# 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
Also, a reconfigurable intelligent surface (RIS)-based beamforming can be applied to ensure the directivity of communication links [47; 95]. To shed light on how NTN can be constructively exploited in SSC, we discuss its potential benefits in the following. * _Relieving spectrum scarcity_: As the number of LEO satellites is rapidly increasing, and while frequency resources are becoming limited, the coexistence of GEO and LEO satellites has been studied to enable aggressive frequency reuse. Frequency co-existence between NTN devices is expected to be a promising approach to alleviating spectrum scarcity. The deployment of NTN should consider interference from the 3D network with the altitude of the NTN devices. As the altitude of the NTN devices increases, the beam width can be wider, thus intensifying inter-beam interference. To manage interference caused by the coexistence of NTN devices, advanced multiple access  
Figure 4: Candidate NTN terminals on SSC. techniques such as non-orthogonal multiple access (NOMA) [96] or rate-splitting multiple access (RSMA) can be applied [97; 98]. * _Power efficient and cost-effective deployment_: Note that LEO satellites have a lower power budget compared to GEO satellites. Transparent LEO satellites can be employed for an efficient power assumption. To fully achieve low-latency, regenerative LEO satellites using the inter-satellite links (ISL) should be exploited. Due to on-board processing in regenerative satellites, effective power management of LEO satellites is critical to ensure their optimal performance. The RIS can be combined with LEO satellite communication systems to ensure ubiquitous connectivity by addressing coverage holes [99]. Note that RISs consume much less power than conventional relay terminals, since RISs do not operate complicated signal processing. In addition, RISs can be easily mounted on NTN devices such as LEOs and UAMs. * _Low latency over the globe_: While a round-trip delay (RTD) of a GEO satellite exceeds 250 ms, the RTD of a LEO satellite at an altitude of 600 km is less than 30 ms [94]. Thus, non-GEO devices can guarantee a faster response time compared to GEO. However, due to the low orbits of the non-GEO satellites, the beam coverage of the non-GEO satellites decreases. Thus, it requires the deployment of a mega-constellation including a large number of non-GEO satellites for global coverage while ensuring a delay as short as a few tens of milliseconds. Recently, integrating satellite and terrestrial networks (ISTN) has sparked a significant interest due to its capability to increase coverage, reliability, and spectrum efficiency [100]. In 6G-enabled SSC, terrestrial infrastructures can be exploited as relay nodes to ensure reliable communication links. Due to obstructions on the ground, there may exist coverage holes that non-GEO satellites cannot cover. Even in the absence of an LOS link, employing RIS on non-GEO satellites or UAM can enable communication with information transmission. In particular, UAMs with RIS, taking advantage of their mobility, can provide a higher probability of LOS in channels between RIS and users. In addition, to alleviate the spectrum scarcity, NTN devices and terrestrial infrastructures need to exploit the same spectrum resources.