Abstract:
A secure apparatus, system, device and method for coding surface acoustic wave identification tags and sensors to enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, a pseudo noise sequence is applied to the orthogonally coded signal for increased security. An orthogonal frequency coding technique is applied to the SAW tag using periodic reflector gratings for responding to an orthogonal interrogation signal to transmit the sensor identification and sensed data. A transceiver interrogates the sensor with a stepped chirp corresponding to the orthogonal frequency coded chip frequency response, receives a response from the SAW device, applies an oppositely stepped chirp to the response and then uses matched filtering to produce a compressed pulse. The orthogonal frequency coding technique has an inherent advantage of processing gain, code division multiple access, spread spectrum and security.

Description:
This application claims the benefit of priority to U.S. Provisional application Ser. No. 60/650,843 filed on Feb. 8, 2005. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to signal encoding and, in particular, to apparatus, systems, devices and methods for generating, distributing, processing and detecting orthogonal frequency coding for surface acoustic wave and silicon tags and sensors for transmission of sensor identification and information. 
     BACKGROUND AND PRIOR ART 
     The surface acoustic wave (SAW) sensor offers advantages in that it is wireless, passive, small and has varying embodiments for different sensor applications. SAW sensors are capable of measuring physical, chemical and biological variables and have the ability to operate in harsh environments. In addition, there are a variety of ways of encoding the sensed data information for retrieval. Single sensor systems can typically use a single carrier RF frequency and a simple device embodiment, since tagging is not required. In a multi-sensor environment, it is necessary to both identify the sensor as well as obtain the sensed information. The SAW sensor then becomes both a sensor and a tag and must transmit identification and sensor information simultaneously. 
     Known SAW devices include delay line and resonator-based oscillators, differential delay lines, and devices utilizing multiple reflective structures. Single sensor systems can typically use a single carrier frequency and a simple coding technique, since tagging is not required. However, there are advantages of using spread spectrum techniques for device interrogation and coding, such as enhanced processing gain and greater interrogation power. 
     The use of orthogonal frequencies for a wealth of communication and signal processing applications is well known to those skilled in the art. Orthogonal frequencies are often used in an M-ary frequency shift keying (FSK) system. There is a required relationship between the local, or basis set, frequencies and their bandwidths which meets the orthogonality condition. If adjacent time chips have contiguous local stepped frequencies, then a stepped chirp response is obtained. 
     The technique disclosed in this patent include a novel spread spectrum coding that uses orthogonal frequency coding for SAW identification tags and sensors which enables unique sensor operation and identification in multi-sensor environments. The OFC technique of the present invention provides a wide bandwidth spread spectrum signal with all the inherent advantages obtained from the time-bandwidth product increase over the data bandwidth. The encoding technique is similar to M-ary in terms of its implementation where transducers or reflectors are built with the desired code. Implementations of the system architecture, the device configuration, and encoding and detection schemes of orthogonal frequency coding are presented. 
     SUMMARY OF THE INVENTION 
     A primary objective of the invention is to provide a new method, system, apparatus and device for applying orthogonal frequency coding to device for communications, tags and sensors. 
     A secondary objective of the invention is to provide a new method, system, apparatus and device that uses orthogonal frequency coding to allow for a wide and ultra-wide bandwidth coding. 
     A third objective of the invention is to provide a new method, system, apparatus and device that uses orthogonal frequency coding and allows for chirp interrogation for increased power. 
     A fourth objective of the invention is to provide a new method, system, apparatus and device that uses orthogonal frequency coding to allow for frequency and binary coding per bit. 
     A fifth objective of the invention is to provide a new method, system, apparatus and device that uses orthogonal frequency coding to allow for a reduced compressed pulse width as compared with pseudo noise (PN) sequence. 
     A sixth objective of the invention is to provide a new method, system, apparatus and device that uses orthogonal frequency coding to provide a secure code for increased data security. 
     A seventh objective of the invention is to provide a new method, system, apparatus and device that adds a pseudo noise (PN) sequence to the OFC coding to provide additional code diversity for tagging. 
     An eighth objective of the invention is to provide a new method, system, apparatus and device for using orthogonal frequency coding to achieve a processing gain. 
     A first preferred embodiment of the invention provides an orthogonal frequency coding technique for applying the orthogonal frequency coding to a surface acoustic wave device for ID tags and sensors. The tag includes a transceiver and at least one bank of reflectors and the reflectors are fabricated so that each reflector produces an orthogonal frequency corresponding to the carrier frequency. The device code is determined by the order of the orthogonal frequencies. 
     For the second embodiment, the novel orthogonal frequency coding technique, pseudo noise (PN) coding is applied to the OFC basis function. In this embodiment, in addition to OFC coding, each chip can be weighted as a±1, giving a PN-OFC coding. In this embodiment, the OFC technique allows for frequency and binary coding per bit and produces a reduced compressed pulse width as compared to the PN code and adds a level of security. 
     For the third embodiment, the orthogonal frequency technique of the present invention is applied to a surface acoustic wave (SAW) device for communication, tagging and sensors in a multi-sensor system. A transceiver interrogates the tag with a stepped chirp and the tag transmits a scrambled signal in response to the interrogation. At the transceiver, an opposite stepped chirp is applied to the tag response to unscramble the code sequence and the resulting signal is match filtered with the coded PN-OFC producing the correlated compressed pulse. 
     Further objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments which are illustrated schematically in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES  
         FIG. 1  is an example of a stepped chirp response. 
         FIG. 2  is an example of an OFC chip frequency response. 
         FIG. 3  is an example of a 7 chip OFC waveform based on the placement of chips. 
         FIG. 4  is a frequency response of a 7 chip OFC device (solid line) and a single carrier (dashed line). 
         FIG. 5  shows the time autocorrelation (½ length) of a single carrier BPSK (dashed line) and a 7 chip OFC (solid line) signals having approximately the same time length. 
         FIG. 6  shows the time autocorrelation (½ impulse length) of a single carrier PN code (dashed line) and a PN-OFC (solid line) signal having a 7 chip Barker code modulating the chips of both signals. 
         FIG. 7  shows the frequency response of a 7 chip PN-OFC signal (solid line) and a single carrier signal (dashed line). 
         FIG. 8  is a block diagram of an example of an OFC SAW system according to the present invention. 
         FIG. 9  is a schematic diagram of an example of a 7 chip OFC SAW tag according to the present invention. 
         FIG. 10  shows an OFC-PN coded waveform (top), an example of a noise-like signal returned in response to a stepped linear up chirp interrogation (center), and the compressed pulse results after application of down chirp and matched filtering (bottom). 
         FIG. 11  shows the compressed pulse produced by matched filtering the unscrambled tag response. 
         FIG. 12  is a schematic diagram of an example of an OFC SAW sensor implementation. 
         FIG. 13  shows two compressed pulses having a differential time delay between pulses. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before explaining the disclosed embodiments of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangements shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. 
     The following is a list of the reference numbers used in the drawings and the detailed specification to identify components: 
     
       
         
               
               
             
           
               
                   
               
               
                 Reference No. 
                 Component 
               
               
                   
               
             
             
               
                 121-127 
                 Bank of reflectors 
               
               
                 140 
                 Transducer 
               
               
                 150 
                 Stepped up chirp signal 
               
               
                 160 
                 Scrambled response signal 
               
               
                 200 
                 OFC SAW Transceiver 
               
               
                 210 
                 Tag 
               
               
                 220 
                 Up-chirp 
               
               
                 230 
                 Tag impulse response 
               
               
                 240 
                 Down-chirp 
               
               
                 250 
                 Receiver 
               
               
                 300 
                 Transducer 
               
               
                 305 
                 Propagation path 
               
               
                 310 
                 Reflectors 
               
               
                 320 
                 Reflectors 
               
               
                 330 
                 Transceiver 
               
               
                   
               
             
          
         
       
     
     It would be useful to review orthogonal frequency before discussing the methods, systems, apparatus and devices for generating, distributing, processing and detecting orthogonal frequency coding according to the present invention. 
     Orthogonal frequencies are used to spread the signal bandwidth. The orthogonality condition describes a relationship between the local chip frequencies and their bandwidths. As an example, consider the stepped linear chirp shown in  FIG. 1 . Seven coherent carriers are used to generate the signal shown. Each chip contains an integer number of carrier half cycles due to the orthogonality condition. Under these conditions, the resulting waveform is continuous. The conditions, however, do not require that the local frequency of adjacent chips, that are contiguous in time, be contiguous in frequency. Instead, the time function of a bit provides a level of frequency coding by allowing a shuffling of the chip frequencies in time. 
     The chip frequency response is shown in  FIG. 2 . These responses are a series of sampling functions with null bandwidths equal to 2·τ −1 . In addition, the sampling function center frequencies are separated by multiples of τ −1 . Coding is accomplished by shuffling the chips to produce a signal such as shown in  FIG. 3 , wherein the adjacent frequencies are not required to be sequential. The code is now determined by the order in which the orthogonal frequencies are used. Both signals occupy the same bandwidth and the coded information is contained within the signal phase. A more complete description of orthogonal frequency coding is given in D. C. Malocha, et al., “Orthogonal frequency coding for SAW device application,” 2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control 50 th  Anniversary Joint Conference, in press, which is incorporated herein by reference. 
     In the example shown in  FIG. 3 , the seven local chip frequencies are contiguous in frequency but are not ordered sequentially in time and the chip weights are all unity. If the local chip frequencies were ordered high to low or low to high, the time sequence would be a stepped down-chirp and up-chirp, respectively. The start of the chip carrier frequency begins at zero amplitude, as seen in  FIGS. 2 and 3 , which is a condition of the orthogonality. 
     The OFC technique of the present invention provides a wide bandwidth spread spectrum signal with all the inherent advantages obtained from the time-bandwidth product increase over the data bandwidth. The OFC concept allows for a wide bandwidth, chirp interrogation, frequency and binary coding per bit, a reduced compressed pulse width as compared to a PN sequence, and a secure code. The OFC technique of the present invention can be applied to ultra-wide-band applications since the fractional bandwidth can exceed 20% and can be used in a multi-tag or sensor environment by using proper coding techniques. 
     In the preferred embodiment, apparatus, systems, devices and methods of the present invention provide an orthogonal frequency coding technique for SAW sensors. The given chip sequence represents the OFC for the bit. If there are J-chips with J different frequencies in a bit, then there are J factorial possible permutations of the frequencies within the bit. A signal can be composed of multiple bits, with each bit having the same OFC or differing OFC. For the case of a signal, J-chips long and having a single carrier frequency, the signal is a simple gated RF burst τ B  long. The frequency responses of a 7 bit OFC (solid line) and a single carrier signal (dashed line) are shown in  FIG. 4 , with both time functions normalized to unity and having identical impulse response lengths. The single carrier, shown as the dashed line, is narrowband and has approximately 17 dB greater amplitude at center frequency, as compared to the OFC (J=7), shown as a solid line, which has a much wider bandwidth. The time domain autocorrelation for the signals is shown in  FIG. 5 . The peak autocorrelation is exactly the same, but the OFC compressed pulse width is approximately 0.28·τ C , as compared with the single carrier compressed pulse width of approximately a bit width, τ B =7·τ C . This provides the measure of the processing gain (PG), which is the ratio of the compressed pulse width to the bit length, or in this case, PG=49. 
     In a preferred embodiment, in addition to the OFC coding, each chip can be weighted as ±1, giving a pseudo noise (PN) code in addition to the OFC, namely PN-OFC. This does not provide any additional processing gain since there is no increase in the time bandwidth product, but does provide additional code diversity for tagging.  FIG. 6  shows the time autocorrelation of a 7 bit Barker code applied to an OFC (solid line) and a single carrier (dashed line) frequency. The PN code has a compressed pulse width of τ C ·2, or a PG PN =7 as compared PG PN-OFC =49 achieved with orthogonal frequency coding. The compressed pulse width of the OFC is a function of the bandwidth spread and not the PN code, yielding comparable pulse-width and side lobe results as shown in  FIGS. 5 and 6  without PN-code and with PN-code, respectively. 
     The PN-OFC has an increased PG and a narrower compressed pulse peak over just the PN sequence, proportional to the bandwidth spreading of the OFC.  FIG. 7  compares the PN-OFC and conventional PN frequency response, the bandwidth is spread based on the OFC design. 
     OFC waveforms can be employed in SAW devices using shorted periodic reflector gratings as shown in  FIG. 8 . Each chip of the OFC waveform is implemented using a shorted periodic reflector grating  121 - 127 . The grating periodicities are chosen so that the grating center frequencies correspond to the chip carrier frequencies. In order to keep the chip length approximately constant, each grating must contain different numbers of electrodes as the periodicity changes. This is a direct result of the orthogonality condition. The equation used to find the grating electrode counts is shown below.
 
 N   j =τ c   ·f   j   (1)
 
This equation shows that the grating electrode count is directly proportional to frequency. In addition, the normalized metal thickness also increases with frequency. Therefore, in a device fabricated with a single metal thickness for all reflectors, the magnitude of SAW reflection for each chip will not be equal.
 
     The OFC SAW transceiver  200  block diagram is shown in  FIG. 9 . The SAW tag  210  is interrogated with a linear stepped up chirp  220  ( FIG. 10  top) possessing the same time length and bandwidth as the tag impulse response  230 . For a given peak amplitude, the chirp provides increased power over a given bandwidth as compared to a simple RF tone burst. A noise-like tag response signal  230  as shown in the center signal in  FIG. 10  is returned from the identification tag  210 . Since orthogonal frequencies are used, the intersymbol interference is drastically reduced when compared with a conventional PN sequence. A band-limited version of the tag&#39;s impulse response results after a down chirp  240  is applied. The signal is then match filtered to produce a compressed pulse as shown in the bottom signal in  FIG. 10 . 
     For example, orthogonal frequency coding is applied to a SAW tag system wherein the tag is designed having a center frequency of 235 MHz, composed of a 3-bit, 7 chip Barker code with τ c =0.1 μsec, using 7 reflectors  121 - 127  each having a different center frequency dependent on the electrode period. For this example, the reflectors  121 - 127  are assumed to have equal reflectivity and have a rect time function response. A device schematic is shown in  FIG. 8 . The input transducer  140  is wideband and its effect is assumed negligible for this example. The OFC tag impulse response  160  has uniform amplitude versus time and is 21 chips long. The tag is interrogated with a linear stepped up-chirp  150  having the same center frequency, time length and bandwidth as one bit. By using a chirp signal  150 , the interrogation signal power is increased over that of a simple RF burst. The re-transmitted signal  160  from the tag is 28 chips long due to the convolution of the interrogation chirp and tag impulse response; producing a noise like signal  160 . The tag response is a spread spectrum signal which is wideband and has inherent security. Since the chips have orthogonal frequencies, there is no intersymbol interference with overlapping chips, unlike a conventional PN sequence. 
     Referring back to  FIG. 9 , at the receiver  250  a corresponding stepped down chirp  240  is applied to the tag response signal  230 , which unscrambles the code sequence producing a reconstructed signal that is approximately 21 chips long and has some amplitude modulation ( FIG. 10  bottom). The signal is then match filtered with the coded PN-OFC producing the correlated compressed pulse as shown in  FIG. 11 . The resulting compressed pulse is approximately 0.28 τ c  long, yielding the processing gain of 49. FIG.  11  shows the ideal convolution of the orthogonal frequency coded signal and the system simulation. The compressed pulses shown in  FIG. 11  are nearly identical, demonstrating that the chirp interrogation signal and matched filter process accurately reconstructs the desired tag signal. 
     The OFC apparatus, systems, devices and methods of the present invention are readily applied to SAW sensing applications. The resulting system offers the advantage of simultaneous sensing and tagging. Measurement of the received sensor OFC signal in a matched filter system, using differential or calibrated delay, allows for sensing of an environmental temperature, pressure, gases, liquids, or bio-agents within range of the surface acoustic device. Application of sensing materials in the propagation path  305  to and from the transceiver  330 , on the transducer  300  or on reflectors  310  and  320  can be used for specific targeted measurands. An example is as shown in  FIG. 12 . 
     In summary, the present invention provides new apparatus, systems, devices and methods for using the OFC technique disclosed above to provide a wide bandwidth spread spectrum signal with all the inherent advantages obtained from the time-bandwidth product increase over the data bandwidth. The OFC technique of the present invention allows for a wide bandwidth, chirp interrogation, frequency and binary coding per bit, a reduced compressed pulse width as compared to a PN sequence, and a secure code. This approach can be applied to ultra-wide-band applications since the fractional bandwidth can exceed 20%. The approach can be used in a multi-tag or sensor environment by using proper coding techniques. A SAW tag example demonstrated the coding approach and showed good auto and cross correlation results. 
     While the invention has been described, disclosed, illustrated and shown in various terms of certain embodiments or modifications which it has presumed in practice, the scope of the invention is not intended to be, nor should it be deemed to be, limited thereby and such other modifications or embodiments as may be suggested by the teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended.