Research On Synchronization For The Uplink Of Multi-User OFDM Systems

Time and frequency synchronization are more important and more difficult to OFDMA system than to OFDM system. In OFDMA system, each user has its own time offset and CFO (carrier-frequency offset), so the synchronization has became a multiple-parameter estimation problem, and the compensation of time offset and CFO has also became more difficult.First in this thesis, basic principle about OFDM is introduced, and a generalized OFDMA is described considering arbitrary subcarrier-assignment. Then the algorithms of time and frequency synchronization based on sub-band subcarrier-assignment are researched, but the interleaved subcarrier-assignment scheme is preferred. The subcarriers are overlapping, but orthogonal to each other such that there is no ICI (intercarrier interference). Carrier frequency offsets between the transmitter and the receiver destroy the orthogonality and introduce ICI. An estimation algorithm based on the signal structure is described for estimation the CFOs of all users using only one OFDMA block. Diversity scheme are also presented to improve the estimation performance. Simulation results illustrate the high accuracy and efficiency of this algorithm.With the estimation results, we must compensate the synchronization errors. The traditional method is feeding back the estimation results to the transmitter with downlink channel, but the performance is not good enough over time-varying radio channels, so we should implement it at the receiver. The algorithms of compensation of synchronization errors at the receiver are derived first for OFDMA system based on sub-band subcarrier-assignment. For the interleaved subcarrier-assignment scheme, a model is formulated to characterize the interference on the generalized OFDMA uplink due to frequency synchronization errors. Least squares and minimum mean square error criteria are used to construct the orthogonal spectral signals from one OFMDA block...