Efficient Coding Scheme Of LT Codes And Its Application In Satellite System

The digital fountain code is a type of rateless code,that is,the length of the codes generated from the original data packet is infinite.These codes can be generated online,their application is not limited by channel changes,and the receiver can recover the original information without retransmission as long as it receives enough codes.Luby Transform(LT)code is the first practical fountain code.This paper studies efficient coding design and decoding of LT code for performance improvement,and its application scheme.The main research results include the following three parts:Firstly,this paper proposes an improved coding scheme based on Poisson Robust Soliton Distribution(PRSD),which improves the performance of Bit Error Rate(BER)of LT coded system.In particular,the unreliability of the low-degree variable nodes has been reduced by averaging the degree of the variable nodes,which results in a considerable performance improvement with soft iterative decoding.Simulation results demonstrated that,not only over Additive White Gaussian Noise(AWGN)but also Rayleigh fading channel,compared with the traditional coding scheme,the proposed scheme could improve both the decoding efficiency and the symbol recovery probability regardless of the degree distribution.Secondly,this paper presents a novel encoding method for the LT code.The proposed method assigns different priorities on selecting neighbouring source nodes of the encoding nodes instead of random selection,when the degree of the encoding node,d-1 the encoder is forced to select a variable node with the highest degree.Wheras d>1,the encoder minimizes the number of low-degree variable nodes and may invoke an even accumulation of the reliability values across the variable nodes,while increases the number of variable nodes in lower categories,and afterwards the ones with the highest category is selected with the highest priority The simulation results demonstrated that compared with the traditional LT encoding method which minimizes the number of low-degree variable nodes,the proposed encoding method can produce fewer low-degree variable nodes regardless of the degree distribution function,thus improving its overall BER performance with soft iterative decoding.Finally,a Hybrid Automatic Repeat reQuest(HARQ)scheme based on Medium Access Control(MAC)layer using LT code and Physical(PHY)layer using Low-Density Parity Check(LDPC)code is proposed.By using LDPC codes defined in DVB-S2 and DVB-S2X standards.the above scheme is applied to satellite communication systems.In addition,An effective cross-layer decoding scheme is proposed,which improves the overall decoding performance by feeding back the accurate soft information of LT codes to LDPC codes.Once the cross-layer joint decoding fails,the decoding is activated by collecting more received symbols.The simulation results investigated in this paper reveal that the proposed cross-layer joint decoding method can improve the decoding performance compared with the traditional tandem decoding scheme.