Study Of Synchronization Algorithm In MIMO-OFDM Systems

The combination of MIMO and OFDM is a promising technology in the broadband wireless communication systems of the future. It can greatly increase the channel capacity of the wireless communication systems and the transmission rate, and is one of the main techniques for4G mobile communications. Synchronization is a critical factor to affect the performance of the MIMO-OFDM system. The thesis mainly researches on the timing synchronization algorithm and the frequency synchronization algorithm in the centralized MIMO-OFDM system and distributed MIMO-OFDM system. The primary contents of the thesis are as following:The principle of MIMO and OFDM systems is discussed, and the modulation and demodulation mechanism of the OFDM system is also studied. The principle of the MIMO-OFDM system is analyzed and its system model is given. The synchronization error models of the centralized MIMO-OFDM system and the distributed MIMO-OFDM system are investigated respectively and its affects on the OFDM and MIMO-OFDM systems are considered. The timing and frequency synchronization methods in the SISO-OFDM system are studied. Several classic synchronization methods in the SISO-OFDM system are discussed and their synchronization performance is also investigated.In the study of the blind synchronization algorithm in the centralized MIMO-OFDM systems, the application of the LS (Least Squares) algorithm in the centralized MIMO-OFDM system is investigated. For the low accuracy of LS algorithm, a new frequency synchronization method based on the MMSE (Minimum Mean Square Error) algorithm is proposed. The operation is carried out under the criterion of MMSE algorithm through using the received signal and the frequency error of the system can be obtained. The comparisons of the synchronization performance and complexity between MMSE algorithm and LS algorithm are made. The simulation results show that the MMSE algorithm can get better bit error rate and mean square error performance than LS algorithm in the same signal-to-noise ratio. The MMSE algorithm has higher complexity than the LS algorithm. Since the frequency offset of the system is related to the subcarrier interval, a tradeoff between performance and complexity of the MMSE algorithm can be achieved by choosing an appropriate subcarrier interval.For the synchronization algorithm in the distributed MIMO—OFDM system, the synchronization method based on training sequence is studied. The new timing and frequency synchronization ways based on the training sequence are put forward, and a set of new training sequence is also designed. Because the training sequences have ideal aperiodic autocorrelation and periodic autocorrelation properties, some time offset and single frequency offset can be estimated through implementing correlation operation of the training sequences and utilizing CP (cyclic prefix) information contained in the received signal. The simulation results indicate that the new proposed method has better performance in the bit error rate and mean square error.