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COMMUNICATION ALGORITHMS IN CONTACTLESS OBJECTS’ IDENTIFICATION SYSTEMS OPERATING WITHOUT AND WITH CHANGES OF OBJECTS LOCALIZATIONS

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P O Z NA N UN I V E R S ITY O F TE C H N O LO GY A C A D E M IC J O U R N AL S

No 54 Electrical Engineering 2007

__________________________________________

Marek GOTFRYD*

Bartosz PAWŁOWICZ*

COMMUNICATION ALGORITHMS IN CONTACTLESS

OBJECTS’ IDENTIFICATION SYSTEMS OPERATING

WITHOUT AND WITH CHANGES OF OBJECTS

LOCALIZATIONS

At present there appears a necessity of design of RFID systems operating at the presence of dynamic change of TAGs location. Such systems are required for identification of road vehicles, the railway transportation and identification of movable elements in manufacturing processes. Below there is presented a comparison between TAGs’ identification algorithms for case of their static layout in reader operation zone and for case of dynamic changes of their localization.

Keywords: RFID, collision, protocol, identification, TAG

1. INTRODUCTION

The RFID (Radio Frequency IDentification) systems for contactless objects’ identification are treated as an alternative for bar codes, chip and magnetic cards because of their numerous advantages. The identification of moving objects is one of the most important fields of usage of such systems. In those systems there exist dynamic changes of location and orientation of TAGs during operation of read-write device (RWD). TAG is a microchip attached to a small antenna that is packaged in a way that it can be applied to an object.

Manufactured at present prototype systems are based on specific, created on demand, communication algorithms, or they work basing on standards such ISO18000 - "RFID handicap item management; Air interface", ISO18000 - "Proximity integrated circuit card" and also on the protocol EPCgen -2 introduced by EPCglobal and approved as ISO 18000-6C standard.

Communication stages in RFID systems designated for identification of TAGs with their static layout and with dynamic changes of their location are different to some extent. The analysis of those differences and of used communication

2007

Poznańskie Warsztaty Telekomunikacyjne Poznań 6 - 7 grudnia 2007

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protocols is the base for evaluation of conditions that should be fulfilled by systems for identification of movable objects in order to obtain the effective exchange of data among all TAGs placed in RWD interrogation zone

.

Fig. 1. Communication in RFID systems operating in different conditions a) from RWD to TAGs and from TAGs to RWD in static condition;b) from RWD to a selected TAG and

from this TAG to RWD in RFID system with dynamic location change of tags The systems of contactless identification of movable objects are being introduced in areas of management and monitoring of road traffic, railway transport as well as in industrial logistic systems (conveyors belts, distribution of goods) [1].

2. SYSTEMS FOR CONTACTLESS IDENTIFICATION

OPERATING AT DYNAMIC CHANGES OF TAGS

LOCALIZATION.

In the identification process it is essential the kind of executed operations in individual stages of exchange of data among elements of system (Fig. 1). The correct exchange of data with selected TAG has to be preceded by its finding and sorting from group of them that are present in zone of system operation.

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Considerations relating the RFID systems operating with static layout of TAGs may be simplified due to apparently infinite time of the presence of TAGs in zone of system operation. It means that the reader/programmer device has always a sufficient time to find all TAGs (regardless their number).

The first stage of communication is originated by a recognition of unique serial numbers of all TAGs (or of the chosen group of them) that are in zone of correct operation of system (Fig.1a), seldom, in specific protocols the situation may be different.

This stage is connected with data transmission from the reader/programmer unit to TAGs, and then from TAGs to RWD unit. Alas, simultaneous transmission by all TAGs leads to frequent collisions that should be detected by the proper coding of data.

The exemplary codes used for data transmission are depicted in Fig. 2. The proper choice of this code in multiple RFID system is crucial for correct detecting of data collisions; some codes are better in this respect (Fig 2. b). Second stage of communication protocol in solution from Fig 1.b consists in the sequential deactivation of already recognized TAGs. In this way the number of active TAGs remaining in the reader interrogation zone decreases so the probability of collisions during the next trials of TAGs recognition goes down. The process lasts so long until all collisions are removed and the last TAG is recognized. Since then it is possible the exchange of information between reader/programmer unit and the internal memory of selected TAG or TAGs.

In RFID systems dealing with moving TAGs the additional factor of the proper operation of system is the time required for execution of all operations and commands during exchange of data between TAGs and reader unit. This time depends on the speed of TAGs movement and the size of reader range of operation [2].

Fig. 2. Exemplary collisions during the reading of data coming from two TAGs, a) coding in NRZ code – collision remains undetected, b) coding in Manchester code – at

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3. COMMUNICATION ALGORITHMS

The mentioned stages of data exchange in multiple RFID systems may be depicted by flowchart diagrams. They are different for the case of static layout of TAGs and for the case of their movement during the identification process.

Fig. 3. A general algorithm of the communication with selected TAG in multiple identification system a) system with static layout of TAGs, b) systems with dynamic

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The algorithm of operation of multiple RFID system for the case of static layout of TAGs is depicted in Fig. 3a. During the first stage of communication many TAGs are transmitting their unique serial numbers, it is presented by the box (1). The process of collisions’ elimination that is equivalent to recognizing of TAGs that are in the reader interrogation zone begins by the conditional instruction (2). In case of some collision there appears a controlled deactivating (remote switching off) of earlier properly recognized TAGs (boxes 3 and 4). In that way during the next cycle of TAGs interrogation, the probability of the repeated collision decreases. After some cycles all TAGs may be recognized in this way and switched off. This leads to the possibility of random choice of one TAG from their accessible group (it is illustrated by the box (5)) and possibility of the required exchange of data between the internal memory of the selected TAG and reader unit (box 6). That data exchange process is the same as in case of a RFID single identification systems. The presented above recognition procedure forms the finite algorithm [3].

In case of dynamic layout of TAGs that enter the reader range interrogation zone and after some time they leave it the above algorithm is modified (Fig. 3b) and becomes an infinite algorithm. Like in the static case the aim of executed operations is a finding of all TAGs and the exchange of data with the selected ones. The algorithm should take into account the continuous appearing of not yet identified TAGs in the reader interrogation zone. It is realized by the additional block (7) while the other blocks (1-6) are the same like in the static case. The block (7) is a conditional block of checking whether there are new not yet recognized TAGs in the reader operation zone. The internal loop of the algorithm represents a recognition of all new (so far unknown) TAGs entering the range of reader operation. This loop may be executed so many times how many new TAGs appeared in the reader zone of operation during the considered time interval. Because of always possible collisions the number of loop executions may be also greater. The external loop illustrates the data exchange with a selected TAG [4].

In design of the communication protocol for identification of TAGs with their dynamic layout may be helpful the following considerations. Often TAGs and reader use so called SLOTTED ALOHA protocol in which TAGs answer to reader interrogations randomly but only in definite timeslots [5]. The number of possible timeslots depends on TAGs type, it may be for instance of order of ten or greater. If we denote:

n – the current number of TAGs in reader interrogation zone,

k – number of timeslots in that TAGs may respond to reader interrogations,

then the probability of the event that one of TAGs will answer in a definite timeslot equals 1/k.

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The probability that in the one, definite timeslot there will be a response from only one TAG (it is equivalent to the absence of collision in this slot and the recognition of this TAG) may be expressed as

1 n 1 k1 1 k1 p − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = (1)

Repeating this consideration for all possible timeslots one can obtain that probability of the recognition of the one definite TAG is equal to

1 n k 1 1 k1 p − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = (2)

Thus the average number nr of recognized TAGs in one interrogation cycle will be

n k 1 1 n 1 n r ⎟ ⋅ ⎠ ⎞ ⎜ ⎝ ⎛ − = − (3)

In dynamic case for good operation of system, the quantity (3) should be greater than the number of new (not yet recognized) TAGs that enter the reader range of operation during the time of recognizing of nr TAGs.

How many cycles of interrogations are needed for recognizing of n TAGs? If initially in the reader interrogation zone there are present n (unidentified) TAGs then after one cycle of interrogation the remaining number of unidentified ones will be (because of (3))

⎪⎭

⎪⎩

⎛ −

=

n

k

1

1

E

n

n

1 n 1 (4)

where E denotes the integer part. After the second cycle of interrogation the quantity of still not identified TAGs will be equal

⎪⎭

⎪⎩

⎛ −

=

− 1 1 n 1 2

n

k

1

1

E

n

n

1 , …etc. (5) The identification of n TAGs finishes when ni = < 0.

In Fig. 4 there are depicted the results of calculations of average number of reading cycles required for identification of all n TAGs provided they respond to

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reader in one of 8 timeslots. The other calculations have shown that increasing of possible timeslots decreases the required reading cycles but not much – for example, for k = 50, n = 16 the average number of cycles is i = 3.

Fig.4. The average number of the reading cycles required for recognizing of n TAGs (k = 8).

It on basis of above results and knowing the duration of one reading cycle it is possible to estimate the time required for recognizing of all TAGs and thus conclude how fast TAGs may move in the reader range zone of operation.

4. CONCLUSIONS

Present problems arising in prototype RFID systems for recognizing of movable objects are being solved by experimental way. These problems do not still allow for commercial implementation of those systems.

In the light of the created legal regulations there is necessary a continuation of complex experimental investigations relating the RFID systems for identification of moving objects aimed to creation of their mathematical models. It would ease the design of such systems regarding the field, electrical and communication conditions of their proper operation.

REFERENCES

[1] Finkenzeller K.: RFID Handbook – Fundamentals and Applications in Contactless Smart Card and Identification, Second Edition, John Wiley & Sons, New York, 2003.

[2] Vogt H.: Multiple Object Identification with Passive RFID Tags, IEEE International Conference on Systems, Man and Cybernetics (SMC '02), October 2002.

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[3] Jankowski-Mihułowicz P., Kalita W., Pawłowicz B.: Communication Algorithm in Anticollision RFID Systems with Inductive Coupling, 30th International Conference and Exhibition IMAPS-Poland’06, pp. 493-496, 24-27 September, Kraków, 2006. [4] Jankowski-Mihułowicz P., Kalita W., Pawłowicz B.: Communication Phases in

Anticollision RFID Systems Dynamic Change of Tags Location, 31st International Conference and Exhibition IMAPS-Poland Chapter, pp. 339-342, 23-26 September, Rzeszów-Krasiczyn, 2007.

[5] Burdet L. A.: RFID Multiple Access Methods, Seminar Smart Environments SS’04, ETH Zurich, Switzerland, 2004

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