基于LDPC和PPM的水下光通信性能分析

Performance Analysis of Underwater Optical Communication Based on LDPC and PPM

在水下光通信中,信号在海水信道中衰减极大,严重影响通信系统的可靠性,因此必须采用优秀的纠错码来降低数据传输的误码率。将低密度奇偶校验码(LDPC)与脉冲位置调制(PPM)相结合,建立水下光通信系统模型,在原有的PPM软解调基础上进行简化,降低计算的复杂度,便于硬件实现。简化后的方法由于不需要知道信道的详细特征,特别适合复杂的海水信道中软信息的提取。Matlab仿真分析比较简化前后系统的误码性能,结果表明,简化后的方法由于简化了软信息的提取,尽管相对于未简化方法造成了系统编码增益的下降,但相对于未编码情形来说仍然具有较高的编码增益;当PPM调制阶数较低时,简化后方法的误码性能甚至优于里德-所罗门码。总体而言,简化软解调的方案特别适合不同应用场合下的水下光通信。

Because the signal attenuation is great in sea channel so as to seriously impact on the performance of communication system, an excellent error correcting code is necessary to reduce the error rate of data transmission in underwater optical communication. A system model of underwater optical communication is established by combining low density parity-check code with pulse position modulation. A simplified soft demodulation based on the general PPM soft demodulation is used to reduce the complexity of the algorithm, which is convenient for hardware implementation. What＇s more, the simplified soft demodulation method is very suitable for the extraction of soft information in sea channel because it does not need the detailed features of channel. The error performance between the general method and simplified method is analyzed by using Matlab software. According to the simulation results, the system code gain by using simplified method is decreased because of the simplified extraction of soft information, but it is still higher than code gain of the uncoded system. When PPM order is lower, the performance by using simplified method is even superior to that by Solomon Reed code. On the whole, the scheme of the simplified soft demodulation based on the combination of LDPC and PPM is very suitable for underwater optical communication in huge ranges of applications.