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A look at RF identification technology

Posted: 17 Mar 2003 ?? ?Print Version ?Bookmark and Share

Keywords:rfid system? radio technology? far-field communication? near-field communication? dipole-antenna?

Radio frequency identification (RFID) is an application of radio technologies in automatic identification area. It comprises an electronic tag and a reader. Generally, the tag contains some electronic data in one of preset formats, and is attached on the surface of the object that needs to be identified in later logistic processes. The reader, also called "reading device", can read out and identify the data saved in the tag so that the object can be automatically identified. The read-out data can be further collected, processed and transmitted by computers or networks.

RFID technologies can be divided into two sections in terms of their operating frequencies:

1. Lower-frequency system: the operating frequencies are lower than 30MHz, and some of the standard frequencies are 125kHz, 225kHz, and 13.56MHz. The RFID systems based on these frequencies usually have well-established international standards to follow. It features lower cost tags, fewer data saved in the tags, shorter reading range, diverse shapes of tags (card, ring, button, and pen shapes) and non-directional antennas.

2. Higher-frequency system: the operating frequencies are higher than 400MHz, with operating frequencies such as 915MHz, 2.45GHz, and 5.8GHz. The system has a longer reading range (suitable for moving objects or multiple-tag packages that need to be identified very fast) and more directional antennas, both in readers and electronic tags.

Critical theories realization

RFID systems are based on the following critical theories and technologies:

1. The communication between tags and readers carries out via electromagnetic wave. Far-field and near-field communications are according to their distances. They also have different communication mechanisms from dipole-antenna radiation patterns derived by solving the Maxwell equation.

2. The coding formats used in the reader's and tag's digital baseband circuits are directly related to some critical concerns like anti-collision of signals, power spectrum utilization and digital clock recovery.

3. There is also a concern regarding the anti-conflict problem in the RFID systems, that is, how to correctly identify the multiple tags individually. Many algorithms and technologies have to be considered in solving the problem.

In an RFID system, near field is the area where the distance between tag and reader is less than one wavelength of the communication carrier. By solving the Maxwell equation regarding dipole-antenna radiation pattern, the energy in near field exists as either electric field or magnetic field, and can be considered as a quasi-static field. Therefore, the near-distance LF communication in an RFID system is completed by the quasi-static field coupling.

Far field, on the other hand, means that the distance between tag and reader is longer than the wave length of the carrier. Far-field communication is often used in 915MHz and 2.4GHz RFID systems. Their reading range can be several meters, while the carrier wavelength is only several centimeters longer. According to the related theory, the far field in a dipole radiation pattern is a radiation field.

In the situation of far field, the communication of a long distance HF RFID system is completed by electromagnetic wave's coupling and reflection between tag's and reader's antennas, called reflection modulation technology. After the electromagnetic wave is transmitted into the air around, some energy of the microwave that reaches the target is absorbed by the tag and the remaining energy of that is reflected from the tag into all the directions at different levels of power.

Some of the reflected energies find their way back to the transmitting antenna. When the tag's antenna resistance matches with its receiving circuit, most of the energy reaching the tag is absorbed; but if it does not match, the energy is reflected. So, the reader can tell the 0/1 signals of the tag by receiving the reflected wave which changes according to the tag's resistance.

- Dr. Hao Min


Shanghai Huahong Integrated Circuit Co. Ltd

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