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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).