M. S. Awan

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.

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.

Analysis and evaluation of optimum wavelengths for free-space optical transceivers

The objective of this paper is to analyse and present the latest results obtained for free-space optics (FSO) within the EU COST Action IC-0802 and within the European Space Agency (ESA) contract. First, the FSO technology is briefly discussed and some performance evaluation criteria for FSO are provided. Some optical signal propagation experiments through the atmosphere (including the recent investigations in airborne and satellite application for FSO) are also shown. In the main part, considerations on suitability of different optical wavelengths are brought into question. The wavelength selection is dependent on the atmospheric effects and on the availability of receiver and transmitter components. Discussion on the available receiver(s) and transmitter(s) includes the focus on advantages and mainly the costs of the different systems. In the final part, we examine the latest practical results (carried out within the COST Action IC-0802) on modelling of the FSO channel under fog conditions and other atmospheric effects. Additionally, recent results, showing major performance improvement, based on the hybrid system and specific modulation and coding schemes are presented.

Prediction of drop size distribution parameters for optical wireless communications through moderate continental fog

Wireless optical communication links (OCL), or free space optics links involving optical ground stations are highly influenced by the earth atmosphere due to the interaction of the optical wave with particles of different size and shape. Fog, clouds, rain and snow cause significant signal attenuation, thus limiting the performance of OCL. In this paper, we consider the behavior of OCL in the troposphere under moderate continental fog conditions, which are important for both ground–ground and ground–space OCL. The impact of the droplet size distribution (DSD) of fog is investigated, by processing laser attenuation measurements carried out in Milan (Italy) and Graz (Austria). Significant differences are observed between measured and predicted attenuation when using standard values for the DSD parameters. Hence, new sets of DSD parameters are proposed to model peak, mean and median values of measured attenuation for moderate continental fog. These, in turn, can be useful to make accurate link availability predictions, thus improving the quality of service design for OCL. Copyright

Comparing the cloud effects on hybrid network using optical wireless and GHz links

Optical wireless link provides high bandwidth solution to the last mile access bottleneck. However, an appreciable high availability of the link is the basic requirement of any communication application. Free space optics (FSO) links are highly weather dependent and fog is the major attenuating factor reducing the link availability. Hybrid networks consisting of free space optics (FSO) link and back up link in the GHz frequency range renders high availability besides providing comparable data rates. The back up link should be nearly immune to fog attenuation for achieving carrier class availability. In this paper effects of fog on FSO and GHz frequency range links are studied so that frequencies with best complementary behaviour can be selected as a back up link.

Cloud attenuations for free-space optical links

Free-space Optics (FSO) or optical wireless offers huge unlicensed bandwidth and is able to deliver the required bandwidth needed in next ten years for the next generation networks (NGN). Moreover, an increasing number of military and commercial access systems rely critically on the propagation of optical energy through the atmospheric free-space. As a result the need for predictable estimates of optical attenuation in low visibility conditions is steadily increasing, especially for fog, rain, clouds and snowfall. Cloud attenuations are the least observed/measured in case of free-space optical wireless links. We investigate here the influence of cloud attenuations on the performance of optical wireless links.

Distribution function for continental and maritime fog environments for optical wireless communication

Free space optics (FSO) links in the troposphere are highly dependent on different atmospheric effects, which vary according to different climatic conditions. For terrestrial FSO links, fog is the most critical of all atmospheric effects causing optical signal attenuations up to 480 dB/km in dense maritime fog environments and 130 dB/km in moderate continental fog environments. In this paper a brief analysis of modified gamma particle size distribution (PSD) parameters for dense maritime fog and moderate continental fog is presented and discussed on the basis of standard reference data available. Moreover, an effort was made to briefly analyse and compare our measured continental and maritime fog attenuation data on the basis of Mie scattering parameters and the standard modified gamma PSD parameters, in order to estimate the values of modified gamma parameters and the total attenuation for the two kinds of fog environments.

Weather Effects Impact on the Optical Pulse Propagation in Free Space

Optical wireless links offer gigabit per second data rates and low system complexity. For ground-space and or terrestrial communication scenarios, these links suffer from atmospheric loss mainly due to fog, scintillation and precipitation. We investigate here the impact of fog, rain and snow effects and evaluate their performance on the basis of attenuation data collected for the optical pulse propagated through the troposphere. I. FSO INTRODUCTION Free-Space Optics (FSO) or optical wireless communication is the concept of transmitting very high bandwidth digital data using laser beam directly through the atmosphere. Recently FSO -links are identified as an attractive alternative to the existing radio links for applications involving ground-to-ground (short and long distance terrestrial links), satellite uplink/downlink, inter- satellite, satellite or deep space probes to ground, ground-to- air/air-to-ground terminal (UAV, HAP etc.). Moreover, growing demands for higher data rates and wider bandwidths from the end user to manipulate multimedia information in the recent years allegorises a challenge for the future Next Generation Networks (NGN). The prime advantages of FSO

Optical wavelengths comparison for different weather conditions

Free space Optic (FSO) links have the tremendous potential to fulfil high data rate requirements of future communication applications. However, the widespread growth of FSO has been hampered by availability and reliability issues. FSO links are highly weather dependent and attenuating factor like fog, rain and snow causes reduced link availability for considerable amount of time. However different optical wavelengths suffer different attenuation. In this paper, the attenuation of different wavelengths FSO links has been analysed under different weather attenuating condition like fog, rain and snow. The attenuation analysis has been compared using measured attenuation data for some of the wavelengths.

Further results on fog modeling for terrestrial free-space optical links

Recent measurement campaigns in Prague, Czech Republic have recorded the specific attenuation caused by fog in terrestrial free-space optical (FSO) links along with visibility, liquid water content (LWC), and integrated particle surface area for operational wavelengths of 1550 and 830 nm. Data recorded during 5 months of measurements has been analyzed in an attempt to validate and improve the empirical models for LWC and visibility-based attenuation and the probability density function (PDF) estimation of the received signal strength (RSS). The results strongly suggest that further attempts in refinement of the empirical modeling for visibility-based attenuation or LWC-based attenuation shall move towards the establishment of a global data bank of fog attenuation measurements in different climatic regions. This data bank can then help develop generic models which could lead to better system design of future terrestrial FSO links. The PDF estimation of RSS puts forth the Kumaraswamy distribution as the best fit for settled continental fog conditions.