Abstract: Multi-path time-varying，high noise and serious attenuation of underwater acoustic channel make the received signals distortion，sudden and random errors make high bit errors rate，it can’t ensure reliability of system.It proposes spread spectrum Underwater Acoustic Communication（UWAC） system based on non-binary Low Density Parity Check（LDPC） codes，which adopts lower triangular form of Quasi cyclic-LDPC（Qc-LDPC）parity-check matrix，the iterative coding and decoding based on symbol can correct the sudden and random errors effectively，and there is no information loss during the transform between bit and symbol when improving underwater limited spectrum efficiency.The theoretical analysis and simulation results show that compared with the binary LDPC code，turbo code and convolution code based on bit，non-binary LDPC code based on symbol can satisfy the bit errors rate of spread spectrum UWAC system with more lower SNR，proves the validity of system.

Key words: non-binary Low Density Parity Check（LDPC）, lower triangular form, spread spectrum, Underwater Acoustic Communication（UWAC）

摘要： 水声信道多径时变、高噪声和严重衰落等特点使接收信号畸变失真，产生的突发和随机错误使系统误码率较高，可靠性难以保证。提出基于多元LDPC码的扩频水声通信系统，采用具有下三角结构的Qc-LDPC码校验矩阵，利用基于符号的迭代编译码算法有效纠正突发和随机错误，且与高阶调制结合提高水下有限频谱利用率时不存在符号与比特转化的信息损失。理论分析和仿真结果表明：相比基于比特的LDPC码、turbo码和卷积码，基于符号的多元LDPC码能以更低信噪比达到扩频水声通信系统误码率指标的要求，证明了方案的有效性。

关键词: 多元低密度奇偶校验, 下三角结构, 扩频, 水声通信