The surface acoustic wave (SAW) sensor offers advantages in that it is wireless, passive, small and has varying embodiments for different sensor applications. Surface acoustic wave (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. See S. E. Carter and D. C. Malocha, “SAW device implementation of a weighted stepped chirp code signal for direct sequence spread spectrum communication systems”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency control, Vol. 47, July 2000, pp. 967-973.
Co-pending U.S. patent application Ser. No. 11/508,674 filed on Aug. 23, 2006, assigned to the same assignee as the present application, teaches weighted surface acoustic wave reflector gratings for coding identification tags and sensors to enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, the Weighted reflectors are variable while in another embodiment the reflector gratings are apodized. The weighting technique allows the designer to decrease reflectively and allows for more chips to be implemented in a device and, consequently, more coding diversity. As a result, more tags and sensors can be implemented using a given bandwidth when compared with uniform reflectors. Use of weighted reflector gratings with OFC makes various phase shifting schemes possible, such as in-phase and quadrature implementations of coded waveforms resulting in reduced device size and increased coding. The device may include a single transducer/antenna pair with a bank of reflectors on one side of the transducer/antenna pair, or a bank of reflectors on both sides of the transducer/antenna pair, or alternatively, a unidirectional transducer may be used to reduce the device loss and size.
U.S. patent application Ser. No. 11/521,708 filed on Sep. 15, 2006, now U.S. Pat. No. 7,623,037 issued on Nov. 24, 2009, assigned to the same assignee as the present application, and which is incorporated herein by reference describes and claims a surface acoustic wave device including a substrate, at least two banks of reflectors fabricated on said substrate for producing at least two contiguous stepped frequencies as an orthogonal coded signal, wherein each of said at least two contiguous stepped frequencies have a different center frequency within a bandwidth and at least two transducer and antenna pairs each having a different tuned center frequency on said substrate, each of said at least two transducer/antenna pairs coupled with one of said at least two banks of reflectors for receiving an orthogonal frequency coded signal generated by a corresponding one of said at least two banks of reflectors, wherein the bandwidth of each transducer/antenna pair is inversely proportional to the number of transducer/antennas pairs used.
Co-pending U.S. patent application Ser. No. 11/703,377 filed on Feb. 7, 2007, assigned to the same assignee as the present application, which is incorporated herein by reference, teaches an orthogonal frequency coded device that includes a substrate, a transducer and plural acoustic tracks each having a bank of reflectors fabricated on the substrate. The plural acoustic tracks are coupled with the transducer and each acoustic track produces a different code sequence with a different delay between a starting chip sequence in each of the different code sequences. The sum of the different code sequences forms an orthogonal coded signal for the device to provide increased coding by including delays in the code sequences.
Each of the banks of reflectors includes a first and second bank of reflectors located on opposite sides of said transducer and coupled with the transducer. Each bank of reflectors includes plural reflectors coupled together each producing an orthogonal frequency within a bandwidth to generate the code sequence for a corresponding one of the plural tracks. A summation of the codes sequences from the plural tracks produces the orthogonal coded signal for the device.
Surface acoustic wave tags, by their passive nature, are purely a reflective device; changing amplitude, phase and/or delay of the interrogation signal over the band of interest. Because of these properties, the SAW tag is equivalent to a radar target, with the exception that the return, or reflected signal, has been modified in amplitude, phase, or delay in a predefined manner. This change in device parameters is similar to the change of a radar target due to movement causing a Doppler shift. Because there is no handshake in the wireless passive system, where device return signal parameters can be actively modified, the return signals may return in overlapping time positions with the desired signal energy, distorting the desired signal and possibly causing false identification or parameter extraction. Since the OFC SAW devices are tagged using spread spectrum techniques for identification, the device identification at the receiver is accomplished using matched filter and correlation techniques, as well as possibly digital signal processing. The device identification is made via its code, and the overlap of other SAW tags/sensors in the range of the transmitter causes code collisions. Code collisions are a difficult problem to mitigate given the asynchronous nature of the passive SAW tag. The co-inventors have researched numerous books and publications, with nothing of real value to aid in addressing the code collision problem for asynchronous, passive, multi□tag environments.
There are numerous publications and books on code auto□and cross□correlation properties. In an active communication system where there is a handshake between transmitter and receiver, the use of signal level adjustment, orthogonal code sets, spatial diversity, and synchronization can optimize a communication link. This active link scenario provides many options to minimize code collision effects. For the SAW passive tag/sensor, the use of orthogonal code sets has little to no value without the availability of transceiver synchronization and signal level adjustment. Moreover, most work to date has been addressing CDMA codes and little effort on orthogonal frequency coding approaches.
For example, T-K Woo, Orthogonal code design for Quasi-synchronous CDMA”, Electronic Letters, September 2000, Vol. 36, #9, 1632 examined orthogonal code design for quasi□synchronous CDMA. The last sentence in the conclusion is, “However, the results for cross□correlation are mixed”. The Woo scheme has a lower variance but a higher mean”. In general, most analysis was done on a statistical basis for CDMA active systems where it can be assumed that the signals are non□stationary. In the case of a fixed SAW device and a fixed interrogator, the signals are stationary and re□interrogating will just continuously give the wrong answer. Moreover, as code delays change with temperature or other measurand, the code collision can change from acceptable to unacceptable.
As another example, Dudzik, et. al., Orthogonal code design for passive wireless sensors, Communications, 2008 28th Biennial Symposium on 24-26 Jun. 2008, pp 316-319, describes orthogonal CDMA code design for passive wireless sensors. Dudzik sets two criteria:
1. Largest peak□to□sidelobe ratio of the auto□correlation response of a given code
2. Smallest maximum peak value of cross□correlation of that code with any other code in the set.
Dudzik, et. al. shows only summary results and a SAW sensor application, but no useful details are given. But also important is the fact that the criteria may not be correct in a passive, multi□sensor environment. It appears that criteria (2) may be overly simplistic, since it is our belief that it is necessary to have the smallest maximum peak value of cross□correlation of that code to the sum of all other codes in the set under the interrogation range. This is a much more stringent and difficult criteria.
Both examples are for CDMA coding and are not directly applicable to orthogonal frequency coding, but it does illustrate the lack of available work on passive systems. The addition of the OFC□PN coding helps the code collision problem, but does not eliminate basic energy considerations and physical limitations.