4 august 2004 peter h. cole rfid world master class

4 August 2004Peter H. Cole RFID World Master Class: Fundamentals in

RFID 1

AUTO-ID LABS

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 @ Adelaide

4 August 2004 Peter H. Cole RFID World Master Class: Fundamentals in RFID 2

AUTO-ID LABS The Auto-ID Labs


AUTO-ID LABS

Heckling is encouraged

Good news


AUTO-ID LABS Outline

RFID Physics

RFID Protocols

Some simple exercises


RFID 5

AUTO-ID LABS

PART 1

MOTIVATION


AUTO-ID LABSTag reading

L a b e l

T r a n s m i t t e r

R e c e i v e rCon

trol

ler

Some application illustrations will be given shortly

Normally a very weak reply is obtained

The black spot


AUTO-ID LABS Motivation

The weak reply


AUTO-ID LABS Electromagnetic fields

Coupling is via electromagnetic fields

There is little margin for poor performance

We must understand their properties


RFID 9

AUTO-ID LABS

PART 2

ELECTROMAGNETIC FIELDS


AUTO-ID LABS Objectives

Outline fundamental electromagnetic theoryOutline concepts fruitful in RFID label developmentAnalyse 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


AUTO-ID LABS 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


AUTO-ID LABS Material state vectors

PED += 0ε

)(0 MHB += µ


AUTO-ID LABS Laws in differential forms

0

-

=⋅∇=⋅∇

∂∂+=×∇

∂∂=×∇

B

D

DJH

BE

ρt

t

Source

Vortex


AUTO-ID LABS The complete lawsFaraday's lawThe 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 MaxwellThe 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.


AUTO-ID LABS 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.


AUTO-ID LABS

Electromagnetic propagation

Magnetic field creates a vortex of electric field

Electric field creates a vortex of magnetic field

Propagation

Electric current creates a vortex of magnetic field


AUTO-ID LABSElectromagnetic waves

They propagate with the velocity of light (Light is an electromagnetic wave)

Velocity c is 300,000,000 m/sWavelength - frequency relation is

c = fλSimple examples: 10 MHz, 30 m; 1000 MHz 300 mm

But not all electromagnetic fields are propagating waves; some are just local energy storage fields


AUTO-ID LABS Boundary Condition: electric field

C h a r g e

C o n d u c t i n g s u r f a c e

E l e c t r i c f i e l d


AUTO-ID LABSBoundary Condition: magnetic field

C o n d u c t i n g p l a n e

o rd i s p l a c e m e n t c u r r e n t

M a g n e t i c f i e l d


AUTO-ID LABSThe basic laws: how they work

Gauss’s lawElectric flux deposits chargeElectric field cannot just go past a conductor, it must turn and meet it at right angles

Faraday’s lawOscillating magnetic flux induces voltage in a loop that it links

V+_

B


AUTO-ID LABSNear and far field distributions

Electric field launched by an electric dipole

There is also a magnetic field not shown


AUTO-ID LABS Fields of a Magnetic Dipole(oh dear)

θββπ

β β cos)(

2

)(

2

4 32

3rj

r er

j

r

MjH −

−=

θβββπ

β βθ sin

)()(

1

)(4 32

3rje

r

j

rr

jMjH −

−+=

θββπ

β βφ sin

)(

1

)(4 2

3rje

rr

jMjE −

+=


AUTO-ID LABSThe radian sphere

At βr = 1, i.e. r = λ/2π, we have the surface of a sphere at critical distance at which

The phase factor e-jβr is one radian

Inside this sphere the near field predominates

Outside this sphere the far field predominates


AUTO-ID LABSNear and Far Fields

The far field is an energy propagating field

Appropriate measure of strength is 0.5 η 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 ω µ0H2

Near field - far field boundary is λ/2π

Examples 100 kHz 500m; 10 MHz 5m; 1000 MHz 50mm


RFID 25

AUTO-ID LABS

PART 3

RFID SYSTEMS


AUTO-ID LABSIssues in RFID Design

Active or passive

Operating frequency

Electric or magnetic fields

Material or microelectronic

Focus on passive systems

Active for the future?


AUTO-ID LABSThe usual way: backscatter

The most popular technologyTag contains a microcircuit and an antennaTag is powered by the interrogation beamFrequency of that beam is chosen for good propagationTag contains an internal oscillatorFrequency of that oscillator is chosen for low power consumptionReply is offset from the interrogation frequency by a small amount


AUTO-ID LABSMicroelectronic Backscatter

Concept can be applied from 10 MHz to 10,000 MHzLow propagation loss points to coupling using the far fieldLow power consumption requires a low frequency microcircuitReply is by modulation of the interrogation frequency

C o n t r o l

c i r c u i tm i c r o

D . c . s u p p l y l i n e

R e s o n a n t c i r c u i t S w i t c h a b l e l o a d

J u n c t i o n c a p a c i t a n c e

L a b e la n t e n n a


AUTO-ID LABSRelevant Issues

Range is determined largely by the ability to obtain sufficient rectified voltage for the label rectifier systemHigh quality factor resonance becomes importantReply is at sidebands of the interrogation frequencyAdaptive isolation has appeared in the patent literature but not practiced


AUTO-ID LABSInteresting features

Near and far fieldsEnergy storage in the near fieldEnergy propagation in the far field

Radian sphere (r=λ/2π) is the boundaryDirectivity in the far field of 1.5No far field radiation in the polar directionPlenty of near field on the polar axis


AUTO-ID LABSField creation structures

Near magnetic field

Near electric field

Far electromagnetic field


AUTO-ID LABSMeasures of exciting field

rv SW β=

In the far field


AUTO-ID LABSThe traditional loop


AUTO-ID LABS Patch antenna


AUTO-ID LABSLabel antennas

Magnetic field – free spaceMagnetic field against metalElectric field – free spaceElectric field against metalElectromagnetic field

Very small antennas respond to either the electric field or the magnetic fieldSomewhat larger antennas respond to both


AUTO-ID LABSPlanar printed coil


AUTO-ID LABSFerrite cored solenoid

)1(1 −+=

ir

irer N µ

µµ


AUTO-ID LABS Electric field bow tie


AUTO-ID LABS Electric field box structure


AUTO-ID LABSElectromagnetic field antenna

Dimensions are no longer a small fraction of a wave length

Operating principles are less clear


AUTO-ID LABSFar field coupling theory

rerr Sg

ASPπλ

4

2

==

2t

4areaunit per flow Power

r

Pg t

π=

areaunit per flow Power ×= err AP

πλ

4

2r

er

gA =

2

4

=

rgg

P

Ptr

t

r

πλ


AUTO-ID LABS

Effective area of a far field receiving antenna

λ / 2

E f f e c t i v e a r e a


AUTO-ID LABSNear field coupling theory

Focus on energy storage fieldsFull electromagnetic theory not neededResonance will enhance power transferVersions for electric or magnetic fields availableFigure of merit for an interrogator will be an energy density per unit volumeFigure of merit for a label antenna will be a volume


AUTO-ID LABS Magnetic field coupling

21

212

1

2

L where

L

MkQQk

P

P ==

Simple result for weakly coupled coils


AUTO-ID LABS

Coupling volume theory for magnetic fields

=

position label at theor interrogat by the

createdpower reactive ofdensity Volume

circuitedshort isit when coil label

untuned in the flowingpower Reactive

cV

=



coilcreation fieldor interrogat theof

inductor in the flowingpower Reactive

dV

211

2 QQV

V

P

P

d

c=


AUTO-ID LABS Some coupling volumes

L

AVc

20µ=

)1(1 −+=

ir

irer N µ

µµ

For a planar coil

volumeits=cV

For a long ferrite cored solenoid Vc is increased by

For a long air cored solenoid


AUTO-ID LABS

Coupling volume theory for electric fields

=

position label at theor interrogat the

by createdpower reactive ofdensity Volume

circuitedopen isit when capacitor

label untuned in the flowingpower Reactive

cV

=



electrodescreation fieldor interrogat the

of ecapacitanc in the flowingpower Reactive

dV

211

2 QQV

V

P

P

d

c=


AUTO-ID LABS Some coupling volumes

For a bow tie, there is no physical volume but there is a Vc depending on the self capacitance and theelectric flux collecting area

For a pair of air cored parallel electrodes Vc= volume enclosed

A dielectric if present reducesthe coupling volume by εr


AUTO-ID LABS

Field configurations for bow tie antenna

Self capacitance Current injection


AUTO-ID LABSSome interesting results

Self capacitance can come from electrostatic field theoryElectric flux collecting area could come from electrostatic modelling, from direct measurement, or as belowHowever, Reciprocity still reigns, and electric flux collecting area can be predicted from radiation resistanceRadiation resistance can be obtained from radiating antenna theoryIt was measured long ago by Brown and Woodward We have employed some of their resultsWe have performed all varieties of experimental confirmation


AUTO-ID LABS

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.


AUTO-ID LABS

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

( ) 3

40 Magnetic

LβQr =

( ) 3

13 Electric

LβQr =

3

2 Electric

3LVc =

2 Magnetic

3LVc =


AUTO-ID LABSOptimum 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.


RFID 54

AUTO-ID LABS

PART 4

RFID PROTOCOLS


AUTO-ID LABS 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.


AUTO-ID LABS Auto-ID Center protocols

The Auto-ID Center definedThe Class 1 UHF protocolThe Class 1 HF protocolThe Class 0 UHF protocol

EPCglobal is definingGeneration 2 UHF protocol


AUTO-ID LABSWhy are they different?

Different field properties at HF and UHFNear and far field – different field confinementDifferent field penetration in materialsDifferent silicon circuit possibilities and costsDifferent electromagnetic regulations

Read only memory technologies enable miniaturisationA high performance UHF system was available and was modified by the Center to manage privacy concerns


AUTO-ID LABSConstraints on protocols

Electromagnetic compatibility regulationsDiffer with frequency range and jurisdictionSome convergence occurring

Reader to reader interferenceReaders confusing tagsReaders blocking other reader receivers

Simplicity (as reflected in chip size) Maybe that influences reliability as well


AUTO-ID LABS Protocols: the major divide

Slotted adaptive roundA version of terminating alohaTags give effectively full replies in random time slots

Tree walkingA systematic exploration of the tag population one or more bits at a time

Differences are degree of randomness and mode of descriptionIn practice a gamble is involved


AUTO-ID LABS Characteristics: contrastsTree walkingMore forward link signalling Prolonged periods of interrupted signalling Partial information of tag population remains relevant

Adaptive round (terminating aloha)Less forward link signallingLong periods of un-modulated reader carrierReader signalling is less No information from one response about others


AUTO-ID LABS 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


AUTO-ID LABS

The HF protocol


AUTO-ID LABS Concept of the adaptive roundLabels reply once per round, in randomly chosen slotsA group of n slots forms a roundThe number of slots in a round varies as neededTags giving already collected replies moved to slot F

B e g i n n i n g o f r o u n d E n d o f r o u n d

S l o t F S l o t 0 S l o t 1 S l o t 2 S l o t 3 S l o t 4 S l o t n - 1


AUTO-ID LABS

120

DESTROYED

UNPOWERED

READY

SLOTTED READ

FIXED SLOT

Destroy

In Field

Before response: Close Slot,Fix Slot

Begin Round and matching mask

Close Slot, Fix Slot, Begin Round

After response:Fix Slot with matching CRC16

Begin Round and matching mask

Write, Begin Round, and not matching mask

Begin Round and not matching mask

After response:, Close Slot, Fix Slot

without matching CRC16

State diagram


AUTO-ID LABS Framing and data symbols

Short start of frame (SSOF)

Long start of frame (LSOF)

Common end of frame (CEOF)

Binary zero

Binary one

Close slot sequence

T = 512/fc = 37.76µs


AUTO-ID LABS Summary: significant aspects

Operation in near field – eavesdropping difficultOperable word wide under harmonised regulationsProduct selection from EPC headerEconomical secure residual reply signallingPerformance near 200 tag/s


AUTO-ID LABS

The UHF protocols


AUTO-ID LABSSome tree concepts

Root (le vel 0, not in tree)

Level 1 MSBLSBLevel 2Level 3

Le vel n

Product 01001

R o o t ( l e v e l 0 , n o t i n t r e e )

L e v e l 1 M S B

L S B

L e v e l 2

L e v e l 3

L e v e l n

P r o d u c t 0 1 0 0 1


AUTO-ID LABSMore tree scanning concepts

R o o t

V i e w i n g p o i n t

D e s c e n t d i s t a n c e

V i e w e d n o d e s


AUTO-ID LABSFurther general tree concepts

R o o t ( n o l e v e l )

L e v e l 0 M S B

L e v e l 1

L e v e l 2

L e v e l n - 1

P r o d u c t 0 1 0 0 1 A t a g c o d e i s d e f i n e d b y i t s d e s c e n t s t r i n g

0

0

0

1

1

Descent strings from root to tags are shown in heavy lines


AUTO-ID LABS

The Class 1 UHF protocol


AUTO-ID LABSTextual description

Based upon “atomic” transactions: almost no memory used in tagTwo important commands: ping, scrollPing 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 commandThat tag is put to sleep to confirm it was the sole respondent Sleep is persistent to ensure protocol immunity against field fading


AUTO-ID LABSDescent string definition

0 1 2 3 4 5 6 7 8 - - - - - - - - - -

P o i n t e r

V i e w i n g l e v e l

L e n g t h V i e w e d l e v e l

R e g i o n o f c o d e m a t c h i n g

M S B o f C R C L S B o f E P C

C R C f o l l o w e d b y E P C

A portion of a descent string is defined by pointer, length and data values supplied in a reader command


AUTO-ID LABSViewing and viewed levels

Root (no level)

The tags

Viewing point (pointer plus length)

Viewed nodes corresponding to the viewing point

More levels of the tree

Singly occupied nodes

Unoccupied nodes

Multipley occupied nodes

Tags descending from the viewed nodes respond

A node is defined by its descent string


AUTO-ID LABSPing bins and scroll waveform

Bin 4(100)

EOF fromCMD

...Setup

Bin 0(000)

Bin 1(001)

Bin 2(010)

Bin 3(011)

Bin 5(101)

Bin 6(110)

Bin 7(111)

TtagreplyNom

// //


AUTO-ID LABS What’s in a ping bin?One or more superimposed eight bit tag responsesResponses come from all tags descended from the viewed node corresponding to the ping binThere may be no, one or more than one tag respondingThe responses are eight bits longInterference between multiple responses is generally visible: if none gamble on a scroll


AUTO-ID LABS

Simulated and actual ping responses

(Zoom of 6th/7th Bins)

-2.00E-01

-1.00E-01

0.00E+00

1.00E-01

2.00E-01

3.00E-01

1.88E-03 1.90E-03 1.92E-03 1.94E-03 1.96E-03

Time (sec)

Sig

nal (

arb)

Class 1 UHF protocolsimulation output

Signal from actual Class 1 UHF tag responding to ping command



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


AUTO-ID LABS

The Class 0 UHF protocol


AUTO-ID LABSSignalling organisation

RTF methodologyReset before tag activityOscillator synchronisation and data command training after resetFast tree descents on three symbols (zero, one null)Three memory pages ID0, ID1 and ID2 for descentID2 contains EPCID1 contains factory programmed random descent stringID0 contains locally generated random string


AUTO-ID LABS

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 inactiveData 0 or 1 again given by reader: causes descent L or R

R o o t ( l e v e l 0 , n o t i n t r e e )

L e v e l 1 M S B

L S B

L e v e l 2

L e v e l 3

L e v e l n

P r o d u c t 0 1 0 0 1


AUTO-ID LABSZero, one and null signals

Reader bit ’0’ (fast bit rate)

Clock start

Total bit exchange time - 12.5 µs, typ.

3µs

response end from tags

1/2 bit cycle for reference

Reader bit ’1’ (fast bit rate)

Clock start

Total bit exchange time 12.5 µs, typ.


6us


Reader ’null’ bit (fast bit rate)

Clock start

Total bit exchange time 12.5 µs, fast mode

( No tag response )

9.5µs



AUTO-ID LABSTag to reader link signals

Tag Backscatter on Reader Data Symbol

Clock start

Tag response

Total bit exchange time

Data Symbol


response start f rom tags

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.



Compact factory programmed read only memorySingle 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 USAVery flexible in signalling: trainable for different jurisdictions


RFID 85

AUTO-ID LABS

PART 5

CONCLUSIONS


AUTO-ID LABS What to take away: 1

Electric and magnetic field concepts

Source and vortex concepts

Frequency wave length relation c = fλ

Near and far field concepts

Radian sphere: size and significance

The weakness of the label reply


AUTO-ID LABSWhat to take away: 2

Boundary conditions near metal

Operating principles of simple antennas

Common antenna designs

Reciprocity concepts

Varieties of protocol


RFID 88

AUTO-ID LABS

The end


RFID 89

AUTO-ID LABS

There is always something beyond the end


AUTO-ID LABS 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


AUTO-ID LABSExercises

Calculate the followingThe 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 π transmitting a power of 1 W at 1 GHz at a distance of 2m.The effective area of a label antenna of gain π/2 at 1 GHz.The available source power from that antenna placed in the field of the first.


RFID 92

AUTO-ID LABS

The real end

4 august 2004 peter h. cole rfid world master class

Technology

field vectors

closed surface

magnetic materials

closed contour

surface bounded

small antennasnear field

b vector

electromagnetic fieldsthere