The physics behind RFID
RFID Journal University: Alan Thorne
Presentation prepared by EasyEPC
April 4, 2005
1
Radio frequency identification
Multiple RFID tags and readers
Radio frequency identification
• Basic overview of RFID
• Fundamental principles and terminology
– Electromagnetic radiation
– Radio communication
– Antennas
• How RFID works
– Basic operation
– Passive, semi-passive and active
– Tags and readers
• Alternative forms of ‘RFID’
Electromagnetic radiation
• Propagation of energy,
in the form of a wave
• Nature can be altered
– Frequency
– Amplitude
Electromagnetic radiation
• Nature of EM radiation first studied in 19th Century
– Maxwell, Hertz and others
• Power in Watts
wavelength
– NB different ways of
measuring Watts!
• Frequency in Hertz…
• … or wavelength in m
• Think in terms of EM ‘spectrum’
wavelength = speed/frequency
speed = 300,000,000 ms-1
eg freq = 918,000,000 Hz (918MHz)
Electromagnetic spectrum
• Grouped into ‘bands’
Radio waves
• Several bands have
similar properties
between them…
Band
designation
Frequency
Wavelength
LF
low
frequency
MF
medium
frequency
HF
high
frequency
VHF
very high
frequency
UHF
ultra high
frequency
SHF
super high
frequency
30–300kHz 300kHz–3MHz 3–30MHz 30–300MHz 300MHz–3GHz 3–30GHz
10–1km
1000–100m
100–10m
10m–1m
1m–0.1m
0.1–0.01m
Radio communication
• Transmitter and receiver
• Electronics plus antenna
• Transfer of:
Antenna
– Information
– Energy
• Often have
transceivers
Transmitter
Receiver
Radio communication
• Modulation
– ‘air interface’
– ‘physical layer protocol’
• Many different techniques
– AM, FM, Manchester, bi-phase…
– May use several simultaneously
Radio communication is either near-field or far-field
• Two very different mechanisms for EM comms
– Can think of as ‘electromagnetic’ vs ‘magnetic’
– Based on distance of communication wrt frequency used
• Far-field (electromagnetic)
– Long distances or high frequencies (e.g. UHF)
– More sensitive to environment (e.g. metal, water)
• Near-field (magnetic)
– Short distances and low frequencies (e.g. LF or HF)
Radio frequency identification
• Basic overview of RFID
• Fundamental principles and terminology
– Electromagnetic radiation
– Radio communication
– Antennas
• How RFID works
– Basic operation
– Passive, semi-passive and active
– Tags and readers
• Alternative forms of ‘RFID’
• Antenna is critical to operation
• Transmit and receive antennas
– Equally important
• Antenna form factor (size, shape) varies
– Near and far field are different
– In far-field, wavelength is important
wavelength/2
Antennas
wavelength = 300M ms-1/915MHz
= 300,000,000/915,000,000
= 33cm
Radiation patterns, range and reliability
• Antennas are usually directional
– Transmit or receive energy in particular direction
– Can map the ‘radiation pattern’
Radiation patterns, range and reliability
• Theoretical complexities
• Practical effects
– ‘Nulls’
– Near-field example
Radiation patterns, range and reliability
• Theoretical complexities
• Practical effects
– ‘Nulls’
– Far-field example
Far-field antenna polarisation
• Two types of far-field antenna
– Linearly polarised
– Circularly polarised
• Linear
–
–
–
–
Energy radiates in a fixed linear fashion
Gives rise to greatest ranges
Tends to generate a narrow beam
Requires alignment of both transmit and receive
antennas
Far-field antenna polarisation
• Two types of far-field antenna
– Linearly polarised
– Circularly polarised
• Circular
–
–
–
–
–
Energy radiates in a rotating fashion
Tends to generate a wider beam
Reduced range
Alignment of antennas less critical
Works better in presence of multipath and scattering
Multiple antennas
• Transceiver may have more than one antenna
– Uses a multiplexer to switch between them
– Only one can be used at a time
• Can increase coverage
Multiple antennas
• Transceiver may have more than one antenna
– Uses a multiplexer to switch between them
– Only one can be used at a time
• Can increase coverage
• Can increase reliability
– Overcome nulls
– Help with antenna alignment
Radio frequency identification
• Basic overview of RFID
• Fundamental principles and terminology
– Electromagnetic radiation
– Radio communication
– Antennas
• How RFID works
– Basic operation
– Passive, semi-passive and active
– Tags and readers
• Alternative forms of ‘RFID’
Multiple tags and multiple readers
• Need to prevent collisions
– When two simultaneous transmissions interfere
• Singulate tags using anti-collision protocol
– Probabalistic approaches
– Deterministic, reader querying (tree-walking)
– Singulated tags become silent
Multiple tags and multiple readers
• Need to prevent collisions
– When two simultaneous transmissions interfere
• Singulate tags using anti-collision protocol
– Probabalistic approaches
– Deterministic, reader querying (tree-walking)
– Singulated tags become silent
• Reader anti-collision needed too
– Listen-before-talk
– Frequency hopping (spread spectrum)
What’s in an RFID tag?
Tag = chip + antenna + substrate
• Chip
– Contains the electronic circuitry on a small piece of silicon
– May be 1mm2 down to 1/100th mm2 (size of a grain of
sand!)
– Also called silicon chip, integrated circuit (IC), nanoblock
– Contains separate parts to do
•
•
•
•
Power collection and supply
Logic to process information from reader and construct replies
Circuits to perform communication with the reader
Memory to store identity and other information
What’s in an RFID tag?
Tag = chip + antenna + substrate
• Antenna
–
–
–
–
Conductive; usually metal foil (e.g. copper)
In far-field, takes the form of a ½ wavelength dipole
In near-field, is a loop of 3-10 turns
Dominant in determining the size of the complete tag
• Susbstrate
– Plastic or paper film
– Acts as physical medium for attaching chip and antenna
What’s in an RFID reader?
Reader = case + electronics + power supply + antenna
• Electronics
– Host computer interface – serial and/or ethernet
– Microcontroller circuitry
– RF transmitter, receiver and antenna switch
• Antenna
– Internal antenna and/or external antenna ports
• Power supply
– Battery or mains transformer
Types of reader
• Multi protocol, multi frequency
– Support for different air interfaces and different operating freqs
• Wireless
– Use separate wireless communication to host computer
– Unlikely to re-use circuitry
– If mains powered then not truly wireless, but easier to install
• Handheld
– Most likely wireless, or requiring periodic synchronisation
• Label printer/applicator
– Automatically applies self-adhesive RFID tags to cases etc
– Most likely prints visible information as well (barcode and
human)
Radio frequency identification
• Basic overview of RFID
• Fundamental principles and terminology
– Electromagnetic radiation
– Radio communication
– Antennas
• How RFID works
– Basic operation
– Passive, semi-passive and active
– Tags and readers
• Alternative forms of ‘RFID’
Alternative types of ‘RFID’
• Presentation has focussed on mainstream ‘RFID’
– Will always be variations between different systems
– Many aspects simplified in order to help explain them
– Alternative approaches already exist, and more will be
developed
• Resonant, LC tags
–
–
–
–
Similar technology to EAS security tags
Limited data storage capacity
No capability for sophisticated functionality
Cheap
Alternative types of ‘RFID’
• Magnetic
– Replace chip with layers of magnetic material
– Very cheap (<1c)
– Robust to high
temperatures
– Limited data storage capacity
– No capability for sophisticated functionality
– Less range
– Limited ability to read multiple tags simultaneously
Alternative types of ‘RFID’
• Surface acoustic wave technology
– Replace complex silicon chip with SAW chip
– Much cheaper
– Tagging metals less
problematic
– Robust to high temperatures
– Typically 2.5GHz operation, i.e. smaller antennas
– Truly simultaneous identification of multiple tags
– Limited data storage capacity
– No capability for sophisticated functionality
– More prone to interference
Radio frequency identification
• Basic overview of RFID
• Fundamental principles and terminology
– Electromagnetic radiation
– Radio communication
– Antennas
• How RFID works
– Basic operation
– Passive, semi-passive and active
– Tags and readers
• Alternative forms of ‘RFID’