Proposing a dynamic scheme of channel estimation for 802.16a OFDM system with the trade off between estimation performance and arithmetic complexity.In addition,we find a set of excellent irregular LDPC codes and examine the performance of them in multipath fading channel.Simulation shows that these codes have nice performance in SUI-3 and SUI-5 fading channels.Furthermore,we find that,under the multipath environments,the performance of LDPC codes is superior to that of the convolution turbo codes which is proposed by IEEE 802.16a standard.