# RF Exposure Assessment by Drone-Based Technology
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline
**E (V/m)** & **LTE 800 DL** & **GSM + UMTS 900 DL** & **GSM 1800 DL** & **UMTS 2100 DL** \\ \hline Min & 0.045 & 0.165 & 0.157 & 0.115 \\ P25 & 0.098 & 0.272 & 0.243 & 0.215 \\ P50 & 0.134 & 0.341 & 0.292 & 0.242 \\ P75 & 0.195 & 0.541 & 0.409 & 0.339 \\ P95 & 0.421 & 0.991 & 0.932 & 0.758 \\ Max & 1.02 & 2.296 & 2.332 & 3.504 \\ Average & 0.18 & 0.5 & 0.4 & 0.3 \\ SD & 0.14 & 0.3 & 0.3 & 0.4 \\ IQ & 0.097 & 0.259 & 0.166 & 0.124 \\ \hline \end{tabular}
\end{table} 
Table 3: Statistical summary of the electric field measured at 20 m height in the four mobile-telephone bands. P25-P95 are the percentiles. SD: standard deviation. IQ: interquartile range. Sample size: 160. However, data transformation techniques can be used to address the lack of normality. One solution to this problem is to transform the original data using functions like logarithms. Converting data to logarithms produces distributions that are approximately normal. In the field of radiocommunications, it is quite common to express units in decibels (dB). In these units, the electric field \(E(dB)\) is related to its value in the international system \(E(SI)\) by the equation \(E(dB)=20\ log[E(SI)]\). In the following, we will use logarithmic units to address the issues that may arise when using non-normal distributions. Using Equation (1), we obtained the experimental variograms corresponding to the four frequency bands and the two sampling taken. Figure 6 shows, as an example, the variogram corresponding to the GSM + UMTS 900 DL band at 20 m height, represented by points. There are 25 points in this variogram and each of them was calculated with between 60 and 320 pairs of experimental data. This allows us, according to [PERSON] and [PERSON] [18], to obtain reliable variograms. It can be observed in Figure 6 that the variance increases up to a certain distance, after which the increase slows down or even stabilizes. We also observe that the extrapolation of the data to the intercept with the y-axis (\(h=0\)) does not pass through the origin. Figure 5: Frequency distribution of the electric-field levels detected in the GSM + UMTS 900 DL band in the sampling carried out at a height of 20 m.  
Figure 6: Theoretical nugget + spherical variogram (solid line) fitted to the experimental variogram (dots) for the GSM + UMTS 900 DL band (20 m height). The dashed line represents the sample variance. Figure 8 shows the electric-field maps at a height of 20 m. The images are semi-transparent so that the buildings can be seen, since their height is lower than that of the maps. In this sampling, carried out with the aerial vehicle, there are no obstacles that prevent the direct line of sight with the antenna, and the field levels are conditioned by the azimuth and inclination of the sectors. It can be observed in this figure that the highest values of the electric field in the four frequency bands are located at the bottom, where the site with mobile-telephone antennas is located. Comparing the electric-field levels in both samples (Figures 7 and 8), we observe that those detected with the PEM-drone system have higher values than those taken at ground level, which is in agreement with the data shown in Table 2. On average, the levels at 20 m were between 11 dB (LTE 800 DL, GSM + UMTS 900 DL, and UMTS 2100 DL bands) and 14 dB (GSM 1800 DL) higher than those taken at ground level. Figure 7: Heat maps of electric-field intensities at ground level in the mobile-phone bands: (**a**) LTE 800 DL, (**b**) GSM + UMTS 900 DL, (**c**) GSM 1800 DL and (**d**) UMTS 2100 DL. The figures were obtained by merging the heat maps with aerial images of the area using Google Earth. The location of the site with sectorial mobile-phone antennas is shown in red along with the orientation of its sectors.