  Although prior works have exploited the UAV's mobility to enhance the
wireless communication performance under different setups, the fundamental
capacity limits of UAV-enabled/aided multiuser communication systems have not
yet been characterized. To fill this gap, we consider in this paper a
UAV-enabled two-user broadcast channel (BC), where a UAV flying at a constant
altitude is deployed to send independent information to two users at different
fixed locations on the ground. We aim to characterize the capacity region of
this new type of BC over a given UAV flight duration, by jointly optimizing the
UAV's trajectory and transmit power/rate allocations over time, subject to the
UAV's maximum speed and maximum transmit power constraints. First, to draw
essential insights, we consider two special cases with asymptotically large/low
UAV flight duration/speed, respectively. For the former case, it is shown that
a simple hover-fly-hover (HFH) UAV trajectory with time division multiple
access (TDMA) based orthogonal multiuser transmission is capacity-achieving,
while in the latter case, the UAV should hover at a fixed location that is
nearer to the user with larger achievable rate and in general superposition
coding (SC) based non-orthogonal transmission with interference cancellation at
the receiver of the nearer user is required. Next, we consider the general case
with finite UAV speed and flight duration. We show that the optimal UAV
trajectory should follow a general HFH structure, i.e., the UAV successively
hovers at a pair of initial and final locations above the line segment of the
two users each with a certain amount of time and flies unidirectionally between
them at the maximum speed, and SC is generally needed.
