# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 8 Challenges of In-Vehicle Measurements  
### Motion Artifacts  
When measuring biosignals unobtrusively, coverage and accuracy can be relatively low compared to clinical modalities. By using methods for sensor fusion, both quantities can be improved by exploiting redundancies in multiple measurement channels. When considering contact-free monitoring of vital signals, motion artifacts always pose a problem [181]. Compared to classical, cable-bound sensing, they are always more severe.  
In particular, radar-based measurement systems are susceptible to motion artifacts, since they derive a vital sign by means of a distance measurement. To cope with this problem, [PERSON] et al. developed a radar system using a transmit and re-transmit approach. The system uses two antennas, one being located in front and one being located behind the patient [182]. Vital signs, in particular respiration and heart rate, will move the patients towards or away from both antennas at the same time (in phase), while artifact movements will cause a out-of-phase movements (towards one antenna and away from the other antenna, and vice versa). The system makes use of these circumstances by first transmitting and receiving a signal with one antenna and then feeding the received signal into the other antenna. When received by this antenna, the signal contains in-phase signal parts associated with the vital signs (constructive interference) and out-of-phase signal parts from the artifact movement (destructive interference). Thus, despite artifact movement, the system can successfully detect heart and respiration activity.  
Furthermore, most other modalities also have specific problems. Camera-based methods, for example, only work if the subject under test is in the field of view. Capacitive biopotentials may suffer from common mode problems resulting from maladjusted clothing combinations (e.g., polyester-cotton, silk-cotton, etc.).  
[PERSON] and [PERSON] [183] have analysed the contribution of lateral and transversal changes in distance between electrode and skin. While global triboelectricity equally affects all electrodes and acts as a common-mode interference, local transversal changes in distance directly affect the coupling capacity of the electrode-skin combination. This can be modelled as a time-dependent coupling capacity.  
[PERSON] et al. demonstrated in 2011 that additional factors contributing to motion artifacts in capacitive ECG are local triboelectric effects on the electrode-body interface. These seem to depend on the relative distance in the triboelectric series [46]. If combined with lateral movements, this relative distance determines how sensitive different fabric combinations are to common mode signals due to local triboelectricity.  
Several groups have proposed algorithmic and technical methods to fight motion artifacts. In 2011, [PERSON] and coworkers proposed the use of adaptive filters for artifact cancellation [184]. Afterwards, [PERSON] et al. [185] and later [PERSON] and coworkers [181] proposed to analyse the electrode-skin interface by injecting high frequent test currents. This means that the interface is characterized by some sort of impedance measurement. In 2016, [PERSON] et al. demonstrated that motion artifacts in radar systems can be greatly reduced using multiple signal classification (MUSIC) [178]. The authors investigated three different driving conditions: halted vehicle, driving at 60 kmh and driving at 80 kmh. Using a classical, FFT-based approach, the average error rate regarding the heart rate was below 3 ppm when the vehicle was halted, but excessive (approx. 25 bmp) at 60 and 80 kmh. Using the MUSIC approach, the error rate was reduced in all three driving conditions and stayed below 3 bpm.