| In wireless communications system, inter-symbol interference (ISI) which caused by the frequency selective fading mainly affects the capacity, transmission speed and correspondence quality of communications system, and the ISI will become more serious especially to high-speed transmission data traffic, for signal width of high speed information flow is shorter. The multi-carrier transmission technique can solve the ISI problem effectively. Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation method which has many advantages such as efficient bandwidth utilization, large transmission capacity, robustness against multi-path delay spread, easy to realize and so on, so the technology can adopt in multi-path environment for high-speed data transmission and has vast application prospect. But in the OFDM system, its carrier frequency offset and timing error sensitive degree are both higher than single carrier system and needing high synchronization precision. Therefore it's very complex to realize OFDM system and become one of difficulties in OFDM system realization.In this thesis, Firstly, the principium and key technologies of OFDM are described briefly, and compared OFDM and single carrier transmission, other multi-carrier technology. Secondly, the synchronization technique of OFDM system is mainly studied, the impact of synchronization errors to OFDM system are analyzed during describing the synchronization principle of OFDM system, then summarizing the synchronization technique and analyzing several kinds of information-aided classic synchronized algorithm and its improved in detail; OFDM system simulation platform is built by Matlab6.5, as well as some synchronization algorithm are simulated. The study shows that the synchronization methods based on circulation prefix does not need the extra auxiliary information, and its algorithm is simple, but its estimated scope is smaller and performance is influenced seriously by the prefix length; The synchronization methods which based on the training sequence has good estimated performance, but designing the fine training sequence is the key; The pilot-based synchronization methods adopt the pilot of channel estimation adequately, but how to effectively use the pilot is difficult.At last, applied the locomotive depot wireless local area network (WLAN) the OFDM, the synchronous system of locomotive depot WLAN based on OFDM is designed, and simulation indicated the performance of group detection, timing and frequency synchronization is validity and accuracy. |
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