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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. Backscatter system. UHF Wave. Tags. Reader. Antenna.

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


Outline

  • Background & Motivation

  • Proposed Approach

  • Evaluation

  • Discussion & Conclusion


Backscatter system

UHF Wave

Tags

Reader

Antenna

Four elements of UHF backscatter system


Backscatter system

Expensive and do most of the jobs.

EPC C1Gen2

Transmit/receive signals

Polarized, ‘fat’(e.g. )

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.


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


Signal features COTS readers provide

RSS & phase are available, e.g. Impinj readers

RSS: not reliable

Phase:


Empirical studies of phase

Gaussian distribution.

Deviation:

Phase v.s. frequency

Phase is stable.


Empirical studies of phase

8 orientations.

Remain at a same level

Phase v.s. tag orientation

Phase is unrelated to tag orientation.


Phase in backscatter system

Periodicity

Linear

Phase v.s. distance


Challenges of using phase

  • Stable, unrelated to orientation, related to distance

  • Tag diversity:

    • Should be applicable for all tags without measuring first

  • Periodicity:

    • Can not get from directly


BackPos - tag diversity

  • Use phase difference from different antennas

  • Eliminate periodicity

    • Get distance difference from phase difference


Eliminate periodicity

  • Distance difference restrict

    • : one

    • : two possible

  • Naive approach

    • Triangle constraint

    • between antennas


Eliminate periodicity

  • Naive approach

    • Triangle constraint: 𝜆/4 between antennas

    • 𝜆≈32𝑐𝑚 (𝑓≈910𝑀𝐻𝑧)

w=25cm

w=25cm

Triangle restrict does not work


BackPos - periodicity

  • Use feasible region to relax antenna distance constraint

    • Antenna has reading zone

    • Just positions in reading zone satisfy distance difference restrict

Reading zone


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

Feasible region


BackPos - periodicity

  • Hyperbola constraint

    • and are two focus points

    • Angle between two asymptote line:

:

Hyperbola restrict


BackPos– distance difference

  • =>

    • or

3

Two possible :

two half hyperbola for each antenna pair


BackPos– hyperbola intersection

  • For three antennas:

    • , -> ,

    • , -> ,

Three possible points:

Hyperbola intersection


BackPos– estimated position

  • Reader: 4 antenna ports

  • If readings are accurate:

  • With error:


Outline

  • Background & Motivation

  • Proposed Approach

  • Evaluation

  • Discussion & Conclusion


Implementation & Evaluation

  • COTS reader

    • Impinj R420 with 4 antennas

    • 138.1cm high

  • COTS tags

    • Alien ‘2×2’ & ‘Square’

  • Ground Truth

    • Laser meter: ~1𝑚𝑚

  • Scenario

    • room

25cm

25cm


Positioning accuracy

Target position: (−43.2, 283.4)

Distance difference error: <1𝑐𝑚

Positioning error: 12.8𝑐𝑚

Distance difference accuracy

Positioning accuracy


Positioning v.s positions

9 targets in a circle

Error increases as 𝑦 increases

Positioning accuracy v.s positions


Positioning v.s positions

9 targets in a circle

Error increases as 𝑦 increases

Positioning accuracy v.s positions


Positioning v.s tag diversity

Random choose positions for targets

‘2×2’ & ‘Square’

Positioning accuracy v.s tag diversity


Outline

  • Background & Motivation

  • Proposed Approach

  • Evaluation

  • Discussion & Conclusion


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


Thank you!Questions?


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