The evolution of non-terrestrial network technologies from 5G-Advanced to 6G

The integration of multilayered non-terrestrial networks (NTNs) into 6G (6G NTN) is anticipated to play a pivotal role in enabling global coverage, connectivity, and resiliency supporting various use cases [1]. While the integration of NTNs into 6G offers numerous benefits, it also presents significant challenges in optimizing key performance indicators (KPIs), particularly those related to radio access technologies (RATs). One of such challenges is spectral efficiency, given the limited available spectrum for satellite communications amid rising bandwidth demands driven by increasing user numbers and evolving services. The efficient utilization of this finite spectrum is critical, requiring further optimization to maximize data throughput. Additionally, the adaptability to heterogeneous networks and varying channel conditions and the capability to operate across different frequency bands are essential. Therefore this chapter focuses on flexible waveform design solutions to alleviate the impediments of the NTN component (Section 5.1), multiantenna technologies to achieve higher spectral efficiency (Section 5.2), and the optimization of radio resource management (RRM) considering a unified system, such as the 6G NTN system (Section 5.3).