Covert Air-Ground Relaying with Blockages

Although deploying unmanned aerial vehicles (UAVs) can provide line-of-sight (LoS) links to extend the coverage, it also poses severe challenges for covert air-ground transmission. In this paper, we investigate the covert air-ground finite-blocklength communication with ground blockages, where the UAV acts as an aerial relay to achieve the long-distance transmission. First, we analyze the warden detection performance with its optimal detection threshold derived, which is the worst case for the legitimate transmission. To maximize the covert transmission rate while satisfying the covertness constraint, the blocklength, the transmit power, and the position of UAV are jointly optimized. By analyzing the monotonicity of transmit power and blocklength with respect to the relative entropy, we derive their closed-form solutions. Then, we analyze the optimal hovering position of UAV from two perspectives of the distance and the elevation angle between UAV and transmitter. Specifically, we obtain an optimal elevation angle with fixed distance and the optimal distance with fixed elevation angle through analyzing the received signal-to-noise ratio and the feasible hovering regions of UAV. Numerical results demonstrate the effectiveness of the proposed covert UAV relaying scheme with ground blockages.