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In this paper we study the impact of unmanned aerial vehicles (UAVs)trajectories on terrestrial users' spectral efficiency (SE). Assuming a strongline of sight path to the users, the distance from all users to all UAVsinfluence the outcome of an online trajectory optimization. The trajectoryshould be designed in a way that the fairness rate is maximized over time. Thatmeans, the UAVs travel in the directions that maximize the minimum of theusers' SE. From the free-space path-loss channel model, a data-rate gradient iscalculated and used to direct the UAVs in a long-term perspective towards thelocal optimal solution on the two-dimensional spatial grid. Therefore, acontrol system implementation is designed. Thereby, the UAVs follow thedata-rate gradient direction while having a more smooth trajectory comparedwith a gradient method. The system can react to changes of the user locationsonline; this system design captures the interaction between multiple UAVtrajectories by joint processing at the central unit, e.g., a ground basestation. Because of the wide spread of user locations, the UAVs end up inoptimal locations widely apart from each other. Besides, the SE expectancy isenhancing continuously while moving along this trajectory.

Base-Stations Up in the Air: Multi-UAV Trajectory Control for Min-Rate Maximization in Uplink C-RAN