# Summary of 3GPP Technical Document: Updates to FS_ULBC Permanent Document

## Document Overview

This contribution updates the FS_ULBC (Ultra Low Bitrate Speech Codec) Permanent Document to align with SA2 conclusions on Key Issue #1 regarding IMS voice call support over NB-IoT via GEO satellite connecting to EPC, as documented in TR 23.700-19.

## Main Technical Contributions

### 1. Reference Updates

The document adds critical new references to align with recent 3GPP work:

- **TR 23.700-19 V1.2.0**: Study on Integration of satellite components in the 5G architecture; Phase 4
- **S2-2509293**: Interim conclusions on KI#1 Support of IMS voice call over NB-IoT NTN via GEO satellite connecting to EPC
- **TR 36.763**: Study on NB-IoT/eMTC support for Non-Terrestrial Networks
- **R1-2506541**: Reply LS on RAN simulation assumptions for ULBC

### 2. End-to-End Simulation Model Updates (Clause 5.2.1.3)

#### 2.1 Architecture and Protocol Stack Changes

The document introduces significant modifications to the end-to-end simulation model:

- **New GEO Channel Model**: Extends the reference LTE scenario (Annex E of TS 26.132) to accommodate GEO satellite access
- **Three Architectural Scenarios Defined**:
  - Reference LTE VoLTE scenario (Figure 5.2.1.3-1)
  - Main GEO scenario with IP transport (Figure 5.2.1.3-2)
  - GEO scenario with Non-IP Data Delivery option (Figure 5.2.1.3-2a)

#### 2.2 Transport Mechanism Agreements

Based on SA2 conclusions in TR 23.700-19:

- **User Plane Transport**: Voice packets shall be transported over NB-IoT (GEO) user plane using DRB and S1-U
- **Single PDN Connection**: Both IMS signaling and IMS voice use a single PDN connection
- **Mandatory Mechanism**: Transport of IP packets (UP/IP) with RoHC recommended
- **Optional Mechanism**: Transport using removal and restoration of parts of RTP/UDP/IP headers (UP/non-IP)

#### 2.3 Simulation Input Parameters

Key parameters updated for GEO scenarios:

- **BLER_tx/BLER_rx**: Block error rates for UL/DL based on error traces from Clause 5.2.2
- **max_tx/max_rx**: HARQ retransmissions (note: HARQ feedback suggested to be disabled for IMS voice over GEO per Release 18)
- **drx_cycle_length**: DRX cycle duration (LTE values 20-40ms, suitability for GEO requires RAN2 confirmation)
- **mis_eNB1_eNB2**: Scheduling time misalignment between eNBs
- **Speech sequence frame length**: Maximum 80ms frame length for GEO (vs. 20ms for LTE)
- **Voice packet size**: Depends on protocol overhead, varies by transport mechanism

#### 2.4 Protocol Overhead Considerations

Two protocol overhead scenarios illustrated:

- **UP/IP with RoHC** (Figure 5.2.1.3-4 left): Mandatory mechanism
- **UP/non-IP with header removal** (Figure 5.2.1.3-4 right): Optional mechanism

**Editor's Note**: Exact overhead for UDP/IP (SA2 scope) and RTP (SA4 scope) for the removal/restoration mechanism requires determination.

### 3. Simulation Assumptions and Open Issues (Clause 5.2.2.4)

#### 3.1 Resolved Issues

| Issue | Resolution |
|-------|-----------|
| **Latitude-Dependent Loss** | Simulation accounts for latitude-dependent scintillation loss using X term (2.2 dB or 0 dB beyond ±20° latitude per TR 38.821) |
| **Elevation Angles** | Both 2.3° and 12.5° angles considered using X term for worst-case scenarios |
| **Simulation Channel Model** | NTN-TDL-C selected |
| **Repetition Numbers** | Specified and reported in simulation |

#### 3.2 Pending Issues Requiring RAN Input

- **UE Power Class**: 23 dBm (specified for NTN NB-IoT) vs. commercial UE range (26-37 dBm) - requires RAN confirmation
- **UL/DL Guard Time**: 1ms assumption needs RAN verification
- **RX G/T for Downlink**: Field observations show 3dB better performance than current RAN assumptions

#### 3.3 Unresolved Issues

- **Candidate TBS Values**: Multiple proposals from Xiaomi, Fraunhofer, Skylo, Dolby, Huawei, Qualcomm, and vivo require evaluation
- **TBS Selection Approaches**: Three approaches in S4aA250072 need discussion
- **Overall Simulation Methodology**: High-level description to be completed after simulation work
- **Protocol Overhead for UP/non-IP**: Exact overhead values for removal/restoration mechanism depend on specific RTP fields selected (SA4 decision)

#### 3.4 Updated Understanding on Protocol Overhead

Based on SA2 agreements:

- **Control Plane transport excluded**: Only User Plane transport considered
- **Mandatory**: UP/IP with RoHC recommended
- **Optional**: UP/non-IP with partial header removal/restoration
- Exact overhead values for optional mechanism pending SA4 decisions on RTP field selection

## Key Dependencies and Cross-WG Coordination

The document identifies several inter-working group dependencies:

- **RAN1**: Physical layer timing, power class confirmation
- **RAN2**: HARQ configuration, DRX cycle parameters, scheduling mechanisms
- **SA2**: UDP/IP overhead for non-IP mechanism
- **SA4**: RTP overhead, frame length confirmation, RTP field selection for header removal

## Editor's Notes

Two critical editor's notes remain:

1. Whether the eNB1-eNB2 delay model for LTE scenarios accurately reflects GEO deployment delays
2. Whether RTP payload size affects the delay-error profile