Based On Ldpc Code For Ofdm Spread Spectrum Multiple Access Research

With the requirement of wireless data and multimedia business,the purpose of the Beyond 3G and 4G mobile communication system turns to be achieving omnipresent,high-caliber,and high-speed data transmission for mobile multimedia.The system requires support to the high-speed transmission in the high carrier and bandwidth,and with the rate in the range from 2Mbit/s to several megabits per second.And the question how to ensure the reliability of the data communication becomes very important,so we need overcome many difficult problems to achieve this goal,The problems mainly exhibit to be multi-path propagation in the wireless fading channel,signal fading caused by the scattering and Inter-Symbol Interference caused by the long multi-path time delay.The technique of Orthogonal Frenquency Division Multiplexing divides the channel into many orthogonal sub-channels,so the data can be transmitted parallel in the sub-channel in order to achieve high-speed transmission.In order to prevent the transmission error,many techniques for correction error have application in the OFDM system,such as convolutional code,Reed-Solomon code,Turbo code,and so on..Efficiency and reliability are two permanent problems in communication systems. Channel coding is an effective way to improve system reliability,LDPC codes are a class of block codes with near Shannon limit performance that can be realized,the performance of LDPC codes with long code length can be even better than Turbo code.For efficiency,link adaptation is a well-known technology to enhance the spectral efficiency of communication system and has been widely applied in wireless communication.It's meaningful to investigate the link adaptive system based on LDPC codes which can make full use of spectral resource while maintaining an acceptable transmission quality.First of all,we had a thorough research on the construction and theoretical analysis of LDPC codes,including the construction of parity-check matrix,external information transter theory and LDPC coded modulation etc,based on the above,in order to improve the efficiency and reliability at the same time,we combined LDPC codes with link adaption technique.The key modulating technology for B3G and 4G mobile communication system will be OFDM technology, and the LDPC-COFDM system constructed by LDPC code applied in OFDM proves to have wide application prospect. In this paper,Iestablished a simulating model for LDPC-COFDM communication system and did some research of LDPC-COFDM system in several aspects : 1) The influence to the system performance adapted with different code length. 2) The influence to the system performance adapted with different code rate.3) The influence to the system performance adapted with different construction.4) The influence to the system performance with different modulation method.And I get some conlusions from the simulation results:the BER performance of the LDPC-COFDM system will be excellent if we choose code lenth,code rate,code length,modulation type and interweave technology reasonably,and it is spectially suitable to extend to utilize in the future mobile communication system.