## 2 FSO Channel Emulator Representing the Impact of Atmospheric Impairments  
In order to emulate the wave-front distortions of an optical beam due to atmospheric fading, an experimental setup that consists of a hardware channel emulator and atmospheric turbulence and/or cloud data acquired based on simulations is utilized. In particular, turbulence fluctuations and/or cloud attenuation causing phase and irradiance random changes can be evaluated based on their influence over the collected instantaneous FSO power valid for IM/DD scheme [11; 22]. Consequently, the channel emulator is limited to a near-space FSO system having a direct detection technique combined with OOK or DPSK modulation, while the rarely used coherent detection scheme is not considered [17]. In some cases, the coherent detection systems are vulnerable to synchronization issues between the Local Oscillator and incoming signal caused by turbulence-induced phase fluctuations. Even though those disruptions due to synchronization are often internally compensated, they should be addressed by applying an additional MZM in the design. The developed hardware FSO fading emulator built with COTS components is given in Figure 1 (i.e., Figure 1a) while its controlled software is provided in Figure 2. Its performance aims to represent the broad spectrum of Mie scattering and atmospherics turbulence scenarios that are featured with 0.1 Hz-1 kHz fading bandwidth and optical attenuation less than 10 dB. In particular, the real dynamic changes of the received FSO optical power are translated here into a laboratory environment leveraging MEMs-based VOA at 1550 nm wavelength with SMF-28 (Single Mode Fiber) having MFD = 10.4 \(\upmu\)m. On the one hand, the transmitted fundamental transverse mode TEM\({}_{00}\) (Gaussian optical beam) in a typical FSO communication scenario is launched into an SMF with 80% efficiency supported as a linearly polarized LP\({}_{01}\) mode. On the other hand, the FSO architectures for near- and deep-space communication are especially based on SMF optical componentsthat provide exceptional performance. Furthermore, the obtained VOA operates within 0-60 [dB] dynamic attenuation range and has 0-1 kHz fading bandwidth (response time up 1 ms), covering a broad scope of atmospherically induced fading. Moreover, the implemented attenuator driver is utilized together with an STM32 Nucleo-F446 RE evaluation board, providing a broad spectrum of capabilities. It incorporates an STM32F4 microcontroller ARM Cortex M4 series, powered by 3.3 V. Furthermore,  
Figure 1: Atmospheric FSO channel emulator: (**a**) hardware interface and VOA; (**b**) measurements providing set attenuation vs. measured attenuation curve of the used VOA. Figure 2: Graphical User Interface (GUI) used for controlling the hardware channel emulator. the attenuator driver board implements external DAC, namely Texas Instruments DAC8571 characterized by 16-bit resolution, 10 \(\upmu\)s settling time, and \(\pm\)4 mV accuracy. Since the VOA has a low input impedance of 100 Ohm, a higher amount of output DAC8571 DAC power is achieved using an AD8532 ARM operation amplifier that plays the role of an impedance transformer. In order to evaluate the performance of the built hardware, set attenuation versus induced optical attenuation into an optical fiber is measured and compared with a reference line that represents the ideal case. The results in Figure 1b offer <0.16 dB attenuation accuracy within the most used 0-5 dB optical attenuation range in terms of turbulence fading. Furthermore, the control is performed with the developed GUI shown in Figure 2. While the software offers a number of settings, the most important are fading frequency (response time) and an option for loading a lookup table with attenuation values. In addition, manually setting an attenuation value is also possible. The used special SPIRICON PYROCAM III beam camera provides self-measured proof of the propagated LP\({}_{01}\) mode in vertical (Figure 3a) and horizontal (Figure 3b) planes, representing real FSO links.