# Summary of S4-250275: On Device Capability Diversity for ULBC

## Overview

This document (revision of S4aA260006) addresses UE capability diversity in NB-IoT NTN deployments for ULBC voice services. It proposes a capability-aware system design approach rather than assuming uniform baseline UE capabilities, accompanied by a pCR to TR 26.940.

## Key Technical Contributions

### 1. UE Capability Diversity Framework

**Identified Capability Dimensions:**

- **Transmit Power Classes:** 
  - Baseline: PC3 (23 dBm)
  - Enhanced: PC2 (26 dBm) or PC1 (31 dBm)
  - Future: up to 37 dBm under study for Rel-20

- **Receive Antenna Configurations:**
  - Standard: Single RX antenna
  - Enhanced: Dual RX antennas (providing ~3 dB gain)

- **Advanced Features:**
  - Improved RF sensitivity
  - Multi-tone NPUSCH transmission capability

**Key Insight:** These capabilities are **optional** and vary across device categories, market segments, and implementations.

### 2. Benefits of Enhanced Capabilities

Enhanced UE capabilities enable:

- **Reduced time-domain resource usage** in half-duplex NB-IoT transmission
- Overcoming limitations of 80 ms SPS periods (excessive BLER and capacity constraints)
- **Multi-tone NPUSCH transmission** for:
  - Higher ULBC bitrates
  - Reduced time-domain resource usage
  - Improved link robustness (reduced packet error rates)

### 3. Capability-Aware Multi-User SPS Scheduling

**Proposed Scheduling Strategy:**

- **Dynamic SPS Assignment:** Enhanced UEs use shorter SPS periods (80 ms) while baseline UEs use longer periods (160/320 ms)
- **Multi-Tone Transmission:** Enhanced UEs utilize multi-tone NPUSCH formats
- **Load Balancing:** Resource allocation prioritized based on UE capability
- **Service Differentiation:** Three-tier service model:
  - **Baseline Service:** Conservative configurations (long SPS, single-tone NPUSCH)
  - **Intermediate Service:** Moderate enhancements (shorter SPS, possible multi-tone)
  - **Enhanced Service:** Higher bitrates and reduced latency (shortest SPS, multi-tone, dual RX)

**Practical Example (Figure 1):**

- **UE Type A (Baseline):** 160 ms SPS, 128 ms single-tone NPUSCH, 950 bits/s net bitrate (TBS 208 bits)
- **UE Type B (Intermediate):** 80 ms SPS, 64 ms NPUSCH (higher TX power), 1100 bits/s net bitrate (TBS 144 bits)
- **UE Type C (Enhanced):** 80 ms SPS, 64 ms NPUSCH, reduced NPDSCH duration (dual RX), 1100 bits/s net bitrate (TBS 144 bits)

### 4. ULBC Bitrate Differentiation

**Proposed Approach:**

- Leverage UE capability diversity for bitrate differentiation
- Align bitrates with service tiers and UE capabilities
- **Recommended minimum set of 3 ULBC target bitrates:**
  - Basic tier: [600 - 1000] bits/s
  - Intermediate tier: [1000 - 1800] bits/s
  - Enhanced tier: [1800 - 3000] bits/s
  - Higher bitrates may be considered in second ULBC standardization phase

**Note:** Actual bitrates subject to ongoing TBS discussions; values >3000 bits/s may become relevant.

## Proposed Changes to TR 26.940 (pCR)

### Section 5.2.4: New Clause on UE Capabilities

Documents capability variations for NB-IoT NTN:

- **Transmit Power Classes:** PC3/PC5 (Rel-18), PC1/PC2 (Rel-19), potential >31 dBm (Rel-20)
- **Receive Antennas:** Single (typical) vs. dual (enhanced)
- **Enhanced Capabilities:** Higher TX power, improved RF sensitivity

### Section 5.2.5: Enhanced Multi-User Considerations

Replaces assumption of uniform UE configuration with capability-aware scheduling:

- **Capability-Aware Resource Allocation:** Different SPS periods based on UE capabilities
- **Multi-Tone Transmission for Enhanced UEs:** Increased bitrate and/or reduced resource usage
- **Dynamic Load Balancing:** Optimized capacity through capability-based prioritization
- **Service Level Differentiation:** Three-tier service model aligned with UE capabilities

Includes Figure 1 demonstrating practical multi-user scheduling scenario with three UE types.

### Section 5.1.2.2: UE Delay Tables

Updates delay estimation tables (5.1.2-2, 5.1.3-1) to include:

- Voice bundling periods: 80, 160, 320 ms
- Codec frame sizes: 20, 40, 80, 160, 320 ms
- Mouth-to-ear delay estimates for GEO-TN and GEO-GEO scenarios

## Recommendations

1. **Adopt capability-aware ULBC system design** rather than assuming single baseline configuration
2. **Agree on minimum set of 3 ULBC target bitrates** for codec evaluation (approximate ranges: [600-1000], [1000-1800], [1800-3000] bits/s)
3. **Document agreed ULBC target bitrates in Pdoc**
4. **Consider higher bitrates in second ULBC standardization phase**

## References

Key dependencies: S4aA260006 (previous version), S4-260144 (TR 26.940 v0.5.1), S4-260255 (ULBC Re-Focus Proposal), TS 36.763 (UE radio transmission/reception), S4-251863 (system capacity), S4aA250112 (error trace methodology), S4aA250118 (RAN simulation results)