The Application Of The LDPC Code In OFDM System

LDPC code is a class of linear block codes which can be defined by the sparse check matrix.It was first discovered by Gallager in 1962,so it is also called Gallager code. Mackey had rediscovered it and proved its ability of approaching Shannon Limit based on Belief-Propagation decoding algorithm.The application of OFDM (Orthogonal Frequency Division Multiple) began in 60 decade 20 century, mainly on the military purpose. In 70 decade, the Discrete Furrier Transform was used in the modulation in multicarrier, which make the OFDM easier to realize. With the development of DSP (digital signal process) and high-speed component, OFDM has been successfully applied in ADSL,VDSL,DVB,DAB,HDTV. From the 90 decade, the research of OFDM reaches the area of wide band transmission in wireless channel.It is advisable to apply the channel code into the OFDM system to decrease the BER (Bit Error Rate). The RS code and convolutional code have been applied. The Turbo code has also been put into the OFDM system as a tentative choice. It shows a great amelioration in the performance of OFDM system after the channel coding. LDPC (low-density parity-check) code was proposed by Gallager in 1962. It has been neglected for a long time. Until 1993,when the turbo code was proposed by the Frenchman Berrou, the LDPC code reappear in the list. In 1996,Mackay has made a great contribution to the research of LDPC code which made a new period for LDPC code. so apply the LDPC code into the LDPC-COFDM system will have promising application.The paper research the system of the LDPC code in to the OFDM,the main content include that:1) Use the BP algorithm,the iterations number influence the system and the rule of the interation number change. 2) The performance contrast of the LDPC code applied into COFDM system and the Turbo code applied into COFDM system.Although long LDPC codes are superior to Turbo codes, it is not the case for short LDPC codes. So the modification of decoding algorithm is essential to the performance improvements for LDPC-COFDM system.