Study On Synchronization Technology Of OFDM System

Orthogonal frequency division multiplexing (OFDM) is multicarriers modulate technology. Because of robustness to interfere and fast frequency selective fading, OFDM modulation has been attracted considerable attention by researchers. OFDM has already been adopted for Digital Video Broadcasting (DVB) and Digital Audio Broadcasting (DAB), high-speed wireless local area networks (WLAN), including IEEE802.11a, HiperLAN/2 and MMAC. However, the OFDM technology has a fatal shortcoming that is very sensitive to the carrier frequency offset caused by misalignment between frequency oscillators because OFDM has a very narrow space between subcarriers for high spectral efficiency. So symbol timing and carriers frequency offset estimation for an OFDM signal is significantly important. The dissertation will study synchronization technique of OFDM system and a few simple and efficient synchronization projects are proposed.At first, history, application and development, properties and key technology of OFDM are described simply; the effect of symbol timing synchronization error, frequency synchronization error and simple synchronization error of an OFDM system is discussed in detail. The study show that symbol timing offset and carriers frequency offset would introduce inter-symbol interference (ISI), frequency offset of carriers would result in inter-carrier interference (ICI). So, in an OFDM system, finding the symbol timing and compensating the carrier frequency offset at the receiver is very important in the recovery of the signal.Second, timing and carrier frequency synchronization has been studied in this paper. Two OFDM timing algorithms are improved in this paper and the improved algorithms overcome the shortcomings of the phantom which result in ambiguity of timing estimation and multi-peak would occur in the shortcomings of T.Schmidl & D.Cox algorithmå’ŒH. Minn algorithm. Two improved algorithms have a peak and the second algorithm has a pulse. Because autocorrelation is used in algorithms, timing recovery of the proposed is robust to carrier frequency offset and multipath. The timing offset of frame and symbol can be estimated simultaneously in time domain, thus reducing the complexity of timing synchronization. The improved algorithms are simple and the overhead is small, only one training symbol is used, the low overhead and fast algorithms are very suit to synchronizing of OFDM system. In order to fast carrier frequency synchronization and improve the transmission efficiency, the training symbols which are used for timing recovery can be used to achieve frequency synchronization. Two training symbols based on frequency synchronization for OFDM transmission system are designed. The shortcoming of complexity of scanning algorithm and low precision of based power detecting algorithm would be avoided by use of the proposed algorithms. The repeating of reference symbol in training symbols extend the rang of frequency estimation. As the improved algorithm is executed in time domain, the carrier frequency acquisition can be fasted. Moreover unbiased estimation range is extend. Estimation range of the first improved algorithm is same as T.Schmidl & D.Cox algorithm's, but the improved algorithm has high precision. Estimation range of the second improved algorithm is more. In addition, As the training symbols are also used in symbol timing, the overhead is small and the algorithms overcome the shortcoming of reducing transmission efficiency caused of conventional method which use different training symbol for timing and frequency synchronization.At last, IEEE802.11a standard is introduced. According the data group structure in the standard, we analyze the character of training sequences in preamble and present a kind of new algorithm. Analytical and simulation results show that the proposed timing algorithm can fulfill timing synchronization,the proposed frequency algorithm has a high precision.