Key Technologies For High-speed Data Transmission And Demodulation

Modem is one of the key technologies in transmission system with high-speed code rate. Considering the practical requirement of satellite communication system and that QPSK system has the advantages of mature technology, good frequency band and the power utility, moderate algorithm complexity and strong anti-interference, the paper mainly discusses QPSK modem system, the coherent demodulated project and the parallel processing algorithm.Carrier recovery and timing synchronization are two key technologies of coherent demodulation. This paper combines the frequency track and phase track method, uses the digital COSTAS loop to realize the carrier recovery. In the case of Doppler serious shift frequency, this paper compares the MLE algorithm and the interpolation algorithm, and then realizes carrier synchronization using the phase track and frequency capture. As for timing synchronization, it proposes an improved GARDNER timing synchronization algorithm .This algorithm calculates time error signal directly from the sampled value and then gets the optimal decision value through the interpolation algorithm, so every symbol can carry on synchronization adjustment .This algorithm has simple principle and fast capture of the time error and easy realization with software. Finally, this paper simulates the above discussed algorithms on MATLAB and System View platform. The simulation results prove that the carrier recovery algorithm has many advantages, such as low BER and strong anti-interference performance.