  Due to the fact that the locations of base stations (BSs) cannot be changed
after they are installed, it is very difficult to communicate directly with
remote user equipment (UE), which will directly affect the lifespan of the
system. Unmanned aerial vehicles (UAVs) offer a hopeful solution as mobile
relays for fifth-generation wireless communications due to the flexible and
cost-effective deployment. However, with the limited onboard energy of UAV and
slow progress in energy storage technology, it is a key challenge to achieve
the energy-efficient communication. Therefore, in this article, we study a
wireless communication network using a UAV as a high-altitude relay, and
formulate a UAV relay deployment optimization problem (URDOP) to minimize the
energy consumption of system by optimizing the deployment of UAV, including the
locations and number of UAV hover points. Since the formulated URDOP is a
mixed-integer programming problem, it presents a significant challenge for
conventional gradient-based approaches. To this end, we propose a self-adaptive
differential evolution with a variable population size (SaDEVPS) algorithm to
solve the formulated URDOP. The performance of proposed SaDEVPS is verified
through simulations, and the results show that it can successfully decrease the
energy consumption of system when compared to other benchmark algorithms across
multiple instances.
