NB transmits the RVs to the UE upon a NACK. Bent-pipe Satellite Donor Base-Station (DgNB) AEQ Controller BackEnd HARQ Controller BackEnd Base-Station/ Buffer/Relay HARQ Controller Front End Figure 5.3.9 Transmission of HARQ RVs to the gNB via satellite backhauling or direct satellite access. 5.3 Non-terrestrial Networks In other scenarios, dual connectivity can gain performance by utilizing L2 aggregations, for example, PDCP aggregation. Hence, it is important to further study the impact on the NR HARQ operation in the case of dual connectivity with an existing terrestrial gNB to include HARQ redundancy decoding at the physical layer and/or L2 aggregation if possible. The impact on HARQ timing, the number of HARQ processes, and the HARQ process ID/synchronization needs to be identified in this case. 5.3.5.3 Timing Advance (TA) According to 3GPP TR 38.811, fast-moving satellites (e.g. in LEO, MEO orbits) cause fast change in the overall distance of the propagation from the UE over satellite to the gNB, and consequently a strong delay drift. In the case of a satellite, the delay drift is quite predictable because the motion of the satellite follows known paths. The technical challenges for NR adaptations are as follows: 1. A strong delay variation is caused by fast-moving satellites (e.g. in LEO up to 7.5 km/s) generating a fast change in the overall distance of the propagation from a UE over satel- lite to a gNB, requiring fast adaptations for the uplink receiver synchronization and uplink transmission timing. 2. The delay is much longer over a satellite link than one TTI. For terrestrial links, the TA typically does not require very fast updates, because the distance of the terminal to the base station only varies slowly due to the terminal mobility. In the case of GEO satellite links, the terminal mobility also dominates the TA requirements. However, this may not be the case, for example, for LEO satellites. Another technical issue that arises is the delay variation over the satellite link being much larger than