RFID Systems and Operating Principles

University of HoustonBauer College of BusinessSpring 2007

Presentation Source: RFID Handbook, Chapter 3

Overview

Please read Chapter 3 of the RFID Handbook for this section

RFID Systems can be categorized based on: Operating principles Frequency

CLASSIFICATION BY OPERATING PRINCIPLE

LC Circuit An LC circuit consists of an inductor, represented by the letter L,

and a capacitor, represented by the letter C. When connected together, an electrical current can alternate between them.

The resonance effect occurs when inductive and capacitive reactances are equal. The word resonance refers to a class of phenomena in which a small driving perturbation gives rise to a large effect in the system.

Applications of Resonance: Tuning: LC circuits are set at resonance for a particular carrier frequency Voltage Magnification Current Magnification Load Impedence

Electronic Article Surveillance (EAS)

Why EAS?

RFID = Identification + EAS Shoplifters steal more than US$10 billion a

year from U.S. retailers ($60 billion worldwide)

Shoplifting means: lost sales higher inventory costs tighter margins

1-Bit Transponders

A bit is the smallest unit of information that can have only two states: “1” = “transponder in interrogating zone” “0” = “no transponder in interrogating zone”

EAS system architecture

Reader antenna Security element (tag) Deactivation device Activator device

Radio Frequency

Components: The radio frequency (RF) uses LC resonant

circuits adjusted to a particular frequency Tags: Modern Systems employ coils etched

between foils in the form of a stick-on label

Radio Frequency Operation:

The reader generates a magnetic field in the radio frequency range

When tag is moves into the vicinity of the magnetic alternating field, energy from the alternating field induces voltage in the tag’s coil (Faraday’s Law)

If the frequency of the reader’s field corresponds with the frequency of the tag’s circuit, the tag’s circuit produces a sympathetic oscillation (also starts to oscillate)

Radio Frequency

Operation:

The current that that flows in the tag’s circuit, as a result of the sympathetic oscillation, ultimately acts against its cause – the magnetic field of the reader

This “resistance” leads to a small voltage drop in the reader’s coil and ultimately leads to decrease in magnetic field strength

To ensure better detection rate, the reader may “sweep” across frequencies: 8.2 MHz+- 10%

Radio Frequency

Deactivation Item is placed into deactivator

Deactivator generates a sufficiently high magnetic field that the induced voltage destroys the foil capacitor of the circuit

Capacitors are designed with intentional short-circuit points, called “dimples”

The breakdown of the capacitor is irreversible

Radio Frequency

Problems: The detection rate can be as low as 70%

The detection rate is heavily influenced by certain materials (especially metal) – affect the resonant frequency of the coil

Both reader antenna and tag must have adequate size to ensure adequate data transmission

Microwave

Operation: Exploits the generation of “Harmonics” by components (e.g.

capacitance diodes)

The harmonic of a sinusoidal voltage A with a frequency fA is a sinusoidal voltage B, whose frequency fB is an integer multiple of fA

Tag receives frequency wave from the reader and “multiplies” the frequency and sends it back to the reader

After receiving the “multiplied” frequency signal, the sensor is able to detect the presence of the tag. (E.g. the sensor tuned to the second harmonic triggers alarm when it detects that frequency)

Microwave

Advantages: If the signal is modulated (ASK, FSK), then

interference from other signals can be prevented – the harmonic is also modulated

Microwave EAS systems are less sensitive to metal parts – typical frequencies used are 915 MHz (Europe), 2.45GHz, or 5.6 GHz

Microwave systems are typically used to protect textiles

Frequency Divider

Operation: Operates in the long wave range at 100-135.5 kHz

Tag derives power from the magnetic field; frequency received from the reader is divided by two by the microchip and send back to the reader

The signal is half the original frequency - subharmonic

Signal can be modulated (ASK or FSK) to filter interference

Tag has to be removed from a product after purchase

Electromagnetic EAS Operate using strong magnetic fields in range of10-20kHz

Due to the extremely low frequency, they are the only systems suitable for products containing metal

Signal contains summation of differential frequency of the extra signals by superimposing additional signals with higher frequencies over main signal

The tags are usually in the form of self-adhesive magnetic strips with lengths ranging from 2cm to 20cm

To deactivate: cashier runs a strong permanent magnet along the metal strip magnetization of the element. Can be reactivated any number of times.

However, system performance depends on tag position

Acoustomagnetics Tags come in the form of small, thin plastic boxes

The box contains two metal strips Hard metal strip Strip made from amorphous metal (can vibrate) Ferromagnetic substances are “magnetostrictive” – change in length due

to magnetization

The strip vibrates at high amplitude at resonant frequency of the system

The strip continues to oscillate even after the reader’s field is switched off - like a tuning fork. Hence, itself generates a magnetic alternating field that can be detected by security system higher sensitivity.

To deactivate the tag, it has to be demagnetized

Transmission Procedures

HDX: data transfer from the transponder to the reader alternates with data transfer from the reader to the transponder

FDX: data transfer from the transponder to the reader takes place at the same time as the data transfer from the reader to the transponder

SEQ: transfer of energy from the reader takes place for a limited period of time. Data transfers occur in between these energy pulses

FDX, HDX, SEQ

Source: RFID Handbook

Advantages of SEQ Systems

The available operating voltage is up to twice that of a comparable half/full duplex systems

The energy available to the chip can take, theoretically any value

Inductive coupling

Almost always operated passively

Frequency range used (wavelength): <135 KHz (2400 m), 13.56 MHz (22.1 m)

Components Electronic data-carrying device – Microchip Large coil area – Antenna

Inductive coupling

Operation: Reader’s antenna coil generates a strong EM field, which

penetrates cross-section of coil

Because frequency used is >>> distance between reader and transponder’s antennae, the EM field can be treated as a simple magnetic alternating field Voltage generated by Inductance

Circuit resonates at transmission frequency of reader – very high current generated in reader by resonance step-up which produce required field strengths for operation

The two coils can also be interpreted as a transformer (distance between coils < 0.16 λ – transponder is in Near Field

Inductive coupling

Efficiency of power transfer between reader and transponder is proportional to: Operating frequency Number of windings (higher frequencies need lower windings) Area enclosed by transponder coils Distance between two coils

Data Transfer from Transponder Reader Load Modulation: switching a load resistor on and off at the

transponder’s antenna controlled by data – changes voltage and hence, amplitude

Sensitivity: Two modulation sidebands sent along with main signal (subcarriers), or subharmonics used

Inductive coupling

Electromagnetic backscatter coupling Operated at UHF frequencies: 868 MHz (Europe) and 915 MHz

(USA); and microwave frequencies: 2.5 GHz and 5.8 GHz

Used for long-range systems Gap between reader and transponder > 1m

To achieve ranges of >15m – backscatter transponders have backup batteries to supply power

To maximize battery power, “stand-by” mode used when transponder moves out of range of reader

The battery of an active transponder never provides power for the transmission of data between transponder and reader. Exclusively serves for supply to microchip.

Electromagnetic backscatter coupling Data transmission Reader

Modulated reflection cross-section Efficiency by which objects reflect EM waves – “Reflection

cross-section”. Objects that are in resonance with wave front that hits them have large reflection cross-section

Proportion of incoming power is reflected. The reflection characteristics are influenced by altering the load connected to the antenna in time with the data stream to be transmitted. The amplitude of reflected power is thus modulated

The reader has a “directional coupler” which differentiates between forward and backward signals

Close coupling

Ranges between 0.1 cm – 1 cm

Transponder inserted into reader or placed on marked surface (“touch and go”)

Allows transponder coil to be precisely positioned in air gap of a ring-shaped or U-shaped core

High freq AC in reader generates high freq magnetic field in core and air gap – which provides power supply to chip in transponder

Close coupling

Frequencies in range 1- 10 MHz used

In contrast to inductively coupled or microwave systems, the efficiency of power transfer is very good

Suited for operation of chips with high power consumption – microprocessors (need 10 mW for operation)

Contact-less smart cards – ISO 10536

Close coupling

Data transfer transponder reader Magnetic coupling: Load modulation with

subcarrier used for magnetically coupled data transfer. Frequency and modulation specified in ISO 10536 standard

Capacitive coupling: Plate capacitors in reader and transponders arranged so that they are exactly parallel to one another – defined in ISO 10536

Electrical coupling

Uses electrostatic fields for transmission of energy and data

Load modulation used to transfer data from transponder to reader

Data Transfer from Reader

All known digital modulation procedures used ASK: Amplitude shift keying (most used) FSK: Frequency shift keying PSK: Phase shift keying

CLASSIFICATION BY FREQUENCY

Basic Types of RFID SystemsFrequency Band Characteristics Typical

Applications

Low

100-500 kHz

Short to medium read range, inexpensive, low reading speed

Access control

Animal/Human identification

Inventory Control

Medium

10-15 MHz

Short to medium read range

Potentially inexpensive

Medium reading range

Access Control

Smart Cards

High

UHF: 850-950MHz

Microwave: 2.4 – 5.8 GHz

Long read range

High reading speed

Line of sight required (Microwave)

Expensive

Railroad car monitoring

Toll collection systems

Agenda

13.56MHz RFID Systems (HF) Operating principles are similar to LF

400-1000MHz RFID Systems (UHF)

2.4GHz RFID Systems (Microwave)

How to select an appropriate RFID System? For each application, there is an appropriate RFID system in terms of:

Operating principles Frequency Range Coupling

Functionality Read-only Read-write Motion-detection

Physical form: Stationary readers Handheld Readers Tunnels, Gates

Cost

13.56MHz RFID Systems

Library RFID System from Tagsys

Tag

Circulation Desk Station

Programming Station Security Gate

13.56MHz – Operating Principles Mostly passive – no battery

Low cost Longer life-time

Inductive coupling is used for data transmission

13.56MHz – Operating Principles RF field at 13.56MHz is not absorbed by water or

human tissue

Sensitive to metal parts in the operating zone (this applies to all RFID systems)

As the magnetic field has vector characteristics, tag orientation influences performance of the system (distance) Rotating fields

Since inductive RFID systems are operated in the near field, interference from adjacent systems is lower compared to other systems

13.56MHz - Tags

Tags are available in different shapes and have different functionality

A few turns (<10) of antenna are sufficient to produce a passive tag low cost

13.56MHz –Shape of Tags

ISO Cards (ISO 14443, ISO 15693)

Durable industrial tags

Thin and flexible smart labels

13.56MHz –Functionality

Memory size (from 64 bit - ID tags to several Kbytes)

Memory types: ROM, WORM/OTP, R/RW

Security mechanisms can be implemented

Multi-tag capability – several tags can be read at once

13.56MHz – Readers Range

“Proximity” (<100 mm) Handheld devices, printers, terminals Small size, low cost

“Vicinity” (<1.5m) More complex Higher power consumption

“Medium range” (<400 mm)

13.56MHz –Physical Form of Readers Application

Mobile

Stationary

13.56MHz –Readers

Readers can have several antennas to allow for: Greater operating range Greater volume/area coverage Random tag orientation

13.56MHz – Conveyor Performance A reader that reads 10 to 30 tags per second

Successful tagging of items on a conveyor running at 3 meters/sec and spaced 0.10 m

13.56MHz – Overall Performance Application fit is the key

Memory size, security level

Smaller operating distances allow faster data transmission, longer operating distances impose lower transmission speed

Greater resistance to noise Outside of the ISM band

400-1000 MHz UHF RFID-Systems (UHF)

Uses EM Propagation The amount of energy collected is a function of the aperture of the

receiving antenna, which in simple terms is related to the wavelength of the received signal

Operating range is dependent on the radiant power of the reader, the operating frequency, and the size of a tag antenna

400-1000 MHz UHF RFID-SystemsOperating Principles

400-1000 MHz UHF RFID-SystemsWave Properties EM waves are related to light and behave in a

similar manner

EM waves can be reflected off radio conductive reflective surfaces, refracted as they pass the barrier between dissimilar electric media, or detracted around a sharp edge

UHF waves have shorter waves and, thus, are more effected when passing objects

400-1000 MHz UHF RFID-SystemsPenetration into Liquids EM waves penetrate into different liquids,

depending on the electrical conductivity of the liquid

Water has high conductivity will reflect and absorb the signal

Oil and petroleum liquids have low conductivity will allow EM to pass

400-1000 MHz UHF RFIDRange Read range depends on:

Transmitter (reader) power Energy requirements of the tags (for passive tags) Absorption factor of materials to which the tag is

attached Tag size

The smaller the tag, the smaller the energy capture area, the shorter the read range

400-1000 MHz UHF RFIDInterference Electrical noise from motors, florescent lights,

etc is minimal at UHF

Noise from other RFID systems, mobile phones, etc.

Frequency Hoping Spread Spectrum (FHSS) can reduce interference

400-1000 MHz UHF RFIDRead Direction UHF allows for directional antennas This allows to direct the signal to particular

groups of tags

Orientation of the tag antenna with respect to the reader’s antenna will impact range (not important for some systems)

Tag Orientation

2450 MHz RFID Systems

2450 MHz RFID Systems

Microwave RFID systems have been in wide-spread use for over 10 years in transportation applications Rail car tracking Toll collection Vehicle access control

2450 MHz RFID SystemsOperating Principles Modulated backscatter Microwave systems operate in the “far field” long

range systems

Microwave signals are attenuated and reflected by materials containing water or human tissue and are reflected by metallic objects It is possible to design tags that work on metallic

objects

Line of sight is not required for operations

2450 MHz RFID SystemsOperating Principles UHF and microwave signals easily penetrate wood,

paper, cardboard, clothing, paint, dirt, and similar materials

Because of short wave length and reflective properties of metal, high reading readability can be achieved in metal-intensive environments

Sensitive to orientation Rotating antennas can solve the problem

2450 MHz RFID SystemsOperating Principles UHF and Microwave systems are allocated

many MHz of spectrum independent operation of different systems, less interference

Microwave systems have a proven record of reliability

2450 MHz RFID SystemsPhysical Form of Tags Tags come in various forms

Tags are smaller than their LF and HF counterparts

3 major types of tags EZ pass type Tags for logistical purposes Thin and flexible smart labels

2450 MHz RFID SystemsTags From 64 bits to several Kbytes ROM, OTP, R/RW All required security levels can be realized Multiple tags can be read in the same zone

2450 MHz RFID SystemsReaders “Proximity” “Vicinity” Handheld Stationary

2450 MHz RFID SystemsPerformance Compared to inductive systems, the UHF and

microwave systems can have longer range, higher data rates, smaller antennas, more flexibility in form factors and antenna design

Object penetration and no line-of-sight readability can be better for LF systems

Conclusion

Operating principles impact: Appropriateness of a particular RFID system for a

particular application

Vulnerabilities of RFID systems Interference Security attack