ATTENUATION CHARACTERIZATION AND MITIGATION OF FADING CHANNEL IN FREE SPACE OPTICAL (FSO) LINK SYSTEMS UNDER ATMOSPHERIC TURBULENCE

Abstract

Free Space Optical (FSO) communication involves the transmission of signal-modulated optical radiation from a transmitter to a receiver through the atmosphere or outer space. The locationvariant atmospheric channel degrades the performance of an FSO system under severe atmospheric conditions such as the presence of fog, rain, haze, smoke, and atmospheric turbulence, thus necessitating local atmospheric attenuation studies. One-year archived visibility data and in - situ measured 1-minute integration time rain rate data at Akure were obtained from Nigerian Meteorological Agency (NIMET) and the Department of Physics, Federal University of Technology, Akure respectively. Fog-induced specific attenuation was computed at five optical wavelengths of 650, 750, 850, 950, and 1550 nm, at two transmittance thresholds of 2% and 5% using both Kruse and Kim Model. The rain-induced specific attenuation was computed using Carbournea Model. The performances of Single-Input Single-Output (SISO) and Multi-Input Multi-Output (MIMO) FSO systems in the presence of different attenuation was studied using the parameters of a commercial optical transceiver (TereScope 5000) in OptiSystem 13.0 simulation software. A maximum fog-induced specific attenuation of about 122.14 dB/km was observed on the 35th, 269th and 317th days of the year with visibility value as low as 0.3 km. Generally speaking, the specific attenuations were observed to reduce with increasing optical wavelengths, however, such improvement was equally observed to decline with increasing link distance. A maximum rain - induced specific attenuation of about 10.93 dB/km was observed in May, 2012. However, the attenuation values were generally observed to be much lower to those that are fog - induced. The performance of the SISO - FSO system was seen to be adequate in the clear sky and short link distances, but deteriorated at reduced visibility, higher turbulence regimes and longer link distances. The performance was however improved with the introduction V of the MIMO technique which showed higher Q-factor values, „eye‟ height, increased received power and lower bit error rate. It was observed that the extent of FSO system improvement was higher at the 2% transmittance threshold than at 5%. The overall result will serve as a good tool for planning FSO systems in the study area.