Optimized Doppler Estimation and Symbol Synchronization for Mobile M-ary Spread Spectrum Underwater Acoustic Communication

Abstract

In mobile underwater acoustic (UWA) communications, the Doppler effect causes severe signal distortion, which leads to carrier frequency shift and compresses/broadens the signal length. This situation has a more severe impact on communication performance in the case of low signal-to-noise ratio and variable-speed movement. This paper proposes a non-data-aided Doppler estimation method for M-ary spread spectrum UWA communication systems in mobile scenarios. The receiver uses the spread spectrum codes dedicated to transmitting signals with different frequency offsets as local reference signals. Correlation operations are performed symbol by symbol with the received signal. The decoding and Doppler estimation of the present symbol are achieved by searching the correlation maximum in the code domain and frequency domain. The length of the current symbol is corrected for the next symbol synchronization using the estimated Doppler coefficient. To optimize the process of Doppler estimation and symbol synchronization, a heuristic search method is used. By adjusting the Doppler factor search step size, setting the threshold value, and using the Doppler factor estimation of the previous symbol, the search range can be significantly reduced and the computational complexity decreased. The Fisher-Yates shuffle algorithm is used to traverse the search range to ensure reliability of the results. Simulation results show that enlarging the frequency-domain search step size in some degree does not affect the decoding accuracy. On 15 May 2021, a shallow-water mobile UWA spread spectrum communication experiment was conducted in Weihai, China. The horizontal distance between the transmitter and the receiver is 3.7–4.0 km, and the communication rate is 41.96 bits per second. The transmitting ship moves at a speed of 0–3 m/s, and the bit error rate (BER) is lower than 1e−3, which is better than that of the sliding correlation despreading method with average Doppler compensation.

Synchronization techniques for underwater acoustic communications

“…In the work by Yang et al, 47 the authors propose a method for Doppler estimation of M ary spread spectrum signals in the underwater acoustic medium. In this method, a locally generated reference signal is correlated with the received signal in a symbol by symbol fashion.…”

PCDE-Sync: A Time Synchronization Mechanism Based on Partial Clustering and the Doppler Effect for Underwater Acoustic Networks

“…Experimental results show that CD-Sync shortens the distance among nodes and accelerates convergence speed while effectively improving synchronization accuracy. Yang et al [ 43 ] introduced a non-data-aided Doppler estimation method for underwater systems with M-ary spread. The simulation results show that the proposed algorithm is better than the sliding correlation estimation method with average Doppler compensation.…”

Short-Block-Length Low-Density Parity-Check Codes-Based Underwater Acoustic Spread-Spectrum Communication System

“…Underwater acoustic channels are complex and variable [1][2][3], and spread-spectrum communication techniques [4][5][6] are often used for robust acoustic communication. Considering the low-latency requirements of the communication system and the frame data volume of underwater acoustic spread-spectrum systems, short-block-length code is generally preferable in terms of its decoding latency and short-frame-data transmission.…”

“…For spread-spectrum systems in underwater acoustics communications, in 2021, Yang G et al addressed the signal distortion problem caused by the Doppler effect in mobile underwater acoustic communication scenarios by using different spread-spectrum codes with different frequency offsets as local reference signals, correlating the received signals one by one and searching for the maximum correlation value in the frequency domain and the code domain in order to complete the signal descaling and Doppler estimation. Simulations and experiments show that, compared to the average Doppler-compensated sliding correlation spread-spectrum method, this method has a better BER performance [5]. Sun D. et al propose a Doppler tracking and compensation algorithm for direct spreading based on the fuzzy function method, which achieves high Doppler estimation accuracy at a 20 dB signal-to-noise ratio through high-precision delay estimation.…”