# RF-EMF Exposure near 5G NR Small Cells
## 2 Materials and Methods
### Measurement Method
Before this study, this type of measurement had only been performed with the FSV setup [10; 11], with which a large amount of measurement traces with minimal blind time (i.e., the time between traces during which nothing is measured) can be saved (thanks to the spectrogram option). However, in this study, the more practical SRM setup was also used in order to determine its validity. During post-processing, the field components \(E_{RE,j}\) (with \(i=x\), \(y\), \(z\)) were retrieved using the setup's antenna factor and cable losses, for both SSB and PDSCH, and the total electric-field strength per resource element was calculated as a vector sum of the three components. Then, the worst-case (downlink) exposure scenario for a certain user arises when all resources allocated to (PDSCH or downlink) data transmission are being used to transmit data towards the user's UE. In that case, the resulting electric-field level \(E_{max}\) over the whole 5G NR channel can be calculated as follows:  
\[E_{max}=\sqrt{f_{TDC}}\;\sqrt{12N_{RB}}\;E_{RE,PDSCH}\;[\mathrm{V/m}], \tag{1}\]  
with \(f_{TDC}\) being an additional factor due to the technology duty cycle (TDC) (i.e., in the case of Time Division Duplex (TDD) the percentage of slots allocated to downlink transmission) (Table 2) and \(N_{RB}\) the number of resource blocks (each resource block contains 12 resource elements) in the channel bandwidth [2] (Table 2). **Step 4. ** With the measurement device in frequency mode, successive traces of the power received in a 100 MHz bandwidth (i.e., the maximum bandwidth for NR signals at frequencies below 6 GHz) around the NR channel's center frequency were captured. For the FSV setup, this measurement was repeated for each electric-field component and post-processing was necessary to average these traces to obtain the total average power over the channel bandwidth. Using the setup's antenna factor and cable losses, the time-averaged electric-field strength, \(E_{avg}\), was then obtained. With the SRM setup, traces for each component were measured successively, and the resulting \(E_{avg}\) was provided at the end by the device. This measurement was performed twice in order to assess two extreme cases. Without an active UE, only the broadcast, control, and other background signals are measured, resulting in \(E_{avg,min}\). When a UE is active and maximizes the downlink traffic from the base station (see Step 3), this measurement results in a maximized time-averaged exposure level, \(E_{avg,max}\). **Step 5. ** Finally, the worst-case (\(E_{max}\)) and maximum time-averaged (\(E_{avg,max}\)) exposure levels were compared to the ICNIRP [17] exposure-limiting guidelines. The comparison to the guidelines is quantified by the exposure ratio \(R\), which was calculated as follows:  
\[R=(\
icefrac{{E}}{{E_{\mathit{ref}}}})^{2}\left[\text{-}\right], \tag{2}\]  
with \(E_{ref}\) the reference level (or limit for maximum permissible exposure, MPE) for the electric-field strength (i.e., 61.4 V/m for the general public and 137.3 V/m for occupational exposures) at the considered frequency, derived from \(S_{ref}=1\) mW/cm\({}^{2}\) for frequencies above 2 GHz [5]. The observed field levels were below the MPE if \(R\leq 1\).