  In this paper, a novel framework for delay-optimal cell association in
unmanned aerial vehicle (UAV)-enabled cellular networks is proposed. In
particular, to minimize the average network delay under any arbitrary spatial
distribution of the ground users, the optimal cell partitions of UAVs and
terrestrial base stations (BSs) are determined. To this end, using the powerful
mathematical tools of optimal transport theory, the existence of the solution
to the optimal cell association problem is proved and the solution space is
completely characterized. The analytical and simulation results show that the
proposed approach yields substantial improvements of the average network delay.
