Date of Award
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Dr. C. R. Carter
This thesis investigates the carrier synchronization problem due to Doppler shift encountered in low data rate, low earth orbit (LEO) satellite communication systems. In particular, two new techniques are developed. The first technique which is referred to as the novel technique, is based on the idea of applying a linearly decreasing frequency sweep to the received signal as a method to reduce the Doppler shift to tolerable levels, whereby conventional carrier synchronization may be employed. This method has shown to be better suited for satellites at larger cross track angles. However, for direct overhead satellite pass, improved Doppler compensation is achieved by utilizing the modified novel technique. Simulations have been carried out to explore the performance of the novel technique. Simulations have been carried out to explore the performance of the novel technique and results have shown that it is capable of reducing Doppler shift by at least a factor of two and performs better for satellites at large cross track angles. The second technique of Doppler compensation is based on Maximum Likelihood (ML) estimation of multiple parameters. This algorithm makes use of a model developed for the phase error introduced due to Doppler frequency shift and Doppler rate, and the two implementable solutions investigated are; (1) the grid search, and (2) the ML quadriphase shift keying (QPSK) Doppler compensator. These solutions have been realized by utilizing approximations for low signal to noise ratio (SNR) conditions. The grid search is a non tracking technique and may be used for coarse Doppler compensation. Unfortunately, simulation results have shown that it is computationally intensive. On another note, a block diagram consisting of a phase rotator, matched filtering and two feedback loops has been developed to represent the ML QPSK Doppler compensator. Three digital implementations of this structure have been analyzed, namely; steepest descent, optimized steepest descent and Newton Raphson method. The first two, have two modes of operation; conventional feedback and adaptive feedback. Simulations have shown that the adaptive optimized steepest descent has the best performance, that is, the shortest lockup time. Comparisons to previous work in this area, have shown that the proposed Doppler compensator has superior performance.
Ah-Thew, G. P., "Doppler Compensation for LEO Satellite Communication Systems" (1998). Open Access Dissertations and Theses. Paper 1060.