The Research Of Carrier Frequency Offset Estimation Algorithms In OFDM System

Orthogonal Frequency Division Multiplexing (OFDM) technology, a multi-carrier transmission technology, has become one of the key technologies of 4G, which resolves wireless communications spectral efficiency and multipath fading problems. Generally, frequency selective fading channel in an OFDM system can be converted into a number of flat sub-channels, reducing the degree of influence of multipath; The compact subcarrier spacing structure in OFDM systems makes the spectral efficiency be greatly enhanced; OFDM technology can be combined with many other technologies, such as the MIMO technique, the combination enhances the transmission speed and the reliability. But the compact structure in OFDM system requires the orthogonality between subcarriers more stringent. Doppler effect and the presence of a frequency offset of system itself destroy the orthogonality of subcarriers. And the non-orthogonality leads to the inter-carrier interfere (ICI), which degrades the system transmission performance. Therefore, this thesis researches the carrier frequency offset (CFO) estimation algorithms and tries to find better CFO estimation algorithm.Firstly, the wireless channel features, the basic principle of OFDM technology, the reasons for CFO and the influences of CFO have been systemically analyzed in this thesis. Secondly, the existing CFO estimation algorithms have been researched, the merits and drawbacks of each algorithm have been summarized, and MATLAB simulation analyses as well as theoretical analyses have been done. Thirdly, based on the analyses of the existing algorithms, the traditional CFO estimation algorithm based on cyclic prefix (CP) and the CFO estimation algorithm based on one cyclic suffix (CS) structure become the focus. And a feature that shortening the time delay between the data which are used to calculate the phase difference can expand the estimation range and reduce estimation accuracy is found. According to this feature, a new guard interval (GI) structure which is called CP+CS structure is provided. Simulation results show that the CFO estimation algorithm based on CP+CS structure not only has nice estimation accuracy and range but also will be better than the traditional CFO estimation algorithm based on CP when the GI is longer. Fourthly, since the CFO estimation algorithm based on CP+CS structure needs more than one OFDM symbols when it is used to estimation CFO, the thesis provides another new GI structure which is called 2CP structure. And simulation results show that the CFO estimation algorithm based on 2CP structure has the same performance as the CFO estimation algorithm based on CP+CS structure has. But the former one can estimate CFO in only one OFDM symbol. Finally, referring to the prioritization schemes for traditional CFO estimation algorithm based on CP to mitigate the’slow-down’phenomenon of the algorithm’s mean square error (MSE) curve with signal-to-noise rate (SNR) in multi-path channel, those schemes are also been studied for the CFO estimation algorithm based on CS/CP+CS. The simulation results show that those schemes are more useful for the CFO estimation algorithm based on CS/CP+CS.