Hardware emulation of satellite-to-ground APD-based FSO downlink affected by atmospheric turbulence-induced fading

By enabling Free Space Optics (FSO) technology as complementary solution to RF systems, the next generation satellite communication that relies on optical links is on the verge. Even though the transition to wireless optical communication is a fact, the space domain is very conservative to such critical changes that call for close evaluation of each system aspect. Since trade-off between costs and efficiency is required, a state-of-the-art laboratory testbed for verification of satellite-to-ground APD-based (Avalanche Photodiode) FSO links subject to atmospheric turbulence-induced fading is proposed in the current paper. In particular, the self-developed hardware channel emulator represents an FSO channel by means of fiber-coupled Variable Optical Attenuator (VOA) controlled by driver board and software. Having addressed real atmospheric Radiosonde Observation (RAOB) databases for Vienna, Austria, highly precise optical attenuation data due to atmospheric turbulence fading are generated and applied into the considered software. The used approach relies on complex analysis simulating atmospheric vertical profile of refractive index structure parameter as well as Gamma-Gamma and Log-Normal scintillation models considering both parameters the telescope aperture and the elevation angle. Along with the FSO channel emulator, the receiver under-test is high-speed 10 Gbps APD photodetector with integrated Transimpedance Amplifier (TIA) that is typically installed in future OGSs (Optical Ground Stations) for LEO/GEO satellite communication. Having considered On-Off Keying (OOK) Intensity Modulation/Direct Detection architecture, the emulated optical downlink is evaluated based on two different data throughputs while atmospheric turbulence induced-scintillations are also taken into account. The overall testbed performance is addressed by a BER tester and a digital oscilloscope, providing high-quality BER graphs and eye diagrams that prove the applied approach for testing APD-TIA in the presence of scintillations. Furthermore, the accuracy of the hardware channel emulator is evaluated by means of calibration measurements as well as beam camera providing measured proof of the propagated high-quality laser beam.