  Properly 3D placement of unmanned aerial vehicle mounted base stations
(UAV-BSs) can effectively prolong the life-time of the mobile ad hoc network,
since UAVs are usually powered by batteries. This paper involves the on-board
circuit consumption power and considers the optimal placement that minimizes
the UAV-recall-frequency (UAV-RF), which is defined to characterize the
life-time of this kind of network. Theoretical results show that the optimal
vertical and horizontal dimensions of UAV can be decoupled. That is, the
optimal hovering altitude is proportional to the coverage radius of UAVs, and
the slope is only determined by environment. Dense scattering environment may
greatly enlarge the needed hovering altitude. Also, the optimal coverage radius
is achieved when the transmit power equals to on-board circuit power, and hence
limiting on-board circuit power can effectively enlarge life-time of system. In
addition, our proposed 3D placement method only require the statistics of
mobile users' density and environment parameters, and hence it's a typical
on-line method and can be easily implemented. Also, it can be utilized in
scenarios with varying users' density.
