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

2. Introduction RFID Tags types:
passive
semi-passive
active
Frequencies: Low (125KHz), High (13.56MHz), UHF (915MHz)
Coupling methods:
I will start with a brief introduction to RFID and its history.
RFID stands for radio frequency identification, which uses radio signals to uniquely identify objects.
An RFID System consists of readers, tags, and back-end servers for information processing.
There are three general types of RFID tags: passive, semi-passive, and active.
Passive tags have no batteries on-board. They use power from the reader for computation and for communication.
Semi-passive tags have batteries on-board, however the batteries are used for data processing only. The power harvested from the reader is still used for communication.
Active tags have batteries on-board and they can use them for both computations and communications.
The three major RFID frequencies are: Low frequency of 125kHz, High of MHz, and ultra high of 915 MHz in the US.
Two main coupling mechanisms used for read-tag communication.
In inductive coupling, the reader creates a magnetic field between itself and the tag, and the tag harvests the power from its field for its operation. In backscatter coupling, or far-field propagation as it is sometimes called, the reader sends a signal to the tag, which tag backscatters back to the reader.
I placed several tags, readers and antennas on this table – please feel free to play with them.
signal
signal
Reader
antenna
Inductive coupling
Backscatter coupling

3. History Radar invented - 1935 EAS invented - early 1960’s
First RFID patent filed
First RFID book published
Auto-ID Center formed
RFID technology originated with Radar, which was invented in 1935.
Electronic Article Surveillance was invented in the early 60s.
In the 70s, first patent on access control technology using RFID was filed.
In 1999 the first book on RFID was published.
In the same year, the Auto-ID Center was formed at MIT, for developing protocols and standards for Electronic Product Code (EPC), to be used as a substitute for bar-code.
Over 100 large companies and organizations, including Wal-Mart and the US Department of Defense, financed these efforts.
In 2004 the Auto-ID center was transformed into a newly formed non-profit EPCglobal organization.
At the end of 2006, the first RFID-enabled game console was marketed.
EPCglobal formed
First RFID game marketed

4. 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
Bar-code scanning requires a line-of-sight visibility, and the scan rate is at most a few bar-codes per second.
On the other hand, RFID does not require line-of-sight, and hundreds of RFID tags can be read per second.
However, these benefits have a price.
RFID tag detection is unreliable due to the ubiquitous radio noise permeating the environment, which can interfere with the readers’ ability to successfully identify tags.
In addition, liquids such as milk, water, juice etc., or metals, can absorb or reflect radio waves, in ways that impede tag detection.
In 2005, Wal-Mart conducted tag detection experiments that showed
only 90% tag detection rate at case level,
95% tag detection rate on conveyor belts,
and only 66% tag detection rate of individual items inside fully loaded pallets.
Our preliminary experimental data with commercial RFID equipment supports these results.
If objects are tagged with multiple tags, the detection rate will be higher.

5. 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

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

7. 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 β
Optimal Tag Placement: 1 4 3 2
One of the reasons multi-tags are effective in improving tag detection is improved expected tag orientation to the reader.
Let beta be the angle between the reader’s signal and the tag.
The voltage generated on-board a tag is proportional to sin(beta) for inductive coupling and to sin^2(beta) for far-field propagation.
The distance at which the reader can detect the tag is proportional to the sixth root of the voltage for inductive coupling, and to the square root of the voltage for far-field propagation.
Therefore, it is important to make the angle beta as close to 90 degrees as possible.
To maximize the grazing angle beta, it is best to position the tags perpendicular to each other for two and three tag ensembles and to position four tags ensembles parallel to the faces of a tetrahedron, a platonic solid.
For this tag positioning, the expected grazing angle beta is as shown on the graphs, assuming uniform signal distribution.
You can see the sharp double digit increase in the expected angle value when the number of tags is increased from one to two and from two to three, but only a single digit angle increase from three tags to four tags.
This suggests that the law of diminishing returns comes into effect pretty quickly.
We computed the expected angle using simulation 1 to 4 tags and analytics for one and for two tags.

8. Equipment and Setup Equipment Setup
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
We will experimentally evaluate multi-tags using equipment from different manufacturers to ensure impartiality of results.
We will use readers by Alien Technology and ThingMagic, and tags by Alien Technology and UPM Raflatac, the leading tag manufacturer is the world.
We will determine tag detection for a cart full of non-metallic and non-liquid objects, about of them.
We will repeat the experiments for metal and liquid objects.
To determine the detection probability, we will rotate a cube with tags attached to its faces in different planes, and perform similar experiments for tetrahedra.
In our experiments, we will vary distances between objects and the reader antennas, vary reader antennas geometry, and vary readers’ emitted power.
We will compare multi-tags with single-tags and multiple readers.

9. Experiments Read all tags in reader’s field Randomly shuffle objects
Compute average detection rates
Variables
reader type
antenna type
antenna power
object type
number of objects
number of tags per object
tags’ orientation
tags’ receptivity
We will experimentally evaluate multi-tags using equipment from different manufacturers to ensure impartiality of results.
We will use readers by Alien Technology and ThingMagic, and tags by Alien Technology and UPM Raflatac, the leading tag manufacturer is the world.
We will determine tag detection for a cart full of non-metallic and non-liquid objects, about of them.
We will repeat the experiments for metal and liquid objects.
To determine the detection probability, we will rotate a cube with tags attached to its faces in different planes, and perform similar experiments for tetrahedra.
In our experiments, we will vary distances between objects and the reader antennas, vary reader antennas geometry, and vary readers’ emitted power.
We will compare multi-tags with single-tags and multiple readers.

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

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

12. Linear Antennas vs. Multi-tags
Power = 31.6dBm
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Object Number
Detection Probability
2 Readers, 2 Tags 84.5%
Δ= 5.2%
Δ=19.8%
1 Reader, 2 Tags 79.3%
Δ=14.4%
Δ=21.3%
2 Readers, 1 Tag %
Δ= 6.9%
1 Reader, 1 Tag %

13. Circular Antennas vs. Multi-Tags
Power = 31.6dBm 1
0.9
0.8
0.7
Detection Probability
0.6
1 Reader, 1 Tag %
2 Readers, 1 Tag %
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%
0.5
0.4
0.3 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Object Number

14. Power Decrease in detection with decrease in power
1 Tag
2 Tags
3 Tags
4 Tags
Decrease in detection with decrease in power
More rapid decrease in detection for circular antennas

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

16. Controlling Variables
Radio noise
Tag variability
Reader variability
Reader power level
Distance to objects & type, # of antennas

17. Anti-Collision Algorithms
Redundant Tags
Connected-Tags
Binary
No Effect
Binary Variant
Randomized
Linear Increase**
No Effect*
STAC
Causes DoS
Slotted Aloha
In this table, we summarize the affect that multi-tags have on some singulation algorithms. Dual-tags will have no affect assuming that they form a single response. Randomized Binary Tree Walking and Slotted Aloha will double the singulation time assuming that each object is tagged with two tags and STAC will cause a denial of service if the algorithm is not modified a little.
* Assuming tags communicate to form a single response
** If all tags are detected

18. 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
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9 1 2 3 4
Number of Tags
Detection Probability
Decrease in solid/non-liquid object detection
Significant at low power
Similar results for linear antennas

19. Multi-Tags on Metals and Liquids
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1 Tag
2 Tags
3 Tags
Antenna #1
Antenna #2
Antenna #1 and #2
Number of Tags
Detection Probability
Power=31.6dBm, Circular Antennas
Power=31.6dBm, Linear Antennas
Power=27.6dBm, Circular Antennas
Power=27.6dBm, Linear Antennas
Low detection probabilities
Drop in detection at low power
Linear antennas outperform circular
Multi-tags better than multiple readers

20. 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%

21. Detection Delta

22. Economics of Multi-Tags
Year
Cost
Rapid decrease in passive tag cost
5 cent tag expected in 2008
1 penny tag in a few years

23. Cost Trends
Time

24. 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

25. RFID Tag Demand Demand drivers Cost effective tag design techniques
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)

26. Applications of Multi-Tags
Reliability
Availability
Localization
Safety

27. More Applications Security Theft Prevention Packaging
Tagging Bulk Materials

28. Conclusion Unreliability of object detection Many useful applications
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

29. Our Research Generalized “Yoking Proofs” 3 Multi-Tags PUF
Inter-Tag Communication
RFID

30. Thank You
Questions?