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The paper investigates the benefits of employing a Variable Data Rate (VDR) scheme in optical Low Earth Orbit (LEO) Direct-to-Earth (DTE) links at system level. In contrast to the Constant Data Rate (CDR) transmission approach adopted in current systems of the kind, the VDR scheme allows to optimize the data rate of the optical link within a satellite pass. Indeed, both the link budget and the atmospheric effects heavily depend on the satellite elevation angle, which varies significantly within a satellite pass. In order to thoroughly carry out the system level comparison between VDR and CDR, the paper first addresses all necessary aspects involved, including channel modeling, waveform and interleaver dimensioning, and link budget issues. For the scenario evaluated in the paper, the average throughput improvement of VDR versus CDR is found to be 100%. The paper also addresses interesting trade-offs concerning possible concepts of operations (ConOps) of the link once the VDR is in place. This preliminary work will be followed up by the Agency with further industrialized technology developments and system refinements.
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