## 2 System Architecture and Assumptions
Moreover, the compatibility of 1550 nm with existing fiber-optical communication infrastructure makes it an appealing choice as it facilitates seamless integration between satellite-based and terrestrial quantum communication networks. Ultimately, the decision between these wavelengths should consider site-specific conditions, including atmospheric scattering and turbulence, as well as the technological ecosystem surrounding the quantum communication infrastructure. While 850 nm offers superior S/N ratios and allows for the use of more accessible detection technologies, 1550 nm provides a more robust channel and easier integration with existing communication systems. The optimal wavelength for a specific satellite-to-Earth QKD link would thus depend on a balance of these factors, where the aim is to maximize both the efficiency and the practicality of the quantum communication channel. Although, up to now, the majority of the satellite QKD links operating in the regime of 850 nm are motivated mainly by the responsivity of silicon-based Single-Photon Avalanche Diodes (SPADs) in this regime and the presence of fewer geometrical losses (almost four-times greater) [49], our analysis will be focused on the 1550 nm regime. The employment of telecom wavelengths provides the benefit of a direct coupling into an SMF, thus enabling the detection of the quantum signal beyond the telescope to a remote location, as well as the capability of using advanced commercial off-the-shelf optical fiber components for the realization of the filtering, classical and quantum signal separation, and polarization encoding analysis. In addition, according to MODTRAN, the transmission efficiency is slightly higher at 1550 nm than at 850 nm, and the background noise, which is present mainly due to sunlight/mononlight, is reduced. In this direction, the feasibility and requirements for distributing the collected space photons via terrestrial telecom fibers is also being studied with respect to the integration of space QKD segments with the terrestrial ground fiber-based QCI segments. Figure 1 provides a schematic overview of the envisioned architecture, where OGSs can be located outside from the terrestrial grid, thus allowing one to take advantage of the high-altitude locations, where there is less artificial background noise, and the effect of turbulence is also reduced. In this way, photons can be transferred from the OGS to the urban environment via advanced low-loss telescope-to-fiber coupling techniques, thus removing the need for the detection of the collected photons in the OGSs and an intermediate trusted node. In the following section, the assumptions underpinning our study are presented. To streamline our simulation process and ensure a focused analysis, we utilized the specific technological and operational parameters defined by the Micius mission as our baseline scenario [50]. By leveraging the established specifications of the Micius mission as a benchmark, we adhere to proven parameters, which can enhance the relevance and applicability of the findings provided by our simulation. For example, we kept, throughout our simulation process, the aperture emitter constant to 30 cm, so unless stated otherwise, these will be the baseline parameters used in our simulation.