Study On Frequency Synchronization Technique For OFDM Systems

The future wireless communication systems are expected to provide more communication services, higher quality and higher transmission rate with limited spectrum resources. Currently, the transmission rate 2Mbit/s proposed in 3G can not satisfy the subscribers. The 3G technique is on the way of evolution toward B3G or 4G. OFDM is a multicarrier modulation technique, which has been regarded as one of the key techniques in physical layer for the next generation of wireless mobile communications.OFDM systems are suitable for high rate data transmission and robust against frequency selective fading, and have high spectrum efficiency. The inter-symbol interference can be easily eliminated. However, there are still some problems need to be solved in OFDM systems. Among these problems, frequency synchronization is the most important and difficult, for OFDM systems are extremely sensitive to the carrier frequency offset that destroys the orthogonality of the sub-carriers. There are many algorithms proposed in the literature. The estimation performance of the algorithms based on the cyclic prefix is not agreeable due to the impact of multipath. Some algorithms utilize special training sequence in frequency domain or time domain. Algorithms in frequency domain just only can estimate the integer offset. Algorithms in time domain can not give acceptable acquisition range and the estimate accuracy simultaneously. Because of inserting the training sequence, the data transmission rate decreases. In wireless mobile communication systems, the channel is often time-varying. Generally, some pilot symbols are inserted for channel estimation. These pilot symbols not only reflect the channel information but also carry the information of frequency offset. Therefore, pilot symbols can be used to estimate frequency offset. The algorithms based on pilot symbols can achieve agreeable tracking performance and do not largely decrease the data transmission rate.In this thesis, we study synchronization technology of OFDM systems achieved by many people in the world. In addition, there are some further studies about frequency synchronization and several improved algorithms for synchronization of OFDM systems are proposed. The simulation results show that the improved algorithms have better frequency tracking performance. Firstly, the wireless communication channel is introduced. The key techniques of OFDM systems are briefly described after illustrating basic principles and characteristics of OFDM systems. Then, the orthogonality of inter-carrier and the cyclic prefix of OFDM systems are presented. Secondly, the influence of synchronization errors on OFDM systems is showed after describing synchronization requirements of OFDM systems. Afterwards, synchronization methods are summarized. Finally, a new algorithm based on phase correction for frequency tracking is proposed after studying maximum likelihood criteria for frequency tracking. Then, two improved algorithms are proposed after summarizing several estimation algorithms based on pilot symbols. One is weighted algorithm which depends on different amplitudes of received signals have different effects on the estimation. Moreover, the weighted coefficient analyzed is SNR. Another is joint algorithm that combines the large range algorithm with the high accuracy algorithm. OFDM systems simulation platform is built in MATLAB and the proposed algorithms are simulated. The simulation results illustrate that the proposed algorithms in this thesis have been advanced in different aspects and they can improve the tracking performance of the system.