# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
###### Abstract
Extending fiber optic connectivity to remote or geographically challenging areas, such as rural regions or mountainous terrain, can be particularly challenging. While fiber optic infrastructure is available in major cities, the processes of obtaining the required permits, trenching, installing, and maintaining  
Figure 1: Backbone, backhaul, and access segments of wireless networks. of fiber optic cables can be complex and time-consuming, which further limits their use in urban environments. For example, to address the user density and high-peak traffic demands in urban areas, small cells, i.e., low-power base stations with small coverage areas, are typically deployed in addition to macro cells. However, it is not possible to extend the fiber optic infrastructure to every small cell, due to space constraints, regulatory issues, and high installation costs. As a result of the aforementioned limitations, and despite the significant ongoing investments in fiber optics, wireless backhauling has become the preferred choice for mobile operators, since it offers greater flexibility in reaching remote or difficult areas. According to market projections [13; 14], wireless backhauling is projected to account for 65% of global backhaul links within the time frame of 2021-2027. The market trends in favor of wireless backhauling give a unique opportunity for NTN-based solutions, which can be particularly cost-effective for rural areas, isolated regions, rugged terrains, or remote islands, which would otherwise be difficult to reach through traditional terrestrial infrastructure. Similarly, NTN-based backhauling might be an ideal solution for small cells in urban areas. The backhaul links discussed above can be considered distribution networks that connect the points of presence to the backbone. The backbone network is typically composed of high-speed fiber optic cables. These cables are laid along major routes and interconnected with network nodes, such as data centers and telecommunication exchanges, to ensure seamless communication between the different parts of the network. It forms the core of the telecommunications network and provides connectivity between different regions, cities, and countries. Non-terrestrial backbone networks can extend connectivity to remote and underserved regions where the terrestrial backbone infrastructure is non-existent or limited. Furthermore, they can enhance the resilience and redundancy of the overall telecommunication infrastructure. By diversifying the network infrastructure with non-terrestrial solutions, the risk of single points of failure or disruptions in communication can be mitigated [15]. In the event of terrestrial infrastructure damage or outages, NTNs can provide alternative routes for communication, ensuring continuity of services. As illustrated in Table 1, NTNs include variants of space-borne and aerial nodes. Space-borne nodes include satellites in Geostationary Orbit (GEO), Medium Earth Orbit (MEO), and Low Earth Orbit (LEO). Airborne nodes take the form of High-Altitude Platform Stations (HAPSs), and Unmanned Aerial Vehicles (UAVs). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Node Type** & **Altitude Range** & **Description** \\ \hline \multirow{6}{*}{**GEO**} & & GEO satellites orbit the Earth at an altitude of 35,786 km above the equator [16]. Their orbit matches the Earth’s rotation, resulting in a synchronized orbit. Therefore, they remain fixed relative to a specific location on the Earth’s surface, appearing stationary in the sky. \\ \cline{2-3}  & & While a GEO has a fixed altitude of 35,786 km, MEO satellites can be found at various altitudes within the range from 2000 km to the altitude of a GEO, based on their specific mission and requirements [16]. Most MEO satellites are found at altitudes between 8000 and 20,000 km. The key advantage of placing satellites in MEO is balancing the coverage area and signal delay. \\ \cline{2-3}  & & The altitude of LEO satellites is up to 2000 km above the Earth’s surface [16].