  Deployment of unmanned aerial vehicles (UAVs) is recently getting significant
attention due to a variety of practical use cases, such as surveillance, data
gathering, and commodity delivery. Since UAVs are powered by batteries, energy
efficient communication is of paramount importance. In this paper, we
investigate the problem of lifetime maximization of a UAV-assisted network in
the presence of multiple sources of interference, where the UAVs are deployed
to collect data from a set of wireless sensors. We demonstrate that the
placement of the UAVs play a key role in prolonging the lifetime of the network
since the required transmission powers of the UAVs are closely related to their
locations in space. In the proposed scenario, the UAVs transmit the gathered
data to a primary UAV called \textit{leader}, which is in charge of forwarding
the data to the base station (BS) via a backhaul UAV network. We deploy tools
from spectral graph theory to tackle the problem due to its high non-convexity.
Simulation results demonstrate that our proposed method can significantly
improve the lifetime of the UAV network.
