Individually identifiable surface acoustic wave sensors, tags and systems
A surface-launched acoustic wave sensor tag system for remotely sensing and/or providing identification information using sets of surface acoustic wave (SAW) sensor tag devices is characterized by acoustic wave device embodiments that include coding and other diversity techniques to produce groups of sensors that interact minimally, reducing or alleviating code collision problems typical of prior art coded SAW sensors and tags, and specific device embodiments of said coded SAW sensor tags and systems. These sensor/tag devices operate in a system which consists of one or more uniquely identifiable sensor/tag devices and a wireless interrogator. The sensor device incorporates an antenna for receiving incident RF energy and re-radiating the tag identification information and the sensor measured parameter(s). Since there is no power source in or connected to the sensor, it is a passive sensor. The device is wirelessly interrogated by the interrogator.
1. A method for constructing sets of codes that interact minimally comprising:
a. Selecting an integer number of bits that comprises the length of a code;
b. Evaluating the autocorrelation properties of binary codes of a said length, and retaining the set of N codes with the highest autocorrelation peak to sidelobe ratios;
c. Evaluating the cross correlation of each code in said set of N codes with each of the other codes in said set of N codes, and retaining those code pairs that have zero cross correlation in the center of the response, to form a set of M code pairs; and
d. Constructing larger sets of codes that have zero or near zero cross correlation at or near the center of the response from the set of M code pairs generated.
2. A method for constructing sets of codes that interact minimally comprising:
a. Generation of preliminary sets of binary codes that cross correlate to zero or near zero at the center of the response;
b. Amplitude weighting bits in each code to modify the cross correlations of the set of codes to produce zero cross correlation over the time range of interest around the center of the response.
3. A method for constructing sets of codes that interact minimally comprising:
a. Selecting at least one primary code with a large autocorrelation peak to sidelobe ratio;
b. Selecting a set of at least two secondary codes that cross correlate to produce a cross correlation response that is zero at one or more points near the center of the cross correlation response;
c. Refining said set of secondary codes to have zero cross correlation over a wider time range by amplitude weighting the bits of said secondary codes; and
d. Constructing a new set of composite codes by multiplying said primary code by the amplitude of each bit of one secondary code selected from said set of secondary codes sequentially and concatenating the resulting bits to form one composite code in said new composite code set, and repeating this process for each secondary code in said set of secondary codes.
4. A method for constructing sets of codes that interact minimally comprising:
a. The method of claim 3 , wherein said set of at least two secondary codes incorporate amplitude weighting of the bits to reduce cross correlation response amplitudes at and around the center of the response.
5. A surface acoustic sensor tag device, comprising:
a. a piezoelectric substrate;
b. at least one first transducer arranged on at least a portion of said piezoelectric substrate wherein said first transducer has electrode structures to implement bits of a spread spectrum code;
c. wherein said first transducer is implemented with multiple parallel acoustic tracks; and
d. at least one second surface acoustic wave element formed on said piezoelectric substrate and spaced from said first transducer along the direction of acoustic wave propagation.