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The Road Towards 6G: A Comprehensive Survey

Wei Jiang, Bin Han, Mohammad Asif Habibi, Hans Dieter Schotten

arXiv:2102.01420v1eess.SP

TL;DR

Explosive mobile-traffic growth, reaching 5016 EB per month by 2030, motivates examining communications beyond 5G. The survey synthesizes 6G drivers, requirements, efforts, architectures, and enabling technologies, concluding that 6G may integrate optical, intelligent, and space-aerial-terrestrial capabilities.

  • Problem

    Mobile traffic is projected to grow explosively, reaching 5016 EB per month by 2030, challenging 5G's ability to accommodate future demand.

  • Method

    The article provides a comprehensive survey of 6G drivers, requirements, research efforts, architectures, and enabling technologies.

  • Results

    The survey concludes that 6G may combine electronic and photonic technologies, AI-driven networking, and integrated space-aerial-terrestrial coverage.

  • Takeaways & Limitations

    6G is envisioned as a connected intelligent platform providing higher-capacity communications and ubiquitous three-dimensional coverage beyond terrestrial networks.

Abstract

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As of today, the fifth generation (5G) mobile communication system has been rolled out in many countries and the number of 5G subscribers already reaches a very large scale. It is time for academia and industry to shift their attention towards the next generation. At this crossroad, an overview of the current state of the art and a vision of future communications are definitely of interest. This article thus aims to provide a comprehensive survey to draw a picture of the sixth generation (6G) system in terms of drivers, use cases, usage scenarios, requirements, key performance indicators (KPIs), architecture, and enabling technologies. First, we attempt to answer the question of "Is there any need for 6G?" by shedding light on its key driving factors, in which we predict the explosive growth of mobile traffic until 2030, and envision potential use cases and usage scenarios. Second, the technical requirements of 6G are discussed and compared with those of 5G with respect to a set of KPIs in a quantitative manner. Third, the state-of-the-art 6G research efforts and activities from representative institutions and countries are summarized, and a tentative roadmap of definition, specification, standardization, and regulation is projected. Then, we identify a dozen of potential technologies and introduce their principles, advantages, challenges, and open research issues. Finally, the conclusions are drawn to paint a picture of "What 6G may look like?". This survey is intended to serve as an enlightening guideline to spur interests and further investigations for subsequent research and development of 6G communications systems.

I. INTRODUCTION

The paper situates 6G as the next stage of mobile communications and surveys prior work toward a comprehensive vision. It highlights the field’s expanding scope, ongoing research activity, and the need to integrate fragmented perspectives.

  • Generational evolution: Mobile communications evolved from 1G voice services toward successive generations supporting higher-rate data and broader connectivity.The paper describes 1G analog voice, 2G digital services, 3G megabit-per-second access, and the later development of LTE.
  • 5G context: 5G extends communication services from humans to machines and things, supporting applications including Industry 4.0, virtual reality, IoT, and automatic driving.This expansion substantially enlarges the potential subscription scale beyond human users.
  • Motivation for 6G: Although debate continues over whether 6G is necessary, academia, industry, and standardization bodies have initiated work on systems beyond 5G.The paper notes emerging next-generation research and activity within the ITU-T Technologies for Network 2030 focus group.
  • Prior research: Earlier 6G studies largely examined individual topics such as machine learning, quantum computing, terahertz communications, AI, vehicular networks, and explainable AI.The cited literature also includes surveys addressing applications, requirements, challenges, and research directions.
  • Prior research: Table I summarizes state-of-the-art contributions related to 6G communication systems.The table is presented as a summary of related works and their major contributions.

B. Contributions

The article’s contributions combine a broad 6G state-of-the-art survey with proposed drivers, scenarios, requirements, architecture, and an organized technology framework.

  • Contributions: The article provides a thorough state-of-the-art analysis and a broad survey of 6G drivers, requirements, efforts, and enabling technologies.It addresses the necessity of 6G, technical KPIs, worldwide activities, and a roadmap toward 2030.
  • Contributions: It proposes three novel 6G use cases: Global Ubiquitous Connectability, Enhanced On-Board Communications, and Pervasive Intelligence.These use cases are presented as additions beyond the consensus of previous works.
  • Contributions: It proposes three usage scenarios: ubiquitous mobile broadband, ultra-reliable low-latency broadband communication, and massive ultra-reliable low-latency communication.The paper labels them uMBB, ULBC, and mULC, respectively.
  • Contributions: The article also envisions integrated three-dimensional coverage using non-terrestrial and terrestrial networks and projects a roadmap for 6G development.The roadmap spans definition, specification, standardization, and regulation.
  • Contributions: It categorizes enabling technologies into New Spectrum, New Networking, New Air Interface, New Architecture, and New Paradigm.The framework organizes technologies rather than simply listing candidates and supports discussion of their principles, challenges, and open issues.

C. Organization of the Article

The article is organized from 6G drivers through requirements, industry efforts and roadmap development, enabling technologies, and conclusions.

  • Organization: Section II analyzes the driving forces for 6G, including mobile traffic and subscription growth, disruptive use cases, and advanced usage scenarios.It establishes the motivation for developing 6G.
  • Organization: Section III examines 6G technical requirements using multiple key performance indicators.The section focuses on KPI-based requirements analysis.
  • Organization: Section IV summarizes industry ambitions and efforts and estimates a development roadmap.The roadmap concerns the evolution of 6G development activities.
  • Organization: Section V presents a complete view of key 6G technologies, followed by conclusions in Section VI.The paper ends by drawing conclusions about what 6G may look like.

II. DRIVERS

The paper identifies growing deployment and demand as reasons to explore a successor to 5G. It frames 6G as a response to future services, traffic, subscriptions, and efficiency requirements.

  • Current 5G context: More than 100 million 5G subscribers were served by over 500,000 deployed base stations in China at the end of 2020.The example illustrates the already large scale of 5G deployment.
  • Need for 6G: The paper argues that academia and industry should explore 6G while concerns remain about whether 5G is already sufficient.This question motivates clarification of the key driving forces for 6G.
  • Drivers: Next-generation development is driven by exponential growth in mobile traffic and subscriptions, disruptive services, and the need for greater cost, energy, spectrum, and operational efficiency.The paper also connects these drivers with advances in AI, terahertz, and large-scale satellite technologies.

A. Explosively Growing Mobile Traffic

Mobile subscriptions and per-user traffic are projected to grow sharply through 2030, driven by expanding device adoption and richer video and immersive applications.

  • The authors argue that 5G may struggle to accommodate the tremendous mobile-traffic volume anticipated in 2030 and beyond.
  • 17.1 billion MBB subscribers are expected worldwide by 2030 as smartphone, tablet, wearable, and VR-device adoption continues.
  • Mobile video already accounts for two thirds of current mobile traffic and is expected to become more dominant.
  • Average monthly mobile-user consumption is projected to rise from around 5 GB in 2020 to over 250 GB in 2030.

B. Potential Use Cases

The survey identifies 6G use cases spanning immersive communication, ultra-responsive interaction, intelligent services, transport, onboard connectivity, and global coverage.

  • Holographic-type communication could require bandwidth on the order of terabits per second even with compression.
  • Extended-reality devices may require over 1.6 Gbps per device, challenging 5G capacity, particularly at the cell edge.
  • The Tactile Internet targets reaction times of 1 ms or less together with high reliability, availability, security, and sometimes throughput.
  • Digital twins provide detailed virtual copies of physical objects and are expected to reach full deployment with 6G networks.
  • Autonomous vehicles, drones, onboard users, and remote or maritime populations motivate stringent connectivity and coverage requirements.
  • The proposed 6G scenarios are intended to support these disruptive applications through enhanced usage scenarios beyond typical 5G categories.

C. Usage Scenarios

The paper argues that 5G’s three usage scenarios do not cover several 6G use cases and proposes a holistic extension with three additional scenarios.

  • 5G usage scenarios: 5G defines eMBB, URLLC, and mMTC for high-data-rate access, mission-critical connectivity, and dense low-power device connectivity.
  • Need for 6G scenarios: The authors state that these 5G scenarios cannot satisfy the technical requirements of the identified 6G use cases.
  • Use-case requirements: Interactive VR requires both ultra-high bandwidth and low latency, while autonomous vehicles and drones additionally require ubiquitous connectivity and high reliability.
  • 6G scenario design: The proposed methodology extends current usage scenarios and forms a complete set by covering their overlapping areas.
  • 6G scenario design: Ubiquitous MBB, or uMBB, is proposed to provide MBB service across the whole Earth’s surface for onboard communications and global connectability.

III. REQUIREMENTS

The paper frames 6G requirements through existing and new KPIs, combining higher capacity, lower latency, broader coverage, denser connectivity, efficiency, and additional service-quality dimensions.

  • 6G is expected to extend 5G requirements while introducing additional KPIs for emerging technological features.
  • Peak data rate is projected to reach up to 1 Tbps, compared with 5G peak rates of 20 Gbps downlink and 10 Gbps uplink.
  • 6G user-plane latency is envisioned to fall from 5G’s 4 ms for eMBB and 1 ms for URLLC to 100 µs or even 10 µs.
  • Connection density is envisioned to increase from 10^6 devices/km^2 in 5G to 10^7 devices/km^2 in 6G.
  • 6G energy efficiency is projected to be 10–100 times better than 5G while reducing overall mobile-industry power consumption.
  • 6G signal bandwidth may reach 1 GHz or higher in THz communications or optical wireless communications.
  • New or extended KPIs include positioning accuracy, ubiquitous coverage, timeliness, security and privacy, and capital and operational expenditure.

IV. ROADMAP AND EFFORTS

6G research is progressing through coordinated institutional, national, and industry efforts alongside a projected roadmap spanning research, definition, specification, regulation, development, and deployment. The survey compares 5G and 6G requirements and summarizes representative initiatives across regions.

  • ITU-T established the Technologies for Network 2030 focus group in July 2018 to study forward-looking network scenarios relevant to 6G.
  • 6G is envisioned to improve eight representative KPIs relative to 5G, including latency decreasing from 1 ms to 0.1 ms and peak rate reaching 1 Tbps, 50 times over 5G.
  • The survey projects a 6G roadmap covering research, definition, specification, spectrum regulation, development, and deployment.
  • Rel. 15 provided the first phase of 5G standards, focusing mainly on eMBB while supporting URLLC through low-latency capabilities.
  • European beyond-5G efforts included eight projects selected through the ICT-20-2019 call, followed by ICT-52-2020 projects explicitly pursuing early 6G research.
  • Finland, North America, China, and South Korea announced or implemented major 6G initiatives involving research, industry coordination, national planning, experimentation, and trials.

V. TECHNOLOGICAL ENABLERS

The survey organizes potential 6G enablers across transmission, networking, architecture, computing, and security. It emphasizes technologies whose principles, advantages, challenges, and open research issues must be considered together.

  • Potential 6G enablers include network softwarization, virtualization, RAN slicing, O-RAN, and post-quantum security.
  • New air-interface technologies include massive MIMO, intelligent reflecting surfaces, coordinated multipoint, cell-free massive MIMO, and new modulation techniques.
  • A new 6G architecture is proposed to provide three-dimensional coverage by integrating large-scale satellite constellations, HAPs, and UAVs with terrestrial networks.
  • 6G also converges communication, computing, and storage resources while integrating AI, blockchain, digital twins, and mobile networks.
  • For each identified technology, the survey introduces its principles, advantages, challenges, and open research issues.

A. New Spectrum

The survey presents new-spectrum options for 6G, spanning higher-frequency radio, optical communications, and improved spectrum utilization. These technologies offer expanded bandwidth but introduce propagation, hardware, interference, mobility, and security challenges.

  • Terahertz and optical spectrum: THz and optical-frequency technologies are expected to provide extremely high bandwidth for 6G radio access networks.THz communications can offer significantly higher bandwidth than legacy technologies, while optical bands provide almost unlimited bandwidth without worldwide regulatory permission.
  • Terahertz and optical spectrum: THz links rely on directive antennas and line-of-sight channels, limiting coverage despite their potential for simultaneous gains in throughput, latency, and reliability.A single THz base station is expected to embed over 10 000 antennas and generate hundreds of super-narrow beams.
  • Terahertz and optical spectrum: THz deployment faces major hardware challenges involving antennas, amplifiers, and modulators, particularly efficient baseband modulation onto high-frequency carriers.Frequency-mixing solid-state systems and spatial direct modulation are discussed as approaches to this problem.
  • Visible light communications: VLC uses LEDs with image sensors or photodiode arrays to provide high bandwidth at low power without electromagnetic or radio interference.The stated power range is 100 mW for 10 Mbps to 100 Mbps.
  • Visible light communications: VLC has weak indoor MIMO gains and requires beam tracking because line-of-sight channels and narrow beams make performance sensitive to user position and mobility.Non-imaging receivers are highly alignment-sensitive, whereas imaging receivers are costly in cost-critical applications.
  • Dynamic spectrum management: Dynamic spectrum management seeks to improve radio-resource utilization by exploiting unused spectrum, extending listen-before-talk principles beyond unlicensed-band access.The survey identifies licensed-spectrum under-utilization as motivation for this approach.

B. New Networking

The survey describes new networking approaches for flexible, virtualized, sliced, open, and secure 6G infrastructures. It emphasizes dynamic RAN resource management, multi-use-case vertical support, and preparation for quantum-era security threats.

  • Softwarization and virtualization: 6G architecture is expected to use softwarization, virtualization, network slicing, and resource isolation to support flexible, intelligent, multi-vendor, and multi-tenant networks.These mechanisms are discussed for both core and RAN domains, alongside privacy and security concerns.
  • Softwarization and virtualization: NFV and SDN provide flexibility and service modularity, but NFV still faces challenges from the increased amount of virtual network functions and stringent heterogeneous-service QoS requirements.SDN is positioned as an important enabler for beyond-5G and 6G management, orchestration, and architecture.
  • RAN slicing: RAN slicing enables operators to divide infrastructure according to end-user and vertical-industry requirements, with CU and DU commonly implemented as VNFs and RU as a PNF.VNFs run on points of presence, while PNFs use dedicated cellular-site hardware.
  • RAN slicing: Static gNB function splitting is inefficient for large numbers of eMBB, URLLC, and mMTC slices, motivating customized and dynamic distribution of radio-processing functions.Each RAN slice subnet can serve a single use-case type, while vertical industries may contain multiple use cases.
  • RAN slicing: RAN slice subnets for multi-use-case vertical industries require an extensive architectural framework and effective management and orchestration of CU, DU, and RU resources.The stated vertical-industry examples include automobile, manufacturing, and power-grid sectors.
  • Open-RAN: O-RAN opens the RAN to standardized multi-vendor operation and ML- and AI-powered hierarchical control, while interoperability, orchestration, validation, and troubleshooting remain open problems.Its design also uses commercial off-the-shelf hardware and virtualization technologies.
  • Post-quantum security: 6G security must account for long-term quantum-computing threats because networks deployed around 2030 are expected to remain in service for several decades.The survey therefore discusses quantum-resistant cryptographic algorithms and technologies, alongside quantum communication as a possible security-enhancing technology.

C. New Air Interface

The survey presents new air-interface directions for 6G, motivated by the limits of conventional MIMO-OFDM at higher frequencies and increasing adaptation demands. Candidate approaches include massive MIMO, intelligent reflecting surfaces, CoMP/cell-free networking, and NOMA, each offering gains alongside deployment or complexity challenges.

  • Higher carrier frequencies create greater propagation loss and lower NLOS path diversity, limiting the remaining potential of conventional OFDM and small-scale active MIMO arrays.
  • Massive MIMO: Massive MIMO is expected to increase system capacity, statistical multiplexing gain, and spectral efficiency while reducing CAPEX/OPEX and energy consumption.
  • Intelligent Reflecting Surfaces: Intelligent reflecting surfaces use programmable material sheets to adaptively modify radio reflections, turning walls, glass, and ceilings into smart radio environments.
  • Intelligent Reflecting Surfaces: IRS passive reflection works across RF and optical frequencies, making it potentially cost beneficial for 6G systems operating across an ultra-broad spectrum.
  • Coordinated Multi-Point and Cell-Free: Cell-free massive MIMO connects distributed single-antenna access points to a central processor for coherent joint service, outperforming traditional cellular massive MIMO while reducing fronthaul signaling.
  • New Modulation: NOMA allows multiple users to reuse the same physical resource block and can provide higher bandwidth efficiency than OMA for massive-link scenarios, but successive-interference-cancellation receivers become increasingly complex.

D. New Architecture

The proposed 6G architecture extends terrestrial connectivity with satellites, high-altitude platforms, and unmanned aerial vehicles to improve coverage, resilience, and flexibility. These non-terrestrial elements also support specialized applications, including disaster relief, remote sensing, wireless charging, and multimedia services.

  • Large-Scale Satellite Constellation: Terrestrial networks cover only a small portion of Earth and struggle with oceans, deserts, extreme topographies, sparse populations, disasters, and expanding IoT connectivity demands.
  • Large-Scale Satellite Constellation: LEO satellites generally operate below 1000 km, reducing propagation latency relative to GEO satellites at 36000 km and lowering propagation loss for direct mobile connectivity.
  • High Altitude Platform: HAPs operate at 17 km to 22 km altitude and can provide larger-area, cost-efficient telecommunications coverage than terrestrial base stations.
  • High Altitude Platform: HAPs can be redeployed or moved on demand, supporting coverage improvement, remote sensing, navigation, temporary events, emergency communications, and disaster relief.
  • The survey identifies terrestrial, satellite, and HAP synergy as a route toward ubiquitous, robust, and resilient connectivity, including HAP backhaul or satellite-signal enhancement.
  • Unmanned Aerial Vehicle: UAVs can act as flying base stations or relays, dynamically supplement fixed terrestrial gNBs and satellites, while wireless power transmission enables joint battery charging and information delivery.

E. New Paradigm

The survey presents AI, blockchain, digital twins, and intelligent edge computing as major 6G paradigms, while identifying open challenges in scaling, automation, privacy, and security.

  • Artificial Intelligence: AI is positioned as a leading 6G technology for network intelligence and mobile AI services.The survey distinguishes AI for Networking from Networking for AI, including deep-edge support for computation-intensive applications on constrained devices.
  • Artificial Intelligence: Federated learning preserves data locality by processing raw data on devices and sharing masked updates for collaborative model training.The masking is designed to prevent individual data processing from exposing information while enabling universal-model adjustment.
  • Blockchain: Blockchain offers immutability, decentralization, transparency, and security-related benefits for resource sharing, virtualization, and privacy services.Potential applications span edge computing, NFV, network slicing, device-to-device communications, and several smart-domain use cases.
  • Digital Twin: Digital twins create one-to-one virtual counterparts that mirror physical objects across their life cycles for monitoring, control, maintenance, prediction, and optimization.Manufacturing and aviation are developing and commercializing digital twins, while healthcare and medicine remain at an initial stage.
  • Digital Twin: Digital-twin deployment in 6G still faces scaling, zero-touch management, AI-based virtualization, security, and privacy challenges.The survey specifically highlights scaling platforms to millions or billions of IoT devices and adapting solutions to diverse industrial scenarios.
  • Intelligent Edge Computing: Edge intelligence integrates AI and machine learning at the network edge to automate tasks and improve service, resource, cost, and complexity outcomes.One proposed use case extends intelligence into NG-RAN to automate network-slice and network-function management and orchestration.
  • Intelligent Edge Computing: Communication performance is constrained by sensing and control-system design because age of information over control and feedback channels depends on command and feedback arrival rates.Those arrival rates correspond to sensor sampling and controller decision rates.

VI. CONCLUSIONS

The survey concludes that 6G is expected to extend beyond 5G through new applications, integrated network paradigms, and broader connectivity. It envisions a radio-optical, AI-enabled, space-aerial-terrestrial, and sustainable system for 2030 and beyond.

  • Conclusions: The survey expects 6G to improve existing 5G applications while enabling holographic communications, pervasive intelligence, global connectivity, and the Internet of Everything.It projects initial 6G deployment in 2030 or earlier based on ongoing academic and industrial development efforts.
  • Conclusions: 6G is envisioned as a radio-optical system using electronic and photonic technologies to exploit terahertz and visible-light spectrum.The vision particularly emphasizes indoor optical wireless coverage for higher capacity and peak data rates.
  • Conclusions: 6G will integrate mobile networking with AI services and AI-driven transmission, optimization, control, and resource management.The survey frames this as two-way synergy: networks provide over-the-air AI applications, while AI improves network operation.
  • Conclusions: Large-scale LEO satellite constellations are expected to extend 6G beyond terrestrial networks into ubiquitous three-dimensional planetary coverage.The envisioned architecture integrates space, aerial, and terrestrial network segments.
  • Conclusions: 6G is envisioned as a smart compute-connect entity that converges communication, computing, storage, big data, sensing, localization, and control.It is also expected to interwork with AI, blockchain, digital twins, quantum computing, and quantum communications.
  • Conclusions: The target 6G system is intelligent, green, sustainable, and secure to support an informationized and intelligent society in 2030 and beyond.This closing vision presents 6G as a broad societal infrastructure rather than only a higher-capacity radio network.
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