1. Turbocharging Ambient Backscatter Communication Aaron Parks Angli Liu Shyamnath Gollakota Joshua R. Smith 1

2. Radio Communication Trends 2 Multi- Antenna Range (10s of km) Reliability Coding Then Now

3. Our Work 3 Can we achieve these techniques on battery-free devices?

4. If Possible, Benefits New Classes of Devices 4 RFID Wearables Localization Ambient Backscatter Severe power constraints

5. Challenge: Expensive Digital Computation 5 Channel estimation Matrix inversion, etc. Multiple antennas Expensive correlation Synchronization, etc. Coding (e.g., CDMA) Requires power- intensive ADCs 100’s of mW Battery-free devices have orders of magnitude less power

6. Our Design Principle Perform computation in the analog domain

7. Contributions Introduce the first multi-antenna cancellation design for battery-free backscatter devices Introduce first analog coding technique for long-range backscatter communication  10 kbps  1 Mbps  2 feet  20 meters

8. Ambient Backscatter TX Ambient Backscatter Communication No additional power No additional spectrum Modulated reflection 8 Ambient Backscatter RX Limited to 10 kbps and 2 feet of range

9. Contributions Introduce the first multi-antenna cancellation design for battery-free backscatter devices  10 kbps  1 Mbps Introduce first analog coding technique for long-range ambient backscatter communication  2 feet  20 meters

10. + h b2 s(t) b(t)h rf2 s(t) + h b1 s(t) b(t) Multi-Antennas Without Digital Computation 10 Bob (RX) Alice (TX) h rf1 h rf2 h b2 h b1 h rf1 s(t) s(t) b(t) 1 2 1 = 2 =

11. s(t)[h rf2 + h b2 b(t)] s(t)[h rf1 + h b1 b(t)] Multi-Antennas Without Digital Computation 11 Bob (RX) Alice (TX) h rf1 h rf2 h b2 h b1 s(t) = = 1 2 h rf2 + h b2 h rf1 + h b1 h rf2 h rf1, b=0b=1 1 2 = 1 2 Decode b(t) using changes in

12. Division in the Analog Domain 12 Commercial analog dividers are power hungry! – Build our own. Log Amplifier Exp. Amplifier ThresholdingSubtractor = exp(log( ) – log( )) 1 2 12 Multi-antenna design without digital computation

13. Contributions Introduce the first multi-antenna cancellation design for battery-free backscatter devices  10 kbps  1 Mbps Introduce first analog coding technique for long-range ambient backscatter communication  2 feet  20 meters

14. How do we Increase Range? 14 Pattern for ‘1’ bit Pattern detected here ‘1’ Add redundancy to data for easier decoding Cross-correlating is too expensive Pattern for ‘0’ bit O(N 2 ) RX

15. No shift  Gives I component How do we Increase Range? Use periodic code 15 No synchronization required Pattern for ‘1’ bitPattern for ‘0’ bit 000000… Receiver simply correlates with: ½ symbol delay  Gives Q component | I | + | Q | = N

16. How do we Increase Range? Analog implementation 16 Simple analog implementation  Low Power Sum Thresholding Circuit Correlator I Correlator Q

17. Our Hardware Prototype 17 Integrated multi-antenna and coding implementation – 422 uW for multi-antenna – 8.9 uW for coding circuit Software-defined behavior – 0.3 bps to 1 Mbps TV, RFID, and solar harvesting

18. What Gains Can Multiple Antennas Provide? 18 10kbps, SIGCOMM ‘13 Multi-antenna, 1 Mbps 100x improvement

19. How do we get orders of magnitude gains by adding an antenna? Last year (SIGCOMM ’13) – Average to eliminate big TV signal Multi-antenna design – Completely cancel TV signal 19 What Gains Can Multiple Antennas Provide? Orders of magnitude increase in rate

20. Can our Analog Code Increase the Range? 20 Transmitter and receiver in line-of-sight SIGCOMM ‘13 Analog Coding 10-100x improvement across all power levels

21. Can our Analog Code Increase the Range? 21 Transmitter and receiver in non-line-of-sight 0 1 2 3 4 TX

22. Conclusions Introduce the first multi-antenna and coding designs for battery-free backscatter devices Provide orders of magnitude increase in rate and range of ambient backscatter Re-design networking primitives with power as a first class citizen -Full-duplex (MOBICOM’14), UWB (?), Random access (?), TCP/IP (?), …