 consists of the RRC, SDAP, PDCP, RLC, and MAC layers, which are explained in detail in Section 3.4. Depending on the architecture options described above, the NTN radio access protocol may be distributed differently across the network nodes. In case of regenerative architecture the radio protocol is fully hosted by the satellite (Figure 5.3.4), while for transparent architecture it is located at the ground station (Figure 5.3.5). The NG-RAN supports the new split architecture comprising a gNB-CU and a gNB-DU, which is described in Section 4.2. This architecture was also investigated for NTNs and is depicted in Figure 5.3.6. In this case the gNB-DU, which hosts the RLC, MAC, and PHY layers is in the satellite, while the gNB-CU, which hosts the RRC, SDAP, and PDCP layers is hosted by the ground station. 5.3 Non-terrestrial Networks NR-Uu Network Figure 5.3.3 Satellite backhauling configuration. Figure 5.3.4 Protocol stack for regenerative architecture, all of gNB functionality is in the satellite. Fraquence Figure 5.3.5 Protocol stack for transparent architecture, all of gNB functionality is in the ground station. Figure 5.3.6 Protocol stack for split architecture, parts of gNB functionality are in the satellite and parts in the ground station. 5 NG-RAN Evolution In principle, this architecture could improve radio performance toward the UE compared with the transparent architecture, as the propagation delays are reduced at least for the time-critical functions (e.g. buffer status information, scheduling, TA, and retransmission, which are part of the MAC layer), while the RRC procedures still experience a larger two-hop delay, which is somewhat less critical. 5.3.4.1 Scheduling and Link Adaptation Scheduling and link adaptation algorithms can be affected significantly by the increased propagation delay of NTNs. GEO satellites are more challenging in terms of propagation delay, while fast-moving LEO satellites are prone to faster channel variations and can cause frequent handovers. The scheduler in the MAC layer mus