12.30 RFID · PDF fileThe physics behind RFID RFID Journal University: ... –...

April 4, 2005

Presentation prepared by EasyEPC

1

The physics behind RFID RFID Journal University: Alan Thorne

Radio frequency identification

Multiple RFID tags and readers


• 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’

• Propagation of energy, in the form of a wave

• Nature can be altered– Frequency– Amplitude

Electromagnetic radiation

wavelength

wavelength = speed/frequency

speed = 300,000,000 ms-1

eg freq = 918,000,000 Hz (918MHz)

Electromagnetic radiation

• Nature of EM radiation first studied in 19th Century– Maxwell, Hertz and others

• Power in Watts– NB different ways of measuring Watts!

• Frequency in Hertz…• … or wavelength in m• Think in terms of EM ‘spectrum’

Electromagnetic spectrum

• Grouped into ‘bands’

Radio waves

• Several bands have similar properties between them…

Band designation

LF low

frequency

MF medium

frequency

HF high

frequency

VHF very high frequency

UHF ultra high frequency

SHF super high frequency

Frequency 30–300kHz 300kHz–3MHz 3–30MHz 30–300MHz 300MHz–3GHz 3–30GHz

Wavelength 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:

– Information– Energy

• Often havetransceivers

Transmitter Receiver

Antenna

Radio communication

• Modulation– ‘air interface’– ‘physical layer protocol’

• Many different techniques– AM, FM, Manchester, bi-phase…– May use several simultaneously

• 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 communication is either near-field or far-field

• 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 w

avel

engt

h/2

wavelength = 300M ms-1/915MHz= 300,000,000/915,000,000= 33cm

Antennas

• 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


• 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

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?

• 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

Tag = chip + antenna + substrate

What’s in an RFID tag?

• 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

Tag = chip + antenna + substrate

What’s in an RFID reader?

• 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

Reader = case + electronics + power supply + antenna

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)

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


• 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


• 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

12.30 RFID · PDF fileThe physics behind RFID RFID Journal University: ... –...

Documents

Transcript of 12.30 RFID · PDF fileThe physics behind RFID RFID Journal University: ... –...