Research On The Cascade Of LDPC Codes And Fountain Codes

As a relatively new channel coding technology,digital fountain codes become one of the research hotspots in the field of communication in recent years,because that it not only owns no fixed code rate and consistent characteristics,but also has the advantages of no feedback,low coding and decoding complexity and resuming interrupted transfer.Low Density Parity Check(LDPC)codes is a kind of linear block code based on sparse parity check matrix.LDPC codes have strong ability of checking and correcting errors,and its performance can approximate Shannon limit.Its decoding complexity is low and hardware implementation is simple.LDPC codes has been applied in the fourth generation mobile communication(4G),and now recognized as the long code coding scheme in the fifth generation mobile communication(5G).In order to resist the interference of the channel in the course of information transmission and ensure the reliability of the communication,especially for some special environment,such as deep space communications,thunder and the situation that has large transmission delay with poor link quality,this paper proposes a cascade coding scheme for fountain codes and LDPC codes,and discusses its performance and superiority.This paper firstly introduces the theoretical knowledge of digital fountain codes and LDPC codes.Then the two practical fountain codes-LT codes and Raptor codes are studied in detail,which involves the influence of the degree distribution on the decoding performance of fountain codes,the performance and advantages and disadvantages of several commonly used decoding algorithms.Moreover,encoding and decoding algorithms of LDPC codes are also simulated and compared.Based on the deep research of LDPC codes and fountain codes,a cascade scheme is proposed.In order to evaluate the cascade scheme,two special channel models are established besides AWGN channel,one is that deep fading randomly occurs for some time,the other is that several short-term signal interruption randomly occurs in a period of time.The non-erasure channel model is equivalent to the erasure channel through inner code deletion mechanism,which is suitable for the application of fountain codes.We simulate the performance of the Raptor-LDPC concatenated codes under different channel models,and try to improve the performance by changing the code length and rate of the inner code or joining the interleaver.In addition,we compare the performance of fountain-LDPC cascade scheme with other coding schemes.The results show that Raptor code,as an improved LT code,has advantages in encoding and decoding complexity and the decoding success probability,so it is more suitable as the outer code of the cascaded code.Besides,the cascaded coding scheme based on the Raptor and LDPC codes has good performance in the AWGN channel and the constructed channels,and its coding gain has greatly improved comparing with the LDPC code's,which provides a solution for the reliable transmission of information in some certain channel environments.Finally,the concrete implementation of Raptor-LDPC codes on DSP is describled,which involves fixed-point quantization,matrix storage,matrix operation and so on.In addition,we discuss the effect of different quantization schemes and quantization series on the decoding performance of Raptor-LDPC codes in depth.It is found that choosing an appropriate quantization scheme can keep the loss caused by quantization very low and reduce the impact on the performance of concatenated codes.Through theoretical research and practical simulation,it can be found that the concatenated scheme which is based on LDPC codes and fountain codes not only has excellent performance,but also has low coding and decoding complexity.What's more important is that the cascade codes changes the traditional automatic feedback retransmission(ARQ)mechanism,greatly shortening the file transmission delay,while ensuring the success rate of decoding.It can be used in deep space communication,deep sea remote underwater acoustic communication and other environments,which can effectively deal with the situation of channel time variation,long transmission delay,high bit error rate and links easily interrupted.