Summary of 3GPP Technical Document: FS_ULBC - Scheduling Timing Uncertainty

1. Background and Motivation

This contribution addresses ambiguities in interpreting RAN1 LS S4-251654 regarding uplink and downlink timing for NB-IoT NTN with GEO satellites. The interpretation of this LS has direct implications on:
- Scheduling timing uncertainty assumptions
- Link capacity calculations

The document proposes clarifications to the Permanent Document (PD) version 0.4.0 to resolve these interpretation issues discussed at SA4 #133-e and subsequent meetings.

2. Main Technical Contributions

2.1 Frame Structure for Dynamic Scheduling

The document maintains the existing frame structure example for Half-duplex FDD with 80ms bundling period:
- NPDSCH duration: 4ms (variable depending on DL SNR)
- Multiple UL frequency allocation options: 1, 3, 6, and 12 tones with 15 kHz per tone
- Allocation choice depends on UL and DL channel capacity

2.2 Semi-Persistent Scheduling (SPS) Frame Structure

Two SPS approaches are presented:

Approach 1 (Figure 5.2.2.3-2):
- NPDSCH can be positioned anywhere within first 15ms
- Maintains minimum 1ms gap to NPUSCH

Approach 2 (Figure 5.2.2.3-3):
- Based on "Cell_specific_Koffset" approach
- Does not depend on "TA report UE capability"

2.3 Gap Composition Between DL and UL

The gap consists of:
1. Processing time + DL-to-UL switching: Minimum 1ms for half-duplex device switching
2. Max differential delay: Accounts for different round-trip delays of UEs in NTN cell
- Typical range: close to 0 to 10.3ms depending on deployment

2.4 Baseline Assumptions for Codec Simulations

Key Changes Proposed:

For 80ms bundling:
- Original assumption: "Max differential delay" is 10ms AND X + Y ≤ 68ms
- Proposed change: Replace with reference to beam size no larger than 1500km
- Note clarifies this corresponds to scenarios where difference between closest and farthest point to satellite is <1500km
- Explicitly states codec can be deployed in scenarios not meeting these constraints

For 160ms bundling:
- Original assumption: "Max differential delay" is 10ms AND X + Y ≤ 148ms
- Proposed change: Replace with reference to beam size no larger than 1500km
- Same flexibility noted regarding deployment in other scenarios

2.5 Important Notes and Editor's Notes

RAN1 LS Clarification:
- Figure 5.2.2.3-1 supportable in most scenarios
- May not be supportable when:
- Cell is very large (e.g., >3000km)
- UE does not support TA report
- Network does not support UE-specific K-offset
- Requires UE configuration with two HARQ processes and HARQ feedback disabled

SPS Design Status:
- RAN1/2 have not yet started SPS design work
- RAN1 cannot currently confirm whether SPS frame structure examples (Figures 5.2.2.3-2 and associated text) will be supported

Editor's Note:
- Range of "Max differential delay" is TBC (To Be Confirmed)

3. Summary of Changes

The primary technical contribution is replacing specific timing constraint assumptions (X + Y values and max differential delay) with a more practical reference based on beam size ≤ 1500km for codec simulation baseline, while explicitly allowing codec deployment in scenarios exceeding these reference conditions. This provides clearer guidance to SA4 while maintaining flexibility for various deployment scenarios.