Digital-IF CW Doppler radar and its contactless healthcare sensing

According to the definition of the Doppler effect, walking and running can generate the frequency shift when the human target is moving forth or back with respect to the radar sensor. Compared with walking and running, the periodic body movement (i.e. breathing and heartbeat) and non-periodic body movement cause the microDoppler effect. The vital sign-induced micro-Doppler effect, which is also called vital Doppler [1], has been widely used for radar-based non-contact vital sign detection. Various types of radar sensors have been developed for non-contact vital sign detection including single-carrier continuous-wave (CW) Doppler radar, frequency-modulated CW radar, stepped frequency CW radar and ultra-wideband radar [2-6]. The homodyne CW Doppler radar is widely used due to its simple structure and low cost [2]. However, it encounters several problems such as quadrature channel imbalance and DC offset [2]. Fortunately, the digital receiver with direct intermediate-frequency (IF)-to-digital conversion (IF sampling) is a suitable alternative [7-13]. In this chapter, the principle of the digital-IF Doppler radar is introduced. Then, the radio frequency (RF) layer, IF layer and optimised baseband signal processing are discussed. Recently, developed from the conventional application of vital sign detection, the digital-IF Doppler radar is widely applied for healthcare sensing. The digital-IF Doppler radar can not only work as a single sensor but also serve as a sensor in the multi-sensor network. After the introduction of radar sensor, two applications, i.e. non-contact beat-to-beat blood pressure (BP) estimation and multi-sensor-based sleep-stage classification, are briefly introduced.