Rain attenuation modeling for outage prediction of free space optic link under tropical weather

Abstract

In present world, optical fiber is the most emerging and prominent high speed medium of communication. However, optical communication can also be established without fiber and it is known as Free Space Optics (FSO). FSO has attracted a lot of attentions for a variety of applications in telecommunications field. FSO is a line of sight technology allowing optical connectivity without requiring fiber cable or security spectrum licenses. Compare to fiber optic, FSO provides lower cost and shorter time of deployment. Whereas comparing to microwave, FSO offers higher speed, broader and unlimited bandwidth with unlicensed spectrum. Since the transmission of FSO is through atmosphere, there is a variety of detrimental features of the atmospheric channel that may lead to serious signal fading and even complete loss of signal altogether. For FSO signal, the most important obstruction is local weather condition such as fog, snow, cloud, haze and rain. In temperate region, numerous studies and experiments have been conducted and the most limiting factors are found fog and snow. In tropical region with the absence of fog and snow, rain and haze are considered to be the limiting factors. ITU-R has recommended FSO attenuation models which are based on data acquired mainly from temperate climate. Hence to investigate those prediction models using tropical data measurement is required urgently to implement FSO in tropics. Therefore, the focus of this research is to propose new specific rain attenuation parameters that best represent tropical weather condition and to investigate the link availability. A FSO link with 850nm wavelength and 6mW transmit power was installed at International Islamic University Malaysia with 800m link distance. A real time rain intensity and visibility measurement system were installed synchronously with FSO link. Laser power was monitored concurrently with rain intensity and haze. From the concurrent measurement, the effects of rain and haze on FSO link were investigated. Based on one year measurement, modified regression coefficients for ITU-R specific rain attenuation model have been proposed. Preliminary analysis on the effect of haze on FSO and availability of FSO link in tropical climate has been done based on measured data. From the observation, link availability of 99.99% can be achieved for a link distance of 1.25km and fade margin of 25dB. The outcome of this research is expected to give a foundation for the design and development of a long range FSO link under tropical weather condition.