"We've built what we believe is the first functioning cell phone that consumes almost zero power," said associate professor Shyam Gollakota.
To achieve this, the team of scientists removed the need to convert analogue signals into digital data. Instead, the battery-free cellphone takes advantage of the 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 analogue radio signals emitted by a cellular base station.
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.
"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 doctoral student Vamsi Talla.
The battery-free phone does still require a small amount of energy of up to 3.5µW to perform some operations.
The researchers demonstrated how to harvest this small amount of energy from two different sources. The prototype is said to operate on power gathered from ambient radio signals transmitted by a base station up to 31 feet away.
Using power harvested from ambient light with a tiny solar cell, the researchers claim the device could communicate with a base station that was 50 feet away.
