Frequency Compensation Method Of Feeder Link Carrier Tracking In LEO Mobile Satellite Communication System

In LEO Mobile Satellite Communication System (LEO-MSS), as the satellite constellation moves rapidly relative to the ground, the propagation of radio wave between the satellite and the earth station employs the traits of large Doppler frequency shift and dynamic characteristics. The Doppler Effect will lead to the shift of center frequency of different communication links and also the shift of spreading code rate, all of which will deteriorate the demodulation performance in digital communication system. Therefore, research on the carrier tracking under high dynamic condition is necessary.Under high dynamic condition, the carrier frequency range will be far beyond the capture bandwidth of PLL used for carrier tracking, resulting in the inability of PLL, which requires the preprocessing on high dynamic signals before carrier tracking so that the high dynamic characteristics can be reduced or even eliminated. Current FLLs with large capture bandwidth have been widely used in actual system, in which high dynamic signals are preprocessed. This paper proposed the Frequency Compensation based Tracking Method on the basis of improvements to carrier tracking method targeted for LEO-MSS. The main idea of the method is to predict the variation of Doppler Shift during communication process with prior information of LEO satellite movement and then compensate for the impact of Doppler Effect. The compensation is open-loop and is very simple, efficient, low resource consumed, while at the same time achieving the reverse implementation of carrier compensation and eliminating the Doppler Shift on the feeder uplink, which effectively reduces the complexity of satellite receiver and user terminal.Based on the research of variation of Doppler Shift in LEO-MSS, this paper analyzed the impact of Doppler Effect on the carrier frequency of communication link and related performance of frequency spreading system, and proposed Frequency Compensation based Feeder Link Carrier Tracking Method, which is then realized in the prototype equipment of signal processing subsystem in Gateway.The Frequency Compensation based Feeder Link Carrier Tracking Method proposed in this paper can improve signal transmission quality of Feeder Link in LEO-MSS, which is of considerable value in engineering application.