Radar-based system overcomes breathing-motion artifact to improve remote sleep monitoring

June 5, 2021
A newly developed method of sleep monitoring uses radar signals rather than wires and sensors. (Shutterstock)

A newly developed method of sleep monitoring uses radar signals rather than wires and sensors. (Shutterstock)

A novel approach to monitoring sleep with remote sensing and radar signals that uses a new active motion suppression technique to address an artifact from breathing motion has been invented by researchers from Arizona State University, potentially providing an alternative to both high-tech wearable devices and the sensors used in clinical sleep studies.

The team filed a patent application, published by the World Intellectual Property Organization on May 6, that details their new method and system for remote sleep monitoring as well as their novel sleep-monitoring device. Their contribution can monitor a sleeper's vital signs, analyzing them in order to classify a subject's sleep state. This can then be used to diagnose sleep conditions such as insomnia and sleep apnea.

Daniel Bliss, a professor at Arizona State University and one of the inventors, told The Academic Times that he and his colleagues have been studying the viability of using radars in elder care for several years in order to take physiological measurements. Radars use radio waves to detect the distance, angle or velocity of objects, including planes, ships, missiles and spacecraft. In this context, they can be used to gather physiological measurements, such as breathing and heart rate.

In recent years, radars have gotten smaller in size and become cheaper to make and use and significantly more capable, Bliss said, explaining that obvious examples of radars, including those used to track the speed of cars on highways and planes at airports, are quite large and high-powered. But the growth of small-scale radars has offered a range of new applications, which have been the focus of Bliss's ongoing research.

"The size of these small radars is getting to the point where they're postage-stamp size," Bliss said. "You might embed it in something bigger, but the device itself is fairly small."

To monitor sleep, the researchers' proposal is that "anytime you would like to know the well-being of an individual, you can use these radars for noncontact measurements of breathing, heart rate and potentially some other things," Bliss said. He added that the measurements can be taken from up to a few meters away from the individual. The system measures not only the overall breathing rate and heart rate of a sleeper but also individual breaths and heartbeats.

The group's invention stands as an alternative to wearable sleep devices, a burgeoning market in the health and wellness technology space, which was valued at more than $12.5 billion in 2020. Sleep tracking has become very popular, with commercial devices from Apple, Fitbit and Tempur-Pedic promising to improve an individual's sleep quality by collecting data on sleep patterns and offering personalized feedback. 

Radars for sleep monitoring that can be mounted near the sleeper, such as the one proposed in this patent, can also improve upon or replace traditional monitoring methods used in clinical sleep studies, which often involve invasive wiring that makes it hard to fall asleep.

"If you could monitor the person who's sleeping without some wires, the monitoring itself is far more useful, and the study itself is far more effective," Bliss said.

"Furthermore, several of these approaches also require overnight monitoring in clinics, increasing cost and discomfort of patients" the researchers said in the patent.

Bliss explained that radar usually sends out radio-frequency signals in pulses that propagate at the speed of light, bounce off of whatever they find, and then return to the radar system. The radio-frequency signals are able to travel about one foot every billionth of a second, and the radar system counts the number of billions of seconds that the radio frequency takes to go out and come back. The device can send out millions of pulses per second and use all those pulses together to track the chest motion of a sleeping subject.

"When you breathe, you move a little bit," Bliss said. "Your chest might move a centimeter. And so, what we can do is, with that radar, we can watch that very subtle motion. Because it takes slightly longer or a slightly shorter amount of time for that round-trip signal."

He continued, "People have been playing with using radars for monitoring physiological health for a while. Our signal-processing tools are at the point where it's actually starting to become viable."

To test their proposed system, the researchers mounted the prototype sleep-monitoring device above subjects sleeping on either a standard soft mattress or a rigid wooden bed frame. They recorded and processed radar response signals for both situations and found that it was easier to see breathing motion from the subjects sleeping on the rigid surface. Bliss explained that while it is more difficult to get an accurate measurement of vital signs from subjects on a soft surface, it is still possible using his team's methods.

"As you develop any system like this, you can get something to work in the lab once," said Bliss. "We are now focused on making it robust."

The researchers noted in the patent application that prior radar-based heartbeat-detection methods have been limited by the challenges associated with subjects lying on a soft surface. Importantly, their invention includes a new active motion suppression technique that can recover the heartbeat signal even when there is interference from the subject's breathing.

"This is certainly viable," Bliss said. "And there are real opportunities to take the academic things that we do and actually start helping people with their lives. I want to take the things we do and go out and make people's lives better."

The patent application, "Methods and systems for remote sleep monitoring," was filed Oct. 27, 2020. It was published May 6 by the World Intellectual Property Organization with the application number PCT/US2020/057452. The earliest priority date was Oct. 28, 2019, and the inventors are Yu Rong, Alex Chiriyath, Arindam Dutta and Daniel W. Bliss. The applicant is the Arizona Board of Regents on behalf of Arizona State University.

Parola Analytics provided technical research for this story. 

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