**1.1. RFID technology and previous works**

RFID (Radio Frequency Identification) is a technology that uses radio frequency signals for purposes of identification and tracking of objects, humans or animals. In passive RFID systems, where tags reuse the energy radiated by the reader, coordination capabilities can be considerably limited [29]. This issue leads to conflicts or collisions between the transmissions of the different elements of an RFID network, i.e., readers an tags. An efficient medium access control layer (MAC) is thus crucial to the correct operation of RFID [3].

Two types of RFID MAC collision can be identified: tag and reader collision. A tag collision arises when several tags simultaneously respond to a given reader request, thus causing the loss of all the transmitted information. To address this issue, tag anti-collision schemes such as ALOHA and binary tree algorithms are commonly employed [3, 31]. Improvements on these solutions have been further proposed by using tag estimation methodologies [14], and modified frame structures [3, 30], among many other approaches in the literature. Two types of reader collision can also be identified: multiple-reader-to-tag collision and reader-to-reader collision [2]. To address these two issues, reader anti-collision algorithms based on scheduling or coverage control have been proposed. Typical scheduling schemes are frequency division multiple access (FDMA) [7] or listen-before-talk (LBT) [8]. Advanced schemes such as Colorwave in [28] and Pulse in [2] implement inter-reader control mechanisms to assist in the collision resolution process. Other approaches such as HiQ in [11] use an analysis of collision patterns over consecutive time-slots to improve scheduling policies. Regarding coverage-based algorithms, two types of scheme can be commonly found: those that reduce the overlapping coverage area between readers (e.g., [12]), and those that monitor interference to adapt power levels accordingly (e.g., [4]).

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©2012 Sámano-Robles et al., licensee InTech. This is an open access chapter distributed under the terms of the
