Ripple II: Faster Communication through Physical Vibration
Live audio streaming (32Kbps)
Stylus transmitter - Microphone receiver
Ring authentication demo (8Kbps)
Ring transmitter - Microphone receiver
Text transmission (80bps)
Smartphone transmitter - Accelerometer receiver
Key research elements
Microphone as the receiver: Our prior work (Ripple-I) used an accelerometer as the receiver. The low bandwidth of the accelerometer chips (800Hz) proved to be the main bottleneck to link capacity, resulting in around 200 bits/s. This paper (Ripple-II) identifies the possibility of using microphones as a vibration receiver for a wider bandwidth.
MAC layer design: We develop a MAC protocol, named 'Proactive Symbol Recovery Protocol', that uses transmitter side information of the interference for symbol level rate control and corrupted symbol retranmission.
Ambient sound cancellation: The use of microphone as the receiver includes both ambient sound and vibration as the noise to the receiver. We develop an algorithm called 'Symbol Selective Adaptive Noise Filtering' (SANF) to reduce the noise at the receiver while minimizing the potential signal distortion.
Sensing interference from vibration motor's Back-EMF
We observe that ambient vibration and sound leaves impact on the movement of the vibra-motor mass which in turn impacts the Back-EMF of the motor. Hence, instead of using additional sensor for interference sensing, we measure the Back-EMF voltage of the vibration generating motor to produce an estimate of the interference to a reasonable granularity of frequency and time.
The following are the recorded sound sample using the microphone and the sound waveform recovered from the vibration motor's back-EMF. The vibration motor was placed within 1ft of the speaker that generated the interference. The figures show a part of their respective spectrograms for comparison.
Sound recorded with microphone
Sound recovered from motor's Back-EMF
The complete hardware design of the Ripple-II platform is given in the figure below:
Finger Ring for Authentication: We envision touch based two-factor authentication – a user wearing a Ripple II ring or watch could touch the smartphone screen and the vibratory password can be conducted through the bones. The core notion generalizes to other scenarios, including unlocking car doors, door knobs, etc.
Tabletop Communication: Multicast communication is often useful – a group picture at a restaurant needs to be shard with everyone in the group; presentation slides need to be shared in a meeting. We envision placing all phones on the table, near each other, and performing one vibratory multicast.
P2P Money Transfer: In developing regions, mobile payments may be viable with basic phones with vibra-motor and microphones. Perhaps a USB stick can transfer data to phones/tablets on physical contact.
 Ripple: Communicating through Physical Vibrations (NSDI, May 2015) [paper, slides]
 Ripple II: Faster Communication through Physical Vibration (NSDI, Mar 2016) [paper]
 Don’t Bump, Shake on It: The Exploitation of a Popular Accelerometer-Based Smart Phone Exchange and Its Secure Replacement Ahren Studer, Timothy Passaro, Lujo Bauer [paper]
 Privacy-Aware Communication for Smartphones Using Vibration Inhwan Hwang, Jungchan Cho, Songhwai Oh [paper]
 Dhwani : Secure Peer-to-Peer Acoustic NFC R. Nandakumar, K. Chintalapudi, V. N. Padmanabhan, R. Venkatesan [paper]
 Ving: Bootstrapping the Desktop Area Network with a Vibratory Ping Joshua Adkins, Genevieve Flaspohler, Prabal Dutta [paper]
Startups with related applications
Bump uses the timings of physical vibration to pair two smartphones for secure data exchange. Here the physical vibration aids addressing and serves as a hint of collocation. [link, link]
Zoosh enables peer-to-peer money transfer between smartphones using ultrasonic audio signal for communication. [link]
Tagtile implements an alternative short distance communication technique between the custom receiver and a smartphone as a part of their loyality and direct marketing solution. [ link, link]