Tag Identification Protocols in RFID Systems

Introduction

An RFID system consists of radio frequency (RF) tags attached to objects that need to be identified and one or more networked electromagnetic readers. The great appeal of RFID technology is that it allows in-formation to be stored and read without requiring either contact or a line of sight between the tag and the reader. For this contact-less feature, RFID technology is an attractive alternative to bar code in the distribution industry and supply chain, since it can hold more data. Two kinds of tags are possible in RFID systems: active or passive. Active tags have storage capabilities and are provided with power sources for computation and transmission. The complexity and cost of mounting a power source onto a tag, the size/weight of a battery, and the difficulty of recharging it, limit the applicability of active tags: these are not practical for use with disposable consumer products, for instance. Passive tags instead rely only on RF energy induced by the electromagnetic waves emitted by the reader. In a typical communication sequence, the reader emits a continuous radio frequency wave. When a tag enters in the RF field of the reader, it receives energy from the field, for modulating the signal according to its stored data. Continuous advancements in protocols and circuit design, make the reliability and the read range of passive tags RFID systems continuously improving. Besides, their cost continues to decrease, thus leading to an increase of their applications.

In RFID systems, a reader recognizes objects through wireless communications with tags, and it must be able to identify all tags as quickly as possible. However, signals of readers or tags may collide, since a shared wireless channel is used. Collisions are divided into reader collisions and tag collisions. The reader collisions problem (Engels et al. 2002), (Waldrop et al. 2003) occurs when two or more readers communicate on the same frequency at the same time (reader-to-reader frequency interference), or when neighboring readers attempt to query the same tag simultaneously (multiple reader-to-tag interference). Frequency interference is avoided by having readers operating on different frequency bands. Multiple reader-to-tag interference can be avoided only by having neighboring readers operating at different times or frequencies. A simple and distributed reader anti-collision protocol is presented in (Waldrop et al. 2003). Reader collisions can be easily solved, since RFID readers, that are entities with abundant memory and computation power, can detect collisions.

The problem approached in this chapter is the tag collision one, that occurs when several tags try to answer at the same time to a reader query. The reader queries the tags for their ID by broadcasting a request message. Upon receiving such message, all tags send an answer back to the reader. If only one tag answers, the reader identifies the tag. If more than one tag answer, their messages will collide on the RF communication channel, and the reader cannot identify these tags. Given the low functional power and energy constraints in each tag, it is unreasonable to assume that tags can communicate with each other directly, and that they can notice their neighboring tags or detect collisions. At the beginning the reader does not know anything about the tags. Each tag i Є 1, ..., n has a unique ID string in 0, I^k, where k is the length of the ID string. Tags anti-collision protocols are required for identification in RFID systems, they specify the algorithms for the reader and the tags so that the reader can collect all the tag IDs. There are two families of protocols for approaching the tag collision problem (Klair et al. 2010): a family of probabilistic protocols, and a family of deterministic ones. The first ones are Aloha-based protocols (Abramson 1970); (Bonuccelli et al. 2006); (Cha & Kim 2006); (EPC standard 2005); (Lee et al. 2005); (Peng et al., 2007); (Schoute 1983); (Vogt 2002); (Wieselthier et al. 1989), the last ones are tree-based protocols (Chiang et al. 2006);(Choi et al. 2005); (Law et al. 2000); (Myung & Lee 2006); (Myung et al. 2006;2007);(Zhang & Vojcic 2005). In addition, there are also hybrid approaches, where randomization is applied in tree schemes (Hush & Wood 1998); (Micic et al. 2005); (Ryu et al., 2007).