Department of Computer Science

University of Virginia

The Practicality of Multi-Tag RFID Systems

Leonid BolotnyyScott Krize

Gabriel Robins

Introduction• RFID

passive semi-passive active

• Tags types:

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

• Coupling methods:

readerantenna

signal signal

Inductive coupling Backscatter coupling

History

• Auto-ID Center formed - 1999

• EPCglobal formed - 2004

• Radar invented - 1935

• EAS invented - early 1960’s

• First RFID book published - 1999

• First RFID patent filed - 1973

• First RFID game marketed - 2006

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 ~ sin2(β), distance ~ (power)1/2

32.7

58.11

47.98

61.86

30

35

40

45

50

55

60

65

1 2 3 4Number of Tags

Ex

pe

cte

d a

ng

le (

in D

eg

ree

s)

4 2

40[ (2 cos ) ( )(2 cos ) ] /

2

x x dx x x dx

2

0[ (2 cos ) ] /(2 )

x x dx

B-field

β

• Optimal Tag Placement:

1

4

32

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

Antenna Pair #1, Power = 31.6dBm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Object Number

Det

ecti

on

Pro

bab

ilit

y

1Tag: 58%

2Tags: 79%

3Tags: 89%

4Tags: 93%

Circular Antennas

Antenna Pair #1, Power = 31.6dBm

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Object Number

Det

ecti

on

Pro

bab

ilit

y

1Tag: 75%

2Tags: 94% 3Tags: 98%

4Tags: 100%

Linear Antennas vs. Multi-tags

Power = 31.6dBm

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Object Number

De

tec

tio

n P

rob

ab

ilit

y

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

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Object Number

Det

ecti

on

Pro

bab

ility

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%

Linear Antennas

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

31.6 30.6 29.6 28.6 27.6 26.6 25.6

Power (dBm)

Det

ectio

n P

rob

abili

ty

Circular Antennas

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

31.6 30.6 29.6 28.6 27.6 26.6 25.6

Power (dBm)D

etec

tion

Pro

bab

ility

1 Tag 2 Tags 3 Tags 4 Tags

Power

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

Importance of Tag Orientation

Uni-polar tags

1 Tag 2 Tags 1 Tag 2 Tags180-same 0.55 0.37180-diff 0.74 0.5290-same 0.67 0.5290-diff 0.80 0.63

Circular Linear

0.47 0.33

Bi-polar tags

1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags180 1 1 0.57 0.790 0.93 1 0.97 1

Circular Linear

0.75 0.53

Controlling Variables1. 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/Metals Power=31.6dBm, With Liquids/Metals

Power=27.6dBm, No Liquids/Metals Power=27.6dBm, With Liquids/Metals

Circular Antenna

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4

Number of Tags

Det

ecti

on

Pro

bab

ilit

y

• 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

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags

Antenna #1 Antenna #2 Antenna #1 and #2

Number of Tags

De

tec

tio

n P

rob

ab

ilit

y

Power=31.6dBm, Circular AntennasPower=31.6dBm, Linear AntennasPower=27.6dBm, Circular AntennasPower=27.6dBm, Linear Antennas

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

Effect of the Number of Objects on Detection Probability

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags 1 Tag 2 Tags 3 Tags

1 Antenna 2 Antennas 3 Antennas 4 Antennas

Det

ecti

on

Pro

bab

ilit

y

15/15 experiment

20/20 experiment

15/15 experiment

20/20 experiment

Metals & Liquids∆ : 3%-13%

Detection Delta

Change in Detection Based on # of Antennas and Tags

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

1 Antenna 2 Antennas 3 Antennas 4 Antennas

Ch

ang

e in

Det

ecti

on

Pro

bab

ilit

y

1 Tag

2 Tags

3 Tags

Anti-Collision Algorithms

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*

Algorithm Redundant Tags Connected-Tags

* Assuming tags communicate to form a single response** If all tags are detected

Applications of Multi-TagsReliability Availability

Safety

Localization

More Applications

Tagging Bulk MaterialsPackaging

Theft PreventionSecurity

Economics of Multi-TagsPassive Tag Cost Trend

$0.00$0.20$0.40$0.60$0.80$1.00

2001 2002 2003 2004 2005 2006 2007 2008 2011

Year

Ta

g C

os

t

Historical Cost Prediction Cost

2001 $1.042002 $0.812003 $0.452004 $0.192005 $0.132006 $0.082007 $0.062008 $0.052011 $0.01

Year Cost

• Rapid decrease in passive tag cost• 5 cent tag expected in 2008• 1 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

Passive Tag Cost Trend

$0.00$0.20$0.40$0.60$0.80$1.00

2001 2002 2003 2004 2005 2006 2007 2008 2011

Year

Ta

g C

os

t

Historical Cost Prediction Cost

• Favorable economics

Our ResearchGeneralized “Yoking Proofs”

3

Multi-Tags

PUF Inter-Tag Communication

RFID

Thank You

Questions?