Abstract:
The present invention relates to a method and apparatus for using a RFID reader network to provide a large operating area, thereby enabling multiple RFID readers to simultaneously operate with minimal interference among all the RFID readers in the same network. The RFID network comprises a plurality of RFID readers connected to a server via a network backbone. Each operating RFID reader is associated with a group of neighboring RFID readers. A neighboring RFID reader is defined as a RFID reader that can detect tag responses for the communication between the operating RFID reader and its RFID tags. Each operating RFID reader sends its neighboring RFID readers at least one Tag Operation (TO) packet before starting to communicate with RFID tags and at least one End of Tag Operation (ETO) packet after the tag operation is completed. To avoid interference, each RFID reader uses a listen-before-talk scheme in which it checks if any TO packet has been sent by neighboring RFID readers before starting radio transmission. If both RFID readers are allowed to transmit, the RFID reader with a neighboring RFID reader just finishing transmission has the higher priority. This approach improves the throughput by taking advantage of the overlapped areas of neighboring RFID readers.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/509,865 filed Oct. 8, 2003, the entirety of which is hereby incorporated by reference into this application. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates generally to RFID (Radio Frequency Identification) systems and more specifically relates to a method and apparatus of using a RFID reader network to provide a large operating area.  
         [0004]     2. Description of the Prior Art  
         [0005]     RFID tags and readers have recently begun to enter the mass market.  FIG. 1  illustrates a typical prior art system in which RFID reader  10  uses multiple antennas  11 ,  12 ,  13  and  14  to detect RFID tags  15  and  16  in a relatively large area, such as a loading dock doorway. The reason for the use of multiple antennas to cover a large area is because the typical operating range of a small, passive RFID tag is only 2 to 3 meters from the antenna.  
         [0006]     A shortcoming of this prior art system is that the antennas and associated multiplexing circuits and cable connections are not reliable and are expensive. Another shortcoming of this prior art system is that only one antenna is allowed to operate at any time. Therefore the reader throughput is low.  
         [0007]     While this prior art system may be suitable for early deployment of RFID applications, it is desirable to provide an improved system for reliability and throughput purposes in which a RFID reader network includes multiple readers.  
       SUMMARY OF THE INVENTION  
       [0008]     In view of the foregoing disadvantages inherent in RFID systems, the present invention provides a method and apparatus for a RFID reader network thereby allowing a large operating area. The method of using a RFID reader network substantially departs from the concept and design of the prior art, and in so doing provides a reliable reader network in which a plurality of RFID readers can simultaneously operate to provide a large area of coverage with minimal interference among the readers in the same network.  
         [0009]     The present invention generally comprises a RFID reader network comprising a plurality of RFID readers connected to a server via a network backbone. In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details set forth in the following description and illustrated in the construction and arrangements of components. The invention is capable of other embodiments and being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of the description and should not be regarded as limiting.  
         [0010]     The present invention provides for a method of using a RFID reader network to enable operation in a large operating area, which overcomes the shortcomings of prior art systems.  
         [0011]     The present invention also provides for a method to enable a plurality of RFID readers to operate simultaneously thereby providing enhanced throughput.  
         [0012]     The present invention also provides for a method to estimate interference among all RFID readers in the network thereby automatically defining a group for each RFID reader.  
         [0013]     The present invention also provides for a method that each RFID reader has one or more neighboring RFID readers listen to RFID tag responses while it is communicating with the RFID tags.  
         [0014]     The present invention also provides for a method of programming a RFID tag with network information to allow for easy network provisioning.  
         [0015]     To the accomplishment of the above, this invention may be embodied in the form illustrated in the accompanying drawings, attention being called to the fact, however, that the drawings are illustrative only, and that changes may be made in the specific construction illustrated. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Various other objects, features and attendant advantages of the present invention will become fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar pars throughout the several views, and wherein:  
         [0017]      FIG. 1  is a schematic diagram of a prior art RFID system.  
         [0018]      FIG. 2  is a schematic diagram of a RFID reader network in accordance with the teachings of the present invention.  
         [0019]      FIG. 3  is a schematic diagram of a hardware block diagram of an embodiment of the RFID reader of the present invention.  
         [0020]      FIG. 4  is a schematic diagram of an embodiment of the RFID reader network including multiple RFID readers.  
         [0021]      FIG. 5A  is a schematic diagram showing a plurality of RFID readers using information to avoid interference.  
         [0022]      FIG. 5B  is a table of group information to avoid interference.  
         [0023]      FIG. 5C  shows short backoff tables for the RFID readers within the same group.  
         [0024]      FIG. 6  is a state diagram of an RFID reader used in the RFID reader network.  
         [0025]      FIG. 7  is a flow chart of a read/write task in the RFID reader&#39;s OPERATION state.  
         [0026]      FIG. 8  is a flow chart of a listen task in the RFID reader&#39;s OPERATION state.  
         [0027]      FIG. 9  is a flow chart of a networking task in the RFID reader&#39;s networking state. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]     Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts.  
         [0029]      FIG. 2  is a schematic diagram of RFID reader network system  20  in accordance with the teachings of the present invention. RFID reader network system  20  comprises a plurality of RFID readers  21   a - 21   n  connected to processing means  22  via network backbone  24 . For example, processing means  22  can be a computer server for processing network information and tag data. RFID readers  21   a - 21   n  communicate with RFID tags  25   a  and  25   b.  It will be appreciated that additional RFID readers and RFID tags can be used in RFID reader network system  20 . RFID reader network system  20  provides a large operating area for communication with RFID tags. For example, the operating area can be in a range of about 1 meter to about 15 meters.  
         [0030]      FIG. 3  is a block diagram of RFID reader  21  integrated with an antenna. RFID reader  21  comprises antenna  30 , RF and baseband circuits  31 , microprocessor and network circuits  32 , one or more memory devices  33 , and clock circuit  34 . RF and baseband circuits  31  provide conventional RF and baseband features. Microprocessor and network circuits  32  provide communication between RFID readers  21   a - 21   n.    
         [0031]      FIG. 4  is a schematic diagram of RFID reader network system  20  including eight RFID readers  21   a - 21   n  connected to server  22  to provide a large operating area for system  20 . For example, this embodiment can be used in a loading dock doorway. RFID reader  21   a  is labeled Reader  1 . RFID reader  21   b  is labeled Reader  2 . RFID reader  21   c  is labeled Reader  3 . RFID reader  21   d  is labeled Reader  4 . RFID reader  21   e  is labeled Reader  5 . RFID reader  21   f  is labeled Reader  6 . RFID reader  21   g  is labeled Reader  7 . RFID reader  21   n  is labeled Reader  8 . RFID readers  21   a - 21   n  use group information to avoid interference between RFID readers.  
         [0032]      FIGS. 5A-5C  illustrate an example of RFID readers  21   a - 21   n  using group information  50  to avoid interference in the embodiment of system  20  illustrated in  FIG. 4 . The groups are formed by sequentially designating each of RFID readers  21  as the operating reader in one network. The operating reader communicates with RFID tags and requests other RFID readers  21  to report the number of RFID tags detected by them to the operating reader. Any reader that detects the RFID tag responses from the operations between the operating RFID readers  21  and RFID tags belongs to the same group associated with the operating reader.  
         [0033]     In this example, it is assumed that each RFID reader  21   a - 21   n  can cause interference only to its immediate or adjacent neighbors. Group information is listed in table  50 . In this example, Group  1 , associated with RFID reader  21   a  (Reader  1 ), comprises RFID reader  21   n  (Reader  8 ), RFID reader  21   a  (Reader  1 ) and RFID reader  21   b  (Reader  2 ). Group  2 , associated with RFID reader  21   b  (Reader  2 ), comprises RFID reader  21   a  (Reader  1 ), RFID reader  21   b  (Reader  2 ) and RFID reader  21   c  (Reader  3 ). Group  3 , associated with RFID reader  21   c  (Reader  3 ), comprises RFID reader  21   b  (Reader  2 ), RFID reader  21   c  (Reader  3 ) and RFID reader  21   d  (Reader  4 ). Group  4 , associated with RFID reader  21   d  (Reader  4 ), comprises RFID reader  21   c  (Reader  3 ), RFID reader  21   d  (Reader  4 ), and RFID reader  21   e  (Reader  5 ). Group  5 , associated with RFID reader  21   e  (Reader  5 ), comprises RFID reader  21   d  (Reader  4 ), RFID reader  21   e  (Reader  5 ), and RFID reader  21   f  (Reader  6 ). Group  6 , associated with RFID reader  21   f  (Reader  6 ), comprises RFID reader  21   e  (Reader  5 ), RFID reader  21   f  (Reader  6 ) and RFID reader  21   g  (Reader  7 ). Group  7 , associated with RFID reader  21   g  (Reader  7 ), comprises RFID reader  21   f  (Reader  6 ), RFID reader  21   g  (Reader  7 ), and RFID reader  21   n  (Reader  8 ). Group  8 , associated with RFID reader  21   n  (Reader  8 ), comprises RFID reader  21   g  (Reader  7 ), RFID Reader  21   n  (Reader  8 ) and RFID reader  21   a  (Reader  1 ).  
         [0034]     A backoff technique is used to resolve contention among different RFID readers  21  to communicate with RFID tags. A long backoff period  51  is used when there are no RFID readers  21  currently operating in the same group. Long backoff period  51  allows RFID readers  21  within the same group to have higher priority to transmit, so that the reader operations of the same group are less likely to be disrupted by the operations of RFID readers  21  in a different group. For example, long backoff period  51  can be in the range of about 5 milliseconds to about 10 milliseconds seconds. An exponential backoff algorithm can also be applied to the long backoff period  51 . For RFID readers  21  in the same group, a short backoff period  54  can be obtained from short backoff tables  501 - 503 . For example, short backoff period  54  can be in the range of about 0 milliseconds to about 5 milliseconds seconds.  
         [0035]     Short backoff tables  501 - 503  contain backoff values related to the interference levels caused by each RFID readers  21  to its neighboring readers. For example, short backoff table  502  residing in RFID reader  21   b  (Reader  2 ) shows that RFID reader  21   c  (Reader  3 ) detects more tag data than RFID reader  21   a  (Reader  1 ) does while RFID reader  21   b  (Reader  2 ) is reading the RFID tags. In this case, RFID reader  21   b  (Reader  2 ) waits for short backoff time  54  of 1 milliseconds after RFID reader  21   a  (Reader  1 ) ceases its operation and short backoff time  54  of 2 milliseconds after RFID reader  21   c  (Reader  3 ) finishes its operation.  
         [0036]     RFID readers  21   a - 21   n  receive an event for starting a read/write operation. After all RFID readers  21   a - 21   n  receive the event for starting a read/write operation, each of RFID readers  21   a - 21   n  performs a long backoff period  51 . After long backoff period  51 , RFID reader  21   a  (Reader  1 ), RFID reader  21   c  (Reader  3 ), and RFID reader  21   f  (Reader  6 ) begin read/write operation  52 . RFID reader  21   b  (Reader  2 ) is part of Group  2  having RFID reader  21   a  (Reader  1 ) and RFID reader  21   c  (Reader  3 ) as members. Therefore, RFID reader  21   b  (Reader  2 ) waits in block  53  until both RFID reader  21   a  (Reader  1 ) and RFID reader  21   c  (Reader  3 ) complete their operations.  
         [0037]     RFID reader  21   d  (Reader  4 ) is part of Group  4  having RFID reader  21   c  (Reader  3 ) and RFID reader  21   e  (Reader  5 ) as members. RFID reader  21   d  (Reader  4 ) waits in block  54  for RFID reader  21   c  (Reader  3 ) to complete its operation in block  59 . RFID reader  21   e  (Reader  5 ) is part of Group  5  having RFID reader  21   d  (Reader  4 ), and RFID reader  21   f  (Reader  6 ) as members. RFID reader  21   e  (Reader  5 ) waits in block  55  until RFID reader  21   f  (Reader  6 ) completes its operation. After RFID reader  21   f  (Reader  6 ) finishes its operation, RFID reader  21   e  (Reader  5 ) performs short backoff window  54  and finds that RFID reader  21   d  (Reader  4 ) is communicating with RFID tags. Accordingly, RFID reader  21   e  (Reader  5 ) waits in block  56  until RFID reader  21   d  (Reader  4 ) completes it operation. RFID reader  21   g  (Reader  7 ) is part of Group  7  having RFID reader  21   f  (Reader  6 ) and RFID reader  21   n  (Reader  8 ) as members. RFID reader  21   g  (Reader  7 ) waits in block  57  for RFID reader  21   f  (Reader  6 ) and RFID reader  21   n  (Reader  8 ) to complete their operations. RFID reader  21   n  (Reader  8 ) is part of group  8  having RFID reader  21   g  (Reader  7 ) and RFID reader  21   a  (Reader  1 ) as members. RFID reader  21   n  (Reader  8 ) waits in block  58  for RFID reader  21   a  (Reader  1 ) to cease its operation. Thereafter, RFID reader  21   n  (Reader  8 ) performs short backoff window  54  before starting operation  52 .  
         [0038]      FIG. 6  is a state diagram of RFID reader  21 . RFID reader  21  stays in IDLE state  60  after powering up. RFID reader  21  moves to NETWORKING state  61  if it receives a networking command or a trigger from an external device, such as a push button, of RFID reader  21 . Networking state  61  is used to execute necessary tasks to form RFID reader network  29 . After RFID reader  21  completes the networking process, it returns to IDLE state  60 . If RFID reader  21  receives an operation command generated by a server or external devices, such as motion detectors, RFID reader  21  moves to OPERATION state  62 . In operation state  62 , RFID reader  21  runs tasks including read and write of data to RFID tags.  
         [0039]      FIG. 7  is a flow chart of an embodiment of a RFID reader  21  read/write task in RFID reader  21  OPERATION state  62 . The task starts from Step  700  and waits for long backoff window  51  in Step  701 . In Step  702 , the task checks whether any other RFID readers are in Tag Operation (TO) mode by detecting incoming TO packets from the other readers in the same group. If there is one or more TO packets, the task waits for the End of Tag Operation (ETO) packets or Cancellation of Tag Operation (CTO), which are sent from other RFID readers in the same group in Step  703 . Thereafter, in Step  704 , the task takes a short backoff period based on the value in the backoff table if one or more of its neighboring readers are just finishing their operations and then goes back to Step  702 .  
         [0040]     If no TO packets are received in Step  702 , the task sends at least one TO packet to the other RFID readers  21  in the same group in Step  705 . Thereafter, the task waits for a short period of time in the range of microseconds in Step  706 . In Step  707 , the task checks if there is any collision due to receiving TO packets from other RFID readers  21  in the group. If there is any collision, the task moves to Step  713  for sending at least one Cancellation of Tag Operation (CTO) packet to the neighboring RFID readers  21 . If there is not any collision, the task moves to Step  708  to communicate with RFID tags, such as by sending read/write commands. In Step  709 , at least one End of Tag Operation (ETO) packet is sent when the task completes the tag operation. After receiving the at least one ETO packet, other neighboring RFID readers  21  send tag information, such as tag IDs and number of tags, detected by them to the task in Step  710 . The task uses this tag information to estimate the interference levels and sends adjustment information to each neighboring RFID reader  21  in the same group in Step  711 . The interference levels of each RFID reader  21  to other neighboring members in the same group is typically proportional to the number of tags detected by these readers while it is in operation. Each neighboring RFID reader  21  uses the adjustment information to update its short backoff table. The task ends in Step  712 .  
         [0041]      FIG. 8  is a flow chart of a listen task in RFID reader  21  OPERATION state  62 . The task starts from Step  80  and waits for an event of the detection of either a TO packet or an event to end the listen task in Step  81 . If a TO packet is received in Step  82 , the task instructs RFID reader  21  to synchronize with other RFID reader  21  in transmission and listen to tag responses until a ETO packet is received in Step  84 . The task then forwards the information of detected tags to RFID reader  21  just finishing transmission in Step  85 . The task receives the group adjustment information to update the short backoff table in Step  86  and goes back to Step  81  to wait for the following events. If the task receives an end of listen event in Step  83 , the task ends in Step  87 .  
         [0042]      FIG. 9  is a flow chart of a networking task in RFID reader  21  NETWORKING state  61 . The task starts from Step  90  and sends a read command to a network tag that contains network information in Step  91 . For example, the network information can be an ID and IP address. In Step  92 , the task waits for a network tag response or a time out. The task determines if a valid network tag response was received in Step  93 . If the tag response is not a valid network tag or a timeout event occurs in Step  92 , the task goes to Step  96  and broadcasts the RFID reader ID as an independent reader. If the task detects a valid network tag in Step  93 , it sends an identification of the RFID reader  21  and network information to server  22  in Step  94 . In Step  95 , the task receives the short backoff table from the server. The task ends in Step  97 .  
         [0043]     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.