  Beyond visual line-of-sight connectivity is key for use cases of unmanned
aerial vehicles (UAVs) such as package delivery, infrastructure inspection, and
rescue missions. Cellular networks stand ready to support flying UAVs by
providing wide-area, quality, and secure connectivity for UAV operations.
Ensuring reliable connections in the presence of UAV movements is important for
safety control and operations of UAVs. With increasing height above the ground,
the radio environment changes. Using terrestrial cellular networks to provide
connectivity to the UAVs moving in the sky may face new challenges. In this
article, we share some of our findings in mobility support for cellular
connected UAVs. We first identify how the radio environment changes with
altitude and analyze the corresponding implications on mobility performance. We
then present evaluation results to shed light on the mobility performance of
cellular connected UAVs. We also discuss potential enhancements for improving
mobility performance in the sky.

Networking and Internet Architecture