An adaptive gain delay-locked loop system for deep sea underwater acoustic communication

The Doppler effect caused by the relative motion of the transmitter and receiver is one of the major challenges in underwater acoustic communication. The delay-locked loop (DLL) can accurately compensate for the Doppler shift of each symbol, making it one of the most practical Doppler compensation techniques in underwater acoustic communication. However, Doppler estimation in conventional delay-locked loops relies on the phase error between symbols before and after the decision, which limits the estimation accuracy and makes it susceptible to decision errors. For deep-sea vertical underwater communication receivers, this article proposes a delay-locking loop based on the Gardner algorithm's timing error with adaptive gain, combined with an adaptive decision feedback equalizer (DFE). It utilizes the Gardner algorithm's timing error for Doppler estimation, which is unaffected by decision errors. This approach has significant advantages in low signal-to-noise ratio (SNR) environments and high-order modulation scenarios. The delay-locked loop uses phase information to adjust the gain coefficient of the second-order loop adaptively, thereby improving Doppler tracking capability. Through simulation, it was verified that the designed system has stronger Doppler tracking ability and noise resistance. The processing of sea trial data indicates that it achieves a gain of 2~5 dB in actual deep-sea vertical underwater acoustic communication. It can achieve 64QAM communication without encoding and 256QAM communication with Turbo equalization at a symbol rate of 5 kbps, demonstrating certain application value.