# Short-Range Noncontact Sensors for Healthcare and Other Emerging Applications: A Review
## 3 Recent Technical Advancements
### Random Movement Cancellation  
The first challenge that hinders the ubiquitous use of the radar technology may be the random body movement. The radar sensing is based on the illumination of radio frequency radiations to the subject. Since the antenna has certain radiation beam-width, all the targets within the radiation coverage would be illuminated and reflect signals back to the radar receiver. If the body is not stationary, its movement will also modulate the radar signal and will be mixed with heartbeat and respiration. The body movement is usually much larger than the heartbeat and respiration, which challenges the extraction of the expected vital sign signals [(69; 70)]. The body movement may be removed in digital signal processing if it has a regular motion pattern. However, in practice, the body most likely moves randomly, which challenges the signal processing techniques to predict the precise motion trajectory. Researchers have proposed several approaches to deal with the noise and artifacts such as the random body movement. A multi-radar system was introduced in [(54)] to detect the subject from different sides of the body. Figure 6 shows the setup of using two radars measuring from the back  
Figure 5: Demodulation of a 0.2 Hz 16 mm displacement movement in 2.4 GHz Doppler radar using (**a**) regular arctangent demodulation, and (**b**) extended DACM algorithm. Phase discontinuity is seen in (**a**) because the phase exceeds the range of \((-\pi/2,+\pi/2)\). and the front of the body simultaneously. The two radars as shown in Figure 6 are identical and both use patch antennas with linear polarization. To mitigate the interference to each other, antennas on one of the radars can be rotated 90 degrees so that the two sets of antennas on the two radars are orthogonal [57]. As heartbeat and respiration are periodic movements with small amplitude relative to the radar wavelength, they have the same movement direction from the two radars' perspective. However, for random body movement, when the body leans to one of the radars, it moves away from the other. That said, the distance change between the body and one radar is opposite as compared to that between the body and the other. By combining the measured signals from the two radars, the noise from random body movement can be mostly canceled [57]. A similar approach is proposed in [10] using two injection locking radars array for cancelling the body movement artifacts. By sweeping the radar's carrier frequency or adjusting the subject's position, it has experimentally proved that the vital sign signals can be successfully detected when the subject is jogging on a treadmill [10]. The sensors array approaches have apparent drawbacks. For example, they inevitably add to the system complexity, cost and power consumption. They are favorable for specific use cases where there is enough space to set up the sensors array. A radar-camera hybrid system using compensation was proposed in [69; 70] to cancel the random body movement, the setup of which is shown in Figure 7. This approach proves the feasibility of using an ordinary smart phone camera to help to cancel the body movement. It is pretty attractive considering the nowadays' popularity of smart phones. It works in such a way that the external phase information that is opposite to the phase information of random body movement is added in the radar received signals. Since the radar measured signal includes phase information of both vital sign and body movement, the added information could be used to only compensate the phase information of the body movement but retain the vital sign signals. Three motion compensation strategies were proposed in [70]: (a) phase compensation at RF front end using a phase shifter; (b) phase compensation using baseband complex signals; and (c) compensation after phase demodulation.