TNO and DLR envision optical free-space communication between ground stations and geostationary telecommunication satellites to replace the traditional RF links for the next generation of Very High Throughput Satellites. To mitigate atmospheric turbulence, an Adaptive Optics (AO) system will be used. TNO and DLR are developing breadboards to validate Terabit/s communication links using an AO system. In this paper the breadboard activities and first results of the sub-systems will be presented. Performance of these subsystems will be evaluated for viability of terabit/s optical feeder links.
The German Aerospace Center (DLR) and the European Southern Observatory (ESO) performed a measurement campaign together in April and July 2016 at Teide-Observatory (Tenerife), with the support of the European Space Agency (ESA), to investigate the use of laser guide stars (LGS) in ground to space optical communications. Atmospheric turbulence causes strong signal fluctuations in the uplink, due to scintillation and beam wander. In space communications, the use of the downlink channel as reference for pointing and for pre-distortion adaptive optics is limited by the size of the isokinetic and isoplanatic angle in relation to the required point-ahead angle. Pointing and phase errors due to the decorrelation between downward and upward beam due to the point-ahead angle may have a severe impact on the required transmit power and the stability of the communications link. LGSs provide a self-tailored reference to any optical ground-to-space link, independently of turbulence conditions and required point-ahead angle. In photon-starved links, typically in deep-space scenarios, LGSs allow dedicating all downlink received signal to communications purposes, increasing the available link margin. The scope of the joint DLR-ESO measurement campaign was, first, to measure the absolute value of the beam wander (uplink-tilt) using a LGS, taking a natural star as a reference, and, second, to characterize the decrease of correlation between uplink-tilt and downlink-tilt with respect to the angular separation between both sources. This paper describes the experiments performed during the measurement campaigns, providing an overview of the measured data and the first outcomes of the data post-processing.
KEYWORDS: Data modeling, Turbulence, Global system for mobile communications, Free space optics, Receivers, Atmospheric turbulence, Atmospheric modeling, Diffusers, Wave propagation, Atmospheric optics
The recently proposed exponentiated Weibull (EW) fading channel model is assessed with partially coherent beams (PCBs) using the Gaussian Schell-model (GSM) theory. The use of a PCB in free-space optical links has been suggested as an atmospheric turbulence mitigation technique, called transmitter aperture averaging. A wave optics code based on the GSM theory is used to generate a PCB propagating in atmospheric turbulence, and the probability distribution function (PDF) is obtained from the simulated irradiance data. This study on the PDF includes the new proposed EW model in the presence of aperture averaging, in weak-to-strong turbulence regime. Point receiver conditions are also analyzed. For completeness of the study, the Lognormal and the Gamma-Gamma models have also been included in the analysis, allowing for a direct comparison to the EW distribution. Using a least-squares curve fitting algorithm, the EW model always has the best fit to simulated PDF data. Thus, the EW fading channel model proves to be valid for PCBs.
Recently a new proposal to model the fading channel in free-space optical links, namely, the exponentiated Weibull (EW) distribution, has been made. It has been suggested that the EW distribution can model the probability density function (PDF) of the irradiance under weak-to-strong conditions in the presence of aperture averaging. Here, we carry out an analysis of probability of fade and bit error-rate (BER) performance using simulation results and experimental data. The BER analysis assumes intensity modulation/direct detection with on-off keying, and new expressions are derived. Data is modeled following the statistics of the EW fading channel model, and compared with the Gamma-Gamma and Lognormal distributions, as the most widely accepted models nowadays. It is shown how the proposed EW model is valid in all the tested conditions, and even outperforms the GG and LN distributions, that are only valid under certain scenarios.
In terrestrial free-space laser communication, aside from pointing issues, the major problem that have to be dealt
with is the turbulent atmosphere that produces irradiance fluctuations in the received signal, greatly reducing
the link performance. Aperture averaging is the standard method used to mitigate these irradiance fluctuations
consisting in increasing the area of the detector, or effectively increasing it by using a collecting lens with a
diameter as large as possible. Prediction of the aperture averaging factor for Gaussian beam with currently
available theory is compared with data collected experimentally and simulations based in the beam propagation
method, where the atmospheric turbulence is represented by linearly spaced random phase screens. Experiments
were carried out using a collecting lens with two simultaneous detectors, one of them with a small aperture to
emulate an effective point detector, while the other one was mounted with interchangeable diaphragms, hence
measurements for different aperture diameters could be made. The testbed for the experiments consists of a
nearly horizontal path of 1.2 km with the transmitter and receiver on either side of the optical link. The analysis
of the experimental data is used to characterize the aperture averaging factor when different values of laser
divergence are selected.