## 8 Fourth, Fifth and Sixth Generations  
As stated in Sections 4.1 and 5-7 that [102, 156, 157, 159, 160, 161, 180, 182, 230, 232, 236, 243, 244] utilizes MIMO, Relaying, Hybrid FSO/X and FSO only techniques, respectively to achieve 5G connection. Furthermore, [PERSON] et al. [245], compare the main differences between 4G, 5G, and 6G, as shown in Table 8. Despite the advantages of the 5G communication systems such as low delay, high efficiency, and low hardware complexity. Moreover, 5G systems will not be able to fulfil the market demand after 2030; 6G will be able to fill the gap caused by the market. The 6G communication system can be summarized in a few points; the communication will be integrated with artificial intelligence, has high availability and reliability with high energy efficiency, low data loss, and high security and privacy. Since that high-capacity backhaul is one of the main requirements for networks, high-speed optical fiber and FSO systems are used to support high-capacity networks. Where FSO is a perfect solution for stable fronthaul/backhaul connection for 5G and 6G, especially for long-range, it can be used for remote and non-remote areas such as space and isolated islands. Moreover, it supports cellular BS connection as it can handle many users with high bandwidth by producing a focused narrow light beam via the light diode transmitter, which allows p2p communication with a high data rate. Therefore, hybrid FSO/RF is the best solution in 6G connectivity to overcome the limitations caused by the atmospheric changes [18].  
\begin{table}
\begin{tabular}{c c} \hline \hline  & **Ref.** \\ \hline Monte-Carlo & [182, 198, 228, 229, 239] \\ \hline Matlab & [198, 199, 230, 244] \\ \hline DC Bias & [191, 207] \\ \hline \hline \end{tabular}
\end{table}
Table 6: Summary of projects used Monte-Carlo, Matlab and DC Bias.  
\begin{table}
\begin{tabular}{c c} \hline \hline
\begin{tabular}{c} **Fading** \\ **Distribution** \\ \end{tabular} & **FSO Link** & **RF Link** \\ \hline GG & [171, 175, 182, 192, 193, 198, 203, 207, 208, 209, 210, 220, 222, 228, 229, 240, 242] \\ \hline Rayleigh & [175, 193, 198, 199, 222] \\ \hline Weibull & [183] & [236] \\ \hline Malaga-M & [173, 225] \\ \hline Nakagami-m & [183, 191, 192, 201, 207, 220, 225, 228, 229] \\ \hline Log-Normal & [161, 191, 219, 239] \\ \hline Negative Exponential & [199] \\ \hline Rician & [171, 203, 208, 209] \\ \hline \hline \end{tabular}
\end{table}
Table 7: Summary of fading distribution used in FSO and RF links.  
As small cells are the main key technique for 5G networks instead of the macrocells, it has some challenges to provide omnipresent backhaul connectivity to all small cells. [PERSON] et al. [246] provide various backhaul solutions to the challenges caused in the backhaul small cells. This article focuses on RAN architecture, where the small cells communicate with the macro BS, not the cloud for backhauling. Additionally, the 5G promises an update in RAN that will support the huge wireless data traffic. [PERSON] et al. [247] implement and test two independent hybrid architectures applied to 5G NR Fiber-Wireless (FiWi) systems with different optical fronthaul approaches with a 25 km fiber link. The first architecture works in non-standalone mode, described as a third-generation partnership project to transmit 4G and 5G concurrently using a unique FiWi system. A filtered OFDM signal at 778 MHz with 10 MHz bandwidth at the 5G transceiver; a long-term evolution (LTE)-advanced signal with five 20 MHz sub-bands centralized at 2.24 GHz; a 5G NR signal at 2.35 GHz with 100 MHz bandwidth are the three waveforms studied in this architecture respectively. However, the second architecture implements RoF, FSO, and wireless technologies merged into a heterogeneous network. The maximum capacity of RoF/FSO and RoF/FSO/Wireless FiWi transmission are 3 and 1.4 Gbps, respectively.