A beacon analysis-based RFID reader anti-collision protocol for dense reader environments

Accepted Manuscript

A Beacon Analysis-Based RFID Reader Anti-Collision Protocol for Dense Reader Environments Ali Assarian , Ahmad Khademzadeh , Mehdi Hossein zadeh , Saeed Setayeshi PII: DOI: Reference:

S0140-3664(18)30056-2 10.1016/j.comcom.2018.06.006 COMCOM 5715

To appear in:

Computer Communications

Received date: Revised date: Accepted date:

19 January 2018 19 May 2018 13 June 2018

Please cite this article as: Ali Assarian , Ahmad Khademzadeh , Mehdi Hossein zadeh , Saeed Setayeshi , A Beacon Analysis-Based RFID Reader Anti-Collision Protocol for Dense Reader Environments, Computer Communications (2018), doi: 10.1016/j.comcom.2018.06.006

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A Beacon Analysis-Based RFID Reader Anti-Collision Protocol for Dense Reader Environments Ali Assarian Department of Computer Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran

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Ahmad Khademzadeh* IT Faculty, Iran Telecommunication Research Center, Tehran, Iran Mehdi Hosseinzadeh Iran University of Medical Sciences, Tehran, Iran & Computer Science, University of Human Development, Sulaimaniyah, Iraq Saeed Setayeshi Amirkabir University of Technology, Tehran, Iran

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* Corresponding Author, e-mail: [email protected]

Abstract

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Radio-frequency identification (RFID) is one of the major technologies for auto-identification in many applications. When multiple readers work together to improve the read rate and reliability, a dense RFID network is created. One of the most important issues to be considered in such environments is reader collision, which leads to reduced throughput. Although different schemes have been proposed to reduce reader collision, they often either require additional hardware or do not use network resources efficiently, or are not consistent with international laws and standards. In this paper, by managing and analyzing beacon messages, a new centralized scheme, which is consistent with European standards and laws, is proposed to reduce collisions and increase throughput. In the proposed scheme, by sending a priority code via a beacon message and making decisions based on the beacon messages received by readers, the available resources are used optimally, and the maximum number of readers is activated in one round. Additionally, in the proposed scheme, the priority code is initialized in such a manner that the readers access the channel fairly. The simulation results and comparison of the proposed scheme with other protocols indicate that throughput increased. Because of the increased throughput and fairness of the proposed scheme, the delay reduced compared with other schemes, and overall, the proposed scheme was superior to other centralized schemes.

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Keywords: Dense RFID System; Beacon message; Reader collision; Resource allocation Introduction

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Radio-frequency identification (RFID) is one of the key elements in the implementation of ubiquitous computing because of features such as flexibility, extensibility, and automatic detection capabilities. Readers and tags are the main components of an RFID system. Wireless readers read the identifier of tags attached to an object and provide access to the object's information by sending the identifier to a backend system. At present, RFID is used in many areas because of its various benefits and features [1]. One of the most important applications of this technology is its use in smart cities. For example, RFID technology is used in the management of smart parking spaces (SPSs). Many

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existing SPSs either require expensive equipment or are semi-automated and require some human interaction. However, use of RFID enables implementation of a fully automatic SPS management system with much lower costs. The arrival and departure of cars and the management of empty parking spaces can be intelligently controlled using RFID techniques [2]. Another practical application of RFID technology is in training management systems (TMSs). Using this technology, the attendance of the students (i.e., the roll call) can be recorded automatically, their motivation for learning can be enhanced, and the teacher can monitor the performance of the classroom in real time [3]. Other RFID applications include the control and monitoring of perishable foodstuffs [4], the Internet of Things (IoT) [5-6], healthcare monitoring [7-8] and the tracking of animals [9].

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Using RFID in diverse applications requires this system to use a large number of readers together in a small geographic area. When a large number of readers are deployed next to each other to increase the read rate of tags and provide better coverage of the environment, a dense RFID system is formed. The formation of a dense RFID system creates new challenges, among which, the issue of reader collision is one of the most important. A collision in an RFID system leads to the reduction of throughput, delays, misread tags, and wasted bandwidth.

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A reader collision is categorized into two types: reader-to-tag collision (RTC) and reader-toreader collision (RRC) (Fig. 1). When two or more readers simultaneously send a query message to a tag located within their interrogation range, that tag cannot respond to any of them. Even if the frequencies of the readers are different, an RTC occurs. In this type of collision, the tag cannot properly listen to the requests sent by the readers (Fig. 1a). Given the high strength of the signal sent by a reader, in addition to a very small interrogation range, each reader also creates a relatively large interference range. If several readers that are placed in the interference range of each other use the same frequency simultaneously, RRC occurs and they disrupt each other's reading operation (Fig. 1b).

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Figure 1. Two kinds of RFID reader collision problems: (a) reader-to-tag collision (b) reader-to-reader collision

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To date, a variety of schemes have been provided to reduce the number of collisions in an RFID system. Some of these schemes use multiple access techniques to manage and allocate resources. They are generally inefficient and do not use the available resources optimally, or allocate resources unfairly to the readers. Others attempt to reduce interference and increase network performance using additional hardware. Using these schemes eliminates the advantages of the inexpensive RFID technology. Some other schemes are not consistent with international standards and cannot be implemented. Another group of proposed schemes overcomes only one of the two aforementioned types of collisions but not both simultaneously. Therefore, an efficient anti-collision scheduling technique is needed to increase the throughput of the system by preventing and reducing interference in a dense UHF RFID network.

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In this paper, a new scheduling scheme is proposed that is based on TDMA/FDMA and both controls and manages the occurrence of collisions in the network through the management and analysis of beacon messages. The novelty of the proposed scheme is that it enables maximum use of the available resources while also attempting to activate the largest possible number of readers in a single round. In the proposed scheme, when a reader receives a beacon message at the same frequency from neighboring readers after sending out a beacon message, the reader does not leave the round and will be given a second opportunity to access the channel via management and analysis of the beacon message. Another contribution of the proposed scheme is that it increases fairness in terms of access to resources. In the proposed scheme, a reader that could not access a channel will have a greater chance of accessing that channel in the next round by sending a priority code through the beacon message. The proposed scheme is centralized, consistent with general European standards and laws, and does not require any additional hardware; hence, it can be implemented in commercial RFID networks. Additionally, the

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proposed scheme is suitable for networks with fixed and mobile readers and can overcome different types of reader collisions.

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The remainder of the paper is organized as follows: In the next section, previous research is presented. The problem statement is given in Section 3. In Section 4, the proposed scheme is explained. In Section 5, the simulation results and efficiency of the proposed algorithm are discussed. Finally, the conclusion is presented in Section 6. 2. Previous research

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Previous research can be categorized from different perspectives. Schemes can be singlechannel or multichannel. Some schemes are suitable only for networks with fixed readers, whereas others are also used in networks with mobile readers. TDMA-based schemes are based on multiple access techniques in which timeslots are divided among the readers. In FDMA-based schemes, the frequency bands are divided among the readers, and the readers act simultaneously and in different frequency ranges. In CSMA-based schemes, readers first listen to the channel and, if the channel is free, they enter competition mode to access the channel. Some schemes combine multiple access techniques. Based on another categorization, the previous schemes can be divided into two groups of distributed and/or centralized schemes. In this paper, the previous schemes are presented based on this categorization.

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2.1 Distributed schemes

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In distributed schemes, readers decide to access the channel locally after exchanging information with each other. Readers typically send control messages to their neighbors within a defined range, and each reader decides to access the channel independently based on this information. Some distributed schemes are presented in the following.

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In Listen Before Talk (LBT) [10], a reader first selects a channel randomly and listens to the channel for 5 ms. If the reader identifies a stronger signal than the threshold, then it means that the channel is occupied and the reader attempts to access another channel. Otherwise, the reader begins to read the tags. LBT is an FDMA-based scheme and can overcome RRC; however, for this scheme, no mechanism has been proposed to overcome RTC.

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In [11], a distributed TDMA-based scheme called the Distributed Color System (DCS) was presented, in which the reader selects one of the timeslots randomly, and in each round, attempts to communicate with the tags at the selected timeslots. If a neighboring reader selects the same timeslot, then a collision occurs, in which case the reader selects a new timeslot and reserves it by sending a „kick‟ message at the beginning of the next round. Because of the fixed number of timeslots, the DCS reduces throughput in both dense networks and networks whose topology is changing. In [12], an upgraded version of the DCS called Colorwave was provided, which is more flexible and in which the number of timeslots is considered as a variable. In this scheme, the reader changes the duration of a round based on the number of collisions it has experienced.

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Therefore, the Colorwave scheme has higher throughput than the DCS in dynamic networks and networks whose topology is unknown; however, the DCS has higher fairness because, in this scheme, the duration of the rounds is fixed for all readers.

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In [13], a DCS-based scheme, called the Probabilistic DCS (PDCS), was presented, which, unlike the DCS, acts as a multichannel scheme. Additionally, the PDCS uses a probabilistic mechanism to increase operational throughput. In the PDCS, after a collision, readers change their timeslot with probability P and, as a result, second-generation collisions are reduced in this scheme compared with the DCS. To minimize new collisions, the optimal value for P is 0.7. One limitation of the PDCS, like the DCS, is the fixed number of timeslots. Additionally, the random selection of timeslots can increase the likelihood of a collision.

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In [14], a new scheme for improving Colorwave was presented. The performance of the Colorwave scheme is heavily influenced by the values that are considered for the variables. In the Colorwave scheme, the „upsafe’ and „downsafe’ threshold values are considered to be close to one and zero, respectively, which makes the readers change the number of timeslots only when network performance is very low. In [14], a new Colorwave configuration, called „the killer configuration,‟ was presented, in which the threshold values are initialized with a mean value, and as a result, readers display selfish behavior when attempting to access the channel. Also in [14], priority management is considered. The limitation of the scheme presented in [14] is that it is suitable only for networks with fixed readers.

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In [15], a CSMA-based scheme called the Anti-collision Protocol for RFID (APR) was presented, whose main purpose is to reduce interference by considering the remaining battery of the readers, and it is suitable for networks with mobile readers. The APR scheme uses a „tag forwarding‟ mechanism, and the CW value is determined based on the remaining battery of the readers; the lower the remaining battery of a reader, the more time is taken for it to access the channel. This feature increases the life of the network. One limitation of the APR scheme is that, over time, with the decrease in the remaining average battery life of the readers, the access time to the channel and, as a result, delays in the network, increase and because of the mobility of the tags, there is a possibility that some of the tags will not be read. 2.2 Centralized schemes

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In centralized schemes, a central server both manages and allocates resources to readers. Readers may communicate with the server wired or wirelessly. In the wireless case, and at some points, the communication frequency between the readers and the central server is considered to be different from the frequency that the readers use to read the tags. In [16], one of the most well-known centralized anti-collision schemes, called the Neighbor Friendly Reader Anti-collision (NFRA), was presented. In this scheme, readers communicate with the central server wirelessly in the 433 MHz frequency band. NFRA is a TDMA-based scheme where a polling server determines the beginning of every round broadcasting an AC

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message to the readers. The AC message carries the K value, which determines the number of timeslots of the competition. Each reader selects a number randomly between 1 and K and waits for the OC messages from the central server. OC messages are sent by the server to determine the start of the competition slot, and have a number that specifies the slot number. A reader, the whose selected number is equal to the number sent by the OC message, sends a beacon message to its neighbors, and if it does not collide, prevents its neighbors from receiving more OC messages from the server by sending an OF message. Once the server finishes sending OC messages, the server waits, and the readers who succeed in accessing the channel start to read the tags. The NFRA scheme is a single-channel scheme that, if implemented in multichannel mode, has higher efficiency. Moreover, in the NFRA scheme, fairness among the readers is not observed because readers with a lower number have a greater chance to access the channel. In [17] and [18], two NFRA-based centralized schemes, called the Geometric Distribution Reader Anti-collision (GDRA) and Distance-Based RFID Reader Collision Avoidance (DRCA), was presented, whose purpose is to multichannel and reduce the problems of this scheme. They are fully described in the next section.

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All the above methods refer to types of interference that occur because of the wireless nature of RFID networks and are inevitable, and all the methods try to control and reduce the incidence of the interference as far as possible. However, in some cases, interference is caused deliberately by an attacker with the aim of reducing the reliability, security, and privacy in the RFID network [19]. In an ultra-high frequency (UHF) passive RFID network, the RFID tags do not have any power supplies and receive their power from the readers. Tags that are activated by a reader should be able to reflect an appropriate amount of power back to the reader. One potential security threat to passive RFID networks is the case where one or more attacker tags can send a stronger signal to the reader and thus weaken the signals sent from other tags, which can then prevent the reader from decoding signals received from normal tags. Alternatively, an adversary can cause electromagnetic jamming by sending a signal at the same frequency to the reader, thus preventing the tags from communicating with the reader. Identification and detection of attacker tags to distinguish them from normal tags is a difficult task. For example, in [20], the incentive for the interference imposed by intruders in a passive RFID network is presented, and an attempt is made to resolve the problem of imposed interference and separate the attacker tags from the conventional tags using a game theoretic approach. 3

Statement of the problem

As mentioned, one of the techniques to reduce interference is to manage the allocation of resources to readers, such as time and frequency. To date, different schemes have been developed based on „resource allocation,‟ and they have aimed to reduce RRCs and RTCs. These schemes have operated as either distributed or centralized, and some of them have exclusively reduced RRC interference, whereas others have reduced both types of interference. However, inefficient „resource allocation‟ in these schemes leads to either the failure of these schemes to

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overcome both types of interference, some existing resources are left unused, or the inappropriate allocation of resources results in an increase in other types of interference.

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GDRA [17] is a centralized scheme that attempts to reduce RRC by managing time and frequency. In the GDRA scheme, a large number of fixed or mobile readers are considered. This is consistent with EPCglobal standard [21] and ETSI EN-302 208 European rules [22]. Readers collect tag information from the network level and send it via a wired or wireless connection to a central server. In this scheme, readers have two bi-static antennas [23], one for sending and one for receiving, and as a result, when sending, readers can identify readers that use the same channel.

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In the GDRA scheme, to reduce RRC, readers randomly select one of the four available frequency channels and finally, using a TDMA scheme, readers that have selected the same channel are coordinated. Like the NFRA scheme, time is divided into sets of identification rounds of T duration, where each round includes two phases of „contention‟ and a reader-to-tag communication (CRT). A central server determines the beginning of every round broadcasting an AC message to the readers, and this message determines the number of slots (K) in the contention phase. When the AC message is received, readers randomly select one of the K slots and, if in the contention phase the reader wins against the set of same-frequency readers, it starts to read the tags in the CRT phase. The duration of each slot (Tslot) is considered to be 5 ms, based on EPC-ETSI.

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Unlike the NFRA scheme, in which the beginning of each slot is determined by the central server via sending the OC message, in the GDRA scheme, considering that each reader knows the duration of a slot, it can, therefore, determine the beginning and end of a slot based on an internal clock. By selecting slot k, a reader avoids listening to the channel up to the (k-2)th slot to save energy. Additionally, in contrast to the NFRA scheme, where readers use the uniform distribution function to select the slots, in the GDRA scheme, readers select one of the slots based on the „sift distributed function‟ [24]. This mechanism helps to increase the likelihood of a unique reader selecting one of the lower slots. Therefore, if many readers compete with each other, most select higher slots and few select lower slots, which results in less interference in the initial slots of the frame. Additionally, in the GDRA scheme, unlike the NFRA scheme, readers do not use a special channel to send the beacon message. Also in the GDRA scheme, the beacon message is sent in the same channel that is randomly selected by the reader. Figure 2 shows an example of the GDRA protocol. After selecting the timeslots, readers that have selected the first timeslot send a beacon message. In the GDRA scheme, if the beacon message does not result in a collision, then the reader enters the CRT phase and starts to read the tags (as in the R3 and R7 readers). However, if sending the beacon message results in a collision, then readers abandon the channel, randomly select another channel, and wait for the next round. As shown in Fig. 2, the R1 and R8 readers, which have selected the third slot, listen to the channel in the second slot, and because they do not receive a signal, assuming the channel is free

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and send the beacon in the third slot in the same frequency channel. In this mode, sending the beacon message leads to a collision and, as a result, these two readers abandon the channel, select another random channel, and wait for the next round. As a result, frequency channel 2 is abandoned and left unused, and the resources are wasted. Additionally, the R1 and R8 readers, despite being potentially able to read with two different frequency channels, stay inactive until the next round because of the collision. This results in fewer readers are becoming active in one round and, as a result, a delay in the reading of tags, or because of the mobility of readers, the opportunity to read the tag is lost.

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In the GDRA scheme, the reader that selects the slot k listens to the channel on the (k-1)th slot. If the channel is occupied in this slot because of a successful reader transmission or the sending of a beacon message, the reader abandons the channel, randomly selects another channel, and waits for the next round. As shown in Fig. 2, reader 2, which has selected timeslot number 3, listens to the channel in slot 2. Considering that reader 2 is in the interference range of reader 7, and this reader is successfully reading the tags in the same frequency channel (channel 3), reader 2 abandons the channel and waits for the next round. This is a second problem with the GDRA scheme: despite the fact that it is a multichannel scheme, it does not use this feature properly because it is possible for reader 2 to read the tags with another channel without the occurrence of any interference. However, this issue is not considered in this scheme, and the readers abandon the channel once they determine that it is busy.

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The DRCA [18] is a distance-based scheme that attempts to overcome the second problem of the GDRA, to some degree. In the DRCA scheme, when a reader selects slot k, it listens to the channel in the (k-1)th slot. However, unlike the GDRA scheme, if the channel is occupied in the (k-1)th slot, the reader calculates its distance to the reader that has occupied the channel based on the received signal strength. If the calculated distance is greater than the value in a manner that the new channel selection does not result in RTC (the distance is greater than 2*Drt, where Drt is the read range), then it gives the reader a second opportunity to access the channel. In this case, the reader increases its slot to k+1 and randomly selects a new channel. As the (k+1)th slot starts, if the channel is not occupied in slot k, then the reader starts to send beacon messages, and if there is no collision between the sent beacon messages, then the reader has the opportunity to read the tags in a new channel. However, if the calculated distance is less than 2*Drt, even if the reader uses a different frequency to read the tags, RTC occurs. Thus, the reader is not allowed to try again, and has to wait until the next round.

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Figure 2. An example of GDRA

Figure 3 shows an example of the DRCA protocol, in which reader 2, which has selected slot 3, listens to the channel in slot 2. Channel 3 is already occupied by reader 7, and based on the DRCA scheme; reader 2 calculates its distance from reader 7 based on the received signal strength. The distance between the two readers is greater than 2*Drt; in fact, the distance between the two readers is such that RTC does not occur. In this case, the reader increases its slot from 3 to 4, and listens to the channel in the third slot by selecting a new channel (channel 4) to access the channel again. However, the scenario is different for reader 4. Reader 4 selects slot 6 and listens to the channel in the fifth slot. Given that the channel is being used by reader 9, reader 4

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calculates its distance to reader 9. In this case, the distance from reader 4 to reader 9 is less than 2*Drt, and even if the frequency changes, RTC occurs, and thus reader 4 is not given a second opportunity to access the channel and has to wait until the next round.

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The problem with the DRCA scheme is that, in some cases, it not only solves the second problem of the GDRA scheme, but giving a second opportunity to readers also makes the DRCA perform even worse than the GDRA. The DRCA scheme is based on the GDRA scheme, and in GDRA, slots are selected using the sift function. The probability of selecting a slot based on the sift function is obtained from (1)

where α is a constant value between zero and one, and δ is a constant value calculated by )

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Where K is the number of slots. Therefore, with respect to relation 1, the higher the k (which is between 1 and K), the greater the likelihood of its selection by the readers. This means that the possibility of readers selecting slots with higher numbers is high in the DRCA and GDRA schemes. By contrast, the DRCA scheme pushes readers that are unable to occupy a channel, if they have the necessary conditions, by giving them a second opportunity to select higher slots (i.e., slots with a higher number), which means that the probability of interference in higher slots increases, and eventually, the channel is abandoned, remains unused, or more readers are inactive.

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As shown in Fig. 3, in the DRCA scheme, the R2 reader is given a second opportunity. The R2 reader listens to channel 4 again in the third slot, and because it does not receive a signal, like the R5 and R6 readers, with the start of the fourth slot, it starts to send a beacon message in channel 4. In this case, the beacon message sent from R2 collides with the beacon messages sent by R5 and R6, and not only does the R2 reader fail to read the tags, but also the R5 and R6 readers because they identify the collision. In this case, channel 4 remains unused, and a higher number of readers remain inactive until the next round. By contrast, in the GDRA scheme, according to Fig. 2, because the R5 and R6 readers are not within the interference range of each other, they actively read the tags in the same channel. In the DRCA scheme, in addition to the fact that the likelihood of a collision in the higher slots increases, the first problem with the GDRA scheme also remains.

Figure 3. An example of DRCA

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Another problem with the DRCA scheme is that, with respect to the algorithm presented in this paper, readers that have the opportunity to access the channel again are allowed to randomly select one of the four existing channels. Therefore, with a 25% probability, a reader may select its previous channel and, to access the channel again, listen in the previous channel. However, this wastes energy since it causes the reader to make a futile attempt at accessing the channel. This research aims to propose a scheme that, in addition to overcoming the problems of the GDRA and DRCA schemes, allocates resources to readers in such a manner that the maximal use

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of available resources is ensured. The proposed scheme also attempts to activate the largest number of readers in one round so that the readers read tags with minimal interference. 4

Proposed scheme

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The main purpose of the proposed scheme is to prevent resource waste through proper resource management. By presenting a beacon-based TDMA/FDMA technique, the proposed scheme allocates the available resources to the readers in such a manner as to minimize interference while the maximum number of readers remains active. Additionally, compared with the GDRA and DRCA schemes, the proposed scheme does not impose any hardware redundancy. The proposed scheme, called Beacon Analysis-based Collision Prevention (BACP), analyzes beacon messages to reduce interference among the readers in dense reader environments.

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To better describe the BACP protocol, we consider a dense RFID network with a large number of fixed/mobile readers, in which the readers operate according to the EPCglobal Class-1 Gen-2 standard. Readers communicate with a central server wired or wirelessly, and the central server announces the start of each round by releasing an AC message to the readers. The duration of each AC is considered to be 2.83 ms. Each round is divided into two main phases: the contention phase, in which readers compete with each other to access the channel by sending a beacon message, and CRT phase, in which the reader connects with the tags if it accesses the channel successfully [25-26]. The contention phase is divided into K slots, in which the duration of each slot is Tslot = 5 ms. Readers are equipped with a bi-static antenna and can, therefore, send a beacon message in each slot, in addition to listening to the channel. The duration of sending a beacon message is TBeacon = 0.3 ms [17-18].

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In the BACP scheme, each slot is divided into several sub-slots, and each reader selects one of the sub-slots in its slot randomly to send a beacon message. Considering the duration of each slot and the time it takes to send a beacon message, each slot can be divided into 16 sub-slots. Each reader sends a beacon message in the selected sub-slot from its slot and listens to the channel to receive beacon messages from neighboring readers during the slot. The high number of sub-slots and a limited number of neighboring readers that send beacon messages in the same slot cause a collision between the sent beacon messages, although with very low probability. As shown in Fig. 4, in the BACP protocol, each reader sends Preference_Code in the beacon message. Each Preference_Code contains Reader_ID and a bit called Prev_state. Each reader of the network has a specific unit called Reader_ID. Prev_state is the most valuable Preference_Code bit (the last bit from the left), and refers to the status of the reader in the previous round. If the reader has successfully read the tags in the previous round, then the value of Prev_state is zero; otherwise, it is one.

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Figure 4. Round Design

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In the BACP protocol, like the DRCA scheme, readers randomly select one of the slots after receiving the arrangement command (AC). If a reader selects slot k, then it listens to the channel in the (k-1)th slot. If the channel is occupied and if it has the required condition (i.e., it is not within the collision range of RTC from the reader that is occupying the channel), it selects a new channel from the remaining frequency channels, increases its selected slot to k + 1, and attempts to access a new channel. However, if the channel is free, as the slot k starts, the reader randomly sends Preference_Code in one of the sub-slots in the form of a beacon message, which eventually results in one of the following scenarios: If the reader does not receive any other Preference_Code from its neighbors until the end of Tslot, this means that no neighboring reader has selected this channel and, therefore, the reader can enter the CRT phase without any interference. In this case, the reader sets its Prev_state to zero for the next round.

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However, if the reader receives Preference_Code from its neighbors while listening to the channel, it compares its Preference_Code with all the received Preference_Codes after completing Tslot. If Preference_Code of the reader is larger than the remaining received Preference_Codes, then the reader enters the CRT phase and attempts to read the tags, and then sets its Prev_state to zero for the next round. Otherwise, the reader calculates its distance based on the signal strength of the beacon message it has received from its neighbors and, if it has the required conditions, it increases its slot to k+2 and randomly selects a new channel from the remaining channels that it has not attempted yet. Then, the reader listens to the channel again in the k+1 slot and attempts to access the channel. If the reader fails to access the channel in this round, it sets its Prev_state for the next round to one. The purpose of defining Prev_state is that, in the competition phase,

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the readers with larger Reader_ID do not always win. In fact, a state of fairness between the readers is created in accessing the channel, so that the readers that have not been able to read the tags in the previous round may have a higher chance to access the channel in this round.

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Figure 5 shows an example of the BACP protocol for one round. Readers randomly select the slots and frequency channels after the central server sends the AC message. Based on the selected channel, readers are divided into the following four categories: Channel 1={R3,R9,R4} Channel 2={R1,R8} Channel 3={R2,R7} Channel 4={R5,R6}

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With the start of the first slot, the R3 and R7 readers send Preference_Code in frequency channels 1 and 3, respectively, listen to the channel until the end of the first slot, and because they do not receive Preference_Code from any of the neighboring readers, they occupy the channel and start reading the tags. As the second slot starts, the R1, R8, and R2 readers listen to the channel. Given that the R7 reader is reading the tags, the R2 reader understands that the channel is occupied and, as a result, calculates its distance to the R7 reader. Because the R2 reader is within the interference range of RRC from the R7 reader, it increases its slot one unit and randomly selects another channel (channel 4, k=4). However, because the R1 and R8 readers do not receive a signal, they identify that the channel is free, and as the third slot starts, they begin to send Preference_Code in one of the selected sub-slots. After the end of the third slot, the R1 and R8 readers compare the Preference_Codes they have received from each other, and because Preference_Code of the R8 reader is larger, this reader occupies the channel (for simplicity and clarity, it is assumed that Prev_state of all readers in this round is zero). However, because Preference_Code of the R1 reader is smaller, this reader calculates its distance to the R8 reader based on the signal strength of the received beacon from that reader, and because the resultant distance is greater than 2*Drt, it increases its slot by two units, selects another frequency channel, and attempts to access the channel again (channel 3, k=5).

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With the start of the fourth slot, because in the third slot the R2, R5, and R6 readers have identified that the channel is free, they start sending Preference_Code in the selected sub-slots. By the end of the fourth slot, R2 compares the received codes from R5 and R6 with its Preference_Code, and because Preference_Code of the R2 reader is smaller, it calculates its distance based on the signals received from the neighboring readers, and because it meets the required conditions, it increases its slot by two units and selects a new channel from the remaining channels (the reader considers the smallest value as the slot change criterion after calculating its distance from the neighboring readers). The R5 reader also compares its Preference_Code with the received codes, and because it has received Preference_Code only from the R2 reader and its Preference_Code is larger, it succeeds in accessing the channel. The same occurs for the R6 reader.

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By comparing Fig. 5 with Figs. 3 and 4, it becomes clear that not only does the BACP protocol activate more readers in one round, it also makes full use of the available resources, and all the available frequency channels are used by the readers.

Figure 5. An example of BACP

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In the BACP method, in a similar manner to the DRCA and GDRA methods, the central server will signal the start of a round by sending an AC message; additionally, unlike the case of the NFRA method, there is no need for a reader to send the overriding frame (OF) message continuously to its neighbors. Therefore, this method will not only reduce the communication overheads, but deletion of the action of sending an OF message will also save energy. In addition, in the BACP method, as per the DRCA method, when a reader is given a second chance to access the channel, the reader only listens to the channel during the same round and in the next slot and, if necessary, will then send a new beacon message. This happens to readers in the GDRA method but in the next round, so in the BACP method, no communication overheads will be imposed on the network. In contrast, in the BACP method, there is no need for any reader to listen to the channel continuously to access a channel, and if the reader has selected the slot k, it will only listen to the channel during the (k−1)th slot, which will also result in reduced energy consumption. The BACP method, like the DRCA and GDRA methods, manages and controls the occurrence of interference by only sending the beacon message between readers and does not require the exchange of information between the readers and the tags to control the RTCs and RRCs. 4.1 The BACP Algorithm

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The BACP algorithm is presented below. Based on the proposed algorithm, when a reader has a beacon message to send, it first randomly selects one of the sub-slots from 1 to MaxSub_slot and then sends its Preference_Code in the selected channel and sub-slot (lines 14–16).

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In the BACP algorithm, when the channel is occupied (line 27), like the DRCA scheme, the reader calculates its distance from the neighboring readers, and if the distance is greater than 2*Drt, in contrast to the DRCA scheme, the reader is allowed to select a channel only from the remaining channels (line 28–31), and if the distance is less than the specified value, to increase its chances of accessing the channel in the next round, initializes its Prev_state to one and waits for the next round (lines 35–36).

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However, when the channel is not occupied (line 38) and if the reader has a beacon to send, there are two scenarios. In the first scenario, if the reader receives Preference_Codes from its neighbors (line 39), it compares its Preference_Code with the received Preference_Codes, and if it is larger, it attempts to read the tags, and to reduce its chances of accessing the channel in the next round, initializes its Prev_state to zero (lines 40–44). Otherwise, the reader calculates its distance from the readers from which it has received Preference_Code and analyzes the obtained value to access the channel again (lines 45–55). In the second scenario, and when the reader does not receive Preference_Codes from its neighbors (line 57), it reads the tags and initializes its Prev_state to zero (lines 58 to 61).

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Algorithm 1: BACP protocol for every reader Ri 1: Set channeli = rand(1,4) 2: Loop 3: Ri listen to the CS and extracts the K value from AC packet that sends by the server 4: set ki = sfit function(K) 5: if (ki == 1) 6: set Bei = true 7: set Li = true 8: else 9: set Bei = false 10: set Li = false 11: end if 12: set Queryi = flase 13: for c = 1 to K 14: if (Bei == true) 15: set sub_sloti = rand (1,MaxSub_slot) 16: Ri send Preference_codei in channeli & sub_sloti 17: end if 18: if ( ki == c + 1) 19: set Li = true 20: end if 21: while (Ri doesn‟t receive an internal clock signal) do 22: if Li == true 23: Ri keeps listening to the channeli 24: end if 25: end while 26: if (Li == true & Queryi == flase) 27: if (channeli is busy) 28: set Di = min (distance Ri and the neighbor readers that used channeli) 29: if (Di > 2 * Drt) 30: ki= ki+1 31: set channeli = rand (remaining channels) 32: set Bei = false 33: set Li = false 34: else 35: set Prev_statei = 1 36: no operation and wait for the next AC packet 37: end if 38: else 39: if (Bei == true & received Preference_code from neighbor readers) 40: if (Preference_codei > received Pereference_codes) 41: set Bei = flase 42: set Queryi = true 43: set Prev_statei = 0 44: Ri starts reader to tag communication in channeli 45: else 46: set Di = min (distance Ri and the neighbor readers that send Preference_codes) 47: if (Di > 2* Drt) 48: ki = ki + 2 49: set channeli = rand (remaining channels) 50: set Bei = false 51: else 52: set Prev_statei = 1 53: no operation and wait for the next AC packet 54: end if 55: end if

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else if (Bei == true & doesn‟t received Preference_code from neighbor readers) set Bei = flase set Queryi = true set Prev_statei = 0 Ri starts reader to tag communication in channeli else set Bei = true end if end if end if end for end loop

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4.2 Complexity Analysis of the BACP Algorithm

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In this section, we analyze the complexity of the BACP algorithm. To enable complexity analysis of the BACP algorithm, we need to count the number of operations executed by each reader when the BACP algorithm is implemented, and we then apply the same analysis to the other state-of-the-art algorithms. The run-time complexity of the worst-case scenario for a given algorithm can be evaluated by examining the algorithm structure and making simplifying assumptions. For this purpose, and in accordance with Table 1, we define run-times for each of the operations that are used in the BACP algorithm.

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Table 1. Operations and consume time

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Operations Set Comparison Listen to the channel Send message Add Receive message Multiplication

Consume Time Tset Tcom Tlis Tsnd Tadd Trec Tmul

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We also imagine a worst case scenario for the reader in which, based on the BACP algorithm and the number of available channels, a reader attempts to reach the channel four times. The number of rounds required for implementation of the algorithm, n, is also considered. Given the above assumptions and an exact examination of the general structure of the BACP algorithm, the temporal complexity of this algorithm is estimated for the worst possible case using formula (3): ( )

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In formula (3), K represents the number of timeslots and is related to the „for‟ loop in line 13 of the algorithm. In the worst case scenario, if K=n in the above relation and all the values considered for the time consumption are replaced using the maximum value (Tmax), formula (3) can be simplified as follows:

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Given the assumptions made above for the GDRA and DRCA algorithms, the temporal complexities of these algorithms in the worst case scenario can be calculated using relations (5) and (6), respectively: (5)

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As a rule of thumb, it can be assumed that the highest-order term in any given function dominates the rate of growth and thus defines the run-time order. Under this assumption, and based on equations (4), (5) and (6), n2 is the highest-order term, so it can be concluded that T(n)=O(n2) and, as a result, the time complexities of all tree algorithms are equal.

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In this part, we focus on the theoretical analysis of the behavior of the proposed method based on the number of readers (R) and the number of timeslots (K). The superiority of the BACP method when compared with the GDRA and DRCA methods means that the proposed method increases the chances of reaching the channel and, as a result, more readers can be active in a single round without the occurrence of interference. For this reason, in this section, we calculate the likelihood that one specific reader among R competing readers will succeed in accessing the channel. Based on the work in [17], the likelihood that a reader can access the channel from among R neighboring readers when using the GDRA method is obtained from formula (7): (

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In the above relation, R is the number of neighboring readers, K is the number of slots, and pk is the likelihood of selection of a slot based on the sift function that was obtained from equation (1). In formula (7), a frequency channel is considered, while the advantage of the GDRA method relative to the NFRA method consists of its use of four frequency channels. For this reason, in formula (8), we calculate a reader‟s chances of successfully accessing a channel based on consideration of the four frequency channels explained in the GDRA method: ( )

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The likelihood of a reader successfully accessing a channel in the DRCA method is obtained from equation (9). In this equation, in addition to consideration of the likelihood of success in the GDRA method, the likelihood of a reader's success with regard to repeat chances is also taken

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into account. In equation (9), for simplicity, the variables A, B, and C are used, and equations corresponding to these variables are presented in the Appendix. ( ) ∑

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The likelihood that a reader will successfully access a channel in the BACP method is also obtained from equation (10). In this equation, MaxSub_slot is considered to be equal to 16 and is likely that the Preference_Code of a reader is larger than that of the neighboring R−1

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reader. In equation (10), for simplicity, the variables D, E, and F are used, and the corresponding equations for these variables are presented in the Appendix.

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In all the above equations, for simplicity, the RTC interference has been set aside.

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Figure 6 shows the likelihood of successfully reaching the channel required based on the equations above. In Fig. 6, the likelihood of accessing the channel when using the GDRA method is shown in two one-channel (f = 1) and four-channel (f = 4) modes. The x-axis also shows the number of neighboring readers, which is increasing. The number of slots (K) in this sample is considered to be eight. As shown in the figure, the likelihood of success when using the DRCA method is similar to that of the GDRA method, and as the number of readers increases, this difference is also minimized. The reason for this is the small value of K, while giving a second chance to the readers only increases the number of readers that have selected larger timeslots; as explained in Section 3, this leads to greater interference. However, the likelihood of accessing the channel when using the BACP method is considerably greater than that when using the other two methods, even for small values of K. The reason for this enhanced likelihood is that the DRCA and GDRA methods do not have mechanisms for control and management of interference when sending the beacon message; in contrast, in the BACP method, through management of the sending of the beacon message and analysis of the

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Preference_Code sent using the beacon message, the likelihood of access to the channel increases for the readers. As shown in the figure, the chance that a reader will access the channel diminishes as the number of neighboring readers increases, which is due to the limited resources (i.e., the numbers of slots and frequency channels) that are available for competition between the readers.

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Figure 6. Probability that a reader wins the contention. (R=5,…, 50 and K=8)

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One of the most important parameters in competition-based methods is the channel efficiency, which can be defined as the time taken to access the channel relative to the entire frame time. In the proposed methods, because each frame is divided into two phases composed of competition and access to the channel, selection of an appropriate K value for the channel will, therefore, contribute to the channel efficiency. This means that when the K value increases, the efficiency of the channel decreases while reducing the K value causes an increase in the channel efficiency. For this purpose, in Fig. 7, the effects of the K parameter on the behavior of the BACP, DRCA, and GDRA methods are presented. In Fig. 7, the number of neighboring readers is considered to be fixed and is equal to 50 in this case. The vertical axis shows the likelihood of successfully accessing the channel and the horizontal axis gives the various values of K. Figure 7 shows that the performance of the BACP method is better than that of the other two methods and the likelihood of successfully accessing the channel is greater when the value of K is low, and the efficiency of the channel is actually high. The reason for this performance difference is that in the other two methods, when the value of K is low, the likelihood of simultaneous transmission of a same-frequency beacon message increases and because no control mechanism exists for these methods, interference will then occur. As shown in the figure, the DRCA method works for small K values in a manner similar to the GDRA because in this mode, and because of the low number of timeslots, giving a chance to the readers then leads to further interference. In addition, the performances of the three algorithms for large K values are almost identical because of the large numbers of timeslots and the use of the sift function in all three methods.

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1

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The probability that a reader wins the contention

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DRCA(f=4) BACP(f=4)

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Figure 7. Probability that a reader wins the contention. (R=50 and K=2, 4, 16, 32, 64,128)

5

Simulation results

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In this section, the simulation results of the proposed scheme in comparison with centralized and multichannel schemes, such as the GDRA and DRCA, are presented. To comprehensively compare our proposed scheme to other schemes, we have considered several criteria for evaluating an anti-collision algorithm introduced in References [13], [27-28] such as throughput, fairness, delay, and energy consumption.

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To simulate the proposed scheme, a space of 1,000 m2 was considered. The identification range of readers depends on their transmission power, and in this simulation, which is consistent with European standards, the transmission power of readers in the UHF band was considered to be 3.2 watts. The assumed transmission power limits the interrogation range of a reader was 10 meters, and the interference range of the readers with each other was 1,000 meters. Additionally, RTC occurs when the distance between readers is less than 20 meters [29]. In this simulation, the readers were considered to be fixed and mobile, and randomly dispersed in the simulation space, and their number varied between 100 and 500. Based on the details stated in [30] and [31], it was assumed that an average of 100 tags was read by a reader in 0.46 seconds, and to prevent interference with the return signal from the tags, the reader read the tags using the frame slotted ALOHA protocol. In Table 2, the simulation parameters are briefly presented, where kr-wins is the number of the slot for which the reader accessed the channel.

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Table 2. Evaluation parameters

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Value 1000 m2 4 3.2 watt 2.83ms 128 5ms 16 0.3ms 0.46s CRT + K * Tslot TCRT = T – Tslot * kr-wins 1000m 10m 20m 100 to 500 0.1*10-6 Jules 1.6*10-6 Jules

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Parameter Simulation Area Number of frequency channels Reader Tx Power AC packet length Number of slots (K) Tslot Number of sub_slot (MaxSub_slot) Beacon packet length CRT phase Identification round (T) Reader to tag communication length (T CRT) Reader to reader interference range Interrogation range Reader to tag collision range Number of readers Amount of energy used to send the beacon message Amount of energy required to listen to the channel

In the following, each of the evaluation criteria mentioned above are briefly described, and then the results of the simulation of the proposed algorithm with the DRCA and GDRA schemes are presented.

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5.1 Throughput

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The most important goal of a communication protocol is to manage a collision in such a manner that the highest number of successful transmissions is achieved and thus a high number of tag recognitions is ensured. Thus, throughput can be considered as an important and suitable criterion for evaluating the performance of a communication protocol. In this paper, throughput is considered to be the number of readers that successfully perform a transmission per second.

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Two scenarios were implemented to evaluate the throughput of the proposed scheme. In the first scenario, a large number of readers are located as fixed and random. Figure 8 shows the results of the first scenario. The horizontal axis shows the number of readers, which was considered for five modes and from 100 to 500 variable readers. The vertical axis shows the throughput for each mode, in which each value is the mean of performing the simulation 100 times. According to this figure, the throughput of the proposed algorithm was significantly greater than that for the other two algorithms. Because the aim of the proposed scheme is that more readers read the tags in a single round without collision, the throughput of the proposed algorithm increased compared with the other two algorithms.

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Figure 8. Throughput of BACP and multi-channel protocols for fixed dense reader environments

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In the second scenario, the number of readers also varied from 100 to 500, with the difference that the readers were mobile. Because the position of a reader in relation to the neighboring readers is always changing in a dynamic network, and the mobility of readers increases the chance of the occurrence of collisions in the network, the throughput of a mobile network is less than that of a fixed network. Figure 9 shows the simulation results of the second scenario. These results are the mean of performing the simulation 100 times. Figures 8 and 9 show the loss of performance of the mobile network compared with the fixed network. According to these figures, the throughput of the BACP algorithm was still higher than that of the DRCA and GDRA schemes in the dynamic network. Simulation results show that the average percentages of throughput in the fixed and mobile dense reader environments increased by more than 12.6% and 17.9% when compared with the DRCA and GDRA methods, respectively.

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Figure 9. Throughput of BACP and multi-channel protocols for mobile dense reader environments

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5.2 Fairness One of the main goals of the proposed scheme is to increase the fairness of access to the channel. The possibility of randomly selecting timeslots equally among the readers can lead to a reduction in fairness among the readers and, consequently, either some of the tags are not identified, or there is an increased delay in reading the tags.

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One of the most widely used indices for measuring fairness is Jain's fairness index [32], which is defined as |

(11)

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Where N is the number of readers and xi is the throughput of the reader i. The index ranges from zero, unfair treatment, to one, fair treatment.

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The proposed scheme attempts to increase the opportunities of readers that fail to access a channel for subsequent rounds by managing resource allocation among them. In the BACP protocol, when the reader does not succeed in accessing a channel in one round, it initializes Prev_state to one. Because this parameter is the most valuable part of Preference_Code, this initialization increases Preference_Code for the reader and, as a result, the reader also has a higher chance to access the channel. By contrast, readers that succeed in accessing a channel in one round initialize Prev_state to zero, which decreases their Preference_Code for the next round, and hence the chances of accessing the channel is given to the readers with a higher Preference_Code.

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To evaluate the fairness of the proposed scheme, two scenarios in an RFID dense system were implemented. In the first scenario, the variable number of 100 to 500 fixed readers was considered. Figure 10 shows the results of the evaluation of the proposed algorithm compared with other protocols. The horizontal axis shows the number of readers, and the vertical axis shows the fairness percentage. The results are the mean of 100 simulations. According to Fig. 10, and considering the priority in the proposed algorithm, fairness in the BACP protocol was much higher than that in the other two protocols, which means that the readers accessed the channel relatively fairly.

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Figure 10. Fairness of evaluated protocols for fixed dense reader environments

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In the second scenario, the readers were considered to be mobile. According to Fig. 11, also, in this case, the fairness of the proposed scheme was better than that of the other two schemes, which was because of the consideration of priority to access the channel. Furthermore, by comparing Figs. 10 and 11, it is concluded that fairness was higher in the mobile network than the fixed network because, in mobile networks, the mobility of readers makes them change their position in relation to their neighbors, and as a result, the differences in the distribution of readers are reduced. Figures 8 and 9 show that the average percentages of fairness in the fixed and mobile dense reader environments increased by more than 13.2% and 13.1% when compared with the DRCA and GDRA methods, respectively.

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Figure 11. Fairness of evaluated protocols for mobile dense reader environments

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5.3 Delay

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In most RFID applications, readers need to continuously identify tags. Therefore, in an RFID system, an anti-collision protocol must schedule time among the readers in such a manner that readers can connect with the tags as soon as possible. As a result, the waiting time is one of the important parameters for evaluating an anti-collision protocol and should be reduced as much as possible [33]. The waiting time for a reader consists of the time between the read query and successful transmission and the average waiting time for a reader is the mean waiting time for all successful transmissions of a reader. In this paper, to evaluate time performance, the overall average reader waiting time (OARWT) in an RFID network is considered.

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Two scenarios were considered for evaluating time performance, and in both scenarios, the results were the mean of 100 simulations. In the first scenario, 100 to 500 readers were placed in the setting randomly and fixed. In Fig. 12, the horizontal axis shows the number of readers and the vertical axis shows the mean waiting time of the entire network. Because in the proposed scheme managing beacon messages allows readers to access the channel even when they receive these messages at the same frequency, in one round, more readers connected with the tags and this feature reduced the overall network delay. By contrast, the proposed scheme prevented the creation of a large gap between the two transmissions of a reader by applying Preference_Code and increasing the fairness of the network and, as a result, the maximum waiting time of a reader was reduced, followed by a decline in the mean waiting time of the entire network. Figure 12 shows the advantage of the proposed scheme in terms of time performance compared with the other schemes. According to this figure, because the number of frequencies used was limited, with the increase in the number of readers, the mean waiting time of the entire network increased.

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Figure 12. Delay of evaluated protocols for fixed dense reader environments

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In the second scenario, readers were considered to be mobile. According to Fig. 13, the proposed scheme also reduced the network delay better than the other two schemes. Additionally, by comparing Figs. 12 and 13, it can be observed that the delay in the mobile network increased slightly compared with the fixed network, which was because of the increased collision risk among the readers. According to the figures, the average percentages of delay in the fixed and mobile dense reader environments decreased by more than 9.9% and 19.6% when compared with the DRCA and GDRA methods, respectively. 70

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Figure 13. Delay of evaluated protocols for mobile dense reader environments

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One of the most important parameters of an RFID network is the power consumption of the readers. In an RFID network, maximum power consumption occurs when the reader scans the tags within interrogation range, and not when it is attempting to access the channel [34]. However, in this section, the amounts of power consumed by readers when attempting to access the channel in the GDRA, DRCA, and BACP methods have been examined and compared. In this part of the simulation, all readers are assumed to be fixed. In addition, given that all three algorithms use an anti-collision mechanism (frame-slotted ALOHA) to read the tags, it was decided that the energy consumed by the readers to scan the tags was not to be calculated. The average energy required to activate all readers at least once and enable these readers to reach the channel was calculated, and the results are shown in Fig. 14. Since one of the most important goals in RFID networks is reading all tags online and without interruption, we have shown in Fig. 14 the average amount of energy required for both the entire network to be covered by active readers and all the tags to be identified. In this section, to calculate the average energy used, we have considered the number of beacon packets, the amount of energy used to send the beacon message, and the amount of energy required to listen to the channel during the channel access mechanism. According to the results shown in the figure, the average energy consumption values for all three methods with low numbers of readers are close to each other because in this

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mode, the number of readers is close to the number of timeslots, and thus the likelihood of channel access is high for all three methods. However, as the number of readers increases, the power consumption of the GDRA method becomes slightly different from that of the BACP method; this occurs because of the consideration of fairness between the readers in the BACP method, which means that readers that were able to access the channel once do not enter the competition mode and thus allow other readers chances to access the channel. In addition, the likelihood of successful channel access in the BACP method is higher than that in the GDRA method, and with the increasing numbers of readers and the analysis of the beacon message in the BACP method, the likelihood that the energy that is used to access the channel would produce the intended results will also increase. The reason why the difference in energy consumption in the BACP method is not so high when compared with that in the GDRA method is that in the BACP method, it is possible that in a single round, a reader may listen to the channel more than once and may also fail to access the channel; this, in turn, will lead to greater energy consumption. As the number of readers increases, the average energy consumed in the DRCA method also increases when compared with that in the GDRA method. This occurs because, when the number of readers in the DRCA method is much higher than the number of timeslots, the likelihood of success in accessing the channel will be reduced, and thus giving a second chance to the readers will only cause these readers to listen to the channel idly (i.e., uselessly); this, in turn, will lead to higher average energy consumption.

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Figure 14. Energy consumption of evaluated protocols to reach the channel

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6

Conclusion

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In this paper, a novel protocol to reduce reader collision in a dense RFID network was proposed. The BACP protocol is a centralized approach that is consistent with the ETSI EN 302 208 standard. One of the main goals of the proposed scheme was the maximum use of available resources. In previous schemes, after sending a beacon message, when a reader received a beacon message with the same frequency from neighboring readers, it abandoned the channel and waited for the next round. However, in the BACP, by managing and analyzing the beacon message, the opportunity was given to the reader to access the channel under such circumstances, in addition to ensuring that the frequency resources available were not wasted. By contrast, more readers were active in one round without the occurrence of any collisions. Another goal of the proposed scheme was to increase fairness in access to resources. In the BACP protocol, a reader that could not access the channel had a greater chance of accessing the channel in the next round by initializing Prev_state to one, and thus increasing the value of Preference_Code sent through the beacon message. To assess the proposed scheme, the proposed algorithm was evaluated from different perspectives and compared with previous schemes. As shown in the simulation results, the proposed scheme significantly improved throughput, reduced network delay, and promoted fairness in the network. The proposed scheme proved to be superior to the other schemes in both fixed and mobile networks. Therefore, the proposed scheme became an efficient scheme for a dense RFID system with fixed and mobile readers by managing and analyzing beacon messages and efficient resource allocation. In future research, how to enable readers to select resources stochastically and not randomly will be investigated.

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References

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