# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 2 FSO Communication Channel Effects and Their Mitigation Techniques
### FSO Communication--Spatial Profile Challenges
#### 2.3.1 Aperture Averaging  
In long-range optical propagation, in favorable weather conditions, to achieve the desired data rate, the power margin can be low. In adverse atmospheric conditions with beam divergence, fading due to variations in atmospheric turbulence and fluctuations in received signals, the installed FSO system must be capable of providing a reliable data link. Because of the variations in the temperature of the Earth's atmosphere, a strong \({C_{n}}^{2}\) is produced, and it decreases as altitude increases [52].  
Because of the increase in temperature fluctuations, a strong influence of \({C_{n}}^{2}\) around the optical signal will occur. The commonly employed technique for reducing the power fluctuation is aperture averaging. Using this technique, the fluctuations in the received signal power is reduced by making use of a suitable size of beam around the receiver aperture. As a smaller aperture collection area is small, the value of the atmospheric turbulence strength and link range are used to determine the variance of the influence of the receiving signal. The proportion of intensity variance (\(\sigma_{I}^{2}\)) and the variance of point receiver is given as [53]  
\[F=\frac{\sigma_{I}^{2}(D)}{\sigma_{I}^{2}(D=0)}, \tag{8}\]  
Figure 4: FSO communication channel factors.  
where \(D\) denotes the aperture of receiver and \(F\) denotes the aperture average factor. As in [54], \(F\) is also calculated as  
\[F=\frac{16}{\pi D^{2}}\int_{0}^{\infty}\frac{b_{1}(\rho)}{b_{1}(0)}K(\rho)\rho d\rho, \tag{9}\]  
where \(b_{1}(\rho)\) denotes the covariance order of the irradiance (\(I\)), \(b_{1}(0)\) is mild turbulence, and \(K(\rho)\) represents the irradiation variation which is identical to the Rytov variance as  
\[K(\rho)=\arccos\left(\frac{\rho}{D}\right)-\left(\frac{\rho}{D}\right)\sqrt{ \left[1-\left(\frac{\rho^{2}}{D^{2}}\right)\right]}, \tag{10}\]  
Commonly, \(D<<0.5(\lambda L)\), \(L\) denotes link range, which is considered as a point receiver, and \(10<<0.5(L/k)\) denotes a flat wave with minor internal scale; thus, \(F\) can be approximated as  
\[F=\left[1+1.07\left(\frac{kD^{2}}{4L}\right)^{\frac{2}{6}}\right]^{-1}, \tag{11}\]  
where wave number is denoted by \(k\) (\(k=2\pi/\lambda\)). According to Equation (11), for smaller apertures, i.e., (\((kD^{2}/4L)<<1\)), \(F\) is 1, and for bigger apertures, \((kD^{2}/4L>>1)\), with an increase in aperture size, there is a decrease in the power variance. Thus, an aperture averaging technique becomes essential for reducing the variations in received power in strong atmospheric conditions. Figure 5 shows a brief illustration of the aperture averaging of the received optical signal. An aperture-averaged optical antenna, e.g., a Cassegrain type telescope, gathers most of the photon particles that reach its aperture [54]. Usually larger-aperture antennas are used to collect the entire photons entering it; hence, the scintillation induced in received power due to turbulence variations are reduced. In practicable FSO system installations, when choosing the aperture of the receiver, there will be a trade-off between the weight, the optical transmitting receiving antenna size, and potential agility.