# SDR-Enabled Multichannel Real-Time Measurement System for In Situ EMF Exposure Evaluation
## 1 Introduction  
Today's 5 th Generation New Radio (5G NR) introduces significant enhancements in characteristics such as flexibility, scalability, and efficiency, both in terms of power usage and spectrum. The end user experiences are boosted by employing new frequency resources and spectrum-sharing techniques combined with strong cell densification. The system 5G NR uses an optimized access scheme based on cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) together with beamforming and Multi-User Multiple Input Multiple Output (MU-MIMO). As they are being implemented, 5G NR systems are designed to operate within several bands comprising frequencies below 6 GHz (most of the rolled out commercial networks in Europe) up to 66-71 GHz [1]. The densification of the mobile network architecture due to the newly implemented 5G NR communication technologies is continuously raising concerns on the possible health effects of EMFs. The technological advent that we are facing nowadays urges for a relevant characterization of user exposure to such complex EMFs. As an example, relevant methodologies for assessing 5G base station exposure were developed in [2, 3]. The standards concerning human exposure to EMFs were recently updated to cover for the emerging applications [4, 5]. According to the standards, specific absorption rate (SAR) measurements must be conducted for compliance with the guidelines' basic restrictions. The SAR is a measure of the rate of EMF energy deposition inside tissues, and restrictions based on this metric consider solely the thermal effect of the EMF. As SAR measurements are impossible to conduct in vivo, the regulatory standards also provide external limits in terms of reference levels that are more-easily assessable. Above 2 GHz, the reference level for the averaged local exposure to EMFs (duration longer than 6 min) is expressed in terms of incident power density (PD) expressed in W/m\({}^{2}\). Additionally, for averaging time intervals shorter than 6 min, ICNIRP introduced the incident energy density (in J/m\({}^{2}\)) as a reference level for local exposure [4]. To determine the incident energy density, one must access the temporal variation of signal power in time, a requirement which becomes complicated as the frequency bandwidth enabled by the modern communications standards continuously expands. Different types of measurement systems are currently available on the market that can experimentally determine either the SAR, the incident electric (E) field strength, or the PD [6]. These include E-field probe-based systems [7], electro-optical probe-based systems [8], and optical fiber thermal sensor-based systems [9]. In addition, a series of non-invasive measurement systems such as infrared measurement systems [10], thermal scanner systems [11], PD measurements [12], and optical SAR systems [13] also exist. According to a recent sounding review in the field of radiofrequency (RF) exposure assessment, the major uncertainties in the exposure assessment are likely to be related to the inherent variability of real-world exposures, rather than imprecision in measurement techniques [14]. Moreover, with the increasing number of mobile subscribers, there are studies that indicate that the user exposure is dominated by the own or neighboring mobile equipment [15; 16]. With 5G NR, the importance of self-produced exposure increases both due to uplink (UL) and downlink (DL) signals contributions [17]. Therefore, this requires extensive in situ EMF measurements to be performed in the proximity of user equipment. Several methods are available for assessing the incident PD in the proximity of a device [18]. The first proposed method requires the measurement of both E and magnetic (H) fields on an evaluation surface. To this extent, E and H fields are measured sequentially and then combined to calculate the local PD value. This activity is extremely time consuming since both field strengths must be measured in all three orthogonal directions, and their time-variability counted continuously.