170-2,200 MHz. As per the ITU, S-band is for use in satellite and terrestrial components of International Mobile Telecommunications (IMT) frameworks. L-band: Certain parts of this band are used for Maximum Segment Size (MSS). It depends on the GSO or NGO composition. Note that 3GPP cannot dictate the use of such bands for NTNs universally. Some countries may have different local use of this band for terrestrial use. The ITU helps to co-ordinate between the countries in such cases for satellite and mobile network operators (https://www.itu.int/en/ITU-R/terrestrial/broadcast/ Pages/Bands. aspx). In Figure 16.17, we see the high-level NTN architecture spanning urban, rural, and remote area coverage. For urban coverage, we see the use of Unmanned Aerial Vehicles (UAVs) or drones. AT&T has tested cell on wings to provide 5G coverage for about 10 square miles using a drone. NTN in a wide sense Not to scale 3GPP work on NTN 3GPP work on UAV Satellite network 600 km & above High-altitude platform station as IMT base station (HIBS) Air-to-ground network 10 km 150 m (191) Ground level Urban Remote Rural Figure 16.17 - NTN network architecture The architecture of 5G and satellite communications Let us look at the two common types of satellite-based NTN communications. In Figure 16.18, we see the first scenario, where only one satellite is involved in the communication and both the sender and receiver are communicating with the same satellite. In this context, UAS refers to unmanned aerial systems and ISL stands for inter-satellite link. Satellite (or UAS platform) Feeder link network Service Gateway equipments Beam foot print Field of view of the satellite (or UAS platform) Figure 16.18 - Scenario 1 of NTN architecture In scenario 2, as shown in Figure 16.19, we see there is more than one satellite involved in the NTN communication. The satellites communicate with each other via the ISL. In this case, the sender and the receiver are communicating with different satellites. In a general case, there can be more than two satel