Accessing Wi-Fi for free at a cafe
MIT has invented a new Wi-Fi technology that can detect people who steal Wi-Fi by triangulating users' exact locations Jason Dorfman/MIT CSAIL

MIT has invented a new system that makes it possible to locate all the people who are trying to use your Wi-Fi, so that people who try to piggyback on a paid public Wi-Fi service without being a customer can be detected and kicked off.

The Chronos system, developed by a group of researchers led by Professor Dina Katabi at MIT's Computer Science and Artificial Intelligence Laboratory (CSAIL), enables a single Wi-Fi access point to locate and pinpoint users who walk into the vicinity– to within tens of centimetres.

So rather than your smartphone searching for available Wi-Fi networks, instead the Wi-Fi router of the airport, shopping mall or coffee shop you visit will find you instead, meaning you could one day be connected to password-less Wi-Fi because the shop or café you're in knows that you're definitely a customer.

At the moment, if you want to use Wi-Fi to triangulate exactly where visitors or customers are in a building, such as the airport, you will need at least four or five different Wi-Fi access points, because current Wi-Fi devices don't have a wide enough bandwidth to calculate the "time of flight" that it takes for data to travel from a user to an access point.

Also, Wi-Fi signals typically bounce off objects and walls, so the user receives multiple copies of the same signal, and each one has a different time of flight.

Wi-Fi triangulation

Instead, Chronos relies on only one Wi-Fi access point, but it is able to calculate the actual distance between the user and an access point by programming the system to jump between the different possible frequency channels of Wi-Fi to make many different measurements of the distance between the access point and the user.

When these measurements are stitched together, the system can calculate the time of flight calculation in one-billionth of a second (0.47 nanoseconds) and then multiply it by the speed of light to calculate the exact angle and distance between the user and the Wi-Fi access point.

"Knowing both the distance and the angle allows you to compute the user's position using just one access point," said MIT PhD student Deepak Vasisht, who is first author on the paper. "This is encouraging news for the many small businesses and consumers that don't have the luxury of owning several access points."

Smart homes and safer drones

The researchers envision the new technology being used for a wide range of functions other than kicking unauthorised users off Wi-Fi, especially as no other sensors are needed to detect people.

"From developing drones that are safer for people to be around, to tracking where family members are in your house, Chronos could open up new avenues for using Wi-Fi in robotics, home automation and more," said Vasisht.

"Designing a system that enables one Wi-Fi node to locate another is an important step for wireless technology."

The open access paper, entitled "Decimeter-Level Localization with a Single WiFi Access Point", was presented at the USENIX Symposium on Networked Systems Design and Implementation on 17 March 2016 in Santa Clara, California.