Trajectory Planning of Unmanned Aerial Relays for Just-in-Time Connectivity in the Sky

Recently, unmanned aerial vehicles (UAVs) have shown potential for a variety of emerging applications with their high mobility and versatility. To conduct the tasks and receive control and command signals, the seamless connectivity for the UAVs is crucial. While the aerial connection can be provided by the existing ground cellular network, the down-tilted antennas of the ground base stations (GBSs) and the beam lobes generated by the antenna pattern lead to inevitable null areas in the airspace, making it challenging to ensure connectivity and safety in the sky. In this paper, we leverage an unmanned aerial relay (UAR) to extend the signal coverage of existing GBSs, enabling connectivity for aerial user equipments (AUEs) in aerial null areas. We design an efficient algorithm, Connectivity-Aware UAR (CARE-U), optimizing the UAR's trajectory to serve AUEs with pre-scheduled tasks. By simulations in both synthetic and realworld settings, the connected time ratio of the AUEs improves up to 66.3 % compared to that without the UAR. Besides, CARE-U outperforms the approach that modifies trajectories of AUEs, showing the great compatibility of CARE-U.