  This paper studies unmanned aerial vehicle (UAV) enabled wireless
communication, where a rotarywing UAV is dispatched to send/collect data
to/from multiple ground nodes (GNs). We aim to minimize the total UAV energy
consumption, including both propulsion energy and communication related energy,
while satisfying the communication throughput requirement of each GN. To this
end, we first derive an analytical propulsion power consumption model for
rotary-wing UAVs, and then formulate the energy minimization problem by jointly
optimizing the UAV trajectory and communication time allocation among GNs, as
well as the total mission completion time. The problem is difficult to be
optimally solved, as it is non-convex and involves infinitely many variables
over time. To tackle this problem, we first consider the simple
fly-hover-communicate design, where the UAV successively visits a set of
hovering locations and communicates with one corresponding GN when hovering at
each location. For this design, we propose an efficient algorithm to optimize
the hovering locations and durations, as well as the flying trajectory
connecting these hovering locations, by leveraging the travelling salesman
problem (TSP) and convex optimization techniques. Next, we consider the general
case where the UAV communicates also when flying. We propose a new path
discretization method to transform the original problem into a discretized
equivalent with a finite number of optimization variables, for which we obtain
a locally optimal solution by applying the successive convex approximation
(SCA) technique. Numerical results show the significant performance gains of
the proposed designs over benchmark schemes, in achieving energy-efficient
communication with rotary-wing UAVs.
