# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 7 Radar-Based Methods  
The term _radar_ actually is an abbreviation refering to _radio detecting and ranging_. This technique uses high-frequent electromagnetic waves that are emitted from a transmitter (Tx) and reflected by the chest surface thus carrying information on human chest displacement and, to some extent and as a function of penetration depths, on inner organ movements. The reflected waves are sensed by a receiver (Rx) and analysed, e.g., in shift of phase, etc. Radar methods are mainly carrying information on movement, which can of course be caused by both respiratory and cardiac activity. As mentioned in [166], the heart can typically cause chest displacements up to \(\pm\) 0.1 mm in relaxed humans (see also [167]), and respiration displacements of between 0.1 mm and several millimeters. Due to this, radar methods provide similiar information as ballistocardiography (BCG) methods. There are two general radar techniques: the pulsed _ultra wide-band_ (UWB) radar, often with a separate transmitter and receiver antenna, which is based on measurements of time-of-flight and derived quantities, and the _continuous wave_ (CW) radar which basically relies on the Doppler effect. In the U.S., the frequency range for UWB radar is regulated by the Federal Communications Commission (FCC) spans 3.1 GHz to 10.6 GHz. By contrast, in Europe, the allowed frequencies range from 3.4 GHz to 4.8 GHz and from 6 GHz to 8.5 GHz [168]. Vital sign monitoring in the frequency range of a few GHz is possible and has been demonstrated for UWB radar mode in many applications [169], including rescue missions (to _locate human subjects burried under earthquake rubble_ or _being covered under snow_ after avalanche accidents [170]). However, in this frequency range, antennas have been quite bulky and also the allowed transmission power is rather limited. Higher frequencies (like microwave radar in the near 60 GHz ISM band) offer some advantages such as higher sensitivity and smaller antennas [171]. However, at such high frequencies, the penetration depth of electromagnetic radiation into biological tissue is almost negligible and the radar system is mainly a BCG sensor. While this may be acceptable for respiration tracking, heart-related motions are very small and hard to detect. Early attempts to use radar for measuring vital signs date back to the 1970s. In fact, in 1971, [PERSON] and [PERSON] presented a 10 GHz radar system mounted on top of an infant incubator aiming at apnea detection [172]. Figure 14: Gradiometer sensor inside a resin block integrated into the driver seat (**a**), modified and extended from [165]. Middle and right figure are magnified views of the sensor from top (**b**) and from the side (**c**) showing the spatial structure of the gradiometer. In 2009, [PERSON] and colleagues reported an accuracy of \(\pm\) 1 heartbeat per minute for heart rate detection using a 2.4 GHz CW radar system [173]. Similarly, in 2009, [PERSON] and colleagues demonstrated in 12 healthy subjects that heart-rate variability (HRV) and respiratory sinus arrhythmia (RSA) can be monitored using continuous Doppler radar in the 2.4 GHz range [174]. In 2010, [PERSON] and colleagues shared their experiences using an impulse radio UWB radar system prototype manufactured by Geozondas, Lithuania, for vital sign monitoring [166]. In this work, the authors emphasized at the mathematics used for heart and respiration signal extraction. They also reported that the measurements were in compliance with the FCC mask (allowed dissipation power) almost everywhere in the 3.1 to 10.6 GHz frequency range. In 2011, [PERSON] et al. presented a measurement method for respiration using a 6 GHz CW-radar system [175]. Despite successfully detecting respiration activity up to 2.5 m, the use in automotive environments might be difficult since the system uses a large double ridge horn antenna. [PERSON] et al.