# Physiological Driver Monitoring Using Capacitively Coupled and Radar Sensors
## 2 Materials and Methods
### Radar Sensor System and Positioning in the Car Seat  
The radar sensor, shown in Figure 3a, consists of a radar unit, two 6 dBi antennas, and the Avnet PicoZed 7015 module which integrates a dual-core processor and a Field Programmable Gate Array (FPGA). The radar unit is based on an FMCW architecture and transmits series of FMCW signals, called chirps, whose instantaneous frequency increases linearly over time. The chirps have a total bandwidth \(B\) of 750 MHz, in the 7.3-8.05 GHz frequency range, involving a radar range resolution of 20 cm. Moreover, they are \(T=102.4\) us long and are transmitted with a pulse repetition interval (PRI) of 3.072 ms, ensuring a sampling rate of the Doppler (vital signs) signal of 1/PRI = 325.52 Hz. The transmitted power is \(-6\) dBm. Details on the radar-based vital signs sensing and on the FMCW theory can be found in the authors' prior art [23; 24].  
Figure 2: Signal processing path applied to the capacitive sensor data including ccECG and reference ECG (**a**) and ccBIOZ and reference respiration signals from a strapped belt (**b**). The signal processing for ccECG and reference ECG in (**a**) includes performance evaluation of the signals without applying artefact handling algorithms (upper path) as well as the result of these algorithms (lower path branch).  
The block diagram of the radar sensor system is show in Figure 3b. The chirp generator was designed through an analog Voltage-Controlled Oscillator (VCO) and a Fractional-N Phase Locked Loop (PLL). Each chirp is divided in two branches by a power splitter. One output feeds the transmitter antenna while the other output is connected to the mixer. The received chirps, which are reflected from the driver, are picked up by the receiver antenna, amplified by a low noise amplifier (LNA), and then mixed with the copies of the transmitted chirps. The resulting baseband signals are filtered and amplified before being acquired and digitized by an analog-to-digital converter (ADC). The latter are preprocessed in the FPGA which performs digital filtering, windowing function, Fast Fourier Transform (FFT), and then sends the resulting complex samples to the laptop by Ethernet.  
The radar was placed behind the driver's seat (Figure 3c,d). In order to maximize the heartbeat detection and to be able to monitor the respiration, the radar was positioned such that its line of sight (LoS) was in the middle of the back at the height of the chest. One advantage of operating at this frequency range is that the seat material does not obstruct the propagation of the electromagnetic waves. An accelerometer is integrated in the radar board to measure vibration of the radar caused by  
Figure 3: (**a**) Photo of the radar sensor; (**b**) Block diagram of the radar sensor system; (**c,d**) Experimental set-up and physical positioning of the radar sensor in the car.  
the car motion. It is fairly assumed that this vibration is very similar to the vibration of the driver's back due to the motion of the car when he/she leans against the seat. The z-axis of the accelerometer coincides with the radar LoS, which gives the highest contribution in the Doppler shift.