Structural Optimization and Adaptive Demodulation for an Underwater Acoustic Backscatter Node

Underwater acoustic backscatter offers a low-power approach to communication with submerged sensor nodes. This study examines a piezoelectric reflector with a steel backing and a lead zirconate titanate (PZT, grade 5H) element and evaluates an exponentially weighted moving average (EWMA) threshold detector for binary amplitude-shift keying (2ASK). An electromechanical model and a two-dimensional axisymmetric finite-element model were used to compare candidate reflector structures near 200 kHz. In tank tests, the received peak-to-peak voltages in the two load states were approximately 900 and 770 mV for a commercial Leqi transducer, 870 and 650 mV for a DYW-63-G transducer, and 870 and 640 mV for the fabricated transducer. These observations correspond to normalized received-voltage contrasts of approximately 14.4%, 25.3%, and 26.4%, respectively; they do not constitute calibrated acoustic reflection coefficients. A 16-bit 2ASK sequence was recovered in laboratory tests with the commercial reference transducers at 100 bit/s. Link-level simulation was used separately to compare fixed and adaptive threshold detection under time-varying channel assumptions.