a mac scheduling algorithm of the rfid reader system ...

Proceedings of IC-NIDC2010

A MAC SCHEDULING ALGORITHM OF THE RFID READER SYSTEM USING MULTIPLE RECEIVING ANTENNAS Jihyung Kim, Sung Sik Nam, Sung Ho Cho Division of Electrical and Computer Engineering, Hanyang University, Seoul, Korea [email protected], [email protected], [email protected] interference because of having transmitters in one more readers.

Abstract In this paper, we propose a Medium Access Control (MAC) scheduling algorithm to efficiently recognize multiple tags in 900MHz Ultra High Frequency (UHF) Radio frequency identification (RFID) reader, when using multiple receiving antennas. According to the timing of received signal of each antenna, the standard of signal priority is changed to the entering order of the received each signal or the higher value of the Signal to Noise Ratio (SNR). Using additional processing in MAC layer with multiple receiving antennas, it can accelerate the recognition speed of tags comparing with by using current standard based on single antenna. It is possible to free reader to tag interference comparing with by using multiple readers. It is also possible to be compatible with current standard.

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Like these, it is not sufficient to have effect of high-speed recognition of tags in the coverage when just using single antenna or multiple readers for the spread in the industrial environment using multiple tags. In this paper, therefore, we present a Medium Access Control (MAC) scheduling algorithm to recognize multiple tags one reader, when using multiple receiving antennas efficiently. The proposed algorithm is operated by dividing into two ways for more fast tag recognition. The proposed algorithm is operated by dividing into two ways for more fast tag recognition. According to the link timing ( T1 ) of each antenna [4], the standard of priority is changed to the entering order of the received each signal or the value of the SNR. In these processes, tag responses by MAC scheduling are able to be not easily treated to ‘Collision’ or ‘No Reply’.

Keywords: RFID; link timing; anti-collision; MAC scheduling; SNR; SIMO; multiple receiving antennas

The proposed algorithm is compatible with EPCglobal C1 Gen 2 Protocol [4], which is standardized as ISO/IEC 18000-6 Type C. First, it is operated by additional method together with existing anti-collision algorithm. Second, it is designed considering link timing for the compatibility of standard. Although multiple antennas are used, a reader only keeps recognizing one tag at a time in the inventory process, like EPCglobal C1 Gen 2 Protocol. Therefore, it means that it is economical because existing tags based on standard can be still reused.

1 Introduction RFID is automatic wireless data collection technology. Especially in 900MHz UHF band, when a reader sends RF signal to passive tags without self-power, the tags reflect the signal with contending information in a memory of them by using the backscattering method. UHF band RFID reader system has fast processing speed comparing to 13.56MHz High Frequency (HF) [1] [2] [3], In conventional method using this physical feature, First, for accelerating the recognition speed of multiple tags in RFID, enhanced anti-collision algorithm has developed to avoid collision of tags based on single antenna [4][5] because EPCglobal Class-1 (C1) Generation 2 (Gen 2) Protocol [5] standard for 900MHz UHF RFID is based on only single antenna. Second, that standard recommends multiple-reader and dense-reader environment to recognize multiple tags in wide coverage [5], which has problem to solve reader to tag

Having multiple receiving antennas in RFID system, it is similar to Single Input Multi Output (SIMO) system structure that the transmitter has a single antenna, and the receiver has a multiple antenna which has a diversity effect [6]. Therefore, it can be free about reader to tag interference comparing with using multiple readers. The further paper has 4 more sections in it. In section 2, we refer to inventory process in RFID System, and we propose a MAC scheduling algorithm. In section 3, we verify the performance

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our algorithm comparing with using single antenna by Matlab simulation. And in the last section we conclude.

decoding corresponding signal or next tags response of the ACK commands from the reader [5]. Nevertheless SNR value is still significant index regardless of ‘Collision’ because of the smaller SNR value according to the increase of range between each antenna and tags.

2 A MAC Scheduling Algorithm 2.1 Original inventory process In EPCglobal C1 Gen 2 Protocol, the inventory process for recognizing tags is as follows. In response to the start command of the round, a tag sends the 16 bit random value to a reader. If that reader successfully received a RN16 signal, it sends an ACK command to the tags within range. Only one tag that sent the RN16 signal transmits Electronic Product Code (EPC) signal to the reader. Next, the reader judges whether the signal reflected from the tag is successful or not by decoding error checking or next sequence with a corresponding tag. In a successful case, the tag is finally identified. And then the Q value is set at the start of round. If the reader received collision signal from the tags or does not received any signals, Q selection algorithm is performed to reset the Q value [5]. However this process is just based on single antenna. To operate multiple receiving antennas, the reader essentially needs to additional work of the system.

Query TX

Reader

Figure 2. Single tag response

Query TX

RN16(1) RN16(2)

RN16(1) Tag(1)

RX RN16(2) RN16(2) RN16(3)

Query

Tag

RN16

[Single Tag Reply] Interrogator

RN16

RN16 RX

Reader

ACK

Tag(2) RN16(3) Tag(3)

Figure 3. Multiple tags response Tag

RN16 T2

T1

T1

2.3 The procedure & features of the proposed Algorithm

[Multiple Tags Reply With Collision] Interrogator

Query

We designed the algorithm according to the link timing ( T1 ) of each antenna, the standard of priority is changed to the entering order of the received each signal or the value of the SNR. When a reader uses multiple antennas, tags sends the random values of 16 bit (RN16) to the reader. After each antenna of the reader received a signal, The Physical (PHY) layer in the reader starts to decode the signals from each antenna. The MAC layer of the reader checks RN16 data error after receiving its information from PHY layer. If each antenna of the reader received a RN16 at the same time, the MAC layer of the reader uses the result value of the antenna selection diversity to determine the priority of the signals. Therefore, when a reader uses multiple antennas, until the Error checking of RN16, it needs to separate and store RN16 signal.

QueryRep No Reply

Collision Detected

Tag

RN16 T1

T2

T1

T3

Figure 1. Link timing [2] 2.2 Comparing multiple tags response with single tag response In the case of recognizing single tag in EPCglobal C1 Gen 2 Protocol, SNR is very important index [4] because a reader does not need to consider the tag with collision state. It just checks two tags responses states (‘Reply’, ‘No Reply’). On the other hand, in the case of recognizing multiple tags in EPCglobal C1 Gen 2 Protocol, a reader should check three tags response states (‘Collision’, ‘Reply’ and ‘No Reply’). At this time, any responses are not necessarily proportional SNR. We only able to know the tags response states of Query commands from a reader, after

Based on this analysis, the proposed algorithm features are as follows.

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RN16 (Selection) means a strong signal than RN16 (Next) in channel environment.

(1) It should keep link timing ( T1 , T2 ) to use Gen 2 Protocol. Therefore, the tags based on EPCglobal C1 Gen 2 are still could be used in our algorithm. (2) If link timing ( T1 ) of each antenna is different, the standard of priority is the entering order of the signal reflected from the tag. (3) If link timing ( T1 ) of each antenna is same, MAC brings the result value of antenna selection diversity. At this time the standard of priority is signal strength because we consider that received power of the reader or SNR is connected with distance between antenna and tags. RFID system does not require real time system as telecommunications system because it is kinds of half-duplex communications. Therefore a reader can use a stored signal if needed. It will be applied to the operation of antenna selection diversity that could be for an adaptive MAC scheduling algorithm. (4) This algorithm is adaptive method because MAC continuously checks the quality of RN16 signal by stages. If it is failed after checking a RN16, MAC checks another RN16. It would be efficient way to minimize disregarding signal reflected from the tag by our MAC scheduling. (5) This proposed algorithm is determined 3n cases (n: the number of antennas). (6) For reference, if it does not need to keep link timing ( T2 ), this could be more efficient method because a reader does not need to send Query command to the next tag and not need to get RN16 command from the next tag. To the next tag, the reader just starts from ACK command. (7) The time delay when ‘Collision’ is more long than time delay when ‘No Reply’. However, as our MAC scheduling considers timing of entering order, ‘Collision’ has priority more than ‘No Reply’ state. Ant#1 Collision

No Reply

Reply

Ant#2

Result by MAC

Collision

Collision

No Reply

Collision

Reply

Reply

Collision

Collision

No Reply

No Reply

Reply

Reply

Collision

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No Reply

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Reply

Reply

Start

Same T1

Compare T1 of Antennas Different T1

Check RN16 (1st)

Success

Fail Fail

Complete Inventory(1st)

Fail

Success

Check RN16 (2nd) Success

Fail

Complete Inventory(2nd) Success Next Round

Q selection algorithm

Bring the Result Value of Antenna Selection Diversity

Check RN16 (Selection)

Success

Fail Fail

Check RN16 (Next)

Fail

Complete Inventory (Next)

Complete Inventory (Selection)

Fail

Success

Success

Success Next Round

Q selection algorithm

Figure 4. A MAC scheduling algorithm

3 Simulation Result We simulated the expected algorithms results to show system efficiency in MAC comparing with using single antenna. (1) Our simulation complies with the average performance result from 10,000 times. (2) We focus on the system efficiency of the MAC layer in RFID reader system, not considering channel environment include distance between antenna and tags. At this time, data rate is 160KHz for reader to tag, Backscatter Link Frequency (BLF) is 160khz for tag to reader, when T2 5 u Tpri and

Table 1. The result by MAC scheduling: 3n (n = 2).

T3

In the following, RN16 (1st) means a received signal before entering RN16 (2nd) signal. And

0 [7].

(3) We use fixed value c = 0.3, init value Q0 = 4 regardless of tags number.

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(4) In case of using the single antenna, its simulation environment is same to the multiple receiving antennas because it is possible to be treated using one receiving antenna in multiple receiving antennas (5) We consider two (dual) receiving antennas in our simulation comparing with using the single antenna. (6) The coverage between multiple antennas is perfectly separated for simulation. As a result, Figure5 shows that the proposed algorithm is more efficient than by using single antenna in the term of tag identification speed. Figure6 and Figure7 show that the proposed algorithm is more efficient than by using single antenna in the term of reducing ‘Collision’ and ‘No Reply’.

Figure 6. Comparing with collision number

4 Conclusion & Future work This paper proposes a MAC scheduling algorithm of the RFID reader system using multiple receiving antennas to efficiently recognize multiple tags with the compatibility of standard EPCglobal C1 Gen 2 protocol. We focus on system efficiency in MAC layer. In proposed algorithm based on multiple receiving antennas, tag identification speed increased about 10 tags per second than using based on single antenna. In multiple receiving antennas, ‘Collision’, ‘No Reply’ responses are reduced comparing with by using single antenna, because ‘Reply’ is selected prior to ‘Collision’, ‘No Reply’. In future, the study will be made about simulation according to more sophisticated channel models using distance time considering multipath fading and probabilistic methods of signal. And from this information, recognition rates and recognition speed of tags according to the number of tags will be also studied. Also, we will consider the system efficiency of the extended system, which have the reader based on SIMO structure and middleware system connecting network.

Figure 7. Comparing with no reply number

Acknowledgements This work was supported by Korea Evaluation Institute of Industrial Technology (KEIT), under the R&D support program of Ministry of Knowledge Economy, Korea.

References [1] P. V. Nikitin, K. V. Rao, and S, Lazar, “An overview of near field UHF RFID,” IEEE International Conference on RFID, pp. 167174, March 2007. [2] R. Glidden, C. Bockorick, S. Cooper, C. Diorio, D. Dressler, V. Gutnik, C. Hagen, D. Hara, T. Hass, T. Humes, J. Hyde, R. Olive, O. Onen, A. Pesavento, K. Sundstrom, and M. Thomas, “Design of ultra low-cost UHF RFID tags for supply chain application,” IEEE Commun. Mag., vol. 42, no. 8, pp.140151, Aug.2004 [3] Daniel M. Dobkin, “The RF in RFID Passive UHF RFID in Practice,” Newnes, 2007 [4] EPCglobal Specification for RFID Air Interface, "EPCTM Radio-Frequency Identity Protocol for Communications at 860MHz960MHz," ver. 1.1.0, Oct. 2007. [5] EPCglobal Specification for RFID Air

Figure 5. Comparing with tag identification speed

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Interface, "EPCTM Radio-Frequency Identity Protocol for Communications at 860MHz960MHz," ver. 1.2.0, Oct. 2008 [6] DJ Love, RW Heath Jr, T Strohmer, "Grassmannian beamforming for multipleinput multiple-output wireless systems," IEEE Transactions on Information Theory, 2003 [7] J. Park, S.H. Cho, “Modified Slot-count (Q) Selection Algorithm for Higher Tag Identification Speed in Class-1 Gen 2 RFID Protocol”, International Conference on Next Generation Wireless Systems, Australia, 2009.

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