This paper analyzes the performance and tradeoffs of an unmanned aerial vehicle (UAV) as a flying base station that provides wireless communications to a given geographical area, coexisting with an underlaid device-to-device (D2D) communication network. The study considers two scenarios: a static UAV and a mobile UAV. For the static case, the average coverage probability and system sum-rate for users are derived as functions of UAV altitude and D2D user density. For the mobile case, the minimum number of stop points the UAV needs to visit to fully 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 which the system sum-rate and coverage probability are maximized, depending on D2D user density. Additionally, enabling the UAV to intelligently move over the target area minimizes the total required transmit power. The tradeoff between coverage and delay, in terms of the number of stop points, is discussed. The paper also presents a system model with a circular area, UAV as a flying base station, and D2D users distributed according to a Poisson point process. The signal-to-interference-plus-noise ratio (SINR) expressions for D2D and downlink users are derived, and coverage probabilities are analyzed. The study shows that increasing UAV altitude can initially decrease D2D coverage probability but may increase it later due to higher LoS probability. The system sum-rate is derived as a function of coverage probabilities and user density. For the mobile UAV case, the minimum number of stop points is determined to achieve full coverage, and the impact of the number of stop points on delay and outage probability is analyzed. Simulation results demonstrate the impact of UAV altitude, D2D density, and SINR threshold on coverage probability and system sum-rate. The results show that increasing D2D density decreases the optimal UAV altitude for maximizing sum-rate, and reducing the fixed distance between D2D transmitters and receivers increases the system sum-rate. The paper concludes that UAV deployment with D2D communications requires careful optimization of altitude, stop points, and transmission power to balance coverage, rate, and interference.This paper analyzes the performance and tradeoffs of an unmanned aerial vehicle (UAV) as a flying base station that provides wireless communications to a given geographical area, coexisting with an underlaid device-to-device (D2D) communication network. The study considers two scenarios: a static UAV and a mobile UAV. For the static case, the average coverage probability and system sum-rate for users are derived as functions of UAV altitude and D2D user density. For the mobile case, the minimum number of stop points the UAV needs to visit to fully 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 which the system sum-rate and coverage probability are maximized, depending on D2D user density. Additionally, enabling the UAV to intelligently move over the target area minimizes the total required transmit power. The tradeoff between coverage and delay, in terms of the number of stop points, is discussed. The paper also presents a system model with a circular area, UAV as a flying base station, and D2D users distributed according to a Poisson point process. The signal-to-interference-plus-noise ratio (SINR) expressions for D2D and downlink users are derived, and coverage probabilities are analyzed. The study shows that increasing UAV altitude can initially decrease D2D coverage probability but may increase it later due to higher LoS probability. The system sum-rate is derived as a function of coverage probabilities and user density. For the mobile UAV case, the minimum number of stop points is determined to achieve full coverage, and the impact of the number of stop points on delay and outage probability is analyzed. Simulation results demonstrate the impact of UAV altitude, D2D density, and SINR threshold on coverage probability and system sum-rate. The results show that increasing D2D density decreases the optimal UAV altitude for maximizing sum-rate, and reducing the fixed distance between D2D transmitters and receivers increases the system sum-rate. The paper concludes that UAV deployment with D2D communications requires careful optimization of altitude, stop points, and transmission power to balance coverage, rate, and interference.