Fluid antenna systems with terrestrial interference for countering UAV covert communications

This paper studies covert communications in the presence of an unmanned aerial vehicle (UAV) acting as an aerial warden, where a terrestrial transmitter communicates with a legitimate terrestrial receiver under ground interference. To enhance transmission reliability, the legitimate receiver employs a fluid antenna (FA) with port selection based on the maximum signal-to-interference-plus-noise ratio (SINR). The UAV adopts an energy-detection-based hypothesis test to detect the covert transmission. Closed-form expressions for the false alarm and missed detection probabilities are derived, based on which the covertness outage probability (COP) is characterized. To tackle the analytical challenges introduced by spatially correlated FA channels, an eigenvalue-based weighted approximation is adopted to evaluate the outage probability at the legitimate receiver. Based on the derived outage and missed detection probability expressions, the success probability of covert communications is further characterized. Numerical results demonstrate that the proposed FA-assisted scheme significantly improves covert communications, and the eigenvalue-based weighted approximation remains highly accurate under various system parameter settings.