Evaluation of 13.56 MHz RFID system performance considering communication distance between reader and tag

Abstract

Keywords

1. Introduction

2. RFID system

3.1. Resonance frequency of RFID tag

Footnotes

References

Acknowledgement

RFID system has become one of the most useful tools for the management of the library. Using electromagnetic coupling, an RFID tag can get power supply by a reader and communicate with it for data exchange. Because the RFID system enables non-contact communication, various services and applications including the management of a library catalogue are possible. However, because the system is affected easily by neighboring environment, the communication performance is low. In this paper, by using 13.56 MHz RFID system, we evaluate the resonance frequency of RFID tag and the communication distance between the reader and the target tag when some tags becoming as interference sources come close to each other, and show that the adjacent tags near the target tag do not only reduce the performance of RFID system but also expand the communication distance.

Automatic identification Radio Frequency Identification RFID resonance frequency communication distance

The progress of the radio technology provides us with many wireless services such as: TV, Radio, mobile phone, and so on. Using a radio wave technique to get the information from goods without direct contact is very important. This technique is called Radio Frequency Identification (RFID) [6].

The RFID technique uses electromagnetic coupling or radio wave for data exchange between the reader/writer and the tag. The RFID system using this technique can be applied in the case when a large quantity of goods is managed. For example, in the library, by using RFID system, we expect the efficiency of the following services to increase: 1) rental of book and the return, 2) collection inventory, 3) search of the book, 4) access control of users [7,18,19]. Furthermore, if RFID system is integrated with smart phones and sensor networks as new services for the library system to trace books and users, the system may send useful information to users.

A lot of applications using RFID are proposed [4,10,17,20,23]. Moreover, the development of RFID devices and performance evaluation using RFID system are performed to realize reliable RFID systems [1–3,5,8,9,13,14,16,21]. For example, at Kyushu University Library, a RFID system was implemented and the usefulness of RFID system was evaluated as a joint study with Mitsubishi Materials Corp. and Checkpoint Systems Inc. [7]. Furthermore, in [8], using 13.56 MHz RFID system based on the international standard of ISO15693, the influence that papers or other RFID tag give to the resonance frequency of an RFID tag have been evaluated. In [9], the performance of a table type RFID reader using 13.56 MHz have been evaluated and the relation between the reading rate and the distance between metallic plate and RFID reader was investigated. In these experiments, it is reported that the performance depends on the reading range of RFID system. Especially, when tags get too close, the performance largely decreased. The performance of the tag system changed by the distance of the bookshelf and the desk including conductive materials such as iron. The main reason of this problem is the shift of the resonance frequency by the influence of other things. Added to these researches, recently, in order to improve the quality of data obtained by RFID tag, RFID data cleaning operation method is considered [22].

In this paper, using 13.56 MHz RFID system based on the international standard of ISO15693, we evaluate the influence that other RFID tags, which become as the interference sources, give to the resonance frequency of the RFID tag and the communication distance between the reader and the target tag. Furthermore, we show the possibility that the adjacent tags near the target tag do not only reduce the communication performance but also expand the communication distance.

The paper structure is as follows. In the next section, we present the RFID system. Then, we give the evaluations of the resonance frequency and the communication distance RFID tag system. Finally, we conclude the paper.

Fig. 1.

Basic concept of RFID system.

An RFID system is one of the technique used for the automatic identification. The automatic identification means to “automatically input bar-code, magnetic-card, RFID data, etc. with the use of hardware and software and not human intervention in order to recognise the content of the data”. Also, the biometrics, OCR, the machine vision are included in this technique. Because RFID system uses the wireless communication is able to get the ID from the tag without touching the tag.

An RFID system is made up of two components as shown in Fig. 1 [6]. One is the RFID tag, which is located on the object to be identified, and another is the reader/writer. The RFID tag normally does not have the power supply to work, so the reader/writer not only exchange the data, but also supply the power and clock signal to the RFID tag. To do this, the RFID tag has an antenna or a coil as a coupling element for the communication. Figure 2 shows a practical example of label type RFID tag using 13.56 MHz. The small black box inside the coil in this figure is an IC chip. The RFID tag has various shapes, for example, label, card, coin, and stick.

RFID systems are classified according to the operating frequency, the physical coupling method and the communication range. For example, available operating frequency is 135 kHz, 13.56 MHz, 900 MHz, and 2.45 GHz. The operating frequency is used properly depending on a purpose.

Fig. 2.

Example of 13.56 MHz RFID tag in practical use.

3. Evaluation of 13.56 MHz RFID system

13.56 MHz RFID systems are widely known and used for the management of a library catalogue at various libraries. For example, in Kyushu University, using 13.56 MHz RFID system, the effects of the introduction of the RFID system to the library was evaluated [7]. In [8], using 13.56 MHz RFID system, the influence that papers or other RFID tag give to the resonance frequency of a target RFID tag have been evaluated. However, in a library, many books with a RFID tag are placed on the bookshelf. In this situation, some tags are located in the access range of reader and affect the communication between the target tag and the reader. In this situation, because tags come close to each other, the interference occurs among tags and a resonance frequency of the target tag and the communication distance between the reader and the target tag change.

In this paper, we assume the situation mentioned above and the resonance frequency of the target tag and the communication distance between RFID reader and the target RFID tag are evaluated when there are some adjacent tags, which become as the interference sources, stand in a single line with the target tag. 13.56 MHz RFID system based on the international standard of ISO15693 is used in this experiment.

13.56 MHz RFID system is connected by the inductive coupling. In this case a tag is combined with a reader in a resonance frequency that corresponds with the transmission frequency of the reader. Generally, RFID tag has the characteristic between induced voltage at a tag coil and frequency [11,12]. If the induced voltage to occur in the transmission frequency of the reader is lower than the minimum of the voltage necessary to drive a tag, the tag does not work. On the other hand, in the case that the resonance frequency of the tag becomes the transmission frequency of the reader, the induced voltage becomes maximum and maximum power is supplied to the tag. In this case, because the induced voltage of the transmission frequency of the reader is larger than the minimum of the voltage necessary to drive a tag, the tag works. The resonance frequency of the tag is affected easy by neighboring environment and a resonance frequency becomes lower. To know the characteristics of the resonance frequency of the tag in real situation is very important to develop the reliable RFID systems.

For this experiment is prepared a testing bench as shown in Fig. 3. Using the grid dip oscillator DELICA DMS-230S2 as shown in Fig. 4, the resonance frequency of the target RFID tag is observed when some tags are in a single line with the target tag as the interference sources. The number of tags becoming the interference sources, which is put on the target, is changed from 1 to 6 and the distance between each adjacent tags is set to d [mm].

Fig. 3.

Measurement image of the resonance frequency of RFID tag affected by another RFID tags.

Fig. 4.

Photo of grid dip oscillator DMC-230S2.

Fig. 5.

Resonance frequency f of RFID tag vs distance d between each adjacent tags.

The experimental results are shown in Fig. 5. In this figure, vertical axis shows the resonance frequency f [MHz] of the target tag, horizontal axis shows the distance d [mm] between each adjacent tags and N is the number of tags becoming the interference sources. The measurements of each situation are carried out five times. The dotted line in Fig. 5 shows the resonance frequency of the target RFID tag when observed alone and is 13.87 MHz.

These results show that tags near the target have higher influence to the resonance frequency of the target tag. The resonance frequency of the target tag becomes low when the distance between each tags is narrow. In addition, the variation of the resonance frequency becomes large in accordance with the increment of tags as the interference sources. On the other hand, the coupling among the target tag and other tags is small when the distance of these is separated more than 50 mm.

Furthermore, we sandwich papers between tags and evaluated the relation between the resonance frequency and the distance between each adjacent tags including the influence of papers. This situation becomes the image that books having a tag are put on the bookshelf. The results are shown in Fig. 6. In this evaluation, the interference tags are 1 or 2 pieces and the space between adjacent tags is filled up using copying papers. The dotted lines in this figure show the resonance frequency which air fills up between adjacent tags, and these results are the same as the result shown in Fig. 5. In the case that papers filled up between adjacent tags, the resonance frequency is decreased more when the distance between adjacent tags is separated more than 15 mm. On the other hand, when the distance between adjacent tags is less than 15 mm, because the coupling between tags becomes larger, the increase of the stray capacitance by the addition of the papers is ignored.

Fig. 6.

Resonance frequency f of RFID tag vs distance d between each adjacent tags.

3.2. Communication distance between reader and target tag

As shown in the previous section, when some tags come close to each other, the interference occurs among tags and the resonance frequency becomes low. In this situation, if the resonance frequency have big difference from the transmission frequency of the reader, because the induced voltage becomes low, the tag may not sufficiently work. In order to communicate with the tag under this condition is needed to give a higher magnetic flux density to the tag, and we may consider the necessity to short the communication distance. In this section, we evaluate the communication distance between the reader and the target tag when the tags becoming as the interference sources are placed between the reader and the target tag.

In this experiment, we use a handmade type RFID reader as shown in Fig. 7. As the reader is used ISO15693 IC tag kit sold by SOFEL, and has an approximately 45 mm in diameter loop coil as the communication antenna of the reader.

Fig. 7.

Photo of handmade type RFID reader.

As the first, we evaluate the basic performance of communication distance between a tag and this RFID reader. Figure 8 shows the measurement image of the communication distance. As shown in this figure, we define an axis of coordinates, and evaluate the maximum communication distance of each axial direction.

The result is shown in Table 1. In this experiment, we use 5 samples and observed the communication distance. The each sample’s result is average of the 10 times observation of each tag. ‘Average’ in this table is average of these samples’ results. When the reader is placed that the magnetic flux may pass through most inside of the antenna coil as shown in Fig. 8(a), the communication distance $L_{x}$ becomes about 90 mm. On the other hand, when the antenna coil is placed beside the RFID tag coil as shown in Fig. 8(b), because the magnetic flux pass through the antenna coil becomes below half, the communication distance also becomes below half and in this case $L_{y}$ and $L_{z}$ are becomes about 40 mm and 31 mm respectively.

Fig. 8.

Measurement image of the communication distance between reader and target RFID tag.

Table 1

Communication distance between reader and target RFID tag

Based on these results, we evaluate the effect of adjacent tags to the communication distance between the reader and the target tag. Figure 9 shows the measurement image of the communication distance between the reader and the target tag. Some tags becoming as the interference sources are put between the target tag and the reader. The number of tags is changed from 1 to 6 and the distance between each adjacent tags is set to d [mm]. In this experiment, as the target tag, we used the tag of the sample number 1 shown in Table 1.

The experimental results are shown in Fig. 10. In this figure, vertical axis shows the communication distance $L_{x}$ [mm] between the target tag and the reader, horizontal axis shows the distance d [mm] between each adjacent tags and N is the number of tags becoming as the interference sources. The measurements of each situation are carried out five times and the average values are shown in this figure. The dotted line in Fig. 10 shows the communication distance between the target RFID tag and the reader when observed alone and is 101 mm.

Fig. 9.

Measurement image of the communication distance between reader and target RFID tag.

Fig. 10.

Communication distance $L_{x}$ between reader and target RFID tag vs distance d between each adjacent tags.

Before carrying out this experiment, because the resonance frequency becomes low by the interference from other tags, we assumed that the communication distance decreased. In fact, when the distance between each adjacent tags is less than 10 mm, in all cases the communication distance was decreased compared with the distance of a single tag case. However, when the distance between each adjacent tags was more than 10 mm, it is possible to have a long communication distance compared with the single tag case. For example, when there were 5 or 6 pieces of tags as the interference source and the distance between each adjacent tags was about 30 mm, the communication distance was increased by about double. Even in the case of one piece of the interference source could expand the communication distance to 1.5 times in the case when the distance between each adjacent tags was about 60 mm.

From these results, we conclude that when there are some tags becoming as the interference sources and stand in a single line with the target tag, the tags sandwiched between the reader and the target tag collect the magnetic flux around the tag, and relay it to the target tag or/and the reader.

Next, we change the position of the tags becoming as the interference sources and evaluate the communication distance between the reader and the target tag. Figure 11 shows the measurement image of the communication distance between the reader and the target tag. In this experiment, 2 pieces of tags are used as the interference sources and change the position of the target tag as shown in Fig. 11. The experimental results are shown in Fig. 12.

In the case of Fig. 11(a), the communication distance between the reader and the target tag is expanded about 1.7 times by the effect of the interference tags, and this result is the same as the result shown when $N = 2$ in Fig. 10. On the other hand, in the case of Fig. 11(c), the communication distance between the reader and the target tag slightly increased, when the distance between each adjacent tags became bigger than 60 mm. In the case of Fig. 11(b), the communication distance is expanded by the interference tags. Furthermore, this result is similar with the result of Fig. 10 when $N = 1$ . From these results, it is clearly shown that when there are tags becoming as the interference sources between the reader and the target tag, the tags have the possibility to expand the communication distance between the reader and the target tag.

Fig. 11.

Measurement image of the communication distance between reader and target RFID tag.

Fig. 12.

Communication distance $L_{x}$ between reader and target RFID tag vs distance d between each adjacent tags.

Finally, we consider about the situation of checking books which stands in line in the bookshelf for collection inventory. In this case, an antenna coil of the RFID reader is put on the side of the tag’s coil as shown in Fig. 8(b) and the information from tags placed near a reader antenna are collected. Figure 13 shows the measurement image of the communication distance between the reader and the target tag of this situation. In this experiment, the reader’s antenna is put on the same plane with the tag’s coil. For example, as shown in Fig. 13, we call the plane where the coil of the tag D makes as the plane D. The length $L_{y}$ between the reader antenna and the tag, the distance L between the reader and the target tag were observed on each plane.

Fig. 13.

Measurement image of the communication distance between reader which is put on the side of the tag’s coil and target RFID tag.

Table 2 shows the communication distance $L_{y}$ between the tag and the reader which is put on the same plane with tag when the reader was able to communicate with target tag. In the case that the reader antenna put on the plane T, when the distance d is larger than 50 mm, the effect of the adjacent tags becomes small and $L_{y}$ becomes about 40 mm and this result gets closer to the result in Table 1. On the other hand, when the distance between adjacent tags become short, $L_{y}$ becomes small by the interference from the adjacent tags. Similarly, we evaluated $L_{y}$ on the plane A, B, C, and D respectively. In these case, when the distance d becomes 40 mm, $L_{y}$ becomes maximum. Using these results, we evaluated the communication distance L between the reader and the target tag and showed in Fig. 14. From these results, when the adjacent tags near the target stand in a single line with the target tag, the adjacent tags can relay the signal from the target tag to the reader which is put on the side of the adjacent tag’s coil. In this experiment, using the adjacent tags we could expand the communication distance between the target tag and the reader to about 180 mm.

This result is very useful for the check of books in the bookshelf. Furthermore, the number of antennas which is used in the intelligent bookshelf can be reduced. The intelligent bookshelf put on the RFID reader and can monitor the situation of books on the bookshelf. Additionally, as if novices can easily make 3D model with combination of basic parts [15], this approach may be useful for expanding the reading performance of a large number of tags which are, for example, in the shopping bag.

Table 2

Communication distance $L_{y}$ between tag and reader which is put on the same plane with tag when reader was able to communicate with target RFID tag

Distance d	Plane

	T	A	B	C	D
10 mm	6	7	1	7	8
20 mm	8	15	8	8	13
30 mm	14	24	13	15	18
40 mm	25	31	17	22	18
50 mm	41	30	9	21	–
60 mm	44	16	7	–	–
[mm]

Fig. 14.

Communication distance L in a straight line between reader and target RFID tag vs distance d between each adjacent tags.

4. Conclusions

In this paper, we evaluated the resonance frequency of RFID tag and the communication distance between the reader and the target tag when some tags becoming as the interference sources came close to each other.

Using a grid dip oscillator, we measured the resonance frequency of RFID tag under various conditions and showed that the tags near the target have a big influence to the resonance frequency of the target tag. Specifically, the resonance frequency of the target tag becomes low when the distance between each tags is narrow and the variation of the resonance frequency becomes large in accordance with the increment of tags becoming as the interference sources. On the other hand, the coupling among the target tag and other tags is small when the distance of these is separated more than 50 mm.

Furthermore, using the handmade type reader, we evaluated the communication distance between the reader and the target tag and showed the possibility to have a long communication distance compared with the single tag case when the tags becoming as the interference sources stand in a single line with the target tag.

In the future work, we want to evaluate in details the influence that many other RFID tags becoming as the interference sources gives to the communication performance of RFID system for various situation and to suggest methods to improve the performance.

I am indebted to Hirotaka Kato for his assistance in experiments.

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