  Enabling the integration of aerial mobile users into existing cellular
networks would make possible a number of promising applications. However,
current cellular networks have not been designed to serve aerial users, and
hence an exploration of design parameters is required in order to allow network
providers to modify their current infrastructure. As a first step in this
direction, this paper provides an in-depth analysis of the coverage probability
of the downlink of a cellular network that serves both aerial and ground users.
We present an exact mathematical characterization of the coverage probability,
which includes the effect of base stations (BSs) height, antenna pattern and
drone altitude for various type of urban environments. Interestingly, our
results show that the favorable propagation conditions that aerial users enjoys
due to its altitude is also their strongest limiting factor, as it leaves them
vulnerable to interference. This negative effect can be substantially reduced
by optimizing the flying altitude, the base station height and antenna
down-tilt. Moreover, lowering the base station height and increasing down-tilt
angle are in general beneficial for both terrestrial and aerial users, pointing
out a possible path to enable their coexistence.
