Their design can sense speech, actuate the earphones, and switch between uplink and downlink communications, all in real time. The current setup requires the cellphone to be used in a similar manner to a typical push-to-talk device, being unable to work in full duplex mode and requiring the user to push and hold a button when speaking into its microphone.
The system optimises transmission and reception of speech while simultaneously harvesting power, enabling the battery-free cellphone to operate continuously. The battery-free device prototype is built using commercial-off-the-shelf components on a printed circuit board It can operate on power that is harvested from RF signals transmitted by a base station 9.4 metres away.
Using power harvested from ambient light with tiny photodiodes, the students have shown that their device can communicate with a base station 15.2 metres away.
They have also performed a Skype call using the battery-free phone over a cellular network, via their custom bridged base station, which they believe is a major leap in the capability of battery-free devices and a step towards a fully functional battery-free cellphone.
A paper describing their methods and findings can be read here
