  The use of drone small cells (DSCs) has recently drawn significant attentions
as one key enabler for providing air-to-ground communication services in
various situations. This paper investigates the co-channel deployment of dense
DSCs, which are mounted on captive unmanned aerial vehicles (UAVs). As the
altitude of a DSC has a huge impact on the performance of downlink, the
downlink interference control problem is mapped to an altitude control problem
in this paper. All DSCs adjust their altitude to improve the available
signal-to-interference-plus-noise ratio (SINR). The control problem is modeled
as a mean-field game (MFG), where the cost function is designed to combine the
available SINR with the cost of altitude controling. The interference
introduced from a big amount of DSCs is derived through a mean-field
approximation approach. Within the proposed MFG framework, the related
Hamilton-Jacobi-Bellman and Fokker-Planck-Kolmogorov equations are deduced to
describe and explain the control policy. The optimal altitude control policy is
obtained by solving the partial differential equations with a proposed finite
difference algorithm based on the upwind scheme. The simulations illustrate the
optimal power controls and corresponding mean field distribution of DSCs. The
numerical results also validate that the proposed control policy achieves
better SINR performance of DSCs compared to the uniform control scheme.
