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.
Highlights
The underwater acoustic (UWA) channel is complex
Some researchers estimate Doppler based on the time-domain broadening/compression caused by the Doppler effect [8,9], which has a strong anti-noise ability
Accurate symbol synchronization and channel equalization are the keys to achieving high-quality Single carrier multiple phase shift keying (SC-MPSK) communication; orthogonal frequency division multiplexing (OFDM)
Summary
The underwater acoustic (UWA) channel is complex. When the source and receiving devices are in relative motion, the influence of Doppler and noise on the communication quality is severe [1,2,3]. Accurate symbol synchronization and channel equalization are the keys to achieving high-quality SC-MPSK communication; OFDM needs to conduct Doppler estimation and compensation with different accuracy step by step [20,21]. The scheme proposed in this paper is based on M-ary spread spectrum (MSS) code modulation and combines symbol-by-symbol Doppler estimation and synchronization to meet the needs of mobile UWA communication. The method presented needs to implement the correlation maximum search in the two-dimensional grid composed of spreading code numbers and different Doppler factors for Doppler factor estimation. We find that when searching in the frequency domain, within a certain range, this method is more tolerant to relatively large step sizes Through this approach, we reduce the number of calculations to nearly 1/20 of the original.
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