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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
Outline

  • Background & Motivation

  • Proposed Approach

  • Evaluation

  • Discussion & Conclusion


Backscatter system
Backscatter system

UHF Wave

Tags

Reader

Antenna

Four elements of UHF backscatter system


Backscatter system1
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 every unique RFID tag

Locating a tag means almost locating everything


Existing approaches
Existing approaches

By distance estimation.

RSS as distance indicator.

Influenced by environment, tag orientation and diversity.


Existing approaches1
Existing approaches

By reference tags(anchor).

Compare fingerprint: RSS(LANDMARC), multipath profile(PinIt)

A lot of deployment work

Granularity of anchors largely determines accuracy.


Outline1
Outline

  • Background & Motivation

  • Proposed Approach

  • Evaluation

  • Discussion & Conclusion


Signal features cots readers provide
Signal features COTS readers provide

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

RSS: not reliable

Phase:


Empirical studies of phase
Empirical studies of phase

Gaussian distribution.

Deviation:

Phase v.s. frequency

Phase is stable.


Empirical studies of phase1
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
Phase in backscatter system

Periodicity

Linear

Phase v.s. distance


Challenges of using phase
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
BackPos - tag diversity

  • Use phase difference from different antennas

  • Eliminate periodicity

    • Get distance difference from phase difference


Eliminate periodicity
Eliminate periodicity

  • Distance difference restrict

    • : one

    • : two possible

  • Naive approach

    • Triangle constraint

    • between antennas


Eliminate periodicity1
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
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 periodicity1
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 periodicity2
BackPos - periodicity

  • Hyperbola constraint

    • and are two focus points

    • Angle between two asymptote line:

:

Hyperbola restrict


Backpos distance difference
BackPos– distance difference

  • =>

    • or

3

Two possible :

two half hyperbola for each antenna pair


Backpos hyperbola intersection
BackPos– hyperbola intersection

  • For three antennas:

    • , -> ,

    • , -> ,

Three possible points:

Hyperbola intersection


Backpos estimated position
BackPos– estimated position

  • Reader: 4 antenna ports

  • If readings are accurate:

  • With error:


Outline2
Outline

  • Background & Motivation

  • Proposed Approach

  • Evaluation

  • Discussion & Conclusion


Implementation evaluation
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
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
Positioning v.s positions

9 targets in a circle

Error increases as 𝑦 increases

Positioning accuracy v.s positions


Positioning v s positions1
Positioning v.s positions

9 targets in a circle

Error increases as 𝑦 increases

Positioning accuracy v.s positions


Positioning v s tag diversity
Positioning v.s tag diversity

Random choose positions for targets

‘2×2’ & ‘Square’

Positioning accuracy v.s tag diversity


Outline3
Outline

  • Background & Motivation

  • Proposed Approach

  • Evaluation

  • Discussion & Conclusion


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
Thank you!Questions?


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