Evolution of RFID
Systems
Peter H Cole
Adelaide Auto-ID Laboratory
23 January 2006
Evolution of RFID systems
1
1
Objectives
• Title is to capture notions of
• How RFID systems have evolved under constraints
• How they might evolve under research
• If desired, some details of research at Adelaide
23 January 2006
Evolution of RFID systems
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Topics
• RFID regulations
• Antenna issues
• Propagation studies
• Protocol issues
• Higher functionality tags
• Signalling waveform design
• Security and authentication
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Evolution of RFID systems
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RFID regulations
• Regulators
– ITU Regions
– ]FCC, EN, Other national bodies
• Usage of radio spectrum and bands
• Regulatory standards
– ISO ETSI
• Australia experimental licence
• Listen before talk regulations
• Synchronisation
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Evolution of RFID systems
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Antenna issues
•
•
•
•
•
•
•
Electromagnetic theory
Coupling calculations
Bode Fano Limit
Antennas for dual frequencies
Antenna size and frequency constraints
Antennas in or near metal
A small antenna example.
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Evolution of RFID systems
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Propagation studies
•
•
•
•
•
Electromagnetic theory
Near and Far fields
Near and far field coupling theories
Some fundamental constraints
Dense RFID reader studies.
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Evolution of RFID systems
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Protocol issues
• Concept of a protocol
• Tag talks first and reader talks first protocols
• Constraints on protocols
– UHF signalling
– HF signalling
• Adaptive round concepts
• EPCglobal C1G2 protocol
• Approaches to advanced HF protocols
– Can HF sustain heavy signalling?
– Ways it might
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Evolution of RFID systems
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Higher functionality tags
• Autonomously networking tags
• Merging of EAS and data tags
• Trigger circuits for battery tags
– Low power approach
• Issues and experiments
– Zero power approach
• Application to theft detection
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Evolution of RFID systems
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Security and Authentication
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•
•
•
Methods for providing security
Levels of security
Burdens on chip design
Burdens on signalling systems
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Evolution of RFID systems
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Overview of Adelaide research
•
•
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•
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Personnel
Classification of projects
Detector research
Data logging reader research
Dense reader environment research
Privacy and security research
Trigger circuit research
Publications
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Evolution of RFID systems
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Antenna Issues
Peter H Cole
Adelaide Auto-ID Laboratory
23 January 2006
Evolution of RFID systems
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11
Outline
• Electromagnetic theory
• How antennas work (approximately)
• Near and far fields
• Near and far field coupling theories
• Significant conclusions about performance
• Bode-Fano limit for efficiency
• Some simple tag designs
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Evolution of RFID systems
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Laws in differential form
∂B
∇× E = ∂t
∂D
∇× H = J +
∂t
∇⋅D = ρ
∇⋅B = 0
Vortex
Source
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Evolution of RFID systems
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Boundary Condition: electric field
Electric field
Charge
Conducting surface
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Boundary Condition: magnetic field
Magnetic field
or
displacement
current
Conducting plane
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The basic laws: how they work
• Gauss’s law
–Electric flux deposits charge
–Electric field cannot just go past a
conductor, it must turn and meet it at
right angles
• Faraday’s law
–Oscillating magnetic flux induces
voltage in a loop that it links
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Evolution of RFID systems
B
+
V
_
16
Fields of a Magnetic Dipole
(oh dear)
jβ M
Hr =
4π
⎛ 2
2 j ⎞ − jβ r
⎜⎜
⎟e
−
cos θ
2
3 ⎟
( β r) ⎠
⎝ ( β r)
jβ 3 M
Hθ =
4π
⎛ j
1
j ⎞ − jβ r
⎜⎜
⎟e
+
−
sin θ
2
3 ⎟
( β r) ⎠
⎝ ( β r) ( β r)
jβ 3 M
Eφ =
4π
⎛ j
1
⎜⎜
+
2
β
r
(
)
(
β
r
)
⎝
3
23 January 2006
Evolution of RFID systems
⎞ − jβ r
⎟⎟e
sin θ
⎠
17
Near and far field coupling
theories
• Common feature: a label driving field is created, how much
signal can be extracted?
• In the near field of the interrogator, the driving field is
mostly energy storage, and the amount radiated does not
affect the coupling, but does affect the EMC regulator.
• Various techniques to create energy storage without
radiating are then applicable.
• Some theorems on optimum antenna size are of interest.
• In the far field of the interrogator, the relation between what
is coupled to and what is regulated is more direct, and
such techniques are not applicable.
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Far field coupling theory
Pr = Aer × Power flow per unit area
Power flow per unit area =
g t Pt
4π r 2
g r λ2
Aer =
4π
gλ2
Pr = S r Ae =
Sr
4π
⎛ λ ⎞
Pr
⎟⎟
= g r gt ⎜⎜
Pt
⎝ 4π r ⎠
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Evolution of RFID systems
2
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Near field coupling theory
Vc =
Vd =
[Reactive power flowing in the untuned label coil when it is short circuited ]
[Volume density of reactive power created by the interrogator at the label position]
[Reactive power flowing in the inductor of the interrogator field creation coil ]
[Volume density of reactive power created by the interrogator at the label position]
P2 Vc
= Q1Q2
P1 Vd
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Measures of exciting field
In the far field
Wv = β S r
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Significant conclusions
• Coupling volumes for well shaped planar electric and
magnetic field labels are size dependent and similar
3
3
2L
Electric Vc =
3
L
Magnetic Vc =
2
• Radiation quality factors for both types of label formed
within a square of side L are size dependent and similar
40
Magnetic Qr =
3
(β L)
13
Electric Qr =
3
(β L)
• These are calculated results for sensibly shaped
antennas
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Evolution of RFID systems
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Optimum operating frequency
The optimum frequency for operation of an
RFID system in the far field is the lowest
frequency for which a reasonable match to
the radiation resistance of the label antenna
can be achieved, at the allowed size of label,
without the label or matching element losses
intruding.
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Bode-Fano Limit
RS
VS
LOSSLESS
MATCHING
NETWORK
ZIN
∫
∞
0
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C
R
1
π
ln
dω ≤
Γ
RC
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Reflection coefficient, |Γ|
Bode-Fano Limit (cont)
1
|Γ| inband
ω1
ω2
∆ω
Angular frequency, ω
Γ inband ≥ e
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−
1
2 ∆fRC
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Bode-Fano Limit (cont)
• Allocated bandwidths for RFID:
• Others:
China: 917 – 922 MHz (Temporary license can be
applied)
Australia: 918 – 926 MHz
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Bode-Fano Limit (cont)
1
Reflection coefficient, |Γ|
Reflection coefficient, |Γ|
• 3 different cases considered:
|Γ| max
1
|Γ| max
865 868
928
902
∆f1
∆f
Frequency, f (in MHz)
Reflection coefficient, |Γ|
(a)
902
928
∆f2
Frequency, f (in MHz)
950 956
∆f3
(b)
1
|Γ| max
865
956
∆f
Frequency, f (in MHz)
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Evolution (c)
of RFID systems
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Bode-Fano Limit (cont)
• Assume R = 1 kΩ, C = 1 pF
• R = 10 kΩ, C = 1 pF (for less power
consuming tag chip in practice)
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A Simple RFID Tag
• Consists of a circular loop antenna with a very simple
matching network
FRONT
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REAR
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Higher Functionality
Tags
Adelaide Auto-ID Laboratory
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Interesting questions
• Merging of EAS and Data tags
• Turning on battery operated tags
– Low power consumption
– Zero power consumption
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Transmitter operated systems
• Some small voltage is generated form transmitted power
• A low power consumption circuit detects that event
• Quality factor issues arise
Rr
Label
antenna
jXs
Rl
D.C. output
line
Junction
capacitance
Ra
jXl
jX B
Bypass and
reservoir
capacitance
Resonant circuit
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Experiments on detuning
1MΩ
10kΩ
RF IN through an
SMA connector
23 January 2006
11.5pF
Rectified voltage to
Oscilloscope using
a BNC connector
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Low and high power sweeps
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A low voltage turn on circuit
• Sensitivity about 5 mV
• Power consumption few nA
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Zero power turn on concept
• Low frequency magnetic field vibrates a magnet
• Piezoelectric converter generates about a volt
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Electroacoustic conversion modelling
• Variables are
– Torque and angular displacement,
– charge and voltage
• Electroacoustic parameters for substances known
• Parameters for structures are calculated therefrom
q
q
+
f
_
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1
Electroacoustic
energy conversion
system
Evolution of RFID systems
2
+
t
_
37
Eletroacoustic conversion
• Structural parameters appear below
⎡ q ⎤ ⎡C11P
=
⎢θ ⎥ ⎢C
⎣ ⎦ ⎣ 21P
23 January 2006
C12 P ⎤ ⎡φ ⎤
⎥
⎢
⎥
C22 P ⎦ ⎣τ ⎦
Evolution of RFID systems
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Structure analysed
Magnet
Piezoelectric material
F
w
P
F
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h
tp
Evolution of RFID systems
tm
39
Result
• Turn-on voltage depends on
– Driving magnetic field
– Electroacoustic parameters
– Some resonance quality factors
VTO =
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2
k eff
2
2
Q m ( Mvµ 0 )
H
Evolution of RFID systems
2
2
C 22 S
1
C J C 22eff
40