Report from Audio SWG AH on FS_ULBC (December 2, 2025)
The Audio SWG met on December 2, 2025, with 35 participants to address FS_ULBC matters, prioritizing non-treated documents from SA4#134. Key discussions covered:
Codec Selection Process: Different approaches outlined (traditional 3GPP, collaborative, MPEG-style, combined). Concerns raised about timeline feasibility and deviation from established 3GPP processes. Offline discussions invited.
Bundling Periods: Impact on user experience discussed for conversational vs push-to-talk scenarios, particularly regarding 320ms bundling period relevance.
Design Constraints: Agreement to collect all design constraint proposals into the PD with references and brackets for collective editing. Stephane R. volunteered to draft.
Codec Bitrate and Capacity: Analysis presented with differing views on realism. Offline meeting to be arranged by simulation coordinator.
Transport Path: Editor's note agreed for PD reflecting SA2 decision on user plane, with IP vs non-IP kept open.
Tandem Conditions: Proposal for separate transcoding function noted, with discussion on design flexibility.
Performance Requirements: Quality targets and reference codecs discussed. Revision suggested based on feedback.
Packet Loss Concealment: DAC experiment results agreed for inclusion in TR. Existing statement on design freedom for bitrate/BLER retained.
RAN1 LS Interpretation: Ongoing disagreement on interpretation. Offline discussions to continue.
Simulation Assumptions: Update agreed for PD referencing simulation process and link budget analysis.
The Audio SWG Chairman, Tomas Toftgård (Ericsson), opened the call at 22:00 CET on December 2, 2025. Standard IPR, antitrust, and consensus principles reminders were provided. Notes were taken by Thomas Stockhammer with co-pilot assistance.
Presenter: Jiayi Xu (China Mobile)
Content:
- Four selection approaches outlined:
- Traditional "winner takes all" (3GPP style)
- Open collaborative (outside 3GPP, e.g., IETF)
- MPEG-style process (call for proposals with collaborative refinement)
- Combined approach (selecting baseline(s) for optimization within 3GPP)
- MPEG/JPEG AI processes described as potential models
- Proposal for call for proposals after TR phase
- Open questions: dataset definition/sharing, Release 20 timeline feasibility, existing vs new codec, backward compatibility
Discussion:
- Stefan B: Questioned motivation for deviating from typical 3GPP work item approach
- Jiayi: Clarified intent to initiate discussion, not push specific direction; open to traditional 3GPP mechanism
- Thomas: Emphasized timeline focus; noted MPEG process typically takes 3-4 years, incompatible with Release 20
- Zhe: Expressed support for collaborative approach
- Lasse: Concerned about proper motivation needed before changing established 3GPP approach; cited successful EVS/IVAS collaborations
Decision: S4aA250128 noted. Offline discussions encouraged.
Presenter: Lasse Laaksonen (Nokia, Dolby)
Content:
- Impact of bundling periods (80, 160, 320ms) on user experience examined
- Longer periods (320ms) significantly affect delay and packet loss
- Clarification needed on target use cases (conversational vs push-to-talk)
- No specific proposal; open for discussion
Discussion:
- Markus: Supported reconsidering 320ms period if 160ms works well for transport; concerns about laggy experience
- Zhe Wang: Agreed on reconsidering 320ms relevance unless required by transport constraints
Decision: S4aA250129 noted.
Presenter: Stephane Ragot (Orange)
Content:
- Proposal to consolidate all design constraint proposals into single document
- Reference EVS approach for handling multiple inputs
- Starting point: EVS codec supporting multiple bandwidths
- 8 kHz likely too low for quality; 48 kHz may be unnecessarily high
- Possibility of single bandwidth to reduce deployment complexity
- Support key sampling rates for interworking
Discussion:
- Huan-yu: Supported collecting proposals; suggested parallel development with radio simulations
- Atti: Supported single sample rate to simplify SDP negotiations and remove bandwidth negotiation complications
- Juan: Suggested separating sample rate and audio bandwidth discussions
- Milan: Clarified unique sampling rate discussion essentially about maximum bandwidth
- Jiayi: Asked about comparing codecs with different sample rates
- Stephane: Referenced extensive EVS testing experience with multi-bandwidth comparisons
Agreement:
- Collect all design constraint proposals into PD with references and brackets
- Collective editing at later stage for fairness and efficiency
- Stephane volunteered to draft initial collection
Decision: S4aA250130 noted.
Presenter: Huan-yu Su (vivo, Samsung, Spreadtrum, MediaTek)
Content:
- Analysis of codec bit rate impact on radio resources and user capacity
- Higher bit rates significantly reduce supported users
- Proposal: limit codec bit rate to maximum 3 kbps for network efficiency
Discussion:
- Liangping:
- Analysis overly pessimistic; referenced Qualcomm contribution showing support for all bit rates with 80ms bundling
- Simulation flawed: only one tone used for 15 kHz subcarrier spacing; more tones should be used for higher TBS
- Using more tones would lower coding rate and improve performance
- Table 5 shows high coding rate (.46) with one tone; three tones would bring below 1/3
- Even current simulation shows multiple users supported at 31 dBm
- Resource allocation decisions should be left to network operators, not fixed by codec specification
- Huan-yu:
- Disagreed; 23 dBm devices are low-end with limitations
- Designing for ideal scenarios inappropriate
- Leaving decisions entirely to operators not advisable; should provide range but not extremes
- Stefan Bruhn:
- Scheduling in figure one not optimized; other schemes could allow better capacity
- Questions on transport block size gaps and uplink/downlink resource balancing
- Dong: Requested Liangping organize offline meeting as simulation coordinator
Decision: S4aA250131 noted. Offline meeting to be organized by Liangping.
Presenter: Huan-yu Su (vivo)
Content:
- Updates based on 3GPP TR 23.700-19 V1.1.0
- Recommendation: use user plane instead of control plane for GEO satellite communication
- Proposed additions: editor's note, updated references, table entry changes
Discussion:
- Jiayi: Asked whether editor's note for Pdoc or TR; concerns about referencing SA1/SA2 Tdocs in TR
- Tomas: Clarified proposal for Pdoc, not TR; referencing acceptable in Pdoc
- Markus: Asked about other interim solutions besides user plane
- Dong: Explained control plane/user plane decision finalized; only IP vs non-IP remains open
- Atti: SA2 agreed IP-based user plane mandatory, non-IP optional
Agreement:
- Add editor's note regarding user plane solution for GEO satellite communication
- Update table as needed
- Keep IP vs non-IP open for future updates
Decision: S4aA250132 noted.
Presenter: Noboru Harada (NTT)
Content:
- Example scenarios for ULBC interworking: native ULBC and tandem with legacy systems
- Table 1: target codecs for tandem conditions (updated)
- Assumption: ULBC operates on single bandwidth; transcoder adjusts for legacy codecs
- Transcoder modifies sampling rates using bandwidth extension
- Proposal: design constraints for transcoding function separate from ULBC end-user terminals
- Transcoder runs on more capable devices (e.g., PC)
Proposals:
1. Reflect tandem condition scenarios in Pdoc
2. Maintain/update target codec list for future VR reference
3. Set specific design constraints for tandem conditions and transcoding
Discussion:
- Noboru: Clarified transcoder operates in separate node, not ULBC device; different complexity/memory constraints applicable
- Tomas: Supported clarification
- Zhe Wang: Asked if proposal requires two sets of technical solutions
- Noboru:
- Not requiring two solutions
- Goal: ensure good quality after transcoding
- Bandwidth extension/conversion not required in ULBC device; can be handled by transcoder
- Provides design flexibility
- Atti: Asked about expected transcoder performance
- Noboru: Not yet at stage of specifying performance requirements; focus on scenarios and constraints
- Stephane: Suggested including EVS Wideband 13.2 channel aware mode in target codec list
Decision: S4aA250133 noted.
Presenter: Atti Venkatraman (Apple)
Content:
- Minimum performance requirements for TR 26.940
- At lowest operating bit rate (1 kbps): speech quality at least as good as ("NWT") AMR-WB at 12.65 kbps
- At higher operating range (~3 kbps): match or exceed EVS super wideband at 13.2 kbps
- Benchmarks using widely deployed codecs
- Starting list of minimum reference codecs/operating points, expandable
Discussion:
- Huan-yu: Concerned about overspecifying bandwidth; setting targets too high (e.g., 24.4 kbps) unrealistic for GEO satellite
- Atti: Clarified not setting 24.4 kbps as target
- Milan: Suggested EVS at lowest bit rate (7.2/8 kbps) as state-of-the-art wideband reference instead of AMR-WB 8.85 kbps; questioned relevance of higher bit rates (23.85, 24.4 kbps)
- Atti: Reference codec list not meant to exclude others; focus on minimum performance requirements similar to current IMS voice services
- Stefan: Asked about feasibility at 3 kbps, especially with packet loss; questioned comparison conditions
- Atti:
- Broad framework similar to EVS SA1 approach
- Detailed tables to be developed for operating points
- Comparison at 3 kbps as general minimum requirement
- Further granularity for clean speech, noisy speech, packet loss conditions
- Minimum quality benchmarks reflecting current IMS experience
- Erik: Questioned if "better than AMR-WB 12.65 kbps" at 1 kbps too challenging for emergency situations
- Atti: Clarified proposal is "NWT" (not worse than), not "better than"
- Lasse: Suggested including bit rates in brackets for flexibility
Agreement:
- Atti to revise proposal incorporating comments (brackets for bit rates, clarified references)
- Updated version for next meeting
Decision: S4aA250135 noted.
Presenter: Markus Schnell (Fraunhofer IIS)
Content:
- Experiment based on DAC codec
- DAC IBM condition at 1.5 kbps with low packet loss rate
- Other configurations from existing TR experiment
- Results: higher bit rates with higher packet loss can outperform lower bit rates with lower packet loss for DAC
- DAC IBM model (trained for low bit rates) outperformed all other conditions
- Conclusions:
- Optimal performance under error-prone conditions achieved by models trained for specific bit rates and block error rates
- Bit rate scalability (as in DAC) comes with significant performance cost at lower bit rates
- Specifically trained models for certain operation modes may be more efficient
Agreement:
- Add new experiment and two proposed observations/conclusions to TR
- Keep existing statement about allowing design freedom regarding bit rate scalability
Decision: S4aA250136 agreed.
Presenter: Liangping Ma (Qualcomm)
Content:
- Updates to PD incorporating round one LS revisions
- Key changes: references, uplink/downlink parameter corrections, modulation types, number of tones for subcarrier spacing, bundling period support, revised notes reflecting LS
- Text added to figures and notes aligning with LS exceptions and conditions
Discussion:
- Tomas: Pointed out uplink/downlink parameter mix-up; questioned correct number of tones and noise figures
- Stefan: Noted downlink section incorrectly referenced UE maximum TX power
- Erik: Disagreed with updating based on specific LS interpretation; meaning not clear; premature before further clarification through offline coordination
- Liangping:
- Clarified two separate issues: RAN1 LS interpretation vs decision on supporting special exception case
- RAN1 LS clearly defines single exception case with three components
- Intent to accurately interpret RAN1 LS
- Supporting special case is separate discussion
- Stefan: Emphasized need for stable assumptions all companies can agree on for consistent simulations; referenced Dallas disagreements; supported continued offline coordination
Conclusions:
- Concerns from Ericsson regarding agreement on proposed changes
- Topic debated in several meetings with different views
- Continue offline discussions between Ericsson, Qualcomm, and interested parties
- Expand discussions beyond bilateral to include all relevant stakeholders
- Thomas noted discussions are very RAN-centric
Decision: S4aA250137 noted.
Presenter: Liangping Ma (Qualcomm)
Content:
- pCR to PD adding two previously agreed Tdocs:
1. Simulation contribution process (detailing assumptions for alignment)
2. Link budget analysis (verified numbers from RAN1 specifications, real-world deployment scenarios)
- Help companies understand possible deployment deviations (e.g., scenarios where certain losses negligible)
Discussion:
- Stefan: Concerned about need for stable simulation assumptions; referenced Dallas disagreements; questioned if addition helps achieve consensus
- Liangping: Document agreed upon; provides details on possible deviations from 3GPP assumptions; helps justify simulation choices; conclusions confirmed by RAN1
Agreement:
- Add proposed text and references to PD
- Include note clarifying document provides analysis of possible deviations from 3GPP assumptions
Decision: S4aA250138 agreed.
Presenter: Liangping Ma (Qualcomm)
Content: Analysis and recommended handling of reply liaisons from other working groups to SA4 on ULBC
Decision: S4aA250139 noted without presentation (priority given to non-treated SA4#134 documents; no time for further consideration). Revision provided as new document.
Presenter: Xuzhou Ye (Bytedance, vivo)
Content: Complexity evaluation of ULBC audio codec
Decision: S4aA250127 noted without presentation (already treated at SA4#134; no time for further consideration).
The chair thanked delegates for their contributions and discussions. The call closed at 23:29 CET.
Meeting focused on FS_ULBC (item 1.7) with documents prioritized from SA4#134.
35 participants from organizations including: Apple, Bytedance, CMCC, Dolby, Ericsson, Fraunhofer IIS, Huawei, Lenovo, MediaTek, Motorola, Nokia, NTT, Orange, Panasonic, Philips, Qualcomm, Samsung, Spreadtrum, Vivo, VoiceAge, Vodafone, Xiaomi, and others.