Abstract-In terrestrial communication networks without fixed infrastructure, unmanned aerial vehicle (UAV)-mounted mobile base stations (MBSs) provide an efficient solution to achieve wireless connectivity. This letter aims to minimize the number of MBSs needed to provide wireless coverage for a group of distributed ground terminals (GTs), ensuring that each GT is within the communication range of at least one MBS. We propose a polynomial-time algorithm with successive MBS placement, where the MBSs are placed sequentially starting on the area perimeter of the uncovered GTs along a spiral path towards the center, until all GTs are covered. Each MBS is placed to cover as many uncovered GTs as possible, with higher priority given to the GTs on the boundary to reduce the occurrence of outlier GTs that each may require one dedicated MBS for its coverage. Numerical results show that the proposed algorithm performs favorably compared to other schemes in terms of the total number of required MBSs and/or time complexity. Index Terms-Unmanned aerial vehicles, mobile base station placement, user coverage, geometric disk cover problem I. INTRODUCTION With their maneuverability and increasing affordability, unmanned aerial vehicles (UAVs) have many potential applications in wireless communication systems [1] . In particular, UAVmounted mobile base stations (MBSs) can be deployed to provide wireless connectivity in areas without infrastructure coverage such as battlefields or disaster scenes. Unlike terrestrial base stations (BSs), even those mounted on ground vehicles, UAV-mounted MBSs can be deployed in any location and move along any trajectory constrained only by their aeronautical characteristics, in order to cover the ground terminals (GTs) in a given area based on their known locations. When the UAV-GT channels are dominated by line-of-sight (LOS) links, the authors in [2] use a K-means clustering algorithm to partition the GTs to be served by p UAVs, while each UAV has a capacity constraint and the unsupported GTs are served by the fixed ground BSs. The authors in [3] adopt a probabilistic LOS channel model and study the 3-dimensional (3D) placement of a single aerial BS to offload as many GTs as possible from the ground BS. In this letter, we assume that the GT locations are known and the UAVs are flying at a fixed altitude H, while the UAV-GT channels are dominated by LOS links whose channel quality mainly depends on the UAV-GT distance. We consider the scenario where no ground BSs are available and the UAVmounted MBSs are backhaul-connected via satellite links, while each MBS has an equivalent coverage radius of r projected on the ground, as shown in Fig. 1 . We thereby focus on the MBS placement problem to provide wireless coverage for all GTs in a given area. This can be formulated as the Geometric Disk Cover (GDC) problem [4] , whose objective is to cover a set of K nodes (GTs) in a region with the minimum number of
In REF , a polynomial-time algorithm for the optimal placement of drones that provide coverage for ground terminals is proposed.
Placement Optimization of UAV-Mounted Mobile Base Stations
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