  In this letter, we propose an energy-efficient 3-dimensional placement of
multiple aerial access points (AAPs), in the desired area, acting as flying
base stations for uplink communication from a set of ground user equipment
(UE). The globally optimal energy-efficient vertical position of AAPs is
derived analytically by considering the inter-cell interference and AAP energy
consumption. The horizontal position of AAPs which maximize the packing density
of the AAP coverage area are determined using a novel regular polygon-based AAP
placement algorithm. We also determine the maximum number of non-interfering
AAPs that can be placed in the desired area. The effect of the AAP energy
consumption on the optimal placement and the analytic findings are verified via
numerical simulations.
