Department of Computer Science University of Virginia The Practicality of Multi-Tag RFID Systems Leonid Bolotnyy Scott Krize Gabriel Robins

Introduction RFID passivesemi-passiveactive Tags types: Frequencies: Low (125KHz), High (13.56MHz), UHF (915MHz) Coupling methods: reader antenna signal Inductive couplingBackscatter coupling

History Auto-ID Center formed EPCglobal formed Radar invented EAS invented - early 1960’s First RFID book published First RFID patent filed First RFID game marketed

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

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

Multi-Tag RFID Use Multiple tags per object to increase reliability of object detection/identification

The Power of an Angle Inductive coupling: voltage ~ sin(β), distance ~ (power) 1/6 Far-field propagation: voltage ~ sin 2 (β), distance ~ (power) 1/2 B-field β Optimal Tag Placement:

Equipment and Setup Equipment Setup –empty room –20 solid non-metallic & 20 metallic and liquid objects –tags positioned perpendicular to each other –tags spaced apart –software drivers –4 linear antennas by Alien Technology –4 circular antennas by Alien Technology –4 circular antennas by ThingMagic

Experiments Read all tags in reader’s 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

Linear Antennas 1Tag: 58% 2Tags: 79% 3Tags: 89% 4Tags: 93%

Circular Antennas 1Tag: 75% 2Tags: 94% 3Tags: 98% 4Tags: 100%

Linear Antennas vs. Multi-tags 1 Reader, 1 Tag 58.0% 2 Readers, 1 Tag 64.9% 1 Reader, 2 Tags 79.3% 2 Readers, 2 Tags 84.5% Δ=21.3% Δ=19.8% Δ= 5.2% Δ=14.4% Δ= 6.9%

Circular Antennas vs. Multi-Tags Power = 31.6dBm Object Number Detection Probability 1 Reader, 1 Tag 75.9% 2 Readers, 1 Tag 91.0% 1 Reader, 2 Tags 94.2% 2 Readers, 2 Tags 99.4% Δ=18.3% Δ=8.4% Δ= 5.2% Δ=3.2% Δ= 15.1%

1 Tag2 Tags3 Tags4 Tags Power Decrease in detection with decrease in power More rapid decrease in detection for circular antennas

Importance of Tag Orientation Uni-polar tags Bi-polar tags

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

Detection in Presence of Metals & Liquids Power=31.6dBm, No Liquids/MetalsPower=31.6dBm, With Liquids/Metals Power=27.6dBm, No Liquids/Metals Power=27.6dBm, With Liquids/Metals Circular Antenna Number of Tags Detection Probability Decrease in solid/non-liquid object detection Significant at low power Similar results for linear antennas

Low detection probabilities Drop in detection at low power Linear antennas outperform circular Multi-tags better than multiple readers Multi-Tags on Metals and Liquids

Varying Number of Objects Experiment 1: 15 solid non-metallic & 15 liquids and metals Experiment 2: 20 solid non-metallic & 20 liquids and metals Metals & Liquids ∆ : 3%-13%

Detection Delta

Anti-Collision Algorithms BinaryNo Effect Binary VariantNo Effect RandomizedLinear Increase**No Effect* STACCauses DoSNo Effect* Slotted AlohaLinear Increase**No Effect* AlgorithmRedundant TagsConnected-Tags * Assuming tags communicate to form a single response ** If all tags are detected

Applications of Multi-Tags ReliabilityAvailability Safety Localization

More Applications Tagging Bulk MaterialsPackaging Theft PreventionSecurity

Economics of Multi-Tags YearCost Rapid decrease in passive tag cost 5 cent tag expected in penny tag in a few years

Cost Trends Time

Business Case for RFID Costs & benefits (business case) –Moore’s 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

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) Increase in RFID tag demand Decrease in RFID tag cost

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

Our Research Generalized “Yoking Proofs” 3 Multi-Tags PUFInter-Tag Communication RFID

Thank You Questions?