Abstract:
Surface acoustic wave (SAW) identification tag discrimination methods including, in one embodiment (1) detecting a signal emanating from an electronic device operating within a SAW tag frequency band; (2) identifying a null period in a transmission pattern in the signal; and (3) effecting communication of information during the null period.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application Ser. No. 60/503,136, filed on Sep. 15, 2003, and entitled “Proposal for a Low Cost RFID Tag,” commonly assigned with the present invention and incorporated herein by reference. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention is directed, in general, to surface acoustic wave (SAW) identification tag discrimination methods and, more specifically, to an integrated system for isolating an response signal from surface acoustic wave radio frequency identification tags and identifying the information encoded on such tags. 
   BACKGROUND OF THE INVENTION 
   To address and overcome inherent existing limitations in prior art radio frequency identification (RFID) tags with respect to cost, data capacity and reliable range, a new technology utilizing SAW devices as identification tags has been developed. SAW tags are described in detail in U.S. patent application Ser. No. 10/024,624, entitled “Surface Acoustic Wave Identification Tag Having Enhanced Data Content and Methods of Operation and Manufacture Thereof,” Hartmann, Clinton S., commonly assigned with the invention and incorporated herein by reference. The principles used to encode data on SAW tags involving simultaneous phase and time shift modulation are described in detail in U.S. patent application Ser. No. 10/062,833, entitled “Modulation by Phase and Time Shift Keying and Method of Using the Same,” Hartmann, Clinton S., also commonly assigned with the invention and incorporated herein by this reference. The principles used to encode data by combining multi-pulse per group modulation with simultaneous phase and time shift modulation are described in detail in U.S. patent application Ser. No. 10/062,894, entitled “Modulation by Combined Multi-pulse per Group with Simultaneous Phase and Time Shift Keying and Method of Using the Same,” Hartmann, Clinton S., also commonly assigned with the invention and incorporated herein by reference. Additional pertinent information regarding SAW identification tags and SAW identification tag readers is set forth in detail in U.S. Pat. No. 6,708,881B1, entitled “Reader for a High Information Capacity Saw Identification Tag and Method of Use Thereof,” Hartmann, Clinton S., again commonly assigned with the invention and incorporated herein by reference. 
   An interrogated RFID tag reflects or retransmits a radio signal in response to an interrogation signal. The returned or reply signal contains data that, when decoded, identifies the tag and any object with which the tag is associated. A SAW device used as an identification tag can be encoded with a large amount of data. When encoded with 64 or 96 bits of data, in accordance with certain electronic product code (EPC) specifications, a reliable system and procedure to accurately identify the tag from a distance is required. Frequently other electronic devices will also be in use in the environment where RFID tags are used. The signals transmitted by these other devices adds to the difficulty in detecting responses to an interrogation pulse. 
   The problem can be best understood in the context of a user that has a large number of objects, each with its own unique identification tag. Added to the fact that a large number of identification tags are returning signals in response to an interrogation pulse, there most probably are other radio frequency signals present. For example, a SAW identification tag system used in a warehouse or shipping facility will most likely be operated in an environment where a wireless local area network (LAN) is also in operation. To identify a specific object among a large number of objects, an interrogation signal will be transmitted that will simultaneously generate a response from each SAW tag on each object. Not only must the SAW tag reader be able to identify the signals being returned from the SAW identification tags, it must also assure that its interrogation signal and the responses to such signal do not interfere with the wireless LAN. In addition, the SAW tag reader must also be able to cope with any signal interference caused by the wireless LAN. Thus, it is important for SAW tags to be encoded in a manner that permits tags to be readily distinguished from each other. It is equally as important that the SAW tag reader be able to discriminate SAW tag responses from other electronic signals and that signals from the SAW identification tag system not interfere with other devices. Methods are needed to encode and read SAW tags so that the unique data on the SAW tags can readily be distinguished. Methods are also needed to permit a SAW identification tag system to operate in an environment where other signals are present. Methods are also needed that permit the operation of SAW identification tag systems in a manner that does not interfere with other devices. 
   Accordingly, what is needed in the art are methods to operate and use a SAW identification tag system in an environment with other signal generating electronic devices and still reliably discriminate between multiple SAW tag responses. 
   SUMMARY OF THE INVENTION 
   To address the above-discussed deficiencies of the prior art, the present invention provides SAW identification tag discrimination methods including, in one embodiment (1) detecting a signal emanating from an electronic device operating within a SAW tag frequency band; (2) identifying a null period in a transmission pattern in the signal; and (3) effecting communication of information during the null period. 
   Thus, the present invention provides a method for operating a SAW identification tag system in an environment where other signals are present which may interfere with either the transmission of SAW interrogation pulses or the receipt of reflected responses to such interrogation pulses. The present invention also permits operation in the same environment as other electronic devices, without interfering with the operation of such devices. For example, if a SAW identification tag system is used in a grocery store with an automatic door opening system operating within the ISM frequency range, the methods described herein permit the SAW tag system to detect the signals transmitted by the door opening system and adjust its operation to overcome any interference caused by the door opening system. By the same token, the SAW tag identification system can be operated in the grocery store without causing repeated opening and closing of the doors. 
   In one embodiment the null period is a frequency null and in another the null period is a time null. When the SAW identification tag system detects conflicting signals, it can communicate during a time null period when the conflicting signal is not present, or, if a frequency null is detected, the system can vary its communication frequency to operate on a non-conflicting frequency. 
   Because of the importance of being able to operate in the presence of other devices and not interfere with such devices, in another embodiment of the invention a relatively low power setting for communicating SAW tag information is provided. In another embodiment, short transmission bursts are used for communication. 
   Another embodiment of the invention provides for the SAW identification tag system to communicate data information synchronous with at least one other electronic device emanating a signal within the SAW tag frequency band. In one embodiment, the communication is synchronized based on time while in another it is synchronized based on frequency. 
   Still another embodiment of the invention provides for a SAW identification tag with at least two SAW tag identification number reflector groups located on its substrate. Also located on the substrate is a first error-checking reflector group dependent upon data contained in one of the at least two tag identification number reflector groups. A second error-checking reflector group is also located on the substrate and is dependent upon data contained in at least a remaining one of the at least two tag identification number reflector groups. Still a third error-checking reflector group on the substrate is dependent upon data contained in the at least the remaining one tag identification number reflector group and the second error-checking reflector group. Yet another embodiment of the SAW identification tag has a synchronizing reflector group on the substrate. 
   In a variation of the above, another embodiment of the invention provides for a SAW identification tag that has at least one tag identification number reflector group located on a substrate with a first error-checking reflector group dependent upon data contained in the at least one tag identification number reflector group. This embodiment has a second error-checking reflector group located on the substrate that is dependent upon data contained in the at least one tag identification number reflector group and the first error-checking reflector group. Another embodiment provides for the SAW identification tag to also have a synchronizing reflector group on the substrate. 
   In still another embodiment of the invention a SAW identification tag has a first reflector group located on a substrate with reflectors that have substantially similar first reflection characteristics. A second group of reflectors located on the substrate has substantially similar second reflection characteristics. In another embodiment, the first reflection characteristics and the second reflection characteristics are substantially similar. 
   The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  illustrates a SAW tag of the type used as an RFID tag; 
       FIG. 2  illustrates a SAW identification tag system operating in an environment where a wireless LAN device and a microwave oven are generating frequencies within the same ISM frequency range of the SAW tag system; 
       FIG. 3  illustrates a representative embodiment of a SAW tag showing the layout of a SAW tag platform with groups of reflector locations on the substrate; 
       FIG. 4  illustrates a SAW tag platform using a “nested” error check protocol; and 
       FIG. 5  illustrates a SAW tag substrate with multiple reflector groups having multiple reflectors in each group. 
   

   DETAILED DESCRIPTION 
   Referring initially to  FIG. 1 , illustrated is a SAW tag  100  of the type used as an RFID tag. The illustrated embodiment provides for a reader antenna  105  that transmits a radio frequency (RF) interrogation signal  110 . The RF signal  110  is received by an antenna  115  on the tag  100  and excites a transducer  120  located on a piezoelectric substrate  130  so that it produces an initial acoustic pulse  140 . As the initial acoustic pulse  140  moves down the surface  135  of the substrate  130 , it encounters reflectors  150  located thereon, causing a reflection of a portion of the initial acoustic pulse  140 . This reflected pulse is herein called a response acoustic pulse  160 . 
   A feature of the illustrated embodiment is that a plurality of reflectors  150  are arranged on the substrate  130  according to time and phase position to yield a plurality of response acoustic pulses  160 . When the transducer  120  receives these response acoustic pulses  160 , an RF response signal  170  is generated that is transmitted through the antenna  115  to be detected by a reader antenna  105 . The SAW tag reader (not illustrated) then determines the identifier in view of predefined time, phase and amplitude parameters detected in the response acoustic pulses  160 . 
   RFID tags, including SAW tags  100 , operate within the industrial, scientific and medical (ISM) frequency band. In the United States, this band is 80 MHz wide with a range of 2.40 to 2.483 GHz. Because this band is used for other applications, principally wireless local area networks (LANs) and Bluetooth™ wireless transceivers, SAW tags must be designed to operate in the presence of, and not unduly interfere with, these applications. Although SAW tags  100  may be designed to operate within a limited frequency band, such as 40 MHz of bandwidth, SAW tags  100 , themselves, will support, and can operate within, a wider bandwidth. 
   Because SAW tags  100  are frequently used within the same environment as other applications are operating using the same ISM frequency band, the possibility of frequency interference is present. Operation in the ISM band thus dictates that SAW tags  100  and SAW tag readers be able to operate within an environment where interference exists. It is also important that the operation of a SAW identification tag system not interfere with other applications in the ISM band. 
   Turning now to  FIG. 2 , illustrated is a SAW identification tag system  200  operating in an environment where a wireless LAN device  210  and a microwave oven  215  are operating within the same ISM frequency range. Coupled to the SAW tag reader  205  is a detection module  206  that detects the signals  211 ,  216  emanating from the wireless LAN device  210  and the microwave oven  215 . The detection module  205  also detects and identifies a null period  221  in the transmission pattern  220  of the wireless LAN device  210  and the microwave oven  215 . The null period  221  may be either a time null period  221  when no signal  211 ,  216  is transmitted, or it may be a frequency null period where a frequency within the ISM band is identified where no signal is being transmitted. The SAW tag reader  205  uses this information to effect a communication of information by either communicating when the conflicting signal  211 ,  216  is not transmitted, or it can change to a null frequency within the ISM range and use that frequency to communicate. 
   For example, if the SAW tag reader  205  detects a conflicting signal  211  from a LAN device  210 , it can synchronize its operation to the operation of the LAN device  210  and become active only during periods of LAN device  210  inactivity. In the case of the microwave oven  215 , which typically operates with 50% duty cycles with 8 millisecond periods of inactivity the SAW tag reader can operate within the periods of inactivity. A SAW tag reader  205  only needs a few microseconds to transmit an interrogation signal  110  and receive response acoustic pulses  160 , which permits it to transmit and read many signals within such 8 millisecond inactive period. Another characteristic of a microwave oven  215  is that it typically operates within relatively narrow bands of energy that are swept across the ISM band during the 8 millisecond activity period. A SAW tag reader  205  can detect the narrowband of energy and avoid those bands while measuring SAW tag responses at other frequencies, even while the Microwave oven  215  is active. In a similar fashion, other applications such as wireless LANs and Bluetooth have significant inactivity times and use only a portion of the total ISM frequency band when they are active. As mentioned above a SAW tag reader  205  can sense the inactive frequency bands and inactive time intervals to ensure reliability of SAW tag reads while simultaneously preventing intrusion into other ISM applications. 
   While operating in an environment where other devices are present, such as a wireless LAN device  210 , it is important that the SAW tag system  200  not interfere with such device. To avoid generating undo interference, a SAW tag reader  205  can be designed to generate relatively low power with very short duration pulses. The combination of low power and short burst implies that, in most instances, a SAW tag reader will not impair wireless LAN devices or Bluetooth™ applications. 
   Where a SAW tag reader  205  is coexisting with another device such as a wireless LAN device  210 , the SAW tag reader  205  can also be synchronized so that it effects communication with a SAW tag  100  synchronous with the signal emanating from the device. This synchronization can be by time, frequency or both. The SAW tag system  200  can also be enhanced so that when the SAW tag reader  205  detects the presence of another system, such as a wireless LAN device  210 , it interoperates with such system and is totally compatible with such device&#39;s access protocols. 
   Turning now to  FIG. 3 , illustrated is a representative embodiment of a SAW tag  100  showing the layout of a SAW tag platform  300  with groups  310  of reflector locations on the substrate. In the illustrated layout  300 , a preamble  320  precedes data groups  310  and provides for functions such as frame and phase synchronization as well as providing data space for SAW tag version information. The groups  310  are separated by time values  315  (labeled t 1  through t 8 ). Each time value  315  interval represents the time between the center of the last reflector position in one group  310  to the center of the first reflector position of the next group  310 . 
   Eight reflector groups  310  are represented. This represents a generic SAW tag platform  300  with a basic 128-bit encoding structure. Some of the groups  310  convey payload data codes while other groups  310  are used for synchronization and error checking. In the instant case, group  311  thru and including group  314  are used to encode payload data or SAW tag identification number data. For purposes of explanation, assume a 64-bit format payload platform is used with four payload groups, Payload 0   311 , S 1   312 , S 2   313  and S 3   314 . Group  315  is the synchronization group and the error check groups are EC 0   316 , EC 1   317  and EC 2   318 . The error check structure described herein involves two useful concepts: error check separation and error check nesting. Thus, ECO  316  performs an error check on different data than EC 1   317  and EC 2   318  and is, thus, totally separate from EC 1   317  and EC 2   318 . The use of separate error checks facilitates manufacturing processes by allowing shared use of ECO dependent masks with multiple mask sets designed for different higher order data fields (e.g. different manager and object fields). EC 1   317  and EC 2   318 , on the other hand, are nested. EC 2   318  performs an error check of the same data as EC 1   317  and on EC 1   317  as well. Thus, the combination of EC 1   317  and EC 2   318  is, in effect, a form of a 32-bit error check. The nested design is more flexible than a conventional 32-bit error check because EC 1   317  can be used strictly for code space separation (i.e. processing gain) while EC 2   318  is used strictly for error checking. In applications wherein the processing gain is unnecessary, the EC 1 /EC 2   317 ,  318  combination can be used for 32-bit error checking. 
   Thus the present invention provides, in one embodiment, for a SAW identification tag  100  that has at least two tag identification number or payload reflector groups  311 - 14  (Payload 0   311 , S 1   312 , S 2   313  and S 3   314 ) located on its substrate. Also located on the substrate is a first error-checking reflector group (EC 0   316 ) that is dependent upon data contained in one of the at least two tag identification number reflector groups  311 - 14 , in this case S 3   314 . A second error-checking reflector group (EC 1   317 ) is also located on the substrate and is dependent upon data contained in SAW tag identification number reflector groups Payload 0   311 , S 1   312  and S 2   313 . While still a third error-checking reflector group (EC 2   318 ) on the substrate is dependent upon data contained in the at least remaining one tag identification number reflector group (Payload 0   311 , S 1   312  and S 2   313 ) and EC  1   317 , the second error-checking reflector group. 
   Referring now to  FIG. 4 , illustrated is a SAW tag platform  400  using a “nested” error check protocol. The SAW tag platform  400  has at least one tag identification number reflector group  410  located on a substrate. A first error-checking reflector group  411  is dependent upon data contained in the at least one tag identification number reflector group  410 . A second error-checking reflector group  412  is located on the substrate that is dependent upon data contained in the at least one tag identification number reflector group  410  and the first error-checking reflector group  411 . 
   Referring now to  FIG. 5 , illustrated is a SAW tag substrate  500  with multiple reflector groups  510  having multiple reflectors  520  in each group  510 . In the interest of encoding multiple data bits with a small number of SAW reflections, data is encoded using pulse positions. Because allowable pulse positions are more finely spaced than the width of an interrogation pulse, pulse position is difficult to discern using only time of arrival detection. Additional discrimination between pulses is achieved by encoding different pulse positions with differing reflected phases. To this end, successive pulses are encoded with successive increments of a phase step. 
   Assuming the first pulse position is encoded with a reference phase of zero degrees, successive pulse positions are encoded with successive multiples of the phase step. Although the phase of a particular reflector  520  is independent of other active reflectors  520 , the actual phase of a particular reflector  520  is dependent on the number of active reflectors  520  preceding it. An important consideration in designing a SAW tag is to minimize the dependence of the phase of a particular pulse on the presence or absence of pervious pulses. This dependence is minimized by making all reflectors in a group identical. 
   Thus, one embodiment of the present invention provides for a SAW tag substrate that has a first group  511  of reflectors  520  located on a substrate with all the reflectors  520  in such first group  511  having substantially similar first reflection characteristics. A second group  512  of reflectors  520  located on the SAW tag substrate  500  also has substantially similar second reflection characteristics. In another embodiment, the first reflection characteristics and the second reflection characteristics are substantially similar. 
   This embodiment is useful in that the starting phase of a signal for subsequent groups is independent of which particular reflectors are active in preceding groups. Use of identical, or nearly identical, reflectors  520  across multiple groups  510  is also beneficial in that it produces early return pulses with higher amplitudes than later return pulses. Stronger amplitude early pulses are desirable because environmental echoes are stronger near the start of a SAW tag response than they are near the end. 
   Also illustrated in  FIG. 5  is a start reflector  530  located in front of the first reflector group  510  that carries a basic data load. A start reflector  530  located about 100 nanoseconds in front of the first slot in the first data carrying reflector group  510  can be used to enhance data synchronization and to measure multi-path reflections in the reading environment. Once the multi-path has been characterized by observing the tag response of this single isolated start-of-tag pulse, the effects of the multi-path can be removed in subsequent data detection processes. 
   Also shown is an end-of-tag reflector  540  located after the last active data group  510 . The end-of-tag reflector  540  produces an output pulse at a fixed time of a predetermined number of slots and is located after the last slot position of the last data group  510 . In addition to using the direct reflection of the end-of-tag reflector  540  for additional synchronization information, it contributes to useful tertiary reflections involving data reflectors near the end of the SAW tag. The additional return from the end-of-tag reflector enhances the SAW tag reader&#39;s ability to detect the acoustic signal return, a particularly useful feature because the first reflections from the reflectors near the end of the SAW tag substrate are typically lower in amplitude than reflections from the early reflectors. 
   An additional advantage of an end-of-tag reflector is its use in relatively short SAW tags. A SAW tag with two data groups can use the end-of-tag reflector in lieu of the Sync code and error correcting codes. The secondary responses of the information tags provide a time-reflected synchronization signal and, because of the redundancy, provide a form of a signal integrity check to add to the confidence of valid SAW tag presence. 
   Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.