NEAR FIELD COMMUNICATION (NFC) A TECHNICAL OVERVIEW

Transcription

1 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, 28 th of May Supervisor Instructor Mohammed Elmusrati Reino Virrankoski

2 2 Acknowledgement This thesis work has been done during the year 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, 20 st of May 2012

3 3 TAPLE OF CONTENTS PAGE ABBREVIATIONS... 8 ABSTRACT Introduction to wireless communication Introduction to NFC Introduction to RFID Thesis Objectives RFID technology overview Components of RFID system Classification of RFID systems RFID coupling mechanism RFID backscatter coupling RFID capacitive coupling RFID inductive coupling Power Sources Radio Frequency bands RFID Standards Physical Principles and Electromagnetism Magnetic Field Strength Magnetic Flux and Magnetic Flux Density Inductance Mutual Inductance Coupling Coefficient Faraday s Law Radio Frequency and Data transmission Radio Frequency Bit Duration... 40

4 4 4.3 NFC Communication Modes NFC Coding and Bit Representation Active Mode Communication Low Rate Data Transmission Using 106kbps High Rate Data Transmission Using 212kbps and 424 kbps Passive Communication Mode NFC Protocol Overview RF Collision Avoidance Response RF Collision Avoidance Frame Format Load Modulation Modulation with Subcarrier Digital Modulation Techniques NFC Applications Operating Modes Application Examples NFC Security Eavesdropping Data Corruption Data Modification Data Insertion Man in Middle Attack NFC Specific key Agreement Conclusion AND FUTURE WORK References APPENDICES Appendix 1. ASK (Amplitude Shift Keying) Appendix 2. PSK(Phase Shift Keying)... 82

5 5 Appendix 3. FSK(Frequency Shift Keying) Appendix 4.NFC key agreement TAPLE OF FIGURES PAGE Figure 1.1 Distance and data rate difference of NFC with other existing wireless technologies Figure 2.1 The three main components of a RFID system Figure 2.2 RFID family tree Figure 2.3 Idea of backscatter coupling Figure 2.4 Operating principles of a backscatter transponder Figure 2.5 Capacitive coupling mechanisms in close coupling system using two parallel capacitive surfaces Figure 2.6 The inductive communication between reader and a tag using coils Figure 2.7 Internal circuits of the communication devices and power supply of transponder from energy of magnetic field generated by the reader 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 Figure 3.3 Relationship between magnetic flux ɸ and flux density B Figure 3.4 Definition of inductance L Figure 3.5 The definition of mutual inductance by the coupling of two coils through a partial magnetic flow Figure 3.6 Induced electric field strength E in different materials from to bottom are: metal, surface, conductor loop and vacuum Figure 3.7 Equivalent circuit diagram for magnetically coupled coils Figure 4.1 Manchester Coding Figure 4.2 Modified Miller Code Figure 4.3 Pulse shape of 100% ASK modulation... 44

6 6 Figure 4.4 Waveform of 10% ASK Modulation Figure 4.5 Initialization and single device detection Figure 4.6 Initial RF collision avoidance Figure 4.7 Response RF Collision Avoidance sequence during the activation Figure 4.8 Initiator and Target Frame Format Figure 4.9 Format of a short frame Figure 4.10 Frame format for 106 kbps Figure 4.11 Frame format for 212AND 424 kbps Figure 4.12 Targets answer to initiator using load modulation in passive communication mode Figure 4.13 Generation of a load modulated signal with a subcarrier Figure 4.14 Modulation products using load modulation with a subcarrier Figure 4.15 Amplitude Shift Keying Figure 4.16 Phase Shift Keying Figure 4.17 Frequency Shift Keying Figure 5.1 Idea of money transaction using NFC Figure 5.2 Some NFC applications Figure 6.1 Bit modification of Modified Miller Code Figure 6.2 The idea of Man in the Middle Attack Figure 6.3 NFC Specific key Agreement... 73

7 7 LIST OF TABLES PAGE Table 2.1 Differences between active and passive mode RFID systems Table 2.2 Comparison of power resource of passive, active and semi passive tags Table 2.3 Common RFID operating frequencies and characteristics Table 2.4 RFID standards for item management (Air interface) (RFID 2002) Table 4.1 Definition of the divisor Table 4.2 NFC Communication Modes between Active and Passive devices Table 4.3 Definition of time intervals shown by Figure Table 4.4 Definition of time intervals in Figure Table 4.5 Command Set Table 4.6 Command set for frame format shown in Figure

8 8 ABBREVIATIONS ASK ATR BPSK CSMA DEP DSL EPC ETSI FCC FSK HF ISO ITU LF LSB MSB NFC NFCID3 PSK QPSK RFID SDD UHF Amplitude Shift Keying Attribute Request Binary Phase Shift Keying Carrier Sense Multiple Access Data Exchange Protocol Deselect Request and Response Electronic Product Code European Telecommunication Standards Institute Federal Communication Commission Frequency Shift Keying High Frequency International Standardization Organization International Telecommunication Union Low Frequency Least Significant Byte Most Significant Byte Near Field Communication Random ID for Transport Protocol Activation Phase Shift Keying Quadrature Phase Shift Keying Radio Frequency Identification Single Device Detection Ultra High Frequency

9 9 UNIVERSITY OF VAASA Faculty of Faculty of technology Author: Naser Hossein Motlagh Topic of the Thesis: Near Field Communication Supervisor: Mohammed Elmusrati Instructors: Mohammed Elmusrati Degree: Master of Science in Technology Department: Department of Computer Science Degree Programme: Degree Programme in Telecommunications Engineering Major of Subject: Telecommunications Engineering Year of Entering the University: 2009 Year of Completing the Thesis: 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

10 10 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.

11 11 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

12 12 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

13 13 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.

14 14 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

15 15 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. RFID Reader Data Energy Clock Transponder 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.

16 16 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.

17 17 Feature Passive RFID Active RFID Power Source External (Reader Provided) Internal (Battery) Tag Readability Only within the area covered by the reader typically up to 3 meters. to 100 meters. Energy A passive tag is energized only when there is a reader present. Can provide signal over an extended range, typically up An active tag is always energized. Magnetic Field Strength High, Since the tag draws power from the electromagnetic field provided by the reader. Shelf Life Very high, in ideal case does not expire over the life time. Data Storage Limited data storage, typically 128 bytes. Size Small Size of the battery Cost Cheap Expensive Table 2.1 Differences between active and passive mode RFID systems 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. 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

18 18 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.

19 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.

20 20 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).

21 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 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

22 22 to the previous coupling the data is transmitted to the reader by the modulating load (Finkenzeller 2003) RFID inductive coupling RFID inductive coupling mechanism which is defined by ISO 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

23 23 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

24 24 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

25 25 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 10 > 100 m > 100m Cost Least expensive More expensive 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.

26 26 Band LF HF UHF (Low High Frequency Ultra High Frequency) Frequency Frequency kHz 3 30MHz 300MHz Typical RFID Frequencies Approximate read range Typical data transfer rate Characteristics Applications 3GHz kHz 13.56MHz 433MHz (or) MHz (or) 2.45 GHz Less than 0.5 meter Less 1bkps than Short range low data transfer rate, Penetrates water but not metals Animal ID Car immobiliser Up to 1.5 meter Approximately 25kbps Higher ranges, reasonable data rate (similar to GSM phone), Penetrates water but not metals Smart Labels Contact less travel cards Access & Security 433MHz = up to 100 meters MHz = 0.5 to 5 meters = 30kbps 2.45 = 100 kbps Long range, high data transfer rate, concurrent read of <100 items, cannot penetrate water or metals Specialist animal tracking Logistics Microwave 2 30GHz 2.45GHz Up to 10 meters Up to 100 kbps Long range, high data transfer rate, cannot penetrate water or metals Moving vehicle toll Table 2.3 Common RFID operating frequencies and characteristics The most important frequency bands which are defined for RFID systems are khz, ISM frequencies around 6.78 MHz, MHz (NFC), MHz, MHz, MHz, MHz, 915 MHz (not in Europe), 2.45 GHz, 5.8 GHz and GHz (Finkenzeller 2003). Consider that the frequency band under 135 khz is not reserved for ISM band. The defined frequency bands in real and actual communication range alternates

27 27 a lot dependent on some factors such as the operating environments and antenna design. Those RFID systems which apply LF and HF frequencies are used for near field communication and the inductive coupling mechanism which already was discussed. UHF and higher frequencies are used for far field communication and for the backscattering coupling. Talking about far field means that this type of communication is based on electronic radio waves, i.e. the reader emits continues signal that is sent back from the transponder s antenna. 2.6 RFID Standards An RFID system applies few numbers of standards where none of them has been universally accepted for this matter the industries which involve RFID applications encounter with some complexity. These standards may be categorized into four levels of international, national, industry and association level. These standards can be applied to cover four key areas of RFID application: Air interface standards which is used for basic tag to reader communication. Data content and encoding i.e. the format of the codes used in tags. Conformance which means testing the RFID system. Interoperability between applications and RFID system. There are several standards define the development of RFID technologies such as: International Organization of Standardization (ISO) Electronic Product Code (EPC) European Telecommunication Standards Institute (ETSI) Federal Communication Commission (FCC) Each of the standardization organization mentioned above defines set of standards for different RFID applications while ISO supports the required standard for RFID frequencies under series of ISO which known as Air Interface Family.

28 28 The complete set of these standards which is released in 2004 where include different specifications that cover all popular frequencies including 135 KHz, MHz, MHz and 2.45GHz are shown in Table 2.4 (RFID 2002). Series Air Interface (Frequency) Part 1 Generic Parameters for Air Interface Communication for Globally Accepted Frequencies Part 2 Low Frequency below 135 KHz Part 3 High Frequency MHz Part 4 Microwave Frequency 2.45 GHz Part GHz Part 6 UHF Frequency MHz Part MHz Table 2.4 RFID standards for item management (Air interface) (RFID 2002) It is good to mention that there were related standards related to RFID technology such as ISO applied for cattle tracking systems which is defined for read range of 10 cm, ISO used for tag based payment also called proximity cards, ISO used for electronic toll collection called vicinity where the sets ISO and 15693operate at 13.56MHz for High Frequency and defined for the read range of 100 to 150 centimeters.

29 29 3. PHYSICAL PRINCIPLES AND ELECTROMAGNETISM To understand Near Field Communication one might need to understand the underlying physical principles of related technology. In addition in previous chapters it is mentioned that the newly developing NFC technology is extension of RFID system. RFID utilizes electromagnetism for communication and in order to understand the process of power and data transfer, in this chapter the theory of electromagnetic waves and the principles of inductive, capacitive coupling will be reviewed. 3.1 Magnetic Field Strength Flow of current which is moving electric charges in wires or in a vacuum generates magnetic field, see Figure 3.1. The magnitude of this field is defined by magnetic field strength and shown by H regardless of the material properties of the space. Figure 3.1 Lines of magnetic flux are generated around every current carrying conductor In the general form we can say that: the contour integral of magnetic field strength along a closed curve is equal to the sum of the current strengths of the currents within it and it is shown by the equation (3.1). To calculate the field strength H of any type of conductor this equation is applied (Finkenzeller 2003).

30 30 (3.1) The following Figure 3.2 is an example shows that how the magnetic flux behaves when a current passes a conductor. The conductor loops are used as magnetic antennas to produce the magnetic alternating field in the devices of inductively coupled RFID system. Figure 3.2 Lines of magnetic flux around a conductor and a cylindrical coil Also in a straight conductor the field strength H along a circular flux line at a distance r is constant. For the straight conductor H can be calculated as: (3.2) The path of field strength along the x axis of a round coil which is the same as conductor loop can be calculated by using equation (3.3). Consider that the magnetic field strength H decreases when the measuring point moves away from the center of the coil axis this is shown by x in Figure 3.2 and then the field strength reduces by 60 in the near field of the coil. (3.3)

31 31 Where in this equation N is number of windings, r is the circle radius and x is the distance from the center of the coil in the x direction. The condition of the validity of this equation is when that and. In addition the transition into the electromagnetic far field happens when x exceeds. Also at distance zero which is the center of antenna this equation can be simplified to (3.4). (3.4) Furthermore to calculate the magnitude field strength path for a rectangular conductor loop with side lengths at distance of x the following equation (3.5) is applied. (3.5) 3.2 Magnetic Flux and Magnetic Flux Density The total number of magnetic flux lines which pass through the inside of cylindrical coil is denoted as magnetic flux. The magnetic flux is defined by measurement of the amount of magnetic field which passes via a given surface and shown by with the unit webers, magnetic flux density shown with B with the unit Teslas (T) and this is a variable related to area A in square meters ( ) where magnetic Flux is given by the equation (3.6) (3.6)

32 32 Figure 3.3 shows the material relationship between flux density B and field strength H. Figure 3.3 Relationship between magnetic flux ɸ and flux density B Also the relationship between flux density B and field strength H is defined by equation (3.7). (3.7). Where in this equation is the magnetic field constant with the value of which describes the permeability of a vacuum. Also is the relative permeability and explains the permeability of a material if it is greater or less than. 3.3 Inductance Always magnetic field is generated when current flows in a conductor and if the conductor is in the form of a coil the magnetic field will be stronger. Any coil has N loops of the same are A when the same current I flows in it. The total flux which is shown by is the sum of the flux generated by N number of coil loops is defined by the following equation (3.8).

33 33 (3.8) The relationship of the total magnetic flux and the current is called inductance L and represented by the equation (3.9) where the inductance of a conductor loop depends on the geometry of the layout and the permeability of the medium that the flux flows through it. Also this representation is shown in Figure 3.4. (3.9) Figure 3.4 Definition of inductance L 3.4 Mutual Inductance Mutual inductance is the physical principle that an RFID system works based on that which RFID relies on this phenomenon for both power and data transfer. Mutual inductance explains the coupling of two circuits with a magnetic field where the unit and dimension of it is the same as the inductance which explained in previous part. It works in a way that if second conductor coils loop with the area located in vicinity of the first conductor loop with the area which current flows in it will be affected by the magnetic flux generated by.

34 34 This will cause some portion of the flux to flow through the second coil where this flux is called coupling flux that connects the two coils inductively. The idea of mutual inductance is represented in Figure 3.5. Figure 3.5 The definition of mutual inductance partial magnetic flow. by the coupling of two coils through a In mutual inductance the quality of the inductive coupling depends on the geometry of the two coils, their position relative to each other and the permeability of the medium between them. The mutual flux which passes through both coils is called the coupling flux and shown by and the mutual inductance is shown by and this is defined as the ratio of which passes through the second coil to the current in the first coil and represented by the following equation (3.10). (3.10). Consider that the same relationship works the other way around, i.e. a current in the second coil will generate a magnetic field that will induce a current in the first coil through the coupling flux. The relationship between the mutual inductance can be shown by the equation (3.11).

35 35 (3.11) In addition the mutual inductance magnetic field following equation (3.12). between two coils is given by the (3.12). And by replacing in equation (3.3) and substituting with, the following equation (3.13) will be obtained. (3.13) Hint that the validity of the equation (3.13) depends on if the x axis of the two coils lie on the same plane and. 3.5 Coupling Coefficient Coupling coefficient is the way of measuring the efficiency of the inductive coupling between two conductor coils. Coupling coefficient is given by the equation (3.14). (3.14) In a way that and if the value of k is close to 0, due to the distance system will have high decoupling, if the value of k is close to 1, system will have high coupling and if the value of k is equal to 1 then both of the coils will be subject to the same magnetic flux.

36 Faraday s Law Any change to the magnetic flux ɸ generates electric field strength where this feature of the magnetic field is described by Faraday s law. The effect of the electric field generated in this manner is dependent on the material properties of surrounding area; Figure 3.6 shows some of these possible effects. Figure 3.6 Induced electric field strength E in different materials from to bottom are: metal, surface, conductor loop and vacuum Inducting electric field in vacuum causes the field strength E to give rise to an electric rotational field. Open conductor loop causes an open voltage build up across the ends of an almost closed conductor loop which is normally called induced voltage. Also metal surface causes free charge carries to flow in the direction of the electric field strength. Faraday s law in its general form is given by the equation (3.15). (3.15)

37 37 Furthermore for a coil with N windings this equation can be represented as (3.16). (3.16) Also a time variant current in the first coil generates a time variant magnetic flux which leads to a voltage being induced in both coils. It is possible to differentiate into to two cases of self-inductance and mutual inductance. For self-inductance the flux change generated by the current change induces a voltage in the same conductor circuit but for mutual inductance, the flux change generated by the current change induces a voltage in the adjacent conductor circuit. Figure 3.7 shows the equivalent circuit diagram for coupled coils where in an RFID system can be the transmitter antenna of the reader and assumed to be the target antenna. Figure 3.7 Equivalent circuit diagram for magnetically coupled coils In this coupling mechanism the current consumption of the chip is symbolized by the load resistor. A time varying flux in the first coil induces a voltage in the second coil due to the mutual inductance M. Also due to the current, a voltage drop is created across

38 38 the coil resistance and this means that the voltage can be measured across. Furthermore and extra magnetic flux against the magnetic flux is generated because of the current which is flowing through. This action and reaction is presented in following equation (3.17). (3.17) And since and are sinusoidal alternating currents the previous equation can be represented as (3.18). (3.18) Also if is replaced by in (3.18) then the equation for can be solved like (3.19). Where (3.19)

39 39 4. RADIO FREQUENCY AND DATA TRANSMISSION For the first time the development and initiation of NFC technology was done by Sony and Philips. This new technology includes an interface and a protocol which is the developed on top of RFID and this is the reason that NFC device is compatible with existing RFID technology. The development of new technology differentiates it from the existing one with some new additional characteristics. First it provides the possibility of bidirectional data transfer and it also provides peer to peer communication. Where in a passive mode, only one device produces the required Radio Frequency for the communication and the other device utilizes the Load Modulation for data transmission and furthermore, NFC supports data transfer between the both active devices. 4.1 Radio Frequency Near Field Communication is a short range and standard based wireless technology which operates in globally available unlicensed of MHz frequency band ( ) and the bandwidth of the system is ±7kHz. The technology supports the data transfer with the rates of 106kbit/s, 212kbit/s and 424kbit/s and still it has potential of higher data rates where are expected in the future. Furthermore the radio transmissions by the technology are half duplex where the same channel is used for both communicating devices. Also to prevent the collision in radio transmission they apply CSMA protocol which it is Carrier Sense Multiple Access and it means sense or before transmit or listen before to talk. NFC is a short range because it is designed for the communication up to 20 centimeters for the maximum range but typically it is used within less than 10cm and practically connection occurs between two NFC devices when they brought to about 4 centimeters of another. This short range provides a high advantage in the communication which it is the security and this feature will be discussed in later chapters.

40 Bit Duration The bit duration in NFC depends on the communication mode and the data rate and a divisor which is defined for the specific mode and the data rate. In addition always the initiator which initializes the communication chooses the initial bit rate. These modes and divisors are shown in Table 4.1 and the bit duration is calculated by the equation (4.1) (NFC Specifications). (4.1) Where is the bit duration, is the carrier frquancy, and D is the divisor. Communication Mode Bit Rate Divisor Active or Passive Active or Passive Active or Passive Active Active Active Active Table 4.1 Definition of the divisor 4.3 NFC Communication Modes NFC enabled devices can communicate in two different modes which they are active and passive modes. A device that can generate its own radio frequency field is called an active device whereas a device which needs to use inductive coupling for data transmission is called a passive device. Active communication mode occurs when the operation is conducted between two active devices and passive communication mode happens when the operation happens between an active and a passive device. In addition applying active

41 41 mode communication both of the initiator and target devices follow the same and similar specifications such as speed of data transmission. Table 4.2 shows these communication modes clearly. Device Modes Active Device Passive Device Active Device Active Communication Mode Passive Communication Mode Passive Device Passive Communication Mode Not Possible Table 4.2 NFC Communication Modes between Active and Passive devices. For communication establishment, the device that starts the communication is called Initiator and the other communicating device that receives the Initiators request and sends back the acknowledgment is called the Target. In addition during the communication the mode cannot be changed or as long as the established one is terminated and this does not mean that transmission speed cannot be speeded up and this can be done by performing a parameter change procedure. As far as the transmission requires energy, so it is not a good idea for the devices that use battery power such as mobile phones to act as active device and its more suitable to act as the passive device (ISO 2004). 4.4 NFC Coding and Bit Representation In communication systems, signals are transmitted by means of different coding techniques where in electrical transmission voltage shall swing between positive and negative level. The representation of these two level transmissions is called digital signal. Also the process of arranging signal symbols into binary pattern is called coding where NFC applies Manchester and Modified Miller coding scheme. Manchester Coding: This coding scheme is the most common data coding method is applied nowadays. The transmission using Manchester coding consequently happens in the middle of each bit period. This coding method depends on two possible transitions at the middle of a symbol period in which a low to high transmission expresses a 0 bit and a high

42 42 to low transmission represents a 1 bit. The idea of bit expression is shown in Figure 4.1 (Paus 2007). Figure 4.1 Manchester Coding Modified Miller Coding: This coding method defines 0 bits and 1 s by the position of a pulse during one bit period; the bit representation is illustrated in Figure 4.2. Figure 4.2 Modified Miller Code As the Figure 4.2 shows, in the coding scheme the start and initiation of the communication happens at the start of the bit duration where a pulse may occur. For bit 1, the pulse may happen in the second half of the bit period where the transition happens in the middle of the bit period from high to low. For 0 bits a pulse may occur at the beginning of the bit period where if 0 bit follows a 1 bit any pulse does not happen during the 0 bit coding. Also in the end of the communication 0 followed by one bit duration without modulation. Furthermore

43 43 in case of No information, the signal may be coded with at least two full bit durations without modulation. 4.5 Active Mode Communication The role of initiator and target allocation is important in NFC communication and data transmission. Always the initiator is the device who is willing to start the communication and the target is the one who receives the initiator s communicator s request and sends back the reply. Furthermore in active mode, the features shall always be same for both of the communicating devices i.e. initiator to target and target to initiator communication. At the lowest data transfer speed supported by NFC, the initial bit rate shall be 106 kbps and for the higher data transmission the bit which are used are 212kbps and 424 kbps which these low and high rates will be discussed in two different sections Low Rate Data Transmission Using 106kbps Discussing the lowest data transfer speed applying NFC chips, the primitive and initial bit rate is 106kbps. For this low rate transmission the initiator device may apply 100% ASK modulation to generate the required pulses where the Figure 4.3 clears out this generation (Ecma 2004).

44 44 Figure 4.3 Pulse shape of 100% ASK modulation Analysing the envelope of the carrier amplitude shows a tedium decrease to less than 5% of its initial value and the remaining is less than 5% for the duration of. The overshoots may still remain 90% and 110% of. In communication the Target may find the end of the pulse when the field exceeds 5% of and before it exceeds 60% of as shown in the Figure 4.3 by and all these defined by time intervals in Table 4.3. Hint that this definition applies to all modulation envelope timings. Pulse Length (Condition) Maximum Minimum Table 4.3 Definition of time intervals shown by Figure 4.3.

45 45 Furthermore for this low bit rate of 106kbps the byte coding may be LSB and for the transferring the data Modified Miller Coding method is applied (Ecma 2004). Also LSB which is Least Significant Byte and it indicates a serial data transmission system that sends LSB before all other bytes High Rate Data Transmission Using 212kbps and 424 kbps Talking about higher data rate transmission means that NFC applies 212kbps ( or 424kbps which they are chosen by the initiator. The modulation scheme is still ASK but with different indexes which they are 8% to 30% of the operating field in which this is referred as 10% ASK. Figure 4.4 describes the modulation waveform and as the figure implies the rising and falling edge of the modulation may be monotonic. In addition the modulation for the transmission during the initialization and single device detection shall be the same. The peak and the minimum values of the modulated signal are defined by a and b. Also the following Table 4.4 explains and summarizes the figure below clearly (Ecma 2004). Figure 4.4 Waveform of 10% ASK Modulation

46 46 Time Intervals / Data Rate 212 kbps 424 kbps 2.0 s max 1.0 s max 2.0 s max 1.0 s max y 0.1(a b) 0.1(a b) 0.1(a b) max 0.1(a b) max Table 4.4 Definition of time intervals in Figure 4.4. Byte encoding of higher data transmission shall be MSB and the coding applied Manchester method with observe amplitude. Also the reserve polarity in the amplitude of the Manchester symbols is allowed. The target shall respond with the same load modulation scheme but the bit duration must be altered to the actual bit rate (Ecma 2004). Also MSB which is Most Significant Byte and it indicates a serial data transmission system that sends MSB before all other bytes. 4.6 Passive Communication Mode Obviously applying passive communication mode means using different specification than the active one. These differences are separated in two parties of communicator devices, initiator and target. In passive mode, in case of initiator to target, the modulation, byte encoding, bit representation and coding for the different bit rates is the same as in active communication mode following the rules for different bit rates discussed in previous sections. But in case of target to initiator communication, the target responds by Load Modulation which generates a subcarrier with frequency of. In addition the load modulation amplitude has to exceed a minimum value relative to the strength of the existing magnetic field. Also bit representation is done by Manchester coding with observe amplitude and bytes are encoded with LSB first for the lower data rate (106 kbps) and MSB first for the high bit rates (212 and 424 kbps).

47 NFC Protocol Overview All the devices that obtain a NFC technology can be either in initiator or target mode where passive devices are always in target mode. In the first step all devices are set to be in target mode by the default but else if the application ask the device to change to initiator mode where the application determines the mode of the communication and the transfer speed. In case of passive mode it performs as single device detection before it starts data transmission. The protocol flowchart for single device detection and general initialization is shown in Figure 4.5. In addition when talking about the target mode it means the device dose not generate any RF field and waits for the field generated by an initiator. Also a device in initiator mode using collision avoidance tries to detect existing RF fields before generating its own field. Applying collision avoidance in communication means that if the initiator is willing to communicate, first it must figure out that there is not any other RF field. This prevents disturbing any other NFC communication. For this the initiator has to wait in silence to check to see if there is any other field is detected or not, before it can start communicating and after waiting the amount of accurate defined guard times, it has the right to start. Exceptionally if two or more targets answer the initiator s field simultaneously then a collision will be detected and simply the frames will be discarded. The following flow chart shown in Figure 4.5 describes the general initialization and single device detection for the active and passive communication mode at different transfer speeds. The communication starts by initiator but while initialization, the initiator may detect a collision when two or more targets transmit their bit patterns at the same time. Therefore as the protocol describes the RF collision avoidance is defined to handle this issue and in order to do not disturb the other current communications existing on the carrier frequency, the initiator should not generate any RF field until the time the existing field is terminated (ETSI 2003).

48 48 Start Initial RF collision Avoidance RF Field detected? Yes No Application switches to initiator mode for Passive communication mode and chooses the transfer speed and performs the initialization and the SDD Application switches to initiator mode for Active communication mode and chooses transfer speed Activation in passive communication mode by NFCID3 (ATR). Activation in Active Communication mode by NFCID3 (ATR) Data exchange protocol (DEP) Figure 4.5 Initialization and single device detection Parameter selection (PSL) De-Activation (DSL, RLS) Terminate

49 RF Collision Avoidance In the communication, the initiator always senses the medium continuously to check the presence of other existing RF fields. If the initiator dose not detect any external RF field within the timeframe TIDT + n TRFW the RF field should go to switch on mode. The following Figure 4.6 demonstrates the initial collision avoidance while initialization (ETSI 2003). Figure 4.6 Initial RF collision avoidance The Figure 4.6 shows a signal which is divided by different time intervals where each of them has its own definition which they are as described below: : is the initial delay time where : is the RF waiting time where n: is the randomly generated number of time periods for where (0 n 3) : is the initial guard time between switching on RF field and start to send command or data frame where

50 50 Furthermore it is good to mention that the generated RF field by the initiator should be switched off in the Active mode and in the passive mode should not be switched off Response RF Collision Avoidance In order to avoid collision of the data transmission by the simultaneous responding of more than one target a Response RF Collision Avoidance is needed addition to the initial RF collision avoidance. Following Figure 4.7 represents the response RF collision avoidance sequence during initialization (ETSI 2003). Also consider that the incoming or outgoing signal is called a sequence. Also the receiving device needs the required information on when to start and stop demodulation and how to recognize a sequence. So, a sequence always starts and ends with a specific bit pattern where later in this chapter this issue will be discussed under the Frame section. Figure 4.7 Response RF Collision Avoidance sequence during the activation Also the Figure 4.7 illustrates a signal which is divided by different time intervals where each of them has its own definition which they are as described below: : is active delay time, sense time between RF off Initiator/Target and Target/Initiator where ( )

51 51 : is RF waiting time where ) n: is the randomly generated number of time period for where (0 n 3) : is the active guard time between switching on RF field and start to send command where 4.8 Frame Format This part of the thesis chapter defines the frame format used during initialization and single device detection. Data which is transmitted between communicating devices is grouped and formed in frames. The shape of the frame between initialization and the data transfer in passive communication mode is different. Also the data frames are transferred in pairs in a way that the initiator initiates the communication followed by the response of the target. The initiator frame format consists of the start, the data itself and the end of the communication. Figure 4.8 shows the initiator and the target frame format. Start of Communication Information (Start) Figure 4.8 Initiator and Target Frame Format End of Communication (End) There are two types of data frames are used for passive communication at the lower rate of 106kbps. During the initialization step, Short frames are used where a short frame consists of 7 bits of data together with the start and the stop bit. Figure 4.9 illustrates the form of a short frame. bit 0 bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 Start Command End Figure 4.9 Format of a short frame Also for the data exchange at the rate of 106kbps, standard frames are utilized, the format of a standard frame is shown in Figure 4.10.

52 52 Transport Data Field SB LEN CMD0 CMD1 Byte 0 Byte 1 Byte 2. Byte n E1 Figure 4.10 Frame format for 106 kbps In the standard frame the start byte SB is set to be 0xF0. The length byte LEN should set to the length of the Transport Data field plus 1. Also the value of LEN must be from the range of 3 to 255. CMD0 and CMD1 are command bytes that are used by the initiator and the response by the targets, these commands are described in Table 4.5 below (Ecma 2004). Mnemonic SENS_REQ SENS_RES ALL_REQ SDD_REQ SEL_REQ SEL_RES SLP_REQ Table 4.5 Command Set Definition Sense Request (sent by Initiator) Sense Response (sent by Target) Wakeup All Request (sent by Initiator) Single Device Detection Request (sent by Initiator) Select Request (sent by Initiator) Select Response (sent by Target) Sleep Request (sent by Initiator) Furthermore E1is applied in order to CRC checking for the frame format of 106kbps. Hint that the LSB of each byte should be transmitted first. Each byte shall be followed by an odd parity bit. The data frames format which are used in passive communication mode at the rates of 212 kbps and 424 kbps are different, this frame structure is illustrated in Figure 4.11.

53 53 Transport Data Field PA SYNC LEN CMD0 CMD1 Byte 0 Byte 1 Byte 2. Byte n E2 Figure 4.11 Frame format for 212AND 424 kbps As shown in the Figure 4.11the communication starts with the preamble sequence (PA) of minimum 48 bits with all logical Zero encoded. Also the SYNC byte which is the synchronization, contains two bytes where these bytes must be set to 0xB2 and 0x4D. The LEN byte is set to the length of the Transport Data field plus 1 and the value of LEN is an integer number range from 3 to 255. E2 is applied for the CRC in the frame format of 212 and 424 kbps. In addition, in active communication, the frame format for initialization does not differ from the frame format for data exchange. Also the command bytes consist of 2 bytes as shown in Figure The first byte is CMD0 and the second byte is CMD1 and the code of the command specifies the Request and Response according to the Table 4.6 (ETSI 2003). Mnemonic Command Bytes Definition CMD0 CMD1 ATR_REQ D4 00 Attribute Request (sent by Initiator) ATR_RES D5 01 Attribute Response (sent by Target) WUP_REQ D4 02 Wakeup Request (sent by Initiator in Active mode ) WUP_RES D5 03 Wakeup Response (sent by Target in Active mode ) PSL_REQ D4 04 Parameter Selection Request (sent by Initiator) PSL_RES D5 05 Parameter Selection Request (sent by Target) DEP_REQ D4 06 Data Exchange Protocol Request (sent by Initiator) DEP_RES D5 07 Data Exchange Protocol Request (sent by Target) DSL_REQ D4 08 Deselect Request (sent by Initiator) DSL_RES D5 09 Deselect Request (sent by Target) RLS_REQ D4 0A Release Request (sent by Initiator) RLS_RES D5 0B Release Request (sent by Target) Table 4.6 Command set for frame format shown in Figure 4.11

54 Load Modulation Request and Respond in NFC is done by Load Modulation and it means the process of amplitude modulating a radio frequency field by varying the properties of a resonant circuit placed within the radio frequency field. Applying the load modulation principles allows that the data from a passive target to be transmitted back to the reader. In a way that if a target with a resonant frequency equal to the transmission frequency of the reader is placed by the magnetic alternating field of the reader s antenna, the target will be powered by the magnetic field. Also if the load resistor is switched on and off at the target, the voltage changes at the reader s antenna because of the impedance changes in the target resulting in amplitude modulation at the reader s antenna. This will happen when the target is placed by the near field of the reader s antenna. Therefor for short it is said that when the initiator is generating the RF field and the Target responds to an initiator command in a load modulation scheme. Following Figure 4.12 demonstrates a scenario of how load modulation is applied in passive communication mode (Philips 2011). Figure 4.12 Targets answer to initiator using load modulation in passive communication mode

55 Modulation with Subcarrier The use of a modulated subcarrier is widespread in radio technology. The subcarrier modulation represents a multilevel modulation in a way that first, subcarrier is modulated with a differential signal and in order to finally modulate the HF transmitter once again with the modulated subcarrier signal. Also instead of switching the load resistance on and off in time with a baseband coded signal, a low frequency subcarrier is first modulated by the baseband coded data signal. ASK, FSK or PSK modulation may be used as the modulation procedure for the subcarrier. The subcarrier frequency itself is normally obtained by the binary division of the operation frequency. For 13.56MHz systems, the subcarrier frequencies 847 khz, 424 khz or 212 khz are normally used. The modulated subcarrier is now applied to switch the load resistor on and off. The best advantage of utilizing a subcarrier becomes clear when the frequency spectrum generation comes to the consideration. Furthermore the load modulation with a subcarrier initially generates two spectral lines at a distance the subcarrier frequency around the operating frequency. This is illustrated in Figure The actual information is now transmitted in the sideband of two subcarrier lines depending on the modulation of the subcarrier with the baseband coded data stream. In addition if load modulation in the baseband were used, on the other hand, the sidebands of the data stream would lie directly next to the carrier signal at the operating frequency (Finkenzeller 2003).

56 56 Figure 4.13 Generation of a load modulated signal with a subcarrier Also due to the weak coupling factor between the reader and the target antenna and the difference between the carrier signal of the reader and the received modulation sidebands, the targets response varies within the range where this range is lower than the voltage generated by the reader. This is shown in Figure In this procedure one of the two subcarrier modulation products can be filtered out and demodulated by shifting the frequency of the modulation sidebands of the data stream. Consider the production of two modulation sidebands at a distance of from the carrier frequency of the reader where to separate the sidebands from the stronger carrier signal, bandpass filtering is applied and then the signal is amplified at the reader due to demodulation process (Finkenzeller 2003).

57 57 Figure 4.14 Modulation products using load modulation with a subcarrier 4.11 Digital Modulation Techniques Analyzing the characteristics of any electromagnetic wave at any point in the magnetic field allows the reconstruction of the message by measuring the change in the reception power, frequency or phase position of the wave where this procedure is called demodulation. The former classical radio technology is involved with analogue modulation procedures. But the modulation of the electromagnetic waves can be done through different techniques which they are amplitude modulation, frequency modulation and phase modulation. Hint that all the other modulation techniques are derived from one of these three sorts of techniques. The procedure applied in NFC system, the data transmission made possible through ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying) and PSK (Phase Shift Keying) in which these methods are reviewed here in short.

58 58 Amplitude Shift keying (ASK) is a type of digital modulation that shows digital data in the form of variation in the amplitude of a carrier wave, as a result of MATLAB modeling this can be seen from the plot in the Figure For binary levels the bit 1 is represented by the standard carrier wave and 0 is represented by a carrier wave with zero amplitude. The type of ASK is called 100% ASK or on-off keying and this one is the very basic type of ASK modulation. Furthermore the percentage describes how much the amplitude is decreased, as the example when saying 30% ASK would mean that a logical 0 reduces the amplitude level to 70% compared to the amplitude level of logical 1. 5 ASK Signal with two Amplitudes Amplitude Time (bit period) Original Digital Signal 1.5 Amplitude Time (bit period) Figure 4.15 Amplitude Shift Keying

59 59 Phase Shift Keying (PSK) is the other type of digital modulation where it demonstrates the digital data in form of variation of the phase of a carrier wave and as a result of MATLAB modeling this can be seen from the plot in the Figure For Binary Phase Shift Keying (BPSK) the shift is 180 degrees and for Quadrature Phase Shift Keying (QPSK) the phase shift is 90 degrees. Hint that applying QPSK over BPSK enables either higher data rates or lower bandwidth depending on the requirement. PSK Signal with two Phase Shifts 1 Amplitude Time (bit period) Original Digital Signal 1.5 Amplitude Time (bit period) Figure 4.16 Phase Shift Keying

60 60 Frequency Shift Keying (FSK) demonstrates the digital data in the form of variation of the frequency of a carrier wave. For Binary Shift Keying (BPSK), a logical 0 is represented by one frequency and a logical 1 is represented by a different frequency. As a result of MATLAB modeling this can be seen from the plot in the Figure FSK Signal with two Frequencies 1 Amplitude Time (bit period) Original Digital Signal 1.5 Amplitude Time (bit period) Figure 4.17 Frequency Shift Keying

61 61 5. NFC APPLICATIONS Near Field Communication (NFC) is a technology for contactless wireless short range communication where the technology is extended over existing Radio Frequency Identification (RFID) technology. NFC establishes communication link by generating magnetic field induction. Figure 5.1 Idea of money transaction using NFC There is wide range of short range applications for NFC and still these applications are growing rapidly in a way that it is impossible to give a complete picture of them. These applications can be defined for mobile and portable devices, PC world and consumer application (Philips 2011), Figure 5.1 shows an idea of NFC application. But especially the use of NFC chips with mobile phones prepares wide and many opportunities by integrating it into mobile devices which this provides a high advantage and functionality.