  Communication at mmWave bands carries critical importance for 5G wireless
networks. In this paper, we study the characterization of mmWave air-to-ground
(AG) channels for unmanned aerial vehicle (UAV) communications. In particular,
we use ray tracing simulations using Remcom Wireless InSite software to study
the behavior of AG mmWave bands at two different frequencies: 28~GHz and
60~GHz. Received signal strength (RSS) and root mean square delay spread
(RMS-DS) of multipath components (MPCs) are analyzed for different UAV heights
considering four different environments: urban, suburban, rural, and over sea.
It is observed that the RSS mostly follows the two ray propagation model along
the UAV flight path for higher altitudes. This two ray propagation model is
affected by the presence of high rise scatterers in urban scenario. Moreover,
we present details of a universal serial radio peripheral (USRP) based channel
sounder that can be used for AG channel measurements for mmWave (60 GHz) UAV
communications.
