Using a Drone Sounder to Measure Channels for Cell-Free Massive MIMO Systems

Abstract

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.

Energy Efficiency of Uplink Cell-Free Massive MIMO With Transmit Power Control in Measured Propagation Channel

“…However, large measurement datasets for CF-mMIMO systems are scarce due to the complexity of setting up and operating a massive number of antennas simultaneously. To address this issue, we recently proposed that a large amount of channel data for CF-mMIMO systems can be measured using a compact channel sounder with a drone acting as a virtual array and released open-source channel data [2].…”

“…where s k is a transmitted symbol of UE k normalized to unit average power, 0 ≤ q k ≤ 1 is the transmit power coefficient, z m ∼ N C (0, 1) is the normalized noise, and ρ is the transmit SNR, i.e., the ratio of the maximum transmitted signal power to the noise power. 2 Likewise, the collection of received signals at all M BS antennas can be written as:…”

“…Therefore, 1×8×2301 channel matrix may be captured at every 5 cm of drone movement. The characteristics of the drone sounder and the channel measurement principle for CF-mMIMO systems are further discussed in [2], [34].…”

“…Hence, the channel coefficients at 2301 subcarriers are regarded as particular realizations of a 3.5 GHz flat channel. If we define each frequency index as i, then 2 is the average large-scale fading with F the total number of frequency indices (F = 2301). Calibration and time invariance of the sounder characteristics over the duration of the measurements were tested, and some frequency points (less than 10% of the total acquired data) exhibiting excessive calibration errors were discarded.…”

“…9 The channel environment may show some variations during the repeated flights along the trajectory, and the path of the drone may not exactly overlap for different RX sites due to elongated measurement time. Please refer [2] for a discussion and measures taken to minimize the impact of such variations.…”