The phone is based on "ambient backscatter," which, according to the researchers, "transforms existing wireless signals into both a source of power and a communication medium." Initially used to develop a backscatter communications-based system called " Passive Wi-Fi " designed to dramatically reduce the power of Wi-Fi transmissions, the concept is being applied to converting specific RF signals into power.
According to a Wired interview with the researchers, they needed to go beyond Passive Wi-Fi to achieve voice calls on their battery-free phone. Given the power requirements of converting human speech to digital, the phone instead uses an all-analog backscatter process.
"If you can communicate using analog technology, you're actually more power efficient," says Vamsi Talla, a research associate at the University of Washington's Sensor Systems Laboratory, led by Joshua Smith, which earlier this year spun out Jeeva Wireless , a company designed to commercialize a passive Wi-Fi system based on backscatter technology.
To save power, the phone uses some external circuitry, which converts and connects to the digital cellular network, located in a nearby prototype basestation. The basestation signals - low-power transmissions on an unlicensed frequency - are used by the phone for its energy harvesting.
That circuitry could be built into commercial Wi-Fi routers or phone towers, say the researchers. If installed in the latter, they say, the system's range could be increased from about 15 feet to "perhaps a kilometer."
Currently the phone prototype has no display and only a touch-sensitive number pad. A standard large touchscreen would increase the phone's power requirements by over one hundred thousand times.
The researchers plan to add an e-ink display and improved call quality to their next-generation device. Pricewise, using off-the-shelf components, the current prototype is said to be "cheaper than a normal phone," and would be even less expensive if mass produced.
For more, see " Passive Wi-Fi: Bringing Low Power to Wi-Fi Transmissions ." (PDF)