Very high data rate acoustic communication - continuous image transmission using hierarchical modulation in shallow water near vertical channels

Real-time communication in underwater environments is essential for a variety of applications, including exploration, conservation, and construction. However, transmitting compressed video streams across various conditions is challenging due to high-frequency attenuation, time-varying Doppler, multipath propagation, and inconsistent throughput. To address these issues and allow for continuous image transmission in rapidly changing channel conditions we propose a communication system that utilizes advanced signal processing techniques, including an adaptive decision feedback equalizer with a circular array, and low-density parity-check channel coding. The use of hierarchical modulation theoretically has been shown to be beneficial for continuous image transmission due to its flexibility in deciding the image quality at the receiver based on the channel conditions. Hierarchical modulation was investigated to see if it can demonstrate the benefits when compared to standard 16-QAM, in an underwater acoustic channel. The proposed system is tested in the dynamic environment of the North Sea, with Doppler velocities of up to 3 knots at a vertical depth of 100 m. The system is used to transmit still images, where the receiver can decide whether the greyscale or colour image is recoverable. Our approach demonstrates a reliable and efficient communication link, achieving transmission of continuous images with rates up to 100 kb/s while compensating for Doppler shifts in a noisy, multi-reverberating environment utilising a hierarchical modulation scheme.