Enabling Practical Backscatter Communication for On-body Sensors - - PowerPoint PPT Presentation

Enabling Practical Backscatter Communication for On-body Sensors

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Pengyu Zhang, Mohammad Rostami, Pan Hu, Deepak Ganesan UMass Amherst

Ubiquitous deployment of on-body sensors

Fitness-band Smartphone Smart Watch

How should we communicate with on-body sensors which have a limited energy budget?

Teeth Sensor Sensor Tattoo Biometric clothing

Wireless radios consume an order of magnitude higher power compared to low power sensors Current state of wireless radio power consumption

Power Consumption (uW)

1 1000 1000000

Communication Sensing

Zigbee BLE Camera ECG Microphone Accelerometer WiFi

Technology trends in wireless radio power consumption Power (mW)

1 10 100

Years

2003 2006 2009 2011 2014

Wireless radio power reduction is slow…

Client Beamforming for Rate Scalability and MU-MIMO Networks, Hang Yu, Rice University

Backscatter — an ultra low power communication primitive Backscatter enables ultra low-power wireless communication

Backscatter reader Backscatter tag Carrier Wave Reflected Signal

TX

AMP LNA

RX logic RF harvester

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Problem: NO reader infrastructure in mobile environment

Fitness-band Smartphone Smart Watch

Related work: leveraging WiFi signals for backscatter

Infrastructure-less backscatter Infrastructure-assisted backscatter WiFi Backscatter (Sigcomm14) Passive WiFi (NSDI16)

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WiFi packets: 01100110… tag bits: 10011010… WiFi transmitter WiFi receiver

Limitation: WiFi signal is much louder than the backscatter signal WiFi Backscatter (Sigcomm 14) — backscatter WiFi signals

WiFi Backscatter (Sigcomm 14) — backscatter WiFi signals

WiFi packets: 01100110… WiFi transmitter WiFi receiver

Limitation: WiFi signal is much louder than the backscatter signal

dB

• 80
• 60
• 40
• 20

20 40 Distance

SNR SINR

70dB

Passive WiFi (NSDI 16) — independent carrier transmitter Limitation: needs a new device to be carried

WiFi receiver tag bits: 10011010… Plug-in devices —> generate the carrier

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Fitness-band Smartphone Smart Watch

Can we leverage multiple WiFi/Bluetooth radios on mobile devices to enable backscatter?

How do we deal with interference?

WiFi/Bluetooth signal backscatter

Power frequency

Strong interference because backscatter channel and WiFi/ Bluetooth channel are same.

How do we deal with interference?

WiFi/Bluetooth signal backscatter

Power frequency

Interference reduces if backscatter channel is shifted away from WiFi/Bluetooth channel

Why does interference reduce?

WiFi/Bluetooth signal backscatter

Power frequency RX data

LNA

baseband filter RF filter

Filters on commodity radios help interference reduction

How to frequency-shift backscattered signals?

Power frequency

backscatter(t) = wifi(t) * tag(t)

Backscatter device

logic RF harvester

WiFi/Bluetooth signal backscatter

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Spectrum when tag backscatters Bluetooth signals

• 100
• 90
• 80
• 70
• 60
• 50
• 40
• 30
• 20

2.41 2.42 2.43 2.44 2.45 2.46 2.47 Signal Strength (dBm) Frequency (GHz)

Bluetooth FS-Backscatter

How to embed bits? — packet-level encoding

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Packet-level Encoding Bit-level Encoding

packet

How to embed bits? — packet-level encoding

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Packet-level Encoding Bit-level Encoding

packet

How to embed bits? — packet-level encoding

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Packet-level Encoding Bit-level Encoding

packet packet packet packet

1 1

packet

How to embed bits? — bit-level encoding

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Packet-level Encoding Bit-level Encoding

packet pac ket packet packet packet packet

1 1

How to embed bits? — bit-level encoding

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Packet-level Encoding Bit-level Encoding

packet

1 1 1 1

packet packet packet packet packet

1 1

How to embed bits? — bit-level encoding

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Packet-level Encoding Bit-level Encoding

packet

1 1 1 1

Longer communication range but lower bitrate Higher data rate but shorter communication range

packet packet packet packet

1 1

packet

How to embed bits? — bit-level encoding

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Packet-level Encoding Bit-level Encoding

packet

1 1 1 1

• 400 bps throughput
• 5m communication range
• 48kbps throughput
• 3.6m communication range

packet packet packet packet packet

1 1

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What about the tag power consumption?

Oscillator Frequency Power Consumption 32kHz 1.48µW 1MHz 326µW 10MHz 2.04mW

Can we perform 20MHz frequency shifting at a few uWs?

Accuracy Power Consumption Low High Low High

Leverage low power ring oscillator

• utput

Use the smallest number of gates to produce the desired frequency shift. +

• +
• +
• 5

10 15 20 25 30 35 40 45

Ring OSC Modulator Transistor Total Power Consumption (uW)

Human body temperature

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Disadvantages — sensitive to temperature variations

OSC Frequency (MHz)

19.4 19.6 19.8 20 20.2

Temperature (Celsius)

35 35.5 36 36.5 37 37.5 38 38.5 39 39.5 40

Human body temperature

OSC Frequency (MHz)

19.4 19.6 19.8 20 20.2

Temperature (Celsius)

35 35.5 36 36.5 37 37.5 38 38.5 39 39.5 40

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Disadvantages — sensitive to temperature variations

Throughput (kbps)

12.5 25 37.5 50

Frequency Variations (kHz)

• 1.20E+03
• 800
• 400

400 800 1200

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FS-Backscatter performance in mobile deployment

0.2 0.4 0.6 0.8 1 10 15 20 25 30 35 40 45 50

CDF Throughput (kbps)

Static Mobile

FS-Backscatter achieves around 22kbps in mobile deployment

Conclusion Enable backscatter for wearables by leveraging multiple Bluetooth and WiFi radios on mobile devices.

WiFi signals Bluetooth signals