Ripple II

Roy and Choudhury, USENIX NSDI ‘16

Design

• Vibra-motor: Linear Resonant Actuator (LRA) driven by a waveform generator.
  • Input signal to the LRA: OFDM(!)

2.2 Microphone as vibration receiver

• Mic: Sound pushes diaphragm, diaphragm vibrates, produces electrical signal, amplified.
  • Bigger frequency range
• Ripple II: Notice mic is sensitive to contact vibrations
• Problem: Interference from air vibrations

Interference Cancellation (Sect. 3.1)

• Cover sound hole: Figure 5: SINR was -10 dB (@ 10 KHz), increases to +25 dB (@ 10 KHz). Generally better at higher frequencies.

• V (contact vibration), S (interference sound), E (electric noise)

  • E comes from common electric supply voltage of mics.
  • Goal: Interference Cancellation (subtract S)
  • System model shown in Figure 6
  • Possible weakness: Physical interfering vibration (i.e. riding in a Jeep off-road, would it work?)

Failed Attempts (Sect. 3.1)

• E has a spatial signature across mics, MIMO! But, can’t estimate spatial signature for interference sound.

Symbol Selective Adaptive Noise Filtering

• See slides: https://www.usenix.org/sites/default/files/conference/protected-files/nsdi16slidesroy.pdf
• Slide 24: Sound interference affects only certain subcarriers
• V1, S1, S_2 not defined
• Personal comm. w/authors:
  • V1 = V(t)H_{V1}, S1 = S(t)H_{S1}, and so on.
  • As vibration from the primary microphone leaks to the secondary microphone, they model the secondary microphone’s signal as a filtered version of the primary.
  • If not affected by ambient sound, this channel gain is entirely the function of the solid medium (e.g. the circuit board where these microphones are mounted) and hence it is static.
  • Primary and secondary symbols are from Mic1 and Mic2 respectively.
• Avoid lower frequency band interference by starting above 500 Hz

OFDM (Sect. 3.2)

• Characterize the channel in Figure 9
  • Multipath components weak, and from motor mass
  • 10 dB max excess delay of 400 us, conservative CP of 1 ms
  • Coherence B/W 480 Hz, subcarrier chosen 40 Hz (conservative)

MAC Layer (Sect. 4)

• Cool idea: Back EMF lets transmitter sense receiver interference like the Ethernet
  • Interference sound induces a tiny current
  • Measure that induced current to motor by voltage drop across series resistor
  • Results in Figure 11 are pretty convincing

Proactive Symbol Recovery (4.3, 4.4)

• Transmitter has better estimate of errored symbols than receiver (see Figure 14).
• Idea: Transmitter sends on every other OFDM subcarrier, more power.
  • Better SNR, half rate, essentially a bit rate adaptation
  • Estimates start and end (Fig 14) of interference by Back-EMF sensing.
• Convolutional coding atop everything adds fall-back layer

Performance Evaluation (S. 5)

• Fig. 17(a) CDF across all noise environments
  • PSR retrasnmits erroneous symbols and improves throughput
  • Recall is weak, so it misses many symbols that should have been retransmitted
    • Expected/desirable? b/c of coding?

Applications

• Finger Ring
• Tabletop comms
• P2P money transfer