Compared to the aerial radio channel, underwater acoustic channel has the characteristics of strong multipath interference, great doppler shift and time variation. All these factors bring more interference to signal phase, that would cause critical disturbance to coherent underwater acoustic communication. In order to decrease the bad influence, adaptive equalizer is adopted as an effective method.

For owing the advantages such as fast rate of convergence, lower computational complexity, and insensitiveness to round off errors, etc., least squares lattice (LSL) adaptive equalizer is suitable for real-time underwater communications. However, adaptive equalizer can't perform perfectly in great Doppler shift situation. In order to achieve better tracking to Doppler frequency shift, FANG Dong, LI Yu, HUANG Haining and YIN Li of Institute of Acoustics, Chinese Academy of Sciences carried out a series of studies. They proposed to insert an equivalent second order phase-locked loop (PLL) into LSL equalizer and put forward two kinds of algorithms, which were named as LSL with post-deposited PLL (LSL_PLL_post) and LSL with pre-deposited PLL (LSL_PLL_pre), respectively.

In order to test the algorithm performance, the RLS, LSL, LSL_PLL_post and LSL_PLL_pre algorithm are simulated on computer. The test result shows that the PLL brings distinct improvement of Doppler tracking ability of LSL adaptive equalizer. And LSL_PLL_pre algorithm is the best of four in circumstance of multi-path and Doppler shift. Furthermore, LSL-PLL-pre algorithm is proved practical and slightly better than LSL_PLL_post algorithm with analysis of experimental data from lake trial.