Research On Construction Methods Of Structured LDPC Codes In Deep Space Communication

Channel coding is one of the most important technologies to overcome the defects of deep space communication information transmission under long transmission distance,large transmission delay,high bit error rate performance requirements and guarantee the reliability of deep space communication information transmission.As a kind of liner block codes,low-density parity-check(LDPC)have been attracted research interests in the scope of deep space communication channel coding.Compared to random LDPC codes,the structured LDPC codes own cycle or quasi cycle and deterministic check matrix form,which is convenient for hardware storage and implementation,research and analysis the performance of the codes.Therefore,it's a worthy issue to construct the structured LDPC codes with high coding gain performance which is suitable for the scope of deep space communication.Based on the features of deep space communication environment,this thesis focuses on the factors affecting the performance of LDPC codes and proposes two methods to construct structured LDPC codes as follows:1.In view of the requirements of low bit error rate and low code rate,a method to construct Quasi Cyclic(QC)LDPC codes with large column weight and low complexity is proposed in this thesis.Firstly,according to analyzing the relation between the shift coefficients of check matrix and column weight,QC-LDPC codes with girth eight,any column weight and flexible code rate is proposed.Then the above method combined with the parity part of quasi dual diagonal structure with low coding complexity,which makes the constructed QC-LDPC codes girth eight and successfully solves the problem of smaller girth in low complexity fast encoding.Simulation results under deep space communication channel indicate that the constructed QC-LDPC codes based on the proposed method have a certain coding gain performance and low coding complexity,compared with the randomly constructed LDPC codes and structured QC-LDPC codes with equal girth size,which is suitable for the practical application of deep space communication.2.In view of the encoding complexity problem of restricting the practical application in deep space communication,a approach to construct QC-LDPC code with big girth and fast encoding is proposed in this thesis.Firstly,according to analyzing the shortcomings of the existence of dual diagonal structure and quasi dual diagonal structure,a fast encoding scheme with low complexity is proposed,which can make it easy to construct big girth QC-LDPC codes.Then,the above method combines with exponent matrix based on greatest common divisor(GCD)algorithm and masking technology,which makes the constructed QC-LDPC codes be compatible with the double peculiarities of big girth and low encoding complexity,and the encoding flexibility is higher.Simulation results show that our proposed codes have a performance boost,which compared with girth eight QC-LDPC codes based on GCD algorithm and the LDPC codes in the DVB-S2 standard with dual diagonal structure.