Design Of Rate-Compatible LDPC Codes Based On Protograph

With the development of wireless communication,people's demand for high-speed,high-performance and highly reliable communication makes the research of channel coding technology moving forward to the goal of approaching Shannon limit,low complexity,easy implementation and link adaptation.The generation and development of low density parity check(LDPC)codes motivate the research toward to this goal a step closer.LDPC codes can not only provide near-capacity performance,but also has the characters of flexible structure and low decoding complexity,which makes LDPC codes has been selected as 5G long code encoding scheme in 3GPP(3rd Generation Partnership Project).Protograph LDPC(P-LDPC)codes is a kind of LDPC codes with good performance.Compared with other LDPC codes,protograph LDPC codes has the advantages of strong expansibility,simple structure,easy hardware implementation,fast decoding and so on.Rate compatible LDPC(RC-LDPC)coding technology can make the communication system dynamically adapt to the time-varying channel to improve the transmission performance of information,and enhances the utilization of power and spectrum.In this thesis,we focus on the study of the construction of rate compatible protograph LDPC codes.The main work is as follows:First,the basic theories of LDPC codes and protograph LDPC codes are introduced.Then extrinsic information transfer(EXIT)analysis theory for LDPC codes and PEXIT(Protograph-based EXIT)analysis theory for protograph LDPC codes are studied.Secondly,the variable node puncture algorithm and the protograph extension algorithm based on PEXIT are studied,which are adopted to design rate-compatible LDPC protographs.The variable node puncture algorithm realizes the transformation of low code rate protograph to high code rate protographs,while the protograph extension algorithm realizes the transformation of high code rate protograph to low code rate protographs.In this way,a middle code rate protograph is chosen as the mother code's protograph,and rate compatible LDPC protographs can be constructed by the two algorithms.The core of the two algorithms is to select the optimal decoding threshold corresponding to each rate by PEXIT,so as to design rate compatible LDPC protographs with excellent performance by a mother code protograph.According to the analysis of existing multiple-edge removal algorithm of protographs,we find that it is easy to cause 4-cycles in the single side protograph.Furthermore,an algorithm that can avoid the obvious 4-cycles generated by removing heavy edges is proposed.Finally,two kinds of extension algorithms are adopted for constructing certain code length protograph LDPC codes from the single side protograph.One is random interleaved extension algorithm,and the other is cyclic shift extension algorithm.We can construct protograph LDPC codes with girth=8 through 4-cycles and 6-cycles detection algorithm by random interleaved extension algorithm.Then we find that the QC-PEG like algorithm with the cyclic extension block as the search unit can not detect the cycles that exist in a graph composed of multiple overlapped search paths.Furthermore,the conditions that can cause undetectable 8-cycles are presented.After that,protograph LDPC codes' check matrixs are constructed by removing the undetectable 8-cycles as much as possible.In the end,based on the proposed algorithm,protograph LDPC codes are designed with information bit length 4096 and 8192,and code rate varying from 1/3 to 5/6.Simulation results show that the codes designed by the cyclic shift extension algorithm can achieve the consistent performance of the codes designed by the random expansion algorithm.