# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
### Connectivity Technologies
\begin{table}
\begin{tabular}{l l l l l l} \hline
**Technology** & **GEO** & **MEO** & **LEO** & **HAPS** & **UAVs** \\ \hline
**Altitude** & \(\approx\) 35,786 km [60] & \(\geq\)2000 km [60] & \(\approx\)160–2000 km [60] & \(\approx\)17–22 km & \(\leq\)17 km \\ \hline
**Latency** & 600–800 ms [61] & 125–250 ms [61] & 30–50 ms [61] & \(\leq\)30 ms & \(\leq\)30 ms \\ \hline
**Coverage** & Global/Very Large & Global/Very Large & Global/Very Large & Regional/Large & Local \\ \hline
**Number of Required** & \(\bullet\) A single CEO satellite can cover a large portion of the Earth’s surface. & \(\bullet\) Multiple (typically between eight and twenty [62]). & \(\bullet\) Large constellation for global coverage & \(\bullet\) Depends on the intended coverage area. & \(\bullet\) Depends on the intended coverage area. \\  & \(\bullet\) Near-global coverage with overlapping footprints can be achieved by a few GEO satellites & \(\bullet\) The coverage radius of a single MEO satellite at an altitude of 20,000 km would be approximately 10,000 km. & & \\ \hline
**Applications** & Broadcasting, Weather monitoring, Communication, GPS & Navigation, Weather forecasting, GPS, and Communication & Earth observation, Remote sensing, GPS, Communication, and Astronomy & Communication, Surveillance, Surveillance, and Monitoring, and wireless access. \\ \hline
**Advantages** & \(\bullet\) Wide coverage area. & \(\bullet\) Lower latency than GEO. & \(\bullet\) Relatively low latency. & \(\bullet\) Lower latency than satellites. & \(\bullet\) Rapid deployment \\  & \(\bullet\) Stationary over a fixed point on Earth. & \(\bullet\) Higher capacity than GEO. & \(\bullet\) Lower cost than GEO/MEO. & \(\bullet\) Lower cost than satellites. & \(\bullet\) Flexibility in coverage. \\  & & \(\bullet\) Better coverage than LEO. & & \(\bullet\) Flexible deployment. & \(\bullet\) Lower cost than satellites. \\ \hline
**Disadvantages** & \(\bullet\) High latency. & \(\bullet\) Moderate latency & \(\bullet\) Limited coverage superlattice. & \(\bullet\) Limited payload capacity & \(\bullet\) Limited endurance \\  & \(\bullet\) Limited capacity. & \(\bullet\) Less coverage than GEO. & \(\bullet\) Requires large constellation for global coverage. & \(\bullet\) Restricted coverage area. & \(\bullet\) Limited coverage area. \\  & \(\bullet\) High cost. & & \(\bullet\) Frequent & \(\bullet\) Affected by handovers. & \(\bullet\) Affected by weather conditions. \\ \hline
**Flexibility** & None & Limited & Limited & High & High \\ \hline
**General Payload Capacity** & High & Moderate & Limited & Limited & Limited \\ \hline
**Endurance** & Years & Years & Years & Several weeks/months & Several hours to Days \\ \hline \end{tabular}
\end{table} 
Table 2: Comparison of satellite and aerial technologies—altitude, latency, coverage, required nodes, applications, advantages, disadvantages, stationary status. FSO communication has been extensively studied in the literature [65; 66; 67; 68; 69; 70; 71; 72; 73; 74]. Most of the earlier works were limited to terrestrial networks. A comprehensive overview of FSO communication systems was presented in a survey by [PERSON] and [PERSON] [65]. Among the topics discussed in [65] were advanced modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems. [PERSON] and [PERSON] [66] presented a review of FSO use in space, discussing ground-to-satellite, satellite-to-ground, and intersatellite communication systems. A discussion of acquisition, tracking and pointing mechanisms for FSO communications was presented by [PERSON] et al. in [67]. The mechanisms were categorized according to their working principles, use cases, and mechanics. The advantages and disadvantages of each were also discussed. In [68], [PERSON] et al. provided a tutorial on fronthauling for cloud radio access networks and presented FSO systems as a possible fronthaul solution. In [69], [PERSON] et al.