FreeRider: Backscatter Communication Using Commodity Radios Pengyu - - PowerPoint PPT Presentation

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FreeRider: Backscatter Communication Using Commodity Radios Pengyu - - PowerPoint PPT Presentation

Aug 03, 2023 303 likes •592 views

FreeRider: Backscatter Communication Using Commodity Radios Pengyu Zhang 1 , Colleen Josephson 1 , Dinesh Bharadia 2 , Sachin Katti 1 Stanford University 1 , UCSD 2 CoNEXT 2017, FreeRider 1 IoT Explosion Data source:

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CoNEXT 2017, FreeRider

FreeRider: Backscatter Communication Using Commodity Radios

Pengyu Zhang1, Colleen Josephson1, Dinesh Bharadia2, Sachin Katti1 Stanford University1, UCSD2

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CoNEXT 2017, FreeRider

IoT Explosion

2 Data source: http://www.businessinsider.com/internet-of-things-billions-of-connected-devices-2014-1 https://www.gartner.com/newsroom/id/3598917

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CoNEXT 2017, FreeRider

IoT has different needs than traditional devices

  • Long battery a priority
  • Must be small and durable
  • Data needs usually modest

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CoNEXT 2017, FreeRider

Power consumption in embedded systems

WiFi radios consume orders of magnitude more power than other system components!

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CoNEXT 2017, FreeRider

Low Power Options

  • Duty cycling (Bluetooth)
  • Super-Narrowband communications (NB-IoT cellular)
  • Passive communication (backscatter)

○ Lowest power ○ RFID

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CoNEXT 2017, FreeRider

Backscatter primer

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Productive vs. Non Productive Excitation

  • Non-productive excitation: fixed sinusoidal

○ RFID, Passive WiFi, Interscatter

  • Productive excitation: excitation signal contains real data

○ Transits both data and a backscatter communication medium ○ HitchHike, Ambient Backscatter and FreeRider

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CoNEXT 2017, FreeRider

FreeRider — use existing WiFi, ZigBee and Bluetooth radios to enable productive backscatter communication

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CoNEXT 2017, FreeRider

FreeRider System Overview

Receivers compare backscatter signal with excitation signal to extract tag data

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CoNEXT 2017, FreeRider

Codewords

WiFi, Zigbee and Bluetooth use a finite set of codewords to represent 0s and 1s

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CoNEXT 2017, FreeRider

Codeword translation

One valid codeword translated to another. This allows commodity radios to receive backscatter

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How to encode information? A binary example

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Original excitation codeword Backscattered codeword Decoded tag data C1 C1 C1 C2 1 C2 C1 1 C2 C2

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CoNEXT 2017, FreeRider

Case study: 802.11g/n codeword translation

  • OFDM has 52 subcarriers
  • 6Mbps rate uses BPSK subcarrier modulation

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BPSK subcarrier

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CoNEXT 2017, FreeRider

OFDM binary codeword translation

Original codeword:

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subcarrier 1 subcarrier 2 subcarrier 3 subcarrier 52 + … + + +

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CoNEXT 2017, FreeRider

OFDM binary codeword translation

180 degree translation:

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subcarrier 1 subcarrier 2 subcarrier 3 subcarrier 52 + … + + +

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CoNEXT 2017, FreeRider

3-dimensional codeword translation

HitchHike only changes the phase. FreeRider can change phase, amplitude and frequency

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CoNEXT 2017, FreeRider

Case study II: Bluetooth codeword translation

GFSK modulation

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CoNEXT 2017, FreeRider

GFSK codeword translation

Let Δf = |f0 - f1|. Shift the incoming codeword by Δf to encode data1. Signals

  • utside the channel are treated interference, solving the double sideband

problem.

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CoNEXT 2017, FreeRider

Hardware Prototype

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SPICE projection of mass-produced power consumption: ~30 uW

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CoNEXT 2017, FreeRider

Managing multiple tags: WiFi case study

How to coordinate multiple tags using WiFi when the tag cannot decode WiFi?

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Packet-width modulation

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  • Packet duration encodes control messages
  • Duration measured by envelope detector
  • Can use commodity transmitters
  • Robust to interference
  • Currently binary symbols, ~500bps
  • Used to implement a framed slotted-Aloha based random-access scheme
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MAC scheme

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Selected results

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Multi-tag evaluation: aggregated throughput

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Single-tag evaluation: throughput

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Operational regime

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CoNEXT 2017, FreeRider

FreeRider Summary

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  • First backscatter system capable of 802.11g, ZigBee and Bluetooth
  • Excitation signal can be simultaneously used for productive communication
  • 3-dimensional codeword translation
  • First WiFi backscatter system to implement and evaluate multiple tags
  • Source code available at: https://github.com/pengyuzhang/FreeRider