Study On Some Key Issues Of OFDM Wireless Communication System

Transfer rate and frequency spectrum utilization are important indicators to measure the performance of communications system. With the development of mobile communications technology, people's need of high speed and high quality multimedia services is daily increasing. How to carry out effective transmission of multimedia services, which is mainly image and video, under limited radio frequency resources and time variant wireless channels, has been a research hotspot of the academia. Because of its good anti-multipath performance and high data transfer rate, Orthogonal Frequency Division Multiplexing (OFDM) multi-carrier modulation technology has been widely used in recent yeas and it is considered as the key technology of next generation mobile communications system. OFDM modulation technology has been adopted by DVB-T(Digital Video Broadcasting-Terrestrial) of Europe, ISDB-T(Intrgrated Services Ditital Broadcasting-Terrestrial) of Japan, China digital audio broadcasting-terrestrial system, China terrestrial digital television transmission standards and the China mobile television standards.Compared with single-carrier communications system, OFDM is sensitive to both synchronization error and channel estimation error. In view of the questions mentioned above, this paper mainly offers the study on synchronization and channel estimation. At present, OFDM system synchronization mainly has two modes. The first one is frequency domain and pilot frequency synchronization mode, which is based on cyclic prefix. The other one is Time Domain Synchronous OFDM (TDS-OFDM) , which is based on pseudonoise (PN) sequence. TDS-OFDM has been used in China terrestrial digital television transmission standards. TDS-OFDM is the focus of the paper.First, this paper presents a method of frequency offset estimation for OFDM System. Previous methods of frequency offset estimation have drawbacks like insufficient estimated accuracy or narrow estimated Range. It's too difficult to give consideration to both of them. However, this new method can make large frequency offset estimation quickly, and the estimated accuracy is close to Cramer-Rao bound.Then, a channel estimation method based on MPSK sequence is provided in this thesis, and it's compared with the channel estimation method based on PN sequence. In AWGN channel and multi-path channel, this method has improved much better than that in PN sequence channel estimation and balanced effect.Finally, we study the channel equalization algorithm for differential modulation OFDM system. When differential modulation is adopted, channel estimation is not needed because the channel information is included in the difference of adjacent symbols. But'channel estimation is not needed'is on the basis of following hypothesis: channels vary slowly, and changes of gap of a frame of data vary a little. When channels vary tremendously, channel information of adjacent symbols is destroyed and now channel estimation and balance is vitally necessary. In this thesis, a method of channel equalization algorithm based on the Terrestrial Mobile Multimedia Broadcasting (T-MMB) is offered, and the simulation results indicate that this method has a favorable equalization effect.