Comprehensive Summary of 3GPP FS_ULBC Permanent Document

Document Overview

This permanent document (p-doc) version 0.45.0 supports the Study Item on Ultra Low Bitrate Speech Codec (FS_ULBC), focusing on developing recommendations for normative work on an ultra-low bit rate codec for voice over Geostationary Orbit (GEO) satellites. The document tracks agreements, open issues, and progress across the study objectives defined in the SID.

1. Introduction and Scope

The study addresses nine key objectives:
- Document application scenarios for ultra-low bit rate communication services
- Study GEO channel characteristics and derive service-related dependencies
- Identify relevant design constraints
- Provide feasibility evidence
- Define performance requirements and test methodologies
- Identify/develop objective measures for design constraint verification
- Identify reference codecs
- Coordinate with other 3GPP groups (SA2, RAN, CT1)
- Define potential normative work item objectives and timeline

Working Procedure:
- Maintains one TR and one p-doc
- Contributions via pCRs
- Brackets restricted to values only
- Open issues documented in p-doc

2. Application Scenarios

2.1 Main Scenario: IMS Voice Call over GEO

Key Technical Assumptions:

UE1 Uplink (UE1 → GEO satellite → Ground station):
- Transmission data rate significantly limited ([1-3] kbit/s)
- Requires ultra-low bit rate codec fitting this transmission rate
- Subject to transmission errors reflecting GEO satellite access
- Delay greater than typical terrestrial networks

UE1 Downlink (Ground station → GEO satellite → UE1):
- Similarly limited transmission data rate
- Subject to similar transmission errors and delay

UE2 Connection (Core Network → UE2):
- Regular TN network transmission data rate available
- Could use existing IMS codec (with transcoding) or same ULBC (transcoding-free)
- Transcoding functionality in core network likely needed for seamless communication across network types

2.2 Sub-Scenario

Both connections (UE1 and UE2) via GEO satellite with significantly limited transmission data rate ([1-3] kbit/s), allowing both transcoded and transcoding-free operation.

3. Channel Characteristics and Service-Related Dependencies

3.1 End-to-End Simulation Model

Methodology:
- Reuses simulation model from TS 26.132 Annex E (LTE reference scenario)
- Adapted for GEO access scenario with "new GEO channel"
- Potential inclusion of Non-IP Data Delivery (NIDD) option

Key Input Parameters:

BLER_tx/BLER_rx: Block error rates for uplink/downlink from RAN simulation

drx_cycle_length: DRX cycle duration (20-40ms for LTE; suitability for GEO TBC with RAN2)

mis_eNB1_eNB2: Scheduling time mis-alignment; determines buffer waiting time

nFrames considerations:
- Frame length: Maximum 80ms assumed for GEO (vs. 20ms for LTE)
- Voice packet size: Depends on protocol overhead (user plane vs. control plane, IP vs. Non-IP NIDD)
- RTP Payload Size: Product of frame length and codec bit rate

Editor's Note: SA2 concluded in TR 23.700-19 that voice packets shall be transported over NB-IoT (GEO) user plane.

3.2 RAN Simulation Model for Error Traces

Objective: Generate multiple loss traces for combinations of:
- Frame loss rate (target BLER)
- Raw bitrate (TBS)
- Voice bundling period
- Doppler spread

Simulation Parameters:
- Number of seeds: 10
- Trace duration: 400 seconds (6.67 minutes)
- Channel consistency: Same channel realizations across all combinations

3.2.1 Link Budget Analysis

Baseline CNR values from TR 36.763:
- UL CNR = 2.6dB (0dBi UE antenna gain, 3.75kHz SCS, 1 tone, 23dBm UE max TX power)
- DL CNR = -3.3dB (0dBi UE antenna gain, 15kHz SCS, 12 tones, 1 UE receive antenna, 23dBm UE max TX power)

3.2.2 Uplink Simulation Parameters

Channel model: NTN-TDL-C [38811]

Elevation angle: 10 degrees (parameters specified in Table 5.2.2.2-1)

Modulation: QPSK, π/2 BPSK

Subcarrier Spacing (SCS): 3.75kHz, 15kHz

Number of tones: 1 for both SCS values

Voice bundling period: 80ms, 160ms, 320ms
- Note: 40ms not considered due to insufficient time for DL transmissions with 3.75kHz SCS

Doppler spread: 1Hz, 5Hz

Target BLER: 1%, 2%, 6%, 10% (fixed target BLER is FFS)

Maximum Achievable SNR:
SNR = (3GPP SET-1 UL SNR) - 10×log₁₀(B/3.75) + (P - 23dBm) + G + [X] dB

Where:
- 3GPP SET-1 UL SNR = 2.6dB
- B = bandwidth (3.75kHz or 15kHz)
- P = max UE TX power (23, 26, 31 dBm)
- G = UE antenna gain difference (0 to -5.5dBi)
- X = TBD (accounts for lower loss, better satellite performance)

TBS Values and PHY Bitrates:

For 80ms bundling:
- TBS: 144, 256, 328, 424 bits
- PHY bitrate: 1.8, 3.2, 4.1, 5.3 kbps
- Codec bitrate: 1.1, 2.5, 3.4, 4.6 kbps (assuming 7 bytes packet header)

For 160ms bundling:
- TBS: 208, 424, 600, 808 bits
- PHY bitrate: 1.30, 2.65, 3.75, 5.05 kbps
- Codec bitrate: 0.95, 2.30, 3.40, 4.70 kbps

For 320ms bundling:
- TBS: 328, 776, 1096, 1544 bits
- PHY bitrate: 1.025, 2.425, 3.425, 4.825 kbps
- Codec bitrate: 0.850, 2.250, 3.250, 4.650 kbps

Notes:
- Packet header counted once regardless of bundled frames
- Loss of single TB means loss of multiple consecutive voice frames
- Need for 320ms bundling to be revisited after channel simulation results

3.2.3 Downlink Simulation Parameters

SCS: 15kHz

Number of tones: 12

Achievable SNR:
SNR = (3GPP SET-1 DL SNR) + G + [Y] dB

Where:
- 3GPP SET-1 DL SNR = -3.3dB
- G = UE antenna gain difference (0 to -5.5dBi)
- Y = TBD (accounts for 2 RX antennas providing up to 3dB gain, lower loss, better G/T values, better satellite performance)

Editor's Note: Four companies reported Y=3 due to better G/T from field measurements (-28.6dB/K vs. -31.6dB/K assumed), but no RAN1 consensus reached.

TBS values: Identical to uplink (Clause 5.2.2.2)

3.2.4 Frame Structure

Dynamic Scheduling Example (80ms bundling, Half-duplex FDD):
- NPDSCH duration: 4ms (variable depending on DL SNR)
- UL frequency allocation options: 1, 3, 6, 12 tones with 15kHz per tone

Semi-Persistent Scheduling (SPS):
- If specified by RAN for NB-IoT NTN
- NPDSCH can be anywhere in first 15ms (maintaining minimum 1ms gap to NPUSCH)
- "Cell_specific_Koffset" approach proposed (not dependent on "TA report UE capability")

Gap between DL and UL consists of:
- Processing time + DL-to-UL switching (minimum 1ms for half-duplex device)
- Max differential delay: [close to 0 to 10.3ms] (TBC)

RAN1 Note: Example frame structures supportable in most scenarios but may not work for very large cells (>3000km) when UE doesn't support TA report and network doesn't support UE-specific K-offset. RAN1/2 have not yet designed SPS.

3.3 Open Issues for NB-IoT GEO Simulation

Issue 1 - UE Power Class: Whether to use specified 23dBm or broader range (26, 29, 31, 33 dBm) - Pending RAN input

Issue 2 - Latitude-Dependent Loss: Scintillation loss (2.2dB or 0dB depending on latitude) - Solved (accounted via X term)

Issue 3 - Elevation Angles: Keeping both 2.3° and 12.5° - Solved (accounted via X term)

Issue 4 - UL/DL Guard Time: 1ms assumption - Pending RAN confirmation

Issue 5 - Candidate TBS Values: Multiple proposals from companies - Unsolved

Issue 6 - Approaches to Select TBS: Three approaches provided - Unsolved

Issue 7 - Overall Simulation Methodology: High-level description needed - Unsolved (to be addressed after simulation completion)

Issue 8 - Simulation Channel Model: NTN-TDL-C vs. NTN-TDL-C5 - Solved (NTN-TDL-C used)

Issue 9 - Protocol Overhead: Clarify packet header for different transport options - Pending RAN2/SA2 confirmation

Issue 10 - Repetition Numbers: Specify and report in simulation - Solved

Issue 11 - RX G/T for Downlink: 3dB better value observed in field - Unsolved

3.4 Alternative Methodology for Determining ULBC Bit Rate

Editor's Note: This methodology remains an open issue.

Proposed Steps:

1. Agree on operation points: Set of maximum achievable receive SNRs covering marginal to error-free operation with NTN-TDL-C fading

2. Define performance requirements for each SNR operation point

3. Agree on source bit rates for each bundling time (80, 160, 320ms) based on transport formats (TBS, SCS, MCS, NRep)

4. Current range: 825-4650 bits/s

5. Granularity appears insufficient and unequal

6. Determine optimum transport format (SCS, MCS, NRep) for each source bit rate based on BLER vs. SNR curves

7. Produce packet loss patterns for each bundling time and source bit rate at relevant SNRs (unknown to proponents during selection)

8. Compare ULBC candidates based on performance requirements at relevant SNRs

Example Workflow:
- Proponent has design at 0.95 kbps and 3.4 kbps
- For 160ms bundling with 7-byte overhead:
- Low rate: TBS = 208 bits
- High rate: TBS = 600 bits
- Select best transport format configuration from available options
- Generate BLER patterns for different UE TX powers (23, 26, 29, 31 dBm)
- Run codec simulation with these patterns
- Evaluate quality (e.g., listening test) with weighted averaging across power settings

Note: Important to test candidates for other conditions beyond NTN NB-IoT (e.g., Terrestrial IMS with 1% BLER, OTT with 0% BLER, extreme conditions with 10% BLER or blockage losses)

3.5 Simulation Results

Table 5-6 documents preliminary results:
- 80ms bundling: Qualcomm submitted S4-251739
- Company A, B, C: TBD

4. Design Constraints

4.1 Complexity and Memory Demands

Target Device Types:
- Handheld mobile phones
- Smart watches
- Smart glasses/head mounted devices
- TCU (Telematics Control Unit)
- CPE (Customer Premises Equipment)
- Vehicles
- Other IoT devices

Recommended Constraints:
- Implementable on DSP/CPU/NPU enabled UE devices
- For low-end DSP-only UEs:
- Complexity: <500 WMOPS (measured on C reference code)
- ROM memory: <20MB assuming 32bit/parameter (or 5M model parameters)

Editor's Notes:
- Definition of "DSP enabled UE devices" needs clarification
- Exact complexity estimation metric and limits are TBD

4.2 Design Constraint Verification

Complexity Verification:
- Constraints may be based on platform-agnostic metrics:
- MACs/FLOPs for AI-based components
- WMOPS for traditional signal processing
- Model size and precision
- Verification process details and timing are FFS

Algorithmic Delay:
- Verification method for AI-based codecs required

5. Performance Requirements

5.1 Scope

Define performance requirements and test methodologies for:
- Speech quality, intelligibility, conversational quality
- Clean speech and noisy speech
- Tandeming with existing IMS voice codecs
- Clean channel and GEO channel conditions
- Identify relevant reference codecs

5.2 Status Tracking

Core influencing factors identified:
- DC: Sample rate and audio bandwidth
- DC: Bitrates (External dependency)
- DC: Frame length
- DC: PLC (External dependency)
- DC: Algorithmic Delay
- DC: Complexity, Memory
- Test Methodologies
- DC: Noise suppression
- DC: DTX/CNG
- DC: Robust Non-Speech
- Evidence DCs
- Reference codec

All items currently have open issues and progress TBD

6. Coordination and Dependencies

6.1 External Dependencies

From RAN:
- HARQ retransmission parameters (max_tx/max_rx)
- DRX cycle length suitability for GEO
- Scheduling parameters (dynamic vs. SPS)
- Frame structure confirmation
- UE power class
- UL/DL guard time
- Protocol overhead
- G/T values for downlink

From SA2:
- Transport path for voice packets (user plane vs. control plane, IP vs. Non-IP NIDD)
- Protocol overhead details
- Transcoding functionality requirements

From RAN2:
- Dynamic scheduling vs. Semi-Persistent Scheduling
- MAC header size (1-byte feasibility)
- Timing parameters

7. Key Technical Contributions

7.1 Simulation Framework Establishment

The document establishes a comprehensive RAN simulation framework for generating error traces:
- Defined methodology using NTN-TDL-C channel model
- Specified uplink and downlink parameters
- Established TBS values and corresponding codec bitrates for multiple bundling periods
- Defined channel consistency requirements across simulations

7.2 Link Budget Analysis

Adopted baseline CNR values from TR 36.763 with provisions for:
- Variable UE power classes
- Latitude-dependent losses
- Elevation angle variations
- Better-than-assumed satellite performance

7.3 Bitrate Determination Methodology

Proposed alternative methodology allowing proponents design freedom:
- Operation point definition based on receive SNRs
- Transport format optimization for each source bit rate
- Packet loss pattern generation
- Comparative evaluation framework

7.4 Frame Structure Definition

Defined frame structures for:
- Dynamic scheduling with Half-duplex FDD
- Semi-Persistent Scheduling options
- Cell_specific_Koffset approach for large cells

7.5 End-to-End Delay-Error Profile Model

Adapted TS 26.132 Annex E model for GEO scenarios:
- Identified required input parameters
- Defined voice packet structure with protocol overhead
- Established relationship between frame length, bundling, and packet loss

8. Open Issues Summary

High Priority (Blocking):
1. Consensus on UE power class (23 dBm vs. higher values)
2. RAN confirmation on frame structures and scheduling
3. SA2/RAN2 confirmation on protocol overhead
4. Selection of candidate TBS values and selection methodology
5. Downlink RX G/T value consensus

Medium Priority:
1. Fixed vs. variable target BLER
2. Need for 320ms bundling option
3. Complexity metric definition and limits
4. Algorithmic delay verification for AI codecs

Lower Priority:
1. Overall simulation methodology description (after completion)
2. Definition of "DSP enabled UE devices"

9. Document Status

Current Version: 0.45.0 (SA4#135, February 2026)

Recent Updates:
- Added 10-degree channel model parameters
- Updated simulation parameters per multiple agreed TDOCs
- Added company simulation results reporting
- Clarified voice packet transport over user plane

Working Status:
- Active study phase
- Collecting simulation results from companies
- Coordinating with RAN and SA2 for parameter confirmation
- Developing design constraints and performance requirements