The Study Of Synchronization Algorithm For OFDM System Based On IEEE802.11a Standard

Synchronization technology, as one of the key technologies in OFDM, has become the hot spot in recent years. The OFDM is more sensitive to the synchronization errors, because of multi-path delay and Doppler shift. The affect of synchronization errors to the OFDM receiver is systematically analyzed. Because the delay and auto-correlation timing synchronization algorithm has the drawback of low precision, so an improved algorithm, which is suit for IEEE802.11a standard, is presented. Finally, delay and auto-correlation algorithm and ESCA coarse frequency offset estimation algorithm, are studied. In order to expand the frequency capture range, an improved coarse frequency offset estimation algorithm is presented, which is based on the correlation of first half and second half of one cyclical short training symbol.The results show that: (1) Symbol timing error caused phase revolution only; Sampling error leads to not only amplitude decrease but also the inter-carrier interference (ICI); Carrier frequency error brings amplitude decrease and orthogonality destruction, which cause the ICI and the lose of SNR. So OFDM is most sensitive to the carrier frequency error. (2) The delay and auto-correlation timing synchronization algorithm can good complete the frame detection and symbol timing. But it can't determine the beginning of OFDM frame accurately, so bring bigger error. The improved algorithm makes the best use of the short training symbol. It can get one sharp correlation peak, and can improve the detection probability and precision of symbol timing. (3) The frequency synchronization based on 802.11a standard divide into coarse and fine frequency offset estimation. In the first stage, the delay and auto-correlation algorithm and ESCA algorithm bases on the identity of the cyclical short training symbols. Their frequency offset estimation range is less than±625KHz. The improved algorithm reduces the correlation window length, calculates the correlation inside one cyclical symbol. The algorithm not only has less calculation, but also expands the frequency offset range to±1.25MHz.During the frequency capture stage, it satisfies the requirement of low capture time and wide capture frequency range. Finally, with the help of fine frequency synchronization algorithm, the frequency precise is acquired.Although the improved timing synchronization algorithm brings more calculation, it has characters of exact symbol timing, simple in structure. The algorithm enhances the performance, it is suit for application. The frequency synchronization algorithm, combined with the improved coarse frequency offset estimation algorithm and fine estimation algorithm, can ensure the estimation precise, expand the estimation range, reduces the capture time. But it is more sensitive to the noise and multi-path delay. When anti-jamming ability is improved, the frequency synchronization algorithm can apply to OFDM synchronization module in IEEE802.11a.