PN Sequence Period Estimation Method for Underwater Direct-Sequence Spread Spectrum Signals Under Low SNR

In non-cooperative DSSS signal reception, the accurate estimation of the pseudo-noise (PN) sequence period is essential for successful despreading and information recovery. In this paper, we propose an average second-order-moment autocorrelation method based on the accumulated code difference function to enhance the estimation accuracy under low-signal-to-noise-ratio (SNR) conditions. The proposed approach involves three key aspects: First, a quadrature receiver is employed to mitigate the impact of the random initial phase on demodulation, enabling the full utilization of the signal energy. Second, a code difference function is constructed using transition information between adjacent spreading codes, and by leveraging the strong correlation between these functions, accumulated processing effectively suppresses the noise influence. Third, the autocorrelation result of the accumulated code difference function displays periodic peaks separated by intervals equal to the PN sequence period, allowing for period estimation through peak interval extraction. In addition, the introduced average-second-order-moment technique addresses the peak loss caused by information code randomness while further smoothing noise. Simulation and experimental results verify the effectiveness and practicality of the proposed method, which outperformed the average-second-order-moment method by about 1.5 dB and the accumulated-power-spectrum-reprocessing method by about 2 dB.