OFDM is a modulation method and key technology of 4G. And because of its advantages such as the orthogonality of subcarriers and its capability to convert the high-speed Serial data to lower-speed Parallel data,OFDM has better performance in the frequency-selective channel than traditional modulation methods. However, it losses the advantage when there is large frequency offset caused mainly by the asynchronism or the high mobility between the transmitter and receiver. The latter factor is known as the doppler effect which may undermine the orthogonality of subcarriers. So many method are being investigated to avoid such an effect.