The Approaches in Solving Passive RFID Tag Collision Problems

Radio Frequency Identification (RFID) systems are being intensively used recently for automated identification. Every object can be detected as one form of an electronic code. At the beginning, the main purpose of RFID tag usage is meant to be an improvement of barcodes. Besides the fact that an RFID tag does not need line of sight to obtain its ID, the tag is also water and dirt resistant. Moreover, it also has a read-and-writable memory chip, which can store much more data than a barcode, and is difficult to be imitated. The above are the main factors that many enterprises and government associations consider to extensively apply the RFID technology to many applications. An RFID tag is composed of two major components: an IC to store data and to handle communication processing and an attached antenna to transmit and receive radio signal. There are several types of RFID tags based on the differences of their power sources and communication methods. In general, a passive RFID tag does not have an internal power supply, and cannot work without collecting continuous wave from a reader. Oppositely, an active RFID tag has an attached battery and can communicate with other tags or reader on its own. A semi-passive tag is a mixed of above two types, which has an external battery for its operating power and yet communicates with reader in the same way as a passive tag does. In an RFID system, a reader is able to communicate with many tags within its coverage. However the tag identification process may fail when multiple tags are sending their data simultaneously. The signals from the tags may interfere with each other and hence the reader may not receive any correct data at all. If this happens, the tags will have to retransmit their data, which wastes the tag reading time and hence degrades the system performance. Such a problem is often called “tag collision” in an RFID system. To overcome the tag collision problem, researchers are still looking for the most effective anti-collision method to achieve high speed detection with nearly 100% data accuracy ID retrieval. The collision problems are usually classified into two types: the reader collision problems and tags collision problems (Burdet 2004; Dong-Her Shih 2006; Okkyeong Bang 2009). In this chapter, we focus on the latter one. The tag collision problems are in conjunction with the anti-collision protocols used in various RFID systems, of which the objective is to retrieve a tag’s ID accurately with low transmission power, low computational complexity, and minimum time delay. In the