# Short-Range Noncontact Sensors for Healthcare and Other Emerging Applications: A Review
## 1 Introduction  
Since the first radars were developed as early as the 1930s [1], wireless sensing using radio frequency (RF) and microwave has become a well-established and mature field of engineering and applied sciences. The radar operation is based on the principle of electromagnetic backscattering [2]. The mainstream radar hardware and computational methods were developed for detection and tracking of large moving subjects at large distances, e.g. air-defense system and marine radars. In radar applications, \"long range\" refers to the distance between the radar and the target in the order of hundreds of meters or even kilometers, while \"short range\" usually means a few meters or less. It was in 1970s that the short-range radar technology was first introduced for healthcare applications of detecting human vital signs [3]. Without any sensor attached to the body, radar is able to wirelessly detect the tiny physiological movements due to heartbeat and respiration [4; 5; 6; 7; 8; 9; 10]. The non-contact detection of vital signs led to several potential applications such as sleep apnea detection [11], physiological monitoring [12], and earthquake rescue [13]. Owing to microwave penetration, the radar technology can detect human activities under debris after earthquakes [6; 13]. In sleep apnea monitoring, if the subject's vital sign signal disappears, an alarm is triggered to either wake up the subject or warn the subject's guardians so they may take actions immediately [11]. Radar is a favorable approach for sleep monitoring, because it can accurately monitor the subject's physiological signals and body movements without any devices being attached to the body. In contrast, the wearable technologies require the subject wear the device during sleep, which may affect the quality of sleeping [14]. Besides the healthcare applications, the radar sensor is also capable of detecting various mechanical movements in a noncontact manner [15; 16; 17], which leads to some emerging applications such as occupancy sensing [18; 19] and gesture sensing for human-machine interaction [20; 21]. According to the system architecture, the radar family is generally classified into the following categories: continuous-wave (CW) radar [22; 23; 24], frequency-modulated continuous-wave (FMCW)radar [25; 26; 27; 28], and impulse radar [29; 30; 31]. Due to its broadband nature, impulse radar is not favorable in terms of system integration and low-power operation. It is not appealing in short-range applications because of the limited time delay from the short backscattering round trip. The short time delay requires an extremely high-speed switch and the broadband impulse needs to be captured by a power-hungry analog-to-digital converter (ADC) with very high speed [32]. In contrast, CW and FMCW allow higher level of system integration and lower power operation, which make them more attractive for mobile and portable applications [33; 34; 35]. FMCW is capable of both ranging and Doppler sensing but CW can only rely on Doppler information to track relative movements. In other words, FMCW radar can measure both absolute distance and relative displacement movements, while CW radar is only able to detect relative motions due to the lack of modulated spectral information [36; 37; 38]. Although both FMCW and CW radars can measure relative displacement motions, CW radar is superior to FMCW because of higher measurement accuracy, less complex hardware architecture and simpler signal processing approaches [39]. Therefore, for applications that only require relative displacement information, CW radar is a better option. Such scenarios include non-contact vital sign detection [40], sleep monitoring [41], mechanical vibration sensing [42], structural health monitoring [43], and so forth. On the other hand, FMCW radar is a better option for applications that need absolute distance information, such as monitoring the versatile life activities [44; 45] and interactive gesture sensing [20; 46; 47].