Constellation symmetry-guided equalization for high-order modulated underwater acoustic communication in time-varying channels

To address the high demodulation sensitivity and poor stability of high-order modulated underwater acoustic (UWA) communication signals caused by complex multipath delay spread, time-varying characteristics, and noise, this paper proposes a high-order modulation UWA equalization method guided by constellation symmetry. Under a receiver framework consisting of a channel shortener cascaded with a decision feedback equalizer (DFE), the proposed method incorporates the symmetry information of high-order constellations to enhance the performance of adaptive equalization iterations. Specifically, by leveraging the geometric prior of the dense and symmetric distribution of high-order constellation points, a cost penalty term reflecting the symmetry offset is introduced into the adaptive iteration process to accelerate convergence. Simultaneously, an error-driven dynamic weighting mechanism is designed by considering the evolution of error trends during the convergence process. Numerical simulations and sea trials verify the superiority of the proposed scheme. In a time-varying shallow-water channel experiment at Xiamen Port, 64-QAM communication was successfully achieved with an effective data rate of 10.94 kbit/s over a 1 km range, with over 90% of data sub-blocks reaching error-free. The results demonstrate that the introduction of the constellation symmetry guidance mechanism significantly enhances the equalization performance and robustness for high-order modulation in UWA communication.