The evolution toward sixth-generation (6G) wireless communications is driving a fundamental rethinking of physical-layer design, with waveform engineering at its core. Unlike previous generations, where orthogonal frequency-division multiplexing (OFDM) dominated, 6G must support an unprecedented mix of extreme data rates (beyond 1 Tbps), sub-millisecond latency, massive connectivity, and resilience in highly mobile and diverse environments. Next-generation waveform design techniques are being explored to overcome the limitations of OFDM in terahertz (THz) bands, high-mobility scenarios, and energy-constrained Internet of Things (IoT) deployments. This article provides an in-depth look at the key challenges, emerging waveform candidates, AI-driven optimization, and the path toward standardization.

Key Challenges in 6G Waveform Design

The demands placed on 6G networks introduce several waveform-level challenges that require novel approaches beyond traditional OFDM and its variants. These challenges span frequency, mobility, energy, and service heterogeneity.

Operation at Millimeter-Wave and Terahertz Frequencies

6G will likely operate across sub-6 GHz, millimeter-wave (mmWave), and sub-terahertz (0.1–3 THz) bands. At THz frequencies, severe path loss, atmospheric absorption, and phase noise necessitate waveforms with high peak-to-average power ratio (PAPR) resilience, efficient power amplifier operation, and robustness to hardware impairments. OFDM’s high PAPR becomes a critical bottleneck, prompting interest in single-carrier and multi-carrier alternatives that can tolerate nonlinear distortion.

Ultra-High Mobility and Doppler Spread

Applications such as high-speed trains (up to 1000 km/h) and unmanned aerial vehicle (UAV) communications introduce large Doppler spreads that destroy orthogonality in OFDM systems. Traditional cyclic prefix (CP) length increases cannot compensate for rapid time-varying channels. Waveforms that are inherently robust in the delay-Doppler domain, such as Orthogonal Time Frequency Space (OTFS), are emerging as leading candidates for such scenarios.

Energy Efficiency and Massive IoT

Massive machine-type communication (mMTC) and ultra-reliable low-latency communication (URLLC) require waveforms that support sporadic, short-packet transmissions with minimal overhead. Energy efficiency is paramount for battery-powered devices. Waveforms must exhibit low out-of-band (OOB) emissions to enable asynchronous, grant-free access and avoid guard bands, while also allowing simple transceiver architectures to keep power consumption low.

Service Diversity and Spectrum Coexistence

6G networks must simultaneously support services with wildly different requirements—e.g., holographic communications (high throughput), autonomous vehicles (low latency), and environmental sensing (integrated sensing and communication, ISAC). Waveforms need to be flexible, possibly using different numerologies or even different modulation schemes within the same band, requiring advanced filtering and windowing to manage inter-service interference.

Emerging Waveform Techniques

Researchers have proposed numerous waveform candidates to address these challenges. Below we examine the most promising ones, with emphasis on their advantages and intended use cases.

Orthogonal Time Frequency Space (OTFS)

OTFS maps information symbols into the delay-Doppler domain rather than the time-frequency domain. This transforms time-varying multipath channels into a nearly time-invariant, sparse representation. The result is a waveform that exhibits exceptional robustness to high Doppler spreads and offers full diversity gain. OTFS is particularly suited for high-mobility scenarios like vehicular and high-speed rail communications. However, its practical implementation requires efficient two-dimensional equalization, and work continues on reducing complexity. For a comprehensive survey of OTFS, see this IEEE paper.

Filtered OFDM (F-OFDM) and Filter Bank Multicarrier (FBMC)

F-OFDM applies filtering per subband, reducing OOB emissions and enabling flexible numerology allocation. FBMC uses prototype filters with excellent time-frequency localization, virtually eliminating OOB leakage but at the cost of orthogonality loss (leading to intrinsic interference). Offset quadrature amplitude modulation (OQAM) is typically used in FBMC to mitigate this. Both techniques are strong candidates for asynchronous, fragmented spectrum access, and are being investigated for 6G low-latency and ISAC applications.

Generalized Frequency Division Multiplexing (GFDM)

GFDM is a flexible block-based multi-carrier scheme that uses circular pulse shaping for each subcarrier, allowing adjustable time and frequency resource granularity. It supports very low OOB emissions and can be configured for low-latency transmission by reducing block length. GFDM is under consideration for IoT-oriented 6G systems where short data bursts and relaxed synchronization are required.

Universal Filtered Multi-Carrier (UFMC)

UFMC filters groups of contiguous subcarriers (subbands) instead of each subcarrier individually, striking a balance between complexity and spectral containment. The filtering length is kept short, preserving burst transmission benefits. UFMC has been evaluated for 5G NR but faced challenges in high-mobility scenarios; new 6G variants incorporate adaptive filter design tailored to channel conditions.

Index Modulation (IM) Waveforms

Waveforms based on index modulation, such as OFDM with index modulation (OFDM-IM), exploit the indices of active subcarriers, subblocks, or spatial dimensions to convey additional information bits. IM enhances spectral efficiency and energy efficiency, especially at low-to-medium signal-to-noise ratios. For 6G massive IoT, variants like multi-mode OFDM-IM offer robustness against frequency selective fading with lower PAPR compared to classical OFDM.

Delay-Doppler Domain Waveforms Beyond OTFS

Alternative delay-Doppler formulations, including orthogonal delay-Doppler modulation (ODDM) and affine frequency division multiplexing (AFDM), aim to achieve similar robustness as OTFS but with reduced receiver complexity. AFDM, for instance, uses chirp-based basis functions that naturally handle time-frequency dispersion. These techniques are still in early research stages but show promise for 6G channel estimation and pilot overhead reduction.

Role of Artificial Intelligence in Waveform Optimization

Machine learning (ML) and artificial intelligence (AI) are increasingly being applied to waveform design, moving beyond classical model-based approaches. Key areas include:

  • Autoencoder-based end-to-end waveform learning: Neural networks jointly optimize the transmitter and receiver (including the waveform) for a given channel and performance metric. This can learn non-orthogonal, non-linear, and task-specific waveforms that outperform human-designed ones.
  • Online parameter adaptation: Reinforcement learning agents dynamically choose waveform parameters (e.g., subcarrier spacing, filter length, modulation order) in response to changing channel conditions, interference levels, and traffic loads.
  • AI-assisted equalization and detection: Deep learning approaches (e.g., deep neural networks, convolutional neural networks, transformers) are used to decode OTFS, FBMC, and other advanced waveforms with lower complexity than optimal equalizers.

AI-driven waveform design is particularly valuable for ISAC, where the same waveform must jointly optimize communication data rate and sensing resolution—a multi-objective problem well suited to data-driven optimization. For a recent overview, refer to this article on AI for 6G waveform design.

Integration with Massive MIMO and Beamforming

The potential of next-generation waveforms is magnified when combined with massive MIMO (multiple input, multiple output) and advanced beamforming. A few critical intersections:

  • Hybrid beamforming with multi-carrier waveforms: At mmWave/THz frequencies, hybrid analog-digital precoding imposes constraints on phase shifters and bandwidth. Waveform designs that allow low-resolution phase shifter operation, such as single-carrier frequency-domain equalization (SC-FDE) with constant envelope, become attractive.
  • Orbital angular momentum (OAM) multiplexing: Vortex waves carrying OAM can increase spectral efficiency but require orthogonal waveforms tailored to the rotational Doppler effect. Work is ongoing to combine OAM with OTFS-like mapping.
  • Distributed MIMO and cell-free architectures: Waveforms that enable coherent joint transmission from multiple access points (APs) need tight synchronization and low OOB emissions to avoid inter-AP interference. FBMC and F-OFDM are being evaluated for such cell-free massive MIMO deployments.

Standardization and Research Landscape

The path from research to commercial adoption runs through standardization bodies such as 3GPP, ITU-R, and IEEE. The International Telecommunication Union (ITU) has outlined a vision for IMT-2030 (6G) with key performance indicators (KPIs) including peak data rates of 200 Gbps and user-experienced rates of 10 Gbps. 3GPP has started study items on 6G in Release 20, with waveform candidates expected to be evaluated in 2025–2027. Several major research projects, including the European 6G flagship Hexa-X-II and the Japanese Beyond 5G Promotion Consortium, are actively investigating waveform design. For the latest ITU recommendations on 6G, visit ITU-R Working Party 5D. The 3GPP study on 6G is tracked via 3GPP's 6G page.

Future Directions and Open Problems

Despite significant progress, several open challenges remain before next-generation waveforms can be deployed in 6G. These include:

  • Complexity reduction: Many advanced waveforms (OTFS, FBMC-OQAM, AI-optimized schemes) require order-of-magnitude higher computational resources compared to OFDM. Efficient hardware implementations (e.g., using fast Fourier transform variants, iterative equalization) are needed.
  • Joint waveform and channel coding: End-to-end design that tightly integrates waveform selection with channel coding, resource allocation, and retransmission protocols can yield significant gains but is extremely complex.
  • Asynchronous and grant-free operation: For massive IoT, waveforms must enable receivers to detect and decode packets without explicit synchronization. This favors non-orthogonal waveforms and code-domain approaches like NOMA combined with waveform shaping.
  • Integrated sensing and communication (ISAC): ISAC requires waveforms that carry data while simultaneously enabling accurate radar-like sensing. Unlike communication, sensing favors low PAPR, high peak power, and specific ambiguity functions. Designing a single waveform that excels at both is a major research area.

Collaboration between academia, industry, and standardization bodies will be essential to translate these innovations into real-world deployment. The waveform selected for 6G may not be a single modulation but a family of configurable waveforms, each optimized for specific scenarios—much like 5G NR supports multiple numerologies within OFDM. The coming years will see intense competition and convergence as the 6G standard takes shape.

Conclusion

The next-generation waveform design for 6G communications is a rich and multi-faceted field, driven by the need to operate at higher frequencies, support extreme mobility, connect massive numbers of devices, and deliver integrated services. Techniques such as OTFS, FBMC, GFDM, UFMC, and index modulation offer complementary strengths, while AI-driven optimization promises to adapt waveforms to dynamic environments in ways not previously possible. These waveforms must also coexist with massive MIMO, beamforming, and emerging concepts like OAM and ISAC. As 3GPP and ITU move toward concrete specifications, the research community continues to push the boundaries of what is possible, ensuring that 6G will not only be faster but more versatile and intelligent than any previous generation.

For those seeking to dive deeper into the mathematical foundations of modern waveform design, this Springer book on waveform design for 5G and beyond provides thorough coverage. Additionally, the 6G World portal curates ongoing research and industrial developments.