  This paper analyzes the energy efficiency of a novel system model where
unmanned aerial vehicles (UAVs) are used to provide coverage for user hotspots
(user clusters) and are deployed on charging pads to enhance the flight time.
We introduce a new notion of "cluster pairs" to capture the dynamic nature of
the users' spatial distribution in order to exploit one of the top advantages
of UAVs, which is the mobility and relocation flexibility. Using tools from
stochastic geometry, we first derive a new distance distribution that is vital
for energy efficiency analysis. Next, we compute the coverage probability under
two deployment strategies: (i) one UAV per cluster pair, and (ii) one UAV per
cluster. Finally, we compute the energy efficiency for both strategies. Our
numerical results reveal which of the two strategies is better for different
system parameters. Our work investigates some new aspects of the UAV-enabled
communication system such as the dynamic density of users and the advantages or
disadvantages of one- or two-UAV deployment strategies per cluster pair. By
considering the relationships between the densities of user cluster pairs and
the charging pads, it is shown that an optimal cluster pair density exists to
maximize energy efficiency.
