# SDR-Enabled Multichannel Real-Time Measurement System for In Situ EMF Exposure Evaluation
## 2 Materials and Methods
### Receiver Section  
To simultaneously measure the three E-field components, a novel measurement solution based on a multichannel SDR having at least 3 synchronized radiofrequency channels was developed. In particular, the USRP (Universal Software Radio Peripheral) N310, state-of-the-art SDR multichannel equipment, was selected to complete the measurement system which is presented in Figure 5.  
The field in the setup presented in Figure 5 was generated by a signal generator model R&S SMBV 100 A equipped with option SMBV-K85 for Long Term Evolution-Advanced (LTE-A) signal generation connected to an Aaronia HyperLog 4060 directive antenna. This solution allowed to simulate an LTE-A uplink signal having a channel bandwidth of 20 MHz (100 Resource Blocks (RB)/slot) with Quadrature Phase Shift Keying (QPSK) modulation format. Figure 4: (a) S parameters measurement setup; (b) S11 measurement setup for the patch antenna. Three RF channels of the USRP were connected to the corresponding elements of the E-field probes. The USRP was connected via a 10 Gb Ethernet connection to the PC running an Ubuntu operating system. A GNU Radio flowchart was constructed to perform channel power (CP) measurements--Figure 6.  
GNU Radio is a free software development framework that provides signal processing functions for SDR. It is used by hobbyist and academia and supports both wireless communications research and operational radio systems. The latest applications of GNU Radio environment include near-field measurements [33], wireless communication system development [34; 35], spectrum sensing and availability [36; 37], and machine learning [37; 38]. The CP was measured based on the SDR capability to access the signal samples in the time domain. The instantaneous power level of the signal was calculated as the square root of the product between the sample power represented in complex form and its conjugate [39]. We have designed a user interface that enables both time and frequency visualization of the measured signal samples, as well as the calculated CP value. The instantaneous power values are inputted to a custom programmed block for data averaging and saving. The limitations of this USRP model in reporting accurate field levels and calibration procedures were analyzed in [28]. The SDR-enabled measurement system was tested by performing measurements on signals emitted by a mobile phone model Huawei P40 Pro (DUT). The DUT was sequentially  
Figure 5: (**a**) SDR-enabled measurement system, (**b**) SA-enabled measurement system. Figure 6: GNU Radio flowchart used for CP measurements using the SDR (USRP N310). measured while operating in Long Term Evolution-Advanced (LTE-A) communication standard and in the IEEE 802.11 ax Wi-Fi standard. During the LTE connectivity, the DUT was connected to the public designated base station and three experimental scenarios were considered:  
* file upload (large video files sent over WhatsApp)
* streaming (YouTube 4k video at 720 p)
* speed test (using a free available speed test app [40]). For the Wi-Fi connectivity, the DUT was connected to an IEEE 802.11 ax Wi-Fi router model TP-Link Archer AX10 operating in the 5 GHz frequency range (placed in the same room at 5 m distance from the DUT). During the Wi-Fi connectivity four experimental scenarios were considered:  
* file download--20 MHz channel bandwidth (BW)
* file upload--20/40/80 MHz channel bandwidth performed by means of a free android application [41]. The E-field probe's sensing volume (pyramid tip) was positioned at 8 cm from the DUT, and the position of the DUT, probes, and router was kept constant during all measurements. Table 1 summarizes the main characteristics of the investigated experimental scenarios. The measured power values were transformed into E-field strength values by applying (3) (extracted from the E-field probe calibration file provided by the manufacturer).