Section: 16.14.2.1 Scheduling and Timing
Content: DL and UL are frame aligned at the uplink time synchronization reference point (RP) with an offset given by NTA,offset (see clause 4.2 of TS 38.213[ NR; Physical layer procedures for control ]  [38]). To accommodate the propagation delay in NTNs, several timing relationships are enhanced by a Common Timing Advance (Common TA) and two offsets and : - is a configured timing offset that is equal to the RTT between the RP and the NTN payload. - is a configured scheduling offset that needs to be larger or equal to the sum of the service link RTT and the Common TA. - is a configured offset that is approximately equal to the RTT between the RP and the gNB. The scheduling offset is used to allow the UE sufficient processing time between a downlink reception and an uplink transmission, see TS 38.213[ NR; Physical layer procedures for control ]  [38]. The offset is used to delay the application of a downlink configuration indicated by a MAC CE command on PDSCH, see TS 38.213[ NR; Physical layer procedures for control ]  [38], and in estimation of UE-gNB RTT, see TS 38.321[ NR; Medium Access Control (MAC) protocol specification ]  [6]. It may be provided by the network when downlink and uplink frame timing are not aligned at gNB. The is also used in the random access procedure, to determine the start time of RAR window/MsgB window after a Msg1/MsgA transmission (see TS 38.213[ NR; Physical layer procedures for control ]  [38]). The Service link RTT, Feeder link RTT, RP, Common TA, and TTA (see clause 16.14.2.2) are illustrated in Figure 16.14.2.1-1. Figure 16.14.2.1-1: Illustration of timing relationship (for collocated gNB and NTN Gateway) The network may configure the HARQ operation as follows: - For downlink, HARQ feedback can be enabled or disabled per HARQ process (as specified in clause 5.3.2.2 and clause 5.7 of TS 38.321[ NR; Medium Access Control (MAC) protocol specification ]  [6]). Disabling HARQ feedback allows scheduling a HARQ process before one HARQ RTT has elapsed since last scheduled. - For uplink, HARQ mode (i.e. HARQ mode A or HARQ mode B) can be configured per HARQ process (as specified in clause 5.4.3.1 and clause 5.7 of TS 38.321[ NR; Medium Access Control (MAC) protocol specification ]  [6]). HARQ mode B allows scheduling a HARQ process before one HARQ RTT has elapsed since last scheduled. NOTE: For the HARQ processes configured with HARQ feedback enabled/disabled, it is up to network implementation to ensure a proper configuration of HARQ feedback (e.g., either all enabled or all disabled) for HARQ processes used by an SPS configuration. For the HARQ processes configured with HARQ mode, it is up to network implementation to ensure a proper configuration of HARQ mode (e.g., either all HARQ mode A or all HARQ mode B) for HARQ processes used by a CG configuration.