## 2 System Architecture and Assumptions  
Driven by already conducted experimental demonstrations, our analysis will be focused on the Discrete Variable (DV)-QKD systems. DV-QKD implementations seem to be technologically ahead since key exchange has been experimentally demonstrated to perform much more efficiently, providing key rates that are orders of magnitude higher compared to entanglement-based key exchange [37]. We selected for our analysis the prepare-and-measure (one-way) scheme and the BB84 protocol with decoy and vacuum states (vacuum, decoy, and signal). DS-BB84 QKD is the best understood and most widely studied scheme and protocol, and it also has the advantage that complete general security proofs are available, which have also been widely scrutinized for correctness [46]. Moving on to the system architecture, satellites across all the operational orbits, i.e., Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Geostationary Earth Orbit (GEO) will be examined. We will investigate their possible limitations, but also their potential use and advantages. Satellites in LEO can be considered a prominent choice due to the reduced geometrical losses that occur due to beam diffraction when compared to the other two orbits, as well as the increased market interest and the everyday increasing number of LEO satellites that are launched. On the other hand, satellites in MEO may be an attractive alternative, especially for locations close to the equator as they offer a larger visibility time window per satellite, thus resulting in a need for shorter satellite constellations to achieve a continuous communication (with at least one satellite at a time in contrast to the LEO case). The penalty for this case is the increased free-space losses in contrast with LEO, but also the resulting low elevation angles for locations far from the equator. Finally, GEO satellites can offer continuous communication employing a single satellite. However, the high altitude results in much higher losses, making the establishment of the quantum link challenging. At this point, it is important to note that our analysis is limited to the downlink scenario to avoid the challenges associated with lossy and turbulent uplink transmission [47; 48]. For satellite QKD systems, there are two principal wavelength regimes that are considered as possible candidates, namely wavelengths in the low end of the near-infrared (NIR) band, i.e., 810-850 nm, and wavelengths near the low end of the short-wave infrared (SWIR) band, i.e., C-telecom band (1530-1565 nm). Comparing the choice of 850 nm versus the telecom C-band for QKD channel in satellite-to-ground links presents a nuanced trade-off between atmospheric conditions, signal-to-noise (S/N) ratio, and technological compatibility. Operating at an 850 nm wavelength can significantly enhance the S/N probabilities and qubit-transmission rates for satellite-to-ground communications. This advantage stems from a combination of higher photon energy, reduced field-of-view (FOV) requirements, and better aperture coupling efficiency when compared to longer wavelengths. MODTRAN simulations suggest that local sky radiances are considerably higher at 850 nm; yet, the noise photon count remains manageable due to the higher photon energy and smaller FOV. The atmospheric transmission at this wavelength is also quite high, reaching about 90% of that observed at 1550 nm. Importantly, the near-visible wavelength of 850 nm permits the use of efficient, compact, and cost-effective silicon-based detectors. On the other hand, the telecom C-band around 1550 nm is recognized for its slightly higher atmospheric transmission efficiency and significantly lower solar noise interference, which are attributed to the inherent properties of Rayleigh scattering and the relative weakness of sunlight intensity at this wavelength. The choice of 1550 nm also benefits from a substantial reduction in background noise, making it a quieter channel for QKD.