The Case Of Weak Turbulent Atmospheric Optical Wireless Communications, The Ldpc Codes

The atmospheric turbulent channel is a bursty-error prone channel, the performance of the atmospheric wireless optical communication can be greatly improved by using the forward error-correct coding. To combat the influence of scintillation, Low-density parity-check codes (LDPC) are adopted to improve the link performance.LDPC codes are linear block codes, so the principle of LDPC codes encoding is the same with linear block codes, the ordinary encoding method of LDPC codes is developed under the principle. But the ordinary encoding method has high encoding complexity and time-delay problems(especially the length of the codes is very large). A novel encoding method -- approximate upper triangular (AUT) form encoding method can solve these problems, many simulation results conducted in the additive white Gaussian noise (AWGN) channel show that the performance is almost the same as the ordinary encoding method.There are two types of LDPC codes decoding algorithms: hard decision algorithms and soft decision algorithms. The typical algorithms of these two types are bit flipping algorithm, belief propagation algorithm and log-likelihood ratios belief propagation algorithm. They are studied in the AWGN channel in this paper, and the comparison of their performances are given in this paper.For the time being, there is little research of applying LDPC codes to the atmospheric wireless optical communication.Different kinds of LDPC codes are studied by using OOK modulation under weak circumstance. The simulation results disclose that LDPC codes have a good capability of correcting bursty errors and can greatly improve the reliability of the communication system. Using LDPC codes can guarantee its realization.