Skip to Content

How Wi-Fi sensing became usable tech

After a decade of obscurity, the technology is being used to track people’s movements.

February 27, 2024
a figure composed of many overlapping planes made of dashed outlines
Pablo Delcan

Over a decade ago, Neal Patwari lay in a hospital bed, carefully timing his breathing. Around him, 20 wireless transceivers stood sentry. As Patwari’s chest rose and fell, their electromagnetic waves rippled around him. Patwari, now a professor at Washington University in St. Louis, had just demonstrated that those ripples could reveal his breathing patterns. 

A few years later, researchers from MIT were building a startup around the idea of using Wi-Fi signals to detect falls. They hoped to help seniors live more independently in their homes. In 2015, their prototype made it to the Oval Office: by way of demonstration, one of the researchers tripped and fell in front of President Obama. (Obama deemed the invention “pretty cool.”) 

It’s a tantalizing idea: that the same routers bringing you the internet could also detect your movements. “It’s like this North Star for everything ambient sensing,” says Sam Yang, who runs the health-sensor startup Xandar Kardian. For a while, he says, “investors just flocked in.”

Fast-forward nearly a decade: we have yet to see a commercially viable Wi-Fi device for tracking breathing or detecting falls. In 2022, the lighting company Sengled demonstrated a Wi-Fi lightbulb that could supposedly do both—but it still hasn’t been released. The startup that made its case to Obama now uses other radio waves. One breathing-­monitor startup, called Asleep, set out to use Wi-Fi sensing technology but has pivoted to using microphones instead. Patwari also started his own company to make Wi-Fi breathing monitors. But, he says, “we got beaten out by Google.” 

Wi-Fi sensing as a way to monitor individual health metrics has, for the most part, been eclipsed by other technologies, like ultra-wideband radar. But Wi-Fi sensing hasn’t gone away. Instead, it has quietly become available in millions of homes, supported by leading internet service providers, smart-home companies, and chip manufacturers. Wi-Fi’s ubiquity continues to make it an attractive platform to build upon, especially as networks continually become more robust. Soon, thanks to better algorithms and more standardized chip designs, it could be invisibly monitoring our day-to-day movements for all sorts of surprising—and sometimes alarming—purposes.

Yes, it could track your breathing. It could monitor for falls. It may make buildings smarter, and increase energy efficiency by tracking where people are. The flip side of this, however, is that it could also be used for any number of more nefarious purposes. Someone outside your home could potentially tell when it’s vacant, or see what you are doing inside. Consider all the reasons someone might want to secretly track someone else’s movements. Wi-Fi sensing has the potential to make many of those uses possible. What’s more, this technology interprets the physical properties of electromagnetic waves, not the encrypted data they carry. It represents a new kind of privacy risk, and one for which safeguards are still being developed.

Google’s Sleep Sensing feature is built into its Nest Hub; it tracks breathing, snoring, and coughing for whoever is sleeping closest to the device, using not Wi-Fi sensing but a radar chip. Otherwise, Google’s approach is basically the same as Patwari’s: first use electromagnetic waves to sense tiny movements, and then use AI to make those movements make sense. The main difference is the length of the waves. Shorter wavelengths offer more bandwidth and thus more accuracy; longer wavelengths allow sensing over greater distances. The waves that ripple out from most Wi-Fi-enabled devices are two or five inches long: they can cover a lot of ground. The waves from Google’s radar chip, in contrast, are just five millimeters long and can provide much more detail. To come even close, Wi-Fi sensing needs to look at how waves from multiple devices interact. But if it can do that, it will combine detail with range—without the need for special radar chips or dedicated devices like wearables. If Wi-Fi sensing becomes a default option in smart devices like lightbulbs—a push that is already beginning to take place—then those devices can start monitoring you. And as Wi-Fi sensing technology improves, these devices can start watching in more detail.

Initially, says Patwari, “[Wi-Fi] resolution was pretty poor.” Locations were accurate to only two meters, and two people chatting next to each other could look like one person. Over the past decade, researchers have been working to squeeze more information from the longer wavelengths used by commercial routers. More important, they are using AI to make sense of metadata that describes how waves scatter or fade, known as “channel state information.” That gives them much more information to work with. Sixteen years ago, “we would be able to know pretty reliably that a person had walked by,” Patwari says. “But now, people are getting gait information—what somebody’s walking pattern is like.” Still, while Wi-Fi sensing is getting more detailed, the reliability of those details remains iffy. “The signal is just not clean enough,” says Yang. 

If Wi-Fi sensing becomes a default option in smart devices—a push already taking place—those devices can start monitoring you.

Meanwhile, AI advances that are helping Wi-Fi sensing improve are also helping radar. Some of the uses that made Wi-Fi sensing exciting a decade ago are now commercially available with dedicated radar devices that use shorter wavelengths.

Inspiren, a radar company working in hospitals and long-term-care facilities, combines data from radars and cameras mounted above beds for fall detection. It both alerts staff to falls and flags the moments when frail patients are most at risk of falling, like when they get out of bed. Yang’s sensor company sells an FDA-cleared medical device that can monitor heart rates from above hospital beds or jail cells—no wearables required. Some of these devices are already in use in Kentucky jails, where the goal is to help prevent overdoses and other medical emergencies. 

Then there’s a much creepier use case: spying through walls. Patwari earned a “Wi-Fi Signals Can See Through Walls” headline back in 2009 when the technology detected motion in another room. In January 2023 new versions of that headline reappeared, this time for a story about Carnegie Mellon researchers who used an AI engine called DensePose to generate body shapes from Wi-Fi signals. (The accuracy was far from perfect.) Radar that senses people through walls has existed for years; it is used by SWAT teams, border patrol, search-and-rescue teams, and the military. 

Yet Daniel Kahn Gillmor, a staff technologist at the ACLU, has flagged Wi-Fi sensing by state actors as a potential privacy concern, particularly for activists. “We have lots of examples of law enforcement overreach,” he says. “If law enforcement gains access to this data and uses it to harass people, it’s another chunk of metadata that can be abused.” 

Wi-Fi sensing is already replacing other motion detection tools. It may also help make some current radar applications widely available—albeit with less reliability in many cases. In both contexts, Gillmor says, it could be used by corporations to monitor consumers, workers, and union organizers; by stalkers or domestic abusers to harass their victims; and by other nefarious actors to commit a variety of crimes. The fact that people cannot currently tell they are being monitored adds to the risk. “We need both legal and technical guardrails,” Gillmor says.

Wi-Fi sensing may also usher in new forms of monitoring. With its longer wavelengths, Wi-Fi could cover more ground than millimeter-wave radar. As the MIT team demonstrated back in 2015, it might eventually detect falls in private homes instead of hospitals. But there is a reason Google’s Nest tracks breathing from one nightstand instead of from every lightbulb in the house: in the real world, context is hard for algorithms to parse. In a hospital room, a fall is probably a fall. In a home, Grandma falling looks a lot like a child jumping off the couch. So researchers are working on ways to reidentify known users. In addition to flagging falls, a tool like that could spot a burglar while a family is on vacation; depending on the settings and the context, it could also spot a teenager coming home after curfew, activists holding a meeting, or—in countries that enforce sodomy laws—two people of the same sex sleeping in the same bed.

Of course, further along the electromagnetic spectrum, security cameras and nanny cams can already track individuals with ease. “You’ve got to remember: the context you get out of the camera is just crazy,” says Taj Manku, CEO of the Wi-Fi sensing company Cognitive Systems. “You see the person’s face. You see whether they’re doing jumping jacks or they’re doing exercise or they’re doing something bad.”

But the mere fact that existing tools may achieve overlapping results, Gillmor says, does not lower the risk: “That way lies privacy nihilism.” 

Whether or not Wi-Fi can beat other sensors at their own games, integrating Wi-Fi sensing with those tools could eventually enhance the strengths of each. For now, commercial providers are taking advantage of Wi-Fi’s range to focus on home security, along with one other area where they believe that Wi-Fi sensing is already the best solution. Spence Maid, CEO of the Wi-Fi sensing company Origin Wireless, puts it this way: “I hate to even say it, but—‘Is Mom alive?’”    

Until a year and a half ago, Emily Nikolich, 96, lived on her own in a condo in New Jersey. Each day, her grandchildren sent her new photos in an app on her tablet. Each day, Emily could spy on her five great-grandchildren, ages newborn to four. Meanwhile, her son Paul Nikolich, 68, was spying on her.

In 2021, Paul installed a Wi-Fi sensing tool from Origin Wireless called Hex Home. Five small, glowing disks plugged in around Emily’s home—with her permission—helped Paul to triangulate her position. He showed me the app. It didn’t track Emily per se; instead, it tracked movement near each disk. But since Emily lived alone, the effect was the same: Paul could easily watch her daily journeys from bed to brunch to bathroom and back.

It was “a relief,” says Paul, to know that his mom was okay, even when he was traveling and couldn’t call. So when Emily moved into an assisted living home last year, the monitors came with her. Hex has learned Emily’s routine; if something out of the ordinary happens—if, for example, she stays in bed all day—it can send Paul an alert. So far, he hasn’t had any. “Fortunately, she’s been doing really well,” he says. 

In practice, Wi-Fi sensing still has a hard time with details. But it is very good at noticing human presence, regardless of walls or furniture. That accuracy, according to Manku? “It’s 100%.” That makes Wi-Fi sensing great for energy management (lightbulb maker WiZ uses it to turn the lights off in empty rooms) and for cutting back on false alarms from home security systems. It can also be helpful in places with aging populations. In Japan, Maid says, “they’re having the mail delivery people knock on doors and make sure people are still alive.” An Okinawa-based company is developing a proof-of-life service using Origin’s technology.

Manku estimates that at least 30 million homes already have some kind of Wi-Fi sensing available. One of Verizon’s new Fios routers now ships with Origin Wireless’s “human presence detection” built in. Stationary smart things already on the network—like lightbulbs, smart plugs, speakers, or Google Nests—can instantly become sensors. Other internet service providers are creating similar offerings; Cognitive Systems partners with more than 160 ISPs. This January, Cognitive Systems announced that its technology will soon be available in many of the cheap smart plugs for sale on Amazon, allowing people to use Wi-Fi sensing through their existing Google, Apple, and Amazon Alexa smart-home apps.

Eventually, the Wi-Fi sensing companies I spoke with would like to go even bigger: serving not just homes and small businesses, but also larger office buildings or stores. Wi-Fi sensing, Manku says, could help firefighters locate people behind smoke too dense to see through; smart HVAC could leave the AC on for people working late. Occupancy data could help companies make post-pandemic downsizing decisions; foot-traffic data could inform in-store product placement. But to be useful in those complex scenarios, Wi-Fi would need to accurately count and locate lots of people. 

Jie Yang, a researcher at Florida State University, is thinking bigger and in a slightly different direction: he is counting and locating people—and then tracking them individually. “Five years ago, most of the work focused on a single person,” Yang says. “Right now, we are trying to target multiple persons, like a family.” Recent research has focused on reidentifying target individuals when multiple people are present, using walking patterns or breathing rate. In a 2023 paper, Yang showed that it was possible to reidentify people in new environments. But for that research to work in the real world, even for just a handful of family members or employees, researchers won’t just need better AI; they will also need better hardware. 

That’s where Emily’s son Paul comes in. 

For the past 22 years, the younger Nikolich has chaired an obscure but influential group within the Institute of Electrical and Electronics Engineers: the 802 LAN/MAN Standards Committee, which sets the technical standards for Wi-Fi and Ethernet compatibility. 

In 2019, Nikolich attended an IEEE dinner in Washington, DC. Ray Liu, Origin Wireless’s founder and a recent IEEE president, was sitting across the table from him, discussing Wi-Fi sensing with another attendee. Nikolich started listening in. He had been thinking about how to wire—and unwire—the internet since around the time URLs were invented. But here, suddenly, was something different. “I was very excited about it,” Nikolich says. 

Nikolich and Liu started talking, and Nikolich expressed his support for a subcommittee devoted to Wi-Fi sensing. Since 2020, the 802.11bf Task Group for WLAN Sensing, led by experts from companies like Huawei and Qualcomm, has been working on standards for chipmakers designed to make Wi-Fi sensing easier. Crucially, when the new standards go into effect, the channel state information that Wi-Fi sensing algorithms use will become more consistent. Right now, that information requires lots of qualifying and debugging. When the new standard comes out in 2025, it will allow “every Wi-Fi device to easily and reliably extract the signal measurements,” Yang says. That alone should help get more Wi-Fi sensing products on the market. “It will be explosive,” Liu believes.

The longer-term use cases imagined by the committee include counting and finding people in homes or in stores, detecting children left in the back seats of cars, and identifying gestures, along with long-­standing goals like detecting falls, heart rates, and respiration.

Where such goals are concerned, three other IEEE subcommittees may also make a difference. The first is 802.11be, better known as Wi-Fi 7. Wi-Fi 7, which rolls out this year, will open up an extra band of radio frequencies for new Wi-Fi devices to use, which means more channel state information for algorithms to play with. It also adds support for more tiny antennas on each Wi-Fi device, which should help algorithms triangulate positions more accurately. With Wi-Fi 7, Yang says, “the sensing capability can improve by one order of magnitude.” 

The Wi-Fi 8 standard, expected in a few years, could lead to another leap in detail and accuracy. Combined with more advanced algorithms, Yang says, Wi-Fi 8 could allow sensors to track not just a few people per router, but 10 to 20. Then, sharing information between routers could make it possible to count and track individuals moving through crowded indoor spaces like airports. 

Finally, a less widely used standard known as WiGig already allows Wi-Fi devices to operate in the millimeter-wave space used by radar chips like the one in the Google Nest. If that standard ever takes off, it could allow other applications identified by the Wi-Fi sensing task group to become commercially viable. These include reidentifying known faces or bodies, identifying drowsy drivers, building 3D maps of objects in rooms, or sensing sneeze intensity (the task group, after all, convened in 2020).

There is one area that the IEEE is not working on, at least not directly: privacy and security. For now, says Oscar Au, an IEEE fellow and member of the Wi-Fi sensing task group who is a vice president at Origin Wireless, the goal is to focus on “at least get the sensing measurements done.” He says that the committee did discuss privacy and security: “Some individuals have raised concerns, including myself.” But they decided that while those concerns do need to be addressed, they are not within the committee’s mandate.

When Wi-Fi signals are used to send data, the information being sent back and forth over the electromagnetic waves can be encrypted so that it can’t be intercepted by hackers. But the waves themselves just exist; they can’t be encrypted in quite the same way.

“Even if your data is encrypted,” says Patwari, “somebody sitting outside of your house could get information about where people are walking inside of the house—maybe even who is doing the walking.” With time, skill, and the right equipment, they could potentially watch your keystrokes, read your lips, or listen to sound waves; with good enough AI, they might be able to interpret them. “I mean,” Patwari clarifies, “the current technology I think would work best is looking inside the window, right?” 

Wherever there is Wi-Fi, walls are now more porous. But right now, the only people who can do this kind of spying are researchers—and people who can replicate their results. That latter group includes state governments, Jie Yang confirms. “It’s likely that this is already happening,” Yang says. “That is: I don’t know that people are actually doing that. But I’m sure that we are capable of doing that.” 

So more than a decade after he first started trying to use Wi-Fi signals to reveal location information, Patwari is now trying to do the opposite. Recently, he completed a project sponsored by the US Army Research Office, designing strategies to introduce noise and false positives into channel state information to make it harder for unauthorized devices to spy. The EU recently sponsored a project called CSI-MURDER (so called because it obfuscates, or kills, the channel state information). There are plenty of reasons to prevent eavesdropping; for one, Patwari says, the US Army might want “to make sure that they can provide Wi-Fi on a base or whatever and not have audio of what’s going on inside the base eavesdropped outside.” 

Plenty of governments already spy on their own citizens, including the US and China—both hubs of Wi-Fi sensing research. That is a risk here too. Even though the most sensitive Wi-Fi sensing data is often stored locally, intelligence agencies could easily monitor that data in person—with or without a warrant or subpoena, depending on the circumstances. They could also access any reports sent to the cloud. For many Americans, though, the bigger privacy risk may come from ordinary users, not from government eavesdroppers. Gillmor notes that the tools already on the market for detecting human presence could create an extra hurdle for people experiencing domestic abuse. “I’m really glad to hear that a stalker would follow the Verizon terms of service, but color me a little bit skeptical,” he adds.

Palak Shah, who leads the social innovation lab at the National Domestic Workers Alliance, says she could imagine upsides for Wi-Fi sensing. “Wage theft is a very common problem in our industry,” she says. A tool that helps nannies, housekeepers, or care workers prove they were in the home could help ensure proper payment. But, she says, “it’s usually the case that things end up being used against the worker even if there’s a potential for it to be used for them,” and “that inherent power dynamic is really hard to disrupt.”

The National Domestic Workers Alliance has helped pass bills in several states to make it illegal to “monitor or record” in bathrooms. In comparison, Wi-Fi sensing is often touted as “privacy protecting” because it does not show naked bodies. But, Gillmor says, “just because it is a sensing mode that humans do not natively have does not mean that it can’t be invasive.”

In another sense, Wi-Fi sensing is more concerning than cameras, because it can be completely invisible. You can spot a nanny cam if you know what to look for. But if you are not the person in charge of the router, there is no way to know if someone’s smart lightbulbs are monitoring you—unless the owner chooses to tell you. This is a problem that could be addressed to some extent with labeling and disclosure requirements, or with more technical solutions, but none currently exist. 

I asked Liu what advice he would give to lawmakers wrestling with these new concerns. He told me one senator has already asked. “This is a technology that can help change the world and make lives better. Elder care, security, energy management—everything,” he says. “Nevertheless, we as a society need to draw a red line. Whatever the red line is—it’s not my job to decide—here is the red line we do not cross.”

Meg Duff is a reporter and audio producer based in Brooklyn. She covers science, technology, and climate change.

Deep Dive


How a simple circuit could offer an alternative to energy-intensive GPUs

The creative new approach could lead to more energy-efficient machine-learning hardware.

Digital twins are helping scientists run the world’s most complex experiments

Engineers use the high-fidelity models to monitor operations, plan fixes, and troubleshoot problems.

How gamification took over the world

Gamification was always just behaviorism dressed up in pixels and point systems. Why did we fall for it?

It’s time to retire the term “user”

The proliferation of AI means we need a new word.

Stay connected

Illustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at with a list of newsletters you’d like to receive.