Summary of S4-260214: Feasible Bitrates for NTN-TDL-C Channel Model with 10-Degree Elevation Angle

Background and Motivation

This contribution addresses the determination of feasible Transport Block Sizes (TBS) for the newly agreed NTN-TDL-C channel model at 10-degree elevation angle, which was adopted at SA4 #134 (documented in S4-252108). Key observations include:

• Two channel models now exist: The original channel model from TR 38.811 Table 6.9.2-3 and the new NTN-TDL-C model for 10-degree elevation
• Channel model validation: The new channel model shows better correlation with field data from NB-IoT GSO service with handheld devices
• Field data shows 1-3 dB gap between 1st and 50th percentile SNR
• New channel model: ~1 dB gap (consistent with field data)
• Initial channel model: ~6 dB gap (less consistent)
• TBS table update requirement: The TBS values in permanent document tables (5.2.2.1-1, 5.2.2.1-2, 5.2.2.1-3) should reflect the union of supported TBS values for both channel models

Simulation Methodology

The contribution evaluates maximum feasible bitrates under worst-case conditions without DMRS bundling, considering two scenarios:

Scenario 1: Ideal Timing

• 80ms bundling period, 2 UE RX, 15kHz SCS, 5Hz Doppler spread
• Without DMRS bundling
• No uncertainty in scheduling timing

Scenario 2: Timing Uncertainty

• 80ms bundling period, 2 UE RX, 15kHz SCS, 5Hz Doppler spread
• Without DMRS bundling
• 10ms uncertainty in scheduling timing (relevant for large cells without UE-specific Koffset or TA report)

Both scenarios target 2% BLER for uplink and downlink.

Simulation Results

Scenario 1 Results (No Timing Uncertainty)

• Maximum TBS: 936 bits
• Uplink: 15kHz SCS, 3 tones, 48ms (N_RU=6, N_rep=2)
• BLER: 1.5% at 31dBm UE TX power
• Downlink: 24ms (N_SF=6, N_rep=4)
• BLER: 1.1% at -3.3dB SNR

Scenario 2 Results (10ms Timing Uncertainty)

• Maximum TBS: 680 bits
• Uplink: 15kHz SCS, 3 tones, 40ms (N_RU=5, N_rep=2)
• BLER: 0.2% at 31dBm UE TX power
• Downlink: 20ms (N_SF=5, N_rep=4)
• BLER: 0.5% at -3.3dB SNR

Proposed Changes to Permanent Document

TBS Table Updates

The contribution proposes adding new TBS values to support the higher bitrates enabled by the new channel model:

For 80ms Bundling Period (Table 5.2.2.1-1)

• Add TBS 936 bits (PHY bitrate: 11.7 kbps, Net bitrate: 11.0 kbps)
• Add TBS 680 bits (PHY bitrate: 8.5 kbps, Net bitrate: 7.8 kbps)
• Add intermediate value between 680 and current maximum (424)

For 160ms Bundling Period (Table 5.2.2.1-2)

• Add corresponding new TBS values scaled appropriately

For 320ms Bundling Period (Table 5.2.2.1-3)

• Add corresponding new TBS values scaled appropriately

Terminology Change

• Change "codec bitrate" to "net bitrate" to clarify that this represents the bitrate available to the codec (after accounting for packet headers), not a required codec operating bitrate

Updated Tables

The proposed tables include:
- Packet header assumption: 7 bytes (with note that final size depends on SA2/RAN conclusions on 1-byte MAC header feasibility)
- Header counting: Packet header counted only once per bundling period, regardless of number of voice frames bundled
- TBS values: Selected from TS 36.213 Table 16.5.1.2-2 for NB-IoT NPUSCH
- Net bitrate calculation: PHY bitrate minus overhead from packet headers

The complete updated tables show TBS values ranging from 144 to 936 bits for 80ms bundling, with corresponding PHY and net bitrates calculated for each bundling period configuration.