  In conventional terrestrial cellular networks, mobile terminals (MTs) at the
cell edge often pose a performance bottleneck due to their long distances from
the serving ground base station (GBS), especially in hotspot period when the
GBS is heavily loaded. This paper proposes a new hybrid network architecture by
leveraging the use of unmanned aerial vehicle (UAV) as an aerial mobile base
station, which flies cyclically along the cell edge to offload data traffic for
cell-edge MTs. We aim to maximize the minimum throughput of all MTs by jointly
optimizing the UAV's trajectory, bandwidth allocation and user partitioning. We
first consider orthogonal spectrum sharing between the UAV and GBS, and then
extend to spectrum reuse where the total bandwidth is shared by both the GBS
and UAV with their mutual interference effectively avoided. Numerical results
show that the proposed hybrid network with optimized spectrum sharing and
cyclical multiple access design significantly improves the spatial throughput
over the conventional GBS-only network; while the spectrum reuse scheme
provides further throughput gains at the cost of slightly higher complexity for
interference control. Moreover, compared to the conventional small-cell
offloading scheme, the proposed UAV offloading scheme is shown to outperform in
terms of throughput, besides saving the infrastructure cost.
