## 4 Results and Discussion
### Measurement Results and Analysis  
Figures 5 and 6 show the overall drive test results for different paths and heights in terms of RSRP and RSRQ, respectively. The flight distance for routes A, B, and C are about 2.3, 2.4, and 2.6 km, respectively, and the flight heights are 65, 85, 105, and 125 m above the takeoff point. The colors indicate the RSRP and RSRQ conditions, which are changing in the \"excellent\" to \"weak\" range, based on Table 2. Based on the results, the link performance depends on the 3D position of the drone relative to BSs, where, generally, the shorter the distance and the lower the altitude, the better the performance. During the drive tests, the mounted phone on the drone was served by 13 BSs. It was observed that the number of serving cells increased by increasing the flight height. The main reason behind this phenomenon is that the drone is mainly served by the sidelobes of BS's antennas when flight height increases. Hence, in higher altitudes, where the drone is in visual line of sight of the adjacent BSs, the probability of serving by the sidelobes of a larger number of BSs increases. However, the serving time from most of the adjacent BSs is not long, and after a short distance, a handover occurs, and another BS serves the drone. In this regard, we only considered the data belonging to the dominant BSs with a sufficient number of measured samples. After preprocessing and data cleansing, 8457 samples were considered for data analysis. Table 4 describes a statistics summary of the considered data. Meanwhile, Figure 7 shows the histograms of distance, angle, RSRP, and RSRQ. Distance is the 2D distance between serving BS and the drone. The elevation angle is the angle between the drone's position and the center of the antenna's main lobe. Finally, height is the drone's altitude above the serving BS's antenna, and X% is the percentage of the data distribution. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Distance** & **Elevation Angle** & **Height** & **RSRP** & **RSRQ** \\  & **(m)** & **(Degree)** & **(m)** & **(dBm)** & **(dB)** \\ \hline
**mean** & 242.195 & 24.534 & 79.547 & \(-\)74.735 & \(-\)11.396 \\
**std** & 141.782 & 17.066 & 28.396 & 5.832 & 3.081 \\
**min** & 10.534 & 0.560 & 6.000 & \(-\)98.000 & \(-\)20.000 \\
**25\%** & 140.183 & 13.090 & 59.000 & \(-\)79.000 & \(-\)14.000 \\
**50\%** & 224.761 & 21.040 & 89.000 & \(-\)75.000 & \(-\)11.000 \\
**75\%** & 322.479 & 30.520 & 99.000 & \(-\)71.000 & \(-\)9.000 \\
**max** & 1045.209 & 82.700 & 119.000 & \(-\)59.000 & \(-\)5.000 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary statistics of considered data. Figure 5: Measured RSRP values at different routes and elevations. Figure 6: Measured RSRQ values at different routes and elevations. Figures 8a and 9a show the 3D distribution of the measured RSRP and SRRQ values versus angle and distance, respectively. As the data shows, the performance of the cellular communication link directly depends on the distance between the drone and the serving BS, as well as the elevation angle or the flight height. For better visualization, Figure 8b,c and Figure 9b,c show the 2D plots of RSRP and RSRQ versus distance and angle, respectively. Results show that by increasing the distance and angle, both RSRP and RSRQ degrade, and degradation can be modeled with either linear or nonlinear models. Note, since the RSRP and RSRQ values depend highly on distance and angle, a 2D plot cannot reflect the impact of each factor on the link performance, so the data distribution seems to be scattered. Figure 8: (**a**) Data distribution of RSRP versus angle and distance. (**b**) RSRP versus distance. (**c**) RSRP versus angle. Figure 7: Histograms of dataset parameters, including (**a**) distance, (**b**) angle, (**c**) RSRP, and (**d**) RSRQ. The results show that most data are distributed at less than 600 m and an elevation angle of fewer than 40 degrees.