Jungyu Park

While long-distance fiber QKD suffers from transmission loss, satellite-to-ground quantum key distribution (Sat-QKD) can extend key distribution through free-space links. However, to make such keys usable in terrestrial QKD networks, a satellite link must be connected to ground fiber infrastructure through wavelength and encoding interfaces. This work studies a simulation framework for Sat-QKD as a key-supply layer for fiber-based QKD networks. The framework focuses on downlink decoy-state BB84 and combines LEO pass geometry, candidate OGS atmospheric data, receiver/detector models, QBER estimation, and finite-key analysis. After satellite-to-OGS free-space collection, an OGS-side optical-conversion interface is modeled before ground fiber extension. The simulator compares direct backhaul, 850-to-1550 nm quantum frequency conversion (QFC), polarization to time-bin degree-of-freedom (DOF) conversion, and the combined QFC + DOF path. The results show that the preferred wavelength depends on the link section: 850 nm provides higher annual finite-key volume in the satellite free-space link, while 1550 nm is favorable for the fiber-extension layer. QFC connects the free-space-favorable 850 nm link to the fiber-favorable 1550 nm layer, and DOF conversion introduces conversion loss but suppresses polarization-drift-induced QBER. The framework is intended as an engineering-level tool for comparing candidate OGS sites and optical-conversion paths for satellite–fiber QKD network extension.