Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> Radio frequency identification systems with passive tags dramatically increase the ability of an organization to acquire a vast array of data about the location and properties of any entity that can be physically tagged and wirelessly scanned within certain technical limitations. However, if multiple tags are to be identified simultaneously, messages from the tags can collide and cancel each other out. One of the popular anti-collision algorithms is the ALOHA-type algorithm, which is simple and has good performance when the number of tags to be read is reasonably small. In this paper we extended our previous research to an efficient dynamic framed slotted ALOHA for RFID passive tags. Some problems related to the ALOHA for RFID passive tags are investigated and corrected. <s> BIB001 </s> Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> In a radio-frequency identification (RFID) system, the dynamic frame length ALOHA protocol is widely adopted to solve the anticollision problem. Analysis for the anticollision problem can be divided into two primary parts. The concern of the first part is how to precisely estimate the number of tags. The other part involves determination of dynamic frame length to achieve maximum throughput or channel usage efficiency. In this paper, we present an accurate method for estimating tag quantity. This method is based on the maximum a posteriori probability decision. We also derive the optimal frame length using radio channel efficiency. Simulation results indicate the tag estimate error of the proposed method is less than 4%. Use of our proposed tag estimate method together with optimal frame length can achieve close to the theoretical maximum throughput of the framed ALOHA algorithm. <s> BIB002 </s> Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> This paper aims to locate multiple tags in the indoor environment by using low cost devices, active Radio Frequency Identification (RFID). A more reliable positioning algorithm using Mahalanobis distance based on Received Signal Strength Indication (RSSI) is proposed to improve location accuracy, since in this method covariance is taken into account to avoid unnecessary mistakes. Reference tags and readers with known location, which is proposed in LANDMARC system, are used to estimate target tag location. The results from conducting extensive simulations demonstrate that the proposed algorithm can achieve more reliable and higher performance than LANDMARC algorithm without using any additional optimal methods. <s> BIB003 </s> Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> One of the main problems that affect the data integrity of passive RFID systems is the collision between the tags. A popular anticollision algorithm which dominates the standards in HF and UHF passive RFID systems is framed slotted Aloha (FSA) and some variations of FSA. Throughput and average time delay of the RFID system which determines the performance/efficiency of the system are reduced rapidly when the number of tags inside the interrogation zone is increased. Using larger frame sizes is not always the solution. This paper discusses and compares the existing protocols, and proposes a variation of FSA, called progressing scanning (PS) algorithm. The PS algorithm divides the tags in the interrogation zone into smaller groups and gives the reader the ability to communicate with each of them. For performance analysis, the PS algorithm was evaluated with the parameters of a typical passive RFID system at 2.5 GHz. The results showed that the PS algorithm can improve the efficiency of the RFID system and provide a reliable solution for cases with a high density of tags in the area (over 800 tags). <s> BIB004 </s> Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> Tag anti-collision is an important issue in RFID systems because the reader must recognize all tags efficiently. In RFID wireless communication systems, tag identification will encounter the capture effect, where a reader decodes a tag ID even when multiple tags simultaneously transmit their signals. This letter proposes a tag anti-collision algorithm - the generalized binary tree protocol (GBT). GBT separates the identification process into several binary tree (BT) cycles to solve the problem caused by the capture effect. Unrecognized tags, hidden by the capture effect in a BT cycle, will be identified in subsequent cycles. The formal analysis of identification delay for GBT is derived and simulation and analytical results show that GBT significantly outperforms other existing algorithms. <s> BIB005 </s> Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> Radio-frequency identification (RFID) has been well established as an effective technology for track and trace applications. In this paper, we go beyond the ID in RFID, and discuss the potential for RFID tags to be used as low-cost sensors by mapping a change in some physical parameter of interest to a controlled change in RFID tag antenna electrical properties. We will also show that it is possible to design the tag antenna to suffer a permanent change in case of violation of a critical threshold in the parameter of interest thereby creating a low-cost threshold sensing mechanism. This can be achieved by inducing controlled changes to the tag antenna geometry parameters or to the antenna boundary conditions, in effect creating a nonelectric memory to monitor state. After identifying the application space for which this class of sensing is well suited, we present details into the design and testing of three different kinds of sensors based on this sensing paradigm. We demonstrate how we use this concept to sense displacements, temperature thresholds, and fluid levels. We will show that RFID-tag-antenna-based sensing has the potential to revolutionize application domains in which there is a need for low-cost, long-lasting, ubiquitous sensors. <s> BIB006 </s> Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> RFID (radio-frequency identification) is an emerging technology with extensive applications such as transportation and logistics, object tracking, and inventory management. How to quickly identify the missing RFID tags and thus their associated objects is a practically important problem in many large-scale RFID systems. This paper presents three novel methods to quickly identify the missing tags in a large-scale RFID system of thousands of tags. Our protocols can reduce the time for identifying all the missing tags by up to 75% in comparison to the state of art. <s> BIB007 </s> Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> The collision caused by tags is one of the most important problems which affect the efficiency of the RFID system. This paper presents the classical ALOHA algorithm in detail and estimates the real-time number of tags according to the collision so as to adjust the frame size of dynamically. Then a new algorithm based on estimating of the tag number is proposed. Analysis and simulation both show that the presented IDFSA algorithm is very effective compared to the conventional algorithms. <s> BIB008 </s> Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> Tag collision arbitration is considered as one of the critical issues in RFID system design. In order to further improve the identification efficiency of tag anti-collision algorithms in RFID systems, several types of dynamic framed slotted ALOHA (DFSA) anti-collision algorithms are analyzed, and a new anti-collision algorithm is proposed. The proposed algorithm has the ability of identifying the time slot distribution which is selected by the tags within a reader's interrogation range in advance. Then the free time slots will be skipped when the reader queries each time slot. Also, the colliding tags will be immediately processed with additional time slots. Simulation results show that the proposed algorithm takes fewer total number of time slots and has a higher efficiency of tag identification compared to the other four DFSA anti-collision algorithms. <s> BIB009 </s> Smart Solutions for RFID based Inventory Management Systems: A Survey <s> Far Field RFID. <s> Capture effect is very common in passive radio frequency identification systems. Even though multiple tags transmit their signals to a reader simultaneously, one of the tags will be successfully identified due to capture effect. However, capture effect may hide some tags. Many existing algorithms that assume no capture effect will miss the hidden tags. In this paper, we extend an existing algorithm to the capture effect environment and propose a novel tag anti-collision algorithm to enhance the efficiency of tag identification. The proposed algorithm adopts an allocation strategy to reduce collision between the hidden tags by capture effect and the other tags. The strategy lets the collided tags resolved in a current cycle immediately. On the other hand, the hidden tags will enter the next cycle. Thus, the resolved tags will not collide with the hidden tags in the next cycle, and the collision will be reduced. The experiment results show that the proposed protocol's identification efficiency outperforms other existing protocols when the occurrence probability of capture effect varies from 0.1 to 0.6. <s> BIB010
To transfer signals, Far-field system utilizes the electromagnetic field that results from the emittance of the radio frequency signal. Far-field systems operate at the UHF region of the spectrum, normally between 840 MHz and 960 MHz, or microwave frequencies, between 2.45 GHz and 24 GHz [8] . A dipole antenna that is imbedded in the reader antenna transmits EM waves to the tag. The EM wave triggers an alternating potential difference once received by the tag's small dipole antenna, which appears across the antenna circuit, this forms an alternative current that is passed through to a diode which is also linked to a capacitor, which will then be rectified as an energy that the tag uses to power up the chip. However, unlike inductive coupling, tags are beyond the range of the reader's EM wave area, and information cannot be transmitted back to the reader using load modulation  . A far-field system's range has some limitations, which can be summarized: 1. The amount of energy received by the tag. 2. The sensitivity of the reader's antenna to the signal received from the tag. As the electromagnetic wage travels, attenuation occurs on both sides of communication, between the antenna and the tag, and the response signal from the tag. The received signal is expected to be weak. The returning energy from the tag is  BIB007 : where r is the distance. Nowadays, the semiconductor revolution has helped in reducing the size of the electronic materials in general, which helped in decreasing the power necessary for these components to consume in order to operate as expected, RFID tag has had its share of this innovation, power consumption continues to decrease, which helped in expanding the RFID operations and usages. In addition, low cost customized RFID tags can be designed and manufactured. These can efficiently read from a wider distance which sometimes can reach up to 6 meters with power usage as low as 100 dBm  . 3.3. RFID based Communications. RFID systems operate in a different range of the spectrum depending on the antenna and tag design, which can vary from 100 kHz to 5.8 GHz. The underlying technology requires the RFID systems to operate in the UHF frequencies occupying the ISM bands 860 -960 MHz, according to frequency band allocation in each country. The read range offered by UHF RFID makes this frequency the most attractive for use by supply chain management. The allocated band at ultra-high frequency (UHF) ranges from 860 MHz to 5.8 GHz   BIB003 . 3.4. RFID Tag. RFID tags are passive, active or semi-passive. The term, passive or active is referred to the power source that the tag uses to extract the required energy to operate. Passive tags retrieve the energy from the electromagnetic signal transmitted from the reader's antenna. The tag retrieves the required energy from the interrogating wave or from a modern antennas have a dedicated port that transmit constant signal to power up tags in the range. The second type of tags is active tag, which have internal batteries as a power source, to enhance the reading range. Passive tags have the lowest price in terms of manufacturing cost, they do not require maintenance, more compact and lighter   . Semi-passive tags are similar to the passive tags in the way they operate, they use backscattering technique to respond to the reader. The main difference is that semi-passive tags have batteries to power the chips that are embedded in the tag, which is used in conjunction with integrated electronic components such as sensors. Semi-passive tags have almost the same characteristics of the passive tag (reading range, operating frequency) BIB003 . RFID tags contain RF transponders with digital memory chips that are uniquely identified. The antenna packaged with a transceiver and decoder, emits a signal activating the tag, refer to the Figure 3 .3   . A brief description of each RFID tag component is mentioned below BIB010 : 1. RF Interface The radioactive component of the tag, which performs the following: • Supply RFID transponders with power by generating the required energy. • Modulating the signal in preparation for the transponder to transmit. • Reception and demodulation of signals received by the transponders. 2. Control Unit As the name implies, this part of the tag controls the tag operation, by performing the following functionalities: • Execution of the application software commands. • Signal interpretation. • Controls the communication with the transponder. • Anti-collision, and encryption can be implemented in the control unit. • Transponder-reader authentication. The capacity of the tag varies depending on the specifications of the vendor. It is very common ti find most modern tags with a capacity of up to 2,048 bits of information  BIB010 . This is enough to store the necessary information required about library items. 3.5. RFID Reader Antenna. As explained earlier, RFID reader antenna transmits signals as electromagnetic wave called radio frequency wave. Antennas can be classified into two main types based on the characteristics of the transmitted signal  : • Linear Polarization: The electromagnetic wave propagates in a single direction, vertically or horizontally, depending on the orientation. These antennas are also referred to as Dipole antennas. This type of antennas is recommended to use when the tag has fixed orientation. Therefore, it is commonly used in hand-held readers in warehouses and inventories. In order for this antenna to work, the reader and the tag must be aligned together. This is a major drawback of this type of tags. • Circular Polarisation: The electromagnetic wave covers the two planes during propagation, in a circle-like motion that resembles the motion of a screw, which makes this type more efficient than the linear polarization for two main reasons: The wide coverage area, and the tag orientation does not have to be inline with the antenna in order to make a successful read. One disadvantage of this antenna is energy loss, since the majority of the energy in the transmitted signal lies within the first few waves, as illustrated in Figure 3 .3. RFID antennas have also a commercial classification that divides antennas into two main groups, based on the number of ports that the antennas have  : • Monostatic Circular: This antenna is equipped with a single port, which is used for transmission and receiving of the RFID signals. • Bistatic Circular: This antenna has a dual ports, one for transmission and one for receiving signals. This type of antennas is more common, but it is more expensive. Both antennas can be manufactured with a Listen Before Talk (LBT) port, which is a dedicated port that listens for signals before sending RFID signals. The read range for both types depends on  : • The available power at the reader/interrogator to communicate with the tags. • The quality of the tag's circuitry, and its ability to capture the released energy to power the tag's components. • The environmental conditions and structures, as the operation frequency increases, the probability of the signal being obstructed by metal or a wall is higher. The released wave from an antenna propagates in space in a circular motion, its strength diminishes as the traveled distance increases. The design of the antenna determines the shape of the delivered wave, as a result the read range and the positive identification of tags are also affected by the distance between the reader and the tag, as well as the orientation of the antenna. In an ideal situation, free of obstructions or absorption objects, the signal strength decay in inverse proportion to the cube of the distance (refer to equation 2) BIB009 . When an antenna transmits a power signal, regardless if it was a constant power signal or a regular RF signal, the tags in the range are powered up and will reply with their identification signal, for a large number of tags, this will cause an issue, it is called collision, which heavily affects the efficiency of identification. 3.6. Collision and Anti-Collision Algorithms. Collision is an undesired interference that occurs between radio signals initiated from two or more radio frequency components that lie within the frequency range of each other. Normally, an inventory system equipped with passive RFID contains a high density of RFID equipment that transmit signals around, collision happens often when two or more radio active components reply to the reader simultaneously. Collision is a critical terminology in RFID networks for the following reason, when the reader antenna transmits the power up signal, which normally happens by sending the read signal, all tags within the reach of reader signal will detect and respond to it with their modulated signals. Collision occurs here as a result of the antenna receiving tens and sometimes hundreds of response signals from the tags. An algorithm is required to "detect" or organize each tag's signal and marshal the signals in orderly manner without interference of signals from other antennas or tags, applying anti-collision algorithms enhances the network throughput by reducing the time required to perform the identification. Various anti-collision algorithms have been proposed, the main two algorithms: (a) ALOHA-based algorithms, the frequency range is divided into time slots, each time slot is assigned to one tag only. (b) Tree-based algorithms that group the tags into a tree with branches that contain subsets. The identification of tags is achieved by iterating through the tree branches  . ALOHA is an algorithm built based on reducing the probability of tag collision by assigning time slots to tags. It utilizes Time Division Multiple Access (TDMA) to implement a collision resolution BIB008 . This document focuses on ALOHA-based algorithms for two main reasons, they work well with Read-Only tags, in other words, they don't require the tag to be reprogrammed to hold extra information. The second reason is simply because they are the most frequently used anti-collision techniques. Dynamic Frame slotted Aloha (DFS-ALOHA) in particular will be discussed in this article. By anticipating the probability of collision to estimate the number of tags in the interrogation zone, the algorithm determines the appropriate frame size depending on the number of tags. To avoid collision, DFS-ALOHA algorithm allocates a random frame for each tag to transmit data in. The bandwidth is divided into frames, and the frames are divided into slots. The system efficiency depends heavily on numerous factors: The amount of tags, the distribution of tags around the antenna, and the estimated frame size BIB001 BIB004  . However, ALOHA-based algorithms have one disadvantage, "Starvation". A tag may not be allocated a time slot to use to load its signal, which makes it undetectable by the surrounding antenna(s), as a result, the tag may not be detected for a a period of time, and sometimes forever BIB002 . Tree-based algorithms are deterministic, they use a search tree to identify tags that fall in the frequency range of the antenna by classifying the tags in the interrogation zone into a tree. The algorithm iterates through the branches until all tags have been identified. Tree-based algorithms are divided into three types: Binary Tree (BT), Query Tree (QT) , and finally Binary and Query Tree, which is a combination of both  BIB001 BIB005 . QT uses tag IDs to group the tags into subsets, then the system iterates through the subsets by dividing all subsets into groups that contain a set of two. The efficiency of the identification process is affected by the distribution of tags in the system. BT uses randomly generated numbers to identify the tag groups, which requires reprogrammable tag memory to store the assigned number in order to make this algorithm more efficient, however, in case of passive RFID reprogramming the tag chips are not programmable BIB006 .