  Wireless access points on unmanned aerial vehicles (UAVs) are being
considered for mobile service provisioning in commercial networks. To be able
to efficiently use these devices in cellular networks it is necessary to first
have a qualitative and quantitative understanding of how their design
parameters reflect on the service quality experienced by the end user. In this
paper we model a network of UAVs operating at a certain height above ground to
provide wireless service within coverage areas shaped by their directional
antennas, with the UAVs using the existing terrestrial base station network for
wireless backhaul. We provide an analytical expression for the coverage
probability experienced by a typical user as a function of the UAV parameters.
Using our derivations we demonstrate the existence of an optimum UAV height
which maximises the end user coverage probability. We then explore a scenario
where the UAVs adjust their individual heights to meet their backhaul
requirements while at the same time attempting to maximise the coverage
probability of the end user on the ground.
