Ofdm Mobile Communication Systems In Time And Frequency Synchronization Of New Technologies

Orthogonal Frequency Division Multiplexing (OFDM), which has gained much attention with its ability to reduce system complexity, is regarded as one of the most potential candidates for the future mobile communication. But some key problems still need to be optimized. It is one of the main disadvantages in OFDM system that signal is very sensitive to frequency offset; even tiny offset makes serious inter-carrier interference. So frequency synchronization is one of the key techniques in OFDM system. Further more, punctual time synchronization is also essential to capture and track the start position of received OFDM signal. Base on traditional techniques, the research target in this paper is to research on time and frequency synchronization in OFDM system and propose some new techniques for the sake of solving the problems.Firstly, this dissertation reviews the history of the development of mobile communication and the key techniques in the next mobile communication generation, shows OFDM is the core technique. Moreover, this chapter points out synchronization is essential to OFDM.Secondly, this dissertation explains synchronization in OFDM system, proposes the concept of time and frequency synchronization to show the importance.For the B3G project of UESTC, this dissertation proposes a scheme of time synchronization for OFDM system. This method makes improvement for the threshold of time synchronization by combining shaped filter and the character of hardware implementation for signal up-sampling. The improvement can reduce the probability of false and omit and enhance the performance.Meanwhile, a fast time synchronization technique with low complexity is proposed by combining the auto-correlation and cross-correlation, which can reduce the complexity of the system while remaining the same performance with traditional method.A blind frequency synchronization technique with high performance is proposed. With this technique, the receiver can estimate the frequency offset and complement it from the received data without any pre-information send by the transmitter, and the residual offset can be corrected effectively to enhance the performance of the system.Meanwhile, a new method is proposed based on pre-correction of frequency offset at the transmitter so that there is no need for frequency synchronization at the receiver, which can reduce the complex of the implementation for receiver in the down-link. Computer simulation shows this method can make the same performance of traditional method.Finally, some conclusion to the whole dissertation is given, and further research issues and possible research directions are pointed out.