RFID WORLD MASTER CLASS

1

• RFID WORLD MASTER CLASS
• FUNDAMENTALS IN RADIO FREQUENCY IDENTIFICATION
• IMPLEMENTING RFID SYSTEMS

Peter H. Cole Professor of RFID Systems at the
University of Adelaide and Director of the
Auto-ID Laboratory _at_ Adelaide
2
The Auto-ID Labs
3
Good news
Heckling is encouraged
4
Outline

• RFID Physics
• RFID Protocols
• Some simple exercises

5

• PART 1
• MOTIVATION

6
Tag reading
The black spot
Normally a very weak reply is obtained
Some application illustrations will be given
shortly
7
Motivation

• The weak reply

8
Electromagnetic fields

• Coupling is via electromagnetic fields
• There is little margin for poor performance
• We must understand their properties

9

• PART 2
• ELECTROMAGNETIC FIELDS

10
Objectives

• Outline fundamental electromagnetic theory
• Outline concepts fruitful in RFID label
  development
• Analyse coupling w RFID labels and interrogators
• Useful across all frequency ranges LF to UHF
• Both large and small antennas
• Near field and far field
• Electric fields, magnetic fields, and
  electromagnetic fields
• Encourage particular ways of thinking
• Assemble all underlying relevant equations

11
The Field Vectors

• A full theory of electrodynamics, including
  the effects of dielectric and magnetic materials,
  must be based on the four field vectors
• Electric field vector E
• Magnetic field vector H
• Electric flux density vector D
• Magnetic flux density vector B

12
Material state vectors
13
Laws in differential forms
Vortex
Source
14
The complete laws
Faraday's law The circulation of the electric
field vector E around a closed contour is equal
to minus the time rate of change of magnetic flux
through a surface bounded by that contour, the
positive direction of the surface being related
to the positive direction of the contour by the
right hand rule. Ampere's law as modified by
Maxwell The circulation of the magnetic field
vector H around a closed contour is equal to the
sum of the conduction current and the
displacement current passing through a surface
bounded by that contour, with again the right
hand rule relating the senses of the contour and
the surface.
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Complete laws (continued)
Gauss' law for the electric flux The total
electric flux (defined in terms of the D vector)
emerging from a closed surface is equal to the
total conduction charge contained within the
volume bounded by that surface. Gauss' Law for
the magnetic flux The total magnetic flux
(defined in terms of the B vector) emerging from
any closed surface is zero.
16
Electromagnetic propagation
Electric current creates a vortex of magnetic
field
Magnetic field creates a vortex of electric field
Electric field creates a vortex of magnetic field
Propagation
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Electromagnetic waves

• They propagate with the velocity of light
• (Light is an electromagnetic wave)
• Velocity c is 300,000,000 m/s
• Wavelength - frequency relation is
• c fl
• Simple examples
• 10 MHz, 30 m 1000 MHz 300 mm
• But not all electromagnetic fields are
  propagating waves some are just local energy
  storage fields

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Boundary Condition electric field
19
Boundary Condition magnetic field
20
The basic laws how they work

• Gausss law
• Electric flux deposits charge
• Electric field cannot just go past a conductor,
  it must turn and meet it at right angles
• Faradays law
• Oscillating magnetic flux induces voltage in a
  loop that it links

21
Near and far field distributions
Electric field launched by an electric dipole
There is also a magnetic field not shown
22
Fields of a Magnetic Dipole(oh dear)
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The radian sphere

• At br 1, i.e. r l/2p, we have the surface
  of a sphere at critical distance at which
• The phase factor e-jbr is one radian
• Inside this sphere the near field predominates
• Outside this sphere the far field predominates

24
Near and Far Fields

• The far field is an energy propagating field
• Appropriate measure of strength is 0.5 h H2
  (power flowing per unit area)
• The near field is an energy storage field
• Appropriate measure of strength is reactive power
  per unit volume 0.5 w m0H2
• Near field - far field boundary is l/2p
• Examples 100 kHz 500m 10 MHz 5m 1000 MHz 50mm

25

• PART 3
• RFID SYSTEMS

26
Issues in RFID Design

• Active or passive
• Operating frequency
• Electric or magnetic fields
• Material or microelectronic
• Focus on passive systems
• Active for the future?

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The usual way backscatter

• The most popular technology
• Tag contains a microcircuit and an antenna
• Tag is powered by the interrogation beam
• Frequency of that beam is chosen for good
  propagation
• Tag contains an internal oscillator
• Frequency of that oscillator is chosen for low
  power consumption
• Reply is offset from the interrogation frequency
  by a small amount

28
Microelectronic Backscatter

• Concept can be applied from 10 MHz to 10,000 MHz
• Low propagation loss points to coupling using the
  far field
• Low power consumption requires a low frequency
  microcircuit
• Reply is by modulation of the interrogation
  frequency

29
Relevant Issues

• Range is determined largely by the ability to
  obtain sufficient rectified voltage for the label
  rectifier system
• High quality factor resonance becomes important
• Reply is at sidebands of the interrogation
  frequency
• Adaptive isolation has appeared in the patent
  literature but not practiced

30
Interesting features

• Near and far fields
• Energy storage in the near field
• Energy propagation in the far field
• Radian sphere (rl/2p) is the boundary
• Directivity in the far field of 1.5
• No far field radiation in the polar direction
• Plenty of near field on the polar axis

31
Field creation structures

• Near magnetic field
• Near electric field
• Far electromagnetic field

32
Measures of exciting field
In the far field
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The traditional loop
34
Patch antenna
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Label antennas

• Magnetic field free space
• Magnetic field against metal
• Electric field free space
• Electric field against metal
• Electromagnetic field
• Very small antennas respond to either the
  electric field or the magnetic field
• Somewhat larger antennas respond to both

36
Planar printed coil
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Ferrite cored solenoid
38
Electric field bow tie
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Electric field box structure
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Electromagnetic field antenna

• Dimensions are no longer a small fraction of a
  wave length
• Operating principles are less clear

41
Far field coupling theory
42
Effective area of a far field receiving antenna
43
Near field coupling theory

• Focus on energy storage fields
• Full electromagnetic theory not needed
• Resonance will enhance power transfer
• Versions for electric or magnetic fields
  available
• Figure of merit for an interrogator will be an
  energy density per unit volume
• Figure of merit for a label antenna will be a
  volume

44
Magnetic field coupling
Simple result for weakly coupled coils
45
Coupling volume theory for magnetic fields
46
Some coupling volumes
For a planar coil
For a long air cored solenoid
For a long ferrite cored solenoid Vc is
increased by
47
Coupling volume theory for electric fields
48
Some coupling volumes
For a pair of air cored parallel electrodes Vc
volume enclosed
A dielectric if present reduces the coupling
volume by er
For a bow tie, there is no physical volume but
there is a Vc depending on the self capacitance
and the electric flux collecting area
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Field configurations for bow tie antenna
Self capacitance
Current injection
50
Some interesting results

• Self capacitance can come from electrostatic
  field theory
• Electric flux collecting area could come from
  electrostatic modelling, from direct measurement,
  or as below
• However, Reciprocity still reigns, and electric
  flux collecting area can be predicted from
  radiation resistance
• Radiation resistance can be obtained from
  radiating antenna theory
• It was measured long ago by Brown and Woodward
• We have employed some of their results
• We have performed all varieties of experimental
  confirmation

51
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 not
  applicable.

52
Significant conclusions

• Coupling volumes for well shaped planar electric
  and magnetic field labels are size dependent and
  similar
• Radiation quality factors for both types of label
  formed within a square of side L are size
  dependent and similar
• These are calculated results for sensibly shaped
  antennas

53
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.
54

• PART 4
• RFID PROTOCOLS

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What is a protocol?

• Signalling waveforms
• Command set
• Operating procedure
• A back end interface
• whereby the identities of a population of tags
  in the field of a reader may be determined, and
  the population otherwise managed.

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Auto-ID Center protocols

• The Auto-ID Center defined
• The Class 1 UHF protocol
• The Class 1 HF protocol
• The Class 0 UHF protocol
• EPCglobal is defining
• Generation 2 UHF protocol

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Why are they different?

• Different field properties at HF and UHF
• Near and far field different field confinement
• Different field penetration in materials
• Different silicon circuit possibilities and costs
• Different electromagnetic regulations
• Read only memory technologies enable
  miniaturisation
• A high performance UHF system was available and
  was modified by the Center to manage privacy
  concerns

58
Constraints on protocols

• Electromagnetic compatibility regulations
• Differ with frequency range and jurisdiction
• Some convergence occurring
• Reader to reader interference
• Readers confusing tags
• Readers blocking other reader receivers
• Simplicity (as reflected in chip size)
• Maybe that influences reliability as well

59
Protocols the major divide

• Slotted adaptive round
• A version of terminating aloha
• Tags give effectively full replies in random time
  slots
• Tree walking
• A systematic exploration of the tag population
  one or more bits at a time
• Differences are degree of randomness and mode of
  description
• In practice a gamble is involved

60
Characteristics contrasts

• Tree walking
• More forward link signalling
• Prolonged periods of interrupted signalling
• Partial information of tag population remains
  relevant
• Adaptive round (terminating aloha)
• Less forward link signalling
• Long periods of un-modulated reader carrier
• Reader signalling is less
• No information from one response about others

61
Characteristics similarities

• Both can select subsets of tags for participation
• Overt selection may reveal what is selected
• Forms of less overt selection are possible
• Tag sleeping has a role in both

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The HF protocol
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Concept of the adaptive round

• Labels reply once per round, in randomly chosen
  slots
• A group of n slots forms a round
• The number of slots in a round varies as needed
• Tags giving already collected replies moved to
  slot F

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State diagram
120
UNPOWERED
In Field
Destroy
Write, Begin Round
,
DESTROYED
READY

and not matching mask
Begin Round

and matching mask
Begin Round

Begin Round
and not matching mask
and matching mask
SLOTTED
After response
READ
Before response
,
Close Slot, Fix Slot
Close Slot,Fix Slot

without matching CRC16
After response
Fix Slot
with matching CRC16
Close Slot, Fix Slot,
FIXED SLOT
Begin Round
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Framing and data symbols
T 512/fc 37.76ms
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Summary significant aspects

• Operation in near field eavesdropping difficult
• Operable word wide under harmonised regulations
• Product selection from EPC header
• Economical secure residual reply signalling
• Performance near 200 tag/s

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The UHF protocols
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Some tree concepts
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More tree scanning concepts
70
Further general tree concepts
Descent strings from root to tags are shown in
heavy lines
71
The Class 1 UHF protocol
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Textual description

• Based upon atomic transactions almost no
  memory used in tag
• Two important commands ping, scroll
• Ping selects a portion of the tree, and asks any
  tags matching that partial selection to respond
• When a single tag seems to be responding, its
  full reply is sought by a Scroll command
• That tag is put to sleep to confirm it was the
  sole respondent
• Sleep is persistent to ensure protocol immunity
  against field fading

73
Descent string definition
A portion of a descent string is defined by
pointer, length and data values supplied in a
reader command
74
Viewing and viewed levels
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Ping bins and scroll waveform
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Whats in a ping bin?

• One or more superimposed eight bit tag responses
• Responses come from all tags descended from the
  viewed node corresponding to the ping bin
• There may be no, one or more than one tag
  responding
• The responses are eight bits long
• Interference between multiple responses is
  generally visible if none gamble on a scroll

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Simulated and actual ping responses
Class 1 UHF protocol simulation output
Signal from actual Class 1 UHF tag responding
to ping command
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Summary significant aspects

• Deep forward link modulation assists immunity to
  reader collisions
• Selection through CRC make the reader
  communication effectively meaningless to
  eavesdropping
• Eight bit ping bin responses provide a look down
  the tree and assist the detection of probably
  singulated tags
• Eight bit ping bin responses per bin tick are an
  appropriate use of turn-around time

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The Class 0 UHF protocol
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Signalling organisation

• RTF methodology
• Reset before tag activity
• Oscillator synchronisation and data command
  training after reset
• Fast tree descents on three symbols (zero, one
  null)
• Three memory pages ID0, ID1 and ID2 for descent
• ID2 contains EPC
• ID1 contains factory programmed random descent
  string
• ID0 contains locally generated random string

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Start of tree traversal

• Data 0 given by reader in tree start state
• Responses from tag MSB
• Data 0 or 1 given by reader causes descent L or
  R
• Tags which have responded with matching 0 or 1
  stay in, and respond according to their MSB-1
  other tags go temporarily inactive
• Data 0 or 1 again given by reader causes descent
  L or R

82
Zero, one and null signals
83
Tag to reader link signals
Standard is bit 0 2.2 MHz, bit 1 3.3 MHz,
chosen as approximate mid points of carrier
positions. In region I, these frequencies are
divided by 2.
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Summary significant aspects

• Compact factory programmed read only memory
• Single level descents of the tree fast
  turnaround
• High reply sub-carrier frequencies make this
  possible (but are also a limitation)
• The projection of interrogator signalling
  spectrum on the receiver pass band is small
• Very fast singultation, around 1000 tags/s USA
• Very flexible in signalling trainable for
  different jurisdictions

85

• PART 5
• CONCLUSIONS

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What to take away 1

• Electric and magnetic field concepts
• Source and vortex concepts
• Frequency wave length relation c fl
• Near and far field concepts
• Radian sphere size and significance
• The weakness of the label reply

87
What to take away 2

• Boundary conditions near metal
• Operating principles of simple antennas
• Common antenna designs
• Reciprocity concepts
• Varieties of protocol

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• The end

89

• There is always something beyond the end

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Further issues

• Electromagnetic ducting
• Waveguides beyond cut off
• Field shapes therein
• Electromagnetic absorption
• We are mostly water
• Other materials
• The coming protocols
• See accompanying paper

91
Exercises

• Calculate the following
• The free space electromagnetic wavelength at 1
  GHz.
• The propagation constant for electromagnetic
  waves at 1 GHz.
• The size of the radian sphere at 1 GHz.
• The radiated power density Sr in Wm-2 for an
  interrogator antenna of gain p transmitting a
  power of 1 W at 1 GHz at a distance of 2m.
• The effective area of a label antenna of gain p/2
  at 1 GHz.
• The available source power from that antenna
  placed in the field of the first.

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• The real end