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Next Generation Wi-Fi and 5G NR-U in the 6 GHz Bands: Opportunities & Challenges

Gaurang Naik, Jung-Min Park, Jonathan Ashdown, William Lehr

arXiv:2006.16534v2cs.NI

TL;DR

Growing demand for unlicensed spectrum creates opportunities and coexistence challenges in the 6 GHz bands. This paper surveys technical, regulatory, and standardization literature on Wi-Fi and NR-U operations, identifies relevant mechanisms and research problems, and reports that FBE LBT can improve user-perceived throughput by 10-50% in the studied deployment scenarios.

  • Problem

    Rising demand for unlicensed spectrum motivates 6 GHz access, but Wi-Fi and NR-U must coexist with incumbent users and one another.

  • Method

    The paper presents a comprehensive survey of research papers, regulatory and standardization contributions, and other documents on 6 GHz unlicensed operations.

  • Results

    10-50% user-perceived throughput improvement is shown for FBE LBT based on the deployment scenario.

  • Takeaways & Limitations

    The survey identifies features, coexistence challenges, and key research problems relevant to efficiently using 6 GHz spectrum while protecting incumbent users.

Abstract

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The ever-increasing demand for unlicensed spectrum has prompted regulators in the US and Europe to consider opening up the 6 GHz bands for unlicensed access. These bands will open up 1.2 GHz of additional spectrum for unlicensed radio access technologies (RATs), such as Wi-Fi and 5G New Radio Unlicensed (NR-U), in the US and if permitted, 500 MHz of additional spectrum in Europe. The abundance of spectrum in these bands creates new opportunities for the design of mechanisms and features that can support the emerging bandwidth-intensive and latency-sensitive applications. However, coexistence of unlicensed devices both with the bands' incumbent users and across different unlicensed RATs present significant challenges. In this paper, we provide a comprehensive survey of the existing literature on various issues surrounding the operations of unlicensed RATs in the 6 GHz bands. In particular, we discuss how key features in next-generation Wi-Fi are being designed to leverage these additional unlicensed bands. We also shed light on the foreseeable challenges that designers of unlicensed RATs might face in the near future. Our survey encompasses key research papers, contributions submitted to standardization bodies and regulatory agencies, and documents presented at various other venues. Finally, we highlight a few key research problems that are likely to arise due to unlicensed operations in the 6 GHz bands. Tackling these research challenges effectively will be critical in ensuring that the new unlicensed bands are efficiently utilized while guaranteeing the interference-free operation of the bands' incumbent users.

I. INTRODUCTION

Regulators are considering opening the 6 GHz bands to unlicensed Wi-Fi and NR-U access to address rising demand for high-throughput, low-latency connectivity. The paper surveys the resulting opportunities, coexistence challenges, and research problems.

  • Motivation: Emerging AR, VR, and mobile gaming applications increase demand for high-throughput, high-reliability, and low-latency Wi-Fi connectivity.The paper identifies the 6 GHz bands as critical for applications with stringent QoS requirements.
  • Motivation: The US considered opening 1.2 GHz and Europe 500 MHz of additional 6 GHz spectrum for unlicensed access.The spectrum is intended for unlicensed RATs including Wi-Fi and NR-U.
  • Coexistence: Wi-Fi and NR-U are expected to share the 6 GHz bands, making coexistence among heterogeneous unlicensed RATs an important design issue.NR-U is designed to operate alongside Wi-Fi, with spectrum sharing described as imminent.
  • Coexistence: Unlicensed devices must also protect incumbent users, with FCC rules requiring AFC coordination in U-NII-5 and U-NII-7 and restricted indoor low-power operation in U-NII-6 and U-NII-8.These rules reflect different protection mechanisms across the US sub-bands.
  • Paper scope: The paper surveys research, regulatory and standardization contributions, and other documents to identify opportunities, challenges, and research problems for 6 GHz unlicensed access.The survey covers materials from bodies including IEEE and 3GPP and notes that some operational details remained subject to change.

II. RELATED WORK

Prior work covers Wi-Fi, NR-U, incumbent services, and regulatory materials, but the paper centers specifically on unlicensed operations in the 6 GHz bands. It organizes relevant features and coexistence challenges across incumbent, heterogeneous-RAT, and adjacent-band interactions.

  • Prior literature: Existing 6 GHz literature was scarce and regulatory information was scattered across FCC, European, and stakeholder documents.The paper therefore reviews both technical literature and regulatory sources.
  • Wi-Fi and NR-U: Earlier Wi-Fi work examined IEEE 802.11ax features including MU-OFDMA, spatial reuse, and Target Wake Time, while IEEE 802.11be development was ongoing.The cited survey literature addressed these topics in relation to IEEE 802.11ax.
  • Wi-Fi and NR-U: Related 5G literature describes NR and its PHY layer, architecture, and operating scenarios, while NR-U requires additional MAC-layer design for unlicensed operation.NR-U inherits the 5G NR PHY layer and is discussed as a cellular-side technology for the 6 GHz bands.
  • Paper distinction: The paper distinguishes itself by focusing on 6 GHz unlicensed operations and contextualizing features according to their relevance in these bands.It also addresses incumbent services and challenges involving incumbent, cross-RAT, and adjacent-band interference.
  • Incumbent users: US incumbent services include critical fixed point-to-point backhaul, nomadic television and cable relay links, fixed satellite services, and low-power UWB systems.Nomadic services can change transmitter and receiver locations frequently, complicating protection of incumbent operations.
  • Incumbent users: European 5.925-6.425 GHz unlicensed-use proposals must account for incumbent fixed and fixed-satellite services, including long-distance mobile-broadband backhaul links.The European band has the same frequency range as the US U-NII-5 band.

B. Unlicensed Technologies

Wi-Fi 6 and its successor, IEEE 802.11be, introduce mechanisms aimed at improving throughput, efficiency, reliability, and latency, including MU-OFDMA, wider channels, MIMO enhancements, and multi-link aggregation.

  • Wi-Fi 6: Wi-Fi 6 uses MU-OFDMA to assign resource units to users according to uplink or downlink traffic demands.In 6 GHz networks without legacy Wi-Fi devices, restricting transmissions to MU-OFDMA can improve MAC efficiency and user throughput in dense settings.
  • IEEE 802.11be: IEEE 802.11be targets AR, VR, gaming, and industrial automation applications requiring high throughput, reliability, and low latency.The standard considers operation across unlicensed spectrum from 1 GHz to 7.125 GHz and retains features introduced in 802.11ax.
  • IEEE 802.11be: IEEE 802.11be considers up to 16 spatial streams, 4096 QAM, multiple-AP coordination, and hybrid automatic repeat request enhancements.These features contribute toward the standard’s high-throughput, high-reliability, and low-latency objectives.
  • Multi-link aggregation: Multi-link aggregation lets stations and access points transmit or receive the same flow concurrently across multiple bands or channels.Independent/asynchronous MLA suits links far apart in frequency, while synchronous/simultaneous schemes address smaller frequency separations and inter-link cross-talk.

2) 5G NR-U:

NR-U is a 3GPP Release 16 unlicensed RAT derived from LTE-LAA that can operate without a licensed primary carrier, but its initial coexistence mechanisms remain largely band agnostic.

  • NR-U background: NR-U is a 3GPP Release 16 RAT and successor to LTE-LAA, with LTE-LAA serving as the starting point for its PHY and MAC procedures.Its channel-access mechanism is based on LTE-LAA, and operation where Wi-Fi presence cannot be excluded uses bandwidths that are integer multiples of 20 MHz.
  • NR-U deployment: NR-U does not require a licensed primary carrier and can therefore be deployed by parties similarly to Wi-Fi access points.In this operating scenario, the NR-U network connects to the 5G Core Network.
  • Channel access: NR-U uses Listen Before Talk, with four categories whose timing and contention procedures vary by regulation and traffic type.Category 2 uses a 25 µs fixed interval, while categories 3 and 4 add a random contention period after a 16 µs interval.
  • 6 GHz outlook: At the paper’s writing, NR-U coexistence mechanisms were band agnostic and no 6 GHz-specific mechanisms had been specified.Future NR-U releases could design mechanisms that leverage abundant 6 GHz spectrum and support fair coexistence with other unlicensed RATs.

C. Technologies in Adjacent Bands

The 6 GHz environment is shaped by adjacent vehicular-communication bands and by channelization choices that enable wider Wi-Fi channels, higher throughput, and more flexible operation around occupied spectrum.

  • Adjacent-band technologies: The 6 GHz unlicensed spectrum is adjacent to the 5.9 GHz ITS band, whose V2X systems support safety- and non-safety-critical vehicular communications.The safety-of-life nature of these applications motivates protection of the ITS band.
  • Adjacent-band technologies: IEEE 802.11p supports many day-1 vehicular safety applications, but its performance remains inadequate for some applications requiring both high reliability and low latency.IEEE 802.11bd is being developed to address this gap using later Wi-Fi PHY and MAC enhancements.
  • 6 GHz channelization: The US provides 1.2 GHz and Europe 500 MHz of additional unlicensed 6 GHz spectrum, substantially expanding available channel capacity.The paper expects this abundance to increase achievable end-to-end throughput.
  • Wider bandwidths: IEEE 802.11be is expected to support 240 MHz and 320 MHz channels, which can be meaningfully used because 6 GHz offers multiple channels of these widths.The US offers seven 160 MHz, four 240 MHz, and four 320 MHz channels; Europe offers three 160 MHz, two 240 MHz, and one 320 MHz channel.
  • Wider bandwidths: Allowing 240 MHz channels preserves higher throughput when part of a 320 MHz channel is busy by bonding the remaining 80 MHz channels.IEEE 802.11be also uses preamble puncturing to omit busy 20 MHz portions of a large channel and transmit over the rest.

2) Higher Order MCS & Preamble Design:

The 6 GHz bands support higher-throughput Wi-Fi through 4096 QAM, reduced preamble overhead, wider channels, and multi-link aggregation (MLA). MLA performance depends on channel separation and traffic conditions, while synchronous operation can disadvantage legacy single-link devices.

  • Higher Order MCS: 802.11be introduces 4096 QAM, whose high SINR requirements can be supported by improved frequency planning in the 6 GHz bands.The larger number of channels can reduce interference and help provide the SINR needed to decode densely packed symbols.
  • Preamble Design: 6 GHz operation allows newer Wi-Fi devices to omit legacy preamble fields because backward compatibility with earlier generations is unnecessary.This reduces preamble overhead for 802.11ax and 802.11be devices operating in these bands.
  • Multi-Link Aggregation: 802.11be can aggregate channels across the same or different bands and transmit packets simultaneously over multiple links.MLA may operate synchronously or asynchronously, depending on the aggregation scenario.
  • Multi-Link Aggregation: 2 to 4 times the single-link throughput is achievable with asynchronous MLA when widely separated 80 MHz channels minimize inter-link cross-talk.With closely spaced channels, synchronous multi-primary MLA can boost throughput by up to 5 times under heavy traffic.
  • Multi-Link Aggregation: Synchronous multi-primary MLA can outperform asynchronous or independent MLA through secondary-link free-riding.The extra access advantage can create unfairness for single-link and legacy 802.11 devices using the secondary link.

B. Latency Enhancement

End-to-end latency in LBT-based Wi-Fi and NR-U consists mainly of queuing, channel access, transmission, and retransmission delays. In lightly loaded settings, channel access is the dominant bottleneck, while MLA and scheduling can reduce latency by providing additional transmission opportunities.

  • Latency Components: LBT-based packet latency comprises queuing, channel access, transmission, and retransmission delays.Propagation and processing delays are treated as much smaller than these four components.
  • Latency Bottleneck: In lightly loaded environments, channel access latency dominates because queuing and retransmission delays are negligible.The worst-case contention process is identified as a major bottleneck for real-time applications.
  • Scheduling: Schedule-based MU-OFDMA and modified channel-access rules can improve packet latency while also increasing offered throughput.These mechanisms are especially relevant to uplink transmissions in Wi-Fi 6 and later generations.
  • MLA Latency Reduction: An order of magnitude reduction in 95-percentile latency is reported when 802.11be devices aggregate two links instead of using a single link.Independent or asynchronous MLA can reduce latency to as low as 25% of the single-link case.
  • MLA Latency Reduction: MLA flushes queued packets whenever any available link becomes idle, avoiding the need to wait for one sole link.The availability of additional contending links is identified as the key factor reducing worst-case latency.

2) Re-transmissions:

Latency-sensitive retransmission strategies trade capacity for reliability, while 6 GHz coexistence mechanisms constrain where and how unlicensed devices transmit. US rules use AFC, indoor-power restrictions, and exclusion zones, but interference effects remain unsettled across incumbent systems.

  • Re-transmissions: Repeating latency-sensitive packets within the latency budget without waiting for acknowledgments can improve reliability and avoid retransmission-induced latency.The approach requires substantial capacity because each packet is transmitted multiple times.
  • Re-transmissions: Greater NR-U sub-carrier spacing shortens symbol duration and, with mini-slot scheduling, can reduce reservation time and packet latency.These choices also improve NR-U air-time efficiency.
  • US Coexistence Rules: Standard-power APs in U-NII-5 and U-NII-7 must avoid incumbent exclusion zones computed using an I/N threshold of -6 dB.The FCC framework requires AFC-based frequency selection before operation.
  • US Coexistence Rules: AFC supplies permissible frequencies and transmission powers after an AP reports location and antenna height, with database queries required once every 24 hours.The system provides power-specific frequency lists, including increments from 21 dBm to 36 dBm.
  • US Coexistence Rules: Indoor LPI devices can operate across the 6 GHz bands without AFC when they remain within specified power limits, relying on building losses to protect incumbents.VLP operation is separately considered for very-low-power indoor and outdoor devices.
  • Evidence on Coexistence: Studies disagree about interference impacts: some report negligible effects, while others estimate possible fixed-service link failures within incumbent exclusion zones.The supplied literature therefore presents coexistence outcomes as unresolved rather than uniform.

2) U-NII-6 and U-NII-8 bands:

In U-NII-6 and U-NII-8, uncertain incumbent locations limit AFC-style protection, so US rules emphasize indoor LPI operation while studies report both effective mitigation and measurable incumbent impacts. European analyses generally find indoor sharing feasible, with additional constraints for some adjacent or overlapping systems.

  • US U-NII-6/8: Uncertain incumbent locations make AFC-like databases inefficient in U-NII-6 and U-NII-8, leading the FCC to allow LPI operation and consider VLP expansion.Indoor restrictions rely on propagation losses such as building entry loss to reduce interference.
  • US U-NII-6/8: UWB users warn that new unlicensed operations could significantly compromise deployed system performance, despite lacking entitlement to protection from external interference.The concern is especially relevant because UWB systems support already-deployed locating, tracking, and industrial applications.
  • US U-NII-6/8: Simulations report that indoor Wi-Fi mitigates interference to Broadcast Auxiliary Services, while nomadic incumbents can still experience negative performance effects.Reported complete link failure and bit-error fractions are as low as 0.2% and 1.1%, respectively.
  • European Coexistence: European coexistence analyses conclude that indoor unlicensed devices operating at 23–24 dBm can share spectrum with minimal impact on several incumbent services.The assessment covers fixed, satellite, ITS, CBTC, radio astronomy, and UWB systems.
  • European Coexistence: With 2025 deployment assumptions, satellite sharing is considered feasible when no more than 5% of unlicensed devices operate outdoors.Adjacent or partially overlapping systems may require tighter spectral masks and restrictions on lowermost channels.

VI. COEXISTENCE AMONG UNLICENSED TECHNOLOGIES

Coexistence between Wi-Fi and NR-U in the 6 GHz bands requires choices about channel access, fairness, and synchronization. The survey considers LBE and FBE LBT, noting both FBE’s potential gains and its vulnerability when mixed with LBE.

  • Coexistence design: 6 GHz coexistence mechanisms can be designed from the ground up rather than inheriting CSMA/CA-like LBT as the baseline.Existing Wi-Fi and LTE-LAA LBT remains the operational baseline, but the greenfield bands create room for alternatives.
  • LBE v/s FBE: LBE devices contend whenever the channel becomes idle, whereas FBE devices contend only at synchronized frame boundaries.FBE transmissions must end by the current frame boundary, while LBE transmission duration is limited by regional regulations.
  • LBE v/s FBE: FBE’s performance depends on synchronized access and transmission alignment with frame boundaries.Maximum MAC efficiency occurs when transmissions last until the end of each frame boundary, which may not hold for every payload size.
  • LBE v/s FBE: FBE devices receive a smaller channel share when coexisting with LBE devices because they must wait for frame boundaries.This disparity motivated 3GPP’s adoption of LBE for LTE-LAA, while NR-U includes provisions for FBE in environments without Wi-Fi.
  • LBE v/s FBE: 10-50% higher user-perceived throughput is reported for FBE LBT in some deployment scenarios.The proposed synchronized approach reduces hidden-node collisions and can support coordinated interference mitigation among devices from the same operator.

2) Contention Parameters:

Wi-Fi and NR-U contention parameters determine channel-access probability, but equal contention settings alone do not ensure fair coexistence. Transmission opportunities and signal-detection choices introduce additional fairness and interoperability issues.

  • Contention Parameters: Wi-Fi and NR-U use access-category-dependent contention parameters, including CWmin, CWmax, IFS, and defer time.Higher-priority packets use shorter waiting intervals and smaller contention windows.
  • Contention Parameters: Wi-Fi and NR-U use matching wait times and contention-window values for corresponding access categories.These settings are intended to equalize the probability of accessing the channel.
  • Transmission Duration: Equal channel-access probability does not guarantee fairness because NR-U can transmit uninterrupted longer than Wi-Fi.The longer NR-U TXOP gives it an advantage in average observed throughput.
  • Transmission Duration: A 6 msec TXOP duration has been reported as achieving fair coexistence between Wi-Fi and NR-U devices.Prior work also links unequal TXOP values to a smaller Wi-Fi channel share in Wi-Fi and LTE-LAA coexistence.
  • Detection mechanisms: Energy detection and preamble detection offer different trade-offs for cross-RAT channel sensing.ED is technology-neutral and simple, while PD can infer transmission duration and support power saving; hybrid approaches combine both.

2) Choice of the Detection Threshold:

The 6 GHz detection-threshold choice remains contested because existing 5 GHz mechanisms can favor one technology depending on network topology. Contributions largely support a common, technology-neutral threshold, while adaptive intra-RAT thresholds are also proposed.

  • Detection threshold: There is no consensus between energy detection and preamble detection for Wi-Fi–NR-U coexistence in the 6 GHz bands.Contributions argue for both mechanisms, reflecting different implementation and interoperability considerations.
  • Detection threshold: 5 GHz detection mechanisms and thresholds are known to be unfair toward one technology or the other depending on network topology.Some proposals nevertheless recommend carrying the existing 5 GHz sharing rules into 6 GHz operations.
  • Detection threshold: A vast majority of surveyed contributions favor a common, technology-neutral detection threshold for 6 GHz coexistence.This position contrasts with proposals to retain the 5 GHz status quo.
  • Detection threshold: Adaptive thresholds can combine a common inter-RAT threshold with lower intra-RAT thresholds to support spatial reuse.One proposal gives inter-RAT detection thresholds of -62/-72 dBm and an intra-RAT threshold of -82 dBm.
  • Detection threshold: MU-OFDMA changes the implications of detection thresholds because uplink stations can transmit without independently contending after an AP wins access.Multiple simultaneous transmitters can also reduce the probability that transmitters are hidden from sensing devices.

VII. ADJACENT CHANNEL INTERFERENCE ISSUES

Adjacent-channel interference in the 6 GHz bands depends on device emissions, regulatory spectral masks, and deployment conditions near incumbent systems. The survey highlights serious C-V2X degradation risks and the need to balance incumbent protection with spectrum utilization.

  • Adjacent-channel emissions: Out-of-band emissions depend on device type, cost, operating band, and regional regulations, with devices required to conform to spectral masks.Higher-end devices generally confine more power to the desired channel through superior RF filtering.
  • Incumbent protection: Outdoor Wi-Fi deployments near C-V2X devices can significantly reduce C-V2X performance, especially when the C-V2X link has low SINR.The reported loss is measured through a decline in Packet Delivery Ratio.
  • Incumbent protection: -27 dBm/MHz interference from unlicensed devices in lowermost U-NII-5 channels is reported as unacceptable at C-V2X receivers.The adjacent 5.9 GHz ITS band supports vehicular safety applications, making interference control important.
  • AFC efficiency: AFC-based exclusion zones protect incumbent receivers in U-NII-5 and U-NII-7 by restricting unlicensed frequencies near incumbent operations.Devices query the AFC database to determine whether their location lies within an incumbent receiver’s exclusion zone.
  • AFC efficiency: Dynamic Incumbent Protection Zones vary spatially with geolocation, time, and frequency to improve utilization while protecting incumbents.Further research is needed to determine whether DIPZ schemes can be adopted in the 6 GHz bands.

2) Interference characterization of VLP and LPI devices:

6 GHz unlicensed operation must balance incumbent protection with useful access, while next-generation Wi-Fi introduces coexistence challenges such as fairness and simultaneous transmit-receive constraints.

  • Incumbent protection: Nomadic incumbent receivers in U-NII-6/8 may be more vulnerable because they can approach unlicensed devices closely.The FCC R&O therefore restricts these bands to indoor-only low-power indoor devices, relying on lower transmit powers and building entry losses.
  • Incumbent protection: Effective interference characterization must combine incumbent link budgets, operating environments, receiver and transmitter locations, transmit powers, and propagation models.The relevant environments include indoor and outdoor stadiums, public parks, and concert halls.
  • Incumbent protection: Without incumbent detection or coordination, strict operational limits may protect incumbents but significantly reduce unlicensed utility.Proposed restrictions include outdoor power limits for very-low-power devices and indoor-only operation for low-power indoor devices.
  • Incumbent protection: Rogue unlicensed transmitters create an enforcement problem requiring device identification, localization, and punitive action.The paper notes that harmful interference from Wi-Fi has persisted despite mitigation schemes in the 5 GHz bands.
  • Wi-Fi coexistence: Synchronous multi-link aggregation can create unfairness because a secondary link may gain access without independently completing contention.Successful transmission on the secondary channel can also reset its contention window, affecting legacy Wi-Fi, 802.11be, and NR-U devices.
  • Wi-Fi coexistence: The simultaneous transmit-receive constraint occurs when an AP transmits on one channel while receiving on another and inter-link crosstalk prevents decoding.Because synchronous MLA is likely needed for channel aggregation in the 5 GHz and 6 GHz bands, IEEE 802.11be requires effective avoidance solutions.

3) FBE Mechanisms for NR-U:

NR-U FBE LBT is being considered for settings where Wi-Fi absence can be guaranteed, but one-shot sensing can cause persistent collisions and cross-RAT coexistence remains unresolved.

  • FBE mechanisms: NR-U specification work emphasizes FBE LBT for scenarios where the absence of Wi-Fi networks can be guaranteed.A factory managed by one operator is an example of such a setting.
  • FBE mechanisms: One-shot FBE sensing can cause persistent collisions because devices with packets simultaneously sense an idle channel and transmit together.ETSI recommendations use sensing without random back-off, corresponding to category 1/2 LBT.
  • FBE mechanisms: Proposed mitigations introduce randomness into sensing, effectively using category 3/4 LBT at each frame interval.These proposals are intended to reduce simultaneous transmissions at frame starts.
  • FBE mechanisms: Key NR-U-only research questions concern contention timing, LBT category, and interference coordination during the contention interval.Candidate mechanisms include directional sensing and transmissions.
  • FBE mechanisms: Cross-RAT FBE LBT requires a global clock, whose performance benefits for Wi-Fi networks require further investigation.NR-U networks may achieve synchronization more readily because their gNBs are likely integrated with cellular networks.
  • FBE mechanisms: FBE LBT adoption remains only at the proposal stage, and coexistence with LBE LBT remains an open problem.Existing coexistence mechanisms must be evaluated or new mechanisms proposed and tested before FBE-based 6 GHz access is considered.
  • Analysis and optimization: MLA and additional 6 GHz links motivate quantitative analysis of whether Wi-Fi can lower tail latency enough for QoS-sensitive applications.The paper proposes extending one- and multidimensional Markov models to analyze MLA under different traffic conditions.

3) Detection Mechanism and Threshold for NR-U and WiFi Coexistence:

NR-U–Wi-Fi coexistence depends strongly on detection thresholds and mechanisms, but suitable values and common signaling remain unsettled and require rigorous, realistic evaluation.

  • Detection thresholds: No consensus exists on the detection threshold that enables fair and efficient NR-U–Wi-Fi coexistence.The threshold directly affects both RATs’ coexistence performance and has implications for their operation.
  • Detection mechanisms: Preamble-based detection requires a common 6 GHz preamble that conveys packet duration and is accurately detectable by both technologies.Duration signaling can let devices sleep for the remainder of a packet, improving energy efficiency.
  • Evaluation: Threshold selection should be evaluated with analytical models, simulations, and large-scale experiments because small studies can mislead.The stated objective is to determine a threshold supporting fair and efficient coexistence.
  • Wi-Fi 6 effects: Wi-Fi 6 MU-OFDMA changes threshold implications because multiple uplink transmitters and mandatory transmit-power control alter sensing and hidden-node probabilities.The resulting NR-U–Wi-Fi coexistence behavior may differ from earlier LAA–Wi-Fi studies based on 802.11ac.
  • Wi-Fi 6 effects: The impact of MU-OFDMA on NR-U–Wi-Fi coexistence remains a critical research subject requiring rigorous modeling, simulation, and experiments.Reduced hidden-node probability is expected to improve both systems’ performance, but this intuition must be verified.
  • V2X coexistence: Adjacent-channel interference near ITS and V2X operations requires realistic evaluation across device deployments, propagation, emissions, link metrics, system metrics, and application metrics.The literature has not reached consensus on whether adjacent-channel interference significantly affects V2X applications.
  • Research direction: The survey identifies efficient, fair, and harmonious coexistence among unlicensed and incumbent users as a central 6 GHz research objective.It emphasizes that all unlicensed RATs will operate in these bands for the first time, allowing coexistence mechanisms to be redesigned.
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