Abstract
To achieve sensitivity, comfort, and durability in vital sign monitoring, this study explores the use of radar technologies in wearable devices. The study first detected the respiratory rates and heart rates of a subject at a one-meter distance using a self-injection-locked (SIL) radar and a conventional continuous-wave (CW) radar to compare the sensitivity versus power consumption between the two radars. Then, a pulse rate monitor was constructed based on a bistatic SIL radar architecture. This monitor uses an active antenna that is composed of a SIL oscillator (SILO) and a patch antenna. When attached to a band worn on the subject’s wrist, the active antenna can monitor the pulse on the subject’s wrist by modulating the SILO with the associated Doppler signal. Subsequently, the SILO’s output signal is received and demodulated by a remote frequency discriminator to obtain the pulse rate information.
Highlights
Heart rate variability (HRV) refers to the variation in the time interval between heartbeats
Because of HRV’s potential of providing critical information regarding mental and physical treatments and fitness tracking [1,2,3,4], the use of wearables to conduct long-term monitoring of heart/pulse rates of subjects has become increasingly commonplace [5], and various consumer products such as Fitbit wristbands and the Apple Watch have been developed in succession
The radial artery pulse on the wrist causes the active antenna to frequency modulated (FM) signals
Summary
Heart rate variability (HRV) refers to the variation in the time interval between heartbeats. The difficulty is further intensified by the requirement of an instrumentation amplifier with high gain, low noise, and low power consumption to amplify tiny output signals for piezoelectric and ECG devices Another common noninvasive detection technology is photoplethysmography (PPG). Common solutions to the clutter problem include introducing a dual-antenna architecture, implementing a single antenna with high-isolation components to prevent crossover issues [23], and generating signals with the same amplitude with clutter but with an inverted phase [24] These methods pose challenges to the battery life and the device form factor that falls within a wearable size.
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