Performance Analysis of Variable Data Rate System for Free Space Optical Low Earth orbit to Ground Scenario

Abstract

The Free Space Optical LEO downlink scenario will enable an enormous increase in data throughput from Earth observation and communication satellites, through its vastly higher data rates. Further advantages of directed optical links is secure key transfer for QKD satellite link, inter-satellite link for data relay, secure communication through minimized signal spread, and more. However, the LEO downlink channel is challenging due to elevation dependent free space loss and atmospheric effects such as absorption and scintillation. Various techniques like adaptive optics, aperture averaging, use of complex coding and very long interleaver techniques, are being used and studied to mitigate such effects. However, the implementation of such complex and expensive solutions for a constant-data-rate system (CDR) is often an additional overhead for the variable channel, as it has to be designed for the worst channel condition, which is around the lowest elevation angles. Therefore, we propose the variable-data-rate (VDR) system as a solution for such a varying channel, where the data rate can be adapted according to the channel conditions dependent on elevation. In this paper, a LEO-to-ground scenario is selected for detailed analysis and a VDR system is designed whose performance is compared with the CDR system for which the optimum data rate and start elevation angle are selected by maximizing the overall throughput. Some literature reports that the VDR system has a factor of 3 advantage over the optimal CDR system, and in this paper, we evaluate this situation in a more practical approach using the STK simulations and Matlab evaluations.