  Unmanned aerial vehicles (UAVs) have found widespread commercial, civilian,
and military applications. Wireless communication has always been one of the
core technologies for UAV. However, the communication capacity is becoming a
bottleneck for UAV to support more challenging application scenarios. The
heavily-occupied sub-6 GHz frequency band is not sufficient to meet the ultra
high-data-traffic requirements. The utilization of the millimeter-wave (mmWave)
frequency bands is a promising direction for UAV communications, where large
antenna arrays can be packed in a small area on the UAV to perform
three-dimensional (3D) beamforming. On the other hand, UAVs serving as aerial
access points or relays can significantly enhance the coverage and quality of
service of the terrestrial mmWave cellular networks. In this paper, we provide
a comprehensive survey on mmWave beamforming enabled UAV communications and
networking. The technical potential of and challenges for mmWave-UAV
communications are presented first. Then, we provide an overview on relevant
mmWave antenna structures and channel modeling. Subsequently, the technologies
and solutions for UAV-connected mmWave cellular networks and mmWave-UAV ad hoc
networks are reviewed, respectively. Finally, we present open issues and
promising directions for future research in mmWave beamforming enabled UAV
communications and networking.
