  Unmanned aerial vehicles (UAVs) are expected to be an important new class of
users in the fifth generation (5G) and beyond 5G cellular networks. In
particular, there are emerging UAV applications such as aerial photograph and
data relaying that require high-speed communications between the UAVs and the
ground base stations (GBSs). Due to the high UAV altitude, the strong
line-of-sight (LoS) links generally dominate the channels between the UAVs and
GBSs, which brings both opportunities and challenges in the design of future
wireless networks supporting both terrestrial and aerial users. Although each
UAV can associate with more GBSs for communication as compared to terrestrial
users thanks to the LoS-dominant channels, it also causes/suffers more severe
interference to/from the terrestrial communications in the uplink/downlink.
This paper studies the uplink communication from a multi-antenna UAV to a set
of GBSs within its signal coverage by considering a practical yet challenging
scenario when the number of antennas at the UAV is smaller than that of
co-channel GBSs. To achieve high-rate transmission yet avoid interfering with
any of the existing terrestrial communications at the co-channel GBSs, we
propose a novel multi-beam transmission strategy by exploiting the
non-orthogonal multiple access (NOMA) technique. Specifically, the UAV sends
each data stream to a selected subset of the GBSs, which can decode the UAV's
signals and then cancel them before decoding the messages of their served
terrestrial users, and in the meanwhile nulls its interference at the other
GBSs via zero-forcing (ZF) beamforming. To draw essential insight, we first
characterize in closed-form the degrees-of-freedom (DoF) under the proposed
strategy. Then, we propose an efficient algorithm to maximize the UAV's
transmit rate subject to the interference avoidance constraints for protecting
the terrestrial users.
