Enabling Practical Backscatter Communication for On-body Sensors Pengyu Zhang, Mohammad Rostami, Pan Hu, Deepak Ganesan UMass Amherst 1
Ubiquitous deployment of on-body sensors Fitness-band Smartphone Smart Watch Biometric clothing Sensor Tattoo Teeth Sensor How should we communicate with on-body sensors which have a limited energy budget?
Current state of wireless radio power consumption 1000000 Power Consumption (uW) WiFi Zigbee BLE 1000 Camera ECG Microphone Accelerometer 1 Communication Sensing Wireless radios consume an order of magnitude higher power compared to low power sensors
Technology trends in wireless radio power consumption Power (mW) 100 10 1 2003 2006 2009 2011 2014 Years 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 Carrier Wave Backscatter reader Backscatter tag TX AMP logic RF harvester RX LNA Reflected Signal Backscatter enables ultra low-power wireless communication
Problem: NO reader infrastructure in mobile environment Fitness-band Smartphone Smart Watch 6
Related work: leveraging WiFi signals for backscatter Infrastructure-less backscatter Infrastructure-assisted backscatter WiFi Backscatter (Sigcomm14) Passive WiFi (NSDI16)
WiFi Backscatter (Sigcomm 14) — backscatter WiFi signals WiFi packets: 01100110… WiFi receiver WiFi transmitter tag bits: 10011010… Limitation: WiFi signal is much louder than the backscatter signal 8
WiFi Backscatter (Sigcomm 14) — backscatter WiFi signals SNR SINR 40 20 0 dB 70dB -20 -40 -60 -80 Distance WiFi packets: 01100110… WiFi receiver WiFi transmitter Limitation: WiFi signal is much louder than the backscatter signal
Passive WiFi (NSDI 16) — independent carrier transmitter Plug-in devices —> WiFi receiver generate the carrier tag bits: 10011010… Limitation: needs a new device to be carried
Fitness-band Smartphone Smart Watch Can we leverage multiple WiFi/Bluetooth radios on mobile devices to enable backscatter? 11
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 baseband RF filter filter Filters on commodity radios help LNA interference reduction RX data
How to frequency-shift backscattered signals? backscatter(t) = wifi(t) * tag(t) Backscatter device logic WiFi/Bluetooth signal RF Power harvester backscatter frequency
Spectrum when tag backscatters Bluetooth signals -20 Bluetooth Signal Strength (dBm) -30 -40 FS-Backscatter -50 -60 -70 -80 -90 -100 2.41 2.42 2.43 2.44 2.45 2.46 2.47 Frequency (GHz) 16
How to embed bits? — packet-level encoding packet Packet-level Encoding Bit-level Encoding 17
How to embed bits? — packet-level encoding packet Packet-level Encoding Bit-level Encoding 18
How to embed bits? — packet-level encoding packet Packet-level Encoding Bit-level Encoding packet packet packet packet 1 0 1 0 19
How to embed bits? — bit-level encoding packet pac ket Packet-level Encoding Bit-level Encoding packet packet packet packet 1 0 1 0 20
How to embed bits? — bit-level encoding packet Packet-level Encoding Bit-level Encoding packet packet packet packet packet 1 0 1 0 0 1 0 1 1 0 1 21
How to embed bits? — bit-level encoding packet Packet-level Encoding Bit-level Encoding packet packet packet packet packet 1 0 1 0 0 1 0 1 1 0 1 Longer communication Higher data rate but shorter range but lower bitrate communication range 22
How to embed bits? — bit-level encoding packet Packet-level Encoding Bit-level Encoding packet packet packet packet packet 1 0 1 0 0 1 0 1 1 0 1 - 400 bps throughput - 48kbps throughput - 5m communication range - 3.6m communication range 23
What about the tag power consumption? Can we perform 20MHz frequency shifting at a few uWs? Accuracy Low Oscillator Power Consumption Frequency 32kHz 1.48µW 1MHz 326µW High 10MHz 2.04mW Low High Power Consumption 24
Leverage low power ring oscillator Use the smallest number of gates to produce the desired frequency shift. + - + - + - output 45 Power Consumption (uW) 40 35 30 25 20 15 10 5 0 Ring OSC Modulator Transistor Total
Disadvantages — sensitive to temperature variations 20.2 Frequency 20 OSC (MHz) 19.8 Human body 19.6 temperature 19.4 35 35.5 36 36.5 37 37.5 38 38.5 39 39.5 40 Temperature (Celsius) 26
Disadvantages — sensitive to temperature variations 20.2 Frequency 20 OSC (MHz) 19.8 Human body 19.6 temperature 19.4 35 35.5 36 36.5 37 37.5 38 38.5 39 39.5 40 Temperature (Celsius) 50 Throughput 37.5 (kbps) 25 12.5 0 -1.20E+03 -800 -400 0 400 800 1200 Frequency Variations (kHz) 27
FS-Backscatter performance in mobile deployment 1 Static 0.8 Mobile 0.6 CDF 0.4 0.2 0 10 15 20 25 30 35 40 45 50 Throughput (kbps) FS-Backscatter achieves around 22kbps in mobile deployment 28
Conclusion WiFi signals Bluetooth signals Enable backscatter for wearables by leveraging multiple Bluetooth and WiFi radios on mobile devices.
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