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.
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 turbulenceinduced
fades. Outages on a longer term (second scale) producing large data loss can be mitigated by packet-layer largeblock,
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.
KEYWORDS: Automatic repeat request, Receivers, Free space optics, Channel projecting optics, Error control coding, Local area networks, Data transmission, Atmospheric propagation, Unmanned aerial vehicles, Signal to noise ratio
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.
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.
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 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.
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.
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.
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