Multi-UAV Replacement and Trajectory Design for Coverage Continuity

In this paper, we present a novel framework to maintain coverage continuity in a unmanned aerial vehicle (UAV)-assisted wireless communication system, when a serving UAV runs out of energy. Service continuity is maintained by launching another fully charged UAV to replace the existing serving UAV. This replacement process must ensure maximal coverage to all ground users. Our objective during this replacement process is to maximize the achievable sum rate of all ground users by jointly optimizing the three-dimensional (3D) multi-UAVs trajectory and resource allocation to the users from the individual UAVs. This is carried out in the presence of the UAV’s constraints on velocity, collision avoidance, and energy availability while considering a more practical and accurate probabilistic line-of-sight (LoS) channel model. This results in a non-convex optimization problem for which an efficient iterative algorithm based on successive convex approximation and alternating optimization is proposed. Numerical results are provided to obtain insights on the UAV trajectories and the effectiveness of the proposed scheme compared to the existing benchmark schemes is shown.