## 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/nsdi16_slides_roy.pdf
- Slide 24: Sound interference affects only certain subcarriers
- V_1, S_1, 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