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New Delay Doppler Communication Paradigm in 6G era: A Survey of Orthogonal Time Frequency Space (OTFS)

Weijie Yuan, Shuangyang Li, Zhiqiang Wei, Yuanhao Cui, Jiamo Jiang, Haijun Zhang, Pingzhi Fan

arXiv:2211.12955v2cs.ITeess.SP

TL;DR

High Doppler in 6G high-mobility environments challenges OFDM and motivates delay-Doppler communication. This survey reviews OTFS fundamentals, advances, ISAC, applications, and open challenges, concluding that OTFS is a promising enabler while identifying latency and resolution issues requiring further study.

  • Problem

    High-mobility channels create severe Doppler effects that degrade OFDM, while 6G applications require reliable communication and integrated sensing capabilities.

  • Method

    The paper provides a comprehensive survey of OTFS, covering delay-Doppler channel representation, fundamentals, transmitter and receiver design, ISAC, emerging applications, and open challenges.

  • Results

    The survey identifies OTFS as a promising high-mobility waveform whose delay-Doppler representation supports communication and sensing within a common domain.

  • Takeaways & Limitations

    OTFS offers a framework for exploiting delay-Doppler channel structure in future wireless systems and for jointly considering communication and sensing.

  • Takeaways & Limitations

    OTFS may incur higher detection latency than OFDM because of block-based data detection, so overall latency comparisons require further study.

Abstract

from arXiv · show

In the 6G era, space-air-Ground integrated networks (SAGIN) are anticipated to deliver global coverage, necessitating support for a diverse array of emerging applications in high-mobility, hostile environments. Under such conditions, conventional orthogonal frequency division multiplexing (OFDM) modulation, widely employed in cellular and Wi-Fi communication systems, experiences performance degradation due to significant Doppler shifts. To overcome this obstacle, a novel two-dimensional (2D) modulation approach, namely orthogonal time frequency space (OTFS), has emerged as a key enabler for future high-mobility use cases. Distinctively, OTFS modulates information within the delay-Doppler (DD) domain, as opposed to the time-frequency (TF) domain utilized by OFDM. This offers advantages such as Doppler and delay resilience, reduced signaling latency, a lower peak-to-average ratio (PAPR), and a reduced-complexity implementation. Recent studies further indicate that the direct interplay between information and the physical world in the DD domain positions OTFS as a promising waveform for achieving integrated sensing and communications (ISAC). In this article, we present an in-depth review of OTFS technology in the context of the 6G era, encompassing fundamentals, recent advancements, and future directions. Our objective is to provide a valuable resource for researchers engaged in the field of OTFS.

I. INTRODUCTION

6G high-mobility applications expose OFDM’s vulnerability to Doppler, motivating OTFS modulation in the delay-Doppler domain. This survey reviews OTFS fundamentals, advances, applications, and future research, including its potential for integrated sensing and communications.

  • Motivation: High mobility in V2V, high-speed rail, maritime, and satellite links produces severe Doppler spread that remains a bottleneck for traditional wireless systems.Examples include speeds up to 300 km/h for V2V and 500 km/h for high-speed rail.
  • Motivation: OFDM handles multipath-induced inter-symbol interference with a cyclic prefix but degrades in doubly selective channels because Doppler shifts cause frequency dispersion.High Doppler shifts shorten channel coherence time and undermine reliable OFDM operation.
  • OTFS rationale: OTFS maps information to the delay-Doppler domain rather than the conventional time-frequency domain, targeting reliable communication in highly dynamic environments.The survey presents OTFS as a waveform initially proposed for high-mobility wireless applications.
  • OTFS rationale: Delay-Doppler signaling spreads each information symbol across the time-frequency domain, enabling full time-frequency diversity with suitable receiver design.The delay-Doppler channel also offers sparsity and stability that can support low-overhead channel estimation and low-complexity detection.
  • OTFS and ISAC: Delay and Doppler parameters directly correspond to sensing range and speed, aligning OTFS’s communication representation with integrated sensing and communications.The survey identifies OTFS as a promising waveform for unifying communication and sensing in the same delay-Doppler domain.
  • Survey scope: The article surveys OTFS history, fundamentals, recent advances, future work, and emerging applications to provide a broad reference for researchers.Its scope includes OTFS-enabled integrated sensing and communications.

II. OTFS PRINCIPLES

The delay-Doppler domain represents wireless channels through physically meaningful delay and Doppler parameters, yielding stability, sparsity, and compactness. OTFS places symbols on a two-dimensional delay-Doppler grid and converts them to time-domain waveforms through OFDM-based or Zak-transform-based processing.

  • DD channel representation: Time-frequency channel representations are effective for OFDM, but no alternative domain provides an eigenvalue decomposition for doubly selective channels in the conventional sense.This motivates representing channel fading through delay and Doppler parameters instead of time and frequency parameters.
  • DD channel representation: Delay and Doppler responses reflect physical scatterer distance and velocity, remaining approximately unchanged within a channel stationarity region.The stationarity region can exceed the typical time-frequency coherence region and supports nearly constant responses across consecutive frames.
  • DD channel properties: The delay-Doppler representation provides a channel snapshot with slower fluctuations, sparse responses from limited scatterers, and compact support bounded by maximum delay and Doppler.These properties connect channel parameters to physical range and speed while reducing the effective region independently of OTFS frame size.
  • DD channel properties: Doppler information separates paths with identical delays, allowing the channel representation to expose full time-frequency diversity.This separability is a key benefit of delay-Doppler modeling over delay-only representations.
  • OTFS modulation: OTFS multiplexes data symbols on a two-dimensional delay-Doppler grid whose dimensions depend on available bandwidth and signal duration.The inverse symplectic finite Fourier transform then converts the delay-Doppler signal to the time-frequency domain.
  • OTFS modulation: The conventional OTFS implementation uses inverse symplectic finite Fourier transform and OFDM processing before applying the corresponding reverse operations at reception.This realizes a two-step conversion between delay-Doppler and time domains through the time-frequency domain.
  • OTFS modulation: Zak-transform-based OTFS directly converts delay-Doppler signals to the time domain, bypassing explicit time-frequency processing.Discrete Zak transform implementations also derive concise input-output relationships for practical wireless channels.
  • Performance fundamentals: OTFS research analyzes diversity, PAPR, and general performance, including full effective diversity with practical shaping pulses and robustness over high-mobility channels.Reported studies also compare OTFS and OFDM under static multipath and practical channel estimation and detection.

III. TRANSCEIVER DESIGN FOR OTFS SYSTEM

OTFS transceiver research addresses practical waveform, window, precoding, channel-estimation, pilot, detection, and MIMO challenges. The section emphasizes a fundamental trade-off: achieving full time-frequency diversity increases detection complexity, motivating lower-complexity algorithms.

  • Transceiver design: Transceiver research spans shaping pulses, TF windows, multi-user precoding, channel acquisition, pilot design, detection, and MIMO processing.These components target practical implementation under non-ideal waveforms, sparse channels, pilot overhead, interference, and multiple antennas.
  • Channel estimation and pilots: Pilot and channel-estimation designs exploit DD-domain structure, sparsity, reciprocity, regularization, or reduced guard space to improve acquisition efficiency and accuracy.Guard spaces protect pilot estimation but depend on maximum delay and Doppler indices, reducing spectral efficiency.
  • Detection design: OTFS detection is more complex than OFDM because each resolvable channel path contributes nonzero entries across the effective channel matrix.Maximum-likelihood detection can have complexity that grows exponentially with the number of resolvable paths.
  • Detection design: Variational and QRD-based SIC detectors improve reported error, convergence, or interference performance under their respective evaluation settings.The variational approach is compared with MPA, while QRD-based SIC is compared with non-SIC detectors and addresses cross-symbol interference.
  • Multiple access designs: OTFS multiple access is robust to severe Doppler spread but introduces non-negligible multi-user interference because the DD-domain channel is convolutional.This combination motivates dedicated multiple-access designs rather than relying only on conventional subcarrier orthogonality.

IV. OTFS-ENABLED INTEGRATED SENSING AND COMMUNICATION

OTFS-enabled ISAC uses a unified delay-Doppler representation for communications and sensing, aligning naturally with range and velocity estimation. The surveyed works develop common signal models, channel-geometry interpretations, and estimation methods for these dual functions.

  • Unified DD-domain framework: OTFS-ISAC represents communication and sensing signals in one DD domain, directly relating transmitted signals to channel responses while resisting delay and Doppler spreads.This unified representation is contrasted with OFDM-based ISAC and supports joint treatment of both functionalities.
  • Reported methods and findings: Reported studies include more accurate velocity estimation than OFDM, maximum-likelihood and Cramér-Rao analyses, and low-complexity off-grid range-velocity estimation.The off-grid IIoT method has complexity dominated by a two-dimensional DFT and includes SINR analysis.
  • Unified DD-domain framework: For sensing, the transmitted signal is known and the channel is inferred; for communications, data symbols are detected given channel-state information.Both tasks can be written using the common model y = Hx + n.
  • Sensing interpretation: OTFS channel estimates expose propagation geometry because delays and Doppler shifts can be converted into range distances and target speeds.Temporal delay estimates and Bayesian estimators such as EKF can support reflector localization and tracking.

V. EMERGING OTFS APPLICATIONS

The survey extends OTFS beyond high-mobility radio links to optical, underwater acoustic, index-modulated, and SAGIN-related applications. Across these settings, studies exploit OTFS’s diversity, delay-Doppler handling, spectral-efficiency, or implementation properties.

  • Application rationale: OTFS is attractive for emerging applications because of low-complexity implementation, sparse channel representation, and resilience to large delays and Doppler spreads.These properties motivate applications across diverse propagation environments rather than only conventional cellular links.
  • Visible-light communications: In visible-light communications, OTFS can outperform OFDM and reduce cyclic-prefix use through two-dimensional domain multiplexing, improving spectral efficiency.A DC-biased optical OTFS design also uses 2D Hermitian symmetry to generate a real-valued signal and reports superiority over OFDM.
  • Underwater acoustic communications: Underwater acoustic communications present severe delay and Doppler spreads, with delays reaching hundreds of milliseconds and complex multipath from sea-surface and sea-floor reflectors.These channel conditions provide a demanding setting for OTFS application studies.
  • Index modulation: OTFS with dual-mode index modulation is designed to balance reliability and spectral efficiency by embedding additional information in activation indices.Related work also studies peak-to-average ratio and iterative receivers using structured priors and channel coding.
  • SAGIN and high-mobility networks: Although no work specifically addresses OTFS-based SAGIN, existing studies consider OTFS in LEO, UAV, vehicular, and high-speed railway networks.These applications reflect the need to support broad coverage and high mobility under evolving wireless-system requirements.

VI. RESEARCH CHALLENGES AND FUTURE WORKS

The survey identifies unresolved challenges for OTFS, including latency, fractional delay/Doppler, user multiplexing, cross-layer design, sensing-to-communications inference, RIS/backscatter integration, and predictive communications.

  • 6.1 Low Latency OTFS Transmission: OTFS may have higher detection latency than OFDM, although sparse quasi-static delay-Doppler channels can reduce overhead and computational complexity.The overall latency comparison between OTFS and OFDM remains open.
  • 6.2 Insufficient Doppler/delay Resolutions: Shortening signal duration reduces latency but introduces fractional Doppler from insufficient Doppler resolution, complicating reliable symbol detection.Fractional delay can also arise with narrow bandwidth, while commonly used MPA detection may degrade on dense factor graphs.
  • 6.3 Multi-user Communications: The appropriate domain for multiplexing multiple users remains unclear because delay-Doppler circular convolution creates a distinctive inter-user interference pattern.The survey proposes designing user multiplexing schemes from the delay-Doppler perspective.
  • 6.4 Cross-layer Design: High-mobility networks motivate cross-layer joint design linking OTFS physical-layer operation with dynamic network topology.Topology information may support waveform and transceiver design, while wireless conditions could inform topology optimization.
  • 6.5 Sensing-assisted Communications: When sensing targets and communication users differ, it remains an open question how much sensing-channel information can infer about the communications channel.Only some targets within a coverage area may be user equipments.
  • 6.6 OTFS-aided RIS/Backscatter Communications: OTFS-aided RIS/backscatter communications remain insufficiently studied, despite interest in time-varying wireless scenarios.Existing work has extensively investigated OFDM-aided RIS/backscatter communications, while OTFS-RIS benefits have not been thoroughly studied.
  • 6.7 Predictive DD Communications: Nearly constant delay and Doppler shifts for approximately constant-speed transceivers enable channel estimation once for multiple OTFS frames and motivate predictive precoding.The survey identifies delay-Doppler predictive communications as a potential research direction.

VII. CONCLUSIONS

The article surveys OTFS as a promising delay-Doppler communication technology for high-mobility wireless systems. It covers fundamentals, performance limits, transceiver design, integrated sensing and communications, applications, and open research challenges.

  • VII. CONCLUSIONS: The survey presents OTFS as a promising enabler for future wireless systems, particularly in high-mobility environments.It aims to support deeper understanding and further exploration among industry and academic researchers.
  • VII. CONCLUSIONS: The article reviews delay-Doppler channel representation, OTFS fundamentals and performance limits, transmitter and receiver design, OTFS-based ISAC, emerging applications, and open challenges.These topics define the survey’s coverage of OTFS in the 6G era.
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