The physics of RFID

Matt ReynoldsFounding PartnerThingMagic LLC

Vision General

• Una breve Historia de RFID• Elementos de un sistema de RFID• Un modelo ideal de Tag and practical

constraints• Un modelo ideal de Lector and practical

constraints• La base de una radio frecuencia

propagación• La base de RF interación con materiales• Conclusiones

Una breve historia de RFID

1862 1886 1942 1948 1972 2003

printinglasers

IC / VLSInetworkingsupply chain scaling

What is an RFID Reader?

Cuatro principales elementos: Tags, Lectores, Antenas, y Network Systems

(eg Savant)

Elementos de un sistema RFID

Variables de un sistema RF

1. Elección de la frecuencia operativa

2. Tag IC, diseño antena tag

3. Lector, diseño de la antena del Lector

4. Proximate materials

5. Fuentes de externas interferencias

Mayor RFID mercados by frequency

US, Canada125KHz13.56MHz902-928MHz

EU Countries125KHz13.56MHz868-870MHz

Japan125KHz13.56MHz950-956MHz

RFID tags en diferentes frecuencias

125 KHz

TI

Philips

Others

13.56 MHz

Tagsys

Philips

TI

Microchip

Others

915 MHz

Intermec

SCS

Matrics

Alien

Philips

TI

2.4 GHz

Intermec

SCS

Hitachi

Anatomia Tag

Substrate

Tag IC

Antenna

Die attach

Diagrama de bloquesTag

Antenna

Power Supply

Tx Modulator

Rx Demodulator

Control Logic(Finite State machine)

MemoryCells

Tag Integrated Circuit (IC)

¿ Que un lector hace?

• Primarias funciones:

Remota alimentación tags

Establecer una bidireccional data link

Inventario tags, filtrar resultados

Comunicación con networked server(s)

Anatomia Lector

915MHzRadio

NetworkProcessor

Digital SignalProcessor(DSP)

13.56MHzRadio

PowerSupply

Diagrama Bloques de un Lector

antennaSubsystem

Band 1

Band ModuleBand 1

dspsubsystem

network processor

rx

tx

data

control

TCP/IP

data

control

antennaSubsystem

Band 2

Band Module Band 2

rx

tx control

data

antennaSubsystem

Band n

Band ModuleBand n

rx

tx control

data

915MHz band module schematic

UHF (915MHz) reader RF section

A passive RFID communication model

Tags

Reader

Power from RF field

ReaderAntenna

Reader->Tag Commands

Tag->Reader Responses

RFID Communication Channel

Limiting factors for passive RFID

1. Lector potencia trasmitida Pr (Gov’t. limited)

2. Reader receiver sensitivity Sr 3. Reader antenna gain Gr (Gov’t. limited)4. Tag antenna gain Gt (Size limited)5. Potencia requerida en un tag Pt (Silicon

process limited)6. Tag modulator efficiency Et

Lectores ->potencia suministrada Tag

Lector

LectorAntena

Tag

Q: Si un lector transmite Pr watios, Cuanta potencia Pt el tag recive a una distancia de separación d?

A: Depende -UHF (915MHz) : Far field propagation : Pt 1/d2

HF (13.56MHz) : Inductive coupling : Pt 1/d6

Separacióndistancia d

Parametros tipicos sistema UHF

• Reader Transmit Power Pr = 30dBm (1 Watt)• Reader Receiver Sensitivity Sr = -80dBm (10 -11 Watts)• Reader Antenna Gain Gr = 6dBi

• Tag Power Requirement Pt = -10dBm (100 microwatts)• Tag Antenna Gain Gt = 1dBi• Tag Backscatter Efficiency Et = -20dB

• System operating wavelength = 33cm (915MHz)

Far field path loss

d

Pr

Pt

Pt = Pr • Gr • Gt • 2

4 π d2

• Two cases: Tag power limited, or reader sensitivity limited.

Well designed systems are tag power limited.

Pt = Pr • Gr • Gt • 2

4 π d2

dmax = sqrt ( Pr • Gr • Gt • 2 )

4 π Pt

dmax = 19.4 meters, theoretical maximum

UHF read range estimation

Reader sensitivity limit

• Let’s assume we can build a tag IC requiring 1 microwatt (100 times better than current practice)

• dmax = 194 meters tag power limit for this hypothetical IC.

Pt->r = Pr • Gr • Gt • Et • 2

4 π d4

Pt->r = 2.65x10 -13 Watts (-95.6dBm)

Noise power in 50 ohm resistor at 500KHz BW=4kTB=-109dBm.

With a practical receiver of NF=3dB, Pn=-106dBm, SNR=10dB.This signal is at the edge of decodability.

Lessons from the simple model

• Since Pt 1/d2 , doubling read range requires 4X the transmitter power.

• Larger antennas can help, but at the expense of larger physical size because G{t,r} Area.

• More advanced CMOS process technology will help by reducing Pt.

• At large distances, reader sensitivity limitations dominate.

RF señales y materiales

Materiales en campo RF puede tener varios efectos:

1. Reflexión / refración

2. Absorción (perdidas)

3. Efectos dielectricos (detuning)

4. Complejos efectos de propagación (photonic bandgap)

Efectos RF en comunes materiales

Material Efectos de una señal RF

Carton Absorption (moisture)

Detuning (dielectric)

Conductive liquids (shampoo)

Absorption

Plasticos Detuning (dielectric)

Metales Reflection

Groups of cans Complex effects (lenses, filters)

Reflection

Cuerpo Humano / animales Absorption

Detuning (dielectric)

Reflection

Effective shielding of UHF signals• Any conductive material exhibits a skin depth

effect

= sqrt ( 2 / ( 2 f 0 ) )

where 0 = 4 x10 -7 H/m.

For aluminum, = 2.65x10 -6 ohm-cm. An effective aluminum shield is only 27 microns thick.

For dilute salt water, = 10 -2 ohm-cm. An effective salt water shield is 1 mm thick.

Conclusions

• There are serious practical limitations to passive RFID read range.

• It is not practical to read a passive UHF RFID tag from Earth orbit.

• Improvements to tag IC design will yield commercially helpful, but probably privacy-insignificant increase in read range.

• UHF RFID signals are easily shielded by

common materials (aluminum foil, antistatic bags, or your hands).