SAW noise-like anti-collision code study

CDMA and OFC structures and coding schemes have typical correlation side lobes based on chip lengths, which makes it challenging to select good sets of codes for passive wireless SAW sensor systems. Presented in this paper is a new and novel SAW Noise-Like Reflector (NLR) with Pulse Position Modulation (PPM) for multi sensor systems with good auto- and cross-correlation, and anti-collision properties. NLR structures have no typical chips; it consists of a random sequence of varying width and pitch electrodes. Such structures are wideband and have very low correlation side lobes, which significantly reduces code collisions and simplifies code selection for a multi sensor system. A typical SAW NLR PPM sensor consists of a wideband transducer and a NLR PPM on one of the acoustic ports of the transducer. A second reflector bank can be added for differential sensor option. While these sensors can be extended to measure a variety of properties, a thermal sensor system was designed within the scope of this publication. This paper will discuss the basic analytic mathematical model used to predict the first order operation of NLR PPM sensors and the semi-random algorithm used to select code sequences with good anti-collision properties. A coupling of modes (COM) model was used to simulate sensors, and predict the performance of the sensor system more accurately. Both analytic mathematical and COM models yield close agreement, which verified that the simple model is a good synthesis tool and confirmed that the NLR PPM sensor design has the desired performance and advantage. Experimental SAW NLR PPM sensors were built at 250 MHz on YZ LiNbO₃. The system operates with an approximately 25% fractional bandwidth. Several sets of devices were fabricated with various codes and PPM parameters. Measured sensor data agreed well with both simple analytic mathematical and COM models. It is shown that this new NLR PPM structures a unique method for device coding and implementing SAW multi sensor spread spectrum systems.