# 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)
   - Current range: 825-4650 bits/s
   - Granularity appears insufficient and unequal

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

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

6. **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