# Analysis of Scintillation Effects on Free Space Optical Communication Links in South Africa
###### Abstract
The first is related to the spacing between the detector's receive apertures. This inherent boresight displacement corresponds to a fixed distance, namely the distance between each received aperture and its associated alignment point. The second is related to the boresight error caused by the building's thermal expansion [14; 15; 16; 17; 18; 19]. Conventionally, FSOC systems employ intensity modulation/direct detection (IM/DD) schemes. Most commercial FSOC links are based on the on-off keying (OOK) modulation schemes due to their low cost and simple implementation. However, FSOC systems employing OOK require adaptive thresholding, which is difficult to implement when combating irradiance fading, hence their sub-optimal performance over atmospheric turbulence channels [11; 13; 20]. Binary phase-shift keying subcarrier intensity modulation (BPSK-SIM) FSOC systems have also been investigated extensively. In spite of their superior BER performance when compared with other coherent and non-coherent modulation schemes, BPSK-SIM FSOC links have poor power efficiency when compared to pulse position modulation (PPM) FSOC links [20; 21]. FSOC systems employing sub-carrier intensity quadrature amplitude modulation (SIM-QAM) have also been investigated. SIM-QAM FSOC links are found to have better spectral efficiency compared to PPM FSOC links which exhibit poor bandwidth performance. SIM-QAM FSOC systems have great potential for future FSOC systems since they deliver a higher data rate without an increase in the required bandwidth due to their inherent attribute of transmitting more bits per symbol [11; 22; 23; 24]. Between April 2015 and February 2016, the First European South African Transmission ExpeRiment (FESTER) was conducted in False Bay, South Africa, to study the influence of atmospheric turbulence on wave propagation [25; 26]. The experiment focused on measuring and modelling optical turbulence, electro-optical system performance, and imaging. Despite the fact that wind direction, wind speed, and the kinematic vertical sensible heat flux all have an effect on optical turbulence, thermal forces were found to have the greatest impact on it, with both exhibiting a direct relationship regardless of the seasons. Additionally, it was discovered that as friction velocity increases, optical turbulence increases. Onshore and offshore wind directions produced differences in the turbulence strength. With onshore conditions during the winter, the turbulence strength is extremely low. Spring brings an increase in the variability of turbulence strength. The highest refractive index structure parameter (\(C_{n}^{2}\)) values above \(10^{-14}\) m\({}^{-2/3}\) may be reached during the summer [25; 26]. The \(C_{n}^{2}\), which is also dependent on the root-mean-square (RMS) wind speed and altitude of a location, is used to characterize atmospheric turbulence as weak, moderate or strong at any point in time [27; 28; 29; 30]. Most of the results obtained in literature [6; 10; 11; 12; 13; 31; 32; 33] assume arbitrary \(C_{n}^{2}\) values or estimate them based on average wind speed measurements for a particular location. In some cases, worst case scenarios of atmospheric turbulence based on the maximum values of wind speed are investigated [29; 33]. However, these measurements are based on data spanning less than 4 years. As a result, they cannot be accurately used to estimate the maximum attenuation due to turbulence-induced irradiance fading. In this paper, the focus is placed on the wind distributions based on data spanning over 8 years for the various locations of interest where FSOC links are to be deployed. This will allow for accurate estimation of the \(C_{n}^{2}\), and consequently, correct calculations of the maximum attenuation due to turbulence, and the performance of various FSOC links during such periods. Therefore, the key contributions of this work are as follows:  
1.