Abstract:
A system and method are disclosed for synchronizing two RFID readers. The system includes a modulation detector to detect a modulated signal produced by a first of the two reader and produces a synchronization signal. The second of the two readers initiates transmission of a signal in response to the synchronization signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is utility application claims the benefit under 35 U.S.C. 119(e) of provisional application Ser. No. 61/761,033 filed on Feb. 5, 2013 entitled “System and Method for Synchronizing RFID Readers Utilizing RF or Modulation Signals” and on provisional application Ser. No. 61/775,328 filed on Mar. 8, 2013 and entitled “System and Method for Synchronizing RFID Readers Utilizing RF or Modulation Signals.” 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the field of RFID. 
     BACKGROUND 
     In the field of radio frequency identification (RFID), where multiple transponder interrogators (readers) are present is a limited area, it is sometimes necessary to synchronize the readers is to reduce or eliminate interference from one reader onto another reader or from two readers onto a transponder. By synchronizing the readers both readers can communicate with transponders in a way that both readers to communicate with their respective transponders without interfering with each other&#39;s signals. 
    
    
     
       DESCRIPTION 
       Description of the Drawings 
         FIG. 1  is a diagram of an exemplary Block Diagram Single RFID system with two readers; 
         FIG. 2  is an exemplary Multi System Block Diagram; 
         FIG. 3  is an exemplary Synchronization Circuit Diagram 
         FIG. 4  is a timing diagram of a synchronization method including a delay; 
         FIG. 5  is a timing diagram of a synchronization method without a delay; 
         FIG. 6  is a timing diagram where reader are synchronized and use different frequencies to avoid interference; and 
         FIG. 7  is a timing diagram for systems having multiple pairs of readers. 
     
    
    
     DETAILED SPECIFICATION 
     This invention consists of a method for using a signal (either at RF or baseband) to synchronize in time two or more RFID readers, and to minimize interference among a group of reader using time and frequency multiplexing. The reason to synchronize the readers is to reduce or eliminate interference from one reader onto another reader or from two readers onto a transponder. By synchronizing the readers both readers can communicate with transponders in a way that allows both readers to communicate with their respective transponders without interfering with each other&#39;s signals. Adding frequency multiplexing to additional groups of readers will allow this configuration to exist in areas with multiple readers such as a toll plaza with multiple traffic lanes. 
     An exemplary synchronization circuit looks for specific patterns in the RF or modulation signal from reader  1 . The patterns are based on the transponder protocol reader  1  is using to communicate. Once the synchronization circuit detects the pattern, the circuit generates a signal or pulse that is sent to reader  2 . This indicates that reader  1  is in the process of transmitting a command. Reader  2  uses that pulse to know when reader  1  is communication with a transponder.  FIG. 2  shows the synchronization circuit using a coupler to supply the RF signal from reader  1 . The dashed line for the modulation signal shows an alternate input to the synchronization circuit. If a baseband modulation signal is available from reader  1  that modulation signal could be used as an input to the synchronization circuit.  FIG. 3  shows a circuit diagram of what is contained in the synchronization circuit. The synchronization circuit takes in either an RF or baseband modulation signal, detects a particular pattern in that RF or modulation signal, and then generates a pulse to create a signal that is synchronized in time with the input RF or modulation signal. 
     The synchronization circuit does not have to be an external circuit. Reader  2  could be modified to look at the RF output or modulation signal of reader  1  to detect the modulation from reader  1  to generate a synchronization signal. 
     System Time Domain and Frequency Multiple Access Plan 
     To maximize efficiency, different reader/transponder systems may need to use different multiple access schemes. As an example, in RFID tolling systems active transponders all use the same frequency to communicate from transponder to reader. These active transponders need to use separate time slots to minimize interference among readers. Backscatter or passive transponders use different RF frequencies to minimize interference among readers. This invention combines the synchronization method above with the concept of using time slots for appropriate protocols and separate frequencies for other protocols to minimize interference and other deleterious effects like cross-lane reads. 
       FIG. 2  shows a system that combines both time and frequency multiple access methods. In the diagram reader  1  in each system uses different time slots for multiple access. Reader  1  in each system use a separate time slot to allow the transponders responses to be received only by the appropriate reader. In practical configurations time slots may be reused after an acceptable physical separation between readers on the same time slot. For example toll systems have been configured with four time slots so that readers on the same time slot are separated by five traffic lanes. Reader  2  in each system is configured on separate frequencies from reader  2  in the other systems. This frequency separation allows readers to operate in overlapping time slots, but still have acceptable interference rejection by using different frequencies. As an example systems have been configured with frequencies of 902.25, 902.75, 903.25, and 903.75 MHz to operate on toll roads with backscatter transponders. As with time multiplexing, with enough physical separation frequencies can be reused. 
     Example 1 
     (Pair of Readers) Delay Eliminate RF Overlap Between Readers 
     With respect to the timing diagram in  FIG. 4 , Reader  2  uses the synchronization signal to start a timer. The timer is of sufficient duration to allow reader  1  to complete the communication process. Once the timer in reader  2  is finished reader  2  may then begin the communication process. This eliminates any interference between readers during the communication process. 
     Example 2 
     (Pair of Readers) No Delay Eliminate Downlink on Uplink Interference 
     With respect to the timing diagram in  FIG. 5 , Reader  2  uses the synchronization signal to immediately start modulation. Reader  2 &#39;s modulation will be finished before reader  1 &#39;s transponder begins to reply to reader  1 &#39;s command. This will eliminate reader on reader interference when a reader is trying to detect a transponder. 
     Example 3 
     (Pair of Readers) Synchronization and Frequency Detection 
     With respect to  FIG. 6 , the synchronization circuit detects the modulation pattern and the RF frequency of reader  1 . This allows reader  2  to use a frequency that is out of band from reader  1  helping to minimize any interference between reader  1  and reader  2 . 
     Example 4 
     Multiple Pairs of Readers 
     In this example each of the synchronization circuits detects the modulation of reader  1 . Reader  2  uses the output of the synchronization circuit to start a delay. The delay for each pair of readers is different, but allows the RF from each of the reader  2   s  in the diagram to operate simultaneously. The RF frequency for each of the reader  2   s  in the diagram is different. This allows the group of reader  1   s  to operate with TDM and the group of reader  2   s  to operate with FDM. This diagram shows that all of the readers  2   s  are operating at the same time on different frequencies. This is not the only configuration that could be used. Each of the reader  2   s  do not have to be aligned in time. The reader  2   s  in the diagram could each operate at different times as long as they did not interfere with the operation of the reader is in their group.