The use of unmanned aerial vehicles (UAVs) is growing rapidly across many civil application domains, including providing wireless coverage. Most studies on UAV-based wireless coverage typically consider downlink scenarios from an aerial base station to ground users. The uplink scenario in which ground wireless devices transmit data to an aerial base station is only considered by few studies. However, the frequency bands that are used in these studies are not Millimeter Wave frequency bands, and this limits the applicability of these applications when one needs to consider 5G networks. In this paper, we are motivated to explore if the placement of UAV can enhance the time durations of uplink transmissions of wireless devices in Millimeter Wave UAV networks. First, we present a realistic Millimeter Wave path loss model and describe the tradeoff introduced by this model. Then, we study the problem of optimal UAV placement, where the objective is to determine the placement of a single UAV such that the sum of time durations of uplink transmissions is maximized. To this end, an algorithm to find the optimal UAV location is proposed. Simulation results are presented to validate the effectiveness of the proposed algorithm.
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