Patent Publication Number: US-8115603-B2

Title: Method for identifying multiple moving tags with multiple readers

Description:
CROSS-REFERENCE(S) TO RELATED APPLICATIONS 
     The present invention claims priority of Korean Patent Application No(s). 10-2006-0070416 and 10-2007-0069613, filed on Jul. 26, 2006 and Jul. 11, 2007, respectively, which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to identification of multiple tags, and more Particularly to a method for efficiently identifying a plurality of moving tags with a plurality of readers. 
     2. Description of Related Art 
     Radio Frequency Identification (RFID) technology has made a rapid development since 2000. Particularly, passive RFID technology of an ultrahigh frequency (UHF) band stands in the limelight in a distribution market. 
     An RFID tag is attached onto an object to be recognized, and an RFID reader identifies information included in the tag. The tag information identified by the RFID reader is transmitted to a server and processed in real-time. Thus, a tag identification rate and tag reading speed of an RFID reader are significant criterion of performance. 
     When a plurality of RFID readers operate in a small area, the performance of the RFID readers is deteriorated due to interference among the RFID readers and repeated identification of a tag by multiple RFID readers. Also, when an object with multiple tags attached thereto is moving at a predetermined speed, the tag identification rate and tag reading speed of an RFID reader are decreased because there is a limit in the number of tags a single RFID reader can identify and the time that moving tags stay in the reading area of the RFID reader. 
     Therefore, it is required to develop a method for identifying multiple tags to improve tag identification rate and tag reading speed of an RFID system and increase the throughput of the RFID system by efficiently identifying a plurality of moving tags. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention is directed to providing a multiple tag identifying method that can improve a tag identification rate and tag reading speed of an RFID system, increase the throughput of the RFID system, and efficiently identify a plurality of moving tags by dividing the moving tags moving at a predetermined speed into a predetermined number of tag groups and allocating each of the tag groups to each of the RFID readers for identification. 
     Other objects and advantages of the present invention can be understood by the following description, and become apparent with reference to the embodiments of the present invention. Also, it is obvious to those skilled in the art to which the present invention pertains that the objects and advantages of the present invention can be realized by the means as claimed and combinations thereof. 
     In accordance with an aspect of the present invention, there is provided a method for identifying a plurality of moving tags by using a plurality of Radio Frequency Identification (RFID) readers, the method which includes the steps of: a) grouping the multiple tags into a predetermined number of tag groups; b) allocating the tag groups to the RFID readers respectively; and c) identifying the tags of each tag group in each RFID reader. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a multiple tag identification method in accordance with an embodiment of the present invention. 
         FIG. 2  is a flowchart describing a multiple tag identification method in accordance with an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     The advantages, features and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. 
       FIG. 1  illustrates a multiple tag identification method in accordance with an embodiment of the present invention. According to the embodiment, Radio Frequency Identification (RFID) readers R 1  to R 5   103  capable of identifying multiple tags attached to a mobile object  101  are connected to a host  105 . Although  FIG. 1  exemplary shows five RFID readers, the present invention is not limited to the number of the RFID readers. 
     The mobile object  101  with tags attached thereto moves at a predetermined speed, and the RFID readers  103  are disposed sequentially in a direction that the mobile object  101  with the tags moves. Each of the multiple RFID readers  103  has an operation sequence according to the sequential disposition. 
     The RFID readers  103  identify tags within a reading area  107  and estimate the number of the tags. The host  105  manages information on the RFID readers, such as the number of available RFID readers, disposition of the RFID readers, and identification capability of the RFID readers. The host  105  receives a tag identification result (whether tag is successfully identified or not), a tag identifier, and the estimated number of tags from the RFID readers  103  in real-time and transmits information needed for tag identification to the RFID readers  103  based on the information transmitted from the RFID readers  103 . 
     Herein, the information needed for an RFID reader to identify tags is related to parameters of an RFID reader-tag air protocol. The RFID reader-tag air protocol may be International Organization for Standardization (ISO)/International Electrotechnical Commission (IEC) 18000-6c but the present invention is not limited to it. 
       FIG. 2  is a flowchart describing a multiple tag identification method in accordance with an embodiment of the present invention. Referring to  FIG. 2 , the host  105  determines whether an RFID reader to identify tags is the first RFID reader and transmits the determination result to the other RFID readers in step S 201 . In  FIG. 2 , n denotes a sequence allocated to each of the multiple RFID readers shown in  FIG. 1 . 
     When the RFID reader to identify tags is the first RFID reader R 1 , the RFID reader R 1  estimates the number of tags and transmits the tag number to the host  105  in step S 203 . The estimation of the number of tags may be carried out based on a slotted-ALOHA-based probability algorithm but the present invention is not limited to it. The step S 203  is optional. Thus, when it does not have to estimate the number of tags, for example, when the number of mobile tags is fixed or the number of tags to be identified is predetermined, the step S 203  is skipped over. 
     When the number of tags to be identified is estimated based on the slotted-ALOHA-based probability algorithm, for example, when z tags communicate with an RFID reader through N slots, the probability that r tags exist in one slot follows binomial distribution, which can be expressed as the following Equation 1. 
     
       
         
           
             
               
                 
                   
                     
                       B 
                       
                         Z 
                         , 
                         
                           1 
                           N 
                         
                       
                     
                     ⁡ 
                     
                       ( 
                       r 
                       ) 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             z 
                           
                         
                         
                           
                             r 
                           
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           1 
                           N 
                         
                         ) 
                       
                       r 
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           1 
                           - 
                           
                             1 
                             N 
                           
                         
                         ) 
                       
                       
                         z 
                         - 
                         r 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     Thus, the average number of tags to be read for one frame or round can be expressed as the following Equation 2, and the average number of empty slots can be expressed as the following Equation 3. 
     
       
         
           
             
               
                 
                   
                     a 
                     1 
                     
                       N 
                       , 
                       z 
                     
                   
                   = 
                   
                     
                       N 
                       · 
                       
                         
                           B 
                           
                             Z 
                             , 
                             
                               1 
                               N 
                             
                           
                         
                         ⁡ 
                         
                           ( 
                           1 
                           ) 
                         
                       
                     
                     = 
                     
                       
                         z 
                         ( 
                         
                           1 
                           - 
                           
                             1 
                             N 
                           
                         
                         ) 
                       
                       
                         Z 
                         - 
                         1 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
             
               
                 
                   
                     a 
                     0 
                     
                       N 
                       , 
                       z 
                     
                   
                   = 
                   
                     
                       N 
                       · 
                       
                         
                           B 
                           
                             Z 
                             , 
                             
                               1 
                               N 
                             
                           
                         
                         ⁡ 
                         
                           ( 
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                           ) 
                         
                       
                     
                     = 
                     
                       
                         z 
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                         ) 
                       
                       Z 
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
     When the Equation 3 is divided by the Equation 2, the following Equation 4 is acquired. 
     
       
         
           
             
               
                 
                   
                     
                       a 
                       0 
                       
                         N 
                         , 
                         z 
                       
                     
                     / 
                     
                       a 
                       1 
                       
                         N 
                         , 
                         z 
                       
                     
                   
                   = 
                   
                     
                       
                         
                           N 
                           ( 
                           
                             1 
                             - 
                             
                               1 
                               N 
                             
                           
                           ) 
                         
                         z 
                       
                       / 
                       
                         
                           z 
                           ( 
                           
                             1 
                             - 
                             
                               1 
                               N 
                             
                           
                           ) 
                         
                         
                           z 
                           - 
                           1 
                         
                       
                     
                     = 
                     
                       z 
                       ⁡ 
                       
                         ( 
                         
                           N 
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                           1 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
     
     The Equation 4 can be expressed as the following Equation 5.
 
 z =( N− 1)/( a   0   N,z   /a   1   N,z )   Eq. 5
 
     In the slotted-ALOHA-based RFID system, the number of tags is estimated by substituting an average expected value a 0   N,z  of an empty slot with an actual value c 0 , and an average expected value a 1   N,z  of an identification slot with c 1  in the Equation 5. In short, the number of tags is estimated based on the following Equation 6.
 
 z =( N− 1)/( c   0 / c   1 )   Eq. 6
 
     In equation 6, denotes the number of tags, N denotes the number of slots, c 0  denotes the actual number of empty slots, and c 1  denotes the actual number of identification slots. The actual number c 0 of empty slots is a predetermined number which is not 0. 
     In step S 205 , the host  105  groups tags by the number of tags estimated in the RFID reader  103 . The host  105  determines the number of tag groups to be acquired by grouping the multiple tags. The tags may be grouped in diverse methods. For example, when the tags are grouped into four groups using tag identifiers, a first group is formed of tags having a tag identifier starting with ‘00,’ whereas a second group is formed of tags having a tag identifier starting with ‘01.’ A third group is formed of tags having a tag identifier starting with ‘10,’ whereas a fourth group is formed of tags having a tag identifier starting with ‘11.’ The position of a tag identifier bit does not have a specific limitation, and the tags are equally distributed into each group. Generally, a tag identifier includes a domain code, a manufacturing company code, an item code, and a serial number. When the item and the manufacturing company of objects are the same, the tag identifier is positioned in a bit corresponding to a serial number. When the objects are diverse, the bit for the tag identifier may be determined arbitrarily. 
     In step S 207 , the host  105  allocates the tag groups to the RFID readers based on the number of available RFID readers and identification capability of each RFID reader. For example, when the first RFID reader R 1  estimates the number of tags, a tag group of ‘00’ is allocated to the second RFID reader R 2 , and a tag group of ‘01’ is allocated to the third RFID reader R 3 . A tag group of ‘10’ is allocated to the fourth RFID reader R 4 , and a tag group of ‘11’ is allocated to the fifth RFID reader R 5 . Herein, there is no specific limitation in the allocation sequence. When there is a difference in the identification capability of the RFID readers, a plurality of tag groups may be allocated to one RFID reader in consideration of the identification capability. For example, the tag group of ‘00’ and the tag group of ‘01’ may be simultaneously allocated to the RFID reader R 2 . 
     The RFID readers  103  identify the tag groups allocated by the host  105  in step S 209 , and the identification algorithm follows the RFID reader-tag air protocol. Each of the RFID readers  103  R 1  to R 5  identifies only the tag group allocated to itself when the object  101  with the tags attached thereto pass through the reading area  107  of each RFID reader. 
     Meanwhile, when it turns out in the step S 201  that the operating RFID reader is not the first RFID reader, in step S 211 , the host  105  determines whether to perform re-grouping based on the tag identification result transmitted from the (n−1) th  RFID reader and the number of tags included in the tag group allocated to the n th  RFID reader in step S 211 . 
     When the (n−1) th  RFID reader identifies all the tags of the tag group allocated thereto, it does not have to perform re-grouping. When not all the tags of the allocated tag group are identified, re-grouping is needed. 
     Also, when the number of tags included in the tag group allocated to the n th  RFID reader does not exceed the number of tags that the n th  RFID reader can identify, the re-grouping is not needed. However, when the number of tags included in the tag group allocated to the n th  RFID reader exceeds the number of tags that the n th  RFID reader can identify, the re-grouping is needed. 
     Therefore, when the re-grouping is not needed, the n th  RFID reader identifies only the tag group that is already allocated thereto in step S 209 . When the re-grouping is needed, that is, when the (n−1) th  RFID reader fails to identify all the tags in the tag group allocated thereto or when the number of tags included in the tag group allocated to the n th  RFID reader exceeds the number of tags that the n th  RFID reader can identify, the host  105  performs re-grouping in the step S 205 , and re-allocates the tag groups in the step S 207 . Then, the n th  RFID reader identifies a tag group that is newly allocated thereto in the step S 209 . 
     Table 1 shows an operation example of the multiple tag identification method according to the present invention. Table 1 shows an example in which five RFID readers identify 80 tags, and it is assumed that each RFID reader can identify up to 25 tags. As shown in  FIG. 1 , the host groups 80 tags estimated by the first RFID reader R 1  into four tag groups ‘00’, ‘01’, ‘10’ and ‘11’ each of which includes 20 tags. The tag groups ‘00’, ‘01’, ‘10’ and ‘11 ’ are allocated to the second RFID reader R 2 , the third reader R 3 , the fourth RFID reader R 4 , and the fifth reader R 5 , respectively. The tags of the four tag groups ‘00’, ‘01’, ‘10’ and ‘11’ are identified by the second to fifth RFID readers R 2 , R 3 , R 4  and R 5 , respectively. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 RFID reader 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 R1 
                 R2 
                 R3 
                 R4 
                 R5 
                 R6 
                 R7 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Object 
                 80 tags are 
                 ‘00’ tag 
                 ‘01’ tag 
                 ‘10’ tag 
                 ‘11’ tag 
                   
                   
               
               
                 with tags 
                 estimated 
                 group is 
                 group is 
                 group is 
                 group is 
               
               
                 passes by R1 
                 (S203) 
                 allocated 
                 allocated 
                 allocated 
                 allocated 
               
               
                   
                   
                 (S205, S207) 
                 (S205, S207) 
                 (S205, S207) 
                 (S205, S207) 
               
               
                 Object 
                   
                 20 tags are 
               
               
                 with tags 
                   
                 identified 
               
               
                 passes by 
                   
                 (S211, 
               
               
                 R2 
                   
                 S209) 
               
               
                 Object 
                   
                   
                 20 tags are 
               
               
                 with tags 
                   
                   
                 identified 
               
               
                 passes by 
                   
                   
                 (S211, 
               
               
                 R3 
                   
                   
                 S209) 
               
               
                 Object 
                   
                   
                   
                 20 tags are 
               
               
                 with tags 
                   
                   
                   
                 identified 
               
               
                 passes by 
                   
                   
                   
                 (S211, 
               
               
                 R4 
                   
                   
                   
                 S209) 
               
               
                 Object 
                   
                   
                   
                   
                 20 tags are 
               
               
                 with tags 
                   
                   
                   
                   
                 identified 
               
               
                 passes by 
                   
                   
                   
                   
                 (S211, 
               
               
                 R5 
                   
                   
                   
                   
                 S209) 
               
               
                   
               
            
           
         
       
     
     Table 2 shows another operation example of the multiple tag identification method according to the present invention. Table 2 shows an example in which seven RFID readers identify 80 tags, and it is assumed that each RFID reader can identify up to 25 tags. As shown in  FIG. 2 , the host groups  80  tags estimated by the first RFID reader R 1  into four tag groups ‘00’, ‘01’, ‘10’ and ‘11’ which include 35 tags, 15 tags, 15 tags, and 15 tags, respectively. The tag groups ‘00’, ‘01’, ‘10’ and ‘11’ are allocated to the second RFID reader R 2 , the third reader R 3 , the fourth RFID reader R 4 , and the fifth reader R 5 , respectively. Herein, when not all the tags of the ‘00’ tag group are identified because there are 35 tags in the ‘00’ tag group, the host re-groups the ‘00’ tag group into a ‘000’ tag group and a ‘001’ tag group and allocates them into sixth RFID reader R 6  and the seventh RFID reader R 7 , respectively. The ‘01’ ‘10’ and ‘11’ tag groups are identified by the third to fifth RFID readers R 3 , R 4  and R 5 , respectively, whereas the ‘000’ and ‘001’ tag groups are identified by the RFID reader R 6  and the RFID reader R 7 , respectively. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 RFID reader 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 R1 
                 R2 
                 R3 
                 R4 
                 R5 
                 R6 
                 R7 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Object 
                 80 tags 
                 ‘00’ tag 
                 ‘01’ tag 
                 ‘10’ tag 
                 ‘11’ tag 
                   
                   
               
               
                 with 
                 are 
                 group is 
                 group is 
                 group is 
                 group is 
               
               
                 tags 
                 estimated 
                 allocated 
                 allocated 
                 allocated 
                 allocated 
               
               
                 passes 
                 (S203) 
                 (S205, 
                 (S205, 
                 (S205, 
                 (S205, 
               
               
                 by R1 
                   
                 S207) 
                 S207) 
                 S207) 
                 S207) 
               
               
                 Object 
                   
                 There are 
                 Present 
                 Present 
                 Present 
                 ‘000’ tag 
                 ‘001’ tag 
               
               
                 with 
                   
                 35 tags. 
                 allocation 
                 allocation 
                 allocation 
                 group is 
                 group is 
               
               
                 tags 
                   
                 Unidentified 
                 is 
                 is 
                 is 
                 allocated 
                 allocated 
               
               
                 passes 
                   
                 tags are 
                 maintained 
                 maintained 
                 maintained 
                 (S211, 
                 (S211, 
               
               
                 by R2 
                   
                 present. 
                 (S211, 
                 (S211, 
                 (S211, 
                 S205, 
                 S205, 
               
               
                   
                   
                 Re- 
                 S209) 
                 S209) 
                 S209) 
                 S207) 
                 S207) 
               
               
                   
                   
                 Grouped 
               
               
                   
                   
                 (S211, 
               
               
                   
                   
                 S209) 
               
               
                 Object 
                   
                   
                 15 tags are 
               
               
                 with 
                   
                   
                 identified 
               
               
                 tags 
                   
                   
                 (S211, 
               
               
                 passes 
                   
                   
                 S209) 
               
               
                 by R3 
               
               
                 Object 
                   
                   
                   
                 15 tags are 
               
               
                 with 
                   
                   
                   
                 identified 
               
               
                 tags 
                   
                   
                   
                 (S211, 
               
               
                 passes 
                   
                   
                   
                 S209) 
               
               
                 by R4 
               
               
                 Object 
                   
                   
                   
                   
                 15 tags are 
               
               
                 with 
                   
                   
                   
                   
                 identified 
               
               
                 tags 
                   
                   
                   
                   
                 (S211, 
               
               
                 passes 
                   
                   
                   
                   
                 S209) 
               
               
                 by R5 
               
               
                 Object 
                   
                   
                   
                   
                   
                 20 tags are 
               
               
                 with 
                   
                   
                   
                   
                   
                 identified 
               
               
                 tags 
                   
                   
                   
                   
                   
                 (S211, 
               
               
                 passes 
                   
                   
                   
                   
                   
                 S209) 
               
               
                 by R6 
               
               
                 Object 
                   
                   
                   
                   
                   
                   
                 20 tags are 
               
               
                 with 
                   
                   
                   
                   
                   
                   
                 identified 
               
               
                 tags 
                   
                   
                   
                   
                   
                   
                 (S211, 
               
               
                 passes 
                   
                   
                   
                   
                   
                   
                 S209) 
               
               
                 by R7 
               
               
                   
               
            
           
         
       
     
     As described above, when the second RFID reader R 2  fails in identifying tags, the tags are re-grouped into the ‘000’ tag group and the ‘001’ tag group before the third RFID reader R 3  operates. According to another operation example, the ‘000’ tag group and the ‘001’ tag group may be allocated to the third RFID reader R 3  and the fourth RFID reader R 4 , respectively, and the existing ‘01’, ‘10’ and ‘11’ tag groups may be allocated to the fifth to seventh RFID readers R 5 , R 6  and R 7 , respectively. 
     Table 3 shows yet another operation example of the multiple tag identification method according to the present invention. Table 3 shows an example in which six RFID readers identify 80 tags, and it is assumed that each RFID reader can identify up to 25 tags. As shown in Table 3, the host groups 80 tags estimated by the first RFID reader R 1  into four tag groups ‘00’, ‘01’, ‘10’ and ‘11’ which include 15 tags, 15 tags, 10 tags, and 40 tags, respectively. The ‘00’, ‘01’, ‘10’ and ‘11’ tag groups are allocated to the second RFID reader R 2 , the third reader R 3 , the fourth RFID reader R 4 , and the fifth reader R 5 , respectively. Herein, when the fourth RFID reader R 4  identifies 10 tags and a total of 40 tags are identified among the entire 80 tags, the host re-groups 40 tags included in the ‘11’ tag group into a ‘110’ tag group and a ‘111’ tag group each including 20 tags, individually, and allocates them into fifth RFID reader R 5  and sixth RFID reader R 6 , respectively. The ‘00’, ‘01’ and ‘10’ tag groups are identified by the second RFID reader R 2 , third RFID reader R 3 , and fourth RFID reader R 4 , respectively, whereas the ‘110’ and ‘111’ tag groups are identified by the fifth RFID reader R 5  and sixth RFID reader R 6 , respectively. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 RFID reader 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 R1 
                 R2 
                 R3 
                 R4 
                 R5 
                 R6 
                 R7 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Object 
                 80 tags 
                 ‘00’ tag 
                 ‘01’ tag 
                 ‘10’ tag 
                 ‘11’ tag 
                   
                   
               
               
                 with tags 
                 are 
                 group is 
                 group is 
                 group is 
                 group is 
               
               
                 passes by 
                 estimated 
                 allocated 
                 allocated 
                 allocated 
                 allocated 
               
               
                 R1 
                 (S203) 
                 (S205, 
                 (S205, 
                 (S205, 
                 (S205, 
               
               
                   
                   
                 S207) 
                 S207) 
                 S207) 
                 S207) 
               
               
                 Object 
                   
                 15 tags are 
               
               
                 with tags 
                   
                 identified 
               
               
                 passes by 
                   
                 (S211, 
               
               
                 R2 
                   
                 S209) 
               
               
                 Object 
                   
                   
                 15 tags are 
               
               
                 with tags 
                   
                   
                 identified 
               
               
                 passes by 
                   
                   
                 (S211, 
               
               
                 R3 
                   
                   
                 S209) 
               
               
                 Object 
                   
                   
                   
                 10 tags are 
                 ‘000’ tag 
                 ‘001’ tag 
               
               
                 with tags 
                   
                   
                   
                 identified 
                 group is 
                 group is 
               
               
                 passes by 
                   
                   
                   
                 (S211, 
                 allocated 
                 allocated 
               
               
                 R4 
                   
                   
                   
                 S209) 
                 (S211, 
                 (S211, 
               
               
                   
                   
                   
                   
                   
                 S205, 
                 S205, 
               
               
                   
                   
                   
                   
                   
                 S207) 
                 S207) 
               
               
                 Object 
                   
                   
                   
                   
                 20 tags are 
               
               
                 with tags 
                   
                   
                   
                   
                 identified 
               
               
                 passes by 
                   
                   
                   
                   
                 (S211, 
               
               
                 R5 
                   
                   
                   
                   
                 S209) 
               
               
                 Object 
                   
                   
                   
                   
                   
                 20 tags are 
               
               
                 with tags 
                   
                   
                   
                   
                   
                 identified 
               
               
                 passes by 
                   
                   
                   
                   
                   
                 (S211, 
               
               
                 R6 
                   
                   
                   
                   
                   
                 S209) 
               
               
                   
               
            
           
         
       
     
     As described above, whether to re-group the tags in the n th  RFID reader is determined based on the tag identification result of the step S 209  in the (n−1) th  RFID reader, that is, the estimation result or identification result in the S 209 . When the number of tags included in an actual tag group exceeds the identification capability of an RFID reader and the (n−1) th  RFID reader fails to identify all the tags of the tag group, the host re-groups the tags in the steps S 205  and S 207  before the n th  RFID reader operates in the step S 209 , and determines the number of tags for a tag group based on the number of tags estimated in the (n−1) th  RFID reader in the steps S 205  and S 207 . Also, when the (n-1) th  RFID reader successfully identifies tags but the n th  RFID reader does not identifies all the tags of a tag group allocated thereto, tags are re-grouped before the operation of the (n+1) th  RFID reader. 
     Table 4 shows a still another operation example of the multiple tag identification method according to the present invention. When objects with tags attached thereto passes by RFID readers at a predetermined time intervals, each of the RFID readers operates as follows. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Reader R1 
                 Reader R2 
                 Reader R3 
                 Reader R4 
                 Reader R5 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1 st  object 
                 The number of 
                   
                   
                   
                   
               
               
                 passes by R1 
                 tags of 1 st   
               
               
                   
                 object is 
               
               
                   
                 estimated 
               
               
                 2 nd  object 
                 The number of 
                 Tags of 1 st   
               
               
                 passes by R1 
                 tags of 2 nd   
                 object are 
               
               
                   
                 object is 
                 identified 
               
               
                   
                 estimated 
               
               
                 3 rd  object 
                 The number of 
                 Tags of 2 nd   
                 Tags of 1 st   
               
               
                 passes by R1 
                 tags of 3 rd   
                 object are 
                 object are 
               
               
                   
                 object is 
                 identified 
                 identified 
               
               
                   
                 estimated 
               
               
                 4 th  object 
                 The number of 
                 Tags of 3 rd   
                 Tags of 2 nd   
                 Tags of 1 st   
               
               
                 passes by R1 
                 tags of 4 th   
                 object are 
                 object are 
                 object are 
               
               
                   
                 object is 
                 identified 
                 identified 
                 identified 
               
               
                   
                 estimated 
               
               
                 5 th  object 
                 The number of 
                 Tags of 4 th   
                 Tags of 3 rd   
                 Tags of 2 nd   
                 Tags of 1 st   
               
               
                 passes by R1 
                 tags of 5 th   
                 object are 
                 object are 
                 object are 
                 object are 
               
               
                   
                 object is 
                 identified 
                 identified 
                 identified 
                 identified 
               
               
                   
                 estimated 
               
               
                 6 th  object 
                 The number of 
                 Tags of 5 th   
                 Tags of 4 th   
                 Tags of 3 rd   
                 Tags of 2 nd   
               
               
                 passes by R1 
                 tags of 6 th   
                 object are 
                 object are 
                 object are 
                 object are 
               
               
                   
                 object is 
                 identified 
                 identified 
                 identified 
                 identified 
               
               
                   
                 estimated 
               
               
                 7 th  object 
                 The number of 
                 Tags of 6 th   
                 Tags of 5 th   
                 Tags of 4 th   
                 Tags of 3 rd   
               
               
                 passes by R1 
                 tags of 7 th   
                 object are 
                 object are 
                 object are 
                 object are 
               
               
                   
                 object is 
                 identified 
                 identified 
                 identified 
                 identified 
               
               
                   
                 estimated 
               
               
                 8 th  object 
                 The number of 
                 Tags of 7 th   
                 Tags of 6 th   
                 Tags of 5 th   
                 Tags of 4 th   
               
               
                 passes by R1 
                 tags of 8 th   
                 object are 
                 object are 
                 object are 
                 object are 
               
               
                   
                 object is 
                 identified 
                 identified 
                 identified 
                 identified 
               
               
                   
                 estimated 
               
               
                 9 th  object 
                 The number of 
                 Tags of 8 th   
                 Tags of 7 th   
                 Tags of 6 th   
                 Tags of 5 th   
               
               
                 passes by R1 
                 tags of 9 th   
                 object are 
                 object are 
                 object are 
                 object are 
               
               
                   
                 object is 
                 identified 
                 identified 
                 identified 
                 identified 
               
               
                   
                 estimated 
               
               
                   
               
            
           
         
       
     
     As described above, when objects with tags attached thereto continue to pass by the RFID readers at a predetermined time interval, the first RFID reader R 1  estimates the number of tags attached to each object, and each of the RFID readers R 2  to R 5  identifies tags of a tag group allocated thereto, when the object of the tag group passes the reading area of the RFID reader. The RFID readers operate in a pipelined architecture. 
     Herein, the pipelined architecture is a method of dividing one processor into a plurality of sub-processors having different functions and capable of processing different data simultaneously. 
     The multiple tag identification method of the present invention improves a tag identification rate and tag reading speed of an RFID system, which may increase the throughput of the RFID system. The method of the present invention can be applied to diverse areas capable of adopting the RFID technology. 
     As described above, the method of the present invention can be realized as a program and stored in a computer-readable recording medium such as CD-ROM, RAM, ROM, floppy disks, hard disks, and magneto-optical disks. Since the process can be easily implemented by those of ordinary skill in the art to which the present invention pertains, further description will not be provided herein. 
     While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.