| One of the requirements of next generation of wireless mobile communication system is to provide the transmission of the multimedia services with higher speed and wider bandwidth. In the wireless environment, the frequency resources, power and wireless fading channel etc, limit higher data transmission. In theses years, OFDM (Orthogonal Frequency Division Multiplexing) technique has been paid more and more attention because it has strong transmission ability in wireless channels, which comes from its own parallel transmission mechanism, inherited robustness to Inter Symbol Interference (ISI) and the diversity with the multi-path channel. Meanwhile, OFDM system has very high channel utilization ratio because of the orthogonal properties among its sub carriers.While the OFDM technique also has some shortcomings such as being sensitive to the time synchronization error and the frequency offset. In detail, at first, in the wireless environment, the receiver has no knowledge about the time delay between the transmitter and itself in general situation. In the OFDM system, the OFDM data is transmitted as one symbol, so in order to do the correct demodulation, the correct start of symbol should be got from the received signals to reduce the effect of ISI. Secondly, the high Frequency utilization and the robust transmission of OFDM are founded on the orthogonal properties of the sub carriers, so if the frequency offset is generated and the orthogonal properties among sub carriers is lost, that will cause the Inter Carrier Interference (ICI) and high BER of system. As a result, in the OFDM system, how to synchronize in a quick and accurate manner is quite an important issue, which affects the performance of the whole system. So the corresponding time and frequency synchronization algorithms should be proposed and realized in the OFDM receiver design. This paper studies the synchronization technique in the OFDM system, which is to do efficient estimation and compensation of time and frequency offset to enhance the system performance.This thesis deeply studies the synchronization algorithm of the OFDM, Simultaneously combine its application in the European Digital TV broadcasting standard for terrestrial (DVB-T). the main contributions of this thesis are as follows:1. Analyzing basic principles of OFDM system and summarizes three kinds of synchronization: symbol synchronization, carrier synchronization and sampling rate synchronization. Besides, analyze and compare existent synchronization algorithms.2. Because the Maximum likelihood algorithm based on cyclic prefixes only can acquires timing offsets and fractional frequency offsets, We can acquire fractional and integral frequency offsets when joint ML algorithm and pilots. By analyses in simulation, it proves that the proved algorithm certainly extend the range of estimation of frequency offset and resolve integral estimation of frequency offset.3. Research on the OFDM synchronization technology in IEEE802.16a criterion. Expound its synchronization project, using Schmidl&Cox algorithm in short preamble and long preamble for obtaining symbol timing and frequency offset estimation. Finally make performance simulation in the synchronization algorithm.4. Expound the systemic frame of DVB-T criterion, frame of OFDM, and pilot insertion, then make simulation in process of modulation and demodulation under condition that ideal synchronization, finally emphasize on research of synchronization technology in DVB-T. |
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