6G    

 

 

 

TDoc Tracking - Frame Structure

For 6G, a clear direction emerges: use 5G NR’s frame structure and numerology as the 6GR baseline to keep 5G–6G MRSS efficient and deployments straightforward, changing only where 6G truly benefits (notably for FR3). In numerology, companies argue to minimize options—ideally one SCS per band/sub-FR and, where possible, align SSB and DL-BWP SCS—retaining 15 kHz for FR1 FDD, 30 kHz for FR1 TDD, and 120 kHz for FR2; for FR3 (~7–15 GHz) they study 30/60/120 kHz, with results indicating 60/120 kHz suit higher bands and modulation orders while 30 kHz remains attractive near ~7 GHz in larger-delay-spread scenarios. Target channel bandwidths widen—at least 200 MHz in upper FR1 and FR3 (with alternatives like 8k FFT or 60 kHz SCS to realize 200 MHz), and 400 MHz in FR2-1—with several proposals capping max FFT at 8k (16k FFS). For low-tier IoT, a minimum 3–5 MHz CBW in low FR1 is suggested to smooth migration from LTE Cat-M while preserving common design, and multiple papers call to simplify BWP. Finally, to cut latency/complexity and improve coverage and MRSS operation, there’s momentum to relax slot-boundary restrictions (e.g., allow PxSCH/PUCCH to span slots) while keeping the slot as a logical timing unit

Design principles & MRSS compatibility

In designing the 6G frame structure, one of the foremost principles is to ensure compatibility with existing NR systems through efficient Multi-RAT Spectrum Sharing (MRSS). To achieve this, the 6G design seeks to reuse NR’s fundamental timing framework—retaining the 10 ms radio frame, 1 ms subframe, and slotted structure—as the baseline. This continuity not only simplifies 5G–6G coexistence but also reduces deployment complexity for operators. At the same time, the emphasis is on simplicity and efficiency, avoiding the excessive flexibility and configuration overheads that complicated NR deployments. Energy efficiency, implementation ease, and forward compatibility across diverse device types are prioritized, with changes introduced only when 6G can deliver tangible benefits, particularly in new spectrum ranges such as FR3. Finally, MRSS enhancements are envisioned as lightweight additions, leveraging strategies like signal sharing or rate-matching around a limited set of NR always-on signals, rather than introducing heavy, custom mechanisms that could constrain 6G design freedom or increase overhead

  • Reuse NR timing as baseline — Keep 10 ms frame / 1 ms subframe / sloted structure to make 5G–6G MRSS efficient and deployments straightforward.
  • Prioritize simplicity & efficiency — Aim for low complexity, energy efficiency, and forward compatibility; change only where 6G truly benefits (e.g., FR3).
  • Minimal MRSS add-ons — Prefer lightweight mechanisms; consider signal sharing vs. rate-matching around NR signals without over-customizing the frame.

Design Principles and Guidelines

  • Lessons learned from NR: 6G design should avoid introducing unnecessary flexibility that was rarely used in NR deployments, since such options added testing and implementation complexity without delivering real benefits.
  • Numerology alignment for MRSS: To make spectrum sharing seamless, the same subcarrier spacing used in NR bands should also be used in 6G, especially for synchronization signals and initial BWPs, to prevent complicated mixed-numerology operations.
  • Avoid performance trade-offs for coexistence: MRSS should not restrict or compromise 6G’s intrinsic design goals. The priority is to keep 6G optimized for its own KPIs while enabling coexistence with NR. Any MRSS solution should minimize penalties on spectral efficiency, aiming for a much smaller impact than earlier LTE–NR DSS approaches.
  • Reuse NR frame elements as logical units: The basic structure of symbols, slots, subframes, and frames from NR can largely be inherited, ensuring straightforward MRSS compatibility and reducing the need for unnecessary redesign.
  • Lightweight MRSS integration: Spectrum sharing between NR and 6G can be supported with simple mechanisms such as rate-matching around always-on NR signals (e.g., synchronization and broadcast signals), rather than complex, heavy-handed approaches.
  • Forward compatibility and FR3 considerations: Adjustments to numerology and frame design may be justified in new spectrum ranges (e.g., 6–15 GHz), where larger bandwidths and new subcarrier spacing choices (30 kHz, 60 kHz) provide clear performance advantages while remaining compatible with MRSS.
  • System simplification goal: A core design principle is to minimize the number of configuration options and reduce the complexity of UE capability reporting, in order to simplify device implementation, signaling, and network operation.

Numerology (SCS/CP)

For numerology design in 6G, the central goal is to simplify operation while ensuring compatibility with existing NR deployments and efficiency in new spectrum ranges. To reduce device and network complexity, a single subcarrier spacing per band or cell is preferred, avoiding the need for frequent SCS switching that complicates RF calibration, synchronization, and scheduling. In the sub-6 GHz range (FR1), the established baseline of 15 kHz for FDD and 30 kHz for TDD should be maintained to maximize reuse of current deployments. In the new upper mid-band spectrum around 7 GHz (FR3), 30 kHz appears suitable, while higher subcarrier spacings such as 60 kHz and 120 kHz are more appropriate for the wider channels and smaller delay spreads expected at higher FR3 frequencies. For FR2, the established 120 kHz SCS remains the practical baseline to balance performance with phase noise resilience. In terms of cyclic prefix treatment, normal CP is to be retained as the default, with extended CP reserved for further study given its limited real-world adoption in NR and its trade-off between coverage extension and spectral efficiency

  • Single SCS per band/cell — Avoid SCS switching to reduce UE/gNB complexity.
  • FR1 baseline SCS — Use 15 kHz for FDD and 30 kHz for TDD where feasible to maximize reuse.
  • FR3 candidate SCS — Study 30/60/120 kHz; favor 30 kHz near ~7 GHz and 60/120 kHz at higher FR3 with tighter delay spreads.
  • FR2 baseline SCS — Retain 120 kHz as the practical baseline.
  • CP treatment — Normal CP as default; extended CP kept for further study.

Design principle and guidelines

  • Minimize unnecessary flexibility: Lessons from NR show that supporting too many subcarrier spacings and CP types adds complexity without being used in practice. 6G should focus only on SCS/CP options with clear deployment value.
  • Align with existing deployments: For spectrum already in use by NR, maintain the subcarrier spacings that are actually deployed (e.g., 15 kHz, 30 kHz, 120 kHz) to enable smooth spectrum sharing and reuse of equipment.
  • Adopt spectrum-tailored numerologies: Instead of offering every scaling step everywhere, select numerologies per frequency range — lower SCS for wide-area coverage, higher SCS for high-frequency bands with limited delay spread and larger bandwidths.
  • Support future-proof bandwidths: Larger FFT sizes (e.g., ≥8K) should be assumed so that 6G can natively support wide carriers (200–400 MHz or more) without relying on complex carrier aggregation.
  • Keep CP simple: Normal CP is sufficient for most scenarios. Extended CP, while studied in NR, was rarely deployed and should only be reconsidered if new evidence shows significant benefits for specific high-delay environments.
  • Ensure consistency across functions: Use the same SCS for synchronization signals and initial BWPs within a band to avoid unnecessary switching between measurement and data reception, reducing implementation overhead.
  • Balance reuse and innovation: Preserve NR-compatible numerologies where they work well, but allow carefully justified new choices in FR3 or beyond to achieve latency, efficiency, or bandwidth gains without fragmenting the ecosystem.

Channel bandwidth (CBW) & FFT size

In developing the 6G framework, the treatment of channel bandwidth (CBW) and FFT sizing is guided by both continuity with NR and the anticipated demands of new spectrum ranges. For maximum bandwidth, the target is set at around 200 MHz in the upper FR1 and FR3 ranges, while FR2-1 can extend up to 400 MHz to accommodate the wider spectrum blocks available at higher frequencies. On the other end of the spectrum, minimum channel bandwidth remains an important design aspect for coverage and compatibility: in FR1, options such as 3, 5, 10, and 40 MHz are retained to support legacy refarmed bands; in FR3, 40 or 50 MHz provides a reasonable lower bound; and in FR2, a minimum of 50 MHz is expected, subject to RAN4’s confirmation. To efficiently process these bandwidths, FFT sizing plays a crucial role. An 8K FFT is considered a practical upper bound for day-one deployment, offering a good trade-off between implementation complexity and spectral flexibility, with the option of 16K FFT reserved for further study. This sizing allows, for example, a 200 MHz carrier to be realized with 60 kHz subcarrier spacing on an 8K FFT, while still supporting narrower or wider bandwidths with consistent numerology choices. In this way, the CBW and FFT design principles strive to balance backward compatibility, implementation efficiency, and the scalability required for future high-capacity 6G deployments

  • Maximum CBW targets — ≥200 MHz in upper FR1 and FR3; 400 MHz in FR2-1.
  • Minimum CBW options — FR1: 3/5/10/40 MHz; FR3: 40/50 MHz; FR2: 50 MHz (subject to RAN4).
  • FFT sizing — Cap around 8k (16k FFS); example: 200 MHz realized with 60 kHz SCS on 8k FFT.

Design principle and guidelines

  • Target wideband efficiency: 6G should support large contiguous carriers, with maximum CBW targets around 200 MHz in upper FR1 and FR3, and up to 400 MHz in FR2-1, to make full use of available spectrum without excessive reliance on carrier aggregation.
  • Preserve flexible minimum bandwidths: Maintain small bandwidth options to ensure coverage and refarming efficiency — 3/5/10/40 MHz in FR1, 40–50 MHz in FR3, and 50 MHz in FR2 — aligning with legacy allocations and enabling gradual migration.
  • Adopt practical FFT sizing: Standardize on an 8K FFT as the baseline for 6G, offering a balance between implementation cost and spectrum scalability, with a 16K FFT left for further study as technology matures.
  • Map CBW to FFT resources efficiently: Ensure that typical deployment cases (e.g., 200 MHz carriers with 60 kHz SCS) fit cleanly within the FFT size, enabling efficient processing while avoiding excessive overhead.
  • Reduce hardware complexity: Favor single wideband carriers where possible, as they simplify scheduling, reduce RF chain duplication, and improve trunking efficiency compared to multi-carrier aggregation.
  • Enable forward scalability: Leave room for larger FFT sizes and wider CBWs in future releases, ensuring the framework can evolve with spectrum availability and traffic demands without disruptive redesign.

Frame/resource grid & scheduling

For 6G, the approach to frame structure, resource grid definition, and scheduling builds directly on the foundations established in NR while addressing inefficiencies that emerged in practice. The basic time grid of a 10 ms frame and 1 ms subframe remains a stable anchor, ensuring backward compatibility and simplifying MRSS with NR. At the same time, greater flexibility is needed in how scheduling operates within this grid. Instead of restricting physical channels such as PxSCH and PUCCH to the boundaries of a single slot, 6G can allow transmissions to extend across slots, while still retaining the slot as the logical timing unit for processing timelines, HARQ codebooks, and control monitoring. In the frequency domain, the physical resource block (PRB) continues as the base allocation unit, but there is recognition that narrower sub-PRB structures may be beneficial for power-limited or coverage-challenged devices, such as deep-indoor IoT or NTN terminals. Another area of simplification is the Bandwidth Part (BWP) framework. While useful in principle for balancing power and flexibility, dynamic BWP switching introduced complexity, latency, and signaling overhead in NR. In 6G, the intent is to streamline BWP usage by focusing only on the most essential configurations, thereby reducing operational churn while still delivering the necessary adaptability for diverse services

  • NR time grid reuse — Preserve 10 ms / 1 ms structure and slot granularity.
  • Relax slot boundaries — Allow PxSCH/PUCCH to span slots while keeping slot as the logical timing unit.
  • Frequency granularity — Keep PRB as base unit; study sub-PRB options (e.g., 4/6 SC) for narrow UEs.
  • Simplified BWP — Focus on essential configs and avoid churn from dynamic BWP switching.

Design principle and guidelines

  • Preserve NR time grid: Retain the 10 ms radio frame and 1 ms subframe as the fundamental timing structure, keeping slot-based granularity to ensure backward compatibility and efficient MRSS with NR.
  • Enable cross-slot transmissions: Allow PxSCH and PUCCH to extend across slot boundaries to improve coverage, reduce DMRS overhead, and support longer transmissions, while still treating the slot as the logical timing reference for control and HARQ.
  • Keep PRB as the core frequency unit: Continue to use the 12-subcarrier PRB as the baseline scheduling granularity, ensuring consistency with NR and simplifying spectrum management across bands.
  • Study sub-PRB allocation: Investigate narrower allocations (e.g., 4 or 6 subcarriers) for coverage-limited UEs such as deep-indoor IoT or NTN devices, enabling power concentration and extended reach without redesigning the core grid.
  • Simplify BWP framework: Reduce complexity by limiting dynamic BWP switching, focusing instead on a streamlined set of essential configurations that balance power savings with flexibility while avoiding unnecessary signaling overhead.
  • Balance flexibility with practicality: Provide enough structural options to support diverse 6G use cases, but avoid excessive scheduling permutations that increase device complexity without real deployment value.

Duplexing & UL/DL configuration

In shaping duplexing and uplink/downlink configuration for 6G, the design direction emphasizes stability and gradual evolution from proven NR mechanisms. Frequency Division Duplexing (FDD) and semi-static Time Division Duplexing (TDD) are expected to serve as the foundation, while fully dynamic TDD is de-prioritized in the initial release to avoid the complexity and signaling overhead that limited its practicality in wide-area NR deployments. A key addition in 6G is the native integration of Sub-Band non-overlapping Full-Duplex (SBFD), with explicit symbol-type support in the frame structure. To contain complexity, a given cell should operate with either dynamic TDD or semi-static SBFD at one time, rather than trying to run both modes simultaneously. At the same time, fine-grain flexibility is not excluded: symbol-level uplink/downlink adaptation remains under study for specific scenarios where it can deliver meaningful benefits, such as high-density small cells or latency-critical services. This approach balances robustness and ease of deployment with the ability to evolve toward more advanced duplexing features as spectrum and device ecosystems mature

  • FDD & semi-static TDD first — De-prioritize fully dynamic TDD in the initial release.
  • SBFD integration — Include symbol-type design; per-cell, operate either dynamic TDD or semi-static SBFD (not both simultaneously).
  • Fine-grain flexibility — Investigate symbol-level UL/DL adaptation where use cases justify it.

Design principle and guidelines

  • Prioritize stable duplex modes: Begin with FDD and semi-static TDD as the foundation for 6G, ensuring reliable performance and manageable complexity in the first release.
  • Defer full dynamic TDD: Avoid the heavy overhead and CLI challenges seen in NR macro deployments by postponing widespread support for fully dynamic TDD until simpler, hotspot-focused use cases are proven.
  • Integrate SBFD natively: Include a dedicated SBFD symbol type in the 6G frame structure to improve uplink coverage in TDD bands and provide UL extension in new spectrum ranges.
  • Per-cell mode exclusivity: Restrict cells to operate either with semi-static SBFD or dynamic TDD, but not both simultaneously, to avoid excessive specification effort and implementation complexity.
  • Explore symbol-level flexibility: Study fine-grained UL/DL adaptation at the symbol level, but constrain its use to scenarios where clear deployment benefits are identified, such as dense urban small cells or low-latency services.
  • Balance efficiency and simplicity: Ensure that duplexing choices maximize UL coverage and spectral efficiency while minimizing UE power consumption and network management overhead.

MRSS operation specifics

For 6G, the specifics of Multi-RAT Spectrum Sharing (MRSS) focus on ensuring efficient coexistence with NR while keeping the overall frame structure clean and simple. Two primary coexistence paths are foreseen: either sharing certain signals, such as reusing NR synchronization or reference signals directly, or adopting a rate-matching approach similar to DSS, where 6G transmissions avoid overlapping with NR’s always-on signals. In both cases, the guiding principle is to minimize special cases in the 6G frame design so that NR signals can be accommodated without excessive modifications or exceptions. Equally important is the reuse of NR’s timing framework, particularly symbol and subframe lengths, so that MRSS operation is straightforward for devices and networks. This continuity lowers complexity in both implementation and deployment, ensuring that 6G can coexist with NR smoothly while maintaining its own design integrity and efficiency

  • Two coexistence paths — Use signal sharing (e.g., reuse NR SSB/CSI-RS) or rate-matching (DSS-like) around NR signals.
  • Keep frame clean — Handle unremovable NR signals with minimal special casing in the frame structure.
  • Timing reuse for MRSS — Reuse NR symbol/subframe lengths to simplify coexistence.

Design principle and guidelines

  • Support dual coexistence methods: Enable MRSS either through direct signal sharing (e.g., reuse of NR SSB or CSI-RS) or via rate-matching around NR always-on signals, allowing operators flexibility in deployment.
  • Keep the frame structure clean: Minimize the need for special-case handling of NR signals by designing MRSS mechanisms that avoid cluttering the 6G frame with excessive exceptions or overlays.
  • Reuse NR timing units: Retain NR symbol, subframe, and frame lengths as the basis for MRSS, simplifying coexistence by aligning 6G transmissions with familiar NR timing grids.
  • Limit spectral efficiency penalties: Ensure MRSS solutions incur only minimal overhead (e.g., low single-digit percentage loss), avoiding the large efficiency penalties seen in earlier LTE–NR DSS.
  • Preserve 6G design freedom: Design MRSS so it does not restrict 6G’s channel or frame structure evolution; NR coexistence should be supported without compromising native 6G performance.

Device tiers & minimum-bandwidth operation

For 6G device design, ensuring accessibility across different tiers of capability is a critical consideration. A key enabler is support for low-bandwidth options in the lower FR1 range, such as 3–5 MHz carriers, which facilitate smooth migration from legacy narrowband systems like LTE Cat-M while still aligning with the broader design used for enhanced mobile broadband (eMBB). This allows constrained IoT-type devices to coexist with high-capacity terminals within a harmonized framework. Another tool to manage diversity is the Bandwidth Part (BWP) concept, which provides a mechanism to tailor active bandwidth to the device’s capability or energy efficiency needs without fragmenting the carrier design. By reusing and streamlining the BWP framework, 6G can flexibly support devices ranging from ultra-low-complexity UEs to advanced multi-Gbps modems on the same spectrum resource, ensuring both inclusivity and efficiency in early and mature deployments

  • Low-bandwidth options — Provide 3–5 MHz in low FR1 to ease migration from LTE Cat-M while retaining common design with eMBB.
  • BWP for diversity — Use BWP to accommodate diverse UE bandwidths and capabilities on the same carrier.

Design principle and guidelines

  • Support narrowband migration: Provide 3–5 MHz operation in low FR1 bands to ensure smooth migration from legacy LTE Cat-M and NB-IoT while aligning with the common 6G frame design.
  • Maintain eMBB alignment: Ensure that low-bandwidth modes remain consistent with the design principles used for wideband eMBB carriers, preventing fragmentation of frame and resource structures.
  • Leverage BWP for diversity: Use streamlined Bandwidth Parts to accommodate diverse UE classes — from low-power IoT to high-performance devices — on the same carrier, without excessive reconfiguration overhead.
  • Balance efficiency and inclusivity: Optimize the framework so that narrowband devices can achieve robust connectivity and long battery life, while wideband devices continue to exploit high-capacity spectrum.
  • Minimize signaling complexity: Simplify the BWP framework compared to NR by reducing redundant configurations and avoiding latency-inducing dynamic switching, keeping device requirements light.
  • Ensure forward compatibility: Design low-bandwidth operation as a scalable subset of the broader system so that device tiers evolve seamlessly with future 6G releases.

Performance, coverage & efficiency

For 6G, performance targets are closely tied to efficiency, coverage, and predictability in operation. From day one, the design should aim to simplify control mechanisms and streamline scheduling to achieve lower device and network complexity, improved energy efficiency, and broader coverage, while simultaneously reducing latency. These foundational efficiency goals ensure that the system delivers tangible benefits without introducing the overheads seen in some NR features that were rarely used in practice. At the same time, predictability in operation is essential for stable performance. Clear interference assumptions and consistent timing references, particularly in TDD configurations, are needed to prevent uncertainty in uplink and downlink scheduling. Aligning the boundaries of configured TDD periods with subframe boundaries provides a clean structure that reduces complexity in both physical and higher-layer operations, improving coexistence, robustness, and deployment flexibility

  • Day-1 efficiency goals — Lower complexity, improve energy efficiency and coverage, and reduce latency via simpler control and flexible scheduling.
  • Predictable operation — Keep interference/TDD timing assumptions clear; align configured TDD period boundaries to subframes.

Design principle and guidelines

  • Prioritize day-1 efficiency: Ensure that the initial 6G release lowers device and network complexity, improves energy efficiency, and extends coverage while reducing latency through simplified control and streamlined scheduling.
  • Simplify control overhead: Avoid excessive signaling mechanisms or rarely used options from NR; focus on lightweight control designs that reduce processing load on UEs and gNBs.
  • Enhance UL coverage: Incorporate design features such as cross-slot transmissions or SBFD that directly address uplink limitations, especially in higher-frequency TDD deployments.
  • Maintain predictable operation: Keep interference assumptions and TDD timing configurations clear, avoiding ambiguity in uplink and downlink operation.
  • Align timing with subframes: Configure TDD period boundaries to align with subframe boundaries, ensuring cleaner operation across layers and improving coexistence with legacy NR systems.
  • Design for scalable efficiency: Balance short-term efficiency targets with long-term adaptability, ensuring the system can evolve toward broader use cases without redesigning the efficiency framework.

Reference