The Practicality of Multi-Tag
RFID Systems
Leonid Bolotnyy
Scott Krize
Gabriel Robins
Department of
Computer Science
University of Virginia
Introduction
• RFID
• Tags types:
passive
semi-passive
active
• Frequencies: Low (125KHz), High (13.56MHz), UHF (915MHz)
• Coupling methods:
signal
signal
reader
antenna
Inductive coupling
Backscatter coupling
History
• Radar invented - 1935
• EAS invented - early 1960’s
• First RFID patent filed - 1973
• First RFID book published - 1999
• Auto-ID Center formed - 1999
• EPCglobal formed - 2004
• 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
B-field
• Optimal Tag Placement:
4
Expected angle (in Degrees)
1
3
2
65
61.86
58.11
60
55
47.98
50


4
2
 [  x(2 cos x) dx   (
0
4
45
40
35
β

32.7
 [  x(2 cos x)dx] /(2 )
2
0
30
1
2
3
Number of Tags
4

2
 x)(2 cos x) dx] / 
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
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
1
0.9
Detection Probability
0.8
0.7
0.6
0.5
0.4
1Tag: 58%
2Tags: 79%
3Tags: 89%
4Tags: 93%
0.3
0.2
0.1
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20
Object Number
Circular Antennas
Antenna Pair #1, Power = 31.6dBm
1
Detection Probability
0.9
0.8
0.7
0.6
1Tag: 75%
0.5
2Tags: 94%
3Tags: 98%
0.4
4Tags: 100%
0.3
1
2
3
4
5
6
7
8
9
10
11
12
Object Number
13
14
15
16
17
18
19
20
Linear Antennas vs. Multi-tags
Power = 31.6dBm
1
0.9
0.8
Detection Probability
0.7
0.6
0.5
0.4
2 Readers, 2 Tags 84.5%
Δ= 5.2%
0.3
1 Reader,
Δ=14.4%
0.2
0.1
Δ= 6.9%
Δ=19.8%
2 Tags 79.3%
2 Readers, 1 Tag
64.9%
1 Reader,
1 Tag
58.0%
9
12
14
Δ=21.3%
0
1
2
3
4
5
6
7
8
10
11
Object Number
13
15
16
17
18
19
20
Circular Antennas vs. Multi-Tags
Power = 31.6dBm
1
0.9
Detection Probability
0.8
0.7
0.6
2 Readers, 2 Tags 99.4%
Δ= 5.2%
0.5
1 Reader,
Δ=3.2%
0.4
Δ= 15.1%
Δ=8.4%
2 Tags 94.2%
2 Readers, 1 Tag
91.0%
1 Reader,
75.9%
1 Tag
Δ=18.3%
0.3
1
2
3
4
5
6
7
8
9
10
11
Object Number
12
13
14
15
16
17
18
19
20
Power
1 Tag
2 Tags
3 Tags
4 Tags
Circular Antennas
1
1
0.9
0.9
Detection Probability
Detection Probability
Linear Antennas
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
31.6
30.6
29.6
28.6
Power (dBm)
27.6
26.6
25.6
31.6
30.6
29.6
28.6
27.6
26.6
Power (dBm)
• Decrease in detection with decrease in power
• More rapid decrease in detection for circular antennas
25.6
Importance of Tag Orientation
Uni-polar tags
180-same
180-diff
90-same
90-diff
Circular
1 Tag
2 Tags
0.55
0.74
0.47
0.67
0.80
Linear
1 Tag
2 Tags
0.37
0.52
0.33
0.52
0.63
Bi-polar tags
1 Tag
180
90
0.75
Circular
2 Tags
1
0.93
3 Tags
1
1
1 Tag
0.53
Linear
2 Tags
0.57
0.97
3 Tags
0.7
1
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
Circular Antenna
1
Detection Probability
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Power=31.6dBm, No Liquids/Metals
Power=31.6dBm, With Liquids/Metals
Power=27.6dBm, No Liquids/Metals
Power=27.6dBm, With Liquids/Metals
0
1
2
3
4
Number of Tags
• Decrease in solid/non-liquid object detection
• Significant at low power
• Similar results for linear antennas
Multi-Tags on Metals and Liquids
0.9
0.8
Power=31.6dBm, Circular Antennas
Power=31.6dBm, Linear Antennas
Power=27.6dBm, Circular Antennas
Power=27.6dBm, Linear Antennas
Detection Probability
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1 Tag
2 Tags
3 Tags
Antenna #1
1 Tag
2 Tags
3 Tags
Antenna #2
Number of Tags
1 Tag
2 Tags
3 Tags
Antenna #1 and #2
• Low detection probabilities
• Linear antennas outperform circular
• Drop in detection at low power • Multi-tags better than multiple readers
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
0.9
Detection Probability
0.8
15/15 experiment
20/20 experiment
0.7
0.6
0.5
0.4
0.3
Metals & Liquids
∆ : 3%-13%
0.2
0.1
0
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
Detection Delta
Change in Detection Based on # of Antennas and Tags
Change in Detection Probability
0.16
0.14
0.12
0.1
1 Tag
0.08
2 Tags
3 Tags
0.06
0.04
0.02
0
1 Antenna
2 Antennas
3 Antennas
4 Antennas
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
Slotted Aloha
Linear Increase** No Effect*
No Effect*
* Assuming tags communicate to form a single response
** If all tags are detected
Applications of Multi-Tags
Reliability
Availability
Localization
Safety
More Applications
Security
Packaging
Theft Prevention
Tagging Bulk Materials
Economics of Multi-Tags
Cost
2001
2002
2003
2004
2005
2006
2007
2008
2011
$1.04
$0.81
$0.45
$0.19
$0.13
$0.08
$0.06
$0.05
$0.01
Passive Tag Cost Trend
Tag Cost
Year
$1.00
$0.80
$0.60
$0.40
$0.20
$0.00
2001
2002
2003
2004
2005
2006
2007
Year
Historical Cost
• Rapid decrease in passive tag cost
• 5 cent tag expected in 2008
• 1 penny tag in a few years
Prediction Cost
2008
2011
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
Increase in RFID tag demand
Decrease in RFID tag cost
• Cost effective tag design techniques
– memory design (self-adaptive silicon)
– assembly technology (fluidic self assembly)
– antenna design (antenna material)
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
Tag Cost
Passive Tag Cost Trend
$1.00
$0.80
$0.60
$0.40
$0.20
$0.00
2001
2002
2003
2004
2005
2006
2007
Year
Historical Cost
Prediction Cost
2008
2011
Our Research
Generalized “Yoking Proofs”
Multi-Tags
3
RFID
PUF
Inter-Tag Communication
Thank You
Questions?