## 3 A Taxonomy of Measuring Methods, Techniques, and Sensors in Physiological-Based DMSs
### RADAR Techniques  
The methods for monitoring physiological parameters based on the radar technique use high-frequency electromagnetic waves that are emitted by a transmitter and reflected from the surface of the chest, thus carrying information about the displacement of the  
Figure 5: BCG/SCG method and sensor placements for acquisition of the driver’s BCG/SCG signal. human chest and, to some extent, also depending on the depth of penetration, about the movements of internal organs. The reflected waves are captured by a receiver and further analyzed to get information about movements caused by both respiratory and cardiac activity. The type and shape of the emitted signal depends on the type of radar. There are four types of radars that can be used to detect drivers' vital signs: continuous wave (CW) radar, frequency-modulated continuous wave (FMCW) radar, step-frequency continuous wave (SFCW) radar, and ultra-wideband pulsed radar (UWB-IR). An important specificity in the operation of radar sensors in DMSs is the high probability of interference with communication devices operating in the same location and sharing the same frequency spectrum, such as 5G, 6G, and IoT devices. To operate normally in the radio spectrum, all radar sensors must be compatible with standards and regulations for unlicensed activity for the respective countries. For Europe, the standards required for compliance are set by the European Telecommunications Standards Institute (ETSI) and European Communication Committee (ECC). For ultra-wideband pulsed radars [80], the ranges 3.4-4.8 GHz and 6-8.5 GHz are used in DMSs, and for continuous wave radars of the types applicable in DMSs [81], the ranges are 24-24.25 GHz and 76-81 GHz. The CW radars emit a continuous, unmodulated, high-frequency signal. The echo signal is received and processed continuously. Their function is based on the Doppler effect, which is used to detect and capture the movements of breathing and heart activity. In DMSs, the CW radars are used because of their cost-effectiveness and relatively simple design. Over the past decade, a number of studies have been published related to radar applications for monitoring vital parameters, such as HR and RR, while driving. The different locations of the sensors are studied, aiming to find the optimal place in the interior of the vehicle [82] (Figure 6). In several studies, the radar system is located about half a meter in front of the driver, with the antenna pointed directly at their torso [83], placed on and around the dashboard and center console, on the steering wheel, or located on the sun visor above the driver's head with a field of view of their chest area [84]. In other studies, the CW [85] or FMCW [86] radar sensors are integrated into the backrest of the driver seat in close proximity to the driver's heart area. This placement ensures optimal detection of the driver's HR and RR parameters by providing a stable and unobstructed line of sight to the driver's chest area. Unlike CW radar, FMCW radar can change its operating frequency during the measurement, thus the transmitted signal is frequency modulated. FMCW radars measure both speed and distance, which makes them suitable for monitoring drivers' breathing and heart rate. In 2022, [PERSON] et al. [87] proposed a system for monitoring RR, HR, and HRV in the presence of drivers' motion artifacts, using commercial millimeter-wave radio. The system performance was evaluated in a real driving environment, and the authors  
Figure 6: Radar method and sensor locations for acquisition of the driver’s RR and HR signals. reported results for RR, HR, and HRV estimations, corresponding to the relative accuracies of 99.17%, 98.94%, and 94.11%, respectively. In 2023, [PERSON] and [PERSON] [88] presented a system with a 60 GHz FMCW radar sensor for continuous measurement of chest wall displacements and monitoring HRV and blood pressure in drivers. The sensor was mounted on the steering wheel and the measurements were performed while driving.