Erich Leitgeb

Weather effects on hybrid FSO/RF communication link

Free space optics (FSO) or optical wireless systems provide high data rate solution for bandwidth hungry communication applications. Carrier class availability is a necessity for wide scale acceptability which is extremely difficult to achieve in the case of optical wireless links. FSO links are highly weather-dependent and different weather effects reduce the link availability. Employing a hybrid network consisting of an FSO link and a back up link in the GHz frequency range renders high availability besides providing comparable data rates. In this paper effects of fog, rain and snow on FSO/GHz hybrid network are studied so that GHz frequencies with best complementary behaviour can be selected as a back up link. As a prime conclusion of the article, it is suggested that free space optical links can be supplemented with 40 GHz RF links to achieve near carrier class availability.

Performance of PPM on terrestrial FSO links with turbulence and pointing errors

In recent work, the error performance of on-off keying has been investigated for free-space optical (FSO) links impaired by both turbulence and pointing loss. In the current letter, the analytical framework is extended to M-ary pulse-position modulation (PPM), providing exact results for M = 2. Since the approach is not directly applicable to PPM signals with M > 2, a closed-form approximation of the average symbol error probability is derived for this case, which is shown via simulation to be tight over a wide SNR range of interest.

Characterization of fog attenuation in terrestrial free space optical links

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 availabil- ity and reliability of terrestrial FSO links, as fog particles scatter the propagating light, causing nonnegligible attenuation. We present mea- surement 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 perfor- mance and enhanced resilience.

Characterization of Fog and Snow Attenuations for Free-Space Optical Propagation

Free Space Optics (FSO) is now a well established access technology, better known for its robustness in transmitting large data volumes in an energy efficient manner. However the BER performance of a FSO ground-link is adversely affected by cloud coverage, harsh weather conditions, and atmospheric turbulence. Fog, clouds and dry snow play a detrimental role by attenuating optical energy transmitted in terrestrial free-space and thus decrease the link availability and reliability. We measured the time variation of received optical signal level during continental fog and dry snowfall over a link distance of 80 m. We perform a detailed analysis of the continental fog and dry snow attenuation results and further characterise them by presenting some useful attenuation statistics and also showing their comparison with the corresponding measured density values collected by a particle sensor device. We propose also an empirical relationship between temperature, relative humidity and optical attenuation values for the continental fog case based on standard curve fitting technique.

Results of attenuation-measurements for Optical Wireless channels under dense fog conditions regarding different wavelengths

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.

Measurement of light attenuation in dense fog conditions for FSO applications

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.

High availability of hybrid wireless networks

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.

OOK Performance for Terrestrial FSO Links in Turbulent Atmosphere with Pointing Errors Modeled by Hoyt Distributions

Terrestrial free-space optical links are corrupted by both turbulence and misalignment losses. It is common practice to model turbulence by a gamma-gamma distribution and misalignments by a Rayleigh distribution, where the underlying assumption is that the jitter in vertical and horizontal directions is the same. In the current letter, we generalize this previous approach by modeling misalignments with a Hoyt distribution, which removes the assumption of an identical jitter variance along the horizontal and vertical axes.

Estimation of the power scintillation probability density function in free-space optical links by use of multicanonical Monte Carlo sampling

Free-space optics (FSO) can provide cost-effective, high-bandwidth, wireless connections. However, atmospheric turbulence may degrade the performance of FSO links by causing intensity and power scintillations at the receiver. Multicanonical Monte Carlo sampling is used in conjunction with the phase screen method to calculate the statistics, and particularly the probability density function (PDF), of the power fluctuations at an FSO receiver. This allows the efficient calculation of the PDF even for very small values with a limited number of iterations. The obtained PDF can be used to characterize the performance of the system in terms of the error probability.

BER and Outage Probability of DPSK Subcarrier Intensity Modulated Free Space Optics in Fully Developed Speckle

In this paper a differential phase shift keying (DPSK) subcarrier intensity modulated (SIM) free space optical (FSO) link is considered in negative exponential atmospheric turbulence environment. To mitigate the scintillation effect, the selection combining spatial diversity scheme (SelC) is employed at the receiver. Bit error rate (BER) and outage probability (Pout) analysis are presented with and without the SelC spatial diversity. It is shown that at a BER of 10-6, a maximum diversity gain 25 dB is predicted. And about 60 dBm signal power is required to achieve an outage probability of 10-6, based on a threshold BER of 10-4.