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

翻译：本文研究了空对地（A2G）系统中的隐蔽通信问题，其中无人机（Alice）可采用全向微波（OM）或定向毫米波（DM）传输模式向地面用户（Bob）传输隐蔽数据，同时需防范对手（Willie）的检测。针对OM和DM两种模式，我们首先进行理论分析，揭示传输速率/传输功率与基本隐蔽性能指标（检测错误概率（DEP）、有效隐蔽速率（ECR）和隐蔽香农容量（CSC））之间的内在关系。为辅助Alice选择传输模式，我们进一步探索在OM和DM模式下以ECR/CSC最大化为目标的传输速率与传输功率优化问题，并提出一种混合OM/DM传输模式，该模式允许无人机根据其所在位置自适应选择OM或DM模式，以实现最大的ECR和CSC。最后，通过大量数值结果展示了所研究A2G系统在不同传输模式下的隐蔽性能，并证明混合OM/DM传输模式在隐蔽性能方面优于纯OM或DM模式。