  This paper studies energy-efficient unmanned aerial vehicle (UAV)-enabled
wireless communications, where the UAV acts as a flying base station (BS) to
serve the ground users (GUs) within some predetermined latency constraints,
e.g., requested timeout (RT). Our goal is to design the UAV trajectory to
minimize the total energy consumption while satisfying the RT requirement and
energy budget, which is accomplished via jointly optimizing the trajectory and
UAV's velocities along subsequent hops. The corresponding optimization problem
is difficult to solve due to its non-convexity and combinatorial nature. To
overcome this difficulty, we solve the original problem via two consecutive
steps. Firstly, we propose two algorithms, namely heuristic search, and dynamic
programming (DP) to obtain a feasible set of trajectories without violating the
GU's RT requirements based on the traveling salesman problem with time window
(TSPTW). Then, they are compared with exhaustive search and traveling salesman
problem (TSP) used as reference methods. While the exhaustive algorithm
achieves the best performance at a high computation cost, the heuristic
algorithm exhibits poorer performance with low complexity. As a result, the DP
is proposed as a practical trade-off between the exhaustive and heuristic
algorithms. Specifically, the DP algorithm results in near-optimal performance
at a much lower complexity. Secondly, for given feasible trajectories, we
propose an energy minimization problem via a joint optimization of the UAV's
velocities along subsequent hops. Finally, numerical results are presented to
demonstrate the effectiveness of our proposed algorithms. ...
