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.

本文考虑了一种新的无人机辅助下的二用户广播信道，旨在通过联合优化无人机的轨迹和传输功率 / 速率分配，以及考虑无人机的飞行持续时间、最大速度和最大发送功率限制，来表征给定飞行持续时间内的的容量区域。

基于无人机的两用户广播信道容量特性分析

Capacity Characterization of UAV-Enabled Two-User Broadcast Channel

In this letter, we study multiuser communication systems enabled by an
unmanned aerial vehicle (UAV) that is equipped with a directional antenna of
adjustable beamwidth. We propose a fly-hover-and-communicate protocol where the
ground terminals (GTs) are partitioned into disjoint clusters that are
sequentially served by the UAV as it hovers above the corresponding cluster
centers. We jointly optimize the UAV's flying altitude and antenna beamwidth
for throughput optimization in three fundamental multiuser communication
models, namely UAV-enabled downlink multicasting (MC), downlink broadcasting
(BC), and uplink multiple access (MAC). Our results show that the optimal UAV
altitude and antenna beamwidth critically depend on the communication model
considered.

本文研究了一种基于无人机的多用户通信系统，其中无人机配备具有可调波束宽度的定向天线，并提出了一种飞行悬停通信协议，通过联合优化无人机的飞行高度和天线波束宽度，针对三种基本的多用户通信模型实现吞吐量最优化。