Chronos WiFi that will locate you

From airports to restaurants to malls to hospitals, people can always be seen fidgeting with their phones to locate WiFi signals.  But what if the chaser becomes the chased? Instead of you finding WiFi, WiFi finds you. Yes, that is indeed possible.

Researchers at MIT's Computer Science and Artificial Intelligence Lab (CSAIL) have developed a system called "Chronos" which enables a single Wi-Fi access point to locate users within tens of centimeters, even in a crowd.  According to the published paper, this signal can follow a person and even block outsiders, a feature that can lead to safer drones, smarter homes, and password-free Wi-Fi. Chronos is the ability to calculate not just the angle, but the actual distance from a user to an access point, as determined by multiplying the time-of-flight by the speed of light.

'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. Designing a system that enables one Wi-Fi node to locate another is an important step for wireless technology," said Ph.D. student Deepak Vasisht.

The team led by Dina Katabi conducted the first experiment in a two-bedroom apartment with four occupants where Chronos correctly identified the room of a resident 94 percent times. In the second experiment,in a cafe, the system was 97 percent correct in identifying in-store customers from out-of-store invaders.

The system has been programmed to jump from channel to channel, gathering many different measurements of the distance between access points and the user. Chronos locates users by calculating the 'time-of-flight' that it takes for data to travel from the user to an access point, which is 20 times more accurate than existing systems. Existing localization methods require four or five Wi-Fi access points, however, for Chronos is 20 times more accurate than them, computing time-of-flight with an average error of 0.47 nanoseconds, or half than one-billionth of a second.

The success of Chronos suggests WiFi-based positioning could help for other situations where there are limited or inaccessible sensors, like finding lost devices or controlling large fleets of drones.

'By devising a method to rapidly hop across these channels that span almost one gigahertz of bandwidth, Chronos can measure time-of-flight with sub-nanosecond accuracy, emulating with commercial Wi-Fi what has previously needed an expensive ultra-wideband radio,' added Venkat Padmanabhan, a principal researcher at Microsoft Research India.

However, there are some glitches in the story. Chronos channel-hopping approach does come with its own set of complications; getting an accurate time-of-flight with this method still isn't easy, due to three sets of delays that happen during the transfer.

First, when you wirelessly send a piece of web data, there is a delay in detecting the presence of the 'packet' that is hard to distinguish from the actual time-of-flight.

To account for it, the team exploits the fact that WiFi uses an encoding method that transmits bits of packets on several even smaller frequencies.

Secondly, if you're indoors the W-iFi signals can bounce off walls and furniture, meaning that the receiver gets several copies of the signal that each experience different times-of-flight. Lastly, the team's channel-hopping approach leads to one other complication - every time Chronos hops to a new band, the hardware resets, adding a delay known as a 'phase offset.' To address this, the team used the fact that in Wi-Fi, you get an acknowledgement back for each data packet that your phone sends.

The team used these acknowledgements to intelligently cancel out the phase offsets.

'Imagine having a system like this at home that can continuously adapt the heating and cooling depending on the number of people in the home and where they are. Eliminating the need for co-operation between WiFi routers opens up many exciting new applications for localization,' said Dina Katabi.