# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 2 FSO Communication Channel Effects and Their Mitigation Techniques
### FSO Communication--Spatial Profile Challenges
#### 2.3.4 Random Focus Spot Mitigations
The output of the signal intensity that governs the pinhole as the spatial filtering is given by  
\[\langle I_{1}\rangle=\frac{1}{4}\frac{W_{G}^{2}}{W_{0}^{2}}SR~{}I_{-1}^{0} \left(0,-L_{f}\right)\exp\,\left(-SR\frac{2(\Delta r_{0})^{2}}{W_{0}^{2}} \right)\left[1-2 cos\,(\
u)J_{0}(\
u)+J_{0}^{2}(\
u)\right], \tag{14}\]  
where \(\langle.\rangle\) denotes mean value, \(SR\) is the Strehl ratio, \(I_{-1}^{0}\) is the signal intensity received in \(X_{-1}\) in the absence of optical turbulence, \(W_{0}\) is the focus spot radius on \(X_{0}\), \(W_{G}\) is the  
Figure 8: The optical propagation induced by optical turbulence leads to beam wandering, beam spreading, and spatial noise modulation and the focus spot becomes random around the optical axis. effective aperture radius on \(X_{-1}\) and \(J_{0}(v)\) is the Bessel order of the first kind. However, this method cannot anticipate a beam wandering whenever the optical propagation is induced by a strong turbulence level. Thus, a focus spot from a receiver lens deviates randomly from the optical axis of the PD. Thus, to improve the spatial filtering technique, in [72; 73], an optical spatial filter (OSF) which is composed of a cone reflector and pinhole was developed and installed before the PD, as shown in Figure 10. The result of this technique has been reported successfully and improved the quality of signal input into the PD where scintillation, fluctuation of signal spectral, and noise modulation were suppressed optimally by the configuration of the cone reflector and pinhole. The cone reflector collects the random deflection of the focus spot from a receiver lens and directed it into the pinhole in order to suppress spatial noise modulation. This technique greatly improved the performance of FSO under the influence of strong turbulence for the DD signal reception. The governing equation for the OSF is  
\[\langle P_{1}\rangle=\frac{\pi}{2}\frac{W_{\rm G}^{2}}{W_{0}^{2}}SR~{}I_{-1}^{ 0}\left(0,-L_{f}\right)\exp~{}\left(-SR\frac{2r_{0}^{2}}{W_{0}^{2}}\right)cos ~{}(\phi_{i})B, \tag{15}\]  
where \(P_{1}\) is the signal power output from the OSF in the detection plane of \(X_{-1}\), \(B\) is the circular aperture function, and \(\phi_{i}=\phi+2\tan^{-1}\left(\frac{D_{C}-D_{p}}{2Z_{C}}\right)\) is the governing of the guided random focus spot into a pinhole with diameter \(D_{p}\) by a cone reflector with diameter \(D_{\rm C}\) and a width of \(Z_{\rm C}\). In an FD method, an OSF, as in [74], is added using a multimode fiber (MMF), as shown in Figure 11. The results of experiments have been reported which show that the quality of the signal input is improved, as indicated by the signal spectral measurements. Thus, the OSF is capable of processing an optical signal with a minimum of scintillation and noise modulation as well as clean signal spectral components, as in Figure 12. Though the OSF fades out the signal, the subsequent optical amplifier compensates for it. An optical signal that is induced by turbulence effects such as beam wandering, beam spreading, and noise modulation can be suppressed by the configuration of the cone reflector and pinhole.