Frequency Synchronization And Adaptive Techniques In OFDM Systems

As the development tendency of future communications techniques, the two key facets of OFDM are researched. The mathematics model of OFDM with frequency offset is analyzed. The main research works and results are listed as follows.1. Based on the contrast to the optimum and three suboptimum adaptive transmission schemes, a simply single user adaptive transmission method is proposed. By using Lagrange formula and conic fitting, it converge optimal point quickly.2. Combined with signal structure of OFDM and modern signal spectrum analysis, three effectively estimation methods of OFDM frequency offset are discussed. Firstly we give the iterative MUSIC method. The choice of iterative sequence and complex coefficient of iterative MUSIC method are given. Relative to the tradition MUISC, it has the character of low computational complexity, quickly convergence and little performance loss. Then the TLS-ESPRIT method is introduced. It is an effectively method for estimation of OFDM frequency offset by using the character of shift unchangeably of two signal sets. Then we discussed the way of increasing distance of two signal sets and generalize the STLS-ESPRIT. It has low estimation errors, especially in multipath channels. Finally we gave the modified Prony method. It overcomes the disadvantage of original Prony methods, which has low ability to resist noise. For the complexity of modified Prony, we pointed the Newton iterative method should converge to the optimum value quickly and has low computational complexity. We proved the consistent of three proposed methods and ML. The estimation error of three methods is analyzed and the performance of three methods is compared with CRLB. We draw the conclusion that TLS-ESPRIT has the best performance in three methods. In the last of chapter, the limitation of above methods is discussed. For ignore channel zero points, the estimation performance should degraded rapidly if certain channel condition is satisfied. The relation of channel zero points and null subcarriers are derived and the theoretic foundation of selection of null subcarriers is given. We deducted the modified subspace algorithm- flexible ESPRIT. It constructs the forward and backward matrix using non-consecutive data. If two matrixes have same arrangement, the multi-zero-points of estimation would avoid. Finally the dynamic block is derived, which overcame the affection of variation of multipath channels.3. Based on the SUMT algorithm, the optimal way to solve the adaptive resources assignment of the multi-user OFDM is pointed. The using of two phases of circulating iterative method decreases the computational complexity effectively. According the study on the sub-optimal schemes, a new sub-optimal resources allocation scheme with fair is proposed. Then the integer plan technique is proposed in the resource allocation under the restrained of minimum rates. Due to the discrete values of power, it can be solved effectively by computer. Finally a scheme of data and voice transmission simultaneity is studied. The scheme arranges the parameters according the QoS. It can be used in mixed data transmission.