# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
### Space-Borne Nodes
For example, the data rate provided by the Iridium system was limited to 10 kbps. This data rate was also shared among multiple users connected to the system [26] [Section 7.2]. In addition, the whole Iridium system of 66 LEO satellites could only serve thousands of users worldwide. Due to their inherent limitations, they could not support many users with high-speed and reliable communications services. Coverage areas were often limited to specific regions or countries. As a rebirth of NTNs, various companies have recently launched LEO mega satellite constellations, e.g., Starlink, Kuiper, and OneWeb, which can be considered a second generation. Starlink, a project led by SpaceX, plans to deploy thousands of small satellites in LEO at an altitude of 550 km [32], creating a vast network that will cover a large part of the planet and communicate with other satellites and ground stations. Starlink will offer download speeds ranging from 25 to 220 Mbps. Most users are expected to experience speeds exceeding 100 Mbps. Upload speeds will fall within the range of 5 to 20 Mbps. Latency is expected to be between 25 and 60 ms on land, while certain remote locations such as oceans, islands, Antarctica, Alaska, and Northern Canada may encounter latencies of more than 100 ms [33]. As of December 2021, Starlink has launched 1969 satellites, serving 140,000 users in 20 countries, and is planning to extend this to a total of 42,000 LEO satellites before the end of this decade [34]. Another mega-constellation under planning is Amazon Kuiper [35]. This constellation is designed to consist of 3236 satellites deployed in LEO altitudes between 590 km and 630 km. These satellites will be positioned at an altitude that is around 67 km closer to the ground than Starlink, which is expected to result in a relatively lower latency. In terms of data rate, it has been reported that Kuiper customer terminals could reach a speed of 400 Mbps, with expectations for further improvements in future versions [36]. The OneWeb LEO satellite constellation [37] consists of 648 satellites positioned approximately 1200 km above Earth. The constellation is designed to primarily serve businesses, governments, phone network operators, and clusters of communities, rather than individual domestic customers. OneWeb utilizes optical inter-satellite communication systems [38], which allow for efficient communication between the satellites in the constellation, enhancing the network's overall performance [38]. The access strategy of the gateway stations in OneWeb's constellation is relatively more complex, due to the repeated coverage of the same gateway station by multiple satellites [39]. The deployment of mega satellite constellations by these companies represents a significant advancement in the field of telecommunications. By leveraging the advantages of satellites in LEO, such as low latency and global coverage, these constellations have the potential to revolutionize internet connectivity and extend cellular services to previously underserved areas. As technology continues to evolve, it is expected that more companies and researchers will join the effort.