# Prediction of RF-EMF Exposure by Outdoor Drive Test Measurements
## 2 Measurement Description
### Drive Test Protocol and Analysis  
The drive test route (shown in Figure 3) is selected to increase the diversity of measurement environments in Paris. The total itinerary is approximately 65 Km in one day, from 9 a.m. to 5 p.m. The objective of the drive test is to conduct measurements covering a large area of Paris within a limited time. In other words, the challenge is to perform the measurements on the three antenna axes at almost the same position, which depends mainly on the driving speed. In our case, the average driving speed is 9 Km/h (i.e., 2.5 m/s), which allows for reducing the discontinuity of 3-axis measurements while bringing the least inconvenience possible to the road traffic. The average distance between two successive measurement points is around 3 or 4 m.  
The GPS information is recorded using a mobile phone application, i.e., 'Geo Tracker' since Tektronix does not have a built-in GPS module. Geo Tracker records Universal Time Coordinated (UTC) and coordinates (Longitude and Latitude) information, which are later used to be synchronized with Tektronix data. Then, the GPS coordinates of each measurement record are obtained by: (1) Interpolation of the GPS information recorded by Geo Tracker based on time synchronization, (2) conversion from longitude and latitude to Universal Transverse Mercator (UTM) coordinates. After the broadband drive test measurement was obtained, the processing and analysis were carried out. First, the E-field levels from RF cellular bands of 4 main French operators were extracted and computed. In Figure 4, we compare the correlation of E-field levels from each band. A strong correlation is observed among bands from the same operator. This could result from a deployment strategy in that each operator tends to put new antennas on an existing site for the consideration of cost. For this reason, we consider two different ANN models for the RF-EMF spatial reconstruction, which is explained in Section 3.1. Figure 3: Drive test route in Paris. According to guidelines given by ICNIRP [23], the reference levels for EMF exposure from 100 KHz to 300 GHz in the whole body case are frequency-dependent since they depend on the equivalent surface of the body, which is linked to the frequency. For example, reference limits at frequencies 400 MHz (\(L_{4 MHz}\)) and 2 GHz (\(L_{2 GHz}\)) are 27.5 and 61 V/m for the general public, respectively. As described in [23], the human body has a higher capability to grab and absorb EM energy in the frequency band (60-100 MHz). In this frequency band, the adult human body size (e.g., height between 1.5 and 1.8 m) is close to a quarter wavelength. Because of that, the admissible maximum power density must be lower than elsewhere. For a frequency above 2 GHz, the human absorption is much more local and less dependent on the frequency. In this case, the admissible maximum power density is constant. In this context, the root squared integration of the E-fields \(E_{1}\) and \(E_{2}\) measured, respectively, at the frequency bands \(f_{1}\) and \(f_{2}\) (given by \(\sqrt{E_{1}^{2}+E_{2}^{2}}\)) cannot be used to check the compliance to the limit. Indeed, the objective is to verify that the sum of the percentages with respect to the relative reference limit is less than 100%. Considering the limits \(L_{1}\) and \(L_{2}\) at \(f_{1}\) and \(f_{2}\), the sources \(S_{1}\) and \(S_{2}\) are, respectively, inducing \((\frac{E_{1}}{L_{1}})^{2}\) and \((\frac{E_{2}}{L_{2}})^{2}\) percent of their corresponding limits. For frequency between 400 and 2000 MHz, limit \(L\) is defined as \(1.375\sqrt{f}\), where \(f\) is the frequency in MHz. Therefore, we adopt the following metric to assess total exposure level [24]:  
\[E_{total}=\sqrt{\frac{N}{i}\frac{f_{ref}}{f_{i}}(E_{i})^{2}}, \tag{1}\]  
where \(N\) is the number of frequency bands and \(f_{ref}\) is the reference frequency of interest. In the present paper, the E-field from each band is normalized to frequency \(f_{ref}=900\) MHz.