# _photonics_
## 4 Simulation Results
### QKD Link Performance of an LEO Satellite Constellation over Greek QCI
#### 4.3.3 Satellite Full Constellation Pass over Greek QCI  
The QKD link availability can be significantly enhanced by considering that each QKD satellite payload can communicate with a number of OGSs that are geographically distributed across the territory [18; 19]. Assuming that each LEO satellite is equipped with three independent QKD Alice stations, three independent QKD links can be established with the three OGSs simultaneously for a period when they are all visible and only if the weather conditions allow it. Therefore, based on the link study in the above paragraphs, these three QKD links can be efficiently established for less than 10 min each day. During the rest of the period, the links could be used for classical communication or for other functions (e.g., imaging). Figure 7 illustrates the downlink normalized SKR between the satellite constellation and the three OGSs across Greece.  
Figure 6: (**a**) Normalized SKR (bps/pulse) over time for a period of 1 month between the satellite constellation and the OGS of Helmos. (**b**) Normalized SKR (bps/pulse) over time for a period of 60 h between the satellite constellation and the OGS of Helmos.  
The differences in the maximum value of the normalized SKRs depicted in Figure 8a were estimated due to the different aperture diameters of the telescopes installed in the three OGSs. Larger aperture diameters lead to lower free-space loss and scintillation loss, and therefore, higher key rates can be distilled from the QKD links. In our study, Helmos OGS, equipped with the largest telescope in Greece with a diameter of 2.3 m, can offer SKRs up to \(3.9\times 10^{-4}\) (\(\mathrm{bps/pulse}\)), which is about four times higher SKRs compared to that offered by Skinakas OGS and more than eight times higher SKRs compared to that offered by Cholomondas OGS.  
In Table 1, the estimated total distilled key bits between each satellite and each ground station for a period of one year are presented, assuming nighttime communication and no link interruption due to clouds and a quantum signal repetition rate of 100 MHz [25; 26]. It is worth mentioning that despite the fast repetition rate at Alice station, the photon loss due to link attenuation is also high. This combination prevents the detectors of Bob station to be saturated due to their dead time, since only a limited number of photons go to reach OGSs' detectors. Higher repetition rates values have been also assumed in other publications [56],  
Figure 8: (a) Key exchange between the satellite and two optical ground stations (OGSs). The satellite establishes two QKD links for each OGS. (b) The satellite sends Key1 \(\oplus\) Key2 to OGS2 with classical data communication. OGS2 uses Key2 to acquire Key1.  
Figure 7: (a) Normalized SKR (\(\mathrm{bps/pulse}\)) over time for a random pass of one satellite over three OGSs. (b) Normalized SKR (\(\mathrm{bps/pulse}\)) over time for a period of two months between one satellite and three OGSs.  
but this comes at the cost of further reducing the detectors gate time opening, possibly resulting in high time jitter loss and difficulties in synchronization [59].  
As expected, the OGSs with bigger apertures are able to distill more key bits. We assumed that these key bits are stored in a memory and can be used to encrypt classical communication data when necessary. Appropriate key management is necessary to assure that the keys are properly stored and their cryptoperiod while they are either stored or in use does not exceed the standard limits set by the National Institute of Standards and Technology (NIST) [60].