The number of publications on this topic has not decreased over several years. This is explained by the rapid development of RFID technology itself, the mastering of new frequency ranges, and the regular emergence of new products and new applications where the contactless identification technology (or Radio Frequency Identification - RFID) allows solving tasks that were previously beyond the capabilities of technical and software tools.

The number of publications on this topic has not decreased over several years. This is explained by the rapid development of RFID technology itself, the mastering of new frequency ranges, and the regular emergence of new products and new applications where the contactless identification technology (or Radio Frequency Identification - RFID) allows solving tasks that were previously unmanageable for technical and software tools.

From Chaos to Order, or the History of the Matter

Radio frequency identification technology emerged about 20 years ago and has developed throughout this period at a pace surpassing that of computer technologies. RFID has improved especially intensively over the past 5-7 years. This can be explained by two factors: first, the development of microelectronics made it possible to implement many ideas previously inaccessible due to purely technological reasons; and second, standards have appeared whose application ensured compatibility of technical solutions from different manufacturers. Before considering specific issues of using contactless identification in various fields of human activity, let us focus on the general principles of RFID systems and the regulatory documents that define and will define the course of design thinking in the near future.

Fundamentals of Technology

For those unfamiliar with RFID technology, we will briefly outline its essence. The physical principles (at least for most frequency ranges) resemble the operation of a transformer or a system of coupled circuits. As is known, if you take two coils and place them not too far from each other, they will exert mutual influence on each other (Fig. 1).

Fig. 1. Operating principle of the "reader-identifier" pair

The reader contains a high-frequency generator G, which powers the reader antenna Lc. Due to the presence of electromagnetic coupling M between the reader antenna and the identifier (card) antenna LK, an alternating voltage is induced in the latter, the magnitude of which depends on the design and the distance between the card and the reader. The induced voltage is used to power the card microchip DK through a rectifier formed by diode VDп and a filtering capacitor Сф. The card microchip DK modulates the voltage in the antenna IK by shunting it with resistor Кш. Due to the coupling of the antennas, the modulation appears in the reader antenna Lc, is detected by diode VDд, and is fed to the reader microchip Dc, which decodes the card code and sends it to the controller via the interface Int. This principle was used in the first passive R/O (Read Only) Proximity cards and readers. Later, identifiers were created that could not only transmit information to the reader but also receive it for programming purposes (writing information to non-volatile memory). From the standpoint of the basic principles of RFID system construction, a modulator appeared in the reader, which modulated the carrier emitted by the reader, and in the card—a detector and reprogrammable non-volatile memory, into which the information transmitted by the reader was written (Fig. 2). Identifiers (cards) using this technology are already called R/W (Read Write), meaning "reading and writing."

Fig. 2. RFID system with Read/Write technology

The first industrial RFID systems were established in the frequency range of 125 kHz. However, with the growing demand for the volume of information transmitted in a short time, higher frequency systems were developed, in particular those operating in the 13.56 MHz range.

Regardless of the frequency range and encoding method, the design of cards operating with RFID technology is approximately the same, as shown in Fig. 3.

Fig. 3. Proximity card device

From the principle of operation of the "card - reader" pair, the conclusion is unequivocal: the greater the reading range we want to ensure, the larger the reader will be and the higher the emitted power must be. For a rough estimate of the potential range of passive RFID systems in the 125 kHz and 13.56 MHz bands, one can base it on the fact that the maximum reading distance of the card code equals the diagonal of the reader's antenna. If anyone tries to convince you otherwise – don't believe it!

Frequencies and standards

To meaningfully engage with all the subsequent material, it is necessary to consider the frequency ranges of RFID systems and the main standards to which practically all modern developments in this field adhere. Let's start with the frequencies. Today, RFID occupies four frequency ranges: 125 kHz, 13.56 MHz, 800...900 MHz, and 2.45 GHz. It is worth noting immediately that the 800...900 MHz range is used much less frequently than the other three, so we will not dwell on it in more detail.

What explains the choice of these frequency values? It is that these are exactly the values taken by the "gaps" in the frequency schedules, which today are filled to the limit for various communication systems of military and broadcasting purposes. In fact, these are the frequencies for which commercial development is allowed in most countries without obtaining permits for frequency use. For example, the 2.45 GHz range is the frequency at which Bluetooth and Wireless LAN operate, that is, wireless networks for consumer use. Naturally, each of the frequency ranges of RFID systems has quite specific characteristics, which are most clearly illustrated by the schematic graphs shown in Fig. 4.

Fig. 4. Dependence of RFID system parameters on frequency

Therefore, each range uses its own signal encoding methods in the "reader - card" pair, its own transmission speeds, and collision resolution algorithms. The anti-collision mechanism is used so that when multiple identifiers are simultaneously within the reader's field, only one necessary for the current moment can be selected for communication.

In older Proximity systems, without such a mechanism, simultaneously presenting two or more cards to the reader resulted in none of them being read. From the following material, it will become clear that many modern applications based on RFID technology simply could not function without this tool.

But let's return to standards, since it is precisely unification and standardization that have always been the driving forces allowing private solutions to be integrated into the global economy. It should be noted immediately that standardization is not an event, but a process that runs parallel to the development of technologies, but once established, standards remain in effect for a fairly long time (to be fair, it should be noted that from a certain point in time, unfortunately, they also become obstacles to progress).

So, each of the mentioned frequency ranges has its own standards with its own level of development. Their most general characteristics are more conveniently presented in tabular form (see table).
 

The table does not mention the 800...900 MHz range because it is used quite rarely and the author is unaware of the standards applicable to this range.

"Non-standard" solutions

Paradoxically, there is currently a colossal number of Proximity cards in circulation that do not comply with any of the standards considered. They were simply developed and put into use before standardization touched the RFID field. Nevertheless, in access control and management systems (ACMS), these cards still hold the main positions, so we will briefly focus on their characteristics. It should be noted immediately that almost all of them operate in the good old 125 kHz range, for which technical implementation was quite accessible even 15 years ago.

The "non-standard" solutions discussed below, by today's standards, have for many years been and to some extent still remain "de facto" standards.

Indala Cards

Indala (a Motorola division) is historically one of the first mass producers of Proximity cards and readers for access control systems.

Cards have a fixed internal card code length of 35 bits, while 26-bit format readers "cut off" the extra part of the card code when converting to the Wiegand format, whereas readers with a longer code length, such as Wiegand 44 (also known as AMicro), "dilute" the output code with bits of constant value. The type (dimension) of the output code for Indala is determined by the reader. Indala identifiers use amplitude modulation of a carrier divided in half, and the circuit implementation of the demodulator in the reader for them is one of the most complex.

HID Cards

Unlike Indala, any HID reader works in all formats.