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IEEE 802.11be-Wi-Fi 7: New Challenges and Opportunities

Cailian Deng, Xuming Fang, Xiao Han, Xianbin Wang, Li Yan, Rong He, Yan Long, Yuchen Guo

arXiv:2007.13401v3eess.SP

TL;DR

Emerging 4k/8k video, VR, AR, and gaming demand throughput and latency beyond IEEE 802.11ax. The paper surveys EHT’s proposed PHY, MAC, and cross-layer technologies, related research, and directions beyond EHT; it reports approximately 90% feedback-overhead savings per user per transmission in one enhanced-feedback study.

  • Problem

    IEEE 802.11ax cannot satisfy the emerging applications’ stringent high-throughput and low-latency requirements, while multi-band operation and QoS provisioning raise additional WLAN challenges.

  • Method

    The paper surveys EHT task-group standardization progress, proposed PHY/MAC solutions, related academic studies, and potential developments beyond EHT.

  • Results

    Approximately 90% overhead per user per transmission can be saved using enhanced explicit feedback in the cited simulation result.

  • Takeaways & Limitations

    EHT combines PHY and MAC enhancements—including multi-link operation, multi-AP coordination, MIMO enhancement, and HARQ—to address emerging WLAN performance requirements.

Abstract

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With the emergence of 4k/8k video, the throughput requirement of video delivery will keep grow to tens of Gbps. Other new high-throughput and low-latency video applications including augmented reality (AR), virtual reality (VR), and online gaming, are also proliferating. Due to the related stringent requirements, supporting these applications over wireless local area network (WLAN) is far beyond the capabilities of the new WLAN standard -- IEEE 802.11ax. To meet these emerging demands, the IEEE 802.11 will release a new amendment standard IEEE 802.11be -- Extremely High Throughput (EHT), also known as Wireless-Fidelity (Wi-Fi) 7. This article provides the comprehensive survey on the key medium access control (MAC) layer techniques and physical layer (PHY) techniques being discussed in the EHT task group, including the channelization and tone plan, multiple resource units (multi-RU) support, 4096 quadrature amplitude modulation (4096-QAM), preamble designs, multiple link operations (e.g., multi-link aggregation and channel access), multiple input multiple output (MIMO) enhancement, multiple access point (multi-AP) coordination (e.g., multi-AP joint transmission), enhanced link adaptation and retransmission protocols (e.g., hybrid automatic repeat request (HARQ)). This survey covers both the critical technologies being discussed in EHT standard and the related latest progresses from worldwide research. Besides, the potential developments beyond EHT are discussed to provide some possible future research directions for WLAN.

I. INTRODUCTION

The survey motivates IEEE 802.11be EHT (Wi‑Fi 7) by rising throughput and latency demands that exceed IEEE 802.11ax, and reviews candidate PHY and MAC technologies addressing these needs.

  • Uncompressed 4k/8k video can require up to 20Gbps, while applications such as VR, AR, and online gaming impose high-throughput and low-latency demands.
  • IEEE 802.11ax does not meet these emerging high-throughput and low-latency requirements, motivating IEEE 802.11be EHT, also called Wi‑Fi 7.
  • EHT is envisioned to add contiguous 240 MHz, noncontiguous 160+80 MHz, contiguous 320 MHz, and noncontiguous 160+160 MHz bandwidth modes, although channelization and tone-plan details remain under discussion.
  • The survey covers PHY and MAC topics including multi-RU support, 4096-QAM, preamble design, multi-link operation, MIMO enhancement, multi-AP coordination, and HARQ.
  • Multi-link aggregation across the 2.4 GHz, 5 GHz, and 6 GHz bands can provide up to 320 MHz bandwidth, but introduces frequency selectivity, coexistence, backward-compatibility, and multi-band operation challenges.

2) Supporting increased spatial streams and MIMO enhancements:

EHT extends WLAN spatial multiplexing and coordination capabilities to support higher-throughput, lower-latency applications. The survey covers these enhancements alongside PHY, multi-link, multi-AP, and reliability techniques under discussion for EHT.

  • Supporting increased spatial streams and MIMO enhancements: EHT recommends increasing the maximum number of spatial streams to 16, extending IEEE 802.11ax spatial multiplexing capabilities.IEEE 802.11ax supports up to eight simultaneous users with an eight-antenna AP through MU-MIMO.
  • Supporting increased spatial streams and MIMO enhancements: EHT enables neighboring APs to share data and control information through wired or wireless links for coordinated transmission.This expands resource use across time, frequency, and spatial dimensions beyond IEEE 802.11ax single-AP transmission.
  • Supporting increased spatial streams and MIMO enhancements: Multi-AP coordination requires low-overhead procedures for sounding, AP selection, and transmission because decentralized coordination can create substantial signaling and processing costs.The challenge is especially relevant to multi-AP networks without a central coordinating node.
  • Supporting increased spatial streams and MIMO enhancements: HARQ is expected to improve EHT reliability and latency by retaining information from erroneous transmissions instead of relying only on MPDU retransmission.Current ARQ discards erroneous MPDUs before retransmission.
  • Supporting increased spatial streams and MIMO enhancements: The survey examines EHT PHY and MAC techniques, including wider channelization, multi-RU support, 4096-QAM, preamble designs, multi-link operations, MIMO, multi-AP coordination, and HARQ.It also discusses related academic progress and possible research directions beyond EHT.

II. PHY ENHANCEMENTS FOR EHT

EHT PHY enhancements target ultra-high throughput and low-latency applications through wider bandwidth, multi-RU assignment, higher-order modulation, and new preamble mechanisms. Key implementation details remain under discussion, including tone plans, RU combinations, signaling, and transmission procedures.

  • Overview: Up to 30 Gbps peak rate is enabled by EHT PHY enhancements for high-throughput and low-latency applications such as video, gaming, AR, and VR.The enhancements and related research are summarized in Table I.
  • A. New Bandwidth Mode: Candidate EHT bandwidth modes include contiguous 240 MHz, noncontiguous 160+80 MHz, contiguous 320 MHz, and noncontiguous 160+160 MHz.These modes address the limited bandwidth available in existing 2.4 GHz and 5 GHz WLAN operation.
  • B. Multi-RU Support: Multi-RU assignment allows a single user to receive multiple resource units, enhancing spectral efficiency beyond IEEE 802.11ax.For bandwidths up to 160 MHz, proposed combinations restrict small RUs to small RUs and large RUs to large RUs.
  • C. EHT Preamble Design: Two EHT preamble formats are considered for SU and MU PPDUs, alongside preamble puncturing intended to improve spectral efficiency.Preamble puncturing must support wider bandwidths and more flexible patterns than earlier WLAN operation.
  • Open Design Issues: EHT still requires further work on tone plans, multi-RU signaling, RU combinations, coding, interleaving, and data transmission procedures.For new 320 MHz and 160+160 MHz modes, some tone-plan and large-RU combinations remain undetermined.

C. EHT Preamble Design

EHT preamble design combines legacy fields for backward compatibility with new signaling and training fields for EHT transmissions. The design supports resource allocation, user-specific information, and MIMO channel estimation, while wider-band puncturing and higher-order modulation introduce additional requirements.

  • C. EHT Preamble Design: EHT preambles retain legacy L-STF, L-LTF, L-SIG, and RL-SIG fields for detection, synchronization, compatibility, and essential signaling.The legacy portion precedes the U-SIG, EHT-SIG, training, and data fields.
  • C. EHT Preamble Design: The U-SIG carries version-independent information such as PHY version, UL/DL status, BSS color, and TXOP duration, plus version-dependent EHT information.The EHT-SIG common field conveys allocation and transmission parameters, while user-specific fields provide dedicated information.
  • C. EHT Preamble Design: The EHT-SIG field uses common and user-specific fields to signal multi-RU or MU transmission parameters, including allocation, coding, MCS, streams, and guard interval.A variable MCS and variable-length EHT-SIG support signaling for multiple users.
  • C. EHT Preamble Design: EHT-STF and EHT-LTF support MIMO channel estimation, with candidate 1x, 2x, and 4x EHT-LTF types.Existing HE-LTFs may be reused for several legacy bandwidth modes, while wider EHT modes require additional designs.
  • C. EHT Preamble Design: Preamble puncturing improves channel utilization and transmission rate but requires more complex hardware operations and flexible patterns above 160 MHz.Proposed patterns extend IEEE 802.11ax approaches to 240/320 MHz and may puncture primary channels.
  • D. Higher-Order Modulation Schemes: 4096-QAM increases the payload per modulation symbol to 12 bits, while feasible configurations remain dependent on transmission and receiver conditions.Reported conditions include beamforming, low stream count, strict EVM requirements, and multiple receiving antennas.

E. Summary of the PHY Enhancements for EHT

EHT targets at least 30 Gbps through PHY enhancements and requires coordinated MAC and cross-layer designs for multi-band operation. The surveyed work covers wideband aggregation, multi-link architectures, and related management interfaces, while noting that many PHY details remain open.

  • PHY enhancements: EHT introduces new bandwidth modes, multi-RU support, enhanced preambles, and 4096-QAM to target at least 30 Gbps.Open design questions include tone plans, multi-RU coding and signaling, 4096-QAM configurations, and preamble designs.
  • Multi-link operation: Existing multi-band designs include Independent MAC and Distributed MAC architectures, while EHT considers a Unified MAC for multi-link aggregation.The existing architectures support session transfer, but single-TID traffic remains restricted to one band or channel and incurs renegotiation overhead.
  • Open issues: The surveyed EHT technologies and research directions remain provisional because standardization has just started and many technical choices are still open.The article also identifies potential developments beyond EHT as future WLAN research directions.
  • PHY–MAC interface: EHT must define PHY–MAC interface parameters and values, including spatial streams and modulation and coding scheme information.The PHY communicates per-PPDU transmission and reception parameters with the MAC through TXVECTOR and RXVECTOR.
  • Multi-link operation: Multi-link operation aggregates resources across 2.4 GHz, 5 GHz, and 6 GHz bands, motivating new spectrum management, coordination, and transmission mechanisms.The section surveys multi-link aggregation technologies and enhanced multi-link MAC architectures.

B. Multi-link Operation over Wideband and Noncontiguous Spectra

Multi-link operation lets Wi-Fi 7 use multiple bands and links for throughput, latency, flexibility, and coexistence. The surveyed designs span channel access, fast switching, control/data separation, and synchronized or asynchronized transmission, but legacy devices and inter-link interference constrain gains.

  • Multi-link aggregation: Multi-link aggregation across 2.4 GHz, 5 GHz, and 6 GHz can parallelize frame transmissions, increasing throughput and network flexibility.Available links may nevertheless be restricted by legacy devices and noncontiguous spectrum conditions.
  • Channel access: EHT channel access must extend single-link CCA and bitmap mechanisms to multi-link devices using channels up to 320 MHz.The required mechanisms had not yet been defined, and multiple-primary-channel access is more complicated because link states and backoff procedures differ.
  • Channel access: Multi-link access designs include one-primary-channel access, temporary-primary-channel access, and independent or multiple-primary-channel approaches.A temporary primary channel can provide additional TXOP opportunities when the primary channel is unavailable, but its dependence on primary-channel status limits idle-channel use.
  • Multi-link transmission: Fast link switching supports coexistence and load balancing by moving control, management, and data traffic to idle, higher-quality links when service quality declines.A failed downlink frame can be retransmitted immediately on another available link, reducing waiting latency and using channel diversity.
  • Multi-link transmission: Dedicated control links separate control and data planes across links, while synchronized and asynchronized modes differ in whether transmission starting times are aligned.Control links coordinate communication over data channels; synchronized transmission permits aligned multi-link starts, whereas asynchronized transmission uses unaligned starts.

C. Summary of Multi-link Operations

Multi-link operations are a key EHT candidate for improving peak throughput, latency, and jitter. The survey organizes the design space around MAC management, multi-link channel access, and transmission options, while identifying complexity, legacy coexistence, and interference constraints.

  • Summary: EHT multi-link operations target improved peak throughput, reduced latency, and reduced jitter.The section investigates multi-link MAC architectures, channel access methods, and transmissions over multiple links.
  • MAC architecture: Efficient multi-link channel access requires new management functions for setup, teardown, MAC addresses, BA/Ack sessions, and security across links.These functions support the enhanced multi-link MAC architecture.
  • Channel access: Multi-link channel access may use one or more primary channels, but differing link states and per-link backoff make multiple-primary-channel access more complicated.The current WLAN channel-access design is based on a single link.
  • Transmission options: Transmission options include fast link switching, control/data separation, independent transmission, and simultaneous transmission for different application use cases.The survey relates these options to coexistence constraints, load balancing, and spectrum utilization.
  • Constraints: Real-world multi-link gains may be limited by legacy devices, design complexity, spectrum utilization, and inter-link interference during simultaneous transmission and reception.Guard separation can reduce spectrum utilization, motivating advanced interference cancellation or suppression.

IV. MULTIPLE INPUT MULTIPLE OUTPUT(MIMO) ENHANCEMENT

EHT MIMO enhancements target up to 16 spatial streams while reducing the sounding and CSI feedback overhead that can otherwise limit their benefits. The survey reviews enhanced explicit and implicit feedback schemes, including several new reduction approaches.

  • MIMO enhancement: EHT considers increasing the maximum spatial streams from eight to 16, theoretically doubling transmission data rate but increasing sounding and feedback requirements.Existing IEEE 802.11ax sounding and feedback mechanisms are insufficient for 16 spatial streams.
  • Current sounding and feedback: CSI feedback overhead is a key channel-sounding issue because lengthy feedback delays interfere with sounding timeliness.The survey describes implicit feedback, several explicit reduction methods, and IEEE 802.11ax feedback reductions.
  • Enhanced feedback reduction: EHT may improve explicit feedback using φ Only Feedback, Time Domain Channel Feedback, Differential Given’s Rotation, or Variable Angle Quantization.The survey also highlights Multiple Component Feedback, Finite Feedback, and Two-way Channel Sounding as new explicit schemes.
  • Enhanced feedback reduction: Multiple Component Feedback splits feedback into components sent at different intervals, and simulations reported approximately 90% overhead savings per user per transmission.The scheme can provide feedback granularity below 20 MHz.
  • Enhanced feedback reduction: Finite Feedback sends a small number of channel-condition bits, potentially through codewords selected from a designed codebook for channel adaptation.Codebooks widely used in cellular networks are identified as a possible feedback-reduction solution.
  • Enhanced feedback reduction: Two-way Channel Sounding lets the AP estimate its outgoing channel from round-trip training and one-way STA training, with low STA complexity.The AP sends Xa to the STA, receives the returned Yas, and combines that signal with Ysa.

2) Enhanced implicit feedback:

Enhanced implicit feedback relies on channel reciprocity but requires calibration because transmitter and receiver chains introduce non-reciprocal impairments. Local AP calibration can avoid STA participation, while calibration accuracy affects MU-MIMO beamforming.

  • Calibration requirement: Implicit sounding eliminates CSI feedback overhead by using channel reciprocity, but RF-chain impairments make actual uplink and downlink baseband channels unequal without calibration.The impairments are slowly and randomly varying, so reciprocity must be explicitly compensated.
  • Local AP calibration: Local AP calibration can calibrate the AP without STA involvement, avoiding reference-signal and channel-information exchange with other devices.This approach is presented as a newer alternative to interactive calibration methods.
  • Calibration procedure: Calibration factors are applied to the channel matrix before calculating the beamforming vector.The relative calibration factor and transmitter/receiver terms describe the calibration relationship among antenna elements.
  • Calibration performance: Less than 3 deg residual calibration error was reported for each element using Least Squares calibration at the AP.The evaluation compared Least Squares and Argos schemes.
  • Calibration performance: For higher numbers of users in downlink MU-MIMO beamforming, AP calibration error must remain below 3 deg.The passage directly links the tighter calibration range to higher-user MU-MIMO operation.
  • Sounding trade-off: For MU-MIMO, the overhead trade-off between implicit and explicit sounding depends on STA count, feedback duration, and uplink sounding-frame duration.The comparison assumes calibration between receiver and sender has already been implemented.

C. Summary of MIMO Enhancements

The survey positions MIMO enhancement and channel sounding alongside multi-AP coordination as central EHT directions. Multi-AP operation can improve dense-network resource use, but requires coordination infrastructure, synchronization, and efficient CSI acquisition.

  • C. Summary of MIMO Enhancements: The MIMO enhancement targets 16 spatial streams and new sounding schemes, combining enhanced explicit feedback with reciprocity-based implicit sounding.Explicit methods support backward compatibility, while implicit methods offer potential overhead reduction but depend on calibration.
  • Multi-AP coordination: Co-channel interference becomes severe as mobile demand grows, motivating AP collaboration through shared scheduling information and CSI.The survey discusses C-OFDMA, CSR, CBF, and JXT as multi-AP transmission modes.
  • Multi-AP architecture: Neighboring-AP coordination creates substantial signaling overhead and processing complexity, motivating centralized architectures with a master AP and slave APs.The master AP manages coordination and scheduling, while slave APs participate in transmissions.
  • Multi-AP architecture: Multi-AP operation needs high-capacity, low-latency backhaul links and a logical processing unit to exchange coordination information and service data in real time.The processing unit coordinates operations such as resource management across distributed APs.
  • Multi-AP challenges: Multi-AP systems face user-selection, oscillator-synchronization, and residual-CFO challenges that can affect coordination and decoding.Even a 20 Hz residual CFO across APs was reported to significantly degrade peak throughput in simulations.
  • Multi-AP sounding: Multi-AP sounding must acquire CSI before transmission while balancing time overhead, channel-estimation accuracy, and AP- and STA-side computational complexity.Explicit sounding raises complexity and feedback concerns, whereas implicit sounding requires calibration.

2) Coordinated multi-AP transmission:

After multi-AP sounding, EHT coordinates distributed AP transmissions through C-OFDMA, CSR, CBF, or JXT. These schemes trade coordination complexity against frequency reuse, interference suppression, and joint transmission capability.

  • Coordinated transmission: The M-AP initiates multi-AP operations, and selected APs then perform coordinated transmission using schemes including C-OFDMA, CSR, CBF, and JXT.The overview separates multi-AP channel sounding from subsequent coordinated transmission.
  • C-OFDMA: C-OFDMA extends OFDMA across multiple APs by coordinating RUs, allowing mutually orthogonal time-frequency resources and reducing RU conflicts.Fractional coordination limits coordination to interference-limited edge STAs while center STAs can use all RUs.
  • CSR: CSR manages spatial reuse through coordinated interference management, including decisions about receiving STAs, beamforming vectors, and transmission power.The comparison concerns resource utilization and interference control in dense deployments.
  • CBF: CBF suppresses inter-cell interference by transmitting toward intended STAs while placing spatial radiation nulls toward non-served STAs.This extends beamforming from independent single-AP operation to coordinated multi-AP interference control.
  • JTX: JTX treats multiple distributed APs and STAs as a virtual MIMO system, with APs transmitting jointly after receiving scheduling and control information from the M-AP.Downlink JTX can use a giant precoder across the combined distributed antenna array.
  • JTX: UL JTX can improve reliability through distributed interference cancellation or joint reception, while centralized CSMA/CA resolves differing channel-status views across APs.Each AP reports its CCA status to the processing unit before a JTX proceeds.

C. Summary of the Multi-AP Coordination Operations

Multi-AP coordination involves centralized or distributed architectures, channel sounding, and transmission-scheme selection. HARQ introduces additional PHY/MAC design choices involving retransmission granularity, alignment, feedback, and overhead.

  • Multi-AP network: Centralized multi-AP networks simplify resource scheduling, time synchronization, and data sharing through a central node.
  • Multi-AP network: Distributed multi-AP networks must achieve tight synchronization while managing channel-sounding complexity and CSI feedback overhead.
  • Multi-AP transmission: C-OFDMA, CSR, CBF, and JXT provide four multi-AP transmission categories with performance varying across interference-limited and noise-limited regions.
  • Multi-AP transmission: Dynamic environments may require adaptive switching among multi-AP schemes according to channel state, SINR, or SNR.
  • HARQ coordination: HARQ designs must address granularity, PHY/MAC processes, CW-to-MPDU alignment, and padding overhead.

B. HARQ Process

The HARQ process combines retransmitted information before decoding, with PHY-level CW processing and MAC-level MPDU processing offering different feedback and compatibility requirements. Design choices include retransmission granularity, feedback, combining, and coding methods.

  • HARQ process: HARQ combines bits from unsuccessful and retransmitted PPDUs using LLRs, requiring knowledge of PPDU parameters to identify combinable transmissions.
  • HARQ process: PHY-level HARQ checks codewords through parity, while MAC-level HARQ checks MPDUs through CRC and can use the existing BA frame for feedback.
  • HARQ feedback: CW-level feedback requires new signaling, whereas MAC-level feedback using the existing BA protocol requires less new feedback design.
  • HARQ process: At PHY level, the AP stores original CWs and retransmits only failed CWs, which the STA combines before forwarding decoded bits to the MAC.
  • Design trade-offs: PHY-level retransmission is preferred for lower signaling and padding overhead, while MAC-level retransmission remains more compatible with existing ARQ.
  • HARQ methods: CC repeats coded MPDUs, PCC modifies CC, and IR retransmits additional parity, with different implementation complexities and gains.
  • HARQ methods: HARQ gains depend on link adaptation, including fixed-MCS, long-term-SNR, and HARQ-feedback-based rate selection.

VII. FUTURE DEVELOPMENT AND RESEARCH OPPORTUNITIES

Beyond EHT, research opportunities include spectrum coexistence, multi-band integration, QoS-aware resource allocation, energy management, and hybrid beamforming. These directions respond to heterogeneous networks, diverse service requirements, and increased device complexity.

  • Spectrum and connectivity: EHT must coexist in the 6 GHz band with heterogeneous technologies such as IEEE 802.11ax and 5G under differing spectrum-access rules.
  • Spectrum and connectivity: Future WLANs may integrate microwave, mmWave, and THz frequencies to provide seamless connectivity across wide and local areas.
  • QoS and intelligence: Machine learning is being considered for recognizing diverse QoS requirements, allocating resources, predicting link status, and switching links adaptively.
  • Energy management: Multilink operation, multi-AP collaboration, and HARQ may significantly increase EHT device power consumption, motivating improved energy-saving mechanisms.
  • Beamforming: Hybrid beamforming can reduce the number of RF chains relative to antennas while balancing flexibility and cost, but requires system-level modeling and assessment.
  • Research scope: The survey reviews EHT proposals including multi-RU, 4096-QAM, multilink operation, MIMO enhancement, multi-AP coordination, and HARQ, alongside research beyond EHT.

APPENDIX A

Appendix A provides an alphabetical reference for technical terms and acronyms used repeatedly throughout the article.

  • APPENDIX A: Appendix A summarizes the article’s main acronyms.The appendix is intended to support interpretation of recurring technical terminology.
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