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A study on a way of connecting battery-free devices to Wi-Fi

The "Internet of Things" is based on sensors that need electricity in order to communicate. In the future, these devices will be able to do without electricity, instead exploiting radio waves already present in the air around them.

October 2014

IMALab connecting battery-free devices to Wi-Fi

You’re in the office, and you want to check whether you turned the oven off before leaving home… you use your smartphone to find your car keys… you need to make a long-distance check on how a manufacturing unit is performing… you want to verify the state of a river in a different area… you’re reorganizing the illumination of an entire city… you’re drawing up emergency plans for critical weather situations: these are all situations involving IoT, the Internet of Things.

What’s involved is a growing global network of sensors that gather data and then transmit it automatically via internet to other devices or machines, or to people, either simply as information or in order to perform remote activation of other devices.

When an object can represent itself digitally, transmitting and receiving, it can be controlled from anywhere. More connectivity generates more data, gathered from more places, offering more ways to increase efficiency and improve safety and security.

Sensors can be embedded in all kinds of everyday objects to help monitor, track and analyze everything: manufacturing plants, energy grids, healthcare facilities, transportation systems, the structural safety of buildings and the health of your heart.

Many sensors are battery-free, but if they need to connect to a Wi-Fi system and send data via internet, then they need electric power. This is the decisive factor currently putting a brake on the total take-off of a system that in other respects is ready to take off.

For some years now, the University of Washington has been conducting studies into ways of making Wi-Fi connectivity systems work without electricity. It has now come up with Wi-Fi Backscatter, a revolutionary communication system that makes use of signals already present in

their surrounding ambient.

Engineers knew well that a degree of power can be generated by harvesting radio signals in a given area, a largely ubiquitous resource today, consisting of radio and TV signals, and also cellular network transmitters. But although the energy collected in this way can be sufficient to run low-powered circuits, a far higher amount of power is needed to actively transmit data via Wi-Fi. At the present time, harvesting ambient radio waves can collect electric power in the order of tens of microwatts… but to transmit data via Wi-Fi requires tens of thousands of times more power: hundreds of milliwatts at least, and more typically around one watt.

The Washington University research team tackled this problem by testing a method for getting devices to communicate without needing to actively transmit. Instead their messages are sent through patterns in the way they scatter signals from other sources — recycling existing radio waves instead of expending energy to generate their own.

The team succeeded in developing an ultra-low power tag prototype, with an antenna and circuitry that enables it to send messages to Wi-Fi-enabled laptops or smartphones while consuming negligible power.

These tags essentially work by "looking" for Wi-Fi signals moving between the router and a laptop or smartphone. They encode data by either reflecting or not reflecting the Wi-Fi router's signals, thereby slightly changing the wireless signal. Wi-Fi-enabled devices like laptops and smartphones are then able to detect these minute changes and thus receive input from the tag.

This concept has now been successfully implemented, and so far the research team’s Wi-Fi Backscatter tag has communicated with a Wi-Fi device at rates of 1 kilobit per second, at a distance of about 2 meters between devices. Currently the team is working on extending the range to about 20 meters and have patents filed on the technology.

In the not-too-distant future, more and more sensors will operate non-stop in our homes and our cities, making it possible for us to access any kind of information at any moment. The problem will then lie in finding a way to deal with this immense mushrooming mass of data, as those grappling with the "big data" syndrome continually remind us.

This highly significant University of Washington research project has been carried out by Bryce Kellogg, Aaron Parks, Shyamnath Gollakota, Joshua R. Smith, and David Wetherall, being funded by the UW Commercialization Gap Fund, the Qualcomm Innovation Fellowship, the Washington Research Foundation, the National Science Foundation and Washington University.

The complete published research text, titled Wi-Fi Backscatter: Internet Connectivity for RF-Powered Devices, is available in PDF format at





University of Washington – August 2014

MIT Technology Review August 2014