Applications Of LDPC Codes In Wireless Optical Communications

Wireless optical communication has become a signifinant research topic in the field ofcommunication nowadays and has a wide application propects. However, the laser transmissionprocess is inevitably affected by the atmospheric and turbulence, and many other factors. All ofthese factors will degrade the quality of wireless optical communication.High anti-interference channel coding technology is a method to solve the problem. LDPC is anow a popular coding scheme. LDPC combining with efficient BP decoding has the performanceof approaching Shannon information bound. But the high complexity of encoding of LDPCrestricts the practical application in communication system. The construnction of parity checkmatrix H is an important factor which affects the performance of LDPC codes. QC-LDPC has alinear encoding complexity and is friendly to hardware implementation.A circulant permutation search algorithm is present in this paper, which can be employed tosearch appopriate circulant permutation to construct QC-LDPC codes with desired girth. Thealgorithm is based on a tree expansion mechanism and a select-and-discard procedure. Byproposed method, a practical (3,4) code with code length n=37368and girth g=18is constructed.The experiment shows that the codes constructed in this paper have large girth and minimumdistance, and the bit error rate performance is excellent.Furthermore, an optimized BP decoding algorithm is presented in this paper. By selectingvariables node logarithmic likehood ratio decision threshold T and the number of parity nodeedges.With the optimized BP decoding algorithm and QC-LDPC under turbulence channel, thesimulation results show that the optimized decoding algorithm simplifies the complexity of thealgorithm on the one hand, and has an available1.5dB channel gain.