1. Can we get Wi-Fi connectivity for 15 µW? Bryce Kellogg

3. Prototype consumes 14 µW of power

4. Why is this hard? Wi-Fi consumes 100 mW of power 10,000x too much

5. RSSI Wi-Fi Backscatter Wi-Fi Device Wi-Fi Backscatter Tag Wi-Fi AP 2 2 4 4 Changing reflections consumes 0.65 µW of power Key Idea: Piggyback on Wi-Fi Packets 1 1 1 0 3 3 5 5 6 6 7 7 8 8 9 9

6. How to work in a network of Wi- Fi devices? How does the Wi-Fi device send data to the tag? How do we design a network with multiple tags?

7. How to work in a network of Wi- Fi devices? How does the Wi-Fi device send data to the tag? How do we design a network with multiple tags?

8. Challenge: Cannot Decode Packet Headers Wi-Fi Device Wi-Fi Backscatter Tag ???

9. Solution: Be Agnostic to Wi-Fi Traffic 00110 10:34:12.1265710:34:12.1266310:34:12.1267810:34:12.1269810:34:12.1271910:34:12.12734 10:34:12.1274810:34:12.1275710:34:12.1277010:34:12.1277310:34:12.1277510:34:12.1277710:34:12.1277910:34:12.1278610:34:12.12789 Bin backscatter bits into time slots Use Wi-Fi timestamp to reconstruct bits Time Slot t Adjust timeslots based on network traffic

10. How to work in a network of Wi- Fi devices? How does the Wi-Fi device send data to the tag? How do we design a network with multiple tags?

11. Encode data as presence/absence of Wi-Fi Packet Wi-Fi Backscatter Tag CTS-to-Self Wi-Fi Device 0 000 0 1 111

12. OFDM: High peak-to-avg power ratio Low Power Wi-Fi Packet Detection Leverage structure of Wi-Fi signal

13. Can detect packets as short as 50 µs Consumes µWs of power Antenna Envelope Detector Peak finder Set threshold Comparator Low Power Wi-Fi Packet Detection

14. How to work in a network of Wi- Fi devices? How does the Wi-Fi device send data to the tag? How do we design a network with multiple tags?

15. Design Request/Response Protocol Wi-Fi Device CTS 11111 0 00 11111 0 00 11111 0 00 None of the tags transmit concurrently Wi-Fi device act as a central coordinator

16. Our Prototype Intel 5300 Wi-Fi Cards Wi-Fi Backscatter Prototype 2.4 GHz Wi-Fi channels Use both RSSI and CSI values

17. Power Consumption MSP430: 5.0 µW RF Switch: 0.65 µW Peak Finder: 9.0 µW Total: 14.65 µW

18. What is the maximum rate and range? 2.2 meters, 1 kbps 3.0 meters, 20 kbps Wi-Fi Backscatter Tag Wi-Fi Device

19. Does AP Location Affect Backscatter Decoding? 33 3 3 3 94% 99% 98% 8.5 m 12.2 m Probability of receiving a backscatter packet correctly AP can be far away or even in a different room

20. Does Backscatter Affect Wi-Fi Decoding? 33 3 3 3 8.5 m 12.2 m

21. Does it affect Wi-Fi Performance? Wi-Fi is robust to multipath environment

22. How does it work in a busy network? Setup in UW Networks Lab: 8 – 15 People Used University Wi-Fi as AP Observed maximum achievable bit rate over period of 8 hours

23. How does it work in a busy network? More network traffic → faster Backscatter rates

24. Related Work RFID Requires dedicated reader and new hardware installation Wi-Fi Backscatter transforms Wi-Fi devices into RFID readers Ambient Backscatter Device to device communication Wi-Fi Backscatter connects devices to existing Wi-Fi networks

25. Conclusion Can enable RF-Powered Internet of Things, connecting next billion devices Feasibility of Wi-Fi connectivity at < 15 µW iotWiFi.cs.washington.edu