## 4 Validation and Comparison  
### Channel Measurement Campaigns  
Most of the existing A2G channel measurement campaigns are predominantly focused on urban scenarios, with relatively fewer studies conducted in rural farmland scenarios. Furthermore, only a few of them consider different UAV heights. In this study, we conduct measurement campaigns of different UAV heights in a farmland scenario.  
The developed A2G channel sounder is illustrated in Figure 3, including a UAV-aided transmitter (TX) and a receiver (RX) fixed on the ground, as shown in Figure 3a and Figure 3b, respectively. The TX comprises a hexacopter UAV, an omnidirectional antenna, a global positioning system (GPS) antenna, and a software-defined radio (SDR) signal transmitting platform comprising a radio frequency (RF) module, a high-power amplifier (HPA), and a lithium battery. The sounding signal is sent through the RF module, HPA, and omnidirectional antenna. The independent lithium battery is used for the power supply for both the HPA and RF modules. The RX consists of an omnidirectional antenna, a GPS module, an uninterruptible power supply, and an SDR signal processing platform comprising an RF module and a baseband processing module. The received signal is captured by the omnidirectional antenna, and then sent into the baseband processing module through the RF module for data processing. Finally, the measurement data are stored in the disk. Data transmission between the SDR and disk is realized via the PCIE bus. The comprehensive specifications of the channel sounder are introduced in Table 1, and more details about the channel sounder can be found in [33].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**System Parameters** & **Values** \\ \hline Supported frequency band & 100–6000 MHz \\ Bandwidth & 100 MHz \\ Transmit power & 32 dBm \\ HPA gain & 42 dB \\ Antenna type & Omnidirectional Antenna \\ Antenna gain & 2.5 dBi \\ Measurement sequence & Single-tone signal/Zadoff-Chu (ZC) sequence \\ \hline \hline \end{tabular}
\end{table}
Table 1: Hardware parameters of the A2G channel sounder.  
The channel measurement was conducted on a farmland at 3.6 GHz, and the ZC sequence is used in the channel sounder. The farmland area was approximately 150 m by 120 m, with sparse roads and low-rise buildings scattered around, resulting in an overall open environment. The RX was positioned at the edge of the farmland with the antenna at a height 2.5 m above the ground. The GPS coordinates for the fixed RX point were 118.57402 E 33.09518 N. Since the UAV normally flies at low altitudes for operations like yield estimation, precise irrigation, and crop monitoring in agricultural scenarios, we selected the UAV height range of 10 m to 30 m for the channel measurement campaigns. The UAV, equipped with the TX, took off from one side of the farmland and flew in a straight line with a 5 m height interval at different heights ranging from 10 m to 30 m and a flight distance of 100 m for each height, as shown in Figure 4. The GPS coordinates for the starting point of the horizontal movement of TX were 118.5736 E 33.09546 N. The receiver only started recording channel data when the UAV flew along the solid yellow line in the figure and stopped recording when it flew along the dashed yellow line. The PL data corresponding to different distances and UAV altitudes were obtained.