  The use of drone base stations to provide wireless connectivity for ground
terminals is becoming a promising part of future technologies. The design of
such aerial networks is however different compared to cellular 2D networks, as
antennas from the drones are looking down, and the channel model becomes
height-dependent. In this paper, we study the effect of antenna patterns and
height-dependent shadowing. We consider a random network topology to capture
the effect of dynamic changes of the flying base stations. First we
characterize the aggregate interference imposed by the co-channel neighboring
drones. Then we derive the link coverage probability between a ground user and
its associated drone base station. The result is used to obtain the optimum
system parameters in terms of drones antenna beamwidth, density and altitude.
We also derive the average LoS probability of the associated drone and show
that it is a good approximation and simplification of the coverage probability
in low altitudes up to 500 m according to the required
signal-to-interference-plus-noise ratio (SINR).
