Summary of 3GPP CR S4-260159: On Codec Bitrate and Capacity Discussion for ULBC

1. Introduction

This CR addresses the TBS (Transport Block Size) and codec bitrate values for ULBC (Ultra Low Bitrate Codec) evaluation, which are currently noted as 'companies reported' in TR 26.940 v0.4.0. The contribution provides analysis on:
- Multiplexed UE number analysis
- Confirmation of TBS/Codec bitrate values

2. Technical Analysis

2.1 Multiplexed UE Number Analysis

The document presents a methodology for calculating supported UE numbers considering:
- TDM (Time Division Multiplexing): Both UL and DL can schedule different UEs in TDM manner
- FDM (Frequency Division Multiplexing): UL can additionally use FDM since NPUSCH may occupy few subcarriers within 180kHz bandwidth
- FDM capacity: 48 UEs for 3.75kHz SCS (single tone)
- FDM capacity: 12 UEs for 15kHz SCS (single tone)
- Bi-directional constraint: Final supported UE number is the minimum of UL and DL capacity

2.2 Capacity Evaluation Results

Analysis conducted under 50-degree elevation channel model with 2% BLER:

Key Observations:
- Observation 1: Higher UE transmit power leads to higher capacity (multiplexed UE number) for a given codec bitrate
- Observation 2: For codec bitrate of ~3kbps, capacity is limited to ~10 UEs with 31dBm UE power. Capacity further degrades with increased bitrate (e.g., ≤5 UEs for 4.5kbps)

Performance characteristics:
- 23 dBm UE power shows very poor capacity
- Performance improves with higher power UEs (26 dBm, 31 dBm)
- Capacity increases with ptime value

2.3 Benchmark Considerations

• SA1 assumes transmission data rate of 1-3kbps as benchmark
• Commercial GEO system (Tiantong) operates within 0.8-2.4kbps range (per clause 5.2.1.3 of reference [1])
• Real-world deployments support focusing on bitrates below 3kbps

Additional analysis provided in Annex assuming 1% BLER under 10-degree elevation channel model.

3. Proposed Changes to TR 26.940

3.1 TBS and Codec Bitrate Tables

The CR proposes specific TBS values selected from TS 36.213 table 16.5.1.2-2 for NB-IoT NPUSCH, with corresponding PHY bitrates and codec bitrates calculated for each bundling period (assuming 7-byte packet header).

Table 1: 80ms bundling
- TBS range: 88-256 bits
- PHY bitrate range: 1.1-3.2 kbps
- Codec bitrate range: 0.4-2.5 kbps

Table 2: 160ms bundling
- TBS range: 120-424 bits
- PHY bitrate range: 0.75-2.65 kbps
- Codec bitrate range: 0.4-2.30 kbps

Table 3: 320ms bundling
- TBS range: 208-808 bits
- PHY bitrate range: 0.65-2.52 kbps
- Codec bitrate range: 0.475-2.35 kbps

3.2 Additional Notes

NOTE 1: Final packet header size depends on SA2 and RAN conclusions, including feasibility of 1-byte MAC header

NOTE 2: Packet header counted only once regardless of bundled voice frames

NOTE 3: Relationship between voice frame duration and bundling time depends on RTP payload design. Loss of single TB means loss of multiple consecutive voice frames when bundled.

4. Proposals

Proposal 1: Agree that codec bitrate should be bound to be less than 3kbps

Proposal 2: Agree to the proposed changes to Section 5.2.2.2 (Uplink simulation parameters) of TR 26.940, including:
- Updated TBS values and PHY bitrates tables
- Voice bundling periods: 80ms, 160ms, 320ms (40ms excluded due to insufficient time for DL transmissions with 3.75kHz SCS)
- Target BLER values: 1%, 2%, 6%, 10%
- Maximum Achievable SNR formula incorporating UE power (23/26/31 dBm), bandwidth, and antenna gain variations

5. Supporting Information

The Annex provides additional multiplexed UE number analysis for different codec bitrates and UE power levels under 10-degree elevation channel model, supporting the main technical conclusions.