# Short-Range Noncontact Sensors for Healthcare and Other Emerging Applications: A Review
## 3 Recent Technical Advancements
### Hybrid FMCW-Interferometry Radar  
Although multiple critical advancements have been made for the conventional microwave radar sensor, it still has some limitations to fully monitor the real-time life activities. The biggest challenge is how to provide the concurrent information of distance and displacement at low cost. The main stream radar architectures, that is, Doppler (interferometry) radars, impulse radars, FMCW radars, and stepped frequency-modulated continuous-wave (SFCW) radars, cannot meet the requirements. For example, Doppler radars uses single-tone RF signal to obtain phase history. Although they have been widely used for high-precision displacement measurement, they can hardly detect the absolute distance information. There is a challenge for a single Doppler radar to distinguish multiple targets in light-of-sight view of the radar. The impulse, FMCW and SFCW radars are capable of detecting the absolute distance. Nevertheless, large bandwidth is needed to achieve the range resolution needed for monitoring in-home life activities and many of them are forced to work at high frequencies such as millimeter wave to archive the required large bandwidth. The associated problems such as linearity, cost, complexity and high attenuation prevent them from ubiquitous applications. A hybrid FMCW-interferometry radar was proposed in [44; 45] for precise 2-D positioning and life activities surveillance. The proposed hybrid radar works in the 5.8 GHz ISM band with a 160 MHz bandwidth. Based on CW architecture, the FMCW mode and the interferometry mode are incorporated together. The FMCW mode is responsible for detecting absolute range while the interferometry mode is sued for sensing the relative displacement motions such as the tiny physiological signals. The two hybrid modes can share the same RF front end architecture as both of them are based on CW operation. To achieve good Tx and Rx isolation, two separate antennas are used for Tx and Rx, respectively. A turntable allows the antennas to scan over 360\({}^{\circ}\) to get the 2-D information. Figure 16 shows the proposed transmitted signal in time domain. The FMCW signal has the same amplitude with the Doppler signal in time domain. The transmitted signal is basically a sequence of chirp period embedded into single-tone interferometry signal. The interferometry signal has a fixed operating frequency, while the FMCW signal is a chirp signal that is an up-ramp linear frequency-modulated signal. At radar receiver, by mixing a copy of the transmitted signal with that backscattered from the subject, the baseband output contains a series of the FMCW signal and the interferometry signal. Strict clocking synchronization is necessary within the radar system between the signal generator, the digitizer, and the baseband signal processer. If the clocking is not well synchronized, the difference between two clocks will accumulate and eventually misalign the actual location of the beat signals that are used for extracting the range information. The range information is calculated based on the baseband beat signal. Figure 15: Microphotograph of the 5 GHz software-configurable Doppler radar sensor [75]. As shown in Figure 16, the beat signal occur with a fixed interval, so that once the location of one beat signal is determined, those of the following beat signals can be easily predicted based on the FMCW burst interval. Details about the signal processing approaches to extract the range and displacement information can be found in [45]. Figure 17 shows the real 2-D location and life activities monitoring. Figure 17a is a panorama photograph of the experimental room, which has an octagon shape and is filled with several objects such as sofa, a table, a vending machine, and a trash can. Figure 17b shows the 2-D location map obtained by FMCW detection mode and the comparison between detection and the actual location is shown in Figure 17c. The outline of the room is clearly plot on the map as well as two people sitting at the directions of 11 o'clock and 9 o'clock.