# Physiological Driver Monitoring Using Capacitively Coupled and Radar Sensors
## 3 Results
### Experimental Results from Radar  
In order to verify that the radar can monitor the vital signs from the back and through the seat, an initial test was performed with the car put in neutral with the engine on. The driver was invited to breathe normally. Figure 6 shows the results of this experiment. The respiration and heartbeat signals extracted with the radar and the references (i.e., belt and ECG) are in agreement. Moreover, in this static condition, as it can be seen from the small acceleration values, the vibrations from the car are negligible so no vibration removal algorithm is necessary.  
Figures 7-10 show a proof-of-concept result of vital signs monitoring while driving at 120 km/h on the highway. Figure 7 shows the time domain ECG signal from the reference and its relative spectrum. The heartbeat peak is clearly visible at 1.43 Hz in the ECG spectrum. Figure 8 shows the radar Doppler Signal after the linear demodulation, the z-acceleration, and their relative spectra. In the Doppler signal, the vital signs contribution is corrupted by the vibration. In fact, although the respiration signal is visible, the heartbeat is buried into the vibration. This is confirmed by the fact that there is no peak at 1.43 Hz in the Doppler signal spectrum while there is an in-band peak at about 1.17 Hz. Figure 9 is the result of the ICA and it shows the separated phase contributions of the vital signs and of the vibration. In the spectrum of the vital signs output of the ICA the 1.43 Hz heartbeat peak is clearly visible. Finally, Figure 10 shows the time domain signals of the extracted vital signs, of the reference, and the x-y-z accelerations. The average heart rate measured with the radar and with the ECG agree.  
Figure 6: Comparison between the vital signs extracted with the radar and the reference when the driver is normally breathing on the seat with the car put in neutral with the engine on. In this test, the vibrations of the car are negligible as indicated by the very small acceleration values. Due to the signal processing methods used, the displacement is expressed in arbitrary units and not in radians.  
Figure 7: Time domain ECG signal from the reference and its relative spectrum. The subject was driving at 120 km/h on the highway.  
Figure 8: Doppler Signal extracted using the linear demodulation technique (expressed in mV instead of radians due to the signal processing method used), the z-acceleration, and their relative spectra. The subject was driving at 120 km/h on the highway. The respiration signal is clearly visible both in the time domain Doppler signal as well as in the spectrum, but the heartbeat signal is buried in the noise. The reference ECG signal (normalized to the Doppler signal level to facilitate comparison) is superimposed in red.  
Figure 9: Result of the ICA algorithm. The output consists of two components: the vibration component and the vital signs component. The vibration component is nearly identical to the input accelerometer signal. In the vital signs component spectrum, the heartbeat spectral peak at 1.43 Hz (86 ppm) has been lifted out of the noise by the ICA algorithm, and agrees with the reference ECG spectrum superimposed in red (normalized to the radar signal level). The amplitudes are expressed in a.u. instead of radians due to signal processing methods used. The subject was driving at 120 km/h on the highway.  
As seen in Figure 10, this proof-of-concept experiment shows that the radar is able to correctly measure the respiration signal and, after applying the proposed motion artefact algorithms, also the average heartbeat during driving. Further validation will be performed, and further improvements to both the signal processing algorithms and radar integration inside the car are expected to improve the performance further, including the measurement of individual beat-to-beat intervals during driving conditions.