  In this paper, we propose an energy-efficient optimal altitude for an aerial
access point (AAP), which acts as a flying base station to serve a set of
ground user equipment (UE). Since the ratio of total energy consumed by the
aerial vehicle to the communication energy is very large, we include the aerial
vehicle's energy consumption in the problem formulation. After considering the
energy consumption model of the aerial vehicle, our objective is translated
into a non-convex optimization problem of maximizing the global energy
efficiency (GEE) of the aerial communication system, subject to altitude and
minimum individual data rate constraints. At first, the non-convex fractional
objective function is solved by using sequential convex programming (SCP)
optimization technique. To compare the result of SCP with the global optimum of
the problem, we reformulate the initial problem as a monotonic fractional
optimization problem (MFP) and solve it using the polyblock outer approximation
(PA) algorithm. Numerical results show that the candidate solution obtained
from SCP is the same as the global optimum found using the monotonic fractional
programming technique. Furthermore, the impact of the aerial vehicle's energy
consumption on the optimal altitude determination is also studied.
