6G Series for 3GPP Release 20 Journey- Waveform of 6G Air interface

The news of first 6G RAN agreement came last month and with it the journey has now begun for NextG Mobile Communication. For the 6G waveform following agreements for UL and DL were made.

Agreement for Uplink waveform:

CP-OFDM and DFT-s-OFDM waveforms as defined in 5G NR are supported as the basis for 6GR for uplink with enhancements/modifications on CP-OFDM/DFT-s-OFDM will be studied as potential additions however, other OFDM based waveforms are not precluded. 

Agreement for Downlink waveform:

CP-OFDM waveform as defined in 5G NR is supported as the basis for 6GR for downlink with enhancements/modifications on CP-OFDM will be studied as potential additions along with DFT-s-OFDM or any other OFDM-based waveform will be studied as a potential additional waveform for downlink.

The decision came after 40 companies submitted their contribution to finalize 6G waveform. The finalized 6G waveform (CP-OFDM and the DFT-S-OFDM) has already proved a be good choice in 5G due to its low PAPR design resulting in higher energy efficiency and robust performance in low coverage scenarios with better spectral efficiency and flexibility in implementation for MRSS with marginal or no hardware update.

Criteria of evaluation:

The criteria of waveform evaluation included (but not limited to) MRSS compatibility, HW Complexity, MIMO compatibility, higher spectrum efficiency, better PAPR, Sensing, positioning and NTN compatibility.

Brief History in 5G

5G transformed wireless communications by replacing LTE's fixed-parameter approach with a flexible, scalable architecture supporting diverse use cases from enhanced mobile broadband (eMBB) to ultra-reliable low latency communications (URLLC) and massive machine type communications (mMTC).

Scalable Numerology Innovation

5G introduced scalable numerology with flexible subcarrier spacing (SCS):

• FR1 (sub-6 GHz): 15/30/60 kHz SCS
• FR2 (above 24 GHz): 60/120/240 kHz SCS

This enables dynamic adaptation to varying frequency bands, channel conditions, and service requirements while efficiently multiplexing concurrent use cases.

Optimized Waveform Selection

The selection of waveforms for 5G was the outcome of a thorough evaluation of several variants of OFDM and DFT-s-OFDM waveforms, such as universal filtered multicarrier (UFMC), filter bank multi-carrier (FBMC), filtered OFDM (f-OFDM), zero-tail DFT-s-OFDM (ZT DFT-s-OFDM) among others.

GAIN: Reduced guard band overhead from LTE's 10% to as low as 2%

Advanced Multiple Access

5G's OFDMA delivers sophisticated capabilities:

• Dynamic Spectrum Sharing (DSS): Seamless 4G/5G coexistence
• Service multiplexing: eMBB/URLLC on shared resources having different QoS requirements
• Contention-based uplink access: Reduced latency and signaling overhead
• Massive MIMO integration: Spatial beamforming for interference management

Hybrid Uplink Strategy

5G employs dual waveform approach:

• DFT-s-OFDM: Coverage-limited scenarios and high-power transmissions
• CP-OFDM: Multi-layer transmissions and simplified multiplexing

OFDM-based waveform (The Ultimate Winner)

Multiple contributors suggested for the adoption of Unified waveform across multiple services with enhanced performance to cater 6G requirements and to minimize complexity and support diverse 6G services such as TN/NTN integration, joint communication and sensing, massive IoT however, the idea of waveform design for ISAC was suggested to be discussed in the separate agenda item of ISAC due to sensing-specific requirements and further technical details.

Worth noting that a proposal of new 6G Radio waveform study was also suggested for multiple specific vertical use cases such as NTN, IoT, V2X, broadcast etc. apart from eMBB to meet the service diversification and performance requirements.

What is expected in 6G

At least Three main enhancements are expected of 6G.

Pillar 1: Advanced Low-PAPR Waveforms

It is expected that a novel DFT-s-OFDM variants to be devised for optimized low-SNR performance based on Pi/2 BPSK which offer significant benefits over those that use QPSK for coverage. The lower PAPR of these waveforms comes at the expense of reduced spectral efficiencies where they can be used. This perceived drawback can however be overcome by changes for example truncated mapping that lead to a new family of waveforms that offer a tunable trade-off between PAPR and spectral efficiency.

Pillar 2: Multi-Layer Transmission

Unlike 4G which offered multi-layer DFT-S-OFDM transmissions in uplink, 5G NR was designed to use CP-OFDM for multi-layer uplink transmissions and restricted the use of DFT-S-OFDM to single-layer transmissions. This appears to be an unnecessary restriction and hinders the UE from transmitting at its full power any time multi-layer transmissions are required.

Figure below presents the throughput gain achieved by switching a rank-2 transmission from CP-OFDM to DFT-S-OFDM for a PC2 UE powered by 2 PC3 PAs. Note that the differences in maximum transmit powers as determined by the MPR values are reflected in the throughput curves.

Pillar 3: Intelligent Scheduling

Currently in 4G 5G networks, the DFT-s-OFDM waveform is constrained by scheduling rules that prioritize low computational complexity and a low Peak-to-Average Power Ratio (PAPR). It is recommended to mitigate these restrictions while enabling more versatile scheduling and a more efficient use of network resources. This shift would allow for a balance between power efficiency and the spectral flexibility needed for modern, dynamic network demands.

By leveraging Flexible DFT Size Adaptive scheduling mechanisms can be improved. In various DFT-s-OFDM stduies it is recommended that allocation-independent DFT sizing by decoupling resource allocation from transform processing constraints shall result in higher UL performance.

Valid DFT sizes to be defined as a product of powers of 2, 3, and 5:

DFT Size = 2^a × 3^b × 5^c (where a, b, c ≥ 0)

For DFT Size Selection for any given allocation, select the largest valid DFT size that is smaller than or equal to the allocation size. This approach maintains DFT-s-OFDM's spectral properties while enabling flexible resource allocation granularity independent of transform constraints.

Conclusion:

The 6G waveform agreement represents a decisive step forward, building on 5G's proven CP-OFDM and DFT-s-OFDM foundation while expecting new capabilities through advanced PAPR optimization, multi-layer DFT-s-OFDM transmission, and flexible scheduling mechanisms. This pragmatic approach enables seamless infrastructure evolution while addressing critical 6G requirements across diverse verticals from massive IoT to integrated sensing and communication (ISAC). 6G waveforms are now positioned to deliver transformative performance gains while preserving the operational continuity essential for successful network evolution.