Free-space optics (FSO) has gained considerable importance in this decade of demand for high-bandwidth transmission capabilities. FSO can provide the last mile solution, but the availability and reliability issues concerned with it have received increasing attention and need thorough investigation. In this work, we present our results on fog attenuation measurement and prediction at wavelengths 650, 850, and 950 nm with peak values up to 500 dB/km. For the attenuation measurement, optical wavelengths are transmitted over the same path of fog in free air to a receiver, measuring the power at every wavelength. The measurement of fog attenuation was performed at the France Telecom R&D test facility at La Turbie (in the south of France). We compare our measurement data with the commonly used light attenuation models (Kruse and Kim models) and with a new proposed model allowing the prediction of fog attenuation for the 0.69- to 1.55-µm spectral band. We present some interesting insights and discuss the wavelength dependence. The practical measurements described promise to validate the models and therefore should lead to more accurate availability prediction for FSO links.
KEYWORDS: Fiber optic gyroscopes, Signal attenuation, Free space optics, Visibility through fog, Visibility, Particles, Data modeling, Modulation, Light scattering, Aerosols
Terrestrial free space optical (FSO) links are based on the simple concept of a light beam carrying information, thus facilitating very high data rates. Fog remains the major hurdle in increasing the availability and reliability of terrestrial FSO links, as fog particles scatter the propagating light, causing nonnegligible attenuation. We present measurement results from our campaigns carried out at the continental city of Graz and at La Turbie near Nice, the southern coast of France. We perform a detailed analysis of the measurement results providing time-series analysis of these fog measurements and a comparison between maritime and continental fogs. Based on our measurement analysis, we provide insight into designing efficient FSO systems, with better performance and enhanced resilience.
Free Space Optics (FSO) has gained considerable importance in this decade of demand for high bandwidth transmission capabilities. FSO can provide the last mile solution, but the availability and reliability issues concerned with it can not be ignored, and requires thorough investigations. In this work, we present our results about light attenuation at 950 and 850 nm wavelengths in continental city fog conditions with peak values up to 130 dB/km and compare them with attenuation under dense maritime conditions with peak values up to 480 dB/km. Dense fog is the most severe limiting factor in terrestrial optical wireless applications and light propagation in fog has properties in the spatial, spectral and the time domain, which are of importance to free-space optic data communication. In 2004 (within a short term scientific mission of COST 270) measurements of very dense maritime fog and low clouds were made in the mountains of La Turbie, close to the coast of southern France. Using the same equipment, the measurements were continued for the conditions of the continental city of Graz, Austria. This campaign was done in the winter months from 2004 to 2005 and 2005 to 2006 and allows us to compare fog properties for different environments, and the impact of snow fall. We provide detail analysis of a fog and a snow event for better understanding of their attenuation behavior.
Free Space Optics (FSO) has gained considerable importance in this decade of demand for high bandwidth transmission capabilities. FSO can provide the last mile solution, but the availability and reliability issues concerned with it have acquired more attention, and a need for thorough investigations. In this work, we present our results about fog attenuation at the 950 and 850 nm wavelengths in heavy maritime fog with peak values up to 500 dB/km. For the attenuation measurement, optical wavelengths are transmitted over the same path of fog in free air to a receiver, measuring the power of every wavelength. The RF marker technology employed takes advantage of modulating every optical wavelength with an individual carrier frequency, allowing to use one optical front end for the receiver and to separate individual wavelengths by electrical signal filters. The measurement of fog attenuation at different wavelengths was performed at the France Telecom R & D test facility at La Turbie. Maritime or advection fog, which caused the light attenuation consists of water droplets of larger diameter in the order of 20 μm and can cause visibilities as low as 30 meters. The visibility was measured using a transmissiometer at 550 nm. We compare our measurement data with the commonly used light attenuation models of Kruse and Kim, and present some interesting insights. The practical measurements described try to validate the models and therefore should lead to a more accurate availability prediction for FSO links.
In this paper Free Space Optics (FSO) in combination with Wireless LAN and Satellite Communications in Civil-Military-Cooperation (CIMIC) is presented. At first a modular communication system developed at the TU Graz is shown, which allows worldwide access to the Internet or other networks by combining Satellite Communications, FSO and Wireless LAN. Wireless LAN offers connectivity to mobile users in a network cell, Free Space Optics allows quick installation of broadband fixed wireless links instead of cables and Satellite Communications provides a backbone between distant locations in the world.
In the second part results of Free Space Optics- and Satellite applications installed at a civil-military exercise in Spring 2004 in Styria (southern part of Austria) are presented. In this civil-military cooperation a mobile Satellite Earth Station (equipped with FSO and Wireless LAN) was used for Videoconferencing between military and civil organisations.
Free Space Optical (FSO) links offer high bandwidth and the flexibility of wireless communication links. However, the availability of FSO links is limited by weather patterns like fog and heavy snowfall. Microwave based communication links operating at high frequencies (40 - 43 GHz) have similar characteristics like high data rates and needed line-of-sight. Link availability for microwave systems is limited by heavy rain. Combining FSO links with microwave links within a hybrid FSO/microwave communication network has the advantage of added redundancy and higher link availability. Measurements over a period of one year show a combined availability of 99.93% for the climatic region of Graz (Austria) which proves that the combination of both technologies leads to a highly available wireless connection offering high bandwidth.
KEYWORDS: Free space optics, Data storage, Video, Signal attenuation, Visibility, Internet, Visibility through fog, Atmospheric optics, Fiber optic gyroscopes, Receivers
The satisfaction of all communication needs from single households and business companies over a single access infrastructure is probably the most challenging topic in communications technology today. But even though the so-called "Last Mile Access Bottleneck" is well known since more than ten years and many distribution technologies have been tried out, the optimal solution has not yet been found and paying commercial access networks offering all service classes are still rare today. Conventional services like telephone, radio and TV, as well as new and emerging services like email, web browsing, online-gaming, video conferences, business data transfer or external data storage can all be transmitted over the well known and cost effective Ethernet networking protocol standard. Key requirements for the deployment technology driven by the different services are high data rates to the single customer, security, moderate deployment costs and good scalability to number and density of users, quick and flexible deployment without legal impediments and high availability, referring to the properties of optical and wireless communication. We demonstrate all elements of an Ethernet Access Network based on Free Space Optic distribution technology. Main physical parts are Central Office, Distribution Network and Customer Equipment. Transmission of different services, as well as configuration, service upgrades and remote control of the network are handled by networking features over one FSO connection. All parts of the network are proven, the latest commercially available technology. The set up is flexible and can be adapted to any more specific need if required.
Free Space Optics (FSO) is an excellent supplement to conventional radio links and fiber optics. It is the broadband wireless solution for closing the "Last Mile" connectivity gap throughout metropolitan networks. FSO is useful in dense urban and urban applications. The realiabilty and availability of the FSO-link is mainly determined by the local atmospheric condition. In this paper the results of the investigations of the research group "OptiKon" on different installed common network-architectures (ring, mesh and star) are presented. An "Optical Repeater" was used, if there was no line of sight between transmitter and receiver. The usage of optical free space propagation wthi Light Emitting Diodes are source instead of conventionally broadband transmission systems like optical fibers or radio links was also investigated at the Department of Communications and Wave Propagation. Using extended light sources like LEDs and/or multiple transmitter optics due to new laser safety regulations allows multiple output power within the same laser class, increasing the power link budget especially for short range FSO systems. In a first step two families of cost-effective systems were developed and realized. One is based on a modular concept using available standard components, allowing demonstrates at 2×10 Mbps data rae. Another is used for the larger distnaces of about 300 m at a specific power margin of 25 dB/km. Systems for higher data rate are under development. Additionally some ideas and their possible realization for point-to-multipoint configuration are considered. In the last chapter a short overivew about the main results of the work of "OptiKom" is discussed.
The coherent wave propagation is affected by the atmosphere in many ways. Several theoretical models for propagation of light through the atmosphere are well known. To predict link availability in different climate zones it is necessary to do field tests for data acquisition. Therefore we have done reliability- and availability-tests on commercial available and also on self-developed optical point-to-point and point-to-multipoint systems. We sent test data at 155 Mbps (STM-1) from one FSO-unit to a distant (2.7 km) FSO-unit. The received data were sent back (loop) to the first unit. Our primary interest in this long-time investigation was the time of link failure, because it turned out that BERs be low in general, less than 10-8 at very bad weather conditions in winter and less than 10-12 at clear sky. In a second measurement campaign we investigated the influence of turbulences in the air. The measurements clearly show variations in the fluctuation of the incoming optical power during a day. In principle there are two periods with strong variations, during the day and during the night, and two periods of rather stable air, these are around sunset and sunrise. The power variations have the highest amplitude and show the fastest changes at noon and they are less distinct and show slower changes in the night. As a medium value we got power variations of 4 dB over the distance of 2.7 km in summer. The duration of fades/scintillations was in the order of 4 to 60 milliseconds at daytime and about 10 to 150 ms in the night.
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