  The use of unmanned aerial vehicles (UAVs) as communication platforms is of
great practical significance in future wireless networks, especially for
on-demand deployment in temporary events and emergency situations. Although
prior works have shown the performance improvement by exploiting the UAV's
mobility, they mainly focus on delay-tolerant applications. As delay
requirements fundamentally limit the UAV's mobility, it remains unknown whether
the UAV is able to provide any performance gain in delay-constrained
communication scenarios. Motivated by this, we study in this paper a
UAV-enabled orthogonal frequency division multiple access (OFDMA) network where
a UAV is dispatched as a mobile base station (BS) to serve a group of users on
the ground. We consider a minimum-rate ratio (MRR) for each user, defined as
the minimum instantaneous rate required over the average achievable throughput,
to flexibly adjust the percentage of its delay-constrained data traffic. Under
a given set of constraints on the users' MRRs, we aim to maximize the minimum
average throughput of all users by jointly optimizing the UAV trajectory and
OFDMA resource allocation. First, we show that the max-min throughput in
general decreases as the users' MRR constraints become more stringent, which
reveals a fundamental throughput-delay tradeoff in UAV-enabled communications.
Next, we propose an iterative parameter-assisted block coordinate descent
method to optimize the UAV trajectory and OFDMA resource allocation
alternately, by applying the successive convex optimization and the Lagrange
duality, respectively. Furthermore, an efficient and systematic UAV trajectory
initialization scheme is proposed based on a simple circular trajectory.
Finally, simulation results are provided to verify our theoretical findings and
demonstrate the effectiveness of our proposed designs.
