Research On LDPC-Coded Modulation Technology In Wireless Optical Communication

FSO(Free-Space Optical)communication,also known as wireless optical communication,is an emerging communication method with various advantages such as the wide communication frequency bandwidth,strong confidentiality,large capacity and low power consumption,and has received extensive attention in recent years.QC-LDPC(Quasi-Cyclic LDPC)codes are very practical and the parity-check matrices have quasi-cyclic characteristics,which can achieve excellent performance while reducing the construction complexity.The FSO communication system combining BICM-ID(Bit-Interleaved Coded Modulation with Interleaved Decoding)technology and QC-LDPC coding technology can not only fully utilize the bandwidth of the system,but also reduce the influence of various disturbance factors such as atmospheric turbulence on system performance,which greatly improves the performance of the system.The main work of this thesis is as follows:(1)The related background of atmospheric channel is introduced,and then several factors affecting the transmission performance of FSO communication system in the atmosphere are analyzed.Finally,the channel model of atmospheric turbulence channel is given.In this thesis,the Gamma-Gamma distribution model is used as the atmospheric turbulence channel model,because the curve value of the Gamma-Gamma distribution model in the strong turbulence part fits the experimental data well,and the applicable atmospheric turbulence intensity range is very wide.(2)Improved QC-LDPC codes are constructed based on the finite field,and a simple masking matrix construction method is used to improve the performance of the codes.Firstly,QC-LDPC codes are designed by using two subsets in the finite field.Secondly,based on the idea of mask,a masking matrix is designed.And then masked QC-LDPC codes are constructed.The simulation results show that the improved QC-LDPC codes have a certain performance improvement.(3)A class of equiprobable non-uniform constellation diagrams and a class of non-equiprobable uniform constellation diagrams are optimally designed and applied to wireless optical communication system in combination with BICM-ID system.Compared with the traditional modulation and demodulation methods,the two types of constellation diagrams designed in this thesis can make the FSO communication system obtain the shaping gain.Firstly,based on the method of spatial geometric mapping,a kind of constellation diagrams is designed by making the point distribution on the constellation approach the discrete Gaussian distribution.The scheme improves the performance compared with the traditional QAM constellation mapping.Secondly,based on the method of probability mapping,a kind of constellation diagrams is designed.By mapping multiple different information sequences to the same constellation point,each constellation point approaches the discrete Gaussian distribution in use probability and optimizes its minimum Hamming distance.The simulation results show that both types of mapping schemes improve the performance of the FSO communication system.