An Improved Variable Parameter Phase-Locked Loop for Single-Carrier Underwater Acoustic Communications

A direct adaptive equalizer (DAE) coupled with a phase-locked loop (PLL) has been proved as an efficient receiver scheme for single-carrier coherent underwater acoustic (UWA) communications. Ever since its birth, numerous improvements have been proposed for the DAE. In contrast, the PLL has been somehow overlooked. A PLL with predetermined coefficients suffers performance degradation when channel conditions change, then a variable-parameter (VP) PLL designed via gradient descent (GD) method was proposed. The GD-VP-PLL is sensitive to parameter initialization though. In this paper, we propose a new VP-PLL scheme in light of the recursive Gauss-Newton (RGN) method. The resulting RGN-VP-PLL has been tested by at-sea experimental data, and showed better performance than the GD-VP-PLL and a PLL with fixed coefficients. Moreover, it is insensitive to initialization of parameters.