# Short-Range Noncontact Sensors for Healthcare and Other Emerging Applications: A Review
## 3 Recent Technical Advancements
### Hardware System Integration
It is possible that the 2.4 GHz interferences may fall in the radar band. However, the co-existence is usually not a severe issue as long as the radar sensor keeps a reasonable distance away from the noise source. As the vital sign signal is very narrow bandwidth, e.g. respiration is less than half a Hz and heartbeat is just over a Hz, the baseband amplifier is configured to be very narrow bandwidth to let the vital sign signals pass but will block the interferers in the 2.4 GHz ISM band. The baseband amplifier essentially has narrow bandwidth with a gain-bandwidth-product of less than a few MHz. If the gain is configured to be high such as 200, the baseband bandwidth would be minimized to just pass vital sign signals. The 2.4 GHz ISM technologies like WiFi have duty cycle in transmission, which further helps to avoid the co-existence issue. A quadrature mixer is used to directly convert the RF signal to baseband. Moreover, the mixer's differential output helps to better reject the common-mode noise and even-order harmonic distortions. The power management ICs includes a 3.3 V 500 mA low-dropout regulator (LDO), which supplies power to all RF and baseband ICs, and a tunable LDO, which is used to tune the VCO to be within the desired frequency band. Figure 12: Block diagram of the 2.4 GHz Doppler radar integrated on the printed circuit board (as shown in the shaded area). The \(I/Q\) signals from baseband amplifier's output would be sampled by the data acquisition system and further processed off-line. Besides the board level system integration, researchers have also made efforts to integrate the radar system on silicon [8; 26; 46; 48; 35]. The first effort of chip design for a biomedical radar dates back to the early 2000s [48]. The vital sign signal is very weak so the Doppler radar needs to be highly sensitive. To make the radar technology closer to real life applications, a 5 GHz software-configurable high-sensitivity Doppler radar system-on-chip (SoC) was proposed in [75]. Figures 14 and 15 illustrate the block diagram and the microphotograph of the high-sensitivity radar chip, respectively. A 3-wire control bus makes the radar software-configurable to set the operation point and the detection range for optimal performance. The proposed radar SoC is highly integrated that all the RF and analog functions are integrated on-chip. The baseband \(I/Q\) output can be directly sampled for digital signal processing. To achieve high sensitivity, special considerations have been taken on important issues such as flicker noise, baseband bandwidth, and the link budget. Analog-digital co-design was also carried out to optimize the system performance. The link budge analysis, which is dedicated to physiological sensing, shows that large gains in the receiver chain and low flicker noise in the down-conversion mixer and baseband are of key importance for the high sensitivity performance in detecting slow signals of respiration and heartbeat. The experimental results show that the designed radar sensor chip has a high sensitivity of better than \(-101\) dBm for sensing physiological signals when there is no random body movement. Figure 14: Simplified block diagram of the 5 GHz software-configurable Doppler radar sensor. Figure 13: Picture of the 2.4 GHz Doppler radar sensor.