The Application Of LDPC Codes In OFDM System

OFDM modulation technique has good performances in withstanding multi-path disturbing and high using of frequency bandwidth. So, it has been applied diffusely, such as ADSL, DVB, DAB, 802.11a, and so on. Nowadays, it becomes the disquisitive hotspot in wireless communications. LDPC has attracted lots of attentions of relational fields because of Mackay who researched newly LDPC which was presented firstly by Gallager in 1963 and rediscovered that LDPC was a good code which could approach the Shannon limit by Mackay in 1996. LDPC has more excellent performance than Turbo codes,convolve codes or others in many aspects.So LDPC codes combining with OFDM would have academic and practical significance.This paper pays its attentions on decoding algorithms of LDPC code, taking out a new LDPC decoding algorithm and applying it into WLAN system basing on OFDM-IEEE802.11a standard .Simulation validated that the new decoding algorithm is feasible and compared to convolutional code, LDPC–OFDM system has ascendant performance.The conventional probability field LDPC BP decoding algorithm is too complex, it has much floating-point calculation such as multiplication and exponential. So, the value of this algorithm is much unstable. Considering these problems, this paper brings forward a new decoding algorithm basing on logarithm field, Log-likelihood decoding algorithm. There is real number additive calculation nearly in this new algorithm which has good stability and the complexity fall down greatly. It is propitious to operate in hardware.Presently, large preferences referring to LDPC are basing on binary modulation, such as BPSK. But in future 3G+ mobile communications, the extensively adopted OFDM technique requires combining with multilevel modulation. So, How to combine the channel coding LDPC with the multi-modulation OFDM is our primary work. But, some parameters would be altered when it has passed through FFT operation in OFDM system. In this situation, above the new decoding algorithm must be ameliorated again. This paper improves above Log-likelihood decoding algorithm which bases on BPSK modulation, called LogBP algorithm. It realized spanning from LDPC-BPSK modulation to LDPC-multilevel modulation in OFDM system.Recently,the research and application of q-ary LDPC have come forth.Getting illumination from the foregoing papers,at the last of all work ,this paper put forward a new decoding algorithm basing on q-ary LDPC.The new algorithm has lower complexity than q-ary LDPC FFT decoding algorithm,and it is propitious to hardwares realization greatly.