关键词: 多元低密度奇偶校验, 下三角结构, 扩频, 水声通信

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）