OFDM水声通信多普勒估计与跟踪方法

水声通信中传统宽带多普勒估计方法难以准确跟踪时变多普勒因子，从而导致正交频分复用(Orthogonal Fre‐quency Division Multiplexing, OFDM)在变速运动通信场景中补偿性能不佳。针对该问题，文章提出了一种基于空载波的多普勒估计与跟踪算法。首先对三频信号做线性调频 Z 变换(Chirp-Z Transform, CZT)得到多普勒先验值，然后利用 OFDM 符号中的空载波结合载波频偏(Carrier Frequency Offset, CFO)搜索补偿技术，把估计的最优 CFO 值转换为宽带多普勒因子，进而计算当前符号的加速度并预测下一符号的速度。通过更新加速度对预测值进行修正，实现每个 OFDM 符号的多普勒估计。数值仿真和湖试结果表明，文中算法不仅能有效跟踪多普勒的变化，在匀速和变速条件下都有较好的补偿性能，而且对帧结构设计要求低，对先验误差不敏感，有利于水声通信系统的工程实现。

Doppler estimation and tracking method for OFDM underwater acoustic communication

In underwater acoustic communication, the traditional wideband Doppler estimation method is difficult to accurately track the time-varying Doppler factor, so the compensation performance of orthogonal frequency division multiplexing (OFDM) in the variable speed communication scene is poor. To solve this problem, a Doppler estimation and tracking algorithm based on null subcarriers is proposed in this paper. Firstly, the Doppler prior values are obtained by Chirp-Z transformation of the three-frequency signal, and then the estimated optimal carrier frequency offset (CFO) values are converted into broadband Doppler factors by using the null subcarriers combined with the CFO search compensation technology in OFDM notation. Then the acceleration of the current symbol is calculated and the speed of the next symbol is predicted. By updating the acceleration, the predicted value is corrected to realize the Doppler estimation of each OFDM symbol. Numerical simulation and lake test results show that the proposed algorithm can not only track Doppler changes effectively, but also has good compensation performance under constant and variable speed motions. Moreover, it has low requirements on frame structure design and is insensitive to prior errors, which is conducive to the engineering realization of underwater acoustic communication system.