## 5 Results and Discussions  
The described channel emulator provides the benefit of introducing atmospheric turbulence effect into an FSO system used hereby for an assessment of a high-end PD whose performance is crucial for next-generation FSO feeder links. In particular, the architecture of the built prototype used for emulation of satellite-to-ground FSO downlinks in controlled laboratory conditions consists of an optical transmitter, SMF-based optical channel emulator, a few fixed optical attenuators, receiver implementing high-end APD-TIA, oscilloscope, and BERT. Its block scheme is provided in Figure 10(a), while the real experiment is depicted in Figure 10(b). The transmitter is a compact-size 1.25 Gbps SFP module with built-in Distributed Feedback (DFB) laser that emits a high-quality optical beam at 1550 nm and 3.5 dBm  
Figure 11: An experimental setup consisting of (**a**) testbed architecture emulating satellite-to-ground FSO link and (**b**) photograph from the real experiment. Figure 10: Experimental setup used for testing the APD-TIA performance at a maximum data rate of 10 Gbps that is based on transmitter with external MZM. average optical power. Furthermore, the extinction ratio of DFB is at a minimum of 9 dB, but lowering the data rates to 1 Gbps and decreasing the driving current lead to 50% better performance, and \(\sim\)14 dB is achieved, which is still comparable with the real MOPA-based architecture providing 20-25 dB. In order to maintain the \(P_{\text{th}}=-5.5\) dBm of the used APD-TIA, three fixed 3 dB Optical Attenuators (OA) are deployed, which, together with the 1 dB internal loss of VOA, leads to a maximum of \(-6.5\) dBm at the receiver. The SFP is plugged into the SFP2 SMA2 evaluation board that both together play the role of a transparent interface translating the incoming binary data into the optical domain based on Non-Return to Zero (NRZ) OOK intensity modulation. In the next stage, atmospheric-turbulence-induced fading is introduced by using the FSO channel emulator as well as the RAOB-based simulated attenuation data for Vienna, Austria (Figure 5). The simulations are accomplished at 100 Hz fading frequency (response time) that is set up in the developed attenuator GUI (Figure 2). Finally, the received signal is processed using a maximum likelihood receiver with hard decision decoding. The performed BER measurements are carried out with Agilent N4906B BER testers (BERT). In addition, the used pulse pattern of BERT for generating the Pseudo-Random Binary Sequence (PRBS) at 1 Gbps that internally modulates the SFP DFB laser is \(2^{31}-1\). In order to have a comparison when APD-TIA also operates at the maximum possible data rate, its BER at 10 Gbps is measured considering the introduced design discussed in Section 4 that uses 10 Gbps MZM. Finally, the output of the tested APD-TIA at 1 Gbps is analyzed with 33 GHz digital oscilloscope DSAV334A Agilent Keysight having 80 GS/s maximum sample rate. The measured and the approximated BER performance based on the introduced experimental setup applying Agilent N4906B BERT (Figure 11) is provided in Figure 12. Both 1 Gbps and 10 Gbps of a slant FSO propagation channel are evaluated with and without atmospheric turbulence. In addition to the accomplished measurements with the introduced MZM-based prototype board in Section 4, an ideal BER curve at 10 Gbps, which is provided in the FBR2178 GB datasheet, is given in Figure 12a. When both curves are compared, there is up to 1.7 dB offset between them, which is explained by the higher quality MZM in the datasheet that is characterized with a minimum 25 dB extinction ratio [20]. Considering the observed power penalties at BER = \(10^{-8}\), when the data rate is decreased from 10 Gbps to 1 Gbps, the appeared offset of 2.2-3 [dB] is due to improved sensitivity of the APD-TIA, once the information bandwidth is reduced. Furthermore, the approximated measured BER curve subject to the introduced tropospheric scintillations (Figure 5) at 1 Gbps throughput is given in Figure 12b.