# A Review-Unguided Optical Communications: Developments, Technology Evolution, and Challenges
## 3 Hybrid RF-FSO Systems Techniques and Their Advanced Achievements
### Radio-over-FSO and Integrated FSO-RF Antenna Systems
Using MZM as a phase modulator, the transfer function of the modulator is reduced to just the production of the phase shifts in accordance with the electric voltage applied as  
\[E_{out}(t)=E_{in}(t)\bigg{[}e^{i\pi\frac{E(t)}{V_{\pi}}}\bigg{]}. \tag{19}\]  
As shown in Equations (16)-(19), simultaneously transmitting RF signals, in numerous wireless applications, over FSO links will address the demand of higher bandwidths for enhanced electromagnetic or photonic communications, remote sensing, and measurement applications. Note that wireless services using Ro-FSO have been made possible for broadband wireless local area networks (WLAN), CDMA, and ISDB. A full-duplex Ro-FSO network is shown in Figure 20. At the Tx, the RF signals containing the standard network service data generated are used for the modulation of the light signal using the optical unit, which includes MZMs, optical amplifiers, WDMs, and optical couplers. The modulated optical beam is launched into the free-space channel using a single-mode fiber (SMF) cable and optical lenses. At the receiver, the reverse operation is carried out in order to regenerate the original transmitted signal. In Ro-FSO systems, to estimate the losses associated with SMF coupling, fading, atmospheric attenuation, and free-space beam geometry, the power margin should be determined. Note that the fading loss is given by [81]  
\[a_{scin}=4.343\bigg{\{}erf^{-1}(2p_{thr}-1)\sqrt{\Big{[}2 ln\Big{(}\sigma_{p}^{ 2}+1\Big{)}\Big{]}}-\frac{1}{2}ln\Big{(}\sigma_{p}^{2}+1\Big{)}\bigg{\}}, \tag{20}\]  
Figure 20: Top level schematic of a full-duplex Ro-FSO system. where \(P_{thr}\) is the probability of outage time, the inversion error function is given by \(erf^{-1}\), and \(\sigma_{p}^{2}\) denotes the power scintillation index which is given by  
\[\sigma_{p}^{2}(D_{RX},L)=exp\left[\frac{0.49\sigma_{R}^{2}}{\left(1+0.65d^{2}+1. 11\sigma_{R}^{\frac{12}{3}}\right)^{\frac{7}{6}}}+\frac{0.5\sigma_{R}^{2}\left( 1+0.69\sigma_{R}^{\frac{12}{3}}\right)^{\frac{-5}{6}}}{\left(1+0.9d^{2}+0.62d^ {2}\sigma_{R}^{\frac{12}{3}}\right)^{\frac{7}{6}}}\right], \tag{21}\]  
where \(\sigma_{R}^{2}=1.23\left(k^{7/6}L^{11/6}C_{n}^{2}\right)\) denotes the Rytov variance parameter, \(d=\left(kD_{RX}^{2}/4L\right)^{1/2}\), \(L\) is the link range, and DRX is the receiver aperture diameter. The optical signal propagation is significantly degraded because of the atmospheric conditions, which results in the decreasing spatial consistency of the light wave. The estimate of the SMF coupling efficiency of the optical beam which is distorted due to atmospheric turbulence without incorporating any PAT techniques, is given by  
Figure 19:0  
where a is the coupling geometry, \(A_{R}/A_{C}\) is the number of speckles (\(A_{R}=\pi D^{2}/4\) and \(A_{C}=\pi\rho c^{2}\)), \(\rho_{c}=\left(1.46C_{n}^{2}k^{2}L\right)^{-3/5}\), and \(I_{0}\) is the first sort of zero-order generalized Bessel function [83]. The beam centroid stability determines the precision and efficiency achieved using an efficient PAT system. Beam coupling, beam dispersion, and link length are the major forms of geometric losses given as  
Figure 19:1  
To ensure link availability at all time and under all weather conditions, one option would be to use a hybrid RF and FSO link [84; 85]. In such a scheme, the preferred link can always be the FSO that can change over to the RF link under conditions of high turbulence, fog, and smog, etc. [86]. In the present day's long-range FSO networks, a radio communication scheme is also used as a gridlock network. Although FSO systems have numerous advantages which includes the ability of managing a heavy density of data traffic, their obtainability fully relies on the channel weather condition and on rapid power fluctuations (signal fading) due to non-uniform temperature and wind fluctuations in the clear channel. A number system on the hybrid RF-FSO has been proposed and investigated [86]. In Ref. [87], a hybrid antenna suitable for both RF and optical signals transmission was proposed. In Ref.