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A Rapid Tag Identification Method with TwoSlots in RFID SystemsYong Hwan Kim, Sung Soo Kim, Kwang Seon AhnDepartment of Computer Engineering, Kyungpook National UniversityDaegu, Korea{hypnus, ninny, gsahn}@knu.ac.krAbstract—RFID is core technology in the area of ubiquitous compu-ting. Identify the objects begin with the reader’s query to the tag at-tached to the subject. When multiple tags exist in the reader’s inter-rogation zone, these tags simultaneously respond to the reader’s query, resulting in collision. In RFID system, the reader needs the anti-collision algorithm which can quickly identify all the tags in the interrogation zone. This paper proposes tree based Rapid Tag Identi-fication Method with Two Slots(RTIMTS). The proposed algorithm rapidly identifies a tag with the information of Two Slots and MSB(Most Significant Bit). Two Slots resolve the tag collision by receiving the response from the tag to the Slot 0 and 1. The reader can identify two tags at once using MSB of information added to the tag ID. With RTIMTS algorithm, the total time of tag identification can be shortened by decreasing the number of query-responses from the reader.Keywords-RFID; Anti-collision; Two Slots; the number of query-responses.I.I NTRODUCTIONRFID(Radio Frequency Identification) is a technology that deciphers or identifies the tag information through a reader (or interrogator) without contact. RFID have become very popular in many service industries, purchasing and distribution logis-tics, industry, manufacturing companies and material flow systems. Automatic Identification procedures exist to provide information about people, animals, goods and products in tran-sit[1][2].The reader receives required information from the tags by sending and receiving wireless signals with the tag. Since the communication between the readers and the tags shares wire-less channels, there exist collisions. The collisions can be di-vided into the reader collision and the tag collision. The reader collision occurs when multiple readers send request signals to one tag, and the tag receives the wrong request signal due to signal interference between readers. The tag collision occurs when more than two tags simultaneously respond to one reader and the reader cannot identify any tags. This kind of collision makes the reader take long time to identify tags within the read-er’s identification range and impossible to identify even one tag[3][4][5] [6].Therefore, the collision is a crucial problem that must be re-solved in RFID systems, so many studies to resolve this prob-lem have been carried out as well as are ongoing. This paper focuses on the tag collision problem which occurs in the case where one reader identifies multiple tags. Figure 1 provides schematizations of reader collision and tag collision.This paper proposes the Rapid Tag Identification Method with Two Slots (RTIMTS), for faster tag identification in mul-ti-tag environment where one reader identifies multiple tags. In the transfer paper[7], the proposed algorithm designs the method that it does without the value extracting procedure of even(or odd) parity bit of ID bit(T pb),the number of identified ‘1’s(T1n), the number of remaining ‘1’s(T rn), and the number of collided bit (T cb) with simple it can predict a tagID. Maximum 4 tag IDs canbe identified on one round by using Two slots.a) The Reader collision b) The Tag collisionFigure 1. The collision problem in RFID SystemII.T HE R ELATED WORKSA. Query TreeQuery Tree(QT) algorithm is a binary tree based anti colli-sion algorithm and has an advantage in easily implementation due to its simple operating mode[8]. QT sets the reader’s query and tag’s response as one round, and identifies tags by iterating the round. In each round, the reader requests prefix to tag’s ID. And when ID matches the prefix, each tag transmits all IDs including prefixes to the reader. At this time, if more than one tag simultaneously responds, the reader cannot recognize tag’s ID, but can recognize that there are currently more than two tags having the prefix. Then the reader adds ‘0’ or ‘1’ to the current prefix and queries the longer prefix to the tags again. When only one tag responds to the reader, it identifies the tag. In other words, the reader adds the prefix by 1 bit until only one tag responds and iterates this process until identifying all the tags within the range. Figure 2 shows the operating process of QT algorithms[10].Figure 2 shows the process that four tags respond according to the readers’ query. In round 1, 2, 3, and 7, the collision oc-curs because more than two tags respond, and in round 4, 5, 8, and 9, tag can be identified because only one tag responds. The digital coding method applied to QT cannot detect the collisionbits. When a collision occurs, the reader adds ‘0’ or ‘1’ to the2009 Eighth IEEE International Symposium on Network Computing and Applications 978-0-7695-3698-9/09 $25.00 © 2009 IEEEDOI 10.1109/NCA.2009.21292current prefix and queries this prefix to the tags. While having a merit of the simple realization, this generates an idle cycle which has no response like a round 6 [10].Figure 2. The operating process of Query Tree alogrithmB. Query Tree with Collision-Bit PositioningThe tag identification process of the Query Tree with colli-sion-Bit Positioning(QT-CBP) algorithm is as follows[9]. If a collision occurs, the reader detects the location of the collided bits and gets the number of the collided bits. If one collision is generated, a reader establishes ‘0’ and ‘1’ for a collided bit and it saves them into memory. If more than two collisions are gen-erated, a reader will create a new query prefix by appending ‘0’ and ‘1’ to queries to a head of location bit where the collisions are generated. And, it saves then into next stack. If a tag doesn't respond, a reader won’t move. Until all tags are identified, the above protocol is reiterated[11].The QT-CBP uses a stack differently that the QT uses a queue, and when one bit collision is generated, a reader identi-fies as two tags are existing. If tag ID has a collision conti-nuously, The QT-CBP will identify this in the same way with the QT, so it has to be improved[11].Figure 3 shows the process of identifying four tag IDs. In the step 3, a collision was generated, however, it detected that a collision was generated on the bit 2 of tag ID. So, it added ‘0’ and ‘1’ to the bit 2 respectively and it identifies two tag IDs. In the step 1 and 2, query prefix is created [11].Figure 3. The operating process of Collision-Bit Positioning alogrithmC. Collision Tracking using Manchester CodeIn the proposed algorithm, the reader must know the posi-tion and the number of collided bits. This can be resolved by using Manchester coding, which defines the value of a bit bythe change in level within a bit window. So, the state where the level doesn’t change doesn’t exist. If such state is tracked, it can be identified as a collision[1]. Figure 4 shows that the colli-sion can be tracked by an individual bit window with Manches-ter coding.Figure 4. Manchester code tracing a collision to an individual bitIn Figure 4, when the identification code of tag 1 is 10011111, and the identification code of tag2 is 10111011, two tags transmit their ID to the reader on the reader’s request. In case of the bits (bit 3 and bit 6) where two tags have different levels, like in Figure 4, the ‘no transition’ state continues in one bit block because positive and negative transitions cancel each other out. This state is not permissible in the Manchester cod-ing system and leads to an error, which is identified as a colli-sion. It is thus possible to track a collision to an individual bit when Manchester coding is applied.III. R APID T AG I DENTIFICATION M ETHOD WITH T WO S LOTS This section proposes the RTIMTS algorithm that can quickly identify multiple tags in the reader’s interrogation zone. Contrary to the existing algorithm which arbitrates for only one tag to respond at the specific point while identifying tags, the proposed algorithm identifies tags using a characteris-tic that a collision occurs between the tags. In other words, the RTIMTS algorithm predicts the tag ID by analyzing the num-ber of collided bits, the information of MSB(Most Significant Bit) and Two Slots. Also, it prevents collision as tags, which have the ID matching to the value of [prefix] corresponding to the reader’s request command, respond their ID from the [pre-fix]+2th bit in Two Slots using the [prefix]+1th bit of ID. The next section explains all the necessary details for the RTIMTS algorithm to operate and the way the RTIMTS algorithm oper-ates, while showing the example in which tags are identified by the RTIMTS algorithm.A. MSB informationThe RTIMTS algorithm identifies the tag using the Serial number which is identified by the collision tracking informa-tion by bit and the bit information of MSB inside tag ID. Figure 5 shows the structure of tag ID for the proposed algorithm.⊕⊕⊕⊕⊕Figure 5. Structure of tag IDIn the RTIMTS algorithm, the tag ID consists of MSB and Serial Number. The serial number is the part to store ID num-ber assigned to the tag, while MSB stores the XORed values of all the bits in the Serial Number. The reader predicts multiple tags using the MSB information and the collision tracking in-formation by each bit. The total length of tag ID is the existing Serial Number length + one bit of MSB. For example, if the Serial Number length is 6 bit, the total length of tag ID is 7bit. The tag ID used in the proposed algorithm has following cha-racteristics.•The MSB is always identified at the first part of the tag identification process. Because the reader’s identificationprocess always proceeds from the highest bit to the lowestone of the tag’s ID.•After the reader identifies the MSB of the tag, the tags responding to the reader’s request in the followingidentification process have the characteristics of beingdivided into two groups according to the XORed val-ue(‘1’ or ‘0’) of all the bits in the Serial Number.•The tags with matching [prefix] value of the reader re-ply on the Two Slots using the [prefix] + 1th bit oftheir tag ID. At this time, if there are two collision bits,the reader predicts two tags using the MSB information.B.Prediction MethodWhen the number of collisions is two, two cases where it is possible to predict according to the MSB information, are as followings.•Prediction 1In the case where the XORed value of a bit is ‘0’, thereader predicts (‘0’, ‘0’) and (‘1’, ‘1’) in two collided bits.•Prediction 2In the case where the XORed value of a bit is ‘1’, thereader predicts (‘0’, ‘1’) and (‘1’, ‘0’) in two collided bits.C.The Reader’s Request command and Tag’s RespondsFigure 6. Flow Chart of the RTIMTSFigure 6 shows a flowchart of the RTIMTS algorithm. The reader always pops the prefix from the stack to send the request command to tags. The tags that match the prefix respond their ID in the corresponding Slot from the [prefix]+2th bit accord-ing to the value of [prefix]+1th bit.An anti-collision resolution in the proposed algorithm works by grouping tags using Two Slots. Therefore, the tag in which the [prefix]+1th bit of its ID is ‘0’ replies in Slot 0, while the tag in which the [prefix]+1th bit of its ID is ‘1’ replies in Slot 1. After receiving the responses from tags, the reader se-quentially processes Slots from Slot 0.The possibility of prediction is checked using the proposed algorithm. When one tag replies in one Slot, the tag is identi-fied. And if there are two collisions because the two tags will reply in one Slot and the MSB information of tag ID is known, the reader can predicts the two tags. If it is impossible to pre-dict, two prefixes, in which ‘0’ and ‘1’ are added to the first collided bit, will be pushed onto the stack before proceeding to the next step. And then identifies all the tag in which a collision occurs with the iteration of above process.D.An example of RTIMTSFigure 7 shows that the way the requests of the reader and the responses of the tags process in the RTIMTS algorithm.Figure 7. Example of the RTIMTSIn the 1st iteration, the reader sends a request to tags with empty string(€) as a factor. Since the € is considered to match with every prefix, all the tags respond their ID in the corres-ponding Slot from the [prefix]+2th bit according to the value of [prefix]+1th bit.In Slot 1, the reader can identify tag 1 and tag 4 using the MSB information like the section III.B. The reader received the response signal, “11010X1X0”, from tags, the XORed value of a bit is 1 ⊕ 1 ⊕ 0 ⊕ 1 ⊕ 0 ⊕ 1⊕ 0 = 0. This case corresponds to Prediction 1, and ‘0’, ‘0’ and ‘1’, ‘1’ can be predicted in two collided bits. Therefore, two tags, “10100100” and “10101110”, are identified. With the proposed algorithm, thetag identification time can be shortened by predicting the tag using the MSB information anti-collision in the cases of two collisions and using Two SlotsIn the 2nd iteration, if the reader transmits “01” as the pre-fix value of a request, tag 2 and tag 3 that match prefix respond the ID in the corresponding Slot from the [prefix]+2th bit. In this case, two tags can be identified because only one tag rep-lies in the corresponding Slot.IV.P ERFORMANCE E VALUATIONIn this section, we evaluate the performance of the pro-posed algorithm in this paper. In the evaluation, we compared with the average number of query-responses of readers and tags of the QT algorithm, the QT-CBP algorithm, and the Anti Col-lision algorithm using Parity Bit(ACPB) algorithm[7] that wereproposed before by planning a simulation program with C# language.In the simulation program, the users can control the length of tag ID, number of tags and allocation of the tag ID. Tag ID lengths of 8 bit were used in the experiments. In each experi-ment, we used random assignment method and sequential as-signment method. We measured the number of query-responses between the reader and tags by changing the number of tags. Since less number of query-responses between the reader and tags represents that the reader can identify the tags faster, we regards the number of query-responses as time in the perfor-mance evaluation.Figure 8 shows the cases that 8 bit IDs are allocated to ran-dom assignment and sequential assignment. As shown in Fig-ure 8 a) and 8 b), the proposed algorithm shows better perfor-mance than other algorithms even if the number of tags in-creases.The results show that in the random assignment of 8 bit tag IDs experiments; the proposed algorithm has overall 79% less query iterations than the QT, 53% less than the QT-CBP, and 50% less than the ACPB. In case of the sequential assignment, the number of query iterations was reduced by about 82% compared to the QT, by about 50% compared to the QT-CBP and the ACPB.a) 8bit random assignmentb) 8 bit sequential assignmentFigure 8. 8 bit random assignment and sequential assignmentV.C ONCLUSIONIn RFID system, the tag collision problem occurs in multi-tag environment where multiple tags are identified. The anti-collision algorithm is necessary to arbitrate the collision and to identify all the tags faster.Added MSB can predict two bit patterns when two colli-sions occur. When the reader sends the request to tags with prefix as a factor, the only tags having the ID whose prefixes match the received prefix, respond. There tags can resolve the collision problem by selecting a Slot using the information of prefix+1th bit.R EFERENCES[1]K. Finkenzeller, RFID Hand Book: Fundamentals and Applications in ContactlessSmart Card and Identification, Second Edition, John Wiley & Sons Ltd, 2003. [2]P. H. 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