Trade-off between maximum cardinality of collision sets and accuracy of RFID reader-to-reader collision detection

As the adoption of the radio-frequency identification (RFID) technology is increasing, many applications require a dense reader deployment. In such environments, reader-to-reader interference becomes a critical problem, so the proposal of effective anti-collision algorithms and their analysis are particularly important. Existing reader-to-reader anti-collision algorithms are typically analyzed using single interference models that consider only direct collisions. The additive interference models, which consider the sum of interferences, are more accurate but require more computational effort. The goal of this paper is to find the difference in accuracy between single and additive interference models and how many interference components should be considered in additive models. An in-depth analysis evaluates to which extent the number of the additive components in a possible collision affects the accuracy of collision detection. The results of the investigation shows that an analysis limited to direct collisions cannot reach a satisfactory accuracy, but the collisions generated by the addition of the interferences from a large number of readers do not affect significantly the detection of RFID reader-to-reader collisions. Introduction Radio-frequency identification (RFID) is increasingly being used in industries and infrastructures for the purpose of automatic identification and tracking [1]. An RFID system includes some RFID readers and many tags. A reader can query tags by means of a wireless communication. The majority of the RFID systems operate at ultrahigh frequency (UHF). RFID is used for many applications, such as traceability [2], item removal detection [3], anti-counterfeit [4] and positioning [5], and for the establishment of smart environments, such as smart retailers [6], smart hospitals [7], and smart universities [8]. Although the need of covering large areas has been partially satisfied by using MIMO RFID readers [9], the majority of the RFID applications, especially the largest, require a dense reader deployment, where RFID readers operate in close proximity. Consequently, UHF RFID systems are easy to suffer from the interference generated during simultaneous interrogation activities [10]. In this *Correspondence: [email protected] †Equal contributors Dipartimento di Automatica e informatica, Politecnico di Torino, Corso Duca degli Abruzzi, 24 10129 Torino, Italy case, a reader can suffer a reader-to-reader collision due to the interference generated by the simultaneous operations of other RFID readers [11]. When the reader queries a tag, the reader-to-reader interference is too strong with respect to the weak signals received from the tag, thus compromising the interrogation. In recent years, many RFID reader-to-reader anticollision protocols have been proposed. The European standard for UHF RFID communicationa proposes listen before talk, an anti-collision protocol based on carrier sense multiple access (CSMA). PULSE is a subsequent CSMA approach that attempts at increasing the throughput by using an additional control channel [12]. The first approach based on time division multiple access (TDMA) is Colorwave [13], which provides a simple and distributed mechanism for scheduling the query sections and is suitable for low-cost readers. More recent techniques have been proposed in order to improve the performance of Colorwave: in [14], a probabilistic parameter improves the collision resolution, and in [15], an adaptable and selfish algorithm strongly increases throughput. In the Neighbor Friendly Reader Anti-collision (NFRA) protocol © 2013 Zhang et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Zhang et al. EURASIP Journal on Embedded Systems 2013, 2013:10 Page 2 of 14 http://jes.eurasipjournals.com/content/2013/1/10 [16], a central server manages a contention among readers to schedule the query sections. NFRA provides high throughput, but it is not suitable to low-cost devices. The same technique has been enhanced in [17], where fairness among readers is improved by giving more opportunities of querying tags to the readers in the densest areas, and in [18], where the readers are scheduled in the contention phase according to the geometric distribution in order to reduce the quantity of empty time slots. Besides, the researches in [19,20] provide a novel and prospective approach to limit the reader-to-reader interference by separating the transmission phase and listening phase of the RFID readers. Although RFID reader-to-reader collisions are a relevant problem, an established method for the analysis and evaluation of reader-to-reader anti-collision protocols does not exist (e.g., [21,22]). The characteristics of the employed interference model are particularly relevant. Even considering the same deployment and the same attempts to query tags, two different models may detect different collisions. Single interference models only consider direct collisions, where the high-power transmission of a reader interferes with the low-power answers of tags to another reader. These models are easy to implement and provide rapid simulations. Additive models do not limit the analysis to direct collisions but consider the sum of the interferences from a group of readers. They are more similar to the real behavior of RFID networks but require more computational effort. This paper investigates the characteristics of the additive interference models for detecting RFID reader-toreader collisions. In particular, the effects of the quantity of readers involved in the collisions (i.e., the cardinality of the collision set) are analyzed. The goal of the paper is to identify to which extent the collisions detected with a specific cardinality affect the accuracy of the results, in order to establish whether all the additive components must be considered for an accurate result, or instead if it is possible to limit the analysis without considering a part of the collision sets. A preliminary analysis about the interference generated by the collision sets with different cardinality has been presented in [23]. The investigation presented in the current work is based both on an analytical analysis and on simulations. The results show that an analysis of anti-collision protocols limited to direct interferences provides a low level of accuracy since many collisions are not detected. However, few collisions are due to collision sets with high cardinality, so the models used for the evaluation of RFID reader-to-reader anti-collision protocols can be limited to small collision sets. The next section describes the state of the art of the RFID interference models. The ‘Experimental setup Experimental setup’ section illustrates the proposed evaluation algorithm and describes the considered scenario. Data obtained from the evaluation are presented in the ‘Experimental results’ section. Final comments are made in the ‘Conclusion’ section. Related works An established dichotomy among the reader-to-reader interference models regards the cardinality of the set of colliding readers [24]. The single interference models assume that all the collisions involve pairs of readers that query tags at the same time. Given a pair of readers, it is always possible to determine if they collide or not independently of the activity of the other readers in the network. Considering the set A of readers in the network, the list of pairs of readers that collide when transmitting simultaneously is a binary relation R on the set A. Since the relation R is a subset of the Cartesian product A × A, it corresponds to a graph, called collision graph. The nodes of this graph are the elements of A, and an edge exists between two nodes x and y if (x, y) ∈ R. On the contrary, the additive interference models evaluate the sum of the power of all the signals received by a reader in order to determinate if a collision occurs. Considering, for example, three readers x, y, and z, according to the single interference models, if x is placed far enough, it is disturbed neither by y nor by z. However, in the additive interference models, if all the three readers query tags at the same time, the combination of the signals emitted by y and z can be powerful enough to interfere with the transmission of x. Therefore, it is not possible to build a single collision graph, i.e., to identify a priori the readers that interfere among them. Instead, for a reader x, a plurality of collision sets exists: each collision set groups the readers that interfere with x only if all of them transmit at the same time. In the following, the main interference models are reviewed. In the adopted notation, reader x is the one that queries tags and that can be subject to reader-toreader interference from other readers in the network. The power of the signal emitted by reader i is denoted as Pi. This signal propagates in the space, and it decays with distance: the signal that arrives at reader j has power Pi,j, with Pi,j < Pi. Single interference models Single interference models consider only the interference between pairs of readers. Themain examples of this family of models are described in the following. Disk graphmodel This model [25] assumes that the readers are equipped with an omnidirectional antenna: their signal propagates in the same way along all the directions. Due to the path loss, there is a specific distance d beyond which the signal Zhang et al. EURASIP Journal on Embedded Systems 2013, 2013:10 Page 3 of 14 http://jes.eurasipjournals.com/content/2013/1/10 emitted by a reader is not powerful enough to feed the circuitry of a tag. That distance is called interrogation range: all the tags that a reader can identify are located within a circle of radius d, whose center is the reader itself. Formally, in the disk graph model, a reader can identify a tag if the following condition holds: