near field communication (nfc) a technical overview

UNIVERSITY OF VAASA FACULTY OF TECHNOLOGY TELECOMMUNICATION ENGINEERING

Naser Hossein Motlagh

NEAR FIELD COMMUNICATION (NFC) A TECHNICAL OVERVIEW

Master´s thesis for the degree of Master of Science in Technology submitted for inspection, Vaasa, 28th of May 2012.

Supervisor

Mohammed Elmusrati

Instructor

Reino Virrankoski

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Acknowledgement This thesis work has been done during the year 2011- 2012. It took me a while to study this interesting new technology which gave me a deep understanding of the topic. Here, I would like to express my sincere appreciation and thanks to my supervisor Professor. Mohammed Elmusrati, head of Communication Systems Engineering at the University of Vaasa for his support in both my thesis and my studies, in which he gave me the idea of the thesis and also this work would not have been possible unless his supervision. Also I would like to thank Dr. Reino Virrankoski for their help and sharing his knowledge and thanks to the teachers of the university which I have done my courses with, while studying and I have learnt a lot from them and in addition thanks to all Telecommunications Engineering group’s staff at university of Vaasa. Furthermore I would like to express my gratitude to all those who gave me the possibility to complete this thesis and also thanks to all authorities of University of Vaasa for providing this great opportunity and study environment. Finally, I would like to thank my family specially my parents who they always give me confidence and hopes when I meet difficulties and problems. Thanks to all my family, my classmates and my friends for all their help and support and making last few years full of memories and achievements.

Naser Hossein Motlagh Vaasa, Finland, 20st of May 2012

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TAPLE OF CONTENTS

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ABBREVIATIONS ................................................................................................................ 8 ABSTRACT ........................................................................................................................... 9 1. Introduction to wireless communication.......................................................................... 10

1.1 Introduction to NFC .................................................................................................. 10 1.2 Introduction to RFID ................................................................................................ 12 1.3 Thesis Objectives ...................................................................................................... 13 2. RFID technology overview .............................................................................................. 14 2.1 Components of RFID system.................................................................................... 14 2.2 Classification of RFID systems ................................................................................ 15 2.3 RFID coupling mechanism ....................................................................................... 17 2.3.1 RFID backscatter coupling .................................................................................. 19 2.3.2 RFID capacitive coupling .................................................................................... 21 2.3.3 RFID inductive coupling...................................................................................... 22 2.4 Power Sources .......................................................................................................... 24 2.5 Radio Frequency bands .............................................................................................. 25 2.6 RFID Standards .......................................................................................................... 27 3. Physical Principles and Electromagnetism ...................................................................... 29 3.1 Magnetic Field Strength ........................................................................................... 29 3.2 Magnetic Flux and Magnetic Flux Density .............................................................. 31 3.3 Inductance ................................................................................................................. 32 3.4 Mutual Inductance .................................................................................................... 33 3.5 Coupling Coefficient ................................................................................................ 35 3.6 Faraday’s Law .......................................................................................................... 36 4. Radio Frequency and Data transmission ......................................................................... 39 4.1 Radio Frequency ....................................................................................................... 39 4.2 Bit Duration .............................................................................................................. 40

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4.3 NFC Communication Modes .................................................................................... 40 4.4 NFC Coding and Bit Representation ........................................................................ 41 4.5 Active Mode Communication................................................................................... 43 4.5.1 Low Rate Data Transmission Using 106kbps ...................................................... 43 4.5.2 High Rate Data Transmission Using 212kbps and 424 kbps ............................... 45 4.6 Passive Communication Mode ................................................................................. 46 4.7 NFC Protocol Overview ........................................................................................... 47 4.7.1 RF Collision Avoidance ....................................................................................... 49 4.7.2 Response RF Collision Avoidance ...................................................................... 50 4.8 Frame Format............................................................................................................ 51 4.9 Load Modulation ...................................................................................................... 54 4.10 Modulation with Subcarrier ...................................................................................... 55 4.11 Digital Modulation Techniques ................................................................................ 57 5. NFC Applications ............................................................................................................ 61 5.1 Operating Modes ...................................................................................................... 62 5.2 Application Examples ............................................................................................... 62 6. NFC Security ................................................................................................................... 65 6.1 Eavesdropping .......................................................................................................... 66 6.2 Data Corruption ........................................................................................................ 67 6.3 Data Modification ..................................................................................................... 68 6.4 Data Insertion............................................................................................................ 70 6.5 Man in Middle Attack ............................................................................................... 71 6.6 NFC Specific key Agreement ................................................................................... 72 7. Conclusion AND FUTURE WORK ................................................................................ 75 References ............................................................................................................................ 77 APPENDICES ...................................................................................................................... 80 Appendix 1. ASK (Amplitude Shift Keying) ................................................................... 80 Appendix 2. PSK(Phase Shift Keying) ............................................................................ 82

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Appendix 3. FSK(Frequency Shift Keying) ..................................................................... 84 Appendix 4.NFC key agreement ...................................................................................... 86

TAPLE OF FIGURES

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Figure 1.1 Distance and data rate difference of NFC with other existing wireless technologies ......................................................................................................................... 11 Figure 2.1 The three main components of a RFID system ................................................... 15 Figure 2.2 RFID family tree ................................................................................................. 16 Figure 2.3 Idea of backscatter coupling ............................................................................... 19 Figure 2.4 Operating principles of a backscatter transponder .............................................. 20 Figure 2.5 Capacitive coupling mechanisms in close coupling system using two parallel capacitive surfaces ................................................................................................................ 21 Figure 2.6 The inductive communication between reader and a tag using coils .................. 22 Figure 2.7 Internal circuits of the communication devices and power supply of transponder from energy of magnetic field generated by the reader ........................................................ 23 Figure 3.1 Lines of magnetic flux are generated around every current carrying conductor 29 Figure 3.2 Lines of magnetic flux around a conductor and a cylindrical coil ...................... 30 Figure 3.3 Relationship between magnetic flux ɸ and flux density B ................................. 32 Figure 3.4 Definition of inductance L .................................................................................. 33 Figure 3.5 The definition of mutual inductance

by the coupling of two coils through a

partial magnetic flow. ........................................................................................................... 34 Figure 3.6 Induced electric field strength E in different materials from to bottom are: metal, surface, conductor loop and vacuum .................................................................................... 36 Figure 3.7 Equivalent circuit diagram for magnetically coupled coils................................. 37 Figure 4.1 Manchester Coding ............................................................................................. 42 Figure 4.2 Modified Miller Code ......................................................................................... 42 Figure 4.3 Pulse shape of 100% ASK modulation ............................................................... 44

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Figure 4.4 Waveform of 10% ASK Modulation .................................................................. 45 Figure 4.5 Initialization and single device detection ............................................................ 48 Figure 4.6 Initial RF collision avoidance ............................................................................. 49 Figure 4.7 Response RF Collision Avoidance sequence during the activation .................... 50 Figure 4.8 Initiator and Target Frame Format ...................................................................... 51 Figure 4.9 Format of a short frame ....................................................................................... 51 Figure 4.10 Frame format for 106 kbps ................................................................................ 52 Figure 4.11 Frame format for 212AND 424 kbps ................................................................ 53 Figure 4.12 Targets answer to initiator using load modulation in passive communication mode ..................................................................................................................................... 54 Figure 4.13 Generation of a load modulated signal with a subcarrier .................................. 56 Figure 4.14 Modulation products using load modulation with a subcarrier ......................... 57 Figure 4.15 Amplitude Shift Keying .................................................................................... 58 Figure 4.16 Phase Shift Keying ............................................................................................ 59 Figure 4.17 Frequency Shift Keying .................................................................................... 60 Figure 5.1 Idea of money transaction using NFC ................................................................. 61 Figure 5.2 Some NFC applications ...................................................................................... 64 Figure 6.1 Bit modification of Modified Miller Code .......................................................... 69 Figure 6.2 The idea of Man in the Middle Attack ................................................................ 71 Figure 6.3 NFC Specific key Agreement ............................................................................. 73

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LIST OF TABLES

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Table 2.1 Differences between active and passive mode RFID systems ............................. 17 Table 2.2 Comparison of power resource of passive, active and semi passive tags ............ 25 Table 2.3 Common RFID operating frequencies and characteristics .................................. 26 Table 2.4 RFID standards for item management (Air interface) (RFID 2002) ................... 28 Table 4.1 Definition of the divisor....................................................................................... 40 Table 4.2 NFC Communication Modes between Active and Passive devices. ................... 41 Table 4.3 Definition of time intervals shown by Figure 4.3. ............................................... 44 Table 4.4 Definition of time intervals in Figure 4.4. ........................................................... 46 Table 4.5 Command Set....................................................................................................... 52 Table 4.6 Command set for frame format shown in Figure 4.11 ......................................... 53

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ABBREVIATIONS

ASK

Amplitude Shift Keying

ATR

Attribute Request

BPSK

Binary Phase Shift Keying

CSMA

Carrier Sense Multiple Access

DEP

Data Exchange Protocol

DSL

Deselect Request and Response

EPC

Electronic Product Code

ETSI

European Telecommunication Standards Institute

FCC

Federal Communication Commission

FSK

Frequency Shift Keying

HF

High Frequency

ISO

International Standardization Organization

ITU

International Telecommunication Union

LF

Low Frequency

LSB

Least Significant Byte

MSB

Most Significant Byte

NFC

Near Field Communication

NFCID3

Random ID for Transport Protocol Activation

PSK

Phase Shift Keying

QPSK

Quadrature Phase Shift Keying

RFID

Radio Frequency Identification

SDD

Single Device Detection

UHF

Ultra High Frequency

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UNIVERSITY OF VAASA Faculty of Faculty of technology Author: Topic of the Thesis: Supervisor: Instructors: Degree: Department: Degree Programme: Major of Subject: Year of Entering the University: Year of Completing the Thesis:

Naser Hossein Motlagh Near Field Communication Mohammed Elmusrati Mohammed Elmusrati Master of Science in Technology Department of Computer Science Degree Programme in Telecommunications Engineering Telecommunications Engineering 2009 2012 Pages: 87

ABSTRACT Near Field Communication (NFC) technology is a new wireless short range communication technique for data transmission between intelligent devices such as mobile phones by integrating a small NFC reader into the cellular phones. This new technology supports the communication link within distance of up to 4 cm. NFC developed over Radio Frequency Identification (RFID), where it uses magnetic field induction to establish a communication link between devices. The main purpose of developing NFC is for the useful application it provides such as wireless payment and ticketing, electronic keys, identification and so on. Applying NFC for these matters is beneficial because of the peer-to-peer communication which exists behind it. For example this technology prepares the possibility of quick set up a Bluetooth or a WLAN connection without any manual configuration. Also wireless payments and identification will be applied worldwide in near future using contactless feature of NFC. The purpose of this thesis is to review the technical aspects of NFC technology such as Radio Frequency (RF) containing Modulation techniques, Underlying Protocol and Frame format, Applications and finally the Security of NFC will be discussed. Keywords: Near Field Communication, RFID

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1. INTRODUCTION TO WIRELESS COMMUNICATION Wireless communication is one of the fastest growing technologies in communication engineering, where this communication can be in long distance or short which is defined under the concept short range wireless communication system. Conceptually wireless communication means transceiving information without using any physical medium. Referring to the innovation of wireless technology, it was in 1897 where Marconi proved the radio’s capability to provide continues contact and since then this technology’s methods and services ever increased (Hioki 2000).Wireless communication can be via radio frequency or microwave i.e. a long range and line of sight by antennas or a short range communication also infrared (IR) which is applied in short range. This communication system can be broadcasting such as television or radio station, a point to point system such as a machine to machine, point to multipoint, cellular networks and other wireless networks. The focus of this thesis report is on short range communication a new technology which is called near field communication which uses electromagnetic waves.

1.1 Introduction to NFC Near Field Communication technology (NFC) was found and initiated by Sony and Philips. NFC is an upcoming technology developed over RFID, in a way that it consists of an interface and protocol are based on RFID which makes NFC device to a part of this standard and compatible with existing RFID technology. It is a new technology that enables a contactless, wireless communication link between devices close to each other less than 4 centimeters for sharing information at a maximum data rate of 424kbps where the difference with the other existing wireless technologies is shown in Figure 1.1.

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Figure 1.1 Distance and data rate difference of NFC with other existing wireless technologies (NFC Forum)

This communication can be either active, passive or both active devices, NFC works by utilizing magnetic coupling between devices. NFC is a new paradigm for the vast majority of cell phone users and is emerging as a near-term reality (Fischer 2009). This technology has a growing business potential technology. For instance it allows people to use their cell phones to pay their travel tickets or pay for their purchases instead of using their bank cards. Also there have been many applications developed by this new technology such as electronic keys, identification, receiving and sharing information or applying it as a set up service. NFC provides the possibility that users can share business cards, make transactions, access information from smart posters or provide credentials for access control systems with a simple touch. Therefore it can be said that NFC provides easy connections, quick transactions, and simple data sharing. The main technical feature of NFC is that it complements many wireless technologies, in a way that it utilizes the key parameters and elements in the existing standards for contactless card technology. The complementing features of enables NFC to be compatible with other existing contactless infrastructure and able the users to use one device with different systems (NFC Specifications). As mentioned NFC is initiated over RFID then it has more communication possibilities. The main characteristic that differentiates NFC from RFID is that the new technology prepares

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bidirectional data transmission between NFC equipped devices. For the communication between the two devices it is just enough to bring them close together or make them touch physically. Then the NFC protocol automatically establishes peer-to-peer link where the devices can be in passive or active mode. In the passive mode only one of the devices generates a RF field while the other device applies the load modulation for data transmission where in later chapters the active and passive modes and the modulation techniques will be discussed Also to get the knowledge about NFC it is required to understand the underlying infrastructure of RFID.

1.2 Introduction to RFID The use of RFID first started over six decades ago by British military in World War II in order to identify army objects such as planes and it was part of refinement of radar. In 1960s RFID was first considered as a solution for commercial use and in 70s and 80s, it was developed for commercial applications. Also later in 1998 a research at Massachusetts institute of technology (MIT) started to find new ways to track and identify objects moving in different physical locations. Nowadays RFID is developed to enable systems to be used for low cost commercial applications. The first developments of RFID were electronic surveillances called tags. RFID systems consist of tags, interrogator or reader. A tag is a microchip attached to an antenna and packaged so it can be attached to an object. The tags obtain a unique serial number and their identification number which this enables it to communicate (receive and send signals) with the reader. The duty of the interrogator (reader) is to emit electromagnetic waves from the antenna in a way that the sent waves are absorbed by the tag and used as energy to power the tag’s microchip in order to enable tag to send a signal which includes the identification number back to the reader. Additionally there are two types of tags, High Frequency (HF) tags which can be integrated at a distance of up to 0.8 meter while Ultra High Frequency (UHF) can be red up to 15 meters from a

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reader. There for comparing HF and UHF tags, HF tags provide more security functionality than the other one by using larger silicon chips (RFID Products). Furthermore tags can have two modes active or passive. In short to explain, when a tag uses transmitter to return information from the reader it is in active mode where most of the active tags are battery powered. Passive tag is the one that does not have the power source and it uses the transmitter electromagnetic waves as its resource.

1.3 Thesis Objectives This thesis report will start with the description of the RFID since Near Field Communication was developed based on that and then the report will cover the overall knowledge about NFC technology. The report will start with introduction to NFC and continue by describing the communication technology with RF and digital interface and modulation. During the thesis chapters the NFC standard will be described in details and it will cover the technical aspects of the technology such as Electromagnetic fields, Radio Frequency (RF), Data transfer, Modulation, Coding Schemes, Protocols, Frame Format, Applications, Security issues of the technology. Finally the thesis will be summarized as a conclusion of this work and few topics will be point out for future research.

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2. RFID TECHNOLOGY OVERVIEW As Near Field Communication is extension of RFID then it is required to overview this technology. As it is in the introduction chapter of the thesis mentioned, RFID system uses Radio Frequencies for the communication in order to identify the tagged objects. This happens when a tagged object enters the read environment of the interrogator, in a way that the reader initializes the communication by sending a signal and the tag absorbs those signals and uses them as its own power energy to send back the stored data. Tags can hold different types of data about the tagged object; these data can include the serial number, time stamps, and configurations and so on. RFID systems can include many readers where all these readers can be constructed on a single network using one controller. Furthermore the same design is possible for a single reader can communicate with many tags simultaneously as an example, nowadays simultaneous communication of 1000 tags per second is possible. Additional to technical consideration of RFID, according to ITU, the frequency ranges for LF (30~300 KHz), HF (3~30MHz), UHF (300 MHz ~3GHz) and microwave frequency band (over 2.45GHz) are defined. LF and HF have short identification ranges, but low cost. UHF and microwave frequency band own long ranges and high data handling rate (Balanis 2005). RFID communicating systems use reader and interrogator devices which these components will be reviewed in the next part of the thesis work.

2.1 Components of RFID system A RFID system consists of three main components. A transponder (tag), a reader (interrogator) and a controller, as shown in Figure 2.1 which each one of them have their own specific duty in RFID communication link. A tag which sometimes called transponder is a microchip (semiconductor chip) attached to an antenna. Tags are attached to the objects which are going to be identified. The tags obtain a unique serial number and their identification number which this enables it to communicate (receive and send signals) with

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the reader. A reader which is called interrogator is a read and write device. It is composed of an antenna, an RF electronic module for transmitting and receiving signals and a control electronics module. Another component of RFID is the controller which mainly is a computer or a workstation which obtains a database and the required software.

Data Clock

RFID Reader

Transponder

Energy

Figure 2.1 The three main components of a RFID system

2.2 Classification of RFID systems Classification of RFID systems are according to the properties of the data carrier (transponder or tag). RFID systems classification is based on two main modes which they are called Active and Passive modes. This classification is shown in Figure 2.2; the figure also represents the most common RFID system frequency categories which will be explained in later in this chapter.

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Figure 2.2 RFID family tree (Atmel 2010: 45)

In active communication mode, active tags have their own power supplies typically using an internal battery which generate their own Radio Frequency signals for data transmission. Passive tags do not have a power supply and they are dependent on the readers in a way that they acquire their own power produced by the field generated by the readers. Therefore it is clear to understand that passive tags are much smaller and cheaper than active ones. Also semi passive or semi active tags have a battery to power the microchips. In a way that semi passive and semi active use a battery to supply the internal operation of the tag but they rely on the RFID reader to supply the power to transmit the signal to the reader. The features of these two communication modes result in some advantages and disadvantages that obtained from the major differences between Passive and Active RFID modes. The summary of these difference are represented in Table 2.1.

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Feature Power Source Tag Readability

Energy

Magnetic Field Strength

Shelf Life Data Storage Size Cost

Passive RFID External (Reader Provided) Only within the area covered by the reader typically up to 3 meters. A passive tag is energized only when there is a reader present. High, Since the tag draws power from the electromagnetic field provided by the reader. Very high, in ideal case does not expire over the life time. Limited data storage, typically 128 bytes. Small Cheap

Active RFID Internal (Battery) Can provide signal over an extended range, typically up to 100 meters. An active tag is always energized. Low, Since the tag emits signals using internal battery source. Limited to about 5 years (The life of a battery). Can store larger amount of data. Size of the battery Expensive

Table 2.1 Differences between active and passive mode RFID systems

2.3 RFID coupling mechanism To create the two discussed passive and active modes in the communication link a mechanism which is called coupling technique is required. Coupling mechanism is the way of communication between RFID tag and reader. There may be different ways for RFID transponder and reader that can communicate but here this thesis report has focused on three main coupling methods which they are:

1- RFID backscatter coupling 2- RFID capacitive coupling 3- RFID inductive coupling

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In addition it is important to consider that the type of coupling method is applied according to the intended application. Each of these methods has its own feature and differs from the others. The type of coupling method effects different aspects of RFID system such as communication distance, frequency range and other elements of RFID hardware. The range or the communication distance of RFID system can be categorized into three areas: -

Close range coupling

-

Remote coupling

-

Long range coupling

Communication of RFID systems on very short range known as close coupling where the range of these type of coupling are up to 1 centimeter. This means that the tag must be pressed against the reader device and this short distance results in some benefits in case of energy absorption by the tag because the tag can gain large amount of energy from the magnetic field. Another advantage of this coupling provides high security for the systems that are in need of this requirement. Furthermore close range communication inductive and capacitive methods are used. Remote coupling typically operate in the range between 1 centimeter and 1 meter. This range usually applied with passive tags and similar to close coupling this range is uses inductive and capacitive methods. Long range RFID communication is used for longer distances than close and remote couplings. Normally the distance range is between 1 m and 10 meters and this range uses the higher frequency which is specified for RFID. Also unlike the previous ranges this coupling applies backscatter coupling method. Therefore this higher distance specifies the sort of tags and communication modes which in this case typically the system contains tags which act in long range with very low power or active tags which contain a power source such as battery. Additional to all the ranges mentioned above systems with greater distances than 10 meters exist.

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2.3.1 RFID backscatter coupling RFID backscatter coupling operates outside of the near field region in a way that this coupling method, the reader propagates radio signals and then the tag receives the signal and applies it by using part of the received signal as its own power resource and reflecting back some energy as the tag’s response toward the reader as the ‘Data’. The following Figure 2.3 illustrates this concept.

Figure 2.3 Idea of backscatter coupling

The behavior of tag when replying the readers signal seems to be interesting. The way how the tag responds readers signal all is dependent on the properties of the tag and some essential factors such as cross sectional area, antenna properties and so on. Antenna play a very important role in receiving and radiation of the signal and how this radiation is done depends on antenna properties by adding or removing a load resistor across the antenna. Furthermore for full duplex communication of the system sometimes a directional coupler is applied to separate the transmitted and received signals in the system. The design of the transponder electronic circuit has the main role in any communication system; therefor here we catch a glimpse on the electronic design of the transponder (Finkenzeller 2003). Figure 2.4 shows the total idea of a transponder electronic circuit and power transmission between communicating devices.

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Figure 2.4 Operating principles of a backscatter transponder

As it is clearly shown in the Figure 2.4 above power P1 is emitted by the signals from the reader antenna and just a small portion of this energy reaches the transponders antenna. Then this power as High Frequency voltage is supplied to the antenna connection and after recertification by the diodes (D1 and D2) this power can be as a turn on voltage for the deactivation and activation of the power saving which this is called power down mode. It is obvious that for this low provided energy for the circuit the diodes must be low barrier Schottky diodes, where this types of diodes lower energies and they have low threshold voltage. After P1 is supplied into the circuit a proportion of the incoming P1 is reflected by the antenna and returned as P2 as the energy source for the reflecting signals. Furthermore the impedance of the chip is ‘modulated’ by switching the chip’s FET. Also as formerly mentioned the reflection features of the antenna can be influenced by changing the load resistor connected to the antenna, this is done due to transmitting data from transponder to the reader. The reflected power P2 from the tag radiates into free space and just a small proportion of this energy is received by the antenna of the reader. This energy witch is in form of the signal “Data” travels in backwards direction by the readers antenna, where this can be decoupled using a directional coupler to the receiver input of the reader (Finkenzeller 2003).

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2.3.2 RFID capacitive coupling RFID capacitive coupling is applied for short range communication for data transmission whenever a close range coupling is required. This mechanism utilizes the capacitive effects to provide the coupling between the communicating devices. Referring to Figure 2.5 it is shown that in this mechanism the plate capacitors are constructed from coupling surface which are separated from each other and these are designed and equipped in both transponder and reader so that when a transponder is inserted, they become parallel to each other.

Figure 2.5 Capacitive coupling mechanisms in close coupling system using two parallel capacitive surfaces

RFID capacitive coupling has the highest performance when smart cards by applying the standard ISO 10536 and inserting it into a reader and it is because the card becomes close to the reader. In this mechanism capacitive coupling uses electrodes to provide the needed coupling instead of having coils or antenna. Therefore it is the responsibility of the capacitors by providing capacitance characteristics between the transponder and the reader to transmit the signal. It works so that the generated AC signal by the reader is taken and rectified by the transponder and applied as the device power resource of the tag and similar

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to the previous coupling the data is transmitted to the reader by the modulating load (Finkenzeller 2003).

2.3.3 RFID inductive coupling RFID inductive coupling mechanism which is defined by ISO 15693 standard is a coupling technique that transmits the energy from a circuit to another via mutual inductance between the two circuits. The Idea of the inductivity is shown in Figure 2.6 which both transponder and reader apply the inductivity feature for the communication and data transmission.

Figure 2.6 The inductive communication between reader and a tag using coils

The operation of the inductive coupling is so that when a transponder is located near by the reader, the transponder antenna coil is coupled by the field produced by the reader antenna coil. The field will cause the production of voltage in the tag and will be rectified and applied for the power of the transponder’s circuit. Also for modulation the transponders circuit alternates the load on its coil where this can be detect by the reader as outcome of mutual coupling. As far as the RFID inductive mechanism is a near field technique the distance between the coils must be kept in the effect range where generally it is considered as 0.15 wavelength of the applied frequency. Hint that inductive coupling applies the low

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frequency this means the frequency must be under 135 kHz. In addition this type of coupling unlike the capacitive and similar to backscatter coupling electronic circuits are used in transponder and the reader so having a look at these internal circuits of these two devices shown by Figure 2.7 will help to better understanding of this coupling mechanism (Finkenzeller 2003).

Figure 2.7 Internal circuits of the communication devices and power supply of transponder from energy of magnetic field generated by the reader

As it is earlier mentioned some small proportion of emitted magnetic field is received by the antenna coil of the tag and by this induction the required voltage is produced in the antenna coil of the transponder. The generated voltage is rectified and applied as power resource for data transmission which is done by microchip. In electronic circuit of the transponder the capacitor C1 is used parallel with the antenna coil of the reader and also the capacitor is chosen so it combines the coil inductance of the antenna to from a parallel resonant circuit with a resonant frequency in which it will corresponds the transmission frequency of the reader. Furthermore the antenna coil of the transponder and the capacitor C1 construct a resonant circuit which is tuned to the transmission frequency of the reader. Therefore the voltage at the transponder’s coil gets to its own maximum level because of

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the resonant set up in the parallel resonant circuit. Also the on and off switch of the load resistance at the transponders antenna influences the voltage to alternate at the reader’s antenna coil and if the on and off switch of the load resistor is controlled by the data, therefore this data can be transmitted to the reader from the transponder where this type of data are called “Load Modulation” (Finkenzeller 2003) where modulation techniques in later chapters will be discussed.

2.4 Power Sources As already discussed during previous parts of the thesis, transponders may obtain their power in various ways. Definitely the power resource plays the main and essential role in the properties of a tag since the energy source of this device specifies the life time, cost and mainly the tag’s potential read range; also this factor determines the functionality that a tag can provide. Furthermore as we already talked about the different modes of an RFID, we have three different modes which they are active, passive and semi passive or semi active; consider that these modes have direct relation to the transponder’s power resources. Active tags obtain their own power like a battery in this case the tag may start and initiate communication with a reader or even with other active tags. Semi passive transponders have an internal battery but unlike the active tags cannot initiate communications. So to establish the communication between communicating devices these types of tags are dependent on the readers to be able of acting. Passive tags do not have their own power source and then they are not capable of initiating the communication. These sorts of tags obtain their required energy for acting by harvesting it from an incoming RF signal where at low frequencies this energy is received inductively and at higher frequency ranges the energy is obtained capacitive. This different tag type with different way of providing power source affects the communication range where semi passive offer a longer reader range than passive attacks but they have higher cost while the passive tags have the shortest

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range. Also this is clear that using batteries cost and this cost is different in different types, Table 2.2 shows the summary of this comparison.

Tag Type

Passive

Semi Passive

Active

(Semi Active) Power Source

Harvesting RF energy

Battery

Battery

Communication

Respond only

Respond only

Respond or initiate

Max Range Cost

10 Least expensive

> 100 m More expensive

> 100m Most expensive

Table 2.2 Comparison of power resource of passive, active and semi passive tags

2.5 Radio Frequency bands RFID systems operate in the unlicensed radio frequency bands known as ISM (Industrial, Scientific and Medical) but the precise frequencies which are defined for RFID may vary depending on the regulations in different countries. There are several frequency bands Europe, Japan and the United states have all designated as ISM, and most the RFID systems operate at these frequencies. These frequency categories and most usual RFID system frequencies are listed in Table 2.3. In general the operating frequencies are organized into four main frequency bands of LF, HF, UHF and Microwave where these frequencies are shown in Table 2.3. This table represents the frequency bands applied in RFID also some other information such as the amount of data rate by each one of these bands furthermore the characteristics and typical applications.

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Band

Frequency

LF (Low Frequency) 30 – 300kHz

Typical RFID 125 – 134kHz Frequencies Approximate read range

Less than 0.5 meter

Typical data Less 1bkps transfer rate

than

Characteristics

Short range low data transfer rate, Penetrates water but not metals

Applications

Animal ID Car immobiliser

HF UHF High Frequency Ultra High Frequency 3 – 30MHz 300MHz – 3GHz 13.56MHz 433MHz (or) 865 – 956MHz (or) 2.45 GHz Up to 1.5 meter 433MHz = up to 100 meters 865 – 956MHz = 0.5 to 5 meters Approximately 433 – 956 = 25kbps 30kbps 2.45 = 100 kbps Higher ranges, Long range, reasonable data high data rate (similar to transfer rate, GSM phone), concurrent read Penetrates water of