Research On Synchronization Problems For Multiuser OFDM Systems

Multi-user Orthogonal Frequency Division Multiplexing (OFDM), which is presented by Orthogonal Frequency Division Multiple Access (OFDMA) and based on OFDM, will be one of the most promising candidates for next generation mobile com- munications. OFDMA inherits not only the advantages but also the disadvantages of OFDM, such as the sensitivity to synchronization errors, especially to the Carrier Frequency Offset (CFO). In OFDMA, CFO can destroy the orthogonality among all subcarriers and induce inter-carrier interference, which further results in the Multiuser Interferences (MUI), and eventually reduces the system performance. Timing Offsets (TO) that results from timing shift, and/or different path delay between multiple users, is another important reason to the self- interference and MUI.The main contents of the thesis are as follows:A review to the history of OFDM is first made, and then the principle of multiuser OFDM as well as the basic structures of transmitters and receivers is introduced, the mathematical model of OFDMA is given, and effects of CFO on multiuser OFDM system are investigated.Interference suppression algorithms in terms of CFO for the uplink of multiuser OFDM systems are studied in depth. The modification to interference suppression approaches is based on Parallel Interference Cancellation (PIC), and a better system performance is obtained by using the proposed Optimum Weights Parallel Inter- ference Cancellation (OWPIC).Different TO scenarios are also discussed. Especially, the detailed analysis of effects of TO on multiuser OFDM and impacts of the simultaneous TO / CFO on multiuser OFDM are addressed, respectively.