| With the development of communication techniques, the rapidly increasing of people demanding, communication capacity demand more requirements. Orthogonal frequency-division multiplexing (OFDM) is a promising technology for its high spectral efficiency and robustness to selective fading. As multiple inputs multiple outputs (MIMO) technology can be adopted to increase system capacity, data rate and transmission quality. Thus, B3G broadband mobile communication systems based on OFDM and MIMO technologies become research hot point now. The efficient and accurate synchronization, especially the carrier frequency synchronization, are critical for MIMO-OFDM system. In this dissertation, research work mainly focus on theory and technique of carrier frequency synchronization in MIMO-OFDM systems with diversities.At first, this dissertation has discussed the characteristics of mobile wireless channel, reviewed the basic theory of OFDM and MIMO techniques and the advantage of MIMO-OFDM combined. Then summarized a variety of diversity techniques in wireless communication, analyzed the development trends of diversity techniques, introducted the diversity synchronization idea. On this basis, summed up frame synchronization, signal synchronization, carrier synchronization and sample synchronization in MIMO-OFDM systems, emphasized the model of OFDM transmission systems and synchronization performance under carrier frequency offset (CFO) condition.Based on the existed research upon SISO-OFDM and MIMO-OFDM, carrier synchronization mainly focus on how to design preamble traning symbols and structure to distinguish various antennas. However, a MIMO system offers spatial diversity, which results from the fact that multiple independently faded copies of each transmitted signal are present at a multi-antenna receiver. This spatial diversity can be exploited to improve the overall signal quality at receiver, resulting in a better performance of transmission systems. The benefits of spatial diversity for the quality of data transmission is well-known, the aims of the research are to examine similar effects on carrier frequency synchronization, which means diversity synchronization. In chapter 5, we first analyze the model of MIMO systems, then designed a feasible preamble structure which contains only one OFDM symbol. Both coarse and fine synchronization are all finished in the one OFDM symbol. Starting from a performance analysis of SISO synchronization, SIMO, MISO and MIMO synchronization schemes are proposed and analyzed step by step. Simulations for the preamble-based synchronization scheme exploiting spatial diversities show that synchronization performance becomes significantly more reliable compared to a system where only single antenna is used within synchronization process. The largest share of the diversity gain is achieved by exploiting receive diversity; exploitation of transmit diversity leads to moderate further improvements. In high signal noise ratio (SNR) condition, severe flooring effect will appear. |
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