Sub-Nanosecond Time of Flight on Commercial Wi-Fi Cards

Technology

The Inventors’ approach is based on the observation that a very wideband radio (e.g., a few GHz) can allow time-of-flight measurement at sub-nanosecond accuracy. While each Wi-Fi frequency band is only tens of Megahertz wide, there are many such bands that together span a very wide bandwidth. Their solution therefore collects measurements on multiple Wi-Fi frequency bands and stitches them together to give the illusion of a wide-band radio. Their key contribution is an algorithm that achieves this, despite the fact that Wi-Fi frequency bands are non-contiguous, and in some cases, a few Gigahertz apart.

Chronos is a software-only solution that demonstrates the performance and practicality of their design by harnessing these algorithms to enable a pair of commercial Wi-Fi devices to locate each other without any support from the infrastructure. Chronos may be applied to personal drones that follow a user around and capture videos of their everyday indoor activities. Such drones can help monitor fitness, activities and exercise of users at home, work or the gym. Chronos allows a personal drone to maintain the best possible distance relative to its user to take optimal videos at the right focus. It achieves this by using the Wi-Fi card on the drone to locate the user’s device.

Problem Addressed

Time-of-flight is the time incurred by a signal to travel from transmitter to receiver and is a highly intuitive way to measure distances using wireless signals. However, attempts at using time-of-flight for indoor localization are not accurate due to fundamental limitations in measuring time on Wi-Fi and other RF consumer technologies. Alternative methods for RF-based indoor localization do not support indoor positioning between a pair of Wi-Fi devices without third party support. The Inventors have designed a set of algorithms to overcome the above limitations and measure the time-of-flight at sub-nanosecond accuracy using off-the-shelf Wi-Fi cards. This system, called Chronos, achieves accurate device-to-device localization without any support from the infrastructure, including the location of access points.

Advantages

• Chronos is the first RF-based positioning system that can measure sub-nanosecond time of flight on commercial Wi-Fi cards, estimating device-to-device distance measurements without any infrastructure support.
• System operates on typical 2/3-antenna Wi-Fi receivers yet delivers state-of-the-art localization accuracy. 

Publications

Vasisht, D., Kumar, S., and Katabi, D. "Sub-Nanosecond Time of Flight on Commercial Wi-Fi Cards." Massachusetts Institute of Technology, May 13, 2015.