Work Sponsored By…
„ Georgia Tech Foundation
High-Frequency RFID:
Applications and Challenges
„ Center for Organic Photonics and Electronics
„ Georgia Research Alliance
„ NSF CAREER Grant ECS-0546955
Prof. Gregory D. Durgin
www.propagation.gatech.edu
Copyright 2006 Georgia Institute of Technology, All rights reserved
For Further Reading…
„ H. Stockman, “Communication by Means of Reflected Power,” in
Proceedings of the IRE, October 1948, p. 11961204.
„ K. Finkenzeller, RFID Handbook: Fundamentals and Applications in
Contactless Smart Cards and Identification, 2nd ed., Wiley Inc., 2003
„ http://www.rfidjournal.com
„ J.D. Griffin, G.D. Durgin, A. Haldi, and B. Kippelen, “Radio Link Budgets for
915 MHz RFID Antennas Placed on Various Objects,” in WCNG Wireless
Symposium ’05, Austin TX, October 2005.
„ D. Kim, M. A. Ingram, and W.W. Smith Jr., “Measurements of Small-Scale
Fading and Path Loss for Long Range RF Tags,” IEEE Transactions on
Antennas and Propagation, vol. 51, no. 8, pp. 17401749, Aug 2003.
„ J.D. Griffin, “A Radio Assay for the Study of Radio Frequency Tag Antenna
Presentation Overview
„ Background
„ Overview of RFID
„ Challenges for RFID Radio Links
„ Challenges for RFID Tags
„ Challenges for RFID Readers
„ Cross-Disciplinary Challenges
„ The Future of RFID
Performance,” Tech. Report PG-TR-050504-JDG, Georgia Tech
Propagation Group MS Thesis, http://www.propagation.gatech.edu, 2005.
„ D.M. Dobkin and S.M. Weigand, “UHF RFID and Tag Antenna Scattering,
Part I: Experimental Results” and “Part II: Tag Array Scattering Theory”,
Microwave Journal of Theory and Techniques, 2006
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
What This Talk Does NOT Address
„ Detailed Signal-Processing Issues
„ Software and Backhaul Issues
„ Database and Inventory Software
„ Privacy and Security Issues
„ Integrated Circuit Design
Copyright 2006 Georgia Institute of Technology, All rights reserved
Background
Copyright 2006 Georgia Institute of Technology, All rights reserved
1
Prof. Durgin Background
The Propagation Group at GT
„ Prof. Durgin has been teaching
for 3 years at Georgia Tech
„ Virginia Tech 1992 – 2000
„ PhD from MPRG in Dec. 2000
„ Post Doc. 2001-2002 at Osaka
University
„ Director of The Propagation
Group at Georgia Tech
„ Authored first Space-
Time/MIMO textbook
„ Frequent consultant to industry
http://www.propagation.gatech.edu
Copyright 2006 Georgia Institute of Technology, All rights reserved
The Propagation Group at GT
Copyright 2006 Georgia Institute of Technology, All rights reserved
Faculty Collaborators
„ Radio Wave Propagation
„ RFID
Emag/
RF
„ Radiolocation (E911)
„ Propagation Modeling
„ Direction Finding and Array Technology
Steffes
Peterson
Kenney
Kippelen
Perry
Marder
„ Space-Time/MIMO Radio Channels
„ Applied Electromagnetics
Chem/
COPE
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
Specific Contributors to this Talk
Joshua Griffin
Joel Prothro
Radio Assay, MIMO,
Multi-Antenna
Interrogation
Organic Dielectrics
for RF, On-Metal
Antenna Performance
(PhD)
(MS)
Albert Lu
Anil Rohatgi
Antennas, RFID
Receiver design,
Applied Emag
Spread Spectrum
and Anti-Collision
What is Far-Field
RFID?
(MS)
(Eng./LumoFlex)
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
2
Historical Perspective
RF Terminology in This Presentation
„ 1935 – Watson-Watt invents RADAR
„ 1948 – Stockman publishes seminal work on backscatter
„ Reader (Interrogator) – a device that retrieves
„
„
„
„
„
radio in Proceedings of the IRE
1973 – Watson’s keylesss doorway entry patent
1980s – First deployments of electronic article
surveillance (EAS)
1993 – First highway toll tags deployed in NY City (EZ
Pass)
1999 – AutoID center established at MIT by Proctor and
Gamble
2003 – Walmart mandate to suppliers for providing RFID
tags on inventory palettes
information from an RF tag
„ Beacon – an RF device that periodically
transmits information without reception
„ Transponder – an RF device that retransmits
without any waveform reconstruction (no
modulation or demodulation capability)
„ Transceiver – an RF device capable of
modulation and demodulation
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
What is an RF Tag?
RF Tag Power Supply Differentiators
„ RFID is just one application of RF Tags
„ Purely-Passive Tags
„ Our Definition of an RF Tag:
„
A low-profile, low-power transponder
„ Actually, there are 3 types of RF Tags
„
„ Battery Assisted Tags
„
Near-field transponder
„ Far-field transponder
„ Full Transceiver
„
Powered solely from received RF signals
Requires rectification of incoming waves
„
Battery is used instead of in-coming RF power
Finite lifetime
„ Energy Scavenging Tags
Super capacitors
Solar cells
„ Thermocouples
„ Piezoelectrics
„
„
Copyright 2006 Georgia Institute of Technology, All rights reserved
Near vs. Far-field RFID Trade-offs
Far-field RFID
Near-field RFID
„ Long range (1-10m)
„ Range less than 1m
„ High-frequency
„ Conventional circuitry
electronics
effective
„ Malevolent, high-loss RF
channel
„ Resistance to on-object
degradations
„ High bandwidth available
„ Small bandwidth
„ Compact, multiple
„ Only one loop antenna
antennas on a tag
possible
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
Example Applications
„ Passive Data Exchange
„ Sensors
Measure glucose levels in the human body
Pace maker monitor
„ Environmental monitoring
„
„
„ Radio Frequency Identification (RFID)
Toll collection
„ Airport luggage security
Access control
„ Medical ID bracelets
„ Livestock tracking
„ Self checkout
„ Inventory management
„
„
Copyright 2006 Georgia Institute of Technology, All rights reserved
3
The RF Tag System (Near-Field)
Inductive Coupling between reader & RF tag
Near-Field RFID Attributes
„ Low Frequency
„ Short Range (always less than 1m)
„ Direct Influence of RFID Tag on Reader
Physically limited in range and orientation.
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Hertzian Dipole (Simplest Radiator)
„ Tiny current element
Copyright 2006 Georgia Institute of Technology, All rights reserved
Radiation Equations
Field solution for a radiating current I∆z
„ Radiates at the Origin
„ Points up (z-direction)
„ Simplest possible
radiation scenario
„ All radiation theory is
based on this simple
result
„ Near-fields fall off 1/r2 and 1/r3
„ Far-fields fall off 1/r (1/r2 in power)
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
The RF Tag System (Far-Field)
Far-Field RFID Attributes
„ Transmitter antenna
„ Higher Frequency
radiates
„ Tag antenna scatters
wave with modulation
„ Receiver antenna
captures information
„ In some architectures,
one antenna for RX & TX
„ Potentially Longer Range (more than 1m)
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„ RFID Tag Effects Sensed via Traveling Wave
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4
Classes of RFID Emitters
The RF Tag System
RF Tag Components
EPC Class
Characteristic
Programmability
Class 0
"Read Only" passive tags
Programmed as part of the semiconductor
manufacturing process
Class 0+
"Write-Once, Read-Many" passive
tags
Extension of standards allows 0+ to be
rewriteable
Class 1
"Write-Once, Read-Many" passive
Programmed once by the customer then
Class 2
Rewritable passive tags
Can be reprogrammed many times
Class 3
Semi-passive tags
Can be reprogrammed many times
Class 4
Active tags
Can be reprogrammed many times
Class 5
Readers
N/A
„ Antennas: Far field Backscatter
„ Printed Inverted F
„ Printed Dipole
Antenna (PIFA)
„ Printed Folded Dipole
„
Patch Antenna
Table 1. RFID tag classifications. (ImpInj)
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
The RF Tag System
PIN Diode Modulation
„ Modulation Circuitry
„ Forward DC Bias => RF Short
„
Load Modulation -- the carrier signal is
modulated by switching an impedance from a
matched condition to an un-matched condition
to alter the reflection coefficient
„ Reverse DC Bias => RF Open
Forward Bias
(RF short)
Reverse Bias
(RF open)
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PIN Diode Modulation
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Measurement of Backscatter Modulation
„ Example PIN diode modulator built by
Propagation Group Researcher Joshua Griffin
„ Connects to antenna units and modulation
waveforms
Sample Output
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Copyright 2006 Georgia Institute of Technology, All rights reserved
5
Challenges for RFID Links
„ Power-Up Link Budget
„ Backscatter Link Budget
„ Radiation Safety Levels
Challenges for RFID Links
„ Polarization Mismatch
„ Small-Scale Fading
„ Multi-Band Operation
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
Conventional RF Link Budget
Highlights of Forward Link Budget
⎛ 4π ⎞
PR = PT + GTX _ reader + GTag − 20 log d − 20 log⎜
⎟ − Object Penalty
⎝ λ ⎠
„ Power loss is to the square of the distance (d2)
„ Losses increase with higher frequency
Reader
Transmit
Antenna Gain
Free-Space
Tag Antenna
Gain
Tag-Reader
Separation
Distance
Measured
In the
Radio Assay
„ Directional antennas are easier to make at
higher frequency
„ Limits power-up on a passive tag
Copyright 2006 Georgia Institute of Technology, All rights reserved
RF Tag Backscatter Link Budget
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RF Tag Backscatter Link Budget
Backscattered
Power
Antenna Gains
At Reader
Transmit
Power
Tag-Reader
Separation Distance
⎛ 4π ⎞
PR = GTX _ reader + GRX _ reader + PT − 40 log d − 40 log⎜
⎟
⎝ λ ⎠
+ 2GTag + 20 log10 ΓA − ΓB − 2 Object Penalty
Tag Antenna
Gain
Copyright 2006 Georgia Institute of Technology, All rights reserved
Reflection Change
Between
Switched Loads
ΓA, B =
Z A, B − Z*0
Z A, B + Z 0
Adjustment
For on-Object
Degradations
Copyright 2006 Georgia Institute of Technology, All rights reserved
6
Highlights of Backscatter Link Budget
Radiation Safety Levels
„ Power loss is to the fourth power of the distance
„ High Transmit Power Required
(d4) in line-of-sight channel
„ Additional losses due to material attachments
„ Directional antennas at the reader make a big
difference in the link budget
„ Must consider RF tag loading effects
„ Limits the actual information exchange between
tag and reader
„
„
Forward Link Power-Up
Weak Modulated Backscatter (1/r4 losses)
„ Limiting Factor is often Radiation Safety
Readers and People share space
Repeated exposure in the work place
„ Microwave safety levels must be strictly observed
„
„
Copyright 2006 Georgia Institute of Technology, All rights reserved
Polarization Issues
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Analogy to a Famous Physics Problem
„ Two polarizers allow light through if they are aligned
„ Example has vertical polarization
„ If one polarizer is set at 90 degrees, there is complete
on all three antennas
„ Optimum for backscatter link
budget
„ Analogy between the reader antennas in the previous
Copyright 2006 Georgia Institute of Technology, All rights reserved
Analogy to a Famous Physics Problem
„ Insertion of a 45-degree polarizer allows some light to
pass through the entire setup.
„ Final polarization has been rotated by 90 degrees.
„ Analogous to the slanted RFID tag setup in previous slide
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blockage
slide – near-zero power transfer.
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Polarization Issues
„ Complete polarization mismatch
on the backscatter link
„ However, excellent isolation
between reader antennas
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7
First Encounter With Wave Fading
Constructive and
destructive
interference
causes voltage
to vary along
different points
in space on the
transmission
line.
Source
Pow er L evel
Polarization Issues
“Mobile” Power Meter Signal
Position
Voltage
Forward
Propagating
Wave
„ 25% Total Power Loss
„ Extremely Good Carrier-
to-Interference Ratio
Copyright 2006 Georgia Institute of Technology, All rights reserved
Power Level
Load
Copyright 2006 Georgia Institute of Technology, All rights reserved
Small-Scale Fading in Radio Links
Propagating
Waves
Backward
Propagating
Wave
Small-Scale Fading in Radio Links
Received Signal
Position
Mobile Receiver
Antenna
The principle is the same as constructive/destructive
interference on the transmission line.
Copyright 2006 Georgia Institute of Technology, All rights reserved
Double Fading in Backscatter Links
Example measurement at 5.85 GHz made
by moving receiver antenna over 1m area.
Copyright 2006 Georgia Institute of Technology, All rights reserved
Tag Antenna Diversity
„ Use multiple antennas on the same tag
„ Switch antennas when one experiences a fade
„ A deep fade prohibits power-up of a tag
„ A fade can be experienced twice on a backscatter link
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
8
Multi-Band Issues
„ 865 MHz, 915 MHz, 955 MHz used in different countries
„ Individual signal may be narrowband, but range of
operability is broadband
Challenges for RFID Tags
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Challenges for RFID Tags
Copyright 2006 Georgia Institute of Technology, All rights reserved
Powering Up a Passive Tag
„ Powering Up a Passive Tag
„ Battery Issues
„ Antenna Design Issues
„ Multi-Tag Coupling
„ On-Metal Degradations
„ On-Dielectric Degradations
„ Tag Diversity
„ Manufacturing Issues
Copyright 2006 Georgia Institute of Technology, All rights reserved
Powering Up a Passive Tag
„ Passive RF chip must convert incoming RF to DC voltage
„ Basic rectifier shown above
„ Half-wave rectifier circuit shown above
„ Ideal Voltage Conversion: VDC = 0.5 x (VAC)pp − VTO
„ VTO is turn-on Voltage for Diode
Copyright 2006 Georgia Institute of Technology, All rights reserved
Powering Up a Passive Tag
„ Voltage Doubler circuit capable of producing twice the
DC ouput with a few more capacitors and diodes
„ Ideal Voltage Conversion: VDC = (VAC)pp − 2VTO
Copyright 2006 Georgia Institute of Technology, All rights reserved
„ Ideal Voltage Conversion: VDC = 2 (VAC)pp − 4VTO
Copyright 2006 Georgia Institute of Technology, All rights reserved
9
Powering Up a Passive Tag
Full RF Tag Hardware Schematic
„ Charge pump rectifier
„ Diodes and capacitors
„ AC voltage converted to DC
„ Voltage stepped up (current
stepped down)
A:
B:
C:
D:
„ Ideal Voltage Conversion:
VDC = N(VAC)pp − 2NVTO
„ Drawbacks
„ Higher complexity
„ Diminishing returns for added
stages
„ Longer charging transient
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Note potential self-modulation problem
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Battery Issues
Flexible Batteries?
„ Cost
„ Fledgling technology
„ Lifetime
„ Difficult to manufacture
„ Disposability
„ Form-factor
Antenna
Modulator
Charge Pump
Signaling
Electronics
batteries > 1.5V
„ Zinc-Carbon structure:
non-rechargeable
„ Supply issues
„ Flexibility
„ Cell reliability is poor
Battery specifications are heavily
influenced by propagation issues
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Antenna Design Issues
„ Bending/exposure limits
Copyright 2006 Georgia Institute of Technology, All rights reserved
Multi-Tag Coupling
„ Wire designs
„ Area antennas
„ Patch antennas
„ Unorthodox designs
„ Each tag has a radar cross-section (RCS)
„ RCS ~ electromagnetic area
„ Overlapping antennas steal each other’s power
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Copyright 2006 Georgia Institute of Technology, All rights reserved
10
Multi-Tag Coupling
On-Object Degradations
„ Coplanar strip has most
fringing fields
„ Realistic scenarios many involve many RFID tags
„ Environment will affect
„ Higher-gain antennas have larger electromagnetic area
operation
„ Trade-off: an efficient individual RFID tag is a neighbor
to other RFID tags
Copyright 2006 Georgia Institute of Technology, All rights reserved
Antenna Design and Manufacture
Copyright 2006 Georgia Institute of Technology, All rights reserved
Key Fabrication Issue: Skin Depth
„ Conductor Materials
Stamped Metal
Electroless Cu, Ag
„ Silver Inks
„ Exotics
„
„
„ Substrates
Papers
PET plastic
„ Liquid Crystal Polymer
(LCP)
„
„
Copyright 2006 Georgia Institute of Technology, All rights reserved
Key Fabrication Issue: Skin Depth
„ Goals:
„ Low Temperature Process
„ Minimal Metal Deposition
„ …but Stay Thicker than the Skin Depth
Copyright 2006 Georgia Institute of Technology, All rights reserved
RF Tag Antennas
„ Planar folded dipole for 915 MHz
„ Flexible PET* Substrate
„ Silver – Electroless Silver
„ Copper – Electroless Copper
„ Rigid FR4 Substrate
„ Baseline – 1 oz. milled copper on FR4 substrate
*Polyethylene
Copyright 2006 Georgia Institute of Technology, All rights reserved
terephthalate
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11
On-Dielectric Degradations
RF Tag Antenna Material Attachment
„ RFID tag pattern changes when placed on
dielectric media
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Radio Assay
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Tag Pattern on a Wooden Slab
„ Picture shows equipment at Georgia Tech for
measurement
„ RF Tag dipole placed on 1-inch plywood
J.D. Griffin, “A Radio Assay for the Study of Radio Frequency Tag Antenna Performance,” Master’s thesis,
Georgia Institute of Technology, 2005.
Copyright 2006 Georgia Institute of Technology, All rights reserved
Balun-Transformer
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Tag Pattern on a Wooden Slab
Back Side
Front Side
„ Balun + Transformer allows an RF tag antenna,
connected with co-planar strip feeds, to be read by 50Ohm coaxial cable
„ Allows emulation of a chip on arbitrary antenna designs
Copyright 2006 Georgia Institute of Technology, All rights reserved
Question: On which side of the antenna is the
dielectric wooden slab?
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12
On-Dielectric Degradations
On-Dielectric Degradations
„ Dielectric material actually draws radiated power into
„ Simple equivalent circuit
medium
„ Usually this is opposite the desired direction of operation
„ Penalty is assessed twice on the backscatter link budget
for explaining dielectric
degradation
„ Lower Impedance
medium draws power
„ Virtually all non-magnetic
materials have higher
medium impedance than
free space
µ0
ε rε 0
η=
Copyright 2006 Georgia Institute of Technology, All rights reserved
On-Metal Degradations
Copyright 2006 Georgia Institute of Technology, All rights reserved
RF Isolation with Engineered Polymers
„ Ideal signaling occurs
when switch between
open and short circuit
„ Formula depends on
radiation resistance of
antenna, Z0
ΓA, B =
Z A, B − Z 0
Z A, B + Z 0
„ Maximum difference
between reflections:
Γopen = +1, Γshort = -1
Incident Wave
Incident Wave
Antenna
Antenna
Dielectric
Air
Ground Plane
Ground Plane
„ Find practical, low cost techniques for making thin,
flexible substrates for RF tags.
„ Substrates have low velocity of propagation that retard
phase progression and isolate printed antennas.
Copyright 2006 Georgia Institute of Technology, All rights reserved
On-Metal Degradation
Ohms
Copyright 2006 Georgia Institute of Technology, All rights reserved
On-Metal Degradations
„ Formula depends on
radiation resistance of
antenna, Z0
Γshort =
Z short − Z 0
>> −1
Z short + Z 0
„ Difference between states
diminished:
Γopen = +1, Γshort ~ +0.9
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on metal impedance drop
(Z0 drops below Zshort)
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13
Tag Pattern for Foil-Coated Wood
Tunable Impedance Matching Network
Pi Network
Back Side
Front Side
Loss of average gain (about 10 dB) and significant
shielding on metal side of the pattern.
Copyright 2006 Georgia Institute of Technology, All rights reserved
Measured E-Plane Antenna Patterns
J. H. Sinksy and C.R. Westgate, “Design of an Electronically Tunable Microwave Impedance Transformer,” in
Proc. of the 1997 IEEE MTT-S Int. Microwave Symposium. Part 2 (of 3), ser. IEEE MTT-S Int. Microwave
Symposium Digest, vol. 2 Denver, CO USA : IEEE, pp. 647-650, 1997.
Copyright 2006 Georgia Institute of Technology, All rights reserved
Measured E-Plane Antenna Patterns
G.S. Smith, “Directive Properties of Antennas for Transmission into a Material Half-Space,” IEEE
Transactions on Antennas and Propagation, vol. 32, pp. 232-246, 1984.
Copyright 2006 Georgia Institute of Technology, All rights reserved
Gain Penalty
Copyright 2006 Georgia Institute of Technology, All rights reserved
Gain Penalty Results
„ AGP – Average gain penalty due to each material
* These values were interpolated to 915 MHz from data of similar
materials
given by :
A.R.V. Hippel, Dielectric Materials and Applications. New York : The Technology
Press of M.I.T. and John Wiley and Sons, Inc., 1954.
** Undiluted antifreeze
*** At approximately room temperature
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
14
Gain Penalty Example
„ Backscatter Communication „ Power Radio Link
Radio Link Budget
Budget
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Challenges for RFID Readers
Copyright 2006 Georgia Institute of Technology, All rights reserved
Challenges for RFID Readers
Directional Antennas
„ Antenna Selection
„ When used to transmit, they focus energy in a
„ Self-Interference
„ Interrogation through Portals
„ Mechanical Spinning
particular direction
„ When used to receive, they reject radiation from
sources outside their major lobe
„ Single-Antenna Readers
Copyright 2006 Georgia Institute of Technology, All rights reserved
Directional Antennas
Copyright 2006 Georgia Institute of Technology, All rights reserved
Industry Portal Design
„ Tagged object moves through portal via conveyor belt
„ Time-varying, space-varying radio channel
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Copyright 2006 Georgia Institute of Technology, All rights reserved
15
Spinning Through Portals
Single-Antenna Readers
„ Object can be spun through the portal to increase
„ Using one antenna for transmit and receive
„ Inexpensive and Compact, but…
„ Need a good match on the antenna
„ Requires excellent, high-isolation coupler
„ Dynamic range issues
successful read rate
„ Spinning provides both space and polarization diversity
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
Single Antenna Architecture
„ Circulator/coupler allows dual-use antenna
„ Design is extremely sensitive to mismatched antenna
„ Coupler must provide extremely high isolation between
ports
The Future of RFID
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
The Future of RFID
Energy Scavenging and Power Storage
„ Energy Scavenging
„ Mechanical Vibrations
„ Low-Cost Fabrication Procedures
„ Thermocouples (body heat?)
„ Organic Electronics
„ Solar Power
„ Localization Capability
„ Supercapacitors
„ Array Interrogation
„ Thin, flexible batteries
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
16
Low-Cost Fabrication
Organic Electronics
„ Silver Inks
Pros
„ Low-temperature
processing
„ Inexpensive fabrication
„ Non-toxic
„ Electroless Metal
Depositions
„ Exotic Materials
„ Organic Substrates
Cons
„ High turn-on voltage
„ Barrier technology issues
„ Low frequency cut-off
„ Temporal degradation
„ Screen Printing
„ Any good ideas?
Exotic antenna: see-through
Indium-Tin Oxide (ITO) dipole
Copyright 2006 Georgia Institute of Technology, All rights reserved
Localization and Array Interrogation
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Spatio-Temporal Backscatter Signaling
„ Arrays for reading RFID tags (transmitter and receiver)
„ Localization with arrays and direction finding
„ Limitless signal-processing possibilities
Copyright 2006 Georgia Institute of Technology, All rights reserved
Copyright 2006 Georgia Institute of Technology, All rights reserved
Key Questions
„ How can spread-spectrum be used in an anti-
collision scheme?
„ How can this scheme lower complexity and
Spread Spectrum for
RFID and Anti-Collision
Copyright 2006 Georgia Institute of Technology, All rights reserved
power consumption?
„ How many simultaneous reads are ultimately
possible?
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17
What is AntiAnti-Collision?
Spread Spectrum TheoryTheory-Encoding
„ Multiple tag backscatter simultaneously
„ Each tag generates a unique
„ How to separate one tag’s information
high-frequency pseudo-random
chipping sequence
„ The sequence is multiplied by
low-frequency data and then
transmitted
„ Signal a(t) represents the data
and c(t) represents the chipping
sequence.
„ The transmitted waveform is
thus
x(t) = a(t) * c(t)
from the sea of backscatter?
Copyright 2006 Georgia Institute of Technology, All rights reserved
Spread Spectrum TheoryTheory-Decoding
Copyright 2006 Georgia Institute of Technology, All rights reserved
M-Sequence Generation
„ The received signal is a
„
„
„
„
superposition of multiple tag
backscatter
y(t)=x1(t) + x2(t) Å high
frequency
We wish to recover x1(t) Å low
frequency
If both chipping sequences
were of the range –1 : 1, then:
y(t)*c1(t) = x1(t) + x2(t) * c1(t)
(low freq) + (high freq)
Low pass filtering the result will
leave behind only x1(t): the
desired data.
Copyright 2006 Georgia Institute of Technology, All rights reserved
„ Easy to generate and decode
„ N shift registers result in 2N-1 length codes
„ Low complexity, low power consumption
„ No need for reception and decoding
Copyright 2006 Georgia Institute of Technology, All rights reserved
DifferentialDifferential-Offset Sequences
Tag Design
„ Each chip is equipped with a set of two PN generators
„ Supports Anti-Collision for up to 255 tags
tapped to create maximal length sequences of 255 bits.
„ The two sequences are XORed together with a set
phase shift between them. This phase shift determines
the ID of the tag
„ ID is set by delaying the clock to the second PN generator
Copyright 2006 Georgia Institute of Technology, All rights reserved
„ Tag IDs are reset able on the fly
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18
Receiver Software
Sequence Convergence
„ The chipping sequences are easily simulated for a
Measured RF Tag
Data
a.) x1(t) = y(t) * c(t)
desired tag using Matlab
„ The received signal is then captured, sampled, and
processed in Labview.
LPF
Received Signal (Single Tag)
Frequency Spectrum
b.) Low frequency component
Copyright 2006 Georgia Institute of Technology, All rights reserved
c.) Low freq Data…(RSS in this figure)
Copyright 2006 Georgia Institute of Technology, All rights reserved
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