# Short-Range Noncontact Sensors for Healthcare and Other Emerging Applications: A Review
## 3 Recent Technical Advancements
### Concurrent Detection of Range and Displacement  
The hybrid FMCW-interferometry radar is capable to detect the absolute range and the relative displacement at the same time. The conventional Doppler radar is only able to capture the Doppler signature due to the lack of bandwidth information. However, by modifying the hardware system architecture, the conventional Doppler radar could be equipped with the capability of ranging. A Doppler radar system with adaptive beam-steering antenna was presented in [76]. With single-tone  
Figure 16: Transmitted signal of the FMCW-interferometry radar. Figure 17: (**a**) The panorama photograph of the experimental environment; (**b**) 2-D location map obtained by FMCW detection mode; (**c**) room layout and objects distribution in the test room; (**d**) the comparison between detection and the actual location. unmodulated transmission, the proposed radar is capable of concurrent detection of vibration and distance at the same time. The detection theory is based on the fact that the DC offset in the baseband signal contains useful information on distance. In prior works [77], dynamic DC offset tracking has been realized to adaptively calibrate out the DC offset for motion imaging and large displacement sensing. The radar system proposed in [76] extracts the phase from the DC offsets and processes the phase difference between propagation paths at different antenna beam-steering angles. The absolute distance information could be accurately calculated with a fast beam-switching antenna in homodyne architecture [76]. As shown in Equation (3), the residual phase in radar baseband signal is given by:  
\[\phi_{1}=4\pi d_{o}/\lambda+\varepsilon_{o}+\Delta\theta\left(t\right) \tag{13}\]  
Since the phase noise of the radar system can be cancelled out due to the range correlation effect [8], the value of the term \(\Delta\theta\left(t\right)\) is \(\propto 1\) and can usually be neglected. \(\varepsilon_{o}\) is the summation of the constant phase shift from the reflection surface and the constant phase delay in Rx. Therefore, the total phase residue is a function of the distance \(d_{o}\). The distance information could be extracted by using an adaptive beam-steering antenna as shown in Figure 18.  
The Doppler radar with beam-steering antenna is able to radiate the beams in two directions alternatively and fast. If the angle of two radiation beams is \(\theta\), the total residual phase with the angle of \(\theta\) of the beam would be:  
\[\phi_{2}=\frac{4\pi d_{o}\cdot sec\left(\theta\right)}{\lambda}+\varepsilon_{o }+\gamma+\Delta\theta\left(t\right)^{\prime} \tag{14}\]  
where \(\gamma\) is the phase difference of the shifter on the antenna when the beam steers. \(\Delta\theta\left(t\right)^{\prime}\) can be negligible due to phase correlation effect. The difference between Equations (13) and (14) is:  
\[\phi_{2}-\phi_{1}=\frac{4\pi d_{o}}{\lambda}\left(\sec\left(\theta\right)-1 \right)+\varepsilon_{o}+\gamma \tag{15}\]  
Once the phase difference as in Equation (15) is known, the distance can be determined as:  
\[d_{o}=\left[\left(\phi_{2}-\phi_{1}\right)-\left(\varepsilon_{o}+\gamma\right) \right]\cdot\frac{\lambda}{4\pi\left(\sec\left(\theta\right)-1\right)} \tag{16}\]  
The total residual phase can be calculated with the desired DC offset values. The phase difference can then be obtained. Figure 19 illustrates the simulation result of the locations of the \(I/Q\) trajectory when the antenna steers beams. The initial total residual phase is \(0.25\pi\) and the trajectory sits in the first quadrant. However, when the antenna beam is steered, the trajectory moves to the forth quadrant and the total residual phase is now \(-0.25\pi\). The phase difference of \(0.5\pi\) could then be used to predict the absolute distance according to Equation (16). Figure 18: The Doppler radar with beam-steering antenna could radiate the signal to the target in two directions with an angle of \(\theta\).