A Robust Time-Frequency Synchronization Method for Underwater Acoustic OFDM Communication Systems

The strong multipath and long delay extensions can lead to large errors in the timing offset estimation and carrier frequency offset estimation of the Orthogonal Frequency Division Multiplexing (OFDM) underwater communication system, resulting in significant performance degradation. Therefore, this paper proposes a robust time-frequency synchronization method for OFDM systems. First, a new preamble based on Constant Amplitude Zero Autocorrelation (CAZAC) sequence is designed, and a new signal timing offset estimation method is developed based on the time domain characteristics of the preamble. Secondly, a two-stage frequency offset estimation (CFO) method is designed in the time domain followed by the frequency domain, which achieves a wide range of high-accuracy frequency offset estimation. Finally, an adaptive threshold signal first-arrival path detection method is proposed to achieve reliable detection of the position of the signal first-arrival path under complex multipath channels. The simulation, pool, and lake experiment results are provided to verify the reliability and effectiveness of this method. Compared with the traditional methods, the proposed method can more accurately obtain the signal timing position and estimate the CFO in complex multipath channels, and at the same time greatly expand the estimation range of the CFO.