In this paper, we consider an aerial UAV base station (UBS) assisted cellular network that consists of a single ground macro base station (MBS), multiple UBSs, and multiple ground terminals. We assume that the MBS transmits in a licensed band while the UBSs use a separate unlicensed band to transmit to the terminals. For the utilization of the UBSs, we propose a cooperative decode-forward (DF) protocol in which multiple UBSs assist the terminals simultaneously, and we formulate a joint optimization problem for minimizing the aggregate gap between the target rates and the throughputs of terminals. To solve the optimization problem, we propose an efficient nested structured algorithm based on the particle swarm optimization algorithm and convex optimization methods. Extensive numerical evaluations of the proposed algorithm is performed to demonstrate the performance of our algorithm and the gain for utilizing UBSs.
