1. BackPos: Anchor-free Backscatter Positioning for RFID Tags with High Accuracy Tianci Liu, Lei Yang, Qiongzheng Lin, Yi Guo, Yunhao Liu

2. Outline Background & Motivation Proposed Approach Evaluation Discussion & Conclusion 2

3. Backscatter system Four elements of UHF backscatter system Reader Antenna UHF Wave Tags

4. Backscatter system Expensive and do most of the jobs. EPC C1Gen2 Expensive and do most of the jobs. EPC C1Gen2 From 300MHz to 3GHz EPC C1Gen2: 900MHz~915MHz From 300MHz to 3GHz EPC C1Gen2: 900MHz~915MHz No battery and weak Tiny and cheap. 5 cents to 15 cents No battery and weak Tiny and cheap. 5 cents to 15 cents

5. Locating every unique RFID tag Locating a tag means almost locating everything

6. Existing approaches By distance estimation. RSS as distance indicator. Influenced by environment, tag orientation and diversity. By distance estimation. RSS as distance indicator. Influenced by environment, tag orientation and diversity.

7. Existing approaches Granularity of anchors largely determines accuracy. By reference tags(anchor). Compare fingerprint: RSS(LANDMARC), multipath profile(PinIt) A lot of deployment work By reference tags(anchor). Compare fingerprint: RSS(LANDMARC), multipath profile(PinIt) A lot of deployment work

8. Outline Background & Motivation Proposed Approach Evaluation Discussion & Conclusion 8

9. Signal features COTS readers provide RSS & phase are available, e.g. Impinj readers RSS: not reliable Phase:

10. Empirical studies of phase Phase is stable.

11. Empirical studies of phase Phase is unrelated to tag orientation.

12. Phase in backscatter system

13. Challenges of using phase

14. BackPos - tag diversity Use phase difference from different antennas Eliminate periodicity – Get distance difference from phase difference

15. Eliminate periodicity

16. Naive approach – Triangle constraint: /4 between antennas – ≈32 (≈910) w=25cm

17. BackPos - periodicity Use feasible region to relax antenna distance constraint – Antenna has reading zone – Just positions in reading zone satisfy distance difference restrict

18. BackPos - periodicity Use feasible region to relax antenna distance constraint – Feasible region: region that all positions satisfy distance difference restrict. – Cover reading zone intersections of antennas

19. BackPos - periodicity

20. BackPos – distance difference

21. BackPos – hyperbola intersection

22. BackPos – estimated position

23. Outline Background & Motivation Proposed Approach Evaluation Discussion & Conclusion 23

24. Implementation & Evaluation 25cm

25. Positioning accuracy Target position: (−43.2, 283.4) Distance difference error: <1 Positioning error: 12.8

26. Positioning v.s positions 9 targets in a circle Error increases as increases

27. Positioning v.s positions 9 targets in a circle Error increases as increases

28. Positioning v.s tag diversity Random choose positions for targets ‘2×2’ & ‘Square’

29. Outline Background & Motivation Proposed Approach Evaluation Discussion & Conclusion 29

30. Discussion & Conclusion Explore phase in backscatter systems Propose BackPos – No reference tags, 12.8cm Implement and evaluate BackPos with COTS devices Limitations: – Not suitable for NLOS scenarios – 4 antennas

31. Thank you! Questions?