Exploration and Modelling of Atmospheric Turbulences Effects for Downlink Scenario of Free Space Optics Communications

. Scintillation, beam wandering, and phase front distortion are the principal impacts of atmospheric turbulence on laser beam propagation. The aperture averaging concept might be used to minimize the effect for the first two, but the third, which is important for high-speed free-space communications, is significantly more challenging. More than ten years ago, the institute of communications and navigation (IKN) of the german aerospace centre (DLR) conducted optical downlink experiments with JAXA's OICETS/Kirari-Japan to study the optical LEO downlink channel and assess the viability of this transmission technology for upcoming applications. The present work will study and give a comparable insight into simulation and experimental results showing a significant relationship elevation dependency for parameters linked to index-of-refraction turbulence.


Introduction
A growing number of scientific researches are interested in free space optics (FSO) communications because of its high data speeds, inexpensive deployment costs, and absence of electromagnetic interference issues.Because of the water particles that are present along the propagation channel, various atmospheric conditions including fog, rain, and snow cause fading.Another phenomenon that frequently deviates and scatters the optical beam and results in multipath-induced dispersion is atmospheric turbulence.Many analytical [1] and experimental investigations [2] were conducted, and a variety of solutions were put forth, in an effort to lessen the impact of turbulence on FSO communication systems.The DLR Optical Ground Station in Oberpfaffenhofen (OGS-OP), close to Munich, had a number of instruments for detecting the impact of index-of-refraction turbulence (IRT) in the atmosphere as well as a data receiver frontend to calculate bit-error distributions [3].The paper will try to investigate and discuss the theoretical predictions as well as PiLab simulation results and the experimental outcome based on KIODO (KIrari Optical Downlinks to Oberpfaffenhofen) project dataset.

Methodology
The downlink data collecting point was the OGS-OP "Optical Ground Station in Oberpfaffenhofen" on the DLR site, where the OGS-OP was a 40-cm Cassegrain telescope supported by an astronomical mount [3].Fig. 1(a) depicts the telescope and the mount and Fig. 1(b) shows the measurement setup in the optical bench behind the telescope is illustrated on the right side.On a metal plate placed behind the telescope are the communication receiver, the tracking camera, the measurement cameras, and a power meter.Two spatially dispersed 5 cm telescopes, one installed on either side of the 40-cm telescope, were used to transmit the beacon beam [3].The beacon beam from the ground station, with a wavelength of 808 nm and a divergence of 5 mrad, and the communication beam from the optical inter-orbit communications engineering test satellite (OICETS), with a divergence of 5 μrad [3], are both used in the KIODO downlink, as shown in Fig. 2.

Results and scientific findings
To some extent, a test of both KIODO measurement campaigns has been performed and represents the summarized results [3], which will help to extract comparable datasets with theoretical and PiLab simulation results.We used PILab code [4,5] (developed by DLR's Institute of Communications and Navigation) to simulate the downlink scenario, at λ = 847 nm for a vertical terrestrial link length L of 40 km and beam waist = 50 mm use standard Hufnagel-Vally (HUFVAL 5/7).More details of parameter definitions are explained in Table 1.In general, predicting the physical behaviour of the turbulent flow produced in a particular system is the goal of turbulence modelling.As a function of elevation, Fig. 4 displays the experimental scintillation index of the intensity (i.e., of a camera pixel [3]).The camera's 12-bit resolution was able to depict the scintillation dynamic in its entirety.The PiLab simulation results are integrated into the same figure where the curve with sky-blue colour (i.e.PiLab Simulation01) shows simulated scintillation index values at night conditions and the purple curve (i.e., PiLab Simulation02) explains the simulation in the daytime.We detect scintillation saturation at low heights, as expected by theories.The significant difference in scintillation strength between the two nights is also intriguing: on KT7, it was four times as strong as on KT4.
The key difference between the two trials is that KT7 took place a few hours before a warm rainstorm.
Figure 5 shows the differential image motion monitor (DIMM) measurements as well as the PiLab simulation results (simulation01→night, simulation02→day) with the theoretical prediction.The standard DIMM procedure was used [3], which assumes no scintillation and Kolmogorov wavefront aberrations and relates the focalspot motion variance to Fried parameter (r0).Furthermore, we should keep in mind that these assumptions may not hold true for measurements with considerable variations.

Concluding Remarks
It is thought that an error-free data transfer would have been possible with a receiving front end that was more resistant to scintillation.We found that observations and predictions from theory and scintillation simulations generally agreed.Scintillation saturation has been seen thanks to measurements taken at low altitudes.Worth mentioning, more than one new simulation tool is under upgrading for the understanding of atmospheric turbulence.Therefore, the deep enhancement of the PILab code simulation structure could help to generate further reliable and solid results of atmospheric turbulenceinduced multiple and overlapping effects of laser beam spreading, wandering and scintillation..

Fig. 1 :
Fig. 1 : Optical Ground Station Oberpfaffenhofen, (a) view of the focal bench of the 40-cm telescope of OGS-OP, that still with a conventional astronomical mount, (b) Instrument setup (the beam is guided to the tracking camera, receiver frontend and measurement instruments) [3].

Fig. 4 :
Fig. 4: Experimental, simulation and theoretical results of intensity scintillation index of the downlink scenario.