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NEWS
Researchers Develop First Battery-Free Cellphone
October 13, 2017

How fantastic would it be not to worry about where you left your cellphone charger, or where is the mobile charging cable, or if your phone was going to die at the most inconvenient time because the battery wasn’t charged?

Well those worries may soon be a thing of the past. Based on information provided by the University of Washington (UW), researchers have invented a cellphone that requires no batteries. The few microwatts of power your cell phone requires are generated from either ambient radio signals or light instead.


UW engineers have designed the first battery-free cellphone that can send and receive calls using only a few microwatts of power. Photo Credit: Mark Stone/University of Washington.

With funding for the research provided by the National Science Foundation (NSF) and Google Faculty Research Awards, the researchers have presented details of the new technology in a paper published in the Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable and Ubiquitous Technologies.

“We’ve built what we believe is the first functioning cellphone that consumes almost zero power,” said co-author Shyam Gollakota, an associate professor in the Paul G. Allen School of Computer Science & Engineering at the UW. “To achieve the really, really low power consumption that you need to run a phone by harvesting energy from the environment, we had to fundamentally rethink how these devices are designed.”

The team of UW computer scientists and electrical engineers eliminated a power-hungry step in most modern cellular transmissions — converting analog signals that convey sound into digital data that a phone can understand. This process consumes so much energy that it’s been impossible to design a phone that can rely on ambient power sources.

Instead, the battery-free cellphone takes advantage of tiny vibrations in a phone’s microphone or speaker that occur when a person is talking into a phone or listening to a call.

An antenna connected to those components converts that motion into changes in standard analog radio signal emitted by a cellular base station. This process essentially encodes speech patterns in reflected radio signals in a way that uses almost no power.

To transmit speech, the phone uses vibrations from the device’s microphone to encode speech patterns in the reflected signals. To receive speech, it converts encoded radio signals into sound vibrations that that are picked up by the phone’s speaker. In the prototype device, the user presses a button to switch between these two “transmitting” and “listening” modes.

Using commercial, off-the-shelf components on a printed circuit board, the team demonstrated that the prototype can perform basic phone functions — transmitting speech and data and receiving user input via buttons. Using Skype, researchers were able to receive incoming calls, dial out and place callers on hold with the battery-free phone.

“The cellphone is the device we depend on most today. So if there were one device you’d want to be able to use without batteries, it is the cellphone,” said faculty lead Joshua Smith, professor in both the Allen School and UW’s Department of Electrical Engineering. “The proof of concept we’ve developed is exciting today, and we think it could impact everyday devices in the future.”

The team designed a custom base station to transmit and receive the radio signals. But that technology conceivably could be integrated into standard cellular network infrastructure or Wi-Fi routers now commonly used to make calls.

“You could imagine in the future that all cell towers or Wi-Fi routers could come with our base station technology embedded in it,” said co-author Vamsi Talla, a former UW electrical engineering doctoral student and Allen School research associate. “And if every house has a Wi-Fi router in it, you could get battery-free cellphone coverage everywhere.”

While the prototype does require a small amount of energy, about 3.5 microwatts, the UW researchers demonstrated how to harvest this small amount of energy from two different sources — power gathered from ambient radio signals as well as power gathered from ambient light with a tiny solar cell.

“That’s been the biggest challenge — the amount of power you can actually gather from ambient radio or light is on the order of 1 or 10 microwatts. So real-time phone operations have been really hard to achieve without developing an entirely new approach to transmitting and receiving speech,” explained co-author Bryce Kellogg, a UW electrical engineering doctoral student.


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