Abstract:
RFID tags are used for many purpose including tracking. RFID interrogators are used to retrieve information from tags. In many applications, a plurality of RFID interrogators are required. Synchronization between interrogators in the same theatre of operation is critical to ensure that their broadcasts do not interfere with each other. In fixed RFID interrogator applications, RFID interrogators can be wired together allowing a channel to synchronize the transmissions of the RFID interrogators. Methods described herein can ensure that synchronization is maintained in the event of the failure of a synchronizing master. Furthermore, additional methods for synchronizing RFID interrogators in wireless applications are described allowing synchronization in the absence of wired connections between interrogators.

Description:
RFIDLATED APPLICATIONS INFORMATION 
       [0001]    This application claims priority under 35 U.S.C. 119(e) to Provisional Patent Application Ser. No. 60/805,423, entitled “An RFID Smart Cabinet and a Multi-Document Read Write Station,” filed Jun. 21, 2006, which is incorporated herein by reference as if set forth in full. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The field of the invention relates generally to Radio Frequency Identification (RFID) systems and more particularly to systems and methods for synchronizing a plurality of RFID interrogators in a theatre of operation. 
         [0004]    2. Background of the Invention 
         [0005]      FIG. 1  illustrates a basic RFID system  100 . A basic RFID system  100  comprises three components: an antenna or coil  104 , an interrogator  102  with decoder  112 , and a transponder, or RF tag  106  which is often electronically programmed with unique information. Antenna  104  emits radio signals  110  to activate and read and write data to tag  106 . Antenna  104  is the conduit between tag  106  and interrogator  102 , which controls data acquisition and communication. Antennas  104  are available in a variety of shapes and size, for example, in certain embodiments they can be built into a door frame to receive tag data from persons or things passing through the door. In other embodiments, antennas  104  can, for example, be mounted on an interstate toll booth to monitor traffic passing by on a freeway. Further, depending on the embodiments, the electromagnetic field, i.e., radio signal  110 , produced by an antenna  104  can be constantly present when, e.g., multiple tags  106  are expected continually. If constant interrogation is not required, then radio signal  110  can, for example, be activated by a sensor device. 
         [0006]    Often antenna  104  is packaged with interrogator  102 . A conventional interrogator  102  can emit radio signals  110  in ranges of anywhere from one inch to 100 feet or more, depending upon the power output and the radio frequency used. When an RFID tag  106  passes through an electromagnetic zone associated with radio signal  106 , it detects radio signal  106 , which can comprise an activation signal. In some embodiments, interrogators can comprise multiple antenna, though typically only one transmits at a time. 
         [0007]    RFID tags  106  come in a wide variety of shapes and sizes. Animal tracking tags, for example, inserted beneath the skin of an animal, can be as small as a pencil lead in diameter and one-half inch in length. Tags  106  can be screw-shaped for insertion, e.g., in order to identify trees or wooden items, or credit-card shaped for use in access applications. Anti-theft hard plastic tags that include RFID tags  106  can be attached to merchandise in stores. Heavy-duty RFID tags can be used to track intermodal containers, heavy machinery, trucks, and/or railroad cars for maintenance and/or tracking purposes. A multitude of other uses and applications also exists, and many more will come into being in the future. 
         [0008]    RFID tags  106  are categorized as either active or passive. Active RFID tags  106  are powered by an internal battery and are typically read/write, i.e., tag data can be rewritten and/or modified. An active tag&#39;s memory size varies according to application requirements. For example, some systems operate with up to 1 MB of memory. In a typical read/write RFID work-in-process system, a tag  106  might give a machine a set of instructions, and the machine would then report its performance to tag  106 . This encoded data would then become part of the tagged part&#39;s history. The battery-supplied power of an active tag  106  generally gives it a longer read and write range. The trade off is greater size, greater cost, and a limited operational life. 
         [0009]    Passive RFID tags  106  operate without a separate external power source and obtain operating power generated from radio signal  110 . Passive tags  106  are consequently much lighter than active tags  106 , less expensive, and offer a virtually unlimited operational lifetime. The trade off is that they have shorter read ranges than active tags  106  and require a higher-powered interrogator  102 . Read-only tags are typically passive and are programmed with a unique set of data, usually 32 to 128 bits, that cannot be modified. Read-only tags  106  often operate as a license plate into a database, in the same way as linear barcodes reference a database containing modifiable product-specific information. Not all passive tags  106  are read-only tags. 
         [0010]    RFID systems are also distinguishable by their frequency ranges. Low-frequency, e.g., 30 KHz to 500 KHz, systems have short reading ranges and lower system costs. They are commonly used in security access, asset tracking, and animal identification applications. High-frequency, e.g., 850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz, systems offer long read ranges, e.g., greater than 90 feet, high reading speeds, and are used for such applications as railroad car tracking and automated toll collection, however, the higher performance of high-frequency RFID systems  100  incurs higher system costs. 
         [0011]    The significant advantage of all types of RFID systems  100  is the noncontact, non-line-of-sight nature of the technology. Tags  106  can be read through a variety of substances such as snow, fog, ice, paint, crusted grime, and other visually and environmentally challenging conditions, where barcodes or other optically read technologies cannot typically be used. RFID tags  106  can also be read in challenging circumstances at high speeds, often responding in less than 100 milliseconds. RFID has become indispensable for a wide range of automated data collection and identification applications that would not be possible otherwise. 
         [0012]    A conventional RFID interrogator  102  comprises an RF transceiver  106  and a decoder  112 . Decoder  112  can, for example, be a micro controller or other processing circuit configured to carryout the required functions. Often, decoder  112  is interfaced with memory  114 . Firmware instructions used by decoder  112  to control the operation of interrogator  102  can be stored in memory  114 , along with RFID instructions that can be communicated to RFID tag  106  and can be used to control acquisition of information from RFID tags  106 . Memory  114  can, depending on the embodiment, comprise one or more memory circuits. 
         [0013]      FIG. 2  shows an example transmission operation of an RFID interrogator. Graph  200  shows a transmission of the RFID interrogator when no data is transmitted. At the start of each frame  202 , interrogator  102  can be configured to transmit frame synchronization pulses  204 , which can have a much shorter width than the period associated with frame  202 . RFID interrogator  102  can transmit data to RFID tag  106  by modifying the frame synchronization pulses, for instance by doubling the pulses to represent a binary “zero” and tripling the synchronization pulses to represent a binary “one.” Graph  220  shows an example of such a transmission method by an RFID interrogator. Double pulses  222  and  230 , which comprise two pulses sent within a short period compared to the frame period; represent the transmission of a “zero.” Triple pulse  226 , which comprise three pulses sent within a short period compared to the frame period, represent the transmission of a “one.” Remaining single pulses  224  and  228  do not represent data and synchronize the associated frames. 
         [0014]    Another method of modifying frame synchronization pulses used by RFID interrogators is to use wider pulses to represent a “zero” and still wider pulses to represent a “one.” Graph  240  shows an example of such a transmission method. The “wider” pulses  242  and  250 , which are still short compared to the frame period, represent the transmission of a “zero.” The “widest” pulse  246 , which is still short compared to the flame period but wider than pulses  242  and  250 , represent the transmission of a “one.” The remaining “normal” width pulses  244  and  248  do not represent data and synchronize the associated frames  202 . 
         [0015]    Graphs  220  and  240  illustrate just two possible examples of communication protocols that can be used to facilitate transmission of data utility system  103 . 
         [0016]    In response to interrogation signals from the interrogator  102 , RFID tags  106  can be configured to respond in the second half of frames  252 . Furthermore, in many embodiments of an RFID interrogation systems  103  both tags  106  and interrogator  102  operate in the same frequency range. The synchronization pulses, whether “normal” or modified to carry data, can serve two additional purposes. First, the pulses can be used to define the boundaries of frames  202  so the tags  106  can respond at the appropriate time. Second, the pulses supply power for passive RFID tags  106 . 
         [0017]      FIG. 3  depicts an interrogation theatre  300  comprising a plurality of interrogators, of which interrogators  310  and  340  are shown for illustrative purposes. In addition, theatre  300  comprises a plurality of tags, of which tags  320 ,  322 , and  344  are shown for illustrative purposes. Tags  320 ,  322 , and  344  can for example, be similar to, or the same as, tag  106  described above. If allowed to operate independently, these readers can severely interfere with each other. To illustrate, in  FIG. 3 , RFID tags  320  and  322  are near interrogator  310 , while RFID tag  344  is near interrogator  340 . Temporally, interrogator  310  has just transmitted its request through its antenna  312  and is now awaiting a response signal from any nearby RFID tags. Because RFID tags  320  and  322  are near to interrogator  310 , they respond with RFID signals  330  and  332 , respectively; however, at approximately the same time, interrogator  340  wishes to interrogate RFID tags nearby such as RFID tag  344 , by transmitting signal  346  through antenna  342 . Since the responses  330  and  332  are on the same frequency as the interrogation signal  346 , and interrogation signal  346  can be of greater power than signals  330  and  332 , interrogator  310  may only detect the signal from interrogator  340  rather than from RFID tags  320  and  322 . 
         [0018]      FIG. 4  depicts the timing of the example given above. Graph  400  depicts interrogator  310  attempting to interrogate nearby RFID tags using the communications protocol illustrated by graph  220 . RFID tag  320  responds and its RF output signal  330  is graphed over time in graph  410 ; however, with an unsynchronized RFID interrogator  340  also attempting to interrogate nearby RFID tags as depicted in graph  420 , associated signal  346  can interfere with signal  330 . As a result, antenna  312  sees the signal depicted in graph  430 , where rather than seeing pulses  412  and  414  of signal  330  (graph  410 ), interrogator  310  is likely to see something like pulses  432  and  436  dominated by the influence of signal  346  (graph  420 ) of interrogator  340 . As a result, interrogator  310  may interpret pulses  434  and  438  of interrogator  340  as coming from RFID tag  320 , or interrogator  310  may just fail to code any signal or may receive corrupted information. 
       SUMMARY 
       [0019]    An RFID system comprises a plurality of synchronized RFID interrogators. Synchronization between interrogators in the same theatre of operation can be critical to ensure that their broadcasts do not interfere with each other. In fixed RFID interrogator applications, RFID interrogators can be wired together to allow synchronization of transmissions of the RFID interrogators. 
         [0020]    In one aspect, synchronization is maintained in the event of t failure of a synchronizing master. 
         [0021]    In another aspect, synchronizing RFID interrogators in the absence of wired connections between interrogators is provided. 
         [0022]    These and other features, aspects, and embodiments of the invention are described below in the section entitled “Detailed Description.” 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Features, aspects, and embodiments of the inventions are described in conjunction with the attached drawings, in which: 
           [0024]      FIG. 1  is a diagram illustrating an exemplary RFID system  100 ; 
           [0025]      FIG. 2  is a diagram illustrating example transmission protocols that can be used in the system of  FIG. 1 ; 
           [0026]      FIG. 3  is a diagram illustrating an exemplary interrogation theatre comprising a plurality of interrogators; 
           [0027]      FIG. 4  is a diagram illustrating example signals and for the theatre of  FIG. 3 ; 
           [0028]      FIG. 5  is a diagram illustrating an example baggage tracking system that includes a plurality of interrogators synchronized in accordance with one embodiment; 
           [0029]      FIG. 6  is a diagram illustrating example signals and timing for the system of  FIG. 5 ; 
           [0030]      FIG. 7  is a diagram illustrating a temporal overview of a self-promotion process for synchronized interrogators in accordance with one embodiment; 
           [0031]      FIG. 8  is a flowchart illustrating an example method for interrogator promotion in accordance with one embodiment; 
           [0032]      FIG. 9  is a flowchart illustrating an example method for adjusting frame synchronization pulses, when interference is detected in accordance with one embodiment; 
           [0033]      FIG. 10  is a diagram illustrating an example embodiment of RFID tag response encoding in accordance with one embodiment; and 
           [0034]      FIG. 11  is a diagram illustrating an example detecting interference in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    In one embodiment, synchronization signal  512  supplied by synchronization master  510  can be used by interrogators  520 ,  530 , and  540  to ensure that the corresponding interrogator signals  524 ,  534 , and  544  do not interfere with reception of signals transmitted by RFID tags  550 - 564 . For example, if graphs  620 ,  630 , and  640  correspond to signals  544 ,  534 , and  524 , respectively, then it can be seen that synchronization signal  512  (graph  610 ) can cause each interrogator to begin transmission at the start of a common frame period. In other words, interrogators  520 ,  530 , and  540  can be configured such that each interrogator upon receipt of a pulse in signal  512 . This can ensure that each interrogator is finished transmitting before the start of the second half of frame  604 , devoted by dashed line  606 , during which responses from RFID tags  550 - 564  we received. Thus, interference signals  524 ,  534 , and  544  with those transmitted from RFID tags  550 - 564  can be avoided. 
         [0036]    As mentioned above,the start  602  of frames  604 , depending on the requirements of a particular implementation, begin some fixed period (Δd) after the rising edge of the pulses comprising signal  512  as illustrated on graph  610 . The delay (Δd) can, for example, be long enough to account for various delays associated with the circuitry comprising interrogators  520 ,  530 , and  540 . 
         [0037]    In order to avoid the problem illustrated in  FIGS. 3 and 4 , for example, interrogators in a theatre of operation  300  can be synchronized as described herein.  FIGS. 5 and 6  illustrate an embodiment of a system  500  with multiple interrogators in a single theatre of operation. In one embodiment, for example, such a system can be employed in a baggage tracking system, e.g., at an airport. 
         [0038]      FIG. 5  depicts a baggage tracking system  500  where plurality of interrogators  520 ,  530  and  540  are synchronized in accordance with the systems and methods described herein. In airport baggage tracking system  500 , the objective is to track the time and identity of each bag that passes by various checkpoints. To facilitate this objective, an RFID interrogator is placed at each checkpoint. Each bag is equipped with a baggage tag comprising an RFID tag. Upon the check-in, each bag is placed on some sort of conveyance mechanism, such as a conveyor belt. RFID tags  550 ,  552 ,  554 ,  556 ,  558 ,  560 ,  562  and  564  represent the RFID tags embedded in the baggage tags affixed on each bag. Each bag traverses the checkpoint monitored by interrogator  540 , then the checkpoint monitored by interrogator  530 , followed by the checkpoint monitored by interrogator  520 . 
         [0039]    Interrogators  520 ,  530 , and  540  are coupled together and to a synchronization master  510 , which is responsible for synchronizing the interrogators. In this embodiment, the coupling is accomplished through wiring  514 . As depicted in  FIG. 5 , synchronization master  510  can be a simple pulse generator; however, in other embodiments one of interrogators  520 ,  530 , and  540  can serve as a synchronization master. The master transmits, e.g., master  510  can be configured to transmit a pulse train  512  to each of interrogators  520 ,  530  and  540 . Interrogators  520 ,  530 , and  540  can be configured, upon receiving the synchronization pulse, to transmit through antennas,  522 ,  532 , and  542 , respectively, a radio signal  524 ,  534 , and  544 , respectively, to interrogate passing RFID tags  550 - 564 . Signals  524 ,  534 , and  544  can be synchronization pulses or can carry information, e.g., using the exemplary communication protocols illustrated in  FIG. 2 . 
         [0040]      FIG. 6  illustrates an example of the signals and synchronization pulses transmitted by interrogators  520 ,  530 , and  540 . Graph  610  depicts synchronization pulse train  512 . Graph  620 ,  630 , and  640  depict the signal outputs of interrogators  520 ,  530 , and  540 , respectively. In some implementations, the start  602  of the RF frames  604  do not correspond precisely with the leading edges of the pulses in graph  610 , because there can be some propagation delay in the circuitry associated with interrogators  520 ,  530 , and  540 . A certain amount of inconsistency in the delay can be tolerated, because responses to interrogation signals are expected in the second half of the frame. As explained in detail below, each interrogator can transmit different signals without interfering with other interrogator&#39;s ability to receive RFID tag responses because regardless of the type of signal, all transmissions by all interrogators are concluded by the start of the second half of the frame  604  as illustrated by dashed lines  606 . 
         [0041]    Thus, RFID interrogators  520 ,  530 , and  540  can be coupled to a synchronization master  510  configured to synchronize transmissions from the interrogators; however, in the event of a failure associated with synchronization master  510 , system  500  can lose its ability to synchronize the signals of interrogators  520 ,  530 , and  540 . In one embodiment, this is avoided by enabling one of the remaining interrogators to become the synchronization master. Accordingly, when employing such a cooperative strategy, an interrogator can be in one of two states a synchronization master or a synchronization slave. Thus, one or more of the interrogators in a system configured to implement such a cooperative strategy must be able to both send and receive a synchronization signal. 
         [0042]      FIG. 7  is a diagram illustrating a temporal view of signals generated in a system employing such a cooperative strategy. In this example, each interrogator in the system is capable of both sending and receiving an interrogator signal. The system begins with an interrogator, or alternatively a signal generator, as a synchronization master configured to generate a synchronization signal as described above and illustrated in graph  710 . In this particular embodiment, there are three slave interrogators whose radio frame synchronization signals are depicted in graphs  720 ,  730  and  740  and whose synchronization signals are depicted in graphs  725 ,  735 , and  745 . At  750 , the synchronization master suffers a failure and ceases to generate the synchronization signal. 
         [0043]    If just one of the interrogators in the system is capable of taking over as master, which is possible depending on the embodiment, then that interrogator will be promoted to master upon detecting the failure of the original synchronization master. 
         [0044]    Such configurations can be sufficient to avoid synchronization failures; however, a potential drawback to such configurations is that there is no mechanism to ensure synchronization should one promoted interrogator subsequently fail, fails to generate of synchronization signal, or fails to be promoted. Thus, it can be preferable, depending on the implementation, for a plurability of interrogators to be capable of promotion to master. In such embodiments, there must be some mechanism for determining which interrogator will become the next master. 
         [0045]    In one embodiment, interrogators do not recognize an outage until a predetermined period of time has expired at  752 . From there each interrogator selects a random period of time to wait before it attempts to become the new synchronization master. Here, the first interrogator selects the interval between  752  and  754 . The second interrogator selects the interval between  752  and  756 , which happens to be a longer interval. The third interrogator happens to randomly pick the interval between  752  and  754 , the same as the first interrogator. These wait intervals should be large compared to the frame period. In another embodiment, each interrogator at some point in its normal process can select a random time out period before registering a failure of the master. To use the same example, the period would be that between  750  and  754  for the first interrogator, between  750  and  756  for the second interrogator, and between  750  and  754  for the third interrogator. 
         [0046]    Each interrogator can be configured to send a pulse after the associated wait period to the other interrogators indicating its attempt to become the master. The other interrogators, upon receiving the pulse, can be configured to remain slaves. The new synchronization master can then send its synchronization signal to the other interrogators. A conflict can arise in the likely event that two or more interrogators pulse at the same time, which would be the case in the example above. In other embodiments, various schemes can be used to avoid such conflicts, or contentions. For example, in some embodiments, collision avoidance schemes can be used. Factors such as skew in the clocks of each interrogator can eventually lead to a dispersion of the pulses generated. At this point, one of the interrogators will be seen as pulsing first relative to the others. This interrogator will then become the master and the others demoted to being slave interrogators. For example, at time  758 , the pulse, generated by the third interrogator begins to trail those of the first interrogator. The third interrogator can be configured to detect that it is no longer the master, and cease to generate synchronization pulses at time  760 . 
         [0047]      FIG. 8  illustrates a flowchart illustrating an example method for interrogator promotion in accordance with the systems and methods. Wait states  810  and  850  represent the general waiting states for an interrogator in the slave state and in the master states, respectively. For example, most interrogators start in wait state  810 . They can transition out of wait state  810  to step  812  if either a synchronization pulse is received from another interrogator or a predetermined period of time has elapsed since a synchronization pulse from a master was expected. This predetermined period is typically much larger than the frame period. If a synchronization pulse is detected, the interrogator remains a slave and can perform its regular duties by sending either a frame synchronization pulse or data to an RFID tag at step  820 , and if appropriate, it can listen for RFID tag responses at step  822 . Upon completion of the frame, the interrogator returns to wait state  810 . On the other hand, if a synchronization pulse from a master has not been detected, at step  814 , a timeout interval is selected, e.g., randomly generated as described above, the timeout interval can be within a predetermined range, which is typically many times the frame period. The interrogator then waits at step  816  for either this timeout period to expire or for a synchronization pulse from another interrogator. 
         [0048]    If a synchronization pulse is received at step  818 , the interrogator remains a slave and can continue to perform its regular duties starting at step  820 ; however, if the timeout expires then the interrogator attempts to become a master at step  854  by transmitting a synchronization pulse to all the other interrogators. It then can continue to perform its regular duties by sending either a frame synchronization pulse or data to an RFID tag at step  856  and then if appropriate, it can listen for RFID tag responses at step  858 . Upon completion of its regular duties, the interrogator returns to wait state  850 . In wait state  850 , the interrogator waits for either the start of the next frame, which is one frame period after it sent the last synchronization pulse to the other interrogators, or for a synchronization pulse from another interrogator. 
         [0049]    In step  852 , if the interrogator detects a start of frame, it transmits a synchronization pulse to the other interrogators in step  854  and the process repeats as before. But if the interrogator detects another synchronization from another interrogator, it ceases to be, a master, becomes a slave, and resumes slave duties at step  820 . This can occur, for example, where the original master whose failure initiated the promotion from slave to master of steps  814 - 854  comes back online. This can also occur if during the promotion from slave to master one or more other interrogators waited the same random interval and were simultaneously promoted to master and over time, the internal clocks of the interrogators are skewed resulting in slight deviations in the pulse interval. 
         [0050]    Though extremely unlikely, there may be a situation where three or more interrogators claim to be masters. Thus, in certain embodiments, each interrogator can be configured to determine under such circumstances that one of the other interrogators is the rightful master, which will cause each interrogator to switch to a slave state. At this point, no synchronization pulses are sent by any interrogator and the process for each interrogator follows the diagram in  FIG. 8  by traversing steps  812 ,  814 ,  816  and  818 . At which point, a new master is selected. Alternatively, skewing that results from differences in the tolerances and errors associated with the circuitry of each interrogator can be relied on to eventually result in one interrogator being promoted over the others as described above. Obviously, the more interrogators involved the longer such a process will take. Therefore, some alternatives as described above that reduces the delay involved can be preferable for selecting among three or more contending master interrogators. 
         [0051]    It should be noted that in another embodiment, a random predetermined timeout greater than the predetermined “master timeout” and less than the sum of the “master timeout” and the “random timeout” range could be used in wait state  810 , thereby combining steps  812 ,  814 ,  816 , and  818  into a single branch point where the detection of a synchronization pulse transitions the interrogator to step  820  and the expiration of this new predetermined timeout promotes the interrogator to a master state by transitioning to step  854 . Such a hybrid timeout period can be used, for example, when an interrogator changes master-slave state, when a new frame is detected, when the tenth new frame is detected, etc. 
         [0052]    Though the above embodiments address the synchronization issues relating to the operation of multiple interrogators in a single theatre of operation, there are many applications where the wired approach described in the preceding examples is not feasible, e.g., where the RFID interrogators are mobile such as those mounted on a forklift in a warehouse tracking application, or those used as hand-held scanners in a shipment tracking application. Accordingly, one or more wireless communication links can be used to achieve synchronization. Any such wireless approach should provide an inefficient use of power and spectrum associated with the wireless communication channel or link. For instance, ideally a master interrogator should be as centrally located as possible; however, in mobile applications, the interrogators can move around in the theatre of operation. This can lead to inefficient use of power and spectrum since a master interrogator needs to generate sufficient power to be detected by even the most remote interrogator in the theatre of operation. But since it is an objective to mitigate interference between nearby interrogators, synchronization need only be enforced when interrogators are close enough to cause interference. Thus, in certain embodiments, interrogator synchronization is only employed when interference from other interrogators is detected. 
         [0053]      FIG. 9  is a flowchart illustrating an example method for adjusting frame synchronization pulses, when interference is detected, in accordance with the systems and methods described herein. In step  910 , the interrogator waits for the start of frame. In step  912 , the interrogator transmits its frame synchronization or data to nearby RFID tags at the start of the frame. The interrogator then waits, in step  914 , for the second half of the frame. At step  916 , the interrogator can attempt to detect any interference from other interrogators, while listening for RFID tag transmissions, if interference is detected at  918 , the interrogator delays, at step  920 , its start of next frame time to coincide with the start of frame it detected from another interrogator at  918 . If no interference is detected, the interrogator processes any RFID tag transmissions it may have received at step  922 . The process then repeats. 
         [0054]    Basically, if two interrogators come close enough to interfere, the interrogator which is first to detect interference adjusts its frame synchronization timing to match the other interrogator. Because, they are out of sync, one interrogator will have to be first in detecting interference. The environment can become much more complicated if more than two interrogators are out of frame synchronization, but realistically that is unlikely, since the frame periods are typically on the order of microseconds and physical movements take a much longer time, so by the time a third interrogator is out of sync with the first two, those two should have synchronized. 
         [0055]    There are many methods of distinguishing interrogation interference with RFID tag transmissions. Most of these methods involve incorporating certain patterns in the transmission protocol. 
         [0056]      FIG. 10  illustrates a specific embodiment of such an encoding. Graph  1010  shows an interrogator&#39;s frame synchronization pulses. An RFID tag can transmit a “one” by sending a pulse in response in the second half of a first frame and no pulse in a second frame as depicted in graph  1030  and a “zero” by sending a no pulse in a first frame and a pulse in the second half of a second frame as depicted in graph  1040 . In the event of no pulse as in graph  1020 , there is no response from an RFID tag. In the event of a pulse in the second half frame of both a first and second frame, as in graph  1050 , interference from another RFID interrogator can be deduced. More complex patterns in RFID tags transmissions can be implemented, but often these complexities lead to many more false readings. 
         [0057]      FIG. 11  is a flowchart illustrating an example method for synchronization that can alleviate some of the confusion associated with detecting interference. In the example of  FIG. 11 , interrogators only attempt synchronization when not expecting RFID tag transmissions. In a practical system, RFID interrogators spend much of their time sending frame synchronization pulses, but not expecting a return transmission. In step  1110 , the interrogator waits for its internal clock to indicate a start of frame. In step  1112 , the interrogator decides if it has a pending transaction with an RFID tag, if so, at step  1114 , it decides whether it is waiting on a random count due to the detection of interference from a previous iteration. If so or if there is no pending transaction, the interrogator transmits a frame synchronization pulse at step  1116 . Otherwise, if there is a pending transaction and the interrogator is not waiting a random count or that count has expired hence no longer waiting, it transmits its data at step  1118 . The interrogator then waits for the second half of the frame at  1120 . At step  1122 , the interrogator can attempt to detect any interference from other interrogators, while listening for RFID to transmissions. 
         [0058]    If interference is detected at  1122 , the interrogator behaves differently depending on whether it is expecting data from an RFID tag. If it is not expecting data at step  1124 , the interrogator delays, at step  1126 , its start of next frame time to coincide with the start of frame it detected from the interfering interrogator at  1122 . If it is expecting data, the interrogator selects a random number of frames to wait in step  1128 , if no interference is detected, the interrogator processes any RFID tag transmissions it may have received at step  1130 . The process then repeats. In the event multiple interrogators are attempting to interrogate at the same time, the random count gives an interval when none of the interrogators are expecting data to synchronize their respective frame synchronization pulses. 
         [0059]    While certain embodiments of the inventions have been described above, it will be understood that the embodiments described are by way of example only. Accordingly, the inventions should not be limited based on the described embodiments. Rather, the scope of the inventions described herein should only be limited in light of the claims that follow when taken in conjunction with the above description and accompanying drawings.