Differentiation Features of RFID Systems
Fundamental Differentiation Features
RFID systems exist in countless variants, produced by an almost equally high number of manufacturers. If we are to maintain an overview of RFID systems we must seek out features that can be used to differentiate one RFID system from another (Figure 2.1).

RFID systems operate according to one of two basic procedures: full-duplex (FDX)/half-duplex (HDX) systems, and sequential systems (SEQ).

In full-duplex and half-duplex systems the transponder’s response is broadcast when the reader’s RF field is switched on. Because the transponder’s signal to the receiver antenna can be extremely weak in comparison with the signal from the reader itself, appropriate transmission procedures must be employed to differentiate the transponder’s signal from that of the reader. In practice, data transfer from transponder to reader takes place using load modulation, load modulation using a subcarrier, and also (sub)harmonics of the reader’s transmission frequency.

In contrast, sequential procedures employ a system whereby the field from the reader is switched off briefly at regular intervals. These gaps are recognised by the transponder and used for sending data from the transponder to the reader. The disadvantage of the sequential procedure is the loss of power to the transponder during the break in transmission, which must be smoothed out by the provision of sufficient auxiliary capacitors or batteries.

The data capacities of RFID transponders normally range from a few bytes to several kilobytes. So-called 1-bit transponders represent the exception to this rule. A data quantity of exactly 1-bit is just enough to signal two states to the reader: ‘transponder in the field’ or ‘no transponder in the field’. However, this is perfectly adequate to fulfil simple monitoring or signalling functions. Because a 1-bit transponder does not need an electronic chip, these transponders can be manufactured for a fraction of a penny. For this reason, vast numbers of 1-bit transponders are used in electronic article surveillance (EAS) to protect goods in shops and businesses. If someone attempts to leave the shop with goods that have not been paid for the reader installed in the exit recognises the state ‘transponder in the field’ and initiates the appropriate reaction. The 1-bit transponder is removed or deactivated at the till when the goods are paid for.

The possibility of writing data to the transponder provides us with another way of classifying RFID systems. In very simple systems the transponder’s data record, usually a simple (serial)

number, is incorporated when the chip is manufactured and cannot be altered thereafter. In writable transponders, on the other hand, the reader can write data to the transponder. Three main procedures are used to store the data: in inductively coupled RFID systems EEPROMs (electrically erasable programmable read-only memory) are dominant. However, these have the disadvantages of high power consumption during the writing operation and a limited number of write cycles (typically of the order of 100000–1000000). FRAMs (ferromagnetic random access memory) have recently been used in isolated cases. The read power consumption of FRAMs is lower than that of EEPROMs by a factor of 100 and the writing time is 1000 times lower. Manufacturing problems have hindered its widespread introduction onto the market as yet.

Particularly common in microwave systems, SRAMs (static random access memory) are also used for data storage, and facilitate very rapid write cycles. However, data retention requires an uninterruptible power supply from an auxiliary battery.

In programmable systems, write and read access to the memory and any requests for write and read authorisation must be controlled by the data carrier’s internal logic. In the simplest case these functions can be realised by a state machine (see Chapter 10 for further information). Very complex sequences can be realised using state machines. However, the disadvantage of state machines is their inflexibility regarding changes to the programmed functions, because such changes necessitate changes to the circuitry of the silicon chip. In practice, this means redesigning the chip layout, with all the associated expense.

The use of a microprocessor improves upon this situation considerably. An operating system for the management of application data is incorporated into the processor during manufacture using a mask. Changes are thus cheaper to implement and, in addition, the software can be specifically adapted to perform very different applications.

In the context of contactless smart cards, writable data carriers with a state machine are also known as ‘memory cards’, to distinguish them from ‘processor cards’.

In this context, we should also mention transponders that can store data by utilising physical effects. This includes the read-only surface wave transponder and 1-bit transponders that can usually be deactivated (set to 0), but can rarely be reactivated (set to 1).

One very important feature of RFID systems is the power supply to the transponder. Passive transponders do not have their own power supply, and therefore all power required for the operation of a passive transponder must be drawn from the (electrical/magnetic) field of the reader. Conversely, active transponders incorporate a battery, which supplies all or part of the power for the operation of a microchip.

One of the most important characteristics of RFID systems is the operating frequency and the resulting range of the system. The operating frequency of an RFID system is the frequency at which the reader transmits. The transmission frequency of the transponder is disregarded. In most cases it is the same as the transmission frequency of the reader (load modulation, backscatter). However, the transponder’s ‘transmitting power’ may be set several powers of ten lower than that of the reader.

The different transmission frequencies are classified into the three basic ranges, LF (low frequency, 30–300kHz), HF (high frequency)/RF radio frequency (3–30MHz) and UHF (ultra-high frequency, 300MHz–3GHz)/microwave (>3 GHz). A further subdivision of RFID systems according to range allows us to differentiate between close-coupling (0–1cm), remote-coupling (0–1m), and long-range (>1m) systems.

The different procedures for sending data from the transponder back to the reader can be classified into three groups: (i) the use of reflection or backscatter (the frequency of the reflected wave corresponds with the transmission frequency of the reader → frequency ratio 1:1); or (ii) load modulation (the reader’s field is influenced by the transponder → frequency ratio 1:1); and (iii) the use of subharmonics (1/n-fold) and the generation of harmonic waves (n-fold) in the transponder.