rrestrial networks, when the DMRS have sufficient density in time. 5.3.5.2 HARQ The impacts on NR HARQ operation due to the long RTT delay of an NTN were also studied in 3GPP TR 38.811. The impacts are considered for the NTN UEs as well as the serving gNBs, when the number of HARQ processes is either extended to satisfy high reliability scenarios or limited/disabled for longer NTN delays. Two options are envisaged for using HARQ in satellite systems: 1. Enhancing the existing HARQ operation to extend the HARQ processing accommodat- ing low to moderate NTN RTT delays like in scenarios C1, C2, D1, and D2 as defined in 3GPP TR 38.821. Here it is important to study the impact on the possible HARQ process numbers, HARQ timers, HARQ process IDs, number of Redundancy Versions (RVs), and the possibility for bundling. These will have a direct impact on the UE, for example in terms of memory and power limitations, as well as the collected gNB signaling and feedbacks for a large number of UEs. 2. Limiting HARQ capabilities and/or disabling HARQ for long RTT delays. For that, it is important to study the direct impact on NR due to disabling the HARQ feature, includ- ing the required signaling for on/off switching of HARQ and possible metrics like the target BLER/QoS, the Modulation Coding Scheme (MCS)/Transport Block Size (TBS) selection, etc. It is also important to consider a backup mechanism to support high QoS requirements in an attainable RTT delay. This should include, e.g. HARQ-less repeti- tions including its impact on the data rate and utilizing terrestrial NTN dual connectivity including its impact on the used timers. These two aspects are described in more detail below. 5.3 Non-terrestrial Networks 5.3.5.2.1 Enhancing the Existing HARO Operation Potential solutions to extend the HARQ operation in satellite systems are proposed in 3GPP RP-180664, while a comprehensive summary is given in 3GPP R2-1817757: Extending the minimum number of HARQ processes. It was agreed in NR to support a flexible number of HARQ process