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Downlink measurement campaigns from the optical downlink terminal OSIRISv1 onboard the LEO satellite Flying Laptop were carried out with the French Observatoire de la Côte d’Azur and with two Optical Ground Stations of the German Aerospace Center. On/off keyed data at 39 Mb/s were modulated on the laser signal, and according telecom reception was performed by the ground stations. The pointing of the laser terminal was achieved by open-loop body pointing of the satellite orientation, with its star sensor as attitude control signal. We report here on the measurements and investigations of the downlink signal and the data transmission.

Optical communication systems are picking up pace in the various industries for its many scenarios in which it can be used. This includes satellite, aircraft and ground systems for which the DLR Institute of Communications and Navigation develops laser terminals. Free Space Optics (FSO) based optical systems have different applications ranging from terrestrial to airborne to space, and thus also the associated requirements to these application domains vary even though the primary functional goals remain the same. Furthermore, quantum communication became of interest for its inherently secure key generation process that allows two nodes to securely communicate with each other and share secrets for long term security. Out of the different use cases of free-space optical laser terminal came the concept of having a modular FSO optical system design that could quickly adapt to the changing needs from mission to mission. The idea of a modularity appeals to many despite its challenges in the beginning, as in the long run the pros outweigh the cons and the gained advantages make the initial effort worth the while. The concept was realised by carefully studying different requirements from various on-going and past projects and thereafter segmenting different aspects of the optical system based on three main categories, namely; the received path, the transmitted path and the shared path. To elaborate modularity in system here; a fast steering mirror (FSM) can be replaced with a fold mirror for a simpler design should the requirements not require it, or an exchangeable CPA with integrated telescope mounted at the aperture of the optical bench to allow for different magnifications and ex-aperture beam diameter, all these without major impact on other subsystems. This greatly reduces the effort involved in re-evaluating system performance in the long run. In this study the design and development of such an optical system is presented that lays its ground in the Institute of Communications and Navigation’s OSIRIS project, but still continues to serve as the primary optical system design of choice for future projects.

Modern satellite missions like CubeSat-missions are characterized by extremely short development time and significant low resource requirements. This can only be reached by reducing classical standards to necessary levels, tailored down for the respective mission. It leads to a rethinking of system engineering in modern space missions following the new space approach, which tailors down the standard processes to a minimum but reasonable level. The Optical Communication Systems Group (OCS) of DLR develops solutions to break the bottleneck caused by limited data-rates of traditional RF-communication systems with Free-Space-Optics (FSO) technologies for earth orbiting platforms. Therefore DLR started 10 years ago the OSIRIS (Optical Space InfraRed downlInk System) program which concentrates on research and development of laser communication terminals for Low-Earth-Orbit (LEO) satellites. The development speed in laser communication accelerated in the last years due to the availability of compact and low-cost flying platforms like pico- and nanosatellites. Therefore OCS follows the new space approach to shorten development times and finish projects much earlier compared to traditional space missions. The OSIRIS4CubeSat (O4C) project has the goal to develop a high data-rate optical communication payload for CubeSats and demonstrates it in a quasi-operational scenario. The development of the payload is in close collaboration with Tesat-Spacecom who sells O4C under the name 'CubeLCT' as a commercial product. Beside the payload development, the functionality of O4C will be demonstrated in a direct to earth downlink from a CubeSat to an Optical Ground Station. Thus, the payload will fly in the PIXL-1 mission on a 3U CubeSat built by GomSpace. After the Launch and Early Operational Phase (LEOP) the satellite will be hand over to German Space Operation Center (GSOC) to demonstrate for the first a time a commanding of a CubeSat by a professional Ground System. The ambitious path from the development of the payload, which started from scratch, to a quasi-operational mission, shows the necessity of a working system engineering to observe all requirements, coordinate the different teams and manage the interfaces to the stakeholder. This paper describes the system engineering based on the new space approach for a modern CubeSat mission. In the beginning of the project the overall requirements for the whole project were defined. This includes the technical requirements for the payload, to ensure the functionality depending on the extreme environmental conditions in space and the requirements for the satellite. Furthermore the operational requirements were defined. Additionally to the internal requirements the needs of the commercialization partner and the possible launch opportunities had to be considered. Beside the requirements, the single work packages were defined to distinguish the several tasks and visualise the interfaces and connections in a work breakdown structure (WBS). Based on the WBS the technical teams for the development were formed. During the concept development and the integration of the payload, the system engineering had to monitor the requirements and coordinate the different technical teams with all their interfaces. Even though the teams were separated in optics, mechanics, electronics and software, their tasks were strongly connected. Beside the internal interface coordination the system engineering had to coordinate the communication on technical level to the external stakeholders like contractee (Tesat), satellite manufacturer (GomSpace), launch provider and operator (GSOC). This paper gives an overview over the processes in the OSIRIS4CubeSat project which represents modern system engineering following the new space approach.

FSO systems present many advantages like high data rate, license-free bandwidth and tap-proof communication allowing the download of vast amounts of data from LEO satellites. However, the atmospheric channel is quite challenging, because of spurious effects such as absorption, scattering and scintillation that in turn vary the link losses in correspondence to the elevation. In order to maximize the downlink throughput, it should start around 5° elevation. This leads to a design with suboptimal performance for higher elevations when constant data rates are used. Therefore, DLR is developing a system to adjust the data rate according to elevation and atmospheric channel conditions. This data rate variation is achieved by determining the maximum rate for higher elevation and then for lowering the data rates a bit-level repetition is performed. The presented system enables a fast transition between the different data rates. Additionally, this system allows the satellite to transmit data at rates even lower than those nominally supported by the physical transceiver. At the receiver side, the system complexity increases as it should be able to acquire, detect, and filter the signal for different data rates. DLR proposes a system that mirrors the operation of its transmitter counterpart by sampling the acquired signal at the maximum data rate. Then an FPGA processes the signal by majority decision algorithm followed by voting system that filters and detects the intended data rate in real-time. This enables replication and parallelization of the filtering and detection processes enabling the automatic detection of the received data rate. In order to provide noise stability, the transition between data rates is governed by a hysteresis process. This scheme allows the detection and selection of the proper data rate in the range of few microseconds for a system operating between 10 Gbps and 1.25 Gbps in steps of factor of 2, ignoring the propagation delays.

The generated amount of data on high flying platforms like aircrafts, satellites and Unmanned Aerial Vehicles (UAV) increases continuously, due to the technical improvement of modern sensor systems. The resulting demands for higher data rates on airborne and space platforms motivates the development of Laser Communication terminals for aircrafts and satellites in the last years. DLRs Institute of Communications and Navigation has a successful track record in developing Free Space Optical (FSO) terminals for flying platforms like stratospheric balloons, aircrafts and small satellites to transfer data from moving platforms down to earth in real-time. Beside the advantages of FSO such as high data rates and a secure transfer channel against Radio Frequency (RF) interferences, a direct line of sight is mandatory for a successful link. Traditional RF-Communication is more robust and less effected by atmospheric disturbances or weather conditions. Thus, new system concepts have been developed to benefit from the provided high data rates of the FSO and the reliability of RF-Communication technologies. As part of this trend, DLR has developed and demonstrated a Hybrid FSO/RF-communication system capable of overcoming the spurious effects of the atmosphere. This paper gives an overview about DLRs current studies and developments with the goal to combine the advantages of FSO and RF-Communication. It discusses possible implementation concepts on different platforms and presents experimental results of the implemented FSO/RF hybrid communication system operating for airborne, optical downlinks at 1Gbps.

Space industry has undergone a significant change over the last years. The development moved from large and costly spacecrafts to cost-efficient designs and shorter development times. While the satellites became smaller, the resolution of high compact sensors increased which led to a high data-volume to be transmitted and increasing demands for higher data rates on small satellites. This motivated for a highly compact version of DLR’s optical communication payload OSIRIS for small LEO satellites. DLR’s Institute of Communications and Navigation has developed the OSIRIS (Optical Space Infrared Downlink System) program starting with payloads on the satellites Flying Laptop of Univ. of Stuttgart and BiROS of DLR. Combining miniaturization to the flight-proven developments with novel concepts, OSIRIS4CubeSat allows integration in a standard CubeSat bus. The development of OSIRIS4CubeSat (industrialized under the product name “CubeLCT”) is conducted in close collaboration with Tesat Spacecom, DLR’s commercialization partner. The first implementation will be demonstrated within the PIXL-1-Mission on a 3Unit CubeSat. Furthermore, OSIRIS4CubeSat (O4C) has been chosen to support scientific missions together with university partners in the field of Quantum Key Distribution (QUBE). In the future, the modular design will enable extensions for optical inter-satellite links. This paper will give an overview about the development of the O4C payload and the current status of the PIXL-1- Mission. Furthermore, it will show the adaptation of the payload for the scientific mission QUBE. Besides these projects, the paper will give an outlook for future extensions of the O4C payload and the necessity of high data-rates in other scenarios such as inter-satellite links.

Global coverage of internet access is essential for digitalization in society, becoming a disruptive technology in industry, education or political participation for example. Satellite communications is a complementary approach to the terrestrial fiber network, which can provide world-wide coverage with few satellites in geostationary orbit or with low-earth-orbit constellations. Optical wavelengths offer multiple THz of available spectrum that can be used to connect satellites to the ground network with high-throughput links, solving the radiofrequency bandwidth bottleneck, without regulations. Cloud covereage and atmospheric turbulence are the main challenge in guaranteeing the same availability as in terrestrial fiber-based systems. While the former can be addressed by site diversity, for the latter, other mitigation strategies are required. Adaptive optics is a common approach to correct for atmospheric phase distortions and ensure stable fiber coupling. However, this approach requires a relatively complex active setup and therefore a collaboration between DLR Institute of Communications and Navigation and Cailabs has been formed to investigate alternative passive solutions for low-complexity ground stations. Coupling into multimode fibers does not require adaptive optics due to the large fiber core, however the coupled signal is distributed into multiple fiber-modes and is therefore incompatible with standard telecommunications components. Cailabs Multi-Plane Light Conversion (MPLC) technology overcomes this issue, selectively demultiplexing the fibermodes into single-mode fibers. Here, DLR’s adaptive optics system and the MPLC technology in a turbulence-relevant environment for GEO communications are compared, investigating the advantages of the MPLC approach for compensating strong turbulence. This paper presents an overview of the measurement setup and analyzes the single-mode fibers outputs of the spatial demultiplexer and the measured phase-distortions from a wavefront sensor.

Optical dense wavelength division multiplexing (DWDM) can provide multiple Tbit/s throughput in the future geostationary satellite communications. In this letter, the proof-of-concept demonstration of this technology under worst case atmospheric channel conditions for satellite communications in geostationary orbit is presented. The highest-to-date throughput of 1.72 Tbit/s was transmitted over 10.45 km distance with passive transmitter pointing and active receiver tracking with active single-mode fiber coupling. This throughput was achieved by modulating 40 DWDM channels with uncoded 43.01824 Gbit/s rate per channel. Direct bit-error-rate and signal fluctuations measurements were carried out to assess the link performance. No forward error correction was used.

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The design and qualification of a retroreflector specifically designed for a CubeSat (CubeL) is described. The CubeSat will be launched to space in 2019 and demonstrate the latest generation of the optical space infrared downlink system developed by the German Aerospace Center together with the industrial partner Tesat-Spacecom. The retroreflector is optimized to allow for a coarse verification of the satellites attitude control system. By analyzing the returning photon count during satellite laser ranging (SLR) when the satellite is operated in station pointing mode attitude information is obtained. To achieve this goal, the entrance face of the retroreflector is recessed by a circular tube-shaped aperture. Due to this recession, the signal reflected from the retroreflector falls off rapidly when the retroreflector is tilted away from the SLR station. From measurements of the retroreflectors far-field diffraction pattern and calculations, we believe that it should be possible to determine the orientation accuracy of the satellite to within ± 2°. The proposed method is an effective and cheap way for coarse attitude control, e.g., for satellites of mega-constellations with any existing satellite laser ranging ground station.

Transmitter diversity is an effective fading mitigation scheme in an optical geostationary satellite feeder uplink. We quantify the performance of the scheme over a slow fading channel where the atmospheric turbulence causes scintillation and wandering of the optical beam resulting in deep fades at the satellite receiver. For given atmospheric conditions and residual beam pointing jitter, we optimize the transmit power of each beam to obtain the minimum overall power scintillation index and achieve a bit error rate gain of 0.46dB considering intensity modulation and direct detection.

Optical Data Links are a very promising Technology for downlinking data from stratospheric platforms to ground or to satellite relais, as well as interconnecting these towards a network: The optical signal does not interfere with any radio communication on ground or from the HAP user links itself, and the free-space loss of such laser links is minimal compared to its RF counterparts. Still, the communication is very secure by physical nature, and even allows the application of QKD principles based on quantum effects. The impact of the atmospheric channel needs to be evaluated and, if necessary, counteracted. Dependent on the link scenario, the atmosphere can influence the optical signal by its index-of-refraction trubulence, leading to fading-like effects. Furthermore clouds can in certain link geometries cause outages which can be compensated by site- and link-diversity. DLR-IKN has evaluated the atmospheric impact both analytically and by experiments, and according mitigation techniques (like physical diversity, Forward Error Correction /FEC, as well as adaptive data repetition techniques) were developed such as high speed modems with FEC and interleaving, as well as ARQ techniques, and also physical diversity and receiver sensitivity slope techniques. German Aerospace Center's Institute of Communications and Navigation sees a great chance for the application of optical links with HAPS in certain application scenarios - either of commercial or institutional nature - and fosters cooperation based on our technology developments.

© 2020 Optical Society of America]. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited. In a downlink scenario, the performance of laser satellite communications is limited due to atmospheric turbu- lence, which causes fluctuations in the intensity and the phase of the received signal, leading to an increase in bit error probability. In principle, a single-aperture phase-compensated receiver, based on adaptive optics, can over- come atmospheric limitations by adaptive tracking and correction of atmospherically induced aberrations. However, under strong turbulence situations, the effectiveness of traditional adaptive optics systems is severely compromised. We have developed an alternative intensity-based technique that corrects the wavefront by iter- atively updating the phases of individual focal-plane speckles, which maximizes the power coupled into a single- mode fiber. Here, we present the proof of concept for this method. We show how this technique offers around 4 dB power gain with fewer than 60 power measurements under strong turbulence conditions. It delivers a good performance in different turbulent regimes, and it shows robustness against severe deterioration of the signal-to-noise ratio.

Optical geostationary orbit (GEO) satellites are one of the means to provide high speed internet and broadband services to even remote areas on the globe. However, the optical beam, as it propagates through the atmosphere, is affected by the atmospheric index of refraction turbulence and pointing errors due to beam wander and mechanical vibrations on the platform which result in fading, hence loss of signal. We present transmit diversity as a fading mitigation technique and use wavelength division to minimize cross interference between the transmitted signals. Optical single sideband (OSSB) scheme is used to increase spectral efficiency (SE) of the system. We demonstrate a scheme where an OSSB signal is produced using commercially available optical filter with tunable bandwidth and center frequency. For a 32Gbps data signal modulated using amplitude shift keying (ASK), we measure the required minimum 6dB and 20dB bandwidths of the optical filter to be 12GHz and 24GHz, respectively. Also, the offset of the filter from the carrier is found to be -11GHz and +10GHz to produce an error free lower and upper OSSB signal, respectively. The SE of the OSSB signal is found to be 1.34 bit/s/Hz. Moreover the stability of the optical filters and carrier ensure reliable signal generation making the OSSB a potential candidate to be used in future free space optical links.

Was High-Altitude-Platform Systems (HAPS) für die Kommunikationsnetze leisten können.

The microwave spectrum has become a highly limited resource in satellite communications owing to an ever increasing demand for bandwidth and capacity. Therefore, a shift to the exploitation of optical carrier frequencies is currently underway. Focusing on high-rate transmissions of payload data from remote sensing satellites, operational systems, like the well-known European Data Relay Satellite system, are based on optical inter-satellite links. Besides, direct-to-earth free-space optical communications from low Earth orbiting spacecraft hold high potential for upcoming space missions through lower complexity. In that regard, we study the viability of the ground-to-space beacon laser signal of optical ground stations to be additionally modulated with tele-command tokens. Such an optical return channel could be variously put into use, for example to trigger automatic repeat requests of payload data downlinks, for jamming-free control of the spacecraft or for high-rate software uploads to its on-board processor. A particular challenge is posed by the unequal fading behavior of the optical channel regarding the down- and uplinks, which cover asymmetric optical pathways through the atmosphere. We define the end-to-end architecture of the communication chain including the transmitter on ground and the space-based receiver. Special attention is given to compatibility with established space data and system standards. Moreover, we examine the effects on the scheduling of satellite control, resulting from a constrained availability of the optical uplink due to cloud blockages. Our analysis aims at the employment of available space protocols for bidirectional optical communications with low earth orbiting spacecraft. Further on, we consider the adoption of upcoming standards to account for the optical fading channel. Certain applications like immediate automatic-repeat-requests for the downlink will require novel, optimized protocols.

An optical communications system employing intradyne reception and offline digital signal processing is tested over a 10.45 km link through the atmosphere. 40 GBaud transmission using binary phase-shift keying in the C-band is demonstrated and compared with laboratory measurements. Simultaneous photodetector measurements show that the turbulence in the atmospheric channel is representative for relevant and worst-case conditions in the geostationary satellite uplink scenario.

Wavefront distortions of optical waves propagating through the turbulent atmosphere are responsible for phase and amplitude fluctuations, causing random fading in the signal coupled into single-mode optical fibers. Wavefront aberrations can be confronted, in principle, with adaptive optics technology that compensates the incoming optical signal by the phase conjugation principle and mitigates the likeliness of fading. However, real-time adaptive optics requires phase wavefront measurements, which are generally difficult under typical propagation conditions for communication scenarios. As an alternative to the conventional adaptive optics approach, here, we discuss a novel phase-retrieval technique that indirectly determines the unknown phase wavefront from focal-plane intensity measurements. The adaptation approach is based on sequential optimization of the speckle pattern in the focal plane and works by iteratively updating the phases of individual speckles to maximize the received power. We found in our analysis that this technique can compensate the distorted phasefront and increase the signal coupled with a significant reduction in the required number of iterations, resulting in a loop bandwidth utilization well within the capacity of commercially available deformable mirrors.

Delayed Frame Repetition (DFR) is a simple and efficient technique for varying the data rate to cope with varying link budget and fading in free space optical LEO downlink (OLEODL) scenario [1]. The system shall run at highest possible data rate in good channel condition and at lower user data rates when the channel gets deteriorated. In OLEODL scenario, the link budget varies according to the elevation [2]. At lower elevations, the signal propagates longer through the atmosphere causing more free-space loss and creating stronger fluctuations by atmospheric turbulence. Therefore, it is efficient to transmit the data at reduced effective rates at lower elevations. Basic idea of DFR is to retransmit frames after certain delay that is larger than the typical fade duration of the channel. In addition to the gain achieved by varying the data rate, this proceeding also provides diversity gain in the scintillation channel. Unlike varying the data rate by varying the pulse length, this technique avoids the need for changing the receiver bandwidth as per the data rate. This paper evaluates the performance of such systems for channel without fading, medium and bad channels using different receivers namely: shot-noise limited (SNL), avalanche photodiode (APD), and thermal limited PIN (Positive-IntrinsicNegative). For combining the retransmitted frames, Equal Gain Combining (EGC) technique is used. This paper also investigates the variation of the delay length between retransmitted frames. Simulation results show that it is more advantageous to use DFR for channels with higher scintillation index (SI), and use longer delay. With strong scintillation, even a net sensitivity gain in energy-per-bit can be achieved.

To overcome data rate limitations of RF communication links with satellites, TNO and DLR envision optical free-space communication feeder links for next generation high throughput satellites. This paper provides a feasibility assessment of such links and the technology needed. The main results of the link budget and the turbulence modeling of terabit/s optical links are presented. Based on these parameters, requirements and status of the link-subsystems are discussed, and a roadmap is presented, aimed at achieving terabit per second optical feeder links.

Direct optical communication links might offer a solution for the increasing demand of transmission capacity in satellite missions. Although direct space-to-ground links suffer from limited availability due to cloud coverage, the achievable data rates can be higher by orders of magnitude compared to traditional RF communication systems. DLR’s Institute of Communications and Navigation is currently developing an experimental communication payload for DLR’s BiROS satellite. The laser terminal consists of a tracking sensor with an uplink channel and two kinds of laser sources: a directly modulated High-Power Laser Diode (HPLD) and an Erbium Doped Fiber Amplifier (EDFA). This paper will give an overview about the hardware of the laser terminal with a special focus on the calibration of the optical system and the space-qualification, including a radiation test especially for the optical components. Further, the data reception and storage on ground station site will be discussed.

Channel reciprocity in a bidirectional turbulent channel implies that the received powers are correlated to a certain extent at both link ends. In this paper, we investigate by numerical phase-screen propagation simulation the variation of the channel correlation under various boundary conditions, namely orthogonal wind speed, link distance, receiver aperture and turbulence-strength profile for inter High Altitude Platform (HAP) FSO links. Finally, we provide an outlook to different Error Control Coding (ECC) methods that make use of the reciprocity effect.

Optical links to geostationary (GEO) satellites suffer from atmospherically-induced beam wander which leads to pointing errors at the satellite causing deep fades. In this paper, we show the benefit of transmitter diversity in reducing the fades caused by beam wander. We derive an analytical expression for the reduction of overall scintillation index for a given number of transmitted beams with Gaussian profile in a multiple-input single-output (MISO) system considering solely the effect of beam wander. The transmitted power, beam divergence angle and pointing jitter are kept as free parameters as in the real situation. Moreover, the optimized ratio of transmitted powers between multiple beams is obtained through simulations for a two-fold transmitter diversity to obtain minimum overall power scintillation index (PSI).

Reliable modelling of the power variations in atmospherically propagated optical waves is important for appropriate design of the necessary fading mitigation techniques like Forward Error Correction (FEC) or Automatic Repeat reQuest (ARQ) algorithms. A simulation tool modeling the combined impact of atmospheric turbulence and pointing jitter is presented and its performance is compared with measured power vectors.

Future Earth Observation satellite data downlinks via directed laser beams are a promising technique to overcome the data repatriation bottleneck in high resolution sensor missions. A challenge for this technology are received power scintillations induced by atmospheric index-of-refraction turbulence (IRT). The strength of these signal variations depends on the IRT height profile, receiver aperture size, wavelength, and link elevation. We analyze measured optical satellite downlink power vectors to verify a model for the dynamic power scintillation loss. Due to their increased sensitivity compared with PIN-receivers, Avalanche Photo Diodes (APD) are usually used within data receivers on ground. This allows bulk receivers to avoid the demanding coupling onto single-mode detectors. As their sensitivity run is defined by the influence of thermal-, and signal-dependent shot-noise, a new formulation for this dependency is evaluated, and the according modified scintillation loss is quantified. This allows appropriate dimensioning of link budgets and the necessary error control mechanisms to enable reliable high-speed data transfer from low earth orbit satellites to the ground.

Free space optical communication is gaining interest because of its various advantages like high data rate, high power efficiency, no license regulation etc. However, one has to overcome the challenge of distortion and fading caused by atmospheric turbulences and bad weather conditions. Free space optical communication channel is a time variant channel. Sometimes, the channel has a very good condition and sometimes atmospheric condition gets very bad. Variable data rate is an efficient solution to cope with such situations. The system would run at maximum data rate in good conditions and can be adapted to work at lower data rate in worse situations. This paper discusses about one of the techniques to lower the data rate called Delayed Frame Repetition (DFR) for fading channels. It reduces the user data rate while maintaining the same channel data rate. The basic idea is to retransmit the data after a delay which is greater than fading length of the channel. Similar to spatial diversity systems, different combining techniques like Selective Combining (SC), Maximum-Ratio Combining (MRC) and Equal-Gain Combining (EGC) are studied. Numerous simulations are presented for MRC technique considering thermal limited PIN detector as a receiver, and for channel modelling artificially generated power vectors that are based on real measurements are used. Overall results show that DFR can be very useful for fading channel. MRC being the most efficient combining technique, it is used for evaluating the performance of DFR for different channel conditions with good, medium, bad scintillation indices, and different delays between retransmitted frames.

Optical links between ground stations and geostationary orbit (GEO) satellites suffer from the atmospherically induced phase piston and intensity fluctuations. This aggravates stable signal detection in the uplink scenario. In this work transmitter diversity is studied as a fading mitigation technique in GEO satellite feeder uplinks. A single sideband scheme is used as a spectrally efficient frequency division technique compatible with intensity modulation and direct detection. A spectral efficiency of 0.56 bit/s/Hz with twofold diversity gaining 2.3 dB at BER = 10-3 was obtained by simulation.

This paper investigates the impact of different Channel State Information (CSI) on code-rate adaptive Hybrid ARQ pro-tocols over inter-HAP (High Altitude Platform) FSO fading channel. Perfect, delayed, reciprocal and fixed-mean CSI are considered in the investigation. We study in particular the transmission efficiency of HARQ protocols where the code rate of a RS code is adapted to different CSI. Simulation results show that adaptive code rate based on correctly chosen CSI significantly outperforms HARQ with a non adaptive mechanism providing improved overall performance.

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.

Free-space optical (FSO) communication is a very attractive technology offering very high throughput without spectral regulation constraints, yet allowing small antennas (telescopes) and tap-proof communication. However, the transmitted signal has to travel through the atmosphere where it gets influenced by atmospheric turbulence, causing scintillation of the received signal. In addition, climatic effects like fogs, clouds and rain also affect the signal significantly. Moreover, FSO being a line of sight communication requires precise pointing and tracking of the telescopes, which otherwise also causes fading. To achieve error-free transmission, various mitigation techniques like aperture averaging, adaptive optics, transmitter diversity, sophisticated coding and modulation schemes are being investigated and implemented. Evaluating the performance of such systems under controlled conditions is very difficult in field trials since the atmospheric situation constantly changes, and the target scenario (e.g. on aircraft or satellites) is not easily accessible for test purposes. Therefore, with the motivation to be able to test and verify a system under laboratory conditions, DLR has developed a fading testbed that can emulate most realistic channel conditions. The main principle of the fading testbed is to control the input current of a variable optical attenuator such that it attenuates the incoming signal according to the loaded power vector. The sampling frequency and mean power of the vector can be optionally changed according to requirements. This paper provides a brief introduction to software and hardware development of the fading testbed and measurement results showing its accuracy and application scenarios.

This paper presents the experimental results of 100Mbps Laser Ethernet Transceivers for high-speed communications in a 142Km free-space optical inter-island link. Round-trip times below 1.6ms and error free transmission at full throughput during several time intervals were demonstrated

In this paper, the performance of decode-and-forward relay-assisted free-space-optical (FSO) communication systems under atmospheric turbulence induced fading and misalignment errors is investigated. To mitigate the adverse effects of the atmospheric turbulence, the aperture averaging receivers are considered both at the relay and destination sides. The atmospheric turbulence induced fading is modeled via the exponentiated-Weibull distribution, which has recently been proposed to characterize an FSO link in the presence of finite-sized receiver aperture. The expression for the moment generating function (MGF) of the instantaneous signal-to-noise ratio is derived. Further, new closed form expression for the outage probability is obtained. Moreover, the new expression for the average symbol error rate of the subcarrier intensity modulated M-ary phase shift keying is obtained using the MGF-based approach. Finally, numerical examples are discussed and all the derived analytical results are corroborated by Monte-Carlo simulations. Relayed FSO Communication with Aperture Averaging Receivers and Misalignment Errors. Available from: https://www.researchgate.net/publication/307931308_Relayed_FSO_Communication_with_Aperture_Averaging_Receivers_and_Misalignment_Errors [accessed Nov 10, 2016].

Effective coupling of the optical field from free-space to optical fiber is an essential prerequisite for modern free-space optical communications systems. It allows for easier system integration with active and passive optical fiber-coupled components as well as for efficient optical field mixing for coherent communications. While coupling into single-mode fiber provides the advantage of using low-noise erbium-doped fiber preamplifiers, its relatively small mode field diameter limits achievable fiber coupling efficiency. Coupling into multimode fiber (MMF) increases the fiber coupling efficiency while introducing other spurious effects the authors have set out to analyze. The study of free-space optical beam coupling in the context of satellite communications will be presented. Here, we assume satellite link scenarios with different elevations, which correspond to different index-of-refraction turbulence (IRT) conditions. IRT gives rise to both intensity and phase aberration of the received optical field, which then causes extended speckle patterns in the focus of the receiver telescope. The speckle field at the fiber input is calculated by means of Fourier transform of the received field. Using dedicated modelling software, study of the fiber coupling efficiency, polarization preservation and high-order mode coupling in different multi-mode fibers is carried out.

Finite-state Markov chains are a useful tool for modelling communication channels with correlated fading and have recently also been applied with success to terrestrial free-space optical communication channels. However, the issue of how such Markov models should be optimised in order to accurately approximate the original continuous fading channel has not been addressed in a systematic manner. In this study, the authors improve on previous proposals by optimising the state space partitioning of the considered models. In particular, they investigate the properties and approximation accuracy of Markov models which are optimised according to information-theoretic considerations. They validate and evaluate their approach using a set of experimental measurements over a 12 km link distance. The obtained results confirm that optimised Markov models can provide better accuracy at lower state complexity, yet there remain shortcomings in capturing the autocovariance of the fading process.

The paper presents an experimental testbed, which will be used for the demonstration of more than 1 Tbps free-space optical (FSO) transmission in a scenario similar to that of GEO uplink. The chosen terrestrial scenario corresponds to worst-case conditions of a GEO uplink scenario according to the atmospheric channel characteristics and the geometry of the scenario. The testbed consists of a DWDM-based FSO link over 26km distance with less than 2° elevation. The communications system consists of several DWDM transmitters, which are multiplexed, amplified and transmitted in a single FSO channel. The transmitter as well as the receiver will be pointed and tracked by a high-precision fine-pointing mechanism. The state of the atmospheric transmission media will be monitored in realtime, allowing us to deepen our understanding of the optical satellite uplink transmission.

Free-space optical (FSO) communications is an attractive alternative to microwave technology in geostationary (GEO) communication satellite feeder link applications, due to the possibility of transmitting information with high data rate, small antenna size, secure communication, and no spectrum licensing requirements. However, optical links through the atmosphere suffer from scintillation effects caused by index of refraction turbulence of the air. It aggravates stable signal detection in the uplink scenario. The benefits of transmitter diversity to mitigate the fading effects in the uplink GEO feeder link are verified by the recently conducted ArtemEx measurement campaign using unmodulated optical beams. In this paper, the lab implementation of the transmitter diversity technique using a 10Gbps data signal and using measured fading vectors from the ArtemEx campaign is presented.

The goal of near real-time delivery of data from spacecraft in polar low Earth orbits to the customers can be achieved by utilization of Arctic and Antarctic hubs in a Ground Station Network. Additionally, the commanding of the satellites from these hubs can provide novel opportunities for spacecraft operations. The challenge of high-rate data repatriation from such remote sites can be met by means of laser communications from ground to geostationary satellites. We present system architecture and operations concept for a setup employing the European Data Relay System (EDRS) for data uplinks from an Antarctic ground station taking into account the impact of local atmospheric conditions.

Binary phase shift keying (BPSK) optical transmission in the C-band with coherent intradyne reception is demonstrated over a long range (10.45 km) link through the atmosphere. The link emulates representative channel conditions for geostationary (GEO) optical feeder up-links in satellite communications. The digital signal processing used in recovering the transmitted data and the performed measurements are described. Finally, the bit-error-rate (BER) results for 10 Gbit/s, 20 Gbit/s and 30 Gbit/s of the outdoor experiments are presented and compared with back-to-back measurements and theory.