Selection Criteria for RFID Systems
There has been an enormous upsurge in the popularity of RFID systems in recent years. The best example of this phenomenon is the contactless smart cards used as electronic tickets for public transport. Five years ago it was inconceivable that tens of millions of contactless tickets would now be in use. The possible fields of application for contactless identification systems have also multiplied recently.

Developers of RFID systems have taken this development into account, with the result that countless systems are now available on the market. The technical parameters of these systems are optimised for various fields of application – ticketing, animal identification, industrial automation or access control . The technical requirements of these fields of application often overlap, which means that the clear classification of suitable systems is no simple matter. To make matters more difficult, apart from a few special cases (animal identification, close-coupling smart cards), no binding standards are as yet in place for RFID systems.

It is difficult even for a specialist to retain an overview of the range of RFID systems currently on offer. Therefore, it is not always easy for users to select the system best suited to their needs.

In what follows there are some points for consideration when selecting RFID systems. 

Smart card OS,cryptographic coprocessor
Smart card OS
Authentication, encryption (state machine)
Read-write EAS
Read-only EAS Fixed code transponder

Passive transponder 135 kHz, 13.56 MHz, 868/915 MHz, 2.45 GHz ISO 15693, ISO 18000 ISO 14223
Active transponder 868/915 MHz 2.45 GHz ISO 18000
ISO 14443 contactless smart card 13.56 MHz
ISO 14443 dual interface smart card

Operating Frequency
RFID systems that use frequencies between approximately 100 kHz and 30 MHz operate using inductive coupling. By contrast, microwave systems in the frequency range 2.45–5.8GHz are coupled using electromagnetic fields.

The specific absorption rate (damping) for water or nonconductive substances is lower by a factor of 100 000 at 100 kHz than it is at 1 GHz. Therefore, virtually no absorption or damping takes place. Lower-frequency RF systems are primarily used due to the better penetration of objects (Sch¨urmann, 1994). An example of this is the bolus, a transponder placed in the omasum (rumen) of cattle, which can be read from outside at an interrogation frequency of <135 kHz.

Microwave systems have a significantly higher range than inductive systems, typically 2–15m. However, in contrast to inductive systems, microwave systems require an additional backup battery. The transmission power of the reader is generally insufficient to supply enough power for the operation of the transponder.

Another important factor is sensitivity to electromagnetic interference fields, such as those generated by welding robots or strong electric motors. Inductive transponders are at a significant disadvantage here. Microwave systems have therefore particularly established themselves in the production lines and painting systems of the automotive industry. Other factors are the high memory capacity (up to 32 Kbyte) and the high temperature resistance (up to 250 ◦C) of microwave systems (Bachthaler, 1997).

The required range of an application is dependent upon several factors:
the positional accuracy of the transponder;
the minimum distance between several transponders in practical operation;
the speed of the transponder in the interrogation zone of the reader.
For example, in contactless payment applications – e.g. public transport tickets – the positioning speed is very low, since the transponder is guided to the reader by hand. The minimum distance between several transponders in this case corresponds to the distance between two passengers entering a vehicle. For such systems there is an optimal range of 5–10cm. A greater range would only give rise to problems in this case, since several passengers’ tickets might be detected by the reader simultaneously. This would make it impossible to allocate the ticket reliably to the correct passenger.

Different vehicle models of varying dimensions are often constructed simultaneously on the production lines of the automotive industry. Thus great variations in the distance between the transponder on the vehicle and the reader are pre-programmed (Bachthaler, 1997). The write/read distance of the RFID system used must therefore be designed for the maximum required range. The distance between the transponders must be such that only one transponder is ever within the interrogation zone of the reader at a time. In this situation, microwave systems in which the field has a directional beam offer clear advantages over the broad, nondirectional fields of inductively coupled systems.

The speed of transponders, relative to readers, together with the maximum write/read distance, determines the length of time spent in the reader’s interrogation zone. For the identification of vehicles, the required range of the RFID system is designed such that, at the maximum vehicle speed, the length of time spent in the interrogation zone is sufficient for the transmission of the required data.