## 1 Introduction
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. Although the underwater acoustic spread-spectrum technique has strong multipath resistance by virtue of the good autocorrelation property of pseudorandom sequences, the multipath delay exceeding the spread-spectrum symbol length and the non-minimum phase underwater acoustic channel still bring serious inter-symbol interference (ISI) to the spread-spectrum signals, which affects the system reception signal-to-noise ratio (SNR) [22]. [PERSON] et al. used the RAKE receiver processing structure in the DSSS system to improve the SNR gain by utilizing the multipath energy [23]. [PERSON] et al. proposed a hypothesis-feedback equalization (HFE) algorithm in order to solve the effects of channel time-variation and ISI on the underwater acoustic spread spectrum, and the idea of judgment-feedback equalization can be applied in underwater acoustic spread-spectrum communication systems [24]. For the application of coding techniques in underwater acoustics communication, [PERSON] et al. studied the performance of LDPC code in positive sound velocity gradient (PSVG), constant sound velocity gradient (CSVG) and negative sound velocity gradient (NVG) underwater acoustic channels, and the simulation shows that LDPC code has the best performance in the NSVG underwater channel, the second best performance in the ISVG underwater channel and the worst performance in the PSVG channel, and that there is a difference of about 4 dB compared to the former two [25]. As researchers recognize the changes in the harshness of the underwater channel environment, channel coding techniques based on adaptive code rate adjustment have been applied in underwater acoustic communication technology. Among them, the fountain code, which can generate arbitrary length characters, is a typical representative. It has the feature of coding without a code rate, which makes it have good application prospects in time-varying underwater acoustic channels. Therefore, in recent years, the main fountain codes LT code (Luby Transform Code), Raptor code, Strider code, etc., in underwater acoustic communication research have been put on the agenda, and there are preliminary experimental results that show that they have good error correction capabilities. [PERSON] et al. applied spatially coupled low-density parity-check (SC-LDPC) code in an underwater acoustic OFDM communication system that has a high communication rate with low equalization complexity. Simulation experiments demonstrate that the performance of protograph-based SC-LDPC code is improved by about 1 dB at 1 \(\times 10^{-4}\) BER compared to the general LDPC code with the same delay under the condition of a shallow water acoustic channel with 1000 m and a 10 KHz bandwidth [26]. In this paper, we study an underwater spread-spectrum system combining the Consultative Committee for Space Data Systems (CCSDS) 231.1-O-1 standard LDPC codes, and verify the system performance through simulations and water pool experimental analysis. The results show that the method proposed in this paper is suitable for underwater acoustic complex channels and the BER performance is improved compared to other existing coding methods. The LDPC coding and spread-spectrum methods used are listed in Table 1.  
The remainder of this paper is organized as follows. Section 2 introduces the CCSDS codes and their encode/decode algorithms. Section 3 describes the DSSS, M-ary spread-spectrum, and cyclic-shift-keyed (CSK) spread-spectrum. Section 4 presents the simulated BER performance of the spread-spectrum coding system, and Section 5 analyzes the experimental results given by this spread-spectrum coding system. Section 6 discusses the results presented in this paper, before Section 7 concludes with a summary of this study.