Research On Synchronization Algorithms In Broadband Wireless OFDM Systems

The next generation mobile communications systems is aiming at achieving high quality and data rate transmission in finite wireless resource. Orthogonal Frequency Division Multiplexing (OFDM) is a promising technology for its robustness to selective fading and high spectral efficiency. Multi-Input Multi-Output (MIMO) technology can be adopted to achieve high throughput. So OFDM and MIMO have now become key technologies for the next generation mobile communication systems. However OFDM and MIMO-OFDM systems are very sensitive to time and frequency synchronization errors, especially the frequency synchronization errors. Correct time and precise frequency synchronization are necessary for OFDM and MIMO-OFDM systems.In this thesis, synchronization theories and algorithms for OFDM and MIMO-OFDM systems are deeply analyzed and researched. Several new synchronization algorithms are proposed and compared with key conventional algorithms. The major achievements are outlined as follows:Firstly, the influence of synchronization errors to OFDM system performance is analyzed. The joint influences of symbol timing offset and carrier frequency offset on OFDM demodulated signals and mean signal to interference energy ratio in the receiver are investigated comprehensively and confirmed by computer simulations, that provides theoretical foundations for synchronization algorithm designs in the following chapters. Compared with previous analysis, the joint influence analyzed here is more common.Secondly the research on the data-aided synchronization algorithms has been analyzed. An integrated synchronization solution is proposed to avoid the timing metric plateau inherent in the conventional data-aided synchronization methods which causes large variance of the timing estimate. A new preamble is designed in the proposed algorithm and timing instant is obtained by correlating the received signals with the known training sequence which has the best correlation properties. Meanwhile, with the help of the integral frequency offset's influence upon the training sequence relativity, the integral frequency offset can be obtained in time domain without FFT transform, that will reduce the computation complexity for the synchronization in OFDM systems. Furthermore the proposed algorithm can obtain the first path of the arrival signal. The valid of the new algorithm is verified in low SNR and multipath fading channel.Thirdly the cyclic prefix (CP) based synchronization algorithms have been investigated. After the analysis of the bad performance of CP based algorithms in multipath fading channel, an improved synchronization algorithms based on CP is proposed. The proposed method uses the ensemble correlation in CP and available pilot symbols used for channel estimation to construct new timing metric and the demands on pilot symbols design are investigated based on the analysis of the ensemble correlation. Compared with conventional CP based synchronization algorithms, the proposed algorithm is robustness to multipath interference and the time for estimating timing is more shoter.Finally the synchronization problem in MIMO-OFDM systems is investigated. A new synchronization algorithm is proposed in localized MIMO channel environment including estimation of symbol timing, integral frequency offset and fractional frequency offset. The new synchronization is achieved using shift-orthogonal training sequence which is simultaneously transmitted from all transmit antennas in the same OFDM time instant. Compared with conventional algorithms, the analysis method gives out better detection properties in terms of higher correct timing detection probability and accurate fractional frequency synchronization even in multipath fading channel.