## 6 Conclusions  
The provided atmospheric FSO link emulator based on VOA significantly profits from the simulated atmospheric-turbulence-induced fading using well-known RAOB databases. Merging together the simulated scintillation data and the developed hardware enables facilitated high-end testing of ground-space FSO links. This breadboard offers not only complete control over the system parameterization but also substantial advantages in view of the extreme costs of a real test mission. Consequently, the equipment is highly beneficial for performance tests of photodetectors that have been under significant scrutiny in the last years. In other words, the device under test is a high-end APD-TIA detector that is studied in typical continental atmospheric-turbulence-induced fading with a mean attenuation value of 3.43 dB and a standard deviation of 0.57 dB. The approach is demonstrated based on BERT and a high-resolution oscilloscope comparing the system performance for 1 Gbps and 10 Gbps. While the 1 Gbps scenario outperforms the 10 Gbps one in case of no turbulence, the BER curve at 10 Gbps without turbulence is featured with up to 2.03 dB better performance when turbulence at 1 Gbps is introduced. While this testbed configuration is utilized for satellite-to-ground downlinks, the modeling of uplinks that also involves the beam wandering effect due to turbulence will be evaluated in further work. Moreover, in order to perform a complete FSO channel modeling, pointing errors and background noise will also be taken into account.  
Figure 13: Performance of the testbed based on Eye diagrams for received optical power between \(-17\) dBm and \(-37\) dBm.  
**Author Contributions:** Conceptualization, H.I.; methodology, H.I., F.M. and E.L.; software, H.I.; validation, F.M. and E.L.; formal analysis, H.I. and F.M.; investigation, H.I.; resources, F.M. and E.L.; data curation, H.I.; writing--original draft preparation, H.I.; writing--review and editing, P.B., F.M. and E.L.; visualization, H.I.; supervision, F.M. and E.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was supported by EU COST Action CA19111: NEWFOCUS.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data available in a publicly accessible repository that does not issue DOIs. Publicly available datasets were analyzed in this study. This data can be found here: [[[https://www.raob.com/data_sources.php](https://www.raob.com/data_sources.php)]([https://www.raob.com/data_sources.php](https://www.raob.com/data_sources.php)); [[https://www.zamg.ac.at/cms/de/klima/produkte-und-services/data-und-statistiken/messdaten](https://www.zamg.ac.at/cms/de/klima/produkte-und-services/data-und-statistiken/messdaten)]([https://www.zamg.ac.at/cms/de/klima/produkte-und-services/data-und-statistiken/messdaten](https://www.zamg.ac.at/cms/de/klima/produkte-und-services/data-und-statistiken/messdaten)); [[https://ds.climate.copernicus.eu/#](https://ds.climate.copernicus.eu/#)]([https://ds.climate.copernicus.eu/#](https://ds.climate.copernicus.eu/#))!/search?text=ERA5&type=datasetin, accessed on 24 March 2022].  
**Acknowledgments:** This work is based upon work from COST Action CA19111 \"European Network on Future Generation Optical Wireless Communication Technologies (NEWOCUS)\", supported by COST (European Cooperation in Science and Technology).  
**Conflicts of Interest:** The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.