Availability modeling of terrestrial hybrid free space optical/radio frequency (FSO/RF) link under tropical climate condition

Abstract

Free-space optical (FSO) links provide gigabit per second data rates, but its availability can easily be affected by fading due to different meteorological conditions such as fog, haze, snow and rain. One of the feasible solutions to overcome this problem is to employ a Radio-Frequency (RF) as a complimentary link to the FSO to improve the link availability. Therefore, to mitigate the atmospheric effects on FSO link led to the development of hybrid FSO/RF systems. The performance of hybrid FSO/RF system is characterized by its link availability and high speed. Link availability consists of many factors including equipment reliability and other internal network design. However the factor which is very difficult to quantify is the fade statistics of atmospheric attenuation due to different weather conditions. Despite the licensed frequency bands, to achieve higher data rates in hybrid FSO/RF, higher frequencies for RF link are the most desirable to maintain high data rates comparable to the FSO link. In heavy rainfall regions, frequencies higher than 10 GHz are relatively sensitive to rain. A key issue in the deployment of hybrid FSO/RF in the tropical regions is the effects of rain on both FSO and RF links. Availability prediction of hybrid FSO/RF with the selection of suitable RF is an indispensable task, especially in tropical areas. Specific rain attenuation models namely, Carbonneau, Japan, Prague, Malaysia (KL) and Malaysia (Johor) proposed for FSO are investigated and a suitable one for the tropical areas is selected. Prediction methods for specific (dB/km) and longer path rain attenuation are available for RF, while only specific attenuation is available for FSO link. In this research, a total path rain attenuation prediction model for FSO is proposed based on effective path length models developed for microwave links. Rain intensity is measured with one-minute integration time at International Islamic University Malaysia (IIUM) campus for three years. Visibility data is also collected with one-hour basis at Subang airport in Kuala Lumpur for three years. Based on the measurements of rain rate, the percentage of time exceeding 0.01% level corresponds to nearly 100 mm/hr, while the highest of 168 mm/hr is observed at 0.000187%. Visibility ranged between 0.1 and 0.5 km under dense haze condition, while the lowest visibility observed as almost 2 km under normal haze condition. Fade margins for FSO and RF are investigated using measured rain intensity and visibility data. For 99.7% availability, FSO fade margin is varied from 6 dB to 31 dB over 1 km to 5 km links under normal haze; while fade margin is varied from 32 dB to 77 dB over 1 km to 5 km links for ≥ 99.999% under rain. Empirical models are also developed and proposed to predict the availability of FSO, RF, and hybrid FSO/RF links over path length ranges of up to 5 km. The RF availability prediction model is developed for frequency range from 10 GHz to 100 GHz using the ITU-R rain attenuation model. FSO availability prediction model is developed as a function of link distance and fade margin; while the RF availability prediction model depends on link distance, fade margin, and radio frequency. The proposed availability models can predict carrier as well as enterprise class availability. Comparison of the availabilities predicted by proposed models show good agreement with those based on tropical climate data and estimated by ITU-R.