  For an unmanned aerial vehicle (UAV) enabled network we investigate the
energy-efficiency (EE) of joint caching and cooperative communication
(Fog-RAN). Since UAVs are battery- and cache-limited, placing the popular
contents in the caches and managing the energy expenditure of UAVs become
crucial. We formulate the energy consumption of UAVs as an aggregate of
communication/caching, hovering, and vertical displacement energies, and then
devise an optimization problem for optimally assigning contents to caches and
choosing the height of UAVs. Adopting tools from stochastic geometry, we also
derive the EE in a numerically tractable form as a function of density, the
radius of the cooperation zone, cache size, main communication/physical
characteristics of UAVs, and influential environmental parameters. We develop
two content placement strategies with low computational complexity. The
conducted numerical results demonstrate that by adopting these algorithms one
is able to improve EE by up to 800% compared to common content placement
schemes, e.g., the least-frequently used (LRU), the most-popular, and Hit-rate.
Furthermore, while under LRU and Hit-rate schemes there is no benefit in
vertically displacing UAVs, under our algorithms one is able to increase EE by
at most 600%. Importantly, via our algorithms one can increase the size of
cooperation zone in order to steadily increase EE, which is not the cases of
LRU, the most-popular, and Hit-rate schemes. We finally observe that there is
optimal values for density and cache-size of UAVs, granting maximum EE.
