The Practicality of Multi-Tag RFID Systems

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The Practicality of Multi-Tag RFID Systems
Leonid BolotnyyScott KrizeGabriel Robins
Department of Computer ScienceUniversity of
Virginia
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Introduction

• RFID

• Tags types

• Frequencies Low (125KHz), High (13.56MHz), UHF
  (915MHz)

• Coupling methods

signal
signal
reader antenna
Inductive coupling
Backscatter coupling
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History
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Object Identification

• Bar-codes vs. RFID
• line-of-sight
• scanning rate

• Unreliability of object detection
• radio noise is ubiquitous
• temperature and humidity
• objects/readers moving speed
• liquids and metals are opaque to RF
• milk, water, juice
• metal-foil wrappers
• object occlusion
• number of objects grouped together
• tag variability and receptivity
• tag aging

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Case Studies

• Defense Logistics Agency trials (2001)
• 3 of moving objects did not reach destination
• 20 of tags recorded at every checkpoint
• 2 of a tag type detected at 1 checkpoint
• some tags registered on arrival but not departure

• Wal-Mart experiments (2005)
• 90 tag detection at case level
• 95 detection on conveyor belts
• 66 detection inside fully loaded pallets

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Multi-Tag RFID

• Use Multiple tags per object to increase
  reliability of object detection/identification

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The Power of an Angle

• Inductive coupling voltage sin(ß),
  distance (power)1/6
• Far-field propagation voltage sin2(ß),
  distance (power)1/2

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Equipment and Setup

• Equipment

• 4 linear antennas by Alien Technology
• 4 circular antennas by Alien Technology
• 4 circular antennas by ThingMagic

• Setup
• empty room
• 20 solid non-metallic 20 metallic and liquid
  objects
• tags positioned perpendicular to each other
• tags spaced apart
• software drivers

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Experiments

• Read all tags in readers field
• Randomly shuffle objects
• Compute average detection rates

• Variables
• reader type
• antenna type
• tag type
• antenna power
• object type
• number of objects
• number of tags per object
• tags orientation
• tags receptivity

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Linear Antennas
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Circular Antennas
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Linear Antennas vs. Multi-tags
2 Readers, 2 Tags 84.5
1 Reader, 2 Tags 79.3
2 Readers, 1 Tag 64.9
1 Reader, 1 Tag 58.0
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Circular Antennas vs. Multi-Tags
Power 31.6dBm
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0.9
0.8
0.7
Detection Probability
0.6
0.5
0.4
0.3
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Object Number
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Power

• Decrease in detection with decrease in power
• More rapid decrease in detection for circular
  antennas

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Importance of Tag Orientation
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Controlling Variables

1. Radio noise
2. Tag variability
3. Reader variability
4. Reader power level
5. Distance to objects type, of antennas

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Detection in Presence of Metals Liquids

• Decrease in solid/non-liquid object detection
• Significant at low power
• Similar results for linear antennas

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Multi-Tags on Metals and Liquids

• Low detection probabilities
• Drop in detection at low power

• Linear antennas outperform circular
• Multi-tags better than multiple readers

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Varying Number of Objects
Experiment 1 15 solid non-metallic 15 liquids
and metals
Experiment 2 20 solid non-metallic 20 liquids
and metals
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Detection Delta
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Anti-Collision Algorithms
Algorithm
Redundant Tags
Connected-Tags
Binary No Effect No Effect
Binary Variant No Effect No Effect
Randomized Linear Increase No Effect
STAC Causes DoS No Effect
Slotted Aloha Linear Increase No Effect
Assuming tags communicate to form a single
response If all tags are detected
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Applications of Multi-Tags
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More Applications
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Economics of Multi-Tags

• Rapid decrease in passive tag cost
• 5 cent tag expected in 2008
• 1 penny tag in a few years

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Cost Trends
Time
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Business Case for RFID

• Costs benefits (business case)
• Moores law
• higher employee productivity
• reduction in workforce
• automated business processes
• workforce reduction

• Tag manufacturing yield and testing
• 30 of chips damaged during manufacturing
• 15 damaged during printing U.S. GAO
• 20 tag failure rate in field RFIDJournal
• 5 of tags purchased marked defective

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RFID Tag Demand

• Demand drivers
• tag cost
• desire to stay competitive

• Cost effective tag design techniques
• memory design (self-adaptive silicon)
• assembly technology (fluidic self assembly)
• antenna design (antenna material)

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Conclusion

• Unreliability of object detection
• radio noise is ubiquitous
• temperature and humidity
• objects/readers moving speed
• liquids and metals are opaque to RF
• milk, water, juice
• metal-foil wrappers
• object occlusion
• number of objects grouped together
• tag variability and receptivity
• tag aging

• Many useful applications

• Favorable economics

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Our Research
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Thank You
Questions?