Construction And Application Of Unequal Error Protected LDPC Codes Under PPM Modulation In Deep Space Optical Communication

Deep space exploration is to understand the unknown space environment by transmitting the remote captured images back to the earth station,so high reliable and efficient image transmission technology is of primary importance.Pulse position modulation(PPM)has the capability of rejecting interference,and low-density parity check(LDPC)codes has the performance of approaching Shannon limit.The iterative decoding and demodulation system composed of them can obtain better error-code performance in the deep space optical communication system.The unequal error protection(UEP)schemes which can effectively protect the more important information in image datas can be used to improve the quality of reconstructed images and the effectiveness of deep space communication.But the existing UEP-LDPC schemes are all designed based on Gaussian channel and non-iterative demodulation system.The performance are not optimal if they were used in the PPM Poisson channel.However,the equal error protected LDPC codes designed based on deep space optical communication do not fully consider the UEP characteristics of the LDPC code.Therefore,the thesis conducts research on the design of UEP scheme for the iterative demodulating and decoding system based on LDPC-PPM.The iterative demodulating and decoding system based on LDPC-PPM is analyzed.The thesis studies the relationship between the degree of variable node and bit error rate.And then propose the design scheme for UEP-LDPC codes based on optimization of the degree of variable node.The degree distributions are optimized hierarchically by using iterative linear programming.According to the characteristics of deep-space optical communication channel and the characteristics of the iterative demodulating and decoding system based on LDPC-PPM,the convergence of degree distribution is analyzed to restrict the selection of degree distribution which can avoid optimization to get a degree distribution with high threshold.The simulation results shows that the performances of the designed two-level and three-level UEP-LDPC codes are superior to the equal error protection LDPC codes designed based on deep space optical communication and the existing UEP-LDPC codes designed based on Gaussian and non-iterative demodulation system.Finally,the designed UEP-LDPC codes are applied to deep space image transmission,and two UEP schemes for image transmission are proposed.One scheme is to use UEP-LDPC to encode image data after unequal error protection mapping.Another UEP scheme for image transmission is based on wavelet compression.The image datas are coded by UEP-LDPC codes after three-level wavelet decomposition,Embedded Zerotree Wavelets(EZW)coded and Huffman coded.The simulation results show that both of the two UEP transmission schemes are superior to equal protection schemes in quality of recovering image.