Hennes Henniger

Fading-loss assessment in atmospheric free-space optical communication links with on-off keying

Link-budget calculations are a common way to assess system parameters, such as the required transmitter power and receiver sensitivity, in free-space optical (FSO) communication systems. One of the biggest challenges for long-range FSO deployment is its signal propagation under turbulent atmospheric conditions, which produce intensity fluctuations. Methods to estimate atmospheric-fading loss in radio-frequency systems cannot be adapted to the FSO channel. Until now no general closed-form methods have been developed to describe the fading loss in such a channel. A method to calculate the losses due to scintillation fading in the threshold approach, based on lognormal statistics of the received power, is presented.

Evaluation of FEC for the atmospheric optical IM/DD channel

Channel measurements were performed by the German Aerospace Center in various near ground optical channels including a 1.5 km horizontal path as well as a 61km path. These measurements clearly showed that the atmosphere causes very slow fading (compared to the high data rates usually used in optical communication systems), which significantly degrades the transmission quality. As transmitter power and receiver sensitivity are limited by the transmission technology, fading depicts a severe problem, that can be reduced by the use of forward error correction schemes (FEC) in order to improve system performance. Therefore FEC was subject of investigations by means of simulations. To figure out which FEC methods are useful for applications in the atmospheric optical channels simulations of standard block codes and interleavers have been done. They were based on data sets taken in the various channel measurements. The simulations point out that only very long interleaving can increase performance significantly.

Multiple wavelength free-space laser communications

Free-space optical communications systems in the atmosphere, based on intensity modulation and direct detection, are heavily affected by fading caused by turbulence cells of varying scale and motion. Several data sets of fading measurements under different scenarios have been recorded demonstrating this effect. In this paper we introduce a form of free-space laser communications involving a source operating on several wavelengths. The goal is to overcome atmospheric interference on a communications link. We have performed simulations using the DLR PILab Matlab toolbox. These indicate the extent to which the turbulence and beam properties interact. Experimental investigations are planned. Further properties are also taken into account, including the choice of appropriate laser bandwidth and wavelengths, the effect of atmospheric absorption from aerosols and molecular absorption lines, as well as effects of atmospheric structure on beam propagation. Possible scenarios for application of this scheme will be presented as well.

Discussion on design aspects for free-space optical communication terminals

Free-space optical communications is an emerging solution to increase link capacity of nondetectable links to multi gigabits per second. FSO is widely believed as to overcome radio frequency spectrum and data rate limitations. However, one of the biggest challenges facing FSO deployment is its optical signal propagation in different atmospheric conditions. This effect causes variable link degradation due to variable attenuation and fading. Atmospheric turbulence produces temporary pockets of air with slightly different indices of refraction. Therefore the laser beam phase-front varies randomly, producing intensity fluctuation. One further key challenge with FSO systems is maintaining transceiver alignment. Using highly directional and narrow beams of light, variable mispointing of the transmit beam, and tracking errors of the receiver will generate additional signal fading. In this article terminal design aspects are discussed, and problems that occur when building FSO terminals are highlighted.

Measurements of the Beam-Wave Fluctuations over a 142-km Atmospheric Path

An optical link has been established between the Canary Islands La Palma and Tenerife. A 1064-nm transmitting laser was located on La Palma whereas a BPSK communication receiver and measurement instruments were installed in ESAs OGS on Tenerife. Beside the demonstration of a high-data-rate coherent signal transmission, the goal of the experiment was to measure the effects of the atmosphere on the beam propagation in order to estimate its impact on optical links. In particular, wavefront distortions have been investigated by means of a DIMM instrument and scintillation was observed by imaging the pupil of the OGS telescope on a CCD camera. Strong scintillation was observed during all the experiment with scintillation peaks at sunsets and sunrises, and saturation at about noon. Because of the narrowness of the beam (15-μrad divergence), beam wander has been a serious issue. Statistical results are compared with theory. Recommendations regarding the specifications of optical coherent systems in such detrimental conditions are given.

Optical communication in free space

The success of free-space optical links operating indoors or in the atmosphere or deep space shows a good perspective of the technology. There is no doubt that free-space technology is ready for wide practical application. Optical links have several advantages in comparison with radio links, namely high transmission rates and high security. Advance in systems for the detection and tracking of moving objects allowed the development of mobile optical wireless links. This paper is focused on satellite and mobile optical links. Included are basic characteristics of such links and current results of international research projects.

Packet-layer forward error correction coding for fading mitigation

In an on/off-keying atmospheric optical communication system, index of refraction turbulence and PAT (pointing, acquisition and tracking) instabilities create strong and very slow fading compared to the usually used high data-rates. With this channel behavior, forward error correction can only work effectively if the codeword-length is longer than the mean duration of fade. Therefore packet-layer coding is a favorable way to implement a code with codewords longer than typical fading events. A code validation platform based on Ethernet transmission technology and packet-layer coding using UDP packets was built up to prove the performance of packet-layer coding. In this paper a demonstrator setup as well as results from a free-space transmission experiment are presented.

Mobil FSO Activities in Europe and Fading Mitigation Approaches

The increasing emergence of data services for mobile applications requires high-speed communication technologies. To this end, free-space optical (FSO) communications technology has the potential to outperform radio frequency (RF) systems. Within the last years, the German Aerospace Center (DLR) has performed several demonstrations of mobile FSO systems. A brief overview of these activities is given in this paper. Mobil FSO mainly suffers from relatively long link outages, produced by temporary obscured laser-beams, pointing-and tracking-errors or deep signal-fades caused by index of refraction turbulence effects. Error correction and retransmission techniques for fading mitigation are discussed in this paper. Comparison of FEC and protocol based error correction for mobile FSO transmission is presented.

Design Considerations for Optical Inter-HAP Links

A perspective for future high data-rate communication systems are quasi-geostationary high altitude platform (HAP) systems, where communications platforms are placed in the stratosphere at altitudes between 20 and 30 km and are kept stationary. Situated over congested urban areas they will be capable of handling high data rate traffic like a geostationary satellite. Due to the specific altitudes of these platforms, communications cross links between either two platforms are not as severely affected by Earth´s atmosphere, as near-ground links are. Hence this scenario is very well-suited for optical inter-platform communications. Nevertheless such optical links suffer from various atmospheric influences that we will discuss here. In particular this paper addresses intensity fluctuations due to atmospheric turbulence, atmospheric attenuation due to absorption and scattering, and background radiation. Platform vibrations and according mispointing and –tracking losses are regarded as well. It turns out that possible links distances are very much limited by vibrations, scintillations and background light. The terminal design is generally a trade-off between those effects. From an extensive investigation of all influence factors, hints for the design of optical cross links in HAP systems will be given in terms of suitable transmitter divergence angles and receiver field of view. Link budget calculations clearly prove the feasibility of optical cross links. However, they as well show the limitations to optical communications in this particular application.

Maritime Mobile Optical-Propagation Channel Measurements

Optical free-space communications (FSO) is an emerging solution to operate very high capacity non-detectable links. The performance of laser communication systems is reduced by random power fluctuations of the received signal. In this paper fade statistics obtained from experimental data were compared with statistical mathematical models and it was found that the log-normal model fits well under a wide range of turbulence conditions. Therefore we investigate in a more practical way the performance of laser transmission in a commercially available mobile communication system. Received power vectors were collected during a ship-to-land communication trial in summer 2008 at the Baltic Sea.