Research On Synchronization And Adaptive Link Technology In OFDM Systems

Modern wireless communication systems are not limited to conversation services. It integrates multimedia communication services, such as video, picture, data and conversation, etc. Broadband, grouping, integration and personality are the development directions of the future wireless communication systems, which demand larger buffer, higher speed and better performance. While wicked wireless environments and scarce frequency resources are challenges to modern communication research and application. OFDM (Orthogonal Frequency Division Multiplexing) matches the modern communication demands for the high frequency efficiency and effective resistance to multipath interference. It is considered as one of the core technologies of the 4th Generation wireless communication systems.However, many technologies must be improved in OFDM systems. The paramount important technologies of them are synchronization and adaptive link. Studied the synchronization error seriously, we found that carrier frequency synchronization is the most important in all OFDM synchronization technologies. This paper devoted to research carrier synchronization and designed a carrier frequency synchronization algorithm. Adaptive link technologies are one of the power methods to improve system performance, which become the hot research points in OFDM systems recently. We studied the hybrid ARQ and dynamic bit allocation and gained much progress.We summed up the synchronization algorithms in three kinds: data-aided algorithms, blind estimation algorithms and half-blind estimation algorithms based on Cyclic Prefix. Carrier frequency synchronization can be divided into two processes: acquisition and tracking. For the acquisition, we analyzed the classical algorithms, pointed the deficiencies and presented the new algorithm, which set the appropriate pilot frame, peeled off the influence of channel and timing errors, and then captured the carrier frequency error. It possessed the characteristics of robustness, efficiency and small complexity.For the tracking process, we described some classical algorithms, such as maximum likelihood estimation algorithms based on CP, symmetry conjugate prefix algorithms and so on. We also analyzed the disadvantages and virtues, and presented a decision directed tracking algorithm. The presented algorithm adjusted the carrier frequency in closed loop using the similarity between the demodulation data and the transmitted data. The simulation results demonstrated that the presented algorithm was belonged to blind estimation algorithms, which possessed high precision and simple operation. Considering the unite estimation about synchronization error and channel impulse response worked better than separating estimation, we presented a unite maximum likelihood estimation algorithm. It is too complex to calculate the two-dimensions equation. So we translated it into two one-dimension equations and approached to the true value by the iteration. It reduced the operation of the system with little performance damage.