CONTACTLESS CARDS
As already described in Section 2.3.3, contactless cards do not require any electrical connection between the smart card and the card terminal in order to transfer energy and data over a short distance. The most important advantages of the contactless card technology are described in Section 2.3.3. In this section we examine the technology and operating principles of contactless cards in more detail. The techniques used with contactless cards for transferring energy and data are not new. They have been common knowledge for many years in radio-frequency identification (RFID) systems, which have been used for a variety of applications, such as animal implants and transponders for electronic anti-start systems for vehicles. There are many techniques for identifying persons or objects at short or even long distances based on radio techniques, and in particular on radar techniques. Among the large variety of technical possibilities, only a small number are suitable for use in smart cards in the ID-1 format (to which we restrict our attention), since all of the functional components must be housed in a flexible card that is only 0.76 mm thick. For instance, fitting flexible batteries into the card body remains an unsolved problem for mass-produced cards. Although flexible batteries with suitable thickness are nowavailable, there is no experience with using such batteries in the field or in mass production. Consequently, we are still limited to passive techniques in which the energy to power the card must be extracted from the electromagnetic field of the card terminal. This limits the useful range to around 1 m.

To make it easier to understand the variety of techniques used, they can be classified according to various parameters. One possibility is to classify them according to the method
used to transfer energy and data. The most commonly used methods are transmission using radio waves or microwaves, optical transmission, capacitive coupling and inductive coupling.
Capacitive and inductive coupling are best suited to the flat shape of a smart card lacking an internal source of power. The systems presently available on the market utilize these methods exclusively, which are also the only ones considered in the relevant group of ISO/IEC standards (10 536, 11 443 and 15 693). Consequently, in this book we limit ourselves to these methods. Just as with contact-type smart cards, a system using contactless cards consists of at least two components, namely a card and a compatible terminal. The terminal can act as a reader or a reader/writer, according to the technology used. As a rule, the terminal includes an additional interface, via which it can communicate with a background system. The following four functions are necessary to allow a contactless card to communicate with a terminal:
–energy transfer to the card for powering the integrated circuit
–clock signal transfer
–data transfer to the smart card
–data transfer from the smart card.

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Figure 3.69 The necessary energy and data transfers between a terminal and a contactless smart card

Many different concepts based on experience with RFID systems have been developed to satisfy these requirements. Most of them are specifically designed for particular applications. For instance, there is a considerable difference between systems where the cards are only a few millimeters away from the terminal in normal use and systems where the cards can by up to a meter away from the terminal. Naturally, when many different solutions specifically designed and optimized for particular applications are developed, they are inevitably mutually incompatible.

Inductive coupling
Inductive coupling is presently the most widely used technique for contactless smart cards. It can be used to transfer both energy and data. Various requirements and constraints, such as radio licensing regulations, have resulted in a variety of actual implementations.

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Figure 3.70 Basic construction of a contactless smart card with inductive coupling

With some applications, such as access control, it is sufficient to only be able to read the data stored in the cards, which makes technically simple solutions possible. Due to their low power consumption (a fewtens of microwatts), the usable range of such cards extends to approximately one meter. Their memory capacity is usually only several hundred bits. If data must also be written, the power consumption rises to more than 100 μW. As a consequence, the range is limited to around 10 cm in the writing mode, since licensing restrictions prevent the emitted power of the writing equipment from being arbitrarily increased. The power consumption of microprocessor cards is even greater and is typically 100 mW. The distance from the terminal is thus even more restricted.

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Figure 3.71 Inlay foil for a contactless smart card with inductive coupling using an etched coil

Independent of their range and power consumption, all cards that employ inductive coupling work on the same principle. One or more coils (usually with large enclosed areas) are incorporated into the card body to act as coupling components for energy and data transfers, along with one or more chips.

Energy transfer
Almost without exception, contactless smart cards are used passively. This means that all of the energy needed for operating the chip in the smart card must be transferred from the reader to the card. This energy transfer is based on the principle of a loosely coupled transformer. A strong high-frequency magnetic field is generated by a coil in the terminal in order to transfer the energy. The most commonly used frequencies are<135 kHz and 13.56 MHz, which correspond towavelengths of 2400mand 22 m, respectively. Thewavelengths of the electromagnetic fields are thus several times greater than the distance from the card to the terminal, which means that the card is located in the near field of the terminal. This allows the loosely coupled transformer model to be used. If a contactless card is brought close to the terminal, a portion of the terminal’s magnetic field passes through the coil in the card and induces a voltage Ui in this coil. This voltage is rectified to provide power to the chip. Since the coupling between the coils in the terminal and the card is very weak, the efficiency of this arrangement is very low. A high current level is thus required in the terminal coil to achieve the necessary field strength. This is achieved by connecting a capacitor CT in parallel with the coil LT, with the value of the capacitor chosen such that the coil and capacitor form a parallel-resonant network whose resonant frequency matches the frequency of the transfer signal.

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Figure 3.72 Using inductive coupling to supply energy to a smart card

Coil LC and capacitor C1 in the card also form a resonant circuit with the same resonant frequency. The voltage induced in the card is proportional to the signal frequency, the number of windings of coil LC and the enclosed area of the coil. This means that the number of turns needed for the coil drops with increasing signal frequency. At 125 kHz, it is 100 to 1000 turns, while at 13.56 MHz it is only 3 to 10.

Data transfer
For transferring data from the terminal to the card, all known digital modulation techniques can be used. The most commonly used techniques are:
–ASK (amplitude-shift keying)
–FSK (frequency-shift keying)
–PSK (phase-shift keying).
ASK and PSK are usually used, since these are especially easy to demodulate.
In the other direction, from the smart card to the terminal, a type of amplitude modulation is used. It is generated by using the data signal to digitally alter a load in the card (load modulation). If a smart card tuned to the resonant frequency of the terminal is brought into the near field of the terminal, it draws energy from this field as previously described. This causes the current I0 in the coupling coil of the terminal to increase, which can be detected as an increased voltage drop across an internal resistor Ri. The smart card can thus vary (amplitude modulate) the voltage U0 in the terminal by varying the load on its coil, for example by switching the load resistor R2 into and out of the circuit as shown in Figure 3.73. If the switching of resistor R2 is controlled by the data signal, the data can be detected and evaluated in the terminal.

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Figure 3.73 A sample circuit illustrating the principle of load modulation, which is used in contactless smart cards for data transmission

Due to the low degree of coupling between the coils in the terminal and the card, the voltage variations induced in the terminal by load modulation are very small. In practice, the amplitude of the usable signal is only a few millivolts. This can only be detected using sophisticated circuitry, since it is overlaid by the significantly larger signal (around 80 dB) transmitted by the terminal. However, if a subcarrier frequency is employed with a frequency of fs, the received data signal appears in the terminal as two sidebands at the frequencies fc ± fs. These can be isolated from the significantly stronger terminal signal by filtering with a bandpass filter and then amplified. After this, they can readily be demodulated. The disadvantage of modulation with a subcarrier is that it requires significantly more bandwidth than direct modulation. It can thus only be used in a limited number of frequency bands.

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Figure 3.74 Load modulation using a subcarrier produces two sidebands separated from the transmission frequency of the terminal by the value of the subcarrier frequency fs. The information is contained in the sidebands of the two subcarrier sidebands, which are produced by modulation of the subcarrier (based on Klaus Finkenzeller [Finkenzeller 02])

Capacitive coupling
If the distance between the card and the terminal is very small, it is possible to transfer data using capacitive coupling. With this type of coupling, conductive surfaces are incorporated into the card body and the terminal such that they act as the plates of a capacitor when the card is inserted in the terminal or placed on the terminal. The capacitance that can be obtained essentially depends on the sizes of the coupling surfaces and their separation. The maximum size is thus limited by the dimensions of the card, while the minimum separation is determined by the insulation required between the coupling surfaces.With an acceptable level of cost and effort, a usable capacitance of several tens of picofarads can be obtained. This is insufficient for transferring enough energy to power a microprocessor. Consequently, this method is used only for data transmission, with the operating power being transferred inductively. This mixed method has been standardized in ISO/IEC 10 536 for ‘close coupling cards’, and it is fully described in Section 3.6.1. As its name says, this method is limited to small coupling distances.

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Figure 3.75 Operating principle of capacitive coupling. The coupling arises from the alternating electrical field between two parallel, electrically conductive surfaces in the card and terminal

Collision avoidance
When contactless cards are used, there is always a possibility that two or more cards may be located in the range of a terminal at the same time. This is especially true for systems with large effective ranges, but it can even happen with systems with relatively small ranges – for instance, two cards might be lying on top of each other and thus be activated concurrently by the terminal. All cards within range of a particular terminal will attempt to respond to commands from the terminal. However, simultaneous data transmissions will unavoidably cause interference and loss of data if suitable countermeasures are not taken. The technical methods used to ensure interference-free data exchanges with multiple cards within the effective range of a card terminal are called collision-avoidance methods or anticollision methods.

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Figure 3.76 Concurrent operation of several cards within the range of a terminal (multiple access) requires using an anticollision method to ensure interference-free data exchanges

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Figure 3.77 The four types of anticollision methods

Exchanging data between many mobile units and a base station is a frequently encountered situation in communications engineering, and it is referred to as ‘multiple access’. A typical example is a mobile telephone network, in which all users located in a particular radio cell concurrently access a single base station. Numerous methods have been developed to allow the signals of the individual users to be distinguished from each other. These anticollision methods can be classified into four types, as shown in Figure 3.77. Space division multiple access (SDMA) attempts to limit or scan the operational area of a terminal in such a way that only one card can be acquired at any given time. Since this method requires very complicated and correspondingly expensive aerials, it is not used for contactless cards. With time division multiple access (TDMA), measures are taken to ensure that the individual cards have different timing behavior so that they can be separately identified and individually addressed by the terminal. This is the most commonly used method, and it has many variants. Two of them, which are standardized in ISO/IEC 14 433-3 for ‘proximity cards’, are described extensively in Section 3.6.3. With frequency division multiple access (FDMA), different carrier frequencies are provided concurrently for multiple transmission channels. However, this technique is technically complicated and thus expensive. Consequently, it is not used for contactless cards. The same considerations also apply to code division multiple access (CDMA).

The present state of standardization
Given the many different techniques used by various manufacturers, standardization (which was initiated in 1988 by ISO/IEC) proved to be difficult and time consuming, as was expected. The responsible working group had the task of defining a standard for contactless cards that is largely compatible with other standards for identification cards. This means that a contactless card can also have other functional components, such as a magnetic stripe, embossing and chip contacts. This allows contactless cards to also be used in existing systems that employ other technologies. As already described, the technical options for transferring energy and data without using contacts essentially depend on the desired distance between the card and the terminal for reading and writing data. It was therefore not possible to create a single standard that provides a single technical solution to all the requirements arising from various applications.

Presently, three different standards describing three different reading ranges have been completed. Each of these standards in turn permits various technical solutions, since the members of the standardization committee could not agree on a single solution. In order to achieve interoperability among the various options, card terminals must support all of these options. Standardization started with ‘close-coupling’ cards (ISO/IEC 10536), since the microprocessors available at that time had relatively high power consumption, making energy transfer over a relatively large distance impossible. The essential parts of this standard have been completed and approved and are described in the following section. In use, this type of card offers only minimal advantages compared with normal contact-type cards, since it must be inserted into a terminal or at least precisely placed on a surface of a card terminal. Furthermore, the structure of the card is complex, which results in high manufacturing costs. Consequently, up to now this type of system has hardly established a significant position in the market.

Table 3.6 Completed ISO/IEC standards for contactless smart cards
Standard Type of contactless smart card Range
ISO/IEC 10536 Close-coupling card Up to approx. 1 cm
ISO/IEC 14443 Proximity coupling card (PICC) Up to approx. 10 cm
ISO/IEC 15693 Vicinity coupling card (VICC) Up to approx. 1 m