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On the Road to 6G: Visions, Requirements, Key Technologies and Testbeds

Cheng-Xiang Wang, Xiaohu You, Xiqi Gao, Xiuming Zhu, Zixin Li, Chuan Zhang, Haiming Wang, Yongming Huang, Yunfei Chen, Harald Haas, John S. Thompson, Erik G. Larsson, Marco Di Renzo, Wen Tong, Peiying Zhu, Xuemin, Shen, H. Vincent Poor, Lajos Hanzo

arXiv:2302.14536v1eess.SP

TL;DR

5G limitations and unresolved requirements motivate ongoing research into 6G. The paper synthesizes the 6G vision, requirements, architecture, technologies, testbeds, and open challenges, critically appraising the field and identifying lessons learned. It reports representative targets including 1 Tbps peak data rate, 10 Gbps user-experienced data rate, and 0.1 ms latency.

  • Problem

    5G cannot fully support emerging needs for global coverage, ultra-high rates, ultra-low latency, dense connectivity, precise positioning, reliability, low power, efficiency, and ubiquitous intelligence.

  • Method

    The paper comprehensively surveys and critically appraises 6G visions, KPIs, scenarios, architecture, technologies, testbeds, verification platforms, and open challenges.

  • Results

    6G targets include 1 Tbps peak data rate, 10 Gbps user experienced data rate, 0.1 ms latency, 10 Gbps/m2 area traffic capacity, and 10^8 devices/km2 connection density.

  • Takeaways & Limitations

    The survey frames 6G as a global, multi-spectrum, application-rich network whose development requires integrated architectures, enabling technologies, testbeds, and further research.

  • Takeaways & Limitations

    The survey addresses a field still in an early exploratory stage, with numerous theoretical and engineering issues remaining unresolved.

Abstract

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Fifth generation (5G) mobile communication systems have entered the stage of commercial development, providing users with new services and improved user experiences as well as offering a host of novel opportunities to various industries. However, 5G still faces many challenges. To address these challenges, international industrial, academic, and standards organizations have commenced research on sixth generation (6G) wireless communication systems. A series of white papers and survey papers have been published, which aim to define 6G in terms of requirements, application scenarios, key technologies, etc. Although ITU-R has been working on the 6G vision and it is expected to reach a consensus on what 6G will be by mid-2023, the related global discussions are still wide open and the existing literature has identified numerous open issues. This paper first provides a comprehensive portrayal of the 6G vision, technical requirements, and application scenarios, covering the current common understanding of 6G. Then, a critical appraisal of the 6G network architecture and key technologies is presented. Furthermore, existing testbeds and advanced 6G verification platforms are detailed for the first time. In addition, future research directions and open challenges are identified for stimulating the on-going global debate. Finally, lessons learned to date concerning 6G networks are discussed.

I. INTRODUCTION

6G research responds to limitations that remain in 5G and builds on worldwide initiatives defining prospective visions, requirements, scenarios, and technologies. This survey critically synthesizes those developments, proposes an architecture, reviews testbeds, and identifies open challenges.

  • 5G is commercially deployed but remains limited in coverage, data rate, latency, connection density, positioning, reliability, energy efficiency, and intelligence.
  • Global 6G initiatives and white papers have proposed visions, requirements, application scenarios, KPIs, and enabling technologies.
  • The survey critically appraises the global 6G vision, KPIs, application scenarios, network architecture, and key technologies, while proposing a promising architecture.
  • It reviews existing 6G testbeds and verification platforms, discusses open research directions and challenges, and concludes with lessons learned from the literature.
  • The emerging 6G vision emphasizes global coverage, broad spectrum use, diverse applications, enhanced sensing, digital integration, intelligence, and strong security.

A. Global Coverage

6G seeks ubiquitous global coverage by extending terrestrial communications into integrated space-air-ground-sea networks. This expansion spans additional spectrum, applications, sensing, digital twins, intelligence, and security considerations.

  • 6G global coverage integrates satellite, UAV, terrestrial, maritime, underwater, and underground communications to provide seamless three-dimensional connectivity.
  • 6G aims to use sub-6 GHz, centimeter-wave, millimeter-wave, terahertz, and optical bands through heterogeneous RF-optical systems and networks.
  • AI, big data, digital twins, and immersive applications are associated with expanded 6G capabilities and tighter integration between physical and digital worlds.
  • The broader 6G vision also includes full-sensory services and endogenous security, while new applications introduce threats such as large-scale data breaches and learning-empowered attacks.

G. Summary

The paper presents 6G as a globally covered network supporting intelligent, full-sensory, and digital-twin applications, while proposing broader KPIs than 5G. Its proposed requirements address data rate, latency, coverage, capacity, and service efficiency.

  • 6G is envisioned to provide global coverage and intelligent, full-sensory, and digital-twin applications with endogenous security.
  • The paper’s 6G vision is comprehensive and adds strong security, full-sensory applications, and digital twins as research interests.
  • 6G KPI evaluation must update 5G indicators and add measures for positioning, sensing, security, and intelligence.
  • The paper proposes 17 6G KPIs, including indicators not previously defined or lacking reference values in existing surveys.
  • 1 Tbps peak data rate, 10 Gbps user experienced data rate, and 0.1 ms latency are proposed 6G targets.
  • 6G targets 10 Gbps/m2 area traffic capacity, 10^8 devices/km2 connection density, and more than 99% coverage.
  • Spectrum efficiency may reach 90 bps/Hz, while network energy efficiency is expected to increase about 100 times over 5G.
  • 6G cost efficiency is projected at 500 Gb/$, compared with an estimated 5G value of 10 Gb/$.

4) Diversified service evaluation:

The paper extends 5G service evaluation toward diversified 6G scenarios and applications, including enhanced mobility, long-lived devices, reliable communication, and wireless data centers. It also considers tactile, digital-twin, and brain-machine-interface applications.

  • 4) Diversified service evaluation:: 6G mobility requirements may exceed 1000 km/h, while smart-scenario sensing devices may require battery life of up to 20 years.
  • 4) Diversified service evaluation:: 6G reliability is application specific; stringent uRLLC scenarios permit only one erroneous bit in 10 million transmitted bits.
  • 4) Diversified service evaluation:: Industry and operators envision diversified 6G applications beyond the three principal 5G scenarios.
  • 4) Diversified service evaluation:: Existing 6G surveys omit some application categories, motivating a comprehensive classification of potential scenarios.
  • 4) Diversified service evaluation:: 6G extends eMBB, mMTC, and uRLLC into further-eMBB, ultra-mMTC, and enhanced-uRLLC while adding scenarios such as meMBB, MBRLLC, and muRLLC.
  • 4) Diversified service evaluation:: THz-enabled wireless data centers are envisioned to use ultra-large bandwidth for ultra-high-rate transmission between cloud servers.
  • 4) Diversified service evaluation:: Novel 6G applications include tactile Internet, digital twins, distributed AI, and wireless brain-machine interfaces.
  • 4) Diversified service evaluation:: A wireless brain-machine-interface system enabled two paralyzed testers to click and type on a tablet using brain signals alone.

5) Holographic communication:

The paper situates holographic communication and other 6G applications within broader requirements for higher performance, expanded scenarios, and a more integrated network architecture.

  • 5) Holographic communication:: Holographic communication would provide high-precision, real-time 3D interaction, requiring very large bandwidth, low latency, and high positioning precision.
  • 6G requirements cannot generally be optimized simultaneously because hardware impairments, propagation conditions, and resource limits create unavoidable performance trade-offs.
  • Scenario-specific requirements prioritize different KPIs; for example, latency, jitter, and reliability matter more than peak rate and connectivity in euRLLC scenarios.
  • 6G application scenarios extend beyond enhanced 5G services to include digital twins, integrated communication, computing and sensing, and distributed AI applications.
  • The proposed 6G architecture evolves from 5G toward 3D multi-network integration, security, communications-sensing-computing integration, green flexibility, and native intelligence.

2) SDN:

The paper reviews SDN and related 6G architectural developments, emphasizing flexible, integrated networks while identifying deployment and orchestration challenges.

  • 2) SDN:: SDN separates network control functions from forwarding functions through distinct application, control, and data planes.
  • 2) SDN:: SDN research has expanded across heterogeneous networks, but optical-device heterogeneity and complexity can delay full transport-network software definition.
  • 2) SDN:: Deploying SDN in 6G raises challenges involving hybrid forwarding, service virtualization, flow management, energy efficiency, handover, and multipath TCP freshness.
  • 6G network architecture is expected to extend 5G service-based architecture toward end-to-end deployment, including potential service-based RAN.
  • 4) Network slicing:: Future network slicing research emphasizes end-to-end systems, specialized resource allocation, and intelligent slicing using AI, digital twins, and deep reinforcement learning.
  • 1) 3D multi-network integrated:: Integrated 3D space-air-ground-sea networks are progressing but still face architectural construction challenges.

2) Secure and trustworthy:

6G security must address novel threats arising from integrated communication, computing, sensing, new services, and terminals. The proposed direction emphasizes endogenous security through a multilateral trust model and architectures that incorporate security from the outset.

  • Secure and trustworthy: 6G faces data-privacy issues, model and algorithm risks, software vulnerabilities, and other novel threats as network architectures and services evolve.
  • Secure and trustworthy: A multilateral trust model is proposed to cover diverse application scenarios and provide more robust, smarter, and scalable security mechanisms.
  • Secure and trustworthy: The multilateral trust network combines bridge, consensus, and endorsement modes, with decentralized consensus as its core and centralized authorization or third-party endorsement also supported.
  • Secure and trustworthy: 6G communication, computing, and sensing functions will be deeply integrated, with network nodes providing transmission, computing, and sensing capabilities.
  • Secure and trustworthy: Cell-free architecture distributes antenna arrays and access points under unified central processing to jointly serve terminals, addressing cellular boundary effects and potentially improving coverage, cost, and efficiency.
  • Secure and trustworthy: Research on self-sustaining networks remains in its infancy, while AI-driven self-sustaining RAN slicing targets autonomous adaptation under unpredictable network conditions.

C. A Novel Promising 6G Network Architecture

The paper proposes a comprehensive 6G architecture that integrates heterogeneous future network components to satisfy evolving requirements and application scenarios. Its design combines ubiquitous computing, flexible network structures, integrated capabilities, and digital-twin-based intelligence.

  • C. A Novel Promising 6G Network Architecture: The proposed architecture connects potential future network technologies and components within one comprehensive framework for heterogeneous 6G networks.
  • C. A Novel Promising 6G Network Architecture: Cloud, fog, and edge computing are combined with hierarchical ubiquitous computing to support flexible resources and low-latency applications.The architecture also further decouples the RAN and separates base-station control and user planes for more flexible resource use.
  • C. A Novel Promising 6G Network Architecture: NFV, SDN, and SBA are expected to evolve toward end-to-end network slicing while network elements gain computing and sensing capabilities.
  • C. A Novel Promising 6G Network Architecture: Digital twins can model the real 6G network, track its changes, and evaluate optimization schemes through real-time mapping, closed-loop simulation, and optimization.
  • C. A Novel Promising 6G Network Architecture: The architecture expands 5G-era design toward intelligent services integrating communication, sensing, computing, and security assurance across available spectra.

A. New Spectrum

6G spectrum research targets higher-frequency and optical bands alongside more dynamic spectrum management to meet growing traffic and connection demands. THz offers large bandwidth but faces substantial hardware, channel, propagation, and safety barriers, while optical wireless provides complementary advantages with device-bandwidth constraints.

  • A. New Spectrum: 5G spectrum congestion and rising traffic demand motivate 6G exploration of THz, optical wireless, and other spectrum resources.
  • 1) THz: THz occupies the 0.1–3 THz gap between mmWave and optical frequencies, offering high frequency and large bandwidth but also high path loss.
  • 1) THz: THz is expected to support ultra-high-rate communication, sensing, imaging, and positioning applications.
  • 1) THz: THz deployment remains constrained by unsuitable hardware, incomplete channel models, unresolved air-interface and MAC techniques, short range, power limits, and health and safety questions.
  • 1) THz: More than 100 Gbps real-time THz transmission was reported in 2022 using a photonics-aided transparent fiber-THz-fiber system.
  • A. New Spectrum: Optical wireless communication offers high-density broadband, ultra-low latency, physical-layer security, zero electromagnetic interference, unlicensed spectrum, and relatively low deployment cost.
  • A. New Spectrum: Optical bands provide three orders of magnitude more spectrum than RF bands, but optoelectronic electrical bandwidth limits their utilization.
  • A. New Spectrum: Dynamic spectrum sharing, cognitive radio, and symbiotic radio are identified as approaches for improving spectrum efficiency and energy efficiency under changing traffic and environments.

2) New coding:

The paper surveys coding, access, antenna, duplexing, orbital-angular-momentum, and related technologies considered for 6G, emphasizing their benefits and implementation challenges. It highlights advances toward higher efficiency, reliability, capacity, connectivity, and coverage while identifying practical barriers.

  • New coding: Advanced ECC decoding and unified circuit designs target improved energy efficiency under 6G’s ultra-low-latency and ultra-reliability requirements.The discussion covers Turbo, LDPC, and polar codes, including list/flip decoding, message-passing approximations, and short-code-length designs.
  • Modern random access: NOMA reuses the same time, frequency, or code resources across terminals to support massive connectivity, low latency, reliability, and throughput.Its main categories are power-domain, code-domain, and interleave-based NOMA; practical deployment still requires low-complexity interference cancellation and stronger security.
  • Ultra-massive MIMO: Ultra-massive MIMO extends antenna arrays to hundreds or thousands, enabling higher spectral and energy efficiency, flexible coverage, and improved positioning accuracy.Its larger arrays provide higher-resolution spatial beams, interference suppression, three-dimensional beam adjustment, and potential non-terrestrial coverage.
  • IBFD: IBFD can theoretically double spectrum efficiency by allowing simultaneous transmission and reception in the same frequency band, but self-interference cancellation becomes harder at wider bandwidths.The paper identifies moderate-complexity, moderate-cost SIC as the primary practical challenge, especially in THz and optical wireless bands.
  • OAM: OAM adds an angular-momentum dimension for modulation and can support higher-capacity MIMO links, with one reported throughput improvement of up to 30.50%.Beam divergence, misalignment, and loss of modal orthogonality under reflection or refraction remain barriers, particularly for non-line-of-sight applications.

2) RIS:

RIS and holographic radio are presented as configurable electromagnetic-environment technologies for improving coverage, efficiency, capacity, sensing, and positioning in 6G. The section also reviews AI-enabled intelligence and identifies unresolved deployment, hardware, modeling, and processing challenges.

  • RIS: RIS uses programmable sub-wavelength metamaterial elements to manipulate incident signals and form controllable reflected or transmitted electromagnetic fields.The paper associates RIS with improved transmission rate, coverage, frequency efficiency, energy efficiency, and deployment flexibility.
  • RIS: RIS is proposed for smart radio environments, massive connectivity, coverage-hole avoidance, green communication, sensing, positioning, and reliability enhancement.Its applications include replacing relays, improving cell-edge transmission, and supporting wireless body sensor networks.
  • RIS: Channel research and system-level studies report RIS-enabled coverage and ergodic-rate improvements in outdoor, indoor, and millimeter-wave cellular settings.Multiple RISs were also reported to significantly increase secondary-user achievable rate in a downlink MISO cognitive-radio system.
  • RIS: RIS deployment still requires work on hardware capabilities, baseband algorithms, network architectures, networking methods, device cost, energy consumption, and deployment scale.These constraints are especially relevant for high-frequency RIS systems.
  • Holographic radio: Holographic radio uses continuous microwave apertures and holographic interference to reconstruct electromagnetic space for spatial multiplexing, sensing, and communication.The paper also describes interference as a potential resource for energy-efficiency enhancement and holographic imaging as a way to reduce channel-estimation overhead.
  • More Capabilities: AI is positioned as an enabler for 6G intelligence across channel modeling, channel estimation, signal detection, resource allocation, interference management, traffic prediction, and congestion control.The paper cautions that AI cannot completely replace conventional methods and that suitable learning techniques must be identified for specific problems.

2) ISAC:

The paper presents ISAC as a 6G integration of communication and perception, motivated by shared resources and increasingly coupled systems. It also identifies unresolved challenges in measurement, modeling, implementation, and security across several emerging technologies.

  • ISAC:: ISAC integrates communication and perception systems to share wireless and hardware resources while improving efficiency.The systems can mutually assist one another in using hardware, spectrum, time, and energy.
  • ISAC:: Communication and perception systems increasingly use similar high-frequency antennas, large apertures, signals, and data-processing methods.
  • ISAC:: ISAC research requires high-precision equipment, appropriate measurement scenarios, suitable frequency bands, channel-correlation analysis, and accurate channel models.Further integration must address hardware, system, waveform, and anti-jamming design jointly.
  • Blockchain:: Blockchain may support decentralized security, privacy, spectrum sharing, network slicing, and resource-sharing applications in 6G networks.Reported applications include DDoS defense, decentralized spectrum management, slice leasing, MEC, IoT, and vehicular information exchange.
  • Blockchain:: Blockchain decentralization introduces relatively long latency, inefficient storage, and limited throughput, restricting its usefulness to certain scenarios.The paper also notes fragmented underlying platforms and a need for unified standardization and regulation.
  • Semantic communication:: Semantic communication transmits extracted semantic information through noisy channels using shared semantic knowledge rather than requiring bit-level error-free delivery.Its practical development still requires reliable semantic elements, error tolerance, resource-efficient processing, interoperable models, and stronger security mechanisms.

A. Testbeds for 6G Channels

6G channel testbeds support the characterization, modeling, and evaluation of new frequencies, scenarios, and technologies. The survey covers simulators and sounders spanning pervasive channel models, multiple spectra, coverage environments, and application-specific deployments.

  • A. Testbeds for 6G Channels: Channel characterization, measurement, and modeling underpin 6G system design, analysis, evaluation, optimization, and deployment.
  • A. Testbeds for 6G Channels: 6G channel testbeds are divided into software channel simulators and hardware channel sounders for new scenarios and technologies.
  • 1) 6G Pervasive Channel Simulator:: The 6G pervasive channel model uses a GBSM framework intended to cover all spectra, global-coverage scenarios, and full-application scenarios.Examples include sub-6 GHz, mmWave, THz, IR, VLC, LEO satellite, UAV, maritime, ultra-massive MIMO, IIoT, and RIS channels.
  • 1) 6G Pervasive Channel Simulator:: 6GPCM enables analysis of mappings among channel-model parameters, channel characteristics, and communication-system performance.The model is described as important for standardization and for studying integrated space-air-ground-sea networks.
  • 2) Channel Sounders:: Channel sounders actively measure unknown propagation environments using a transmitter, receiver, and data-acquisition unit.Key properties include bandwidth, delay range, channel-snapshot repetition rate, and dynamic range, with bandwidth trading off delay resolution against equipment cost or complexity.
  • 2) Channel Sounders:: Existing sounders cover sub-6 GHz, mmWave, THz, optical wireless, UAV, maritime, mine, massive-MIMO, RIS, IIoT, and ISAC scenarios.

1) Testbeds for mmWave:

The surveyed 6G technology testbeds demonstrate practical platforms for mmWave, THz, RIS, ISAC, and distributed massive-MIMO systems. Reported experiments address coverage, bandwidth, sensing, beam control, and high-throughput communication.

  • Testbeds for mmWave:: LuMaMi28 is a real-time mmWave massive-MIMO testbed with 16 transceiver chains, beamswitchable user antennas, and measurements involving static and mobile users.
  • Testbeds for mmWave:: Photonics-assisted mmWave technology addresses atmospheric-attenuation-related coverage limits using Ka-band phased arrays and FPGA-based automatic beam tracking.The demonstrated terminal movement range was ±50°.
  • Testbeds for mmWave:: TeraNova demonstrates true-THz ultra-broadband wireless communication by converting intermediate-frequency signals between 1 and 1.05 THz.The system also characterized THz channels and thermal and absorption noise near an absorption-defined window above 1 THz.
  • Testbeds for mmWave:: A photonics-aided fiber-THz-fiber 2×2 MIMO system at 370 GHz provided a real-time 100 GbE streaming platform.The platform supported real-time movie and live-surveillance video streaming.
  • Testbeds for RIS:: A 1100-element RIS prototype at 5.8 GHz achieved 26 dB power gain over a copper-plate benchmark in indoor scenarios.
  • Testbeds for RIS:: RIS testbeds demonstrated configurable reflected-wave direction, improved BER and SNR, and antenna gains of 21.7 dBi at 2.3 GHz and 19.1 dBi at 28.5 GHz.
  • Testbeds for ISAC:: A 5G-Advanced ISAC sensor exceeded 500 meters of integrated sensing distance and achieved 100% vehicle and people detection accuracy.
  • Testbeds for mmWave:: A cloud-based cell-free distributed massive-MIMO testbed supported 128 × 128 antenna demonstrations with 10.185 Gb/s throughput and over 100 b/s/Hz spectrum utilization.

6) Testbeds for OWC:

6G testbeds span optical wireless links, comprehensive verification platforms, and emerging channel and theory research. They combine technologies such as OWC, RIS, space-air-ground-sea integration, and high-frequency bands to evaluate performance and coverage.

  • 6) Testbeds for OWC:: OWC testbeds comprise LiFi for indoor multiuser networking, FSO for outdoor point-to-point links, and OCC using smartphone cameras as detectors.OWC leverages optical transmitter and receiver devices beyond the RF spectrum.
  • 6) Testbeds for OWC:: 105 Gbps was demonstrated by a WDM LiFi system using visible and infrared wavelengths.The demonstrator was showcased at CES 2022.
  • 6) Testbeds for OWC:: 13.16 Tb/s over 10.45 km and 14 Tb/s over 220 m were demonstrated for high-speed optical terrestrial and short-range links.These demonstrations used commercial coherent fiber-optic transceivers and a two-dimensional optical system, respectively.
  • 6) Testbeds for OWC:: OCC can reach around 1 Mbps while exploiting smartphone camera sensors and their rolling-shutter effect.Its integrated-camera advantage supports light-based wireless links despite optical sensor limitations.
  • 6) Testbeds for OWC:: The “TKµ” comprehensive verification platform integrates cell-free ultra-massive MIMO, RIS, space-air-ground-sea networking, and mmWave/THz techniques.These components target spectral efficiency, coverage, interference control, positioning, and high peak rates.

3) Unified Baseband Processing:

6G baseband research moves beyond isolated channel-coding modules toward iterative, joint, unified, and automatically generated processing architectures. This system-level perspective addresses diverse scenarios while balancing reliability, latency, complexity, flexibility, and hardware efficiency.

  • 3) Unified Baseband Processing:: Enhanced coding schemes, simplified decoding algorithms, and unified implementations are needed for flexible 6G scenarios.Optimizing only a single channel-coding module may not satisfy stringent 6G requirements.
  • 3) Unified Baseband Processing:: Iterative receivers exchange soft information among channel estimation, MIMO detection, NOMA detection, channel decoding, and source decoding.They can provide higher capacity and link reliability, but repeated module usage increases latency and computational complexity.
  • 3) Unified Baseband Processing:: Unified hardware architectures can improve baseband flexibility, compatibility, and hardware efficiency compared with isolated module designs.System-level design avoids additional hardware-resource costs caused by separately implemented modules.
  • 3) Unified Baseband Processing:: Customized EDA tools can automatically generate baseband circuit designs from customer-required performance.Such tools target diverse and complex 6G applications while lowering circuit-design entry barriers.
  • 3) Unified Baseband Processing:: Research on trade-offs among new 6G KPIs remains limited, especially for specific application scenarios.The paper calls for analysis connecting antennas, coding, and other factors with delay, reliability, and capacity.
  • 3) Unified Baseband Processing:: Space-air-ground-sea networks, network AI, integrated communication-sensing-computing, security, immersive interaction, and green technologies remain active research challenges.The cited challenges include network planning, shared AI data, large multisensory traffic, practical security, and low-power operation.

7) Digital Twin Network:

The paper frames digital twins as a key technology for 6G’s all-digital vision while emphasizing broader lessons for scenario-based design, technology planning, basic research, and green efficiency. Its conclusion presents 6G as an expanding research field with substantial open challenges.

  • 7) Digital Twin Network:: Digital twins can support 6G applications such as body-area networks and digital twin cities, while also making communication networks more secure, efficient, intelligent, and visualized.The paper describes digital twins as supporting both applications and communication-network development.
  • 7) Digital Twin Network:: 6G green-network research includes low-EM-field evaluation and near-zero-power technologies such as RF energy harvesting, backscattering, and low-power computing.These directions address EMF security, battery-capacity limits, and environmentally friendly networks.
  • 7) Digital Twin Network:: 6G channel sounders face construction challenges from THz, optical, ultra-wideband, ultra-high-speed, and ultra-massive-MIMO scenarios.High-frequency power amplifiers and ADC/DAC sampling-rate limits constrain measurement systems.
  • 7) Digital Twin Network:: Fusion is an emerging testbed trend, including combinations such as ISAC with THz for high communication rate and sensing accuracy.Comprehensive testbeds must continuously evolve as 6G key technologies develop.
  • 7) Digital Twin Network:: Scenario-based design calls for refined multi-component optimization tools that identify Pareto-optimal operating points for specific applications.6G is expected to proliferate operational modes supporting more specific scenarios and vertical industrial applications.
  • 7) Digital Twin Network:: Reasonable planning should distinguish mature technologies such as THz, RIS, and cell-free solutions from less mature digital twins, semantic communication, and metaverse technologies.The paper links staged planning to reduced costs and maximized network capabilities.
  • 7) Digital Twin Network:: The conclusion reports a critical appraisal of 6G visions, technologies, testbeds, open challenges, and lessons learned while noting that 6G research remains in its infancy.The paper organizes future directions around fundamental research, green networks, key technologies, and testbed development.
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