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A Vision of 6G Wireless Systems: Applications, Trends, Technologies, and Open Research Problems

Walid Saad, Mehdi Bennis, Mingzhe Chen

arXiv:1902.10265v2cs.ITcs.NI

TL;DR

With 6G’s architecture and performance components still largely undefined, this paper develops a forward-looking vision spanning applications, technologies, services, and research needs. It concludes that 6G should be guided by a diverse portfolio rather than additional high-frequency spectrum alone.

  • Problem

    6G research faces open problems in high-frequency mobility and transceivers, reliable AI, and integration of RF and non-RF links.

  • Method

    The paper constructs a holistic 6G vision by identifying applications, trends, performance metrics, disruptive technologies, services, and a research roadmap.

  • Results

    The paper lays out a 6G vision that outlines trends, challenges, associated research, and recommendations for the next decade.

  • Takeaways & Limitations

    6G should be driven by a diverse portfolio of applications, technologies, and techniques rather than treated as exploration of additional high-frequency spectrum alone.

Abstract

from arXiv · show

The ongoing deployment of 5G cellular systems is continuously exposing the inherent limitations of this system, compared to its original premise as an enabler for Internet of Everything applications. These 5G drawbacks are currently spurring worldwide activities focused on defining the next-generation 6G wireless system that can truly integrate far-reaching applications ranging from autonomous systems to extended reality and haptics. Despite recent 6G initiatives1, the fundamental architectural and performance components of the system remain largely undefined. In this paper, we present a holistic, forward-looking vision that defines the tenets of a 6G system. We opine that 6G will not be a mere exploration of more spectrum at high-frequency bands, but it will rather be a convergence of upcoming technological trends driven by exciting, underlying services. In this regard, we first identify the primary drivers of 6G systems, in terms of applications and accompanying technological trends. Then, we propose a new set of service classes and expose their target 6G performance requirements. We then identify the enabling technologies for the introduced 6G services and outline a comprehensive research agenda that leverages those technologies. We conclude by providing concrete recommendations for the roadmap toward 6G. Ultimately, the intent of this article is to serve as a basis for stimulating more out-of-the-box research around 6G.

I. INTRODUCTION · II. 6G DRIVING APPLICATIONS, METRICS, AND NEW SERVICE CLASSES

The paper argues that 5G has not fully realized its IoE-enabling premise amid rapidly expanding applications, motivating a disruptive 6G system tailored to their performance requirements. It presents a forward-looking vision connecting applications, trends, metrics, technologies, new services, and a roadmap toward 6G.

  • I. INTRODUCTION: 5G’s revolutionary IoE promise remains unrealized despite progress supporting rate-hungry eMBB services.The paper distinguishes 5G’s evolutionary eMBB progress from its unrealized revolutionary outlook as an IoE carrier.
  • I. INTRODUCTION: Early 5G rollouts may rely on sub-6 GHz mobility support, while smart-city IoE applications remain debatable beyond basic IoE and URLLC services.5G can support basic IoE and URLLC services such as factory automation, but its ability to deliver tomorrow’s smart-city applications is questioned.
  • I. INTRODUCTION: Emerging IoE applications—including XR, telemedicine, haptics, flying vehicles, brain-computer interfaces, and connected autonomous systems—challenge 5G’s short-packet, sensing-based URLLC focus.The paper identifies an unprecedented proliferation of services that disrupts the original 5G goal.
  • I. INTRODUCTION: 6G is needed as a disruptive system inherently tailored to IoE performance requirements and accompanying technological trends.Its drivers combine established trends such as densification, higher rates, and massive antennas with emerging services and wireless devices including wearables, implants, and XR devices.
  • I. INTRODUCTION: The article’s central contribution is a forward-looking 6G vision identifying the applications, trends, performance metrics, and technologies driving the 6G revolution.The vision also delineates new 6G services and provides a research roadmap with recommendations for moving beyond current 5G systems.
  • II. 6G DRIVING APPLICATIONS, METRICS, AND NEW SERVICE CLASSES: 6G will emerge from new applications and technological trends that shape performance targets and redefine standard 5G services.The paper frames innovative applications as drivers of every cellular generation, including 6G.
  • II. 6G DRIVING APPLICATIONS, METRICS, AND NEW SERVICE CLASSES: The section proceeds from 6G-motivating applications to ensuing technological trends, target performance metrics, and new service requirements.This sequence establishes the structure for defining 6G deployment motivations and service classes.

A. Driving Applications behind 6G and Their Requirements … 4) Blockchain and Distributed Ledger Technologies (DLT):

The paper identifies four emerging application domains as key determinants of 6G performance beyond traditional services. It outlines requirements spanning immersive human-centered experiences, autonomous systems, brain-computer interfaces, and scalable distributed sensing.

  • A. Driving Applications behind 6G and Their Requirements: Four new application domains, beyond traditional services such as live multimedia streaming, will determine 6G system performance.The passage frames these domains as the primary application drivers behind 6G.
  • 1) Multisensory XR Applications:: XR applications across AR, MR, and VR require 6G to overcome 5G’s inability to provide fully immersive experiences with very low latency for data-rate-intensive applications.A full immersive XR experience requires joint wireless, computing, storage, and perceptual design.
  • 1) Multisensory XR Applications:: QoPE is proposed to combine human perceptual factors with classical QoS and QoE measures for multisensory XR services.Relevant factors include brain cognition, body physiology, and gestures; the brain may not distinguish latency measures within the URLLC regime.
  • 2) Connected Robotics and Autonomous Systems (CRAS):: CRAS applications include drone delivery, autonomous cars, drone swarms, vehicle platoons, and autonomous robotics, creating control-driven latency and possible HD-map transmission requirements.CRAS are not merely another short-packet uplink IoE service, and QoPE applies to them as well.
  • 3) Wireless Brain-Computer Interactions (BCI):: Wireless BCI and implants extend brain-computer interaction beyond healthcare applications such as controlling prosthetic limbs or nearby computing devices.These developments introduce new use-case scenarios requiring 6G connectivity.
  • 4) Blockchain and Distributed Ledger Technologies (DLT):: Blockchain and DLT are characterized as disruptive IoE technologies and next-generation distributed sensing services.Their connectivity needs require a synergistic mix of URLLC and mMTC to provide low latency, reliable connectivity, and scalability.

B. 6G: Driving Trends and Performance Metrics

The paper identifies seven driving trends for 6G, spanning extreme data rates and volumetric efficiency, smart environments, distributed data, autonomous networks, converged functions, and post-smartphone devices. Collectively, these trends impose new performance targets to be achieved through beyond-5G evolution and a revolutionary 6G step.

  • Driving Trends: 6G must deliver another 1000x increase in data rates, targeting around 1 Terabit/second, while exploring frequencies beyond sub-6 GHz and supporting pervasive high reliability.XR and BCI applications motivate the data-rate target and spectrum expansion.
  • Driving Trends: Because 6G serves ground and aerial users, spectral and energy efficiency must evolve from areal to volumetric measures in bps/Hz/m3/Joules.The envisioned system encompasses smartphones, XR/BCI devices, and flying vehicles.
  • Driving Trends: 6G communications will increasingly use smart large intelligent surfaces and environments, including electromagnetically active structures such as walls, roads, and entire buildings.Metamaterials are identified as an example technology enabling these environments.
  • Driving Trends: The data revolution will shift toward massive, distributed small data, requiring 6G to harness both big and small datasets and motivating machine learning beyond classical big data analytics.These datasets support network functions and new services across the infrastructure.
  • Driving Trends: 6G must progress from self-organizing networks to self-sustaining networks that intelligently manage network operations, resources, and optimization.CRAS and DLT technologies motivate the immediate need for intelligent SON.
  • Driving Trends: 6G will converge communications, computing, control, localization, and sensing into a multi-purpose system while extending beyond smartphones to wearables, integrated headsets, and smart body implants.XR and BCI applications fuel the shift toward devices receiving direct sensory inputs from human senses.
  • Performance Metrics: Collectively, these trends impose new performance targets and requirements that will be met through beyond 5G evolution and a revolutionary 6G step.The paper presents these as two implementation stages.

C. New 6G Service Classes … III. 6G: ENABLING TECHNOLOGIES

The paper proposes four new 6G service classes that extend or replace classical 5G categories, followed by disruptive technologies intended to enable and guarantee their performance.

  • C. New 6G Service Classes: New technological trends will reshape classical URLLC, eMBB, and mMTC while introducing additional 6G services.These service classes are summarized in Table II.
  • 1) Mobile Broadband Reliable Low Latency Communication:: MBRLLC unifies high data rates, reliability, and low latency by supporting arbitrary requirements across the rate-reliability-latency space.It targets applications such as XR, wireless BCI, and CRAS, for which eMBB and URLLC distinctions are insufficient.
  • 2) Massive URLLC:: mURLLC scales URLLC across devices by merging 5G URLLC with legacy mMTC.This creates a reliability-latency-scalability tradeoff requiring departure from average-based network designs.
  • 3) Human-Centric Services:: Human-centric services require QoPE targets tied to human users rather than raw rate-reliability-latency metrics.Wireless BCI exemplifies this class, requiring QoPE metrics defined as functions of QoS and QoE.
  • 4) Multi-Purpose 3CLS and Energy Services:: MPS jointly deliver 3CLS services and potentially wireless energy transfer for small devices.They require joint uplink-downlink designs targeting control, computing, energy-transfer, and localization performance.
  • 4) Multi-Purpose 3CLS and Energy Services:: 6G must jointly support 3CLS services and their derivatives, particularly for applications such as CRAS.The service class combines control, computing, communication, localization, and sensing capabilities.
  • III. 6G: ENABLING TECHNOLOGIES: A cohort of disruptive technologies must be integrated into 6G to enable the proposed services and guarantee their performance.This requirement introduces the paper’s enabling-technologies section.

1) Above 6 GHz for 6G – from Small Cells to Tiny Cells: … 4) Edge AI:

The paper envisions 6G as an integrated system combining higher-frequency tiny cells, multi-band transceivers, large intelligent surfaces, and distributed edge AI. These technologies target seamless connectivity, improved efficiency and rates, autonomous network control, and new services.

  • 1) Above 6 GHz for 6G – from Small Cells to Tiny Cells:: 6G will extend mobile connectivity beyond sub-6 GHz by developing mobile mmWave and eventually exploiting terahertz frequencies, requiring a shift from small cells to tiny cells.Higher frequencies are motivated by higher data rates and SEE anywhere, anytime.
  • 2) Transceivers with Integrated Frequency Bands:: Dense high-frequency tiny cells will be complemented by multi-mode base stations integrating microwave, mmWave, and THz bands for seamless wide-area and local connectivity.The integrated approach addresses the connectivity limitations of dense high-frequency tiny cells alone.
  • 3) Communication with Large Intelligent Surfaces:: 6G will move beyond traditional massive MIMO toward large intelligent surfaces and smart environments that provide massive wireless communication surfaces for heterogeneous devices.LISs are presented as an initial leap from traditional massive MIMO and enable holographic RF communication.
  • 4) Edge AI:: 1000x in bps/Hz/m3/Joules (volumetric) is the stated 6G spectral and energy efficiency gain with respect to today’s networks.The corresponding progression shown is 10x in bps/Hz/m2/Joules for 5G and 100x in bps/Hz/m2/Joules for beyond 5G.
  • 4) Edge AI:: 1 Tbps is the stated 6G rate requirement, while 6G end-to-end delay is specified as < 1 ms.The table lists 1 Gbps and 100 Gbps for 5G and beyond 5G, respectively, and 5 ms and 1 ms end-to-end delays.
  • 4) Edge AI:: Edge AI will push network intelligence and learning algorithms onto edge devices, enabling distributed autonomy and supporting integrated 6G services, 3CLS, and potentially new frame structures.The paper describes this development as a new edge AI leap and calls the broader model collective network intelligence.

5) Integrated Terrestrial, Airborne, and Satellite Networks:

6G will integrate terrestrial, airborne, and satellite networks into a single wireless system. Drones can extend connectivity to hotspots and infrastructure-scarce areas, while LEO satellites and CubeSats support connectivity, backhaul, and wide-area coverage.

  • Airborne network support: Drones can provide connectivity to hotspots and areas with scarce infrastructure, complementing terrestrial networks.Drone-carried base stations and tethered balloons can serve temporary hotspots.
  • Candidate technologies: Trials of tiny THz cells are identified among the methods associated with future integrated wireless networks.The passage lists tiny THz-cell trials alongside temporary airborne hotspots.
  • Integrated network architecture: Integrating terrestrial, airborne, and satellite networks into a single wireless system will be essential for 6G.The integration includes drones, terrestrial base stations, LEO satellites, and CubeSats.
  • Satellite connectivity: LEO satellites and CubeSats can connect drones and terrestrial base stations while providing backhaul support and additional wide-area coverage.Both drones and terrestrial base stations may require satellite connectivity with low-orbit satellites.

6) Energy Transfer and Harvesting:

6G is envisioned as a cellular system that provides energy alongside 3CLS, using wireless energy transfer and complementary energy-centric technologies.

  • 6) Energy Transfer and Harvesting:: 6G could provide energy alongside 3CLS as a core cellular capability.The paper frames energy provision as part of Trend 6.
  • 6) Energy Transfer and Harvesting:: As wireless energy transfer matures, 6G base stations could supply basic power to devices, especially implants and sensors.This capability is identified as Trend 7.
  • 6) Energy Transfer and Harvesting:: Energy harvesting and backscatter are proposed as complementary components of 6G.These are described as adjunct energycentric ideas.

7) Beyond 6G: … 2) Exploring Integrated, Heterogeneous High-Frequency Bands:

The paper outlines a forward-looking agenda spanning technologies beyond 6G, fundamental rate-reliability-latency and SEE analysis, and integrated heterogeneous high-frequency networks. It highlights quantum and non-RF communications, quantitative system-performance needs, and unresolved mmWave, THz, and multi-band coexistence challenges.

  • 7) Beyond 6G:: Technologies maturing alongside 6G may influence research and standardization toward the end of the 6G timeframe.The paper identifies these technologies as potential contributors beyond the core 6G development process.
  • 7) Beyond 6G:: Quantum computing and communications could enable security and long-distance networking, while RF links may integrate with optical, neural, and molecular channels.The paper expects major quantum-realm research efforts to intersect with 6G and identifies heterogeneous RF/non-RF integration as another beyond-6G direction.
  • IV. 6G: RESEARCH AGENDA AND OPEN PROBLEMS: The proposed 6G research agenda builds on trends identified in Section II and enabling technologies identified in Section III.The agenda is summarized in Table III.
  • 1) 3D Rate-Reliability-Latency Fundamentals:: 6G requires fundamental 3D analysis of rate-reliability-latency tradeoffs and SEE to quantify spectrum, energy, and communication requirements.This analysis is intended to support the applications identified as 6G drivers, with prior work providing an initial step.
  • 2) Exploring Integrated, Heterogeneous High-Frequency Bands:: mmWave and THz deployment creates open problems in mobility, transceiver design, propagation modeling, and severe THz path loss.For mmWave, high mobility is central; for THz, transceivers need high power, high sensitivity, and low noise figure.
  • 2) Exploring Integrated, Heterogeneous High-Frequency Bands:: New networking approaches are needed after resolving physical-layer issues for mmWave and THz systems.The passage identifies networking as a further direction following work on high-frequency physical-layer aspects.
  • 2) Exploring Integrated, Heterogeneous High-Frequency Bands:: Research should study coexistence of THz, mmWave, and microwave cells across all network layers.This direction builds on early work such as.

3) 3D Networking: … 6) QoPE Metrics:

The paper identifies open research problems for 6G spanning 3D networking, smart LIS communication, reliable AI-network co-design, and QoPE metrics that incorporate human and control-system factors. These directions support emerging applications, including wireless BCI systems leveraging multiple human cognitive senses.

  • 3) 3D Networking:: 6G must support communications and user service in 3D space, including 3D base stations such as tethered balloons and temporary drones.Research priorities include measuring and modeling 3D propagation environments and developing new 3D frequency and network-planning approaches.
  • 4) Communication with LIS:: Smart LIS environments will combine active frequency-selective surfaces, metallic passive reflectors, passive or active reflect arrays, and reconfigurable metasurfaces.Open problems include optimized deployment and AI-powered operation, alongside analysis of rate, latency, reliability, and coverage.
  • 5) AI for Wireless:: Jointly designing ML algorithms and wireless networks is a key 6G research area, including massive small-data analytics and AI-based SSNs.The SSNs are realized using reinforcement learning and game theory.
  • 5) AI for Wireless:: 6G research must develop low-latency, high-reliability, scalable AI and reliable infrastructure for critical application tasks.The paper identifies reliable operation of ML algorithms over 6G as necessary for delivering the applications described in Section II.
  • 6) QoPE Metrics:: QoPE metrics should integrate physical factors from human physiology for HCS services and from control systems for CRAS services.This research is especially important for emerging devices associated with Trend 7.
  • 6) QoPE Metrics:: Developing QoPE requires real-world psychophysics experiments and rigorous mathematical expressions combining QoS, QoE, and human perceptions.Theoretical development can draw on operations research, including multi-attribute utility theory.
  • 6) QoPE Metrics:: 6G will enable a new breed of wireless BCI applications that leverage multiple human cognitive senses.This application direction motivates QoPE metrics that account for human physiological factors and perceptions.

7) Joint Communication and Control: … 10) RF and non-RF Link Integration:

The paper identifies four open directions for 6G: co-designing communication with control and broader 3CLS functions, developing adaptive AI-driven protocols, and integrating RF with non-RF links. These directions respond to limitations of radio-centric 5G design and incomplete end-to-end exploration.

  • 7) Joint Communication and Control:: 6G must co-design wireless communication and control to support CRAS and preserve control-system stability.Wireless-link performance should be optimized for control-system stability, and control requirements should reciprocally shape link design.
  • 7) Joint Communication and Control:: This communication-control co-design was overlooked in 5G because 3GPP and IEEE fora traditionally focused on radio.
  • 8) 3CLS:: The joint-design scope should expand to all 3CLS functions: computing, communication, control, localization, sensing, energy, and mapping.The paper calls for end-to-end exploration of their interdependence and joint performance requirements.
  • 8) 3CLS:: 3CLS research includes multi-modal sensor fusion for 3D-image reconstruction and navigation in unknown environments for robots and autonomous driving.
  • 9) 6G Protocol Designs:: 6G requires radical protocol redesigns, including AI-driven signaling, scheduling, and coordination that replace rigid 5G protocols based on predetermined network parameters and frame structures.
  • 9) 6G Protocol Designs:: Unlike conventional 5G protocols, 6G protocols should continuously adapt to the current and projected state of the wireless environment.
  • 10) RF and non-RF Link Integration:: 6G will converge RF and non-RF links, including optical, visible light communication, molecular, and neuro-communication links.The joint design of these RF/non-RF systems remains an open research area.

11) Holographic Radio: · V. CONCLUSION AND RECOMMENDATIONS

The paper presents holographic radio as a 6G capability enabled by LIS and related structures, while recommending a broader 6G agenda spanning new services, architectures, applications, and analytical approaches.

  • 11) Holographic Radio:: RF holography, including holographic MIMO and spatial spectral holography, enables control of the electromagnetic field across physical space.LIS and similar structures make these capabilities possible through spatial wave field synthesis and closed-loop field control.
  • 11) Holographic Radio:: Holographic radio can improve spectrum efficiency and network capacity while integrating imaging with wireless communication.Its realization remains a widely open research area.
  • V. CONCLUSION AND RECOMMENDATIONS: The paper identifies LIS communication, 3CLS, holographic radio, and related topics as research avenues for the next decade.These avenues extend beyond topics expected to arise through natural 5G evolution.
  • V. CONCLUSION AND RECOMMENDATIONS: Recommendation 1 calls for enabling MBRLLC and mobility management at high-frequency mmWave bands and beyond, including THz.This is presented as a first step toward 6G.
  • V. CONCLUSION AND RECOMMENDATIONS: Recommendation 2 calls for replacing radio-centric system design with end-to-end 3CLS co-design orchestrated by an AI-driven intelligence substrate.The recommendation describes a shift away from the 3GPP-style radio-centric approach.
  • V. CONCLUSION AND RECOMMENDATIONS: Recommendation 3 frames 6G as driven by a diverse portfolio of applications, technologies, and techniques rather than additional high-frequency spectrum alone.The paper explicitly rejects treating 6G as a simple capacity-focused spectrum expansion.
  • V. CONCLUSION AND RECOMMENDATIONS: Recommendations 4 and 5 envision smart surfaces communicating with human-embedded implants and require 3D, fine-grained performance analysis beyond simple averaging.The analysis should address tails, distributions, and QoPE.
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