The optimal placement of multiple unmanned aerial vehicles (UAVs) equipped with directional antennas acting as wireless base stations that provide coverage to several ground users is analyzed. In this research, the optimal placement of unmanned aerial base stations that provide coverage for the ground terminals (users) is analyzed and uses the case study of an MTN malfunctioning terrestrial base station where UAVs are deployed to give coverage to the ground users as the base station problem is being rectified by the maintenance MTN engineers First, the downlink coverage probability as a function of altitude and antenna gain is derived based on the probabilistic line of sight or non-line of sight links (Los/NLoS) links. Next, using circle packing theory, the 3-D locations of the UAVs are determined in the way that the total coverage area is maximized while maximizing the coverage lifetime of the UAVs. Given a desired geographical area that needs to be covered by multiple UAVs, an efficient deployment approach is proposed based on the circle packing theory that leads to maximum coverage while each UAV uses a minimum transit power. The results show that the optimal altitude and locations of the UAVs can be determined based on the number of available UAVs, the antenna gain, and beamwidth. Results also show that to mitigate interference, the altitude of the UAVs must be properly adjusted based on the beamwidth of the directional antenna as well as the coverage requirements. Furthermore, the minimum number of UAVs required to guarantee a target coverage probability for a given area is determined.

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