Development of Algorithm for Demodulator for Processing Satellite Data Communication

摘要

Quadrature Phase Shift Keying (QPSK) is the preferred modulation scheme for satellite data communication. A two stage estimation scheme for the demodulator for processing satellite data is proposed in this paper, where carrier frequency estimation is followed by timing recovery under training. Frequency offset estimation is performed by finding the slope of the best fit line through the unwrapped phase. Since this carrier frequency estimation technique gives an approximate estimate of the frequency offset, there still remains some frequency offset to be corrected. This is done in the second stage, where least mean square algorithm is employed to track the variations in frequency and estimating the symbols. Estimation of the correct sampling instant is based upon finding the minima of the absolute of the received training pattern and then calculating the maximum energy instant from it. An overview of the whole system architecture is presented and its performance is evaluated. BER curves for different frequency and timing offsets are also presented. Some tasks in the system may serve as bottlenecks to higher data rates owing to their computationally intensive nature. These tasks are identified and based upon that, the proposed algorithm is partitioned into two parts. One part is to be implemented on FPGA and consists of high computational complexity modules and the other is to be coded on a DSP processor.