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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.Index Terms-Cell-free (distributed) massive MIMO, drone (UAV) channel sounder, air-to-ground (A2G) experiment, open source channel measurement data, AP deployment strategies 1 Relevant studies might use the framework of CF-mMIMO, or might employ other names, such as distributed MIMO, network MIMO, cooperative multi-point (CoMP), distributed antenna system (DAS), etc.

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 airto-ground massive MIMO channels in an urban environment. Our results provide a platform for future investigations and possible enhancements of massive MIMO systems.

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