F. Nadeem

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.

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

The huge bandwidth of optical Wireless link motivates to use it for high data rate demanding communication applications. However, an appreciable high availability of the link is the basic requirement of any communication link. Free Space Optics (FSO) links are highly weather dependent. One of the important attenuating factors is snow that causes reduced the link availability. The use of back up link can provide the solution. Hybrid networks of Free Space Optics (FSO) link and back up link in the GHz frequency range improves the availability besides providing comparable data rates. The back up link should be nearly immune to different attenuation factors including snow for achieving carrier class availability. In this paper effects of dry snow on FSO and GHz frequency range links are studied so that frequencies with best complementary behaviour can be selected as a back up link. The specific attenuation of different FSO lengths has also been analysed using measured results.

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

Comparing the rain 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 that reduces 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 rate. The back up link should be nearly immune to weather attenuation for achieving carrier class availability. Among all the attenuation factors for GHz back up link, rain is the most significant. In this paper effects of rain on FSO and GHz frequency range links are studied and some measurement results are presented. This analysis can help in selection of frequencies with least rain attenuation as a back up link.

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.

Comparison of link selection algorithms for free space optics/radio frequency hybrid network

The great potential of free space optics (FSO) communication motivates to use it for future requirement of high bandwidth links. However, the widespread use of this technology has been hampered by the reduced availability because of weather effects. To overcome these shortcomings a back-up link can be used so that the combined hybrid network may achieve carrier class availability. A hybrid, FSO/wireless LAN (WLAN) self-synchronising architecture is presented, which provides transparent connectivity without protocol overhead. The switching from one link to another needs efficient algorithms to achieve the optimal bandwidth utilisation while maintaining availability. Generally, the received signal strength is considered as a parameter for switching between the links. In this study, link selection has been analysed for a variety of algorithms, including filtering and hysteresis, using measured data of fog events. The combination of time and power hysteresis is found to be the optimum solution.

Comparing the Life Time of Terrestrial Wireless Sensor Networks by Employing Hybrid FSO/RF and Only RF Access Networks

The increased interest in long lasting wireless sensor networks motivates to use Free Space Optics (FSO) link along with radio frequency (RF) link for communication. Earlier results show that RF/FSO wireless sensor networks has life time twice as long as RF only wireless sensor networks. However, for terrestrial applications, the effect of weather such as rain or snow on optical wireless communication link is major concern, that should be taken into account in the performance analysis. In this paper, life time performance of hybrid wireless sensor networks is compared to wireless sensor networks using RF only for terrestrial applications and weather effects of rain and snow.