  Unmanned Aerial Vehicles (UAVs), popularly called drones, are an important
part of future wireless communications, either as user equipment that needs
communication with a ground station, or as base station in a 3D network. For
both the analysis of the "useful" links, and for investigation of possible
interference to other ground-based nodes, an understanding of the air-to-ground
channel is required. Since ground-based nodes often are equipped with antenna
arrays, the channel investigations need to account for it. This study presents
a massive MIMO-based air-to-ground channel sounder we have recently developed
in our lab, which can perform measurements for the aforementioned requirements.
After outlining the principle and functionality of the sounder, we present
sample measurements that demonstrate the capabilities, and give first insights
into air-to-ground massive MIMO channels in an urban environment. Our results
provide a platform for future investigations and possible enhancements of
massive MIMO systems.
