  In this paper, we investigate the downlink performance of a three-tier
heterogeneous network (HetNet). The objective is to enhance the edge capacity
of a macro cell by deploying unmanned aerial vehicles (UAVs) as flying base
stations and small cells (SCs) for improving the capacity of indoor users in
scenarios such as temporary hotspot regions or during disaster situations where
the terrestrial network is either insufficient or out of service. UAVs are
energy-constrained devices with a limited flight time, therefore, we formulate
a two layer optimization scheme, where we first optimize the power consumption
of each tier for enhancing the system energy efficiency (EE) under a minimum
quality-of-service (QoS) requirement, which is followed by optimizing the
average hover time of UAVs. We obtain the solution to these nonlinear
constrained optimization problems by first utilizing the Lagrange multipliers
method and then implementing a sub-gradient approach for obtaining convergence.
The results show that through optimal power allocation, the system EE improves
significantly in comparison to when maximum power is allocated to users (ground
cellular users or connected vehicles). The hover time optimization results in
increased flight time of UAVs thus providing service for longer durations.
