Optical wireless communications over long-range atmospheric links experiences strong fading that heavily influences the performance of communication systems. Most research on this topic is focused on simulation or measurement of the link performance in terms of the bit error ratio. In this work a statistical channel model derived from measurements is used for simulations of the link performance on packet layer. For analysis of a possible improvement of packet layer performance by error protection techniques like forward error correction and automatic repeat request, additional simulations are done. All simulations are done for several communication scenarios like the maritime environment, land mobile and air-to-ground links

Compared with traditional communication technologies like wired or radio frequency communications, optical wireless communication has a unique fading behavior of the received signal, that does not allow to use existing channel models without modification. In this paper the statistics of received optical power obtained from experimental data are compared to often used statistical mathematical models. These models are the log-normal and the gamma-gamma distribution. It was found that the gamma-gamma gives better firs to the measured data, but the quality of the log-normal fits is sufficient for most needs. This means that the log-normal distribution can be used for the development of simplified channel models which have a better mathematical tractability than the models based on the gamma-gamma distribution.

Free-space optical (or optical wireless) communications represent an attractive technology for the realization of high-bandwidth wireless communications. However, for propagation through the atmosphere the characteristics of the optical signal are different from those of the signals from, e.g., fiber channels or radio-frequency wireless channels, and therefore the error characteristics on these links are also different. For evaluating fading mitigation techniques for optical wireless communications a channel model is needed that can be easily used by protocol designers. Existing channel models for optical wireless communications are based on atmospheric propagation theory and therefore require a deep physical understanding of the optical propagation through the atmosphere. The goal of this publication is to develop a simplified approach for modeling the received power dynamics of the atmospheric free-space optical channel. The proposed model consists of a random number generator and a low-pass filter and is therefore easy to implement and use. This approach is only valid for systems utilizing intensity modulation with direct detection, but this limitation is acceptable since most commercially available systems use this modulation format. The channel model is developed based on the statistics of received power measurements from a maritime-mobile link, a land-mobile link, and a satellite downlink.

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.

Optical wireless communications is an emerging technology for various types of high-bandwidth wireless communications. Several application scenarios involving links between aircraft, high altitude platforms, unmanned aerial vehicles and others have been proposed over the years. For simplicity of the propagation models and mathematical tractability often horizontal links with homogenous propagation characteristics along the path are used for calculations of atmospheric effects. This simplification assumes a flat earth model, where the ground profile underneath the optical link is ignored. For long range links also a curved earth model is used, but again the shape of the ground below the link is ignored. For modeling atmospheric turbulence, it is quite common to use the Hufnagel-Valley model. This model is based on the height above ground and therefore it is sensitive to the ground profile. The impact of the flat earth or curved earth assumptions on the scintillation of the propagating beam is discussed within this work. It is shown that the ground profile has the strongest impact on links at lower altitudes, while for higher altitude links a curved earth model is sufficient. The flat earth model can be best used at altitudes of around 10000 m above mean sea level.

Mobile free-space Laser communications is the next frontier for net-centric connectivity, as bandwidth, spectrum and security issues drive its adoption as an adjunct to RF communications. However, key technology challenges must be addressed and mission requirements understood for it to emerge as a meaningful capability for dynamic tactical communications environments. We present a brief overview of the current military communication infrastructure and future needs, before highlighting terrestrial Lasercomm technology status and trends, and lessons learned from several experiments and field tests. Optical link service parameters and metrics are assessed in the context of available architecture and techniques, while the effectiveness of various technologies to mitigate atmospheric turbulence, poor weather and pointing/tracking inaccuracies is analyzed for improved reliability. Finally, the ramifications of cost and SWaP (size, weight and power), critical to tactical Lasercomm deployment, are considered in narrowing down these technology options.

This paper presents an extensive insight into error protection techniques for free space optical links, focusing in particular in aeronautic stratospherically applications. The long distances present in these scenarios along with challenging atmospherically conditions present significant obstacles that degrade link performance. Thus it is imperative to apply highly efficient error protection scheme to avoid unacceptably high loss rates. The goal was to design a point-to-point data link layer error protection protocol that allows user-transparent bridging of Fast Ethernet data transmission over the optical fading channel in an high altitude inter platform link environment.

Free-space directional communication links (FSO) experience short-term link outages or fades because of atmospheric turbulence, and longer term link outages because of obscuration resulting from either atmospheric conditions, beam pointing errors, or temporary line-of-sight obstructions for links from mobile or static nodes. Various approaches can be used to mitigate these effects. Physical-layer techniques, such as dynamic thresholding, time delayed diversity, and data encoding can significantly reduce the effects of short term (millisecond scale) outages caused by deep turbulence-induced fades. Outages on a longer term (second scale) producing large data loss can be mitigated by packet-layer large-block, error protection techniques. In this paper, we will first introduce physical-layer mitigation techniques. Second we present experimental data comparing the latencies and throughput of different means of packet-based error protection techniques. We will discuss the influence of error protection techniques to quality of service issues like error probability and delay and further compare this with service requirements given by the application.

This paper addresses various aspects for designing and implementing a bridging protocol for reliable data transfer between two local area networks over the high data rate slow-fading free-space optical channel. First the service requirements of widely spread applications of today's life like voice communication, video streaming or file transfer are given and the resulting constrains for data transfer protocols are compiled. Then, based on the physical characteristics of different communication scenarios and the compiled results, the feasibility of these services on optical free-space links is studied. Finally different protocol design aspects are discussed and an implementation concept for bridging data of different services over free-space optical links is presented.

In free-space optical (FSO) communications the transmitted signal is subject to different fading effects. These effects can cause short-term outages of a few milliseconds, caused by atmospheric turbulence-induced fading, and long-term outages up to a few seconds duration caused by line of sight obstructions or pointing errors. To mitigate these effects, several different approaches have been presented in the past. At the physical layer, forward-error correction (FEC), dynamic thresholding, and time-delayed diversity (TDD) have been shown to be reasonable solutions. At higher layers, FEC has also been demonstrated to be a possible solution, but it imposes a penalty on channel throughput. For bidirectional communications, automatic repeat request (ARQ) protocols are proposed to be a more efficient solution. In this paper we will investigate physical layer delay diversity as well as link layer FEC and ARQ.

The ATENAA-Project[4] aims on the development and validation of several technologies that are able to offer broadband communication services to civil aircraft passengers. The proposed Mobile-Ad-Hoc-Network (MANET) in conjunction with broadband data links would allow high data rate services, like In-Flight-Entertainment such as Video-On- Demand or High-Speed Internet Connections. Besides that, flight-relevant information with high amounts of data, e.g. high-resolution weather maps, could be made available for airplane-captains and -crews. This paper is intended to give an overview about the optical data link system that has been investigated in the scope of ATENAA. It provides a broadband communication link to enable the mentioned services. The developed validation platform consists of a Fixed Terminal (FT) as well as a Mobile Terminal (MT) mounted on top of an airplane simulator. A reliable Fast-Ethernet service has successfully been demonstrated. Furthermore, the results of a measurement campaign, which was performed in Oberpfaffenhofen near Munich at the End of January 2007, are presented and an outlook to the follow-up project MINERVAA is given.

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.

The mobile free-space optical channel mainly suffers from relatively long link outages, produced by short-term blockings of the line-of-sight (obstacles, clouds), pointing- and tracking-errors or deep signal-fades caused by index of refraction turbulence effects. This paper discusses the applicability of commonly used communication protocols like UDP, TCP, ARQ and the SCPS-TP from the Space Communications Protocol Standards (SCPS) in various scenarios. The performance of the protocols in the selected scenarios is evaluated using the simulation software OMNeT++. The simulations are based on channel measurements from the three FSO demonstrations FASOLT (61 km Ground - Ground link), KIODO (LEO satellite downlink), and ATENAA (land-mobile link)- and from ongoing measurements at the German Aerospace Center (DLR) (short-range Ground - Ground) as part of the MINERVAA project. Based on the simulation results, recommendations for protocols in free-space optical communication scenarios are given.

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.

Earth-observation (EO) satellite missions using high-resolution optical or radar sensors are producing an immense amount of data which needs to be send down to earth. The fraction of satellite operational time in future missions is therefore clearly limited by the downlink-capability. The current X-band architecture is facing its technological limitations in terms of data rate while causing increased demand on antenna-sizes and transmit power. This bottleneck can be overcome by direct optical downlinks from EO-satellites to the ground with multi-gigabit data rates. According optical satellite terminals will be extremely small and light-weight and will require few transmit power, but one drawback is the link blockage by thick clouds. This can be overcome either by ground station diversity and careful site selection or by using optical terminals onboard high altitude platforms which serve as relais-stations for the satellite. Here we present feasibility and expected performance of these two optical scenarios and propose according space and ground station architectures.

As free-space laser communications systems proliferate due to improved technology and transmission techniques, optical communication networks comprised of ground stations, aircraft, high altitude platforms, and satellites become an attainable goal. An important consideration for optical networks is the ability of optical communication terminals (OCT) to quickly locate one another and align their laser beams to initiate the acquisition sequence. This paper investigates promising low-cost technologies and novel approaches that will facilitate the targeting and acquisition tasks between counter terminals. Specifically, two critical technology areas are investigated: position determination (which includes location and attitude determination) and inter-terminal communications. A feasibility study identified multiple-antenna global navigation satellite system (GNSS) systems and GNSS-aided inertial systems as possible position determination solutions. Personal satellite communication systems (e.g. Iridium or Inmarsat), third generation cellular technology (IMT-2000/LTMTS), and a relatively new air traffic surveillance technology called Autonomous Dependent Surveillance-Broadcast (ADS-B) were identified as possible inter-terminal communication solutions. A GNSS-aided inertial system and an ADS-B system were integrated into an OCT to demonstrate their utility in a typical optical communication scenario. Testing showed that these technologies have high potential in future OCTs, although improvements can be made to both to increase tracking accuracy.

A high bitrate optical downlink was performed by the stratospheric optical payload experiment (STROPEX), a part of the EU CAPANINA project. The STROPEX objectives were to design and build the necessary hardware to demonstrate an optical backhaul downlink from a stratospheric platform to the ground and to carry out channel measurements on the link. A successful measurement campaign at ESRANGE, near Kiruna (Sweden), achieved all these objectives. The transportable optical ground station received an almost error free 1.25 Gbit/s data signal from the payload over a distance of 64.1 km with a bit error rate of better than 10-9. This paper gives an overview of the stratospheric optical payload experiment including the airborne free-space experimental laser terminal (FELT) and the transportable optical ground station (TOGS). Additionally, the successful measurement campaign is described and the operation of the experiment is discussed. Finally, selected measurements are presented highlighting significant data transmission results, pointing and tracking accuracy, and thermal subsystem effectiveness.

A high bitrate optical downlink was performed from a stratospheric balloon. The objectives of the stratospheric optical payload experiment (STROPEX), a part of the EU project CAPANINA, were to plan and build the necessary hardware to demonstrate and carry out channel measurements on an optical backhaul downlink from a stratospheric platform test bed to the ground. During the stratospheric campaign at ESRANGE, near Kiruna, Sweden, all these objectives were achieved. The transportable optical ground station received a nearly error free 1.25 Gbit/s data signal from the stratosphere. The distance was 64.1 km and the bit error rate better than 10-9. This paper gives an overview of the optical terminal, the successful trial, and scientific results. First, the developed optical terminal with precise autonomous acquisition abilities is explained. Next, the experiment, trial realization, and boundary conditions are outlined. Finally, the principles of operation of the turbulence instruments, which monitored the characteristic atmospheric parameters during data transmission, are explained and measurement data are presented.