quipment are envisaged. NR-Uu Network Figure 5.3.1 Access network based on NTN platform with transparent payload. 5 NG-RAN Evolution NR-Uu Network NG over SRI Figure 5.3.2 Access network based on NTN platform with regenerative payload. 5.3.3.2 Access Network with Regenerative NTN Payload The architecture in Figure 5.3.2 differs from the previous one owing to the choice to embark the gNB directly on board the satellite. This requires more computational power in the satellite payload, but it has numerous benefits; for instance, it halves the propagation delay between the UE and the gNB with respect to the transparent solution. It is important to note (see Figure 5.3.2) that the Satellite Radio Interface (SRI) between the gNB and the satellite gateway on-ground is used as a transport network for the NG interface. 5.3.3.3 Transport network based on NTN For the sake of completeness, a third NTN based architecture is reported in Figure 5.3.3. In this configuration, a satellite backhaul is used between the core and terrestrial access net- work providing transport capabilities for the NG interface (e.g. N1/N2/N3 reference points). In this case the satellite system transparently conveys all needed 3GPP protocols on all rele- vant interfaces, effectively being just part of the transport network. Furthermore, the radio link can be either based on 3GPP RAT or not. This scenario is not considered further in the chapter, since it does not impact the 3GPP defined radio access network. 5.3.4 NTN radio protocol A key issue to be resolved within the scope of the 3GPP work on non-terrestrial networks (NTN) is to enhance the 5G New Radio (NR) radio access protocol to support the long laten- cies for satellite communication. While one of the design goals of the NR air interface is extremely fast processing to enable Ultra Reliable Low Latency Communications (URLLC) services with a one-way delay of 1 ms, the same protocol needs to work over non-terrestrial satellite networks with several 100 ms propagation delay. The NR protocol stack