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New Battery-Free Cellphone Uses Ambient Power to Make Calls

06 July 2017

Researchers at the University of Washington have invented the cell phone that can work without batteries. This is a huge step toward moving beyond chargers, cords and dying cell phones. The battery-free cellphone harvests the few microwatts of power required to work from either ambient radio signals or light.

The research team has made Skype calls using the battery-free phone. This demonstrates that the prototype, which is made of commercial, off-the-shelf components, can receive and transmit speech and communicate with the base station.

"We've built what we believe is the first functioning cell phone 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."UW engineers have designed the first battery-free cellphone that can send and receive calls using only a few microwatts of power. (Mark Stone/University of Washington)UW engineers have designed the first battery-free cellphone that can send and receive calls using only a few microwatts of power. (Mark Stone/University of Washington)

The team of computer scientists and electrical engineers has managed to eliminate 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 a lot of energy, making it nearly impossible to design a phone that can go battery-free and rely on other sources for power.

The battery-free cell phone takes advantage of the tiny vibrations from the phone’s microphone or speaker that occur naturally when a person is making a call. An antenna connected to those components converts the motion into changes in a standard analog radio signal that is emitted by a cellular base station. This process encodes speech patterns in reflected radio signals in a way that uses almost no power.

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

The team used off-the-shelf components on a printed circuit board to demonstrate that the prototype can perform basic phone functions like transmitting speech and data and receiving user input via buttons. The researchers were able to receive incoming calls, dial out and place callers on hold with Skype on 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 also designed a custom base station that transmits and receives the radio signals. That technology could conceivably be integrated into standard cellular network infrastructure or Wi-Fi routers that are currently 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."

The battery-free phone does require a little bit of energy in order to perform some operations. The current prototype has a power budget of 3.5 microwatts. The UW researchers have demonstrated how to harvest this small amount of energy from two sources. The phone prototype can operate on power that is gathered from ambient radio signals that are transmitted by a base station up to 31 feet away. Using this power from the ambient light with a rice-sized solar cell, the prototype could communicate with a base station up to 50 feet away.

Three are other battery-free technologies that rely on ambient energy sources, like temperature sensors or an accelerometer in order to conserve power with intermittent operations. These devices take a reading and then “sleep” for a minute or two while they are harvesting enough energy to perform the upcoming task. With the battery-free phone, a phone call requires the device to operate continuously while the conversation is happening.

"You can't say hello and wait for a minute for the phone to go to sleep and harvest enough power to keep transmitting," said co-author Bryce Kellogg, a UW electrical engineering doctoral student. "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."

The next step for the team is to focus their research on improving the battery-free phone’s operating range and encrypting conversations in order to make them secure. The team is also focusing on a way to stream video and adding a visual display feature using low-power E-ink screens.

A paper on this research was published on July 1st 2017 in the Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable and Ubiquitous Technologies.



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