Velocity analysis for UHF RFID vehicle license plateMO Ling-fei1, QIN Chun-fang1 TANG Xiao-fen2

(1 State Key Laboratory of Industrial Control Technology, Department of Control Science and Engineering Zhejiang University, Hangzhou, 310027, China)

(2 Huawei Technologies Co. Ltd., Shenzhen, 518129, China) lfmo@iipc.zju.edu.cn

Abstract—Ultra-high-frequency radio frequency identification (RFID UHF) vehicle license plate is a promising technology for traffic management because of its special advantages. In practical applications, the reading effect is closely related to the vehicle velocity. This paper analyzed the effective vehicle velocity for the steadily reading of the UHF RFID vehicle license plate. The smallest time for one single tag to be effectively read, the relationship between the tags reads and the vehicle velocity, the maximum vehicle velocity to be effective read, are analyzed and measured. Measurement results were in good agreement with the theoretical analysis. Studies have shown that the reading stability of the UHF RFID vehicle license plate is closely related to the effective read range of RFID tags, readers reading cycle and the vehicle velocity. In general vehicle velocity, UHF RFID vehicle license can be effectively read

Keywords- RFID; Tag; vehicle; vehicle license plate

I. INTRODUCTION

Radio frequency identification (UHF RFID), a remote automatic identification technology via radio frequency [1], has large application in traffic control. It can be read and written simultaneously in a long distance with very high rate and low cost without battery no matter the tag is soiled or dirty. RFID technology has been widely applied in vehicle management, such as highway and car park management. [2-7]. Furthermore, electornizaiton of the vehicle license plate is one of the crucial technologies for the information-based management of vehicles in the future. Therefore, imbedding the UHF RFID tag into the license plate can not only enhance the efficiency of vehicle and license plate management but also is beneficial to prevent the occurrence of fake license plate and theft, strengthen peccancy monitoring and realize automatic charging and enter passing without stopping. However, because the reader antenna for the vehicle license plate is normally installed at the two sides of the road or above the road, it can't read towards the tag directly, thus the read range of the tag is limited, within which the reading stability and reliability of the UHF RFID vehicle license plate are greatly affected by the vehicle velocity.

This paper investigated the impact of the vehicle velocity on the reading stability of the UHF RFID vehicle license plate by means of measuring the reader's reading times of the given vehicle passing a certain read range at various velocities. The reading cycle and the effective reading rate of the vehicle license plate were theoretically analyzed first. Next, the reading cycle of a single tag via a radio frequency analyzer was analyzed followed by the measurement of the relationship between the vehicle velocity and the reading

times. Finally, the conclusion and the prospects were drawn on the UHF RFID vehicle license plate. It is studied that the reading stability of the UHF RFID vehicle license plate is closely related to the reading cycle of the reader and the vehicle velocity. In addition, the UHF RFID vehicle license plate can be read by existing commercial readers when the vehicle drives at a proper velocity.

II. THEORETICAL ANALYSIS

The actual read range of the vehicle license plates imbedded with the UHF RFID tag is determined by the tag performance, the reader performance and the reader antenna placement. Considering that this paper is mainly focused on the impact of the vehicle velocity on the reading performance (reading times), the read range can be supposed to be a fixed value and only the reading times of the given vehicle passing this fixed read range at different velocity shall be taken into account.

Reading of the vehicle license plate is shown in Figure 1. The reader antenna is hanged above the road to read the license plate downwards. The effective read range of the license plate is �d.

In order to read the vehicle license tag effectively, the effective read range (�d) of the tag shall >0 and the reader can read the tag within this range stably. Whether the tag can be read by the reader within the limited read range is determined by the reading cycle and rate of the reader as well as the travel speed of the tag.

o

h

Reader antenna

Vehicle license plate

Valid read range �

Figure 1. Reading of the UHF RFID vehicle license plate

Provided that the vehicle velocity is v, the time that this vehicle needs to drive through is

dtv�

� � (1)

Whether the tag can be read by the reader within the effective time �t is depended on the reader’s performance in two aspects:

2010 International Conference on Optoelectronics and Image Processing

978-0-7695-4252-2/10 $26.00 © 2010 IEEE

DOI 10.1109/ICOIP.2010.242

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2010 International Conference on Optoelectronics and Image Processing

978-0-7695-4252-2/10 $26.00 © 2010 IEEE

DOI 10.1109/ICOIP.2010.242

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2010 International Conference on Optoelectronics and Image Processing

978-0-7695-4252-2/10 $26.00 © 2010 IEEE

DOI 10.1109/ICOIP.2010.242

722

1) Query cycle of the reader. The UHF RFID tag is working by means of passive query and the reader sends query command to the tag periodically. If there is no response, the reader will pause for a while. If there is a response, the reader will read the tag in accordance with the protocol. Figure 2 shows the reading cycle of the UHF RFID reader for single tag. If the query cycle of the reader is Tquery,the time for a tag to be read shall be:

queryt T� � (2)

Reader’s query time

Reader’s pause time

Tag reading time

Tquery Tread

No tag response Tag response

Time

Figure 2. UHF RFID reader’s reading cycle (single tag)

2) Reading rate of the reader. When the reader queries a tag, it will respond by the tag and transmit the tag data according to the communication protocol. The whole process needs a reading time Tread. The reading process of a single tag in conformity with EPC Gen2 protocol is shown in Figure 3 [8]. The reader will continue to send out Select and Query command after it sends out continuous waves for a while. If tag response is received, a RN16 data will be sent back. Next, the reader will send out ACK command and the tag will give back EPC data, which will be confirmed by the reader. If the reading cycle is not long enough, the reader can't make confirmation effectively, thus the reading fails.

Time

Rec

eive

sign

al

Continuous waves

Select Query RN16 ACK PC+EPC+CRC16 QueryReq

Figure 3. UHF RFID single tag reading (single cycle)

Therefore, the time for a complete and effective tag reading shall be:

query readt T T� � � (3)

If the vehicle drives through the effective read range �dof the UHF RFID license plate, its velocity shall be:

query read

dvT T

��

� (4)

The above are the conditions for the reader to read the tag signal a time. In practical application, the tag has to be read several times to ensure reading stability and reliability of the vehicle license plate. We can see from formula (1) and (3) that if the vehicle drives through the effective read range at the velocity of v, the reading times of the tag shall be:

query

read

t Tn

T� �

� query readt T T� � � (5)

III. READING CYCLE MEASUREMENT OF SINGLE TAG

In order to effectively analyze the impact of the vehicle velocity on the reading times of the license plate tag, the query cycle of the reader and the reading time of a single tag shall be measured first.

By using the NI-100 RFID Test System provided by NI Company, the UHF RFID tag reading and response signal waveform can be measured effectively. This system is built on the basis of the NI radio frequency module, which has the function of software radio. This measurement only used the reception function of this system. The system structure is shown in Figure 4. It is composed of NI-5640R intermediate frequency processing module and NI-5600 down converter. The NI-5600 down converter is connected to a receiving antenna to receive the RFID reader’s query signal and the tag’s response signal. The NI-5640R intermediate frequency processing module is used for the collection of the real-time intermediate frequency signals and the background processing. The baseband signal waveform can be easily observed via this system.

PXI Chassis

PCI Chassis

Computer Labview

NI-5640R

NI-5600

IFReceiving antenna

Tag

RFID reader

Figure 4. Reading cycle measurement of single tag

The CSL-461 4-Port EPC Class1 Gen2 UHF RFID reader produced by CSL (CONVERGENCE SYSTEMS LIMITED) was applied. This type of reader adopts Impinj (Impinj®, Inc.) technology with good reading accuracy and rate and is applicable in the environment with many closely spaced readers. During reading, the maximum reading rate is 640kbp with only one tag. The test results are indicated as follows:

1. The query cycle of the reader is 24ms without any tag, which is shown in Figure 5.

2. The reading cycle of a tag is 6.7ms, which is shown in Figure 6.

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24 ms

Query cycle

Figure 5. Measurement result of the reader’s query cycle

6.7ms

Reading cycle

Figure 6. Measurement result of the reading cycle of the reader for one tag

IV. MEASUREMENT OF THE EFFECTIVE READING TIMES OF THE VEHICLE LICENSE PLATE

In order to verify the impact of the vehicle velocity on the reading times of the license plate tag, a measurement bench was built up in the open air for field test. The structure of the measurement bench is shown in Figure 7. The measurement bench is door-shaped with a height of 2.5m and a width of 4m. In order to improve the reading stability, a reading antenna erected on one side of the road and another antenna hanged above the road were used. The latter is located at the center of the beam of the measurement bench, and the former is fastened on the stand column of the measurement bench 1m above the ground. The deflection angle of the surface antenna is 60°. The vehicle passed through the measurement bench at different velocities to measure the reading times of the license plate tag.

Because commercial electronics vehicle license plate with RFID tag has not been produced yet, a self-made UHF RFID anti-metal tag designed with microstrip antenna was attached on the vehicle license plate to simulate practical application. The setup of the vehicle license plate is showed in Figure 8.

Hanged reader antenna

Side reader antenna

Figure 7. Measurement bench for reading times of vehicle license plate

Vehicle license plate RFID tag

Figure 8. Setup of the vehicle license plate

The reader is the same as the one used in the reading cycle measurement. Its output power is 30dBm and the gain of the antenna is 6dBi. Then, the radiation power is 36dBm, which is 4W EIRP.

According to the vehicle velocity range given in the theoretical analysis, the vehicle velocity shall meet the formula (4) as the vehicle passes the measurement door. If the vehicle drives too fast, the tag can not be read accurately. In addition, as the vehicle velocity increases, the effective reading times of the tag reduce accordingly.

The effective read range �d of the vehicle license plate was measured first. According to the setting of the measurement bench and the reader, the effective read range�d=4m. As for a single tag, the reading rate of the reader is 150 reads/s based on the reader’s parameters setting in this paper, that is Treads=1/150(s)=6.7ms. According to formula (5), we can find out that the tag’s reading times at different vehicle velocities shall be:

( / )query query

read read

t T d v Tn

T T� � � �

� � (6)

The test vehicle drove passing the measurement bench at the velocity of 10km/h, 15km/h, 20km/h, 25km/h, 30km/h, 35km/h, 40km/h and 45km/h respectively. In order to ensure the accuracy of the measurement, the vehicle at each velocity was tested 3 times. The measurement results are shown in Table 3. We can see that the measurement data have good repeatability and the measurement values are in

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conformity with the theoretical computed values. The reading times of the vehicle are far more than the critical time (one time), which shows that the vehicle could be read reliably as it passed the measurement bench. Therefore, mis-reading and failed reading can be avoided.

From the above analyses we can see that the time for one single reliably reading is 24ms+6.7ms=30.7ms. Provided that the effective read range is 4m, the maximum vehicle velocity is approximately 130m/s, about 469km/h.

Table 1 Measurement results of the reading times of the vehicle at different velocity

Velocity

(km/h) Theoretical Test value Test value 2 Test value 3

10 216 174 166 181 15 144 136 145 119 20 108 172 114 107 25 86 87 78 99 30 72 89 70 65 35 62 62 73 49 40 54 62 41 55 45 48 53 59 54

V. CONCLUSION

This paper analyzed the reading performance of the UHF RFID license plate at different driving velocities. It is found out through theoretical analysis and measurement that the reading stability and effective reading times of the vehicle license plate are closely related to the vehicle velocity. Therefore, it is concluded as following:

1. The effective reading of the UHF RFID vehicle license plate is affected by the read range, vehicle velocity, reader’s query cycle and reading cycle. In order to ensure reliable reading of the tag, the effective reading time shall exceed the sum of the reader’s query cycle and a single tag’s reading cycle.

2. The effective reading times of the vehicle license plate is not only affected by the read range and the vehicle velocity but also decided by the reading cycle of the reader, demonstrating the tag reading capability within the effective reading time.

3. The maximum vehicle velocity can be deduced according to the minimum effective reading time (the sum of the reader's query time and a single tag’s reading cycle). The maximum vehicle velocity can reach over 400km/h under the single tag mode according the performance of the measurement reader.

Therefore, the effective application of the UHF RFID vehicle license plate requires properly design of the reader’s effective read range, effective reading time and query cycle, a single tag’s reading cycle as well as the vehicle velocity. However, the UHF RRID vehicle license plate can meet the requirement of the vehicle velocity in practical use.

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