Battery-Free Mobile Devices Draw Power From Thin Air

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Researchers at the University of Washington (UW) have devised a way to communicate wirelessly without drawing power from a battery or from a power outlet.

The technique, called "ambient backscatter," is explained in a paper published at the Association for Computing Machinery's Special Interest Group on Data Communication 2013 conference in Hong Kong, which began on Tuesday, Aug. 13.

The paper's authors, Vincent Liu, Aaron Parks, Vamsi Talla, Shyamnath Gollakota, David Wetherall and Joshua R. Smith, describe the technique as "a new form of communication that provides connectivity between computers out of what is essentially thin air."

[ Will Google make use of this new form of communications? Read Google Glass Adds Path, Evernote Voice Commands. ]

Ambient backscatter is a form of power harvesting that takes existing radio frequency waves, such as broadcast TV signals, and turns them into both a source of power and a medium for communication. It works by reflecting the signals to exchange information.

The technique, the paper says, avoids the maintenance burden of batteries and the dedicated power infrastructure required to run other forms of low-power communication, like RFID and NFC, enabling "a bevy of new applications that were previously impossible or at least impractical."

Joshua R. Smith, an associate professor of computer science and engineering at UW, said in a phone interview, "One of the things I think would be a good match for this technology would be detecting leaks. You can imagine building a whole bunch of these into a roof because they're battery free. You'd never need to worry about accessing them again."

He also suggested that ambient backscatter sensors would be useful for chemical plants that need to conduct fence-line monitoring to ensure that no harmful gases are being emitted.

The technology could open up new possibilities in wearable computing, ubiquitous sensors and self-powered surveillance devices. Related work on ambient backscatter, published in April, describes how the technique can be used to create a battery-free wireless microphone.

The UW researchers created a prototype smart card payment application and a prototype grocery store application for determining whether products are out of place on store shelves. They demonstrated data communication rates of 1 Kbps at up to 2.5 feet -- this is not a long-range communications technology.

Shyamnath Gollakota, an assistant professor of computer science and engineering at UW, said in a phone interview that while a 1 Kbps communicate rate meant ambient backscatter wouldn't be well suited for transmitting rich media, ongoing research indicates that transmission rates of 1 Mbps should eventually be possible.

The paper notes that while it's illegal to broadcast random signals in the regulated TV spectrum, battery-free backscattering devices like RFID tags are unregulated and not tested by the Federal Communications Commission because they merely reflect existing signals rather than emitting new ones. The authors assert ambient backscattering is similarly lawful.

Ambient backscatter, however, can interfere with TV receivers when a backscatter device is close to the TV. A backscatter transmitter within 2.3 inches of a TV produces noticeable glitches. High data transmission rates can extend this area of interference. With 7.2 inches or more between a backscatter transmitter and a TV receiver, no interference was detected, regardless of transmission rate.

Gollakota insists the technology would be well-suited for environments with a lot of wireless signals, thanks to the integration of a collision avoidance protocol called carrier sense that coordinates backscatter transmitters to ensure that they are polite and only broadcast when they're not interfering with other signals.

Gollakota says that the technology could be commercialized in a few years.