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Dissertation by master`s work
1. Introduction
Potential application of RFID technology is limited only by imagination of the person. Although there is an opinion that RFID is best suited to manage the network marketing industry, or for using in the consumer packaged goods. But a range of applications of RFID goes far beyond the borders of these areas of application.
RFID-systems allow to read out the information which is out of visibility limits. The identification code is stored in a tag consisting of a microchip attached to the antenna. Transponder or reader has a connection with tag through telecommunication.
As the functionality of RFID-tags as a method of gathering information, it is very close to bar codes, the most widely used today for marking, but the functionality far exceed them.
2. Aims and objectives
Using technology raises a number of problems whose solution is aimed at Master's work.
In process of magnifying of number of objects which are marked, the probability of reading tags at the same time increases. The probability of collisions of signals accordingly grows. Reliability of communication and high-speed performance in system RFID are closely coupled to anticonflict algorithms. Similarly, to construct a reliable RFID-system should take into account a number of restrictions on its technical parameters. The main technical features of RFID are range, speed, reliability and electromagnetic compatibility.
Thus, in my master's work following problems will be solved:
1) Analysis of a conflict while reading multiple tags
2) Review of existing anticollision algorithms
3) Analysis of the impact of restrictions on the technical parameters of the equipment
4) Analysis of the impact of noise and interference on the system
5) Creating a simulation model of the system
3. Alleged scientific novelty
During Master’s work following results will be received:
1) An RF model of the RFID system
2) Methods to increase the reliability and range of the system
4. Analysis of existing solutions
Contrary to popular delusion RFID-reader can conduct simultaneously an information exchange only with one tag. As the tag is the simple carrier of identification number, there is a problem of exact reading of this number. If in a working area of the input reader there is a unique tag, it is not required any commands. The tag simply transmits data containing in it at sufficient power. But if in the working area of the reader there are a lot of tags — their signals interfere. Such overlap of signals is called as a collision, and results of reading are lost more often. To avoid conflicts of RFID system requires the formation of teams, based on certain protocols. Such reports usually name anticonflict protocols or algorithms.
The anticonflict algorithms used in systems RFID, are similar to ways of the permission of conflict situations of multiple communication access and with various network protocols, including Aloha and family of CSMA (Carrier Sense Multiple Access) protocols. Realization of anticonflict algorithms in RFID, however, is limited by low power consumption and small volume, and sometimes with full absence of memory of a tag. In addition, the algorithms must be optimized, taking into account the small energy tags that do not reduce the range in the case of passive tags, or to increase the lifetime of the battery in the case of active tags. Besides that tags are capable to co-operate only with the input reader, use of methods CSMA is impossible. Moreover, variations of parametres of the channel of propagation of a signal in wireless communication channels much more similar variations in wire channels — burst noises is extremely severe influence rather burst transmissions of communication between the input reader and a tag. Complexity of algorithms also should be minimised, as this leads to an increase in equipment cost.
Despite of the fact that all anticonflict algorithms are unique and have the merits and demerits, it is enough to consider two families of algorithms — SuperÒag and QT. The given algorithms are their typical representatives. Both algorithms are realised in time domain. However algorithm SuperÒag is probabilistic, and QT — deterministic.
Two types of algorithms can be compared by two criteria: technical characteristics and practical realization. As to technical characteristics, it is necessary to pay especial attention to two factors — rate of identification and behaviour of equipment at variation of number of tags in the course of reading. As to practical realization hardware realization and questions of the software (commands) is especially important.
By consideration of algorithms SuperTag it is possible to note, that at junction from variant ST.std.free to variant ST.std.off, increase of time of the permission of collisions depending on number of tags is watched close to linear. It is caused by additional functions of variant ST.std.free which switch off tags after their identification and-or switch off tags when one of them answers. Both these functions lead to reduction of number answering in the field of tags and, means, have the linear tendency. High-speed performance of algorithm ST.fast.off depending on number of tags in the field close to the linear.
In algorithm QT.ds we see essential distinctions in characteristics at consecutive and a random distribution of numbers of tags.
Comparing characteristics of two algorithms, we will note, that they are close at small number of tags in the field. At magnifying of number of tags in the field appear non-linearity.
From all observed algorithms, ST.fast.free demands the least number of commands from the input reader. It can be very useful in those frequency ranges where rigid procedural limitings act. Direct navigation of algorithm QT.ds, as well as in case of ST.fast.off, demands transfer of considerable number of commands. Generally, the more volume of transmitted commands, the more widely a demanded frequency band and that above probability of origination of errors.
Each of algorithms — SuperTag and QT.ds, has the unique merits and demerits. Algorithm QT.ds has ability to select tags with certain numbers. Algorithms SuperTag, at least, ST.std.off, demand smaller number of commands of the reader, more narrow bandwidth of frequencies and, hence, it is possible to assume the least probability of errors.
Unique lack of algorithm QT.ds and other reports with binary navigation is their inability in retrieval time to identify tags which arrive again in the area of reading. This feature can be important or not important depending on concrete application of equipment RFID.
The problem of algorithms SuperTag and others, coupled to a casual response time, consists in necessity to appoint the maximum detention period of the answer of a tag. This time can be transmitted a tag or to be put in pawn in manufacture.
6. Effect of restrictions on the range of RFID.
Range of RFID is mainly confined to the tension radiated electromagnetic fields and electrodynamic properties of the spread of the signal in the environment. Tension radiated fields directly is limited by administrative rules (regulations), as well as indirectly, through the administrative rules and restrictions on the width of the spectrum of the signal. Field is heavily dependent on the mutual orientation and location of tags and readers, the type of antenna. Administrative regulation limits the tension field and power density in certain areas of space, even if these parameters can be achieved by technical means within a wide range.
Once the tension is defined fields or power density at a certain point in space, the next problem is receiving power. In order to achieve the maximum induced voltage or power avaitag, it is necessary to optimally orient the antenna relative to the tag field emitted. In practice, this can be a very difficult task. Field intensity or power density is perceived by the antenna tag, given its geometry and orientation. With a fixed geometry and orientation of the antenna operation of tags depends on the parameters of its electronic circuit. Especially important impedance antenna matching circuit and the load.
In the reverse link, from the tag to the reader, produced modulation signal information stored in the memory tag. Level modulated signal thus considerably below the level of the signal reader. However, due to complex signal processing in the reader, it usually does not restrict the range of the system.
7. Conclusions
Since the purpose of passive RFID systems is a simple reading of identification codes that are stored in tag, and in the field of reading a lot of tags, protocols must be optimized with respect to anticollision algorithms. When considering anticollision algorithm the important characteristic is not only the speed of identification, but also the desired set of commands, the operation in the presence of noise and hardware implementation. Furthermore, in terms of command and anticollision algorithms is very important to influence the width of the spectrum.
Technical characteristics of equipment, directly or indirectly dependent on the fundamental limits. When designing the tags and the development of standards is evident a high degree of relationship between the technical characteristics of equipment constraints, configuration of the system and its specific application.
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