Deep Learning-Based Joint Channel Equalization and Symbol Detection for Air-Water Optoacoustic Communications

The optoacoustic effect is triggered by directing an optical signal in the air (using laser) to the surface of water, leading to the generation of a corresponding acoustic signal inside the water. Careful modulation of the laser signal would enable an innovative method for direct communication in air-water cross-medium scenarios experienced in many civil and military applications. In order to achieve a high data rate, a multilevel amplitude modulation scheme can be used to generate different acoustic signals to transmit multiple symbols. However, accurately demodulating these acoustic signals can be challenging due to multipath propagation within the harsh underwater environment, inducing inter-symbol interferences. This paper proposes a deep learning-based demodulation technique that uses a U-Net for signal equalization and a Residual Neural Network for symbol detection. In addition, fine-tuning at the receiver side is also considered to increase the demodulation robustness. The proposed deep learning model has been trained with our laboratory constructed dataset containing eight levels of optoacoustic signals captured from three different underwater positions. The model is validated using two datasets containing severe interference due to multipath-generated echoes and reverberations. The results show that our demodulation model achieves 96.6% and 91.7% accuracy for the two datasets, respectively, which significantly surpasses the 72.9% and 65.30% accuracy achieved by the conventional peak detection-based technique.