This paper analyzes the deployment of an unmanned aerial vehicle (UAV) as a flying base station to provide wireless communications to a given geographical area, focusing on the co-existence between the UAV and an underlaid device-to-device (D2D) communication network. The authors derive an analytical framework for coverage and rate analysis, considering two scenarios: a static UAV and a mobile UAV. For the static UAV, the average coverage probability and system sum-rate are derived as functions of the UAV altitude and D2D user density. For the mobile UAV, the minimum number of stop points required to cover the area is computed using the disk covering problem. The overall outage probability of D2D users is also derived, considering multiple retransmissions. Simulation and analytical results show that optimal UAV altitudes exist for maximizing system sum-rate and coverage probability, depending on D2D user density. Additionally, the total transmit power of the UAV is minimized by intelligently moving over the target area. The tradeoff between coverage and delay, in terms of the number of stop points, is discussed.This paper analyzes the deployment of an unmanned aerial vehicle (UAV) as a flying base station to provide wireless communications to a given geographical area, focusing on the co-existence between the UAV and an underlaid device-to-device (D2D) communication network. The authors derive an analytical framework for coverage and rate analysis, considering two scenarios: a static UAV and a mobile UAV. For the static UAV, the average coverage probability and system sum-rate are derived as functions of the UAV altitude and D2D user density. For the mobile UAV, the minimum number of stop points required to cover the area is computed using the disk covering problem. The overall outage probability of D2D users is also derived, considering multiple retransmissions. Simulation and analytical results show that optimal UAV altitudes exist for maximizing system sum-rate and coverage probability, depending on D2D user density. Additionally, the total transmit power of the UAV is minimized by intelligently moving over the target area. The tradeoff between coverage and delay, in terms of the number of stop points, is discussed.