# Key Technologies for 6G-Enabled Smart Sustainable City
## 4 Key Candidate Technologies for Future SSC
### Prime Technologies for SSC
#### 4.2.3 Non-Terrestrial Networks for Zero-Coverage Networks  
It is envisaged that UAM is one of the main transportation within the 6G-enabled SSC. To support aerial devices and provide greater coverage than currently deployed 5G systems, 6G wireless networks should be designed to support both NTN and terrestrial networks (TN) [7]. NTN has sparked significant global interest since it can provide high-speed Internet services even in rural, ocean, sea, and mountain areas [45]. Based on flying altitude and coverage area, NTN devices can be categorized into UAM, low-earth orbit (LEO) satellites, medium-earth orbit (MEO) satellites, and geostationary orbit (GEO) satellites (see Figure 4). LEO satellites get closer to Earth's surface to ensure faster response times compared to GEO satellites. Specifically, the round-trip delay (RTD) of a GEO satellite is around 500 ms, while that of a LEO satellite is less than 100 ms.  
As the orbit of LEO satellites gets closer to Earth, the size of their footprint decreases, resulting in more limited coverage. For example, at an altitude of 550 km, the beam coverage reaches approximately 0.2% of the entire region [46]. Achieving global coverage would necessitate deploying a mega-constellation comprising a large number of LEO satellites. NTN technologies are expected to offer seamless connectivity for 6G-enabled SSC. To achieve a fully connected SSC in a cost-effective manner, the following issues need to be addressed. **Channel realization due to high-mobility NTN devices**: Most NTN devices move at high speeds from the perspective of ground devices [45]. The mobility of NTN devices induces a significant Doppler shift in wireless channels. The fast-changing wireless channels pose a critical challenge that must be addressed to ensure seamless radio connections in NTN networks. The wireless channel models should be reevaluated to capture diverse channel characteristics and time-varying properties in NTN networks. **Frequent handover in 3D coverage**: High-speed movement of NTN devices causes a very short visible period. For example, the maximum visible time of LEO satellites is up to 25 min to a ground terminal. Due to the continual movement of the NTN coverage area, frequent handover will be required even for the fixed ground terminal. Also, the cell size can be varied depending on the elevation angle of the NTN devices. Thus, conventional handover techniques for terrestrial networks become incongruuous in the new era of aerial and space networks. **Integrated TN and NTN architecture**: When satellites are closer to earth, the coverage of the satellites becomes smaller. Therefore, LEO satellites should work together to provide sufficient coverage to a designated area, which requires a complicated protocol and backhaul connection. A multi-layered architecture that integrates the satellite network, the airborne network, and the terrestrial network can provide better coverage and performance over the satellite network only [93]. This is because the airborne network can bridge the LEO/GEO satellite systems to the ground terminals. To obtain this full benefit of the multi-layered architecture, efficient protocols including network mobility management and resource allocation should be developed. **Weak LOS environments**: Although the channels of NTN devices are dominated by line-of-sight (LOS) components in most aerial spaces, non-LOS components also occur near the ground in the channel model for NTN devices. Note that LOS probability depends on several factors such as the environmental information and elevation angle. For example, for an elevation angle of 10\({}^{\circ}\), the LOS probability is lower in urban areas (24.6%), while the LOS probability is higher in suburban and rural areas (78.2%) [94]. Specifically, in weak LOS environments (i.e., urban areas), blockage can hinder the direct link between NTN devices and ground users. To remedy this problem, UAM can be exploited as the relay node.