# Analysis of Scintillation Effects on Free Space Optical Communication Links in South Africa
###### Abstract  
The performance of free space optical communication (FSOC) systems is severely degraded by certain atmospheric conditions prevalent in places where they are deployed, in spite of their numerous advantages. In clear weather conditions, the random fluctuation in the atmosphere's refractive index causes substantial scintillation losses to transmitted optical signals. It is therefore imperative to estimate the potential losses due to atmospheric turbulence in locations where FSOC links are to be deployed. This will provide the necessary fade margin for FSOC systems so that designed links withstand such atmospheric disturbances. In this paper, statistical analysis of wind speed data collected for various cities of South Africa is used for calculating the corresponding refractive index structure parameter (\(C_{n}^{2}\)). These \(C_{n}^{2}\) values, as well as the zero inner scale and infinite outer scale model and finite inner and finite outer scale model, are used in computing the scintillation indices not exceeding 50%, 99%, 99.9%, and 99.99% of the time for the investigated locations. The Lognormal and Gamma-gamma distribution models are then employed for the computational analysis of the irradiance fluctuations and channel characteristics while considering the effect of pointing errors for weak and moderate to strong turbulence regimes. Finally, derived mathematical expressions for outage probabilities and bit error rate (BER) performances for FSOC links, employing various intensity modulation and direct detection (IM/DD) schemes, are presented. title=false, numbersep=0 pt, numbersep=0  
when compared to the rigours encountered in the installation of fibre-optic infrastructures, high resistance to signal eavesdropping, and low latency communication since the velocity of light in the atmosphere is about 40% faster than in the fibre-optic cable [7; 8]. In spite of these advantages, the performance of FSOC systems is severely affected by atmospheric impairments. Dense fog, haze, and snow storms are known to cause the aerosol scattering of optical signals and consequently degrade the availability of FSOC links [9]. In clear weather, atmospheric turbulence or scintillation is the most significant cause of impairment in received signal quality [10]. Atmospheric turbulence causes fluctuations in air temperature, pressure, density, and humidity, which results in rapid variations in the atmosphere's refractive index. The impact of these changes leads to irradiance fluctuations of received information signals [11]. Other turbulence effects on FSOC link performance are disruptions in the coherence of the laser beam and distortions in the optical wave front. Optical wave front distortions result in laser beam broadening, uneven beam energy redistribution within a cross-section of the laser, and beam wander [12]. Improving the bit error rate (BER) performance of FSOC systems during these adverse weather situations is the major challenge in the design of FSOC links [13]. In addition, the misalignment between FSOC transceivers cause pointing errors, which increase the performance degradation of FSOC links. These misalignments arise from either mechanical vibrations in the system as a result of wind or building movement or errors in the tracking system. The displacement of the laser beam along vertical (elevation) and horizontal (azimuth) directions, which are generally expected to be independent Gaussian random variables, result in pointing errors [14; 15; 16; 17; 18; 19]. Beam width, boresight, and jitter are the three fundamental components of a pointing error. The beam width is the beam waist (radius computed at \(e^{-2}\)), while the jitter is the random offset of the beam centre at the detector plane produced by building motion, minor earthquakes, and dynamic wind loads. The boresight denotes the fixed displacement between the beam centre and the alignment point. It should be noted, however, that boresight displacements are of two kinds: inherent boresight displacement and additional boresight error.