PaperlayerView exact revision on arXiv

Source-linked AI summary

A Prospective Look: Key Enabling Technologies, Applications and Open Research Topics in 6G Networks

Lina Bariah, Lina Mohjazi, Sami Muhaidat, Paschalis C. Sofotasios, Gunes Karabulut Kurt, Halim Yanikomeroglu, Octavia A. Dobre

arXiv:2004.06049v2eess.SP

TL;DR

Emerging applications demand wireless systems with ultra-high reliability, extremely high data rates, and ultra-low latency beyond current 5G capabilities. The paper surveys a 6G roadmap centered on five enabling technologies and identifies their requirements, challenges, and open research problems.

  • Problem

    The paper addresses whether 5G can accommodate emerging applications requiring ultra-high reliability, extremely high data rates, ultra-low latency, and expanded spectrum beyond 52.6 GHz.

  • Method

    The paper synthesizes state-of-the-art literature to study the 6G vision and five technologies, including programmable metasurfaces, their applications, requirements, challenges, and open problems.

  • Results

    The paper outlines terahertz, programmable metasurface, drone-based, backscatter, and tactile Internet technologies as components of a prospective 6G roadmap.

  • Takeaways & Limitations

    The study frames 6G research around integrating diverse wireless technologies while addressing spectrum, heterogeneity, security, resource, energy, and mobility challenges.

Abstract

from arXiv · show

The fifth generation (5G) mobile networks are envisaged to enable a plethora of breakthrough advancements in wireless technologies, providing support of a diverse set of services over a single platform. While the deployment of 5G systems is scaling up globally, it is time to look ahead for beyond 5G systems. This is driven by the emerging societal trends, calling for fully automated systems and intelligent services supported by extended reality and haptics communications. To accommodate the stringent requirements of their prospective applications, which are data-driven and defined by extremely low-latency, ultra-reliable, fast and seamless wireless connectivity, research initiatives are currently focusing on a progressive roadmap towards the sixth generation (6G) networks. In this article, we shed light on some of the major enabling technologies for 6G, which are expected to revolutionize the fundamental architectures of cellular networks and provide multiple homogeneous artificial intelligence-empowered services, including distributed communications, control, computing, sensing, and energy, from its core to its end nodes. Particularly, this paper aims to answer several 6G framework related questions: What are the driving forces for the development of 6G? How will the enabling technologies of 6G differ from those in 5G? What kind of applications and interactions will they support which would not be supported by 5G? We address these questions by presenting a profound study of the 6G vision and outlining five of its disruptive technologies, i.e., terahertz communications, programmable metasurfaces, drone-based communications, backscatter communications and tactile internet, as well as their potential applications. Then, by leveraging the state-of-the-art literature surveyed for each technology, we discuss their requirements, key challenges, and open research problems.

I. INTRODUCTION · A. 6G VISION AND REQUIREMENTS

The paper motivates 6G by exponential mobile-data growth and emerging IoE services whose simultaneous reliability, data-rate, and latency demands may exceed 5G capabilities. It envisions 6G as an AI-enabled network supporting extreme connectivity, immersive applications, tactile feedback, autonomous environments, and highly reliable low-latency communications.

  • I. INTRODUCTION: Mobile data is projected to reach about 5 zettabytes per month in 2030 as IoE connects billions of people and devices across smart applications.The stated applications include smart homes, smart cities, and e-health.
  • I. INTRODUCTION: Emerging IoE services span XR, flying vehicles, haptics, telemedicine, autonomous systems, and human-machine interfaces.These services are enabled by massive connectivity and data-centric operation.
  • I. INTRODUCTION: These services require ultra-high reliability, extremely high data rates, and ultra-low latency simultaneously over uplink and downlink, pushing 5G toward its limits.The paper speculates that this could occur within 10 years of 5G’s launch.
  • I. INTRODUCTION: 6G research must address sub-THz bands, rate-reliability-latency trade-offs, flexible network architectures, and intelligent holistic orchestration.These challenges arise from the heterogeneous requirements of future cutting-edge services.
  • A. 6G VISION AND REQUIREMENTS: 6G is envisioned to introduce disruptive wireless technologies and innovative network architectures for abundant autonomous services and next-generation connectivity.The vision extends beyond 5G toward emerging service trends.
  • A. 6G VISION AND REQUIREMENTS: 6G should offer extreme data rates and ultra-high throughput despite changing device densities, spectrum, infrastructure availability, traffic patterns, or emergency conditions.This requirement targets massive-scale connectivity under extreme conditions.
  • A. 6G VISION AND REQUIREMENTS: 6G should provide immersive quality, per-user capacity, and unified quality of experience for AR and VR applications in retail, tourism, and education.The requirement concerns the targeted quality of immersion for these applications.
  • A. 6G VISION AND REQUIREMENTS: 6G should combine AI-enabled context-aware communications, submillisecond latency for tactile feedback, and more than 99.9999% reliability for intelligent and mobile applications.Examples include smart structures, autonomous transportation, smart industry, e-health, and flying vehicles.

B. RELATED WORK AND CONTRIBUTIONS · II. KEY ENABLING TECHNOLOGIES FOR 6G NETWORKS · A. MILLIMETER-WAVE AND TERAHERTZ COMMUNICATIONS

The survey frames 6G around enabling technologies rather than primarily use cases, covering five disruptive technologies, their applications, challenges, and future research directions. This section introduces spectrum scarcity and mmWave communications as part of the technological foundation for stringent 6G requirements.

  • B. RELATED WORK AND CONTRIBUTIONS: Earlier 6G studies examined prospective use cases, trends, requirements, technologies, and challenges, including terrestrial–satellite integration, spectrum utilization, AI, and mobility management.
  • B. RELATED WORK AND CONTRIBUTIONS: This survey instead approaches 6G through enabling technologies, presenting a holistic conceptual overview of revolutionary paradigms for realizing the 6G vision.
  • B. RELATED WORK AND CONTRIBUTIONS: The survey covers backscatter communications, tactile internet, and aerial networks among five disruptive technologies, while examining applications, challenges, and future research guidance.
  • II. KEY ENABLING TECHNOLOGIES FOR 6G NETWORKS: Future 6G technologies are intended to enable unprecedented network functionalities and applications with stringent latency, reliability, energy, efficiency, and capacity requirements.
  • A. MILLIMETER-WAVE AND TERAHERTZ COMMUNICATIONS: Spectrum scarcity is a major 6G challenge because broadband penetration and emerging use cases impose rigorous bandwidth requirements.
  • A. MILLIMETER-WAVE AND TERAHERTZ COMMUNICATIONS: 3GPP Release 17 extends 5G New Radio physical-layer support beyond 52.6 GHz, reaching 71 GHz, toward higher-frequency communications.
  • A. MILLIMETER-WAVE AND TERAHERTZ COMMUNICATIONS: mmWave communications operate over 30-300 GHz with wavelengths of 10 to 1 mm, enabling compact large-element antenna arrays and narrow directional beams.
  • A. MILLIMETER-WAVE AND TERAHERTZ COMMUNICATIONS: mmWave deployment faces costly small-component manufacturing and atmospheric-absorption attenuation as high as 15 dB/km, limiting transmission range.

2) State-of-the-Art · B. OPTICAL WIRELESS COMMUNICATIONS

The surveyed state of the art spans mmWave and THz wireless systems, emphasizing blockage mitigation, channel and hardware design, and the challenges of reliable THz operation. Optical wireless communications are presented as a 6G-enabling technology with high-speed terrestrial, space, and underwater applications, alongside VLC, LiFi, OCC, and FSO implementations and impairment-related research.

  • 2) State-of-the-Art: Human blockage can degrade mmWave links by 20-30 dB, motivating relay selection and scheduling protocols to reduce blockage effects.Blockage is especially severe when obstacles are larger than the short mmWave wavelength.
  • 2) State-of-the-Art: NOMA-based mmWave research examines secrecy rates and performance in scenarios including drone communications, massive MIMO, and simultaneous wireless information and power transfer.
  • 2) State-of-the-Art: mmWave research also focuses on channel modeling, transceiver design, and antenna design.These areas are summarized as core research directions in Table 3.
  • 2) State-of-the-Art: THz communications offer wider bandwidth and high data rates, but require new beamforming, tracking, hardware, massive MIMO, and intelligent-surface solutions.Dynamic and precise tracking of THz-enabled devices remains an open research problem.
  • 2) State-of-the-Art: Outdoor THz deployment is challenged by molecular absorption, rain, and other atmospheric losses, making antenna and transceiver design essential.The research community has therefore investigated THz antenna and transceiver designs before effective deployment.
  • 2) State-of-the-Art: THz channel modeling must characterize LOS and NLOS propagation, scattering, free-space loss, molecular absorption, mobility, and channel-state-information estimation.Deterministic models are more accurate than stochastic models but suffer from high computational complexity and a need for geometrical environmental information.
  • B. OPTICAL WIRELESS COMMUNICATIONS: Optical wireless communication is positioned as a 6G technology enabling broadband terrestrial, space, and underwater connectivity with ultra-high bandwidth, low latency, high data rates, and physical-layer security.Spatial reuse and immunity-related advantages are also identified among OWC features.
  • B. OPTICAL WIRELESS COMMUNICATIONS: VLC supports indoor connectivity over up to 20 m, reaching 10 Gbps with LEDs and 100 Gbps with LDs, while LiFi provides bidirectional communication using visible-light downlink and infrared or RF uplink.OCC uses LED transmitters and camera or image-sensor receivers for indoor positioning and navigation, whereas FSO delivers around 10 Gbps over reasonable distances around 1 km.

C. PROGRAMMABLE METASURFACES FOR WIRELESS COMMUNICATIONS · 1) Metasurfaces

Programmable metasurfaces are presented as adaptive structures that can manipulate electromagnetic-wave propagation, enabling flexible wireless environments for future communications. Their tunable functions support connectivity, interference mitigation, diversity, beam management, physical-layer security, polarization control, and phase adjustment.

  • 1) Metasurfaces: Metasurfaces use periodically repeated artificial conductive meta-atoms to manipulate electromagnetic waves across a wireless link.The meta-atom is the basic building block of a rectangular metasurface tile.
  • 1) Metasurfaces: Programmable metasurfaces dynamically alter meta-atom designs through external stimuli such as binary switches, using tunable components including MEMS switches or CMOS transistors.These mechanisms provide adaptivity to the metasurface structure.
  • 1) Metasurfaces: Tunable metasurfaces introduce an additional degree of freedom that facilitates massive connectivity, interference mitigation, and enhanced diversity.These capabilities provide flexibility for future wireless communications.
  • C. PROGRAMMABLE METASURFACES FOR WIRELESS COMMUNICATIONS: Programmatically controlled wireless environments enable broad functionalities at mmWave and THz bands, where attenuation and scattering otherwise challenge communication efficiency.Walls and furniture are examples of indoor objects that scatter rays.
  • 1) Metasurfaces: Metasurfaces steer impinging waves toward desired directions by manipulating refraction or reflection indices, overriding outgoing directions defined by Snell’s Law.This directional control is one of the summarized metasurface functionalities.
  • 1) Metasurfaces: Beam splitting divides an incident wave into customized orthogonal beams directed toward multiple users simultaneously.The beams are steered in multiple directions to serve multiple users.
  • 1) Metasurfaces: Wave absorption can reduce incident-wave power by 35 dB, helping block unauthorized devices, prevent eavesdropping, and optimize physical-layer security.The functionality adjusts metasurface properties to minimize reflection or refraction.
  • 1) Metasurfaces: Metasurfaces can control impinging-wave polarization and alter the carrier phase through wave-polarization and phase-control functionalities.Polarization control manipulates oscillation orientation, while phase control changes the carrier phase.

2) Artificial intelligence (AI)-empowered Metasurfaces · D. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS · 1) Drone-assisted Wireless Communications

AI and ML are positioned as essential to programmable metasurfaces because they enable adaptive, coordinated, scalable, and automated operation. Drone-based communications are a key 6G driver, with mobile aerial platforms supporting coverage, capacity, reliability, low latency, and connectivity in challenging environments.

  • 2) Artificial intelligence (AI)-empowered Metasurfaces: AI can identify operation policies and dynamically adapt metasurface parameters for heterogeneous wireless applications with diverse networks and QoS requirements.The cited examples include data-driven adaptation of network parameters such as coding rates.
  • 2) Artificial intelligence (AI)-empowered Metasurfaces: ML-enabled metasurfaces can coordinate complex functions while maintaining desired global behavior and reducing scalability, energy, and overhead costs.The approach also supports self-organization and automation of metasurface maintenance, management, and operational tasks.
  • 2) Artificial intelligence (AI)-empowered Metasurfaces: Deep learning can map estimated channel state information at a user location to optimal unit-cell configurations for metasurface signal focusing.Adaptive control and coordination of multiple metasurfaces for multiple users was also demonstrated.
  • D. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS: Connected autonomous vehicles and drone-based communications drive 6G research toward ultra-low latency and unprecedented communication reliability.Research efforts in connected and autonomous vehicle and UAV-based communication systems are growing in academia and industry.
  • 1) Drone-assisted Wireless Communications: Drones are foundational aerial-network elements whose mobility and flexibility support imminent and futuristic wireless applications, while improving coverage and transmission rates.3GPP standardization activities are ongoing to integrate drones into future wireless networks.
  • 1) Drone-assisted Wireless Communications: Cellular-enabled drones operate as user equipments for mining, oil and gas, transportation, surveying, and monitoring applications, reaching 160 km/h in urban and rural environments.They require reliable, low-latency links between drone-UEs and ground base stations.
  • 1) Drone-assisted Wireless Communications: Wireless infrastructure drones extend network coverage or capacity through drone base stations, aerial relays, and aerial backhaul for cellular networks.These platforms can provide temporary connectivity in hard-to-reach, rural, disaster, event, or harsh environments and robust high-speed backhaul.

2) Air-Ground-Space Integrated Networks · 3) Drone-based Multi-access Edge Computing · 4) SWIPT-enabled Drones

The paper presents air-ground-space integration as an evolution of wireless infrastructure for diverse IoE use cases, while drone-based MEC and SWIPT address computation offloading, mobility, and energy constraints. It also identifies processing, battery, security, and broader network-design issues as central challenges for realizing drone-network potential.

  • 2) Air-Ground-Space Integrated Networks: Air-ground-space network integration is envisioned to support diverse IoE use cases with different QoS requirements as generated data grows exponentially.Existing terrestrial, aerial, and satellite networks may not cope with the massive volume of generated data.
  • 2) Air-Ground-Space Integrated Networks: Drone-based systems are positioned as an integrated infrastructure for applications spanning advantages, categories, and challenges.These dimensions are summarized in the paper’s overview figure.
  • 3) Drone-based Multi-access Edge Computing: MEC enables resource-constrained mobile devices to offload computation tasks to the edge of cellular networks.MEC provides cloud-computing capabilities at the cellular edge and is identified as a potential 5G technology.
  • 3) Drone-based Multi-access Edge Computing: Drones with MEC capabilities can reduce network congestion, accelerate application deployment, and provide uninterrupted services for highly mobile users without handover.Large-scale coverage and line-of-sight connections support mobility management in drone-enabled networks.
  • 3) Drone-based Multi-access Edge Computing: Limited drone storage and battery capacity constrain onboard processing, potentially excluding computation-intensive applications such as real-time image processing.Cloud computing is proposed to address these execution limitations.
  • 4) SWIPT-enabled Drones: Short built-in battery lifetime limits drone utilization and cruising duration, motivating trajectory design, location adjustment, and power-allocation optimization.The limitation follows from drones’ restricted size and weight, which constrain energy storage.
  • 4) SWIPT-enabled Drones: UAV-assisted SWIPT systems can transmit data to ground receivers while sending the same RF signals to wireless-power-harvesting energy receivers, which may be eavesdroppers.Resource-allocation optimization was formulated for this setting.
  • 4) SWIPT-enabled Drones: Drone-network realization requires optimizing battery lifetime, charging mechanisms, and energy consumption while addressing security, privacy, architecture, image processing, interference, and storage.The surveyed literature includes encryption and authentication mechanisms alongside these practical and system-design issues.

E. THE NEXT FRONTIER FOR IOE: BACKSCATTER COMMUNICATIONS AND WIRELESS POWERED NETWORKS … 4) Quantum BackCom

Backscatter communications support energy-efficient IoE connectivity by reflecting ambient signals while integrating wireless power, cognitive radio, visible light, and quantum techniques. The section also identifies security, scalability, access, channel, resource-allocation, and energy-harvesting challenges for future BackCom networks.

  • E. THE NEXT FRONTIER FOR IOE: BACKSCATTER COMMUNICATIONS AND WIRELESS POWERED NETWORKS: Backscatter transceivers reflect ambient TV or Wi-Fi signals and consume orders of magnitude less power than conventional systems, making them strong candidates for low-power IoE networks.Their low power consumption enables communication among connected devices with limited battery lifetime.
  • 1) Radio-frequency (RF)-Powered BackCom Networks: RF energy harvesting replenishes wireless-device energy from ambient or dedicated RF sources through dedicated harvesting circuits.The approach includes wireless-powered communications and simultaneous wireless information and power transfer.
  • 2) RF-Powered Cognitive Radio Networks and Ambient BackCom: RF-powered cognitive-radio networks harvest energy from active primary-user signals and use it for secondary-user transmissions, but performance depends strongly on primary-user signal availability.This paradigm integrates RF energy harvesting with cognitive-radio operation.
  • 3) Visible Light BackCom: Visible light communications use LED intensity modulation and direct photodiode detection, offering communication security, spatial reuse, and immunity to RF interference.These properties make visible light suitable for critical environments such as hospitals and industrial plants.
  • 4) Quantum BackCom: Quantum BackCom uses entangled signal-idler photon pairs, transmitting and backscattering the signal photon while retaining the idler photon at the receiver.The quantum setting is anticipated to improve 6G and next-generation IoT performance and security.
  • 4) Quantum BackCom: BackCom transceivers face eavesdropping and jamming threats, while conventional encryption and digital signatures may be impractical under their power and complexity constraints.These limitations motivate research into security mechanisms that provide fully secure communications.
  • 4) Quantum BackCom: Existing BackCom networks are not optimized for massive low-power IoT deployments, motivating orthogonal and non-orthogonal access schemes such as NOMA and rate-splitting multiple access.These schemes target massive connectivity alongside high energy and spectral efficiency.
  • 4) Quantum BackCom: Open BackCom research topics include channel modeling and estimation, resource allocation, and wireless energy harvesting.The paper summarizes these areas as timely open research topics for BackCom systems.

F. TACTILE INTERNET · III. DRIVING APPLICATIONS OF 6G TECHNOLOGIES · A. MILLIMETER-WAVE COMMUNICATIONS

The section presents Tactile Internet as a 6G frontier for M2P and M2M applications, requiring extremely low latency, high reliability, and secure communications. It also identifies millimeter-wave communications as a scalable enabler for wireless backhaul, wearables, imaging, and tracking.

  • F. TACTILE INTERNET: Tactile Internet targets M2P and M2M interactions across healthcare, education, smart manufacturing, and other Industry 4.0 applications.Its envisioned uses also include VR/AR, intelligent transportation, and robotics.
  • F. TACTILE INTERNET: Tactile Internet infrastructure must provide extremely low end-to-end latency, high reliability, and data security without compromising latency through demanding encryption.The current communication infrastructure cannot meet these stringent use-case requirements.
  • F. TACTILE INTERNET: Meeting Tactile Internet requirements motivates disruptive beyond-5G technologies spanning THz communications, novel wireless architectures, and AI-enabled communication networks.These technologies are proposed to catalyze use cases with unique requirements.
  • III. DRIVING APPLICATIONS OF 6G TECHNOLOGIES: The applications associated with the discussed technologies are intended to realize the vision of 6G systems.The section highlights potential applications across the aforementioned technologies.
  • A. MILLIMETER-WAVE COMMUNICATIONS: Wireless backhaul offers a scalable alternative to fiber for connecting densely deployed heterogeneous small-cell base stations because fiber installation is becoming uneconomical.The proposed approach uses wide, underutilized millimeter-wave bandwidth.
  • A. MILLIMETER-WAVE COMMUNICATIONS: Millimeter-wave links support wearable devices such as smart watches, AR/VR glasses and helmets, healthcare gadgets, and motion trackers requiring higher data rates and longer battery lifetime.Relaxed transmission-range constraints can make communication between wearables and a smart receiver efficient.
  • A. MILLIMETER-WAVE COMMUNICATIONS: At 60 GHz, reflected signals and highly directional beams enable precise imaging and tracking while reducing interference.Miniaturized antenna arrays support integration into small receivers such as smartphones and tablets.

B. TERAHERTZ COMMUNICATIONS · C. OPTICAL WIRELESS COMMUNICATIONS · D. METASURFACES FOR WIRELESS COMMUNICATIONS

The paper surveys applications of THz and optical wireless communications, alongside metasurface-enabled wireless power transfer and body sensor networks. These technologies target high-capacity, secure, health-related, transportation, energy-harvesting, and wearable-device use cases.

  • B. TERAHERTZ COMMUNICATIONS: THz communications can support secure drone communications through higher capacity, increased mobility, and extremely narrow beams that reduce eavesdropping probability.Large antenna arrays enable coverage extension and narrow-beam transmission.
  • B. TERAHERTZ COMMUNICATIONS: THz-enabled nano-sensors could monitor blood ions, cholesterol, infections, and cancer biomarkers before forwarding data to a phone or medical device.The passage identifies glucose and sodium among the monitored ions and describes THz as a health-care candidate.
  • B. TERAHERTZ COMMUNICATIONS: THz communications are envisioned for bandwidth-intensive WLAN/WPAN applications, including holographic video conferencing and ultra-high-speed data transfer.The proposed interconnection spans fiber-optical networks and wireless devices or personal wireless devices.
  • C. OPTICAL WIRELESS COMMUNICATIONS: Optical wireless communications can use transportation LEDs for illumination and vehicle-to-vehicle or infrastructure communication of speed, navigation, and traffic information.The application is intended to realize safer and smarter transportation systems.
  • C. OPTICAL WIRELESS COMMUNICATIONS: OWC can provide airplane passengers with audio/video transmission, instant messaging, and data exchange through LED passenger lighting.The proposed use embeds wireless communication in travelers’ lighting.
  • C. OPTICAL WIRELESS COMMUNICATIONS: In underground mining, VLC offers indoor positioning suited to enclosed environments, with low cost, low interference, and high data rates for locating miners.The application addresses assistance after cave collapses, chemical leaks, and gas explosions.
  • D. METASURFACES FOR WIRELESS COMMUNICATIONS: Metasurfaces can steer and concentrate electromagnetic waves to improve wireless power transfer and energy harvesting for low-power devices and sensors.The application addresses the challenge of harvesting sufficient energy over wireless channels.
  • D. METASURFACES FOR WIRELESS COMMUNICATIONS: Metasurface-based textiles enable energy-efficient and secure WBSNs by guiding signals around clothing to connect wearable devices, reducing power dissipation and improving battery life and data rates.The approach uses conductive metasurface textiles fitted into regular clothing, and is expected to boost received signal compared with conventional technologies.

E. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS · F. BACKCOM AND ENERGY HARVESTING · G. TACTILE INTERNET

The sections present drones as flexible platforms for rescue, delivery, and underwater missions; BackCom as an energy-efficient sensing technology; and the Tactile Internet as an enabler of low-latency, haptic interactions across industry and society.

  • E. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS: Drones support search-and-rescue missions because they are more flexible and faster to deploy than manned vehicles.
  • E. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS: Drone delivery offers fast, cost-effective, and reliable package transport, with 83% of Amazon packages weighing below 2.5 kg.FedEx’s average package weight is less than 5 kg.
  • E. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS: Underwater drones perform military and civil missions including oil-spill exploration, identification, and intensive studies of marine organisms and ecosystems.
  • F. BACKCOM AND ENERGY HARVESTING: Battery-less backscatter sensors with energy harvesting enable smart-home functions such as gas-leak, smoke, carbon-oxide, movement, and garbage-level monitoring.Smart dustbins can report garbage levels to collection trucks.
  • F. BACKCOM AND ENERGY HARVESTING: BackCom sensors support energy-efficient, low-cost smart cities through air-quality monitoring and parking-traffic management.They can monitor pollution and noise and indicate available parking slots.
  • G. TACTILE INTERNET: The Tactile Internet enables industrial automation by controlling rapidly moving machinery through networked sensors and actuators while meeting end-to-end latency requirements.Industrial control processes also impose varying latency, reliability, data-rate, and security demands.
  • G. TACTILE INTERNET: Tactile Internet applications include shared haptic VR, real-time AR augmentation, location-independent healthcare, and distance education with multimodal interaction.Examples include tele-surgery, remote diagnosis using haptic feedback, and instant musical-instruction corrections.

IV. CHALLENGES AND FUTURE DIRECTIONS … C. OPTICAL WIRELESS COMMUNICATIONS

The paper identifies open research issues across prospective 6G technologies, spanning mobility, interference, blockage, transceiver and channel modeling, and optical wireless networking. Optical wireless challenges include secure hybrid networks, multiple access under practical impairments, and FSO line-of-sight constraints.

  • IV. CHALLENGES AND FUTURE DIRECTIONS: The section surveys open research issues associated with the previously presented potential 6G technologies and highlights their research challenges.
  • A. MILLIMETER-WAVE COMMUNICATIONS: User mobility in mmWave networks causes channel variations, rapid load fluctuations, recurrent handovers, and severe capacity impacts, requiring adaptive coding and sophisticated association mechanisms.
  • A. MILLIMETER-WAVE COMMUNICATIONS: Dense access-point deployment in mmWave small cells creates interference, while blockage can disconnect sessions, reduce reliability, and increase reconnection overhead and latency.
  • B. TERAHERTZ COMMUNICATIONS: THz communications require transceiver architectures operating across the entire THz band to combat severe path loss and enable high sensitivity and power gain.
  • B. TERAHERTZ COMMUNICATIONS: Accurate THz channel models must capture molecular absorption, spreading loss, weather effects, and severe frequency selectivity that existing low-frequency models cannot represent.
  • C. OPTICAL WIRELESS COMMUNICATIONS: Secure hybrid RF/optical networks require efficient physical-layer security mechanisms despite the presence of legitimate users and eavesdroppers.
  • C. OPTICAL WIRELESS COMMUNICATIONS: Limited modulation bandwidth and peak optical power constrain OWC spectral efficiency, while optical multiple access must be evaluated under ambient light, fading, shadowing, and imperfect channels.
  • C. OPTICAL WIRELESS COMMUNICATIONS: FSO deployment is challenged by line-of-sight requirements, atmospheric turbulence, and geometric and misalignment losses, motivating optical reconfigurable intelligent surfaces.

D. METASURFACES FOR WIRELESS COMMUNICATIONS · E. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS

The paper identifies unresolved design challenges for metasurfaces, including dynamic reconfiguration, programmable interfaces, higher-order modulation, and wireless power transfer. Drone-based networks require new architectures, energy-efficient operation, collision avoidance, and accurate aerial-channel models.

  • D. METASURFACES FOR WIRELESS COMMUNICATIONS: Metasurfaces require further design investigation to realize their full potential, particularly for dynamic structure reconfiguration supporting diverse wireless functionalities.
  • D. METASURFACES FOR WIRELESS COMMUNICATIONS: Advanced multi-functional metasurfaces must switch electromagnetic behavior rapidly, with low-complexity control software supporting high-mobility environments.The system convergence rate may not fit within the surrounding wireless environment’s coherence time.
  • D. METASURFACES FOR WIRELESS COMMUNICATIONS: High-order modulation and novel waveforms are needed for high-data-rate metasurface communications, whose transmitters currently use single-carrier low-order schemes.Examples include binary and quadrature phase-shift keying.
  • D. METASURFACES FOR WIRELESS COMMUNICATIONS: Wireless power transfer is proposed for perpetual energy replenishment as short battery life remains a major challenge for wireless devices.
  • E. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS: Drone-based aerial networks need new planning, evaluation, and resource-allocation architectures integrating terrestrial base stations with drone-based user equipment.Existing terrestrial base-station antennas do not support ultra-low-latency requirements for high-elevation-angle aerial users.
  • E. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS: Energy constraints limit mobile-enabled drones because solar energy and small built-in batteries are their only power sources, especially during continuous monitoring missions.
  • E. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS: Drone deployments require dynamic collision-avoidance schemes because buildings and obstacles create hazards, while restricted flight zones can increase interference and collision probability.
  • E. DRONE-BASED COMMUNICATIONS AND AUTONOMOUS SYSTEMS: Reliable cooperative aerial networks require robust channel models characterizing air-to-air and air-to-ground communication links across frequency bands.

F. BACKSCATTER COMMUNICATIONS AND ENERGY HARVESTING · G. TACTILE INTERNET · V. CONCLUSION

The sections identify unresolved challenges in backscatter communications, tactile Internet realization, and emerging 5G limitations. Key concerns include interference, security, self-interference, haptic-device capability, multimodal integration, ultra-reliability, and ultra-low latency.

  • F. BACKSCATTER COMMUNICATIONS AND ENERGY HARVESTING: Backscatter systems require simple, highly efficient security solutions because limited resources cannot support conventional encryption and digital signatures.Their simple modulation and coding schemes also expose them to security and jamming attacks.
  • F. BACKSCATTER COMMUNICATIONS AND ENERGY HARVESTING: Ambient backscatter inevitably interferes with licensed users, motivating protocols that guarantee no or minimal interference.Research has addressed interference modeling and compensation schemes.
  • F. BACKSCATTER COMMUNICATIONS AND ENERGY HARVESTING: Full-duplex ambient backscatter must mitigate substantial self-interference when the same antenna simultaneously transmits and receives.Developing self-interference mitigation schemes remains an open research issue.
  • G. TACTILE INTERNET: Haptic devices need greater degrees of freedom, cost effectiveness, and simultaneous kinesthetic and tactile control for envisioned tactile Internet applications.Sensors and actuators already enable interaction with real or virtual objects, but current devices remain insufficient.
  • G. TACTILE INTERNET: Bandwidth-limited networks make haptic data compression essential, yet tactile Internet still requires further haptic codec-design research.Compression solutions aim to improve system reliability and user experience.
  • G. TACTILE INTERNET: Integrating visual, haptic, and auditory feedback requires novel multiplexing schemes because modalities differ in latency, sampling, transmission rate, and priority.The schemes must temporally integrate multiple modalities with different priorities.
  • G. TACTILE INTERNET: 10−7 outage is the target for tactile Internet ultra-reliability, while subms end-to-end latency requires latency budgeting and optimization across air-interface, backhaul, and core networks.Packet losses, outages, and latency can directly disrupt users’ activities and sensations.
  • V. CONCLUSION: Emerging applications and initial 5G implementation and testing reveal continuing challenges involving ultra-high reliability, extremely high data rates, and ultra-low latency.Bio-interface applications are among the developments exposing these limitations.
Loading 2004.06049v2…