Mini-Circuits ZHL-42 PA Tx Vivaldi Agilent 83622B RF Signal Generator 3 GHz 2.98 GHz Hittite HMC753 LNA Rx Vivaldi Agilent E8257D RF Signal Generator 100 MHz Ref. Out DDS Quadrature DDS 20 MHz Sin Cos 10 MHz Ref In FPGA Decimation Filter 2 MHz 2 MHz ADC Data I Data Q Multiplier Hittite HMC213 19-23 MHz Band Pass Mixer Sample CLK 60 MHz CLK Ref. CLK Figure 3. Block diagram of the CW radar prototype [21] © 2013 IEEE. Third Layer Second Layer RO4350 FR4 First Layer (a) Antenna Array 24-GHz Radar Chip Baseband Circuit Bluetooth Module MCU y z x (b) (c) Figure 4. Photograph of prototyped radar (a) the structure of three-layered PCB (b) top view (c) bottom view [27] © 2016 IEICE. received signal with the local oscillator signal. Despite this, the CW radar in Figure 3 enables the elimination of low frequency noise and interference, at the cost of added complexity. 48 IEEE CIRCUITS AND SYSTEMS MAGAZINE A recent study [22] demonstrated that a machine learning-based algorithm can be used to model heart beat signal using respiration artifacts. For this purpose, a CW radar is used to measure the results, which confirms that the heartbeat signal is independent and separable from respiration signal. Another interesting study in [23] proposed a time domain detection method using a CW Doppler radar to reduce the time required to obtain a high resolution spectrum of RR and HR. This process typically takes around 30 seconds. The proposed time domain peak detection algorithm is capable in acquiring RR and HR in a breathing cycle (of around 5 seconds). Meanwhile, the design of an SDR prototype for vital sign sensing and detection based on CW radar is presented in [25]. This research is aimed at investigating the validity of remotely sensed of RR and HR. Besides this, another interesting detection method based on CW radar is presented in [26]. Heart rate variability was able to be monitored in real time. The proposed method uses a combined frequency- and time domain technique to obtain the beat-to-beat interval. The low computational complexity of the proposed algorithm makes its application in real time possible. Next, in [27], a CW radar system was developed and implemented on a printed circuit board (PCB) to conduct vital sign detection measurement. It applies a 24 GHz transceiver chip (BGT24MTR11 from Infineon) with homodyne architecture. The prototyped three-layered PCB is shown in Figure 4. A proposed heart beat model and heart rate variability detection method was presented in [28] based on a 2.4 GHz bistatic quadrature CW Doppler radar. The FIRST QUARTER 2021