Study On Synchronization Algorithms In OFDM Systems

The combination of MIMO and OFDM techniques is regarded as one of the best transmission methods in the future wireless broadband communication. It not only solves the problems of multi-path fading channel and bandwidth efficiency well, but also enlarges the system capacity and achieves highly reliable transmission. Timing synchronization and frequency synchronization are the key effects which influence the performance of OFDM system. This paper introduces the joint timing-frequency synchronization algorithms in SISO/MIMO-OFDM systems thoroughly and comprehensively,and focuses on the training symbol aided synchronization algorithms in OFDM systems.Its main contents of the dissertation are expressed as follows:Firstly, the influences of time-offset and frequency-offset are analyzed. In addition, simulations are made to get an obvious understanding of influences.Secondly, two typical joint timing-frequency synchronization algorithms, the maximum likelihood estimation (MLE) method based on cyclic prefix (CP) and Schmidl's algorithm based on training sequence, are studied separately. Through simulation, the performance and disadvantages of the algorithms are acquired.Thirdly, based on special symmetric pilot sequence and known frequency synchronization methods, an improved joint timing-frequency synchronization method is presented here. Simulation results show that the performance is greatly improved comparing with Schmidl's algorithm in AWGN channel and the algorithm is also available in multi-path channel. In practical applications, the inherent CP and the MLE method also can be used to get a better estimation.Finally, aiming at the centralized MIMO-OFDM system, a new training sequence structure is given. Each transmit antenna's training sequence is created in the frequency domain and Zadoff_Chu sequence is used as sub-carrier of the frequency domain, which makes the training sequence have favorable orthogonal performance in both time and frequency domain. The simulations indicate that the given synchronization scheme in this paper has low algorithm complexity and high estimate precision.