# SDR-Enabled Multichannel Real-Time Measurement System for In Situ EMF Exposure Evaluation
## 1 Introduction
To overcome this issue, a phase reconstruction algorithm was proposed in [19]. The method refers to the measurement of the E-field component (both amplitude and phase) at a larger distance from the evaluation surface followed by the field back-projection. These systems can be used in laboratory conditions with controlled experimental setups to characterize antennas or perform compliance assessments. However, the mentioned methods are not applicable for in situ measurements, in the proximity of mobile devices, that require the simultaneous retrieval of the orthogonal vector-field components. Most commercially available field probes that can be used for frequency selective measurements are unidirectional [7], but some electronically switched isotropic probes/antennas are also available [20; 21]. The size of the probes (415 mm \(\times\) 87 mm--by R&S) does not recommend them for proximity measurements because of the low spatial resolution and significant field alteration. To measure the total field strength arising from all directions, typical isotropic sensors consist of three orthogonally aligned probes that are electronically switched on [22]. However, besides the probe's dimension, the dwell time or switching time of current isotropic antennas (10 \(\upmu\)s) [20] appears to be inadequate for the fast symbol variation of 5G NR technology--where the symbol duration is 16.67 \(\upmu\)s for a 60 kHz subcarrier spacing [23]. To solve the issue of the simultaneous retrieval of E-field vector components, we proposed a sensor prototype based on orthogonally positioned miniature near-field probes. The close positioning of the probes makes them susceptible to mutual coupling even in the receiving mode [24]. Thus, we can analytically analyze the mutual coupling phenomenon and evaluate the mutual coupling errors based on an original approach that requires the measurement of the probes scattering parameters (S). The proposed sensor must be connected to suitable frequency selective equipment to enable a real-time multichannel EMF field measurement with an adequate time response corresponding to the minimum time variations of the investigated communication technologies. Traditionally, for frequency-selective in situ measurements, spectrum analyzers (SA) are employed [25; 26]. With recent technological development, multi-channel instruments such as oscilloscopes [27] and software-defined radio (SDR) transceivers [28; 29] became a viable alternative for EMF measurements. Therefore, in this paper, we propose an SDR-enabled real-time multichannel measurement system to be used for in situ EMF exposure evaluations. The measurement system is analyzed, characterized, and compared to the standard solution of using SAs and a multichannel digital oscilloscope (DO). After calibration and validation, the SDR-enabled measurement system was used for an in situ measurement campaign of a mobile device operating in the LTE-A and IEEE 802.11 ax communication standards under different operating scenarios. The motivation of this research originates from the need to obtain synchronous real-time isotropic responses from the orthogonal field dipoles. This is required to eliminate the switching time spans between the sensor elements and to obtain a true real-time isotropic capability. Moreover, the designed measurement system must be flexible, agile, and customized for in situ measurement campaigns. Therefore, the novelty of our approach consists of a multichannel measurement system capable of retrieving simultaneous E- and/or H-field components. This advances the current isotropic measurement systems which are based on switching between the probes by providing the real-time simultaneous determination of highly stochastic signals on all three spatial axes followed by the further computation of the total E-field strength. In addition, we comparatively analyzed the measurement systems' performance, cost, and suitability with respect to the accurate evaluation of 5G/6G-emitted fields in the proximity of mobile devices.