Research On The Encoding Algorithm Of QC-LDPC For CO-OFDM System

With the development of technology and popularization of information,the demand for cloud computing and high-definition video data is bigger and bigger.Therefore,the network bandwidth requirements are also increasing,as well as the optical fiber communication.Coherent optical orthogonal frequency division multiplexing(CO-OFDM)system with "super channel" technology can improve the spectrum utilization in the condition of sacrificing small transmission distance.It is one of the most potential technologies in optical communication system to satisfy the requirements of optical communication,such as "super capacity" and "long distance".With the in-depth study of the 5G technology,the high standard of ultra-fast download rate,zero delay and zero packet loss has become the basic requirements for the realization of the Internet of Everything.It is gradually established,the corresponding communication standards,in order to fit the reliability of high-speed communications.Low density parity check(LDPC)code as the encoding scheme of data channel in long code is radiated as the international mobile communications standard of the third generation partnership project(3GPP)with the eighty-seventh meeting in 2016,which has become the inevitable trend for scholars' research about the channel encoding technology.LDPC code is divided into two categories: random code and structured code.It has lower error platform and lesser coding complexity based on structured LDPC code,which makes it easier for the system to constitute longer LDPC codes.Quasi-Cyclic LDPC(QC-LDPC)code is a kind of typical structured LDPC code,which is easy to encode and realize,and greatly enhances the reliability of communication.In this paper,we deeply study the encoding algorithm of QC-LDPC for CO-OFDM system,and the main work is as follows:First of all,fully master the theory of QC-LDPC code through analyzing the coding rules and methods.It will have close to the Shannon limit of the outstanding performance when the LDPC code has a good structure,and must be able to make the system reliability has been a qualitative leap.In the course of the study can be found in Invertible Subset LDPC(IS-LDPC)code that has the same excellent reliability with QC-LDPC code in IEEE 802.16 e standard because of its special codes,and then get IS-QC-LDPC code which the application of IS-LDPC codes to quasi-cyclic structure.Secondly,the check matrix of a good LDPC code must ensure that it has the maximum girth length and the minimum number of short loops in the bipartite graph.In order to optimize the IS-QC-LDPC code and reduce the correlation between codes,theOIS-QC-LDPC code that is the improved QC-LDPC code is proposed and the corresponding coding algorithm is also proposed by optimizing the offset of the basic parity check matrix,reducing the number of short loops and increasing the girth.In this paper,the OIS-QC-LDPC code is optimized,and both the double diagonal property of the check matrix of QC-LDPC code in IEEE 802.16 e is fused and the invertible property of IS-LDPC code structure is kept.It is divided into three steps to establish check matrix construction method of OIS-QC-LDPC code.Firstly,get the basic parity check matrix based on the degree distribution considering the variable nodes and check nodes,using incremental length extension progress edge growth algorithm.In this paper,the distribution of parity check matrix of QC-LDPC codes in IEEE 802.16 e standard is used.Secondly,update the offset of each cyclic matrix in the basic parity check matrix and maintain the invertible structure characteristic in the meantime.Finally,the check matrix is extended by the cyclic matrix.Finally,the basic principles of CO-OFDM system are studied by analyzing the optical fiber communication technology and the basic principles of OFDM system,and then apply OIS-QC-LDPC code to the CO-OFDM system.The simulation results show that the OIS-QC-LDPC code based coding algorithm proposed in this paper has more reliable communication performance than IS-QC-LDPC code and QC-LDPC code with the IEEE 802.16 e standard.The simulation results show that the OIS-QC-LDPC code performs better reliability than the other two codes in different code length,modulation mode and distance.For one thing,under the same conditions of QPSK modulation,when the code length is 30720,the OIS-QC-LDPC code is increased about 0.7d B OSNR compared with the IS-QC-LDPC code,and rather than IEEE 802.16 e about 1 d B when the bit error rate is210-.For another,under the same conditions of 16 QAM modulation,OIS-QC-LDPC code is much lower than 1.5d B to improve the IS-QC-LDPC of OSNR when the bit error rate is210-,rather than the IEEE 802.16 e QC-LDPC code.With the change of the speed of the curve,the gap will be bigger and bigger.Furthermore,the OIS-QC-LDPC code can be transmitted at a distance of about 70 km,while the other two codes transmission conditions are only the length of 40 km when the bit error rate is37 10-?.At the same time,this paper also verifies that the algorithm reduces the number of short cycles.When the code length is 30720 or 15360,the improved algorithm has less girth with 4-length and 6-length compared with the original algorithm,and to a certain extent,the number of the 8-length is smaller than before.