  The use of unmanned aerial vehicles (UAVs) that serve as aerial base stations
is expected to become predominant in the next decade. However, in order for
this technology to unfold its full potential it is necessary to develop a
fundamental understanding of the distinctive features of air-to-ground (A2G)
links. As a contribution in this direction, this paper proposes a generic
framework for the analysis and optimization of the A2G systems. In contrast to
the existing literature, this framework incorporates both height-dependent path
loss exponent and small-scale fading, and unifies a widely used
ground-to-ground channel model with that of A2G for analysis of large-scale
wireless networks. We derive analytical expressions for the optimal UAV height
that minimizes the outage probability of a given A2G link. Moreover, our
framework allows us to derive a height-dependent closed-form expression and a
tight lower bound for the outage probability of an \textit{A2G cooperative
communication} network. Our results suggest that the optimal location of the
UAVs with respect to the ground nodes does not change by the inclusion of
ground relays. This enables interesting insights in the deployment of future
A2G networks, as the system reliability could be adjusted dynamically by adding
relaying nodes without requiring changes in the position of the corresponding
UAVs.
