Ripple II: Faster Communication through Physical Vibration

We envision physical vibration as a new modality of data communication. In NSDI 2015, our paper reported the feasibility of modulating the vibration of a smartphone’s vibra-motor. When in physical contact with another smartphone, the accelerometer of the second phone was able to decode the vibrations at around 200 bits/s. This paper builds on our first prototype, but redesigns the entire radio stack to now achieve 30 kbps. The core redesign includes (1) a new OFDM-based physical layer that uses the microphone as a receiver (instead of the accelerometer), and (2) a MAC layer that detects collision at the transmitter and performs proactive symbol retransmissions. We also develop two example applications on top of the vibratory radio: (1) a finger ring that transmits vibratory passwords through the finger bone to enable touch based authentication, and (2) surface communication between devices placed on the same table. The overall system entails unique challenges and opportunities, including ambient sound cancellation, OFDM over vibrations, back-EMF based carrier sensing, predictive retransmissions, bone conduction, etc. We call our system Ripple II to suggest the continuity from the NSDI 2015 paper. We close the paper with a video demo that streams music as OFDM packets through vibrations and plays it in real time through the receiver’s speaker.