# 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.