Recently, RFID (Radio Frequency IDentification) has been widely used in many applications as an alternative technique of the existing bar code technique by using wireless frequency. Especially, RFID systems are spread far after the standardization in EPCglobal Class 1 Gen. 2. With increasing interests of RFID systems, there have been many challenging issues not yet solved. Especially, collisions among signals such as reader collisions and the tag collision are the typical trouble for development of the RFID systems. By resolving the collision problems, the read rate and throughput can be improved.
The tag collision problem arises when two or more tags transmit their data to the same reader simultaneously. There are two basic approaches to solve the tag collision problem: ALOHA-based protocols and Tree-based protocols. ALOHA-based tag anti-collision scheme is standardized by EPCglobal. In this thesis, the tag collision is not focused on.
Reader collisions are split into the reader-to-tag collision and the reader-to-reader collision. The reader-to-tag collision occurs in the case that the reading range among multiple readers overlaps and the readers are trying to interrogate a particular tag at the same time. In this case, the tag is not read by any reader though readers use different frequency bands, respectively.
The reader-to-reader collision occurs when a reader receives signals from a tag and other readers simultaneously. When a signal of a reader is transmitted to the other reader with a weak response signal of the tag which is previously issued by the reader concurrently, the reader cannot identify the tag. Especially, this kind of collision occurs even when the reading range among readers is not overlapping.
However, in addition to reader collisions, there is another serious problem being raised recently in RFID systems. As the number of readers increases, the available channels are relatively scarce due to the fixed number of channels. Like this, the ca...