  Measurements of the propagation channel form the basis of all realistic
system performance evaluations, as foundation of statistical channel models or
to verify ray tracing. This is also true for the analysis of cell-free massive
multi-input multi-output (CF-mMIMO) systems in real-world environments.
However, such experimental data are difficult to obtain, due to the complexity
and expense of deploying tens or hundreds of channel sounder nodes across the
wide area a CF-mMIMO system is expected to cover, especially when different
configurations and number of antennas are to be explored. In this paper, we
provide a novel method to measure channels for CF-mMIMO systems using a channel
sounder based on a drone, also known as a small unmanned aerial vehicle (UAV).
Such a method is efficient, flexible, simple, and low-cost, capturing channel
data from thousands of different access point (AP) locations within minutes. In
addition, we provide sample 3.5 GHz measurement results analyzing deployment
strategies for APs and make the data open source, so they may be used for
various other studies.
