  Agility and resilience requirements of future cellular networks may not be
fully satisfied by terrestrial base stations in cases of unexpected or
temporary events. A promising solution is assisting the cellular network via
low-altitude unmanned aerial vehicles equipped with base stations, i.e.,
drone-cells. Although drone-cells provide a quick deployment opportunity as
aerial base stations, efficient placement becomes one of the key issues. In
addition to mobility of the drone-cells in the vertical dimension as well as
the horizontal dimension, the differences between the air-to-ground and
terrestrial channels cause the placement of the drone-cells to diverge from
placement of terrestrial base stations. In this paper, we first highlight the
properties of the dronecell placement problem, and formulate it as a 3-D
placement problem with the objective of maximizing the revenue of the network.
After some mathematical manipulations, we formulate an equivalent
quadratically-constrained mixed integer non-linear optimization problem and
propose a computationally efficient numerical solution for this problem. We
verify our analytical derivations with numerical simulations and enrich them
with discussions which could serve as guidelines for researchers, mobile
network operators, and policy makers.
