## 3 FSO Systems for 5G Backhauling Network
[PERSON] in [100] demonstrated that modern MMW radios can attain Gbps speed owing to the broad radio bandwidth made possible by the frequency allocation method [101]. The estimated lifespan of the E-band radio connections is 3-5 years. However, when optical cable reaches a radio node, the investment is not wasted. As 5G networks are spreading everywhere, 5G aims to reach gigabit-level in cellular networks. However, there are a lot of challenges that disallow 5G wireless backhaul networks to work effectively. [PERSON] et al. [102] analyze wireless backhaul networks by using small cell, millimeter-wave (MMW) and Multiple-Input Multiple-Output (MIMO) communication technologies to attain Gigabit transmission rates in 5G networks strategies for promoting 5G wireless backhaul networks in a way that uses low energy and offers high throughput. The wireless backhaul traffic in upcoming 5G+ networks is set to be examined using two common small cell scenarios. Furthermore, two common small cell scenarios are used to examine the energy efficiency of wireless backhaul networks. According to numerical findings, in 5G wireless backhaul networks, the distribution solution uses less energy than the central solution. If the new distribution network design is used in the next 5G wireless backhaul networks, a real difficulty would materialize. However, there are solutions to overcome 5G network challenges which can be summarized as follows; distribution of cell architecture and protocols to allow wireless data forwarding in ultra-dense small cell networks, make use of MMW communications that are highly recommended as they transfer huge traffic in 5G networks, utilize cooperative small cell groups as it will decrease handover failure rate in small cell networks and applying FSO into the system which have high energy-efficient data transmission schemes [35]. Results imply that the distribution solution is more energy-efficient compared to the central solution in 5G networks, where distribution solution is all backhaul communication that is forwarded to a specific small cell base station (BS) because there is no macrocell BS to gather all backhaul traffic from small cells. Small cells resemble WiFi in many ways, but they can use cellular standards, unlike WiFi. Depending on the cell's bandwidth, maximum output power, coverage area, user count, deployment scenario, and connection to the backhaul, small cells can be categorized as femto, pico, or micro. Table 3 below compares small cells with macrocells. \begin{table}
\begin{tabular}{c c c c c} \hline
**Parameters** & **FSO** & **Optical Fiber** & **Microwave Radio** & **Coaxial Cable** \\ \hline Installation & Moderate & Difficult & Difficult & Moderate \\ \hline Data Rate & Gbps & Independent & Gbps & Mbps \\ \hline Security & Good & Very good & Poor & Good \\ \hline Connectivity & P2P, P2 MP short & P2P, P2 MP short & P2P short reach & Multidrop short reach \\ \hline Cost & Low & Very High & Very High & High \\ \hline Maintenance & Low & Low & Low & Moderate \\ \hline Bandwidth & Very High & Very High & Medium & High \\ \hline Spectrum License & Not required & Not required & Required & Required \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of FSO with Different Systems [97; 100].