Bernhard Epple

Simplified Channel Model for Simulation of Free-Space Optical Communications

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 Satellite Downlinks to Optical Ground Stations and High-Altitude Platforms

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.

Broadband backhaul communication for stratospheric platforms : The stratospheric optical payload experiment (STROPEX)

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 of 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 64.3 km with a bit error rate of better than 10-9. This paper gives an overview of the stratospheric optical payload experiment, focusing on the airborne free-space experimental laser terminal (FELT). Additionally, the successful measurement campaign is described and the operation of the experiment is outlined.

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.

Using a GPS-aided inertial system for coarse-pointing of free-space optical communication terminals

In the Capanina project, it has been shown that it is possible to use solely GPS positions for coarse-pointing of a stratospheric free-space optical communication terminal. Although this system design has been shown to be functional, it has to deal with acquisition and reacquisition times of up to 20 seconds. Whenever the line of sight is blocked, the Capanina terminal has to do a reacquisition of the partner terminal. In land-mobile systems, e.g. transmission between a moving vehicle and a fixed station, where objects like trees or buildings can frequently block the line of sight, this type of acquisition/reacquisition is not acceptable. In this paper a revised system design will be shown, which reduces the initial acquisition time to below a second by using information from a GPS-Aided Inertial System (GPS-INS). It will also be shown how reacquisition times and coarse-pointing errors can be reduced to a minimum by using feedback from the optical fine-pointing device. Finally a demonstrator setup and some test results will be presented.

Concepts for fast acquisition in optical communications systems

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/UMTS), 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.

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.

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.

Optical high-capacity satellite downlinks via high-altitude platform relays

Earth-observation (EO) satellite missions produce a large amount of data using high-resolution optical or radar sensors. During the last decades the amount of data has steadily increased due to improved sensor technologies with increased temporal resolution, sensor resolution, and pixel count. As a consequence EO satellite missions have become limited by the downlink data rates of microwave communication systems, which are inhibited by spectrum restrictions, manageable antenna sizes, and available transmit power. Optical downlinks from EO satellites with data rates of several Gbps mitigate the limiting effects of microwave communication systems; however optical links do not provide the necessary link availability through the atmosphere due to cloud blockage above the ground station. Apart from diversity concepts with several ground stations or satellite networks, a stratospheric High Altitude Platform (HAP) could act as a relay station to forward the optical communication beam over the last 20km through the atmosphere to the ground station, where short-range, high data-rate microwave systems are feasible. This paper will discuss the capabilities of HAP and GEO relay stations to increase the downlink capacities of LEO satellites. Environmental aspects for the deployment of HAP relays and regulatory/technology issues for a microwave downlink on the last 20km to the ground will be discussed.

A cross-layer approach to mitigate fading on bidirectional free space optical communication links

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.