  In this paper, we study an aerial drone base station (DBS) assisted cellular
network that consists of a single ground macro base station (MBS), multiple
DBSs, and multiple ground terminals (GT). We assume that the MBS transmits to
the DBSs and the GTs in the licensed band while the DBSs use a separate
unlicensed band (e.g. Wi-Fi) to transmit to the GTs. For the utilization of the
DBSs, we propose a cooperative decode--forward (DF) protocol in which multiple
DBSs assist the terminals simultaneously while maintaining a predetermined
interference level on the coexisting unlicensed band users. For our network
setup, we formulate a joint optimization problem for minimizing the aggregate
gap between the target rates and the throughputs of terminals by optimizing
over the 3D positions of the DBSs and the resources (power, time, bandwidth) of
the network. To solve the optimization problem, we propose an efficient nested
structured algorithm based on particle swarm optimization and convex
optimization methods. Extensive numerical evaluations of the proposed algorithm
is performed considering various aspects to demonstrate the performance of our
algorithm and the gain for utilizing DBSs.
