Research On Synchronization Algorithms For High Order QAM Demodulation

Compared to the conventional low order modulation methods, high order QAM modulation is widely used in the digital wide band system due to the high utilization efficiency of the system bandwidth. However, with the increasing of the modulation order, the Euclidean distance between the adjacent symbol becomes smaller, which will reduce the anti-interference ability of the system and increase the difficulty of the synchronization at the same time. Therefore, the research on corresponding synchronization technology has become more and more popular. In particular, the accuracy of the measurement in the vector signal analysis system is determined directly by the accuracy of the synchronization block, which means that the traditional synchronization algorithms with the characteristics of small capture range, slow convergence rate and large residual error have a limited application in this situation. As a result, it is very important to realize the large scale acquisition and high precision synchronization of the high order QAM modulation in the vector signal analysis system.In this paper, in order to establish a large scale acquisition and high precision synchronization scheme for the vector signal analysis system with the square QAM constellation range from 16 QAM to 1024 QAM, the key timing synchronization algorithms and carrier synchronization algorithms with open loop structure and closed loop structure are deeply studied and improved. At the theoretical level, the equivalent baseband mathematical model of the digital modulation and demodulation is firstly established to lay the foundation for the following research. Secondly, to satisfy the requirement of the accuracy and the stability of the high order QAM system in timing synchronization, the performance of the existing open loop nonlinear timing estimation algorithms, the closed loop Gardner timing recovery algorithm and the polynomial interpolation filter are studied. By analyzing the self-noise arising from the bandlimited system, a timing jitter reduction method based on the prefilter design is proposed to improve the accuracy of the both timing estimation algorithms above. Thirdly, to satisfy the requirement of the accuracy and the acquisition range of the high order QAM system in carri er synchronization, this paper presents a multistage closed loop carrier synchronization correction structure based on DFT+PFDPLL+DDPLL and a multistage open loop carrier synchronization correction structure based on DFT+ PFD+DD+Viterbi phase estimation algorithm. The simulation results show that the joint multistage carrier synchronization schemes can improve the accuracy of the open loop frequency offset estimation algorithms and expand the estimation range of the system at the same time.Finally, in the practical application level, this paper designs a complete squre 16QAM~1024QAM synchronization schem e for vector signal analysis system based on the research above, where the final bottom test and top interface design is achieved in the CVI platform using C language. The theoretical analysis and the actual signal test show that the residual error introduced by the proposed synchronization scheme can be ignored. Compared to the well-known Keysight 89600 VSA software, our scheme has a similar accurancy performance but performs a larger measurement range of frequency offset under the same conditions. As a result, the proposed synchronization scheme can achieve high precision and large capture range of synchronization.