmission mode outperforms the pure OM or DM mode in terms of covertperformance.

Covert Communication in Hybrid Microwave/mmWave A2G Systems with Transmission Mode Selection