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Abstract
Orthogonal Frequency Division Multiplexing (OFDM) has become a popular technique for high bit rate wireless systems. High spectral efficiency and robustness to frequency selective fading channels are two major features of this technology. However, it is very sensitive to synchronization errors, especially frequency and timing errors. Timing errors lead to inter symbol interference and inter channel interference which degrades the performance of the systems. Therefore, timing synchronization plays a major role in the physical layer design of OFDM systems.
In this thesis, first the effect of both timing and frequency synchronization errors on OFDM systems is studied. Then the timing synchronization in downlink Mobile WiMAX as an example ofOFDM systems is investigated. Timing synchronization in Mobile WiMAX downlink is realized in two steps, initial timing synchronization and fine timing synchronization. Initial timing synchronization is performed once at the beginning of a connection between Base Station and Mobile Station to detect the presence of frames and to give a coarse estimation to the start of the OFDM symbols in the frame. On the other hand, fine timing synchronization is applied every frame to estimate fine timing errors caused by the mobility or drifts between the transmitter and receiver sampling clocks.
In this thesis, we proposed new efficient methods for both the initial and fine timing synchronization. Although the new methods are applied and evaluated for the Mobile WiMAX, they can be used for other OFDM systems. All the new methods are depending on frequency domain processing, in contrast to the existing algorithms which depend on time domain algorithms that use computationally intensive correlation algorithms. We also studied the interfering effects from other nearby Mobile Stations on initial timing synchronization and the interfering effects from other nearby Base Stations on fine timing synchronization. The new methods are compared to the existing methods in different channel conditions. Simulation results show that our methods are robust and efficient.