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