  As essential aerial platforms, unmanned aerial vehicles (UAVs) play an
increasingly important role in broad wireless connectivity and high-data-rate
transmission for future communication systems. Notably, various communication
scenarios are involved in UAV communications, such as intercommunications
between UAVs and communications with the ground user equipment, the cellular
base station, and the ground station, to name a few. However, existing works
mostly focus on a single communication scenario, a designated channel type, and
a specific operating frequency, thus urgently requiring a comprehensive
understanding of multi-scenario, multi-frequency, and multi-type UAV channels.
This article pours attention into the essentials of corresponding air-to-air
(A2A) and air-to-ground (A2G) channels in UAV communications. We first identify
the latest key challenges of channel modeling for UAV communications. We then
provide the state of the art for A2A and A2G channel properties and models
based on extensive measurement campaigns. In particular, we conduct realistic
case studies to further demonstrate critical channel characterizations and
machine learning-based modeling methods. Last but not least, potential
directions are widely discussed for paving the way towards more accurate and
effective channel models for UAV communications.
