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The physics of RFID
Matt ReynoldsFounding PartnerThingMagic LLC
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Overview
• A brief history of RFID• Elements of an RFID system• An ideal tag model and practical
constraints• An ideal reader model and practical
constraints• The basics of radio frequency propagation• The basics of RF interaction with materials• Conclusions
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A brief history of RFID
1862 1886 1942 1948 1972 2003
printinglasers
IC / VLSInetworkingsupply chain scaling
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What is an RFID Reader?
Four main elements: Tags, Readers, Antennas, and Network Systems
(eg Savant)
Elements of an RFID system
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RF system variables
1. Choice of operating frequency
2. Tag IC, tag antenna design
3. Reader, reader antenna design
4. Proximate materials
5. Sources of external interference
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Major RFID markets by frequency
US, Canada125KHz13.56MHz902-928MHz
EU Countries125KHz13.56MHz868-870MHz
Japan125KHz13.56MHz950-956MHz
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RFID tags at different frequencies
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
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Tag anatomy
Substrate
Tag IC
Antenna
Die attach
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Tag block diagram
Antenna
Power Supply
Tx Modulator
Rx Demodulator
Control Logic(Finite State machine)
MemoryCells
Tag Integrated Circuit (IC)
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What does a reader do?
• Primary functions:
Remotely power tags
Establish a bidirectional data link
Inventory tags, filter results
Communicate with networked server(s)
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Reader anatomy
915MHzRadio
NetworkProcessor
Digital SignalProcessor(DSP)
13.56MHzRadio
PowerSupply
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Reader block diagram
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
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915MHz band module schematic
UHF (915MHz) reader RF section
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A passive RFID communication model
Tags
Reader
Power from RF field
ReaderAntenna
Reader->Tag Commands
Tag->Reader Responses
RFID Communication Channel
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Limiting factors for passive RFID
1. Reader transmitter power 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. Power required at tag Pt (Silicon process
limited)6. Tag modulator efficiency Et
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Reader->Tag power transfer
Reader
ReaderAntenna
Tag
Q: If a reader transmits Pr watts, how much power Pt does the tag receive at a separation distance d?
A: It depends-UHF (915MHz) : Far field propagation : Pt 1/d2
HF (13.56MHz) : Inductive coupling : Pt 1/d6
Separationdistance d
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Typical UHF system parameters
• 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)
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Far field path loss
d
Pr
Pt
Pt = Pr • Gr • Gt • 2
(4 π)2 d2
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• Two cases: Tag power limited, or reader sensitivity limited.
Well designed systems are tag power limited.
Pt = Pr • Gr • Gt • 2
(4 π)2 d2
dmax = sqrt ( Pr • Gr • Gt • 2 )
(4 π)2 Pt
dmax = 5.8 meters, theoretical maximum
UHF read range estimation
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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)2 d4
Pt->r = -99dBm
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.
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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.
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RF signals and materials
Materials in the RF field can have several effects:
1. Reflection / refraction
2. Absorption (loss)
3. Dielectric effects (detuning)
4. Complex propagation effects (photonic bandgap)
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RF effects of common materialsMaterial Effect(s) on RF signal
Cardboard Absorption (moisture)
Detuning (dielectric)
Conductive liquids (shampoo)
Absorption
Plastics Detuning (dielectric)
Metals Reflection
Groups of cans Complex effects (lenses, filters)
Reflection
Human body / animals Absorption
Detuning (dielectric)
Reflection
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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.
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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).