## 3 Radio Wave Propagation Analysis
### Results of the Channel Analysis  
We performed radio wave propagation analysis using the ray-tracing method of RapLab (Kozo Keikaku Engineering Inc., Tokyo, Japan) [45] using the created model. The transmitting and receiving antennas in the analysis were placed in the same locations as in the measurements. Among the calculation results using the ray-tracing method, we focused mainly on the following items: the received level between both ends of the transmitting and receiving antennas, the direction and path power of the direct wave, and the direction and path power of each scattered wave. Here, the received level and path power include spatial loss and the radiation gain in the direction of the incoming wave in the transmitting and receiving antennas. As an example of the analytical results, the path for \(D=5\) m and \(H=10\) m in the S environment is shown in Figure 7a, while the path for horizontal installation and \(H=10\) m in the C environment is shown in Figure 7b. The shapes drawn in green and yellow show the directivity of the transmitting and receiving antennas which are the half-wavelength dipole antennas. The blue lines show the paths with a power difference of less than 60 dB from the direct wave. It can be seen from Figure 7a that the number of paths shown is very small because there is only one building in the propagation environment. In contrast, as shown in Figure 7b, there are a lot of scattered waves through surrounding buildings. Hence, it is expected that the received power will vary greatly depending on the environment. Figure 8 shows the cumulative number of paths up to the power difference defined as that between the direct wave and each scattered wave. This analysis focused on the cases with the shortest and longest distance between the UAV and the receiving antenna, at altitudes of \(H=10\) m and \(H=30\) m. Figure 8a,b show the results for the S and C environments, respectively. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Simple Propagation Environment** & **Complex Propagation Environment** \\ \hline Frequency \(f\) & \multicolumn{2}{c}{2.5 GHz} \\ Transmitting antenna & Half-wavelength dipole antenna \\ Receiving antenna & Half-wavelength dipole antenna \\ \hline Ground material & Concrete \\ Relative permittivity \(\varepsilon_{r}\) & \multicolumn{2}{c}{5.31} \\ Conductivity \(\rho\) [S/m] & \multicolumn{2}{c}{0.194} \\ \hline Wall material & Brick & Concrete \\ Relative permittivity \(\varepsilon_{r}\) & 3.91 & 5.31 \\ Conductivity \(\rho\) [S/m] & 0.024 & 0.194 \\ \hline Maximum number of reflections & \multicolumn{2}{c}{3} \\ Maximum number of diffractions & \multicolumn{2}{c}{1} \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Configurations of the analysis. To support the results of Figure 7, the cumulative number of paths up to the power difference in Figure 8b is quite a bit larger than that in Figure 8a regardless of the analysis conditions. A possible reason for this trend is that there are many scattered waves caused by surrounding buildings. As shown in Figure 8a, the cumulative number of paths up to the power difference of \(H=10\) m (yellow and yellow-green lines) is greater than that of \(H=30\) m (orange and green lines) regardless of the distance between the antenna and Building C. The reason is that when \(H=10\) m, the scattered waves arriving at the receiving antenna, except for the single reflection from the ground which has the largest path power, are the reflected waves through Building C. In contrast, when \(H=30\) m, the scattered waves, except for the single reflection from the ground, are the diffracted waves through Building C because the UAV altitude is more than twice the height of Building C, so the reflected waves through Building C do not impact the receiving antenna. Therefore, it can be considered that the diffraction loss is greater than the reflection loss.