Laser acoustic cross-medium communication demodulation method based on sliding RMS filter

Cross-medium information transmission poses a significant challenge in establishing an integrated sea-air network due to the impedance mismatch between water and air. Conversion-type cross-medium transmission methods, such as translational radio frequency acoustics and laser-induced acoustics, utilize electromagnetic waves as information carriers in air and acoustic waves in water, facilitating long-distance cross-medium communication. Laser-induced acoustics for cross-medium communication has garnered significant attention in recent years due to its advantages, including minimal environmental impact, long transmission distances, and the elimination of relays. Laser acoustic signals are highly suitable for pulse position modulation (PPM) due to their short duration and concentrated energy. Due to the high single-pulse energy of the laser in cross-medium communication, its repetition frequency is typically low. To maintain the transmission rate, the encoding positions in PPM are often closely spaced. The inherent randomness of laser acoustic signals in both the time and frequency domains complicates their demodulation. To accurately determine the occurrence time of laser acoustic signals, we propose a laser-induced acoustic cross-medium communication demodulation method based on a sliding RMS filter, leveraging the energy concentration characteristics of laser acoustic signals for demodulation. Initially, the absolute value of the signal is computed, followed by filtering through multiple sliding RMS filters in series to derive its energy distribution. As the energy distribution of laser sound is uneven over time, offset compensation is applied. The local maximum is computed within the eight coding areas, and the positions where the local maximum exceeds the predefined threshold are identified as the coding positions obtained through demodulation. Numerical simulations indicate that the proposed demodulation method achieves a time resolution on the order of 10μs, with a bit error rate remaining below 10⁻³ at a water depth of 500m. To validate the demodulation method, experiments were conducted in a pool at depths ranging from 1m to 5m. In the experiments, the laser operated at a repetition frequency of 60 Hz, with a single-pulse energy of 380 mJ. Experimental results demonstrate that the demodulation method is effective for cross-medium communication across various environments and sound pressure levels, maintaining a bit error rate below 10⁻⁴ at a distance of 5m. The implementation of laser-induced acoustic cross-medium communication offers a novel approach to establishing an integrated sea-air network.