Transcript BackPos
BackPos:
Anchor-free Backscatter Positioning for RFID Tags with High Accuracy
Tianci Liu, Lei Yang, Qiongzheng Lin, Yi Guo, Yunhao Liu
Outline • • • •
Background & Motivation
Proposed Approach Evaluation Discussion & Conclusion 2
Backscatter system Reader Antenna UHF Wave Four elements of UHF backscatter system Tags
Backscatter system Expensive and do most of the jobs. EPC C1Gen2 Transmit/receive signals Polarized, ‘fat’(e.g. 25 × 25𝑐𝑚 2 ) From 300MHz to 3GHz EPC C1Gen2: 900MHz~915MHz No battery and weak Tiny and cheap. 5 cents to 15 cents
Locating every unique RFID tag Locating a tag means almost locating everything
Existing approaches By distance estimation.
RSS as distance indicator.
Influenced by environment, tag
orientation and diversity.
𝑃 𝑅𝑋,𝑟𝑒𝑎𝑑𝑒𝑟 = 𝑃 𝑇𝑋,𝑟𝑒𝑎𝑑𝑒𝑟 𝐺 2 𝑟𝑒𝑎𝑑𝑒𝑟 𝐺 2 𝑡𝑎𝑔 cos 2 𝜃 𝑝𝑜𝑙 𝜆 4𝜋𝑑 4
Existing approaches By reference tags(anchor).
Compare fingerprint: RSS(LANDMARC), multipath profile(PinIt)
A lot of deployment work
Granularity of anchors largely determines accuracy.
Outline • • • • Background & Motivation
Proposed Approach
Evaluation Discussion & Conclusion 8
Signal features COTS readers provide • • • RSS & phase are available, e.g. Impinj readers RSS: not reliable Phase: 𝜃 = 2𝜋 − 2𝜋 2𝑑 𝜆 + 𝜃 𝑅𝑒𝑎𝑑𝑒𝑟 + 𝜃 𝐴𝑛𝑡𝑒𝑛𝑛𝑎 + 𝜃 𝑇𝑎𝑔 𝑚𝑜𝑑 2𝜋
Empirical studies of phase Phase is stable.
Gaussian distribution.
Deviation: 5.7°
Empirical studies of phase 8 orientations.
Remain at a same level ~200° Phase is unrelated to tag orientation.
Phase in backscatter system Periodicity Linear 2𝜋~𝜆/2 𝜃 = 2𝜋 − 2𝜋 2𝑑 𝜆 + 𝜃 𝑅𝑒𝑎𝑑𝑒𝑟 + 𝜃 𝐴𝑛𝑡𝑒𝑛𝑛𝑎 + 𝜃 𝑇𝑎𝑔 𝑚𝑜𝑑 2𝜋
Challenges of using phase • Stable, unrelated to orientation, related to distance • • Tag diversity: 𝜃 𝑇𝑎𝑔 – Should be applicable for all tags without measuring first Periodicity: 𝑚𝑜𝑑 2𝜋 – Can not get 𝑑 from 𝜃 directly 𝜃 = 2𝜋 − 2𝜋 2𝑑 𝜆 + 𝜃 𝑅𝑒𝑎𝑑𝑒𝑟 + 𝜃 𝐴𝑛𝑡𝑒𝑛𝑛𝑎 + 𝜃 𝑇𝑎𝑔 𝑚𝑜𝑑 2𝜋
BackPos - tag diversity • Use phase difference from different antennas Δ𝜃 = 2𝜋 − 2𝜋 2Δ𝑑 𝜆 + Δ𝜃 𝐴𝑛𝑡𝑒𝑛𝑛𝑎 𝑚𝑜𝑑 2𝜋 • Eliminate periodicity – Get distance difference from phase difference
Eliminate periodicity • • Distance difference restrict – |Δ𝑑| < Δ𝑑 𝑑𝑖𝑓 – Δ𝑑 𝑑𝑖𝑓 = 𝜆/4 : one Δ𝑑 – Δ𝑑 𝑑𝑖𝑓 = 𝜆/2 : two possible Δ𝑑 Naive approach – Triangle constraint – |𝑇𝐴 1 − 𝑇𝐴 2 | < d ant between antennas = Δ𝑑 𝑑𝑖𝑓
Eliminate periodicity • Naive approach – Triangle constraint: 𝜆 /4 between antennas – 𝜆 ≈32 𝑐𝑚 ( 𝑓 ≈910 𝑀𝐻𝑧 ) 𝑑 𝑎𝑛𝑡 > 𝑤 = 25𝑐𝑚 ≫ 8𝑐𝑚 𝑜𝑟 16𝑐𝑚 w=25cm
BackPos - periodicity • Use feasible region to relax antenna distance constraint – – Antenna has reading zone Just positions in reading zone satisfy distance difference restrict 𝜃 𝑙𝑜𝑏𝑒 = 4𝜋 𝐺
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 𝑑 𝑎𝑛𝑡
BackPos - periodicity • Hyperbola constraint – 𝐴 1 and 𝐴 2 are two focus points – 𝑃𝐴 1 − 𝑃𝐴 2 < Δ𝑑 𝑑𝑖𝑓 – Angle between two asymptote line: > 𝜃 𝑙𝑜𝑏𝑒 𝑑 𝑎𝑛𝑡 = 40𝑐𝑚 : Δ𝑑 < Δ𝑑 𝑑𝑖𝑓 𝜆 = 2
BackPos – distance difference • Δ𝑑 < Δ𝑑 𝑑𝑖𝑓 – Δ𝜃 = |𝜃 1 = 𝜆 => 2 − 𝜃 2 | < 2𝜋 𝜃 1 – 𝜃 1 ≥ 𝜃 2 or 𝜃 1 < 𝜃 2 0 𝜋 𝜃 2 𝜃 1 2𝜋 3 𝜋 4𝜋 Two possible Δ𝑑 : two half hyperbola for each antenna pair
BackPos – hyperbola intersection • For three antennas: – – 𝐴 1 , 𝐴 2 𝐴 2 , 𝐴 3 -> 𝑐 1 , 𝑐 3 -> 𝑐 2 , 𝑐 4 Three possible points: 𝑅 = {𝑇 1 , 𝑇 2 , 𝑇 3 }
BackPos – estimated position • Reader: 4 antenna ports – 𝐴 1 , 𝐴 2 , 𝐴 3 → 𝑅 1 – 𝐴 2 , 𝐴 3 , 𝐴 4 → 𝑅 2 • • If readings are accurate: {𝑇} = 𝑅 1 ∩ 𝑅 2 With error:
Outline • • • • Background & Motivation Proposed Approach
Evaluation
Discussion & Conclusion 23
Implementation & Evaluation • • • • COTS reader – – Impinj R420 with 4 antennas 138.1cm high COTS tags – Alien ‘2 × 2’ & ‘Square’ Ground Truth – Laser meter: ~1 𝑚𝑚 Scenario – 500 × 800 𝑐𝑚 2 room 25cm
Positioning accuracy • • • Target position: (−43.2, 283.4) Distance difference error: <1 𝑐𝑚 Positioning error: 12.8
𝑐𝑚
Positioning v.s positions • • 9 targets in a circle Error increases as 𝑦 increases
Positioning v.s positions • • 9 targets in a circle Error increases as 𝑦 increases
Positioning v.s tag diversity • • Random choose positions for targets ‘2 × 2’ & ‘Square’
Outline • • • • Background & Motivation Proposed Approach Evaluation
Discussion & Conclusion
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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