If Digital Demodulator Carrier Synchronization Algorithm

Synchronization is a very important problem of intermediate-frequency(IF) digital modem. Performance of demodulation system mainly depends on the synchronization technology. Carrier synchronization, as an important branch of synchronization technology, is the foundation of coherent demodulation. This paper focuses on carrier synchronization algorithms of IF digital modem.Based on the theories of IF digital modem, the influence of carrier frequency offset to the performance of IF digital demodulation system is analyzed in the paper,with emphasis upon carrier frequency and phase synchronization algorithms. The principle and performance of cross product AFC and Costas tracking loop is analyzed and compared. Then, method of frequency-detecting aid of Costas loop is studied, and the loop based on Costas cross-over loop and Cross-Dot product AFC algorithm is Constructed.Among these carrier synchronization algorithms with open loop structure, FFT frequency offset estimation method, frequency offset estimation based on the signal's phase and maximum likelihood estimation algorithm are the focus of research. Performance of the algorithm is analyzed and compared, from estimation range, estimation mean and the estimation precision. A new maximum likelihood estimation algorithm is put forward. Simulation results show that this algorithm has large estimation range and high precision. Besides, a new scheme of carrier synchronization is proposed, combining frequency estimation based on FFT with Costas loop. This synchronization scheme has good acquisition and tracking performance, and can be easily implemented. Furthermore, carrier lock detector algorithms are analyzed.In addition, the carrier synchronization scheme combining frequency estimation based on FFT with Costas loop and self-normalizing carrier lock detection algorithm have been successfully implemented on FPGA.