A Novel Solution to the Reader Collision Problem in RFID System

Dong Wang IEEE Student Member Jianwei Wang IEEE Student Member Yuping Zhao IEEE Member

School of Electronics Engineering and Computer Science Peking University Beijign PR. China

dongwang@pku.edu.cn carfield@pku.edu.cn Yuping.zhao@pku.edu.cn

Abstract-Radio Frequency Identification (RFID) is one of the hottest research topics in recent communication area. In the RFID system, the Reader Collision Problem (RCP) is considered to be the bottleneck of the system throughput and reading efficiency. In this paper, a novel RCP solution-Central Cooperator (CC)-RFID is proposed to reduce the reader collisions by sharing the tag information among adjacent readers in RFID network. Both analytical and simulation results show that the proposed schemes improve the reading speed and efficiency dramatically compared to current popular RCP solutions.

Key Words- RFID； RCP； MAC； Anti-Collision

I. INTRODUCTION

With the rapid development of RFID technology, many applications now require several readers to operate in close proximity of each other. In such a case, if several readers work simultaneously, the signal from one reader may interfere with the signal from others. For example, when more than two readers communicate with one tag, the tag will not be able to respond to any readers. The interference between adjacent readers is called Reader Collision Problem (RCP) in RFID systems.

Current solutions to the RCP problems can be classified into two categories: scheduling based approach [1]~[3] and coverage based approach[4]. As for the scheduling based approach, the available system resources, such as frequencies and time are allocated properly among readers to prevent readers from transmitting simultaneously. This kind of approach can reduce the possibility of reader collisions effectively, however it requires the system to establish and maintain information over the network, which will be time and energy consuming. As for the coverage based approach, the reading ranges of readers are adapted dynamically to reduce the overlapped area between adjacent readers as much as possible. This kind of approach increases the space re-used ratio, but it usually needs a central node to calculate the distance between every two readers and adjust their reading ranges, which will increase the complexity of realization and cost of the system.

In RFID systems, the tags read by adjacent readers contain exactly the same information. However, none of the current RCP protocols takes this information redundancy into

consideration. If the same tags could be read by one reader and shared with others, the reading efficiency in RCP environment will be further increased. Based on this fact, the paper proposed a novel RCP scheme-Central Cooperator (CC)-RFID to improve the performance of multiple readers RFID system.

II. A NOVEL CC-RFID SYSTEM

In CC-RFID system, with the introduction of the central operating device –Central Cooperator (CC), the present ‘multiple points to multiple points’ (MP2MP) collision problem is converted into two ‘multiple points to one point’ (MP2P) classical collision problems. Moreover, the reading queries from several readers could be multiplexed by CC and the same tag information could be stored and shared among adjacent readers.

A. CC-RFID Achitecture Located in the central position of RFID system as shown in

figure 1, the CC communicates with readers and tags simultaneously, receiving the readers’ requests and reading tags. Thus, CC controls the entire working process of RFID system.

Fig.1 CC-RFID System Architecture

Sponsor-LG Electronics, Korea

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CC is made up of 6 modules as shown in figure 2:1) CC Receiver Module1: to receive queries from readers. 2) CC Relay Module: to multiplex and relay readers’ queries 3) CC Sender Module1: to send out readers’ queries to tags. 4) CC Receiver Module2: to receive the feedback from tags; 5) CC Storage Module: to store the data from tags; 6) CC Sender Module2: to send tags’ data back to readers in a sub-carrier.

CC ReceiverModule1

CC RelayModule

CC SenderModule1

CC ReceiverModule2

CC StorageModule

CC Sender Module2

Fig. 2 Modules of Central Cooperator (CC)

B. CC-RFID Working Scheme In CC-RFID, the system provides two kinds of working

schemes, which are based on different information handling procedures carried out in CC. In the first working scheme, the CC multiplexes several readers’ requests into a single query, which contains all tags to be read by the requesting readers. Then CC sends out the query to read tags and tags respond to each reader separately. While in the second working scheme, the CC reads and stores the tag’s information, sharing it with all requesting readers. Meanwhile CC updates the tags information periodically to keep the data from being out of date.

CC-RFID scheme 1

The flow chart of the first working scheme is shown in figure 3. Firstly, in the ‘Reader to CC’ block, multiple readers compete for the access to the channel and send their reading requests to CC. Then the request information goes into the CC ‘Schedule and Multiplexing’ block, in which CC demodulates the request information and multiplexes them into a single reader’s query. After that, through the ‘CC to Tag’ block CC sends the multiplexed query to read tags. In the ‘Tag Feedback’ block, tags receive the query and feed back their identification information to CC. Finally in the ‘CC to Reader’ block, CC selects the tag response information by examining the original requests from readers and sends it to each reader separately. When all readers get their information, the reading process is over.

Reader to CC CC Schedule & Multiplexing CC to Tag

Tag Feedback CC to Reader CC Module

Fig. 3 The first working scheme of CC-RFID

CC-RFID scheme 2

The flow chart of the second scheme is shown in figure 4. The working procedure of CC-RFID system is divided into two independent communication processes. One is the reading communication carried out between readers and CC, while the other is the updating process going on between CC and tags. As for the first process, in the ‘Reader to CC’ block, multiple readers compete to send CC their reading requests. And in the following ‘CC Search’ block, CC receives readers’ requests and looks through the CC Storage Module for the corresponding tags’ ID requested by readers. After collecting all the required information, CC sends them back to readers through the ‘CC Feedback’ block and the reading process is over. However, since the tags information stored in CC may be outdated as time goes by, a renewal timer is set up in the ‘CC Timer’ block to start the second communication process which will update the latest tag information to CC periodically. In this communication process, CC sends updating query by the ‘CC Update’ block to read tags in its coverage. Then in the ‘Tag Response’ block, tags respond their latest information to CC, which will keep the information stored in the CC Storage Module up to date.

Reader to CC CC Search CC Feedback

CC Timer CC Update

CC Module

Tag Response

CC Module

(a) Reading Communications between Readers and Tags

(b) Updating Process between Readers and Tags

Fig.4 The second working scheme of CC-RFID The two working schemes can be selected to adapt to

different RFID application environments. As for the first scheme, it is appropriate to apply in scenarios where tags have

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a high mobility and their information changes quickly. While the second scheme performs better in scenarios where tags change slowly but the reading speed is considered to be more critical.

In short, CC-RFID simplifies the complex ‘MP2MP’ access problem as two ‘MP2P’ classical problems by introducing a Central Cooperator (CC) into the system, making the anti-collision protocol design for RCP an easier job to accomplish. Furthermore, CC could both multiplex several readers’ requests together and share the tags’ information among readers, which increases the reading efficiency dramatically. Additionally, CC-RFID provides two working schemes that can be applied in either mobile critical or time critical systems. Last but not least, CC is a powered device, it can charge tags in its vicinity to extend the reading range of low power readers.

III. SIMULATION AND PERFORMANCE COMPARISON

We carry out simulation experiments in OPNET modeler to

verify the improvement in reading efficiency of CC-RFID over current popular RCP protocol PULSE [3]. To guarantee the fairness of the comparison, the simulation parameters are set exactly the same for two schemes as shown in table 1.

Table 1 Simulation Setup

PULSE, with the basic idea of adding an extra beacon

channel to the system, is considered to be one of the most effective RCP solutions which have both high channel throughput and system efficiency [3]. And in simulations, we would like to use the time slots consumed by all readers to finish reading all tags in its coverage as the metric to evaluate the reading efficiency of RFID system with RCP. The smaller number of time slots are consumed, the higher reading efficiency is. The simulation results in both PULSE protocol and CC-RFID scheme is shown in figure 5 and figure 6.

As shown in figure 5, the simulation results show that timeslots needed for reading all tags in reader’s area as a function of the number of readers. It is seen that the reading efficiency of CC-RFID is higher than PULSE protocol. When there are 50 readers in adjacent area and 100 tags in every reader’s coverage, the time slots consumed by CC-RFID is only half of that in PULSE protocol. When tag’s number reaches 200, the time slots used by CC-RFID is just one third of that in PULSE protocol. By multiplexing readers’ requests and sharing tags’ information, the CC-RFID scheme improves the reading efficiency over PULSE in a great sense. Meanwhile, it is shown that the first working scheme in CC-RFID system consumes a little more time slots than the second one. This is due to the fact that, in the first working scheme, CC reads tags based on readers’ requests. The reading frequency and speed are higher than the updating process in the second scheme.

10 15 20 25 30 35 40 45 500

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RCP Performance Comparison (Tag Number :100)

RFID System in PULSECC-RFID Working Scheme 1CC-RFID Working Scheme 2

Fig. 5 RCP Performance Comparison (Tag Number: 100)

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RCP Performance Comparison (Tag Nmuber: 200)

RFID System in PULSE Protocol

CC-RFID working scheme 1

CC-RFID working scheme 2

Fig. 6 RCP Performance Comparison (Tag Number: 200)

PULSE Scheme CC-RFID Scheme

No. of readers

10～50 10～50

Tags to be read

100，200 100，200

Data channel frequency

915MHz 915MHz

Control channel

frequency

930MHz 930MHz

Data rate 100Kbps 100Kbps Antenna Polarity

Omni-directional Omni-directional

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IV. CONCLUSION

This paper proposed a novel RCP solution-CC-RFID scheme in RFID system. By introducing the central device-CC, a complex collision problem is converted into two classical collision problems. Furthermore, reader’s requests could be multiplexed at CC and tag’s information could be shared. Additionally, CC-RFID does not change the air interface and can be easily applied in current RFID system. Both analytical and simulation results show that the reading speed and efficiency of CC-RFID scheme is highly improved over current popular RCP protocol PULSE. Therefore, CC-RFID is an effective RCP solution in RFID system.

ACKNOWLEDGMENT

This work is financially supported by LG Electronics Korea.

REFERENCES

[1] Waldrop, J.; Engels, D.W.; Sarma, S.E.; ‘Colorwave: an anticollision algorithm for the reader collision problem’

Communications, 2003. ICC '03. IEEE International Conference on Volume 2, 11-15 May 2003 Page(s):1206 - 1210 vol.2 [2] Junius Ho; Engels, D.W.; Sarma, S.E.; ‘HiQ: A Hierarchical Q-Learning Algorithm to Solve the Reader Collision Problem’ Applications and the Internet Workshops, 2006. SAINT Workshops 2006. International Symposium on 23-27 Jan. 2006 Page(s):88 - 91 [3] Shailesh M.Birari ‘Mitigating the Reader Collision Problem in RFID Networks with Mobile Readers” Dissertation for Master of Technology, Indian Institute of Technology, India, 2005 [4] Joongheon Kim; Wonjun Lee; Jieun Yu;Effect of localized optimal clustering for reader anti-collision in RFID networks: fairness aspects to the readers Computer Communications and Networks, 2005. ICCCN 2005. Proceedings. 14th International Conference on 17-19 Oct. 2005 Page(s):497 - 502 [5] “ Information technology automatic identification and data capture techniques – Radio frequency identification for item management Air interface - Part 6: Parameters for Air interface communications at 860-960 MHZ,” Final Draft International Standard ISO 18000-6.

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