# Short-Range Noncontact Sensors for Healthcare and Other Emerging Applications: A Review
## 3 Recent Technical Advancements
### Signal Distortion Analysis and Compensation
To deal with that, the RF tuning was implemented using a path of an attenuator and a phase shifter at the RF front end of the Doppler radar system, as shown in Figure 8. By doing so, it adds a portion of the transmitter signal back to the receiver so as to cancel out most of the DC offset at mixer output. To further calibrate the remaining DC offset and to minimize the quantization noise impact from ADC, the baseband fine-tuning circuitry is added to adaptively adjust the amplifier bias to the desired level. With the two tuning stages, it allows both high gain amplification and maximum dynamic range the baseband stage. With the proposed DC tuning architectures, the Doppler radar is able to precisely measure the low-frequency movement even if the motion trajectory has stationary moment. The proposed DC coupled radar was experimentally verified in the lab environment. Both the DC radar and AC radar were used to measure the actuator that was programmed to perform sinusoidal movement with a short period of stationary moment in between two adjacent cycles. The experimental results, as shown in Figure 10, illustrates that the DC radar matches with the actuator very well by successfully preserving the stationary information. However, owing to the fact that the AC coupling capacitors cannot hold the charge for a long time and they tend to discharge over the stationary moment, the stationary information is distorted in AC radar. Figure 9: Block diagram of the Doppler radar with DC coupled architectures including RF coarse-tuning and baseband fine-tuning. Using DC coupled radar inevitably adds to the hardware complexity. Several approaches have been proposed to use AC coupled radar for accurate displacement measurement without adding to radar architecture complexity. Reference [72] presents a method of calibrating \(I/Q\) signals in AC coupled radar to achieve high accuracy displacement measurement. The calibration is based on data-based imbalance compensation [59] and radius correction [73]. Nevertheless, the limitation is that a severely distorted ribbon-shaped \(I/Q\) trajectory may not be corrected to recover the actual displacement due to the fact that the displacement is directly translated from the length of the trajectory [58]. A more advanced technique of digital post-processing (DPoD) was proposed to compensate for the signal distortions in the digital baseband domain [58]. Without any cumbersome hardware modification, the simple quadrature direct-conversion receiver architecture can be used to detect the complete pattern of slow periodic motions. As many authors may know, pre-distortion technique is widely used in power amplifier's linearization. In contrast to that, the proposed digital DPoD technique applies signal compensation in the digital baseband to recover the signal information that may be lost in an AC coupled receiver. The signal distortion is compensated in the digital domain by an algorithm whose system response is the inverse function of that of the high-pass filter at AC coupled baseband. That being said, the DPoD technique tends to \"linearize\" the high-pass characteristics of the AC coupling so that the AC coupled baseband imparts virtual \"all-pass\" characteristics to the input signals. Therefore, the signal integrity can be preserved using the low-cost low-complexity AC coupled architecture without any hardware modifications. The DPoD technique was experimentally evaluated by using an AC coupled radar to measure a healthy adult at rest. It is known that the respiratory motion for an adult at rest includes a short period of stationary moment, which means that the respiration tends to rest for a moment at the end of expiration [33]. The experimental result in Figure 11 shows that, without DPoD, the information of stationary moment is lost so the respiration signal was distorted. After applying DPoD, the stationary moment is recovered so the complete respiratory patter is restored. Figure 11: Respiration motion measured by AC coupled Doppler radar with and without using the proposed DPoD technique.