Synchronization Techniques For Multi-user OFDM Systems

Nowadays, people have higher demand on the quality and the bandwidth for mobile communication than ever. The technique of OFDM has been widely recognized for its high spectrum efficiency and advantages in dealing with frequency selective fading. Moreover, OFDM is highly flexible, as it can not only be combined with the traditional accessing schemes to form multi-user OFDM system such as OFDM-TDMA and OFDM-CDMA, but also take advantage of the its orthogonality between the subcarries to realize uplink multiple access, the so called OFDMA system.Multi-user OFDM system is sensitive to the three kinds of synchronization error just as the general OFDM system, that is carrier frequency offset, symbol timing error and sampling frequency offset. What’s more, such errors may produce ICI and ISI as well as MAI, which may decrease the system performance further. So, multi-user OFDM system requires more on the synchronization techniques.Supported by the national key 863 project of China– research and development of the TDD mode new-generation mobile communication system (B3G TDD), the dissertation has focused on the synchronization problems in multi-user OFDM system. It includes the impact of synchronization error on the system, conclusion and summarization of the mechanism and algorithms suitable for multi-user OFDM system.Firstly, after a brief introduction of the principle and mathematical model of OFDM system, the concept, system model of multi-user OFDM system and comparison among the different features under different type of access was given.Then, the impact of carrier frequency offset, symbol timing error and sampling frequency offset on the system was analyzed in detail. It was focused on the downlink channel. Concerning the uplink ones, the theoretical analysis of MAI was emphasized.Subsequently, systematic conclusion was given to the typical synchronization algorithms for OFDM system put forward at home and abroad. The principles, rules and features were clarified. As for the multi-user system, these algorithms can be applied directly to the downlink channel, but for uplink, the separation of the parameters of different users and the synchronization mechanism are more important.Finally, a synchronization scheme for B3G TDD multi-user communication was put forward. The synchronization was implemented in the synchronization slot, whether in downlink or in uplink. The downlink synchronization contains coarse timing, fine timing, integral carrier frequency offset synchronization, fractional offset and sampling frequency offset estimation with compensation. While designing the uplink synchronization scheme, the mechanism was set up in the first step. In the period of accessing, users use downlink synchronization to made their synchronization error under control and in data transferring, the base station uses uplink synchronization to do the timing track. Also, an algorithm for the uplink timing error track was proposed by means of the methodology used in channel estimation for MIMO-OFDM. The effectiveness of the synchronization scheme was proved by data analysis and matlab simulation.