Lasercomm Activities at the German Aerospace Center’s Institute of Communications and Navigation

The German Aerospace Center (DLR) has a heritage of more than 25 years in working on optical inter-satellite and satellite-to-ground links. The Institute of Communications and Navigation (IKN), as a research organization of DLR, has developed coherent homodyne BPSK transmission schemes with world record sensitivity as they are now implemented in the space-proven Laser Communication Terminals (LCT) for the European Data Relay System (EDRS). Further research being pursued at IKN includes the development of transmission systems optimized for atmospheric scenarios such as LEO downlinks, aircraft downlinks and inter-HAP links (High Altitude Platforms). For such scenarios with extreme index-of-refraction turbulence, robust adaptive optics technologies have been investigated and suitable data transceivers have been tested. Furthermore, several verification campaigns with prototype flight terminals and optical ground stations (fixed and transportable) have been performed in recent years, providing a large data basis for optimizing the long-range FSO technology

Improved Markov Models for Terrestrial Free-Space Optical Links

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

Adaptive HARQ with Channel State Information in Inter-HAP FSO Links

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

Free-Space Optical Data Receivers with Avalanche Detectors for Satellite Downlinks Regarding Background Light

Data Receiver Frontends using Avalanche Photodiodes are used in optical Free-Space Communications for their effective sensitivity, large detection area, and uncomplex operation. Precise control of the high-voltage necessary to trigger the avalanche effect inside the photodiode depend on the semiconductor excess noise factor, temperature, received signal power, background light, but also the subsequent transimpedance amplifier thermal noise behavior. Several prerequisites have to be regarded and are explained in this document. We focus on the application of avalanche photodiodes as data receivers for on/off-keying modulated bit streams with 50% duty cycle. Also, an experimental verification of the receiver performance with background light is demonstrated

Performance Analysis of Adaptive Hybrid ARQ for Inter-HAP Free-Space Optical Fading Channel with Delayed Channel State Information

In this paper, we present simulative performance analysis of Hybrid Automatic Repeat reQuest (ARQ) with adaptive rate Reed-Solomon (RS) code under delayed Channel State Information (CSI) at the transmitter over inter-HAP (High Altitude Platform) FSO fading channel. The combined effect of atmospheric turbulence and temporal behaviour of the channel were investigated. The performance analysis is determined in terms of Transmission Efficiency (TE) via event based simulations. The results enable us to analyse the benefit of adaptive rate HARQ in presence of delayed CSI so as to maximize the transmission efficiency

Optical Data Downlinks from Earth Observation Platforms

The increasing resolution of earth observation sensors will require much higher data rates for the data downlink in future than is feasible with conventional RF-technology. This applies for earth observation satellites as well as for aeronautic observation platforms, such as aircraft or stratospheric high altitude platforms. The most promising solution for this data downlink bottleneck is the application of optical free space transmission technologies. DLR has built diverse atmospheric flight terminals and performed several trials of optical downlinks from space (together with partnering organizations) as well as from atmospheric carriers in recent years. Here we present and compare results of such communication system trials

Adaptive Symbol-Rate Free-Space-Optical Communications

Free space optical communications through fading atmospheric channels can achieve improved throughput by using adaptive transmission techniques. This paper considers the use of adaptive symbol-rate and transmit-power methods. We use an idealised model of channel coding that assumes zero errors can be achieved above some SNR threshold and adjust the transmission rate and power as the channel varies to operate above the required threshold. Results are presented for two noise models appropriate to non-coherent photodetection. Substantial performance gains are demonstrated compared to non-adaptive transmission. Most of the benefit from adaptive power control can be obtained with modest peak to average power constraints

Quantifying the Effect of Atmospherically-Induced Pointing Errors in Optical Geostationary Satellite Feeder Links Using Transmitter Diversity

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 though simulations for a two-fold transmitter diversity to obtain minimum overall power scintillation index (PSI)

High-Altitude-Platform Systems und deren Vernetzung mit optischer Freistrahl-Kommunikation

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

Link budget calculation in optical LEO satellite downlinks with on/off-keying and large signal divergence: A simplified methodology

Direct-to-Earth transmissions with Optical On/Off-Keying are becoming the method of choice to realize telemetry downlinks from low Earth orbit satellites at highest data-rates. Here, we review the calculation procedure for a practical assessment of the mean link budget in this space-ground data communication technology. We present a comprehensive survey of the dynamic orbital and beam-pointing effects as well as the impacts from atmospheric attenuation on the link performance. The paper provides an exhaustive review of the formulas commonly used, and propounds a recipe to reliably estimate the received power on ground. An overview of typical data transmitter terminals, transmission channel parameters, and the according optical ground stations is provided. Comparison with measured received powers over transmitter elevation angle, and the respective design estimates serves for verification