Research On Synchronization And Equaliazation Of Shortwave OFDM System

In HF communication, the dominant obstacle to further increase the data rate of transmission is the channel distortion caused by multipath spread and Doppler shift. Orthogonal Frequency-Division Multiplexing (OFDM) has been under intensive research as an effective way against multipath fading and has become a hot topic of research in the field.One of OFDM's drawbacks is its sensitivity to the errors of symbol and carrier synchronization, which damages the orthogonality among its sub-carriers, thus causes ICI and severe deterioration in demodulation performance. Because of selective fading caused by mutipath and other factor, OFDM need equalization to compensate the channel characteristic so that the performance loss of demodulation could be minimized. This thesis is devoted to a study on synchronization and equalization of OFDM, which are the critical technologies in the field, while laying emphasis on its application in HF.A new frequency offset estimation algorithm is present based on auto-correlation matrix, where diagonality criterion is used and fractional frequency offset tracking is implemented by minimizing the modular sum of off-diagonal elements while integral frequency offset is estimated exactly based on the position movements of diagonal elements. Based on above algorithm a new method of symbol timing is designed and implemented, the main idea of which is to minimize the modular sum of diagonal elements corresponding to virtual carriers.In the aspect of channel equalization, several types of blind channel estimation algorithms are discussed first. In view of the slow varying of HF channel and peculiar structure of 39-tone OFDM system, a semi-blind channel estimation algorithm named IFFT-Filtering and Correlation-Detection Method is proposed, where iterative processing is introduced which simplifies the computation and improves the estimation performance.Finally, a complete demodulation scheme targeting at a segment of practical intercepted 39-tone OFDM signal is described and discussed in detail including detection,synchronization and channel equalization. Test results show that the algorithms proposed and discussed in this thesis are feasible and effective.