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White Paper on Broadband Connectivity in 6G

Nandana Rajatheva, Italo Atzeni, Emil Bjornson, Andre Bourdoux, Stefano Buzzi, Jean-Baptiste Dore, Serhat Erkucuk, Manuel Fuentes, Ke Guan, Yuzhou Hu, Xiaojing Huang, Jari Hulkkonen, Josep Miquel Jornet, Marcos Katz, Rickard Nilsson, Erdal Panayirci, Khaled Rabie, Nuwanthika Rajapaksha, MohammadJavad Salehi, Hadi Sarieddeen, Tommy Svensson, Oskari Tervo, Antti Tolli, Qingqing Wu, Wen Xu

arXiv:2004.14247v1eess.SP

TL;DR

Future 6G must support broadband use cases ranging from extreme capacity to high-mobility connectivity, but higher-frequency operation and advanced architectures remain technically challenging. This white paper surveys infrastructure, spectrum, and protocol or algorithmic enablers, concluding that broadband connectivity will combine distributed networks, expanded spectrum, and coordinated processing techniques.

  • Problem

    Future applications require broadband connectivity with high data rates and specialized characteristics beyond the capabilities of existing cellular generations.

  • Method

    The paper surveys PHY and MAC layer methodologies and expected infrastructure, spectrum, and protocol or algorithmic enablers for 6G broadband access.

  • Results

    The paper identifies distributed cell-free infrastructure, sub-6 GHz through visible-light spectrum use, and coding, interference-management, caching, and broadcasting techniques as broadband enablers.

  • Takeaways & Limitations

    6G broadband connectivity will require coordinated use of terrestrial and satellite networks, higher-frequency links, and multiple complementary efficiency techniques.

  • Takeaways & Limitations

    Higher-frequency 6G operation faces hardware, waveform, impairment-mitigation, and computational-complexity challenges, while several proposed techniques remain open research areas.

Abstract

from arXiv · show

This white paper explores the road to implementing broadband connectivity in future 6G wireless systems. Different categories of use cases are considered, from extreme capacity with peak data rates up to 1 Tbps, to raising the typical data rates by orders-of-magnitude, to support broadband connectivity at railway speeds up to 1000 km/h. To achieve these goals, not only the terrestrial networks will be evolved but they will also be integrated with satellite networks, all facilitating autonomous systems and various interconnected structures. We believe that several categories of enablers at the infrastructure, spectrum, and protocol/ algorithmic levels are required to realize the intended broadband connectivity goals in 6G. At the infrastructure level, we consider ultra-massive MIMO technology (possibly implemented using holographic radio), intelligent reflecting surfaces, user-centric and scalable cell-free networking, integrated access and backhaul, and integrated space and terrestrial networks. At the spectrum level, the network must seamlessly utilize sub-6 GHz bands for coverage and spatial multiplexing of many devices, while higher bands will be used for pushing the peak rates of point-to-point links. The latter path will lead to THz communications complemented by visible light communications in specific scenarios. At the protocol/algorithmic level, the enablers include improved coding, modulation, and waveforms to achieve lower latencies, higher reliability, and reduced complexity. Different options will be needed to optimally support different use cases. The resource efficiency can be further improved by using various combinations of full-duplex radios, interference management based on rate-splitting, machine-learning-based optimization, coded caching, and broadcasting.

Executive Summary

The paper examines broadband connectivity for future 6G systems across extreme capacity, higher typical rates, and high-mobility scenarios, combining terrestrial and satellite networks.

  • 6G broadband use cases range from peak data rates up to 1 Tbps to broadband connectivity at railway speeds up to 1000 km/h.
  • Terrestrial networks will evolve alongside satellite integration to support autonomous systems and interconnected structures.

1 Introduction

The paper frames 6G as a potential successor built around technologies beyond 5G, organized across infrastructure, spectrum, and protocol or algorithmic enablers for broadband connectivity.

  • 1 Introduction: Infrastructure enablers include extreme densification, cooperating access points, distributed processing and caching, network slicing, and multi-access edge computing.
  • 1 Introduction: Spectrum evolution combines sub-6 GHz coverage with higher carrier frequencies, extending beyond mmWave toward visible light for high-capacity point-to-point links.
  • 1 Introduction: The paper identifies integrated terrestrial and satellite networks, including UAVs and LEO microsatellites, as contributors to coverage and network offloading.
  • 1 Introduction: 6G broadband connectivity is the paper’s specific focus, targeting very high data rates up to the Tbps range through PHY and MAC layer methodologies.
  • 1 Introduction: Protocol and algorithmic enablers span coding, modulation, waveforms, duplex, interference management, machine learning, coded caching, broadcasting, and full-coverage broadband connectivity.
  • 1 Introduction: The paper concludes that these broadband concepts remain subject to substantial research and technological challenges before the 6G vision can be reached.

2 Use Cases, Key Performance Indicators, and Spectrum

6G broadband connectivity targets diverse use cases, from extreme-capacity links and immersive applications to high-mobility autonomous systems, with specialized KPI groups rather than one universal profile. The paper outlines spectrum expansion from sub-6 GHz coverage toward mmWave, THz, and visible-light bands, while highlighting major implementation challenges at higher frequencies.

  • Use Cases: 6G use cases span extreme-capacity xHaul, enhanced hotspots, short-range device-to-device links, smart rail mobility, extended reality, industrial automation, and autonomous mobility.These applications combine different data-rate, latency, reliability, coverage, energy, and mobility requirements.
  • Use Cases: Extreme-capacity xHaul targets fixed symmetric point-to-point links that aggregate many users’ traffic and require high bandwidth and spectral efficiency.Wireless xHaul complements fiber deployment where fiber penetration or extension costs limit backhaul capacity.
  • Use Cases: Smart rail and autonomous mobility extend broadband connectivity to trains, vehicles, and other systems operating at speeds up to 1000 km/h.Rail applications include surveillance, passenger connectivity, broadcasting, and remote control, while autonomous vehicles require higher rates, reliability, and low latency.
  • Key Performance Indicators: 6G retains familiar KPIs but sharpens their targets and groups them according to specialized application requirements that must be fulfilled together.Because not all requirements can be supported simultaneously, networks are expected to be real-time configurable across different KPI groups.
  • 6G Spectrum: Spectrum evolution combines sub-6 GHz coverage with higher-frequency bands for high-capacity links, extending through mmWave and THz toward visible light.Higher frequencies provide wider bandwidths but introduce unresolved hardware, waveform, impairment-mitigation, materials, and computational-complexity challenges.

3.1 Ultra-Massive MIMO and Holographic Radio

Ultra-massive MIMO seeks higher spatial resolution and multiplexing by using physically large, digitally controlled arrays, while holographic radio replaces discrete beam-space descriptions with spatially continuous electromagnetic processing. These approaches offer greater spatial dimensionality but introduce substantial implementation, modeling, computation, and algorithm-adaptation challenges.

  • Beamforming beyond the Beam-Space Paradigm: Conventional beam-space models based on predefined angular beams are restrictive for near-field channels, arbitrary array geometries, and imperfect hardware.The paper notes that these restrictions motivate channel estimation beyond three-dimensional far-field approximations.
  • Ultra-Massive MIMO: Ultra-massive MIMO uses physically large panels to focus signals narrowly and enable spatial multiplexing across many dimensions.The beamforming dimensionality equals the number of antennas, while beamwidth decreases as array aperture increases.
  • Continuous-Aperture Antennas: Continuous-aperture antennas can improve beamforming accuracy and reduce sidelobes, but dense active implementations incur high cost, energy use, or infeasible RF-feed complexity.Holographic radio is presented as a possible solution to the active continuous-aperture implementation problem.
  • Holographic Radio: Holographic radios use diffraction-based processing with many spatial dimensions, interference patterns rather than beams, and Fresnel-Kirchhoff modeling of spatial channel correlation.Accurate performance evaluation requires computational electromagnetics and computational holography.
  • Holographic Radio: Holographic radio can generate waveforms also achievable by appropriately shaped discrete arrays, but optical processing and massive data volumes create processing and algorithm-adaptation challenges.Machine learning and heterogeneous optoelectronic architectures may be needed for low-latency, reliable operation, while adapting physical-layer algorithms to optics remains difficult.

3.2 Intelligent Reflecting Surfaces

Intelligent reflecting surfaces are reconfigurable, low-complexity surfaces that shape incident waves to improve propagation, especially at high frequencies. Their benefits come with practical constraints involving physical size, beam squint, hardware resolution, broadband control, and channel estimation.

  • Implementation: IRSs can provide low-cost, low-complexity, and low-energy alternatives to active arrays because metasurfaces generally require no traditional RF chains.Metasurfaces use controllable metamaterials to manipulate electromagnetic properties.
  • IRS Operation: An IRS reconfigures sub-wavelength elements to control phase, amplitude, and polarization, forming beams or focusing reflected signals toward selected locations.Its reflection coefficients can be jointly optimized with transmitters and receivers for end-to-end spectral or energy efficiency.
  • Use-Case Boundary: The most promising IRS use case is short-range communication, particularly in sub-THz and THz bands, because practical surfaces must be physically large and face beam-squinting.An IRS reflects without amplification and generally adds no noticeable propagation delay, apart from possible channel delay-spread increases.
  • Capabilities: IRSs can increase signal strength, improve channel rank, suppress interference, and enhance multicasting in single-user and multi-user MIMO.The paper also identifies applications including cognitive radio, wireless power transfer, physical-layer security, and backscattering.
  • Open Challenges: Practical IRS deployment still requires study of hardware resolution, frequency-selective phase patterns, real-time control, and channel estimation without passive pilot transmission.The paper highlights performance losses from low-resolution hardware and the difficulty of joint active-passive beamforming optimization.

3.3 User-Centric and Scalable Cell-Free Networking

Cell-free massive MIMO distributes access points that jointly serve users to reduce cell-edge disparities and support more uniform performance. Its gains and added capabilities depend on dense deployment, coordination, fronthaul, and redesigned access procedures, creating scalability and deployment constraints.

  • Cell-Free Architecture: Cell-free massive MIMO combines distributed APs and joint service to target nearly uniform rates and seamless handover regardless of user position.It integrates elements of massive MIMO, small cells, user-centric network MIMO, and coordinated multipoint transmission and reception.
  • Distributed Deployment: Distributed APs place users near at least one strong access point, reducing pathloss while serving each UE with a reasonable number of good antennas.This differs from traditional massive MIMO, where users may connect to extremely large antenna arrays.
  • Performance: Cell-free massive MIMO has been shown to vastly outperform traditional small-cell and cellular massive MIMO for the majority of users, but it is not intended to increase peak broadband rates.Peak rates are described as achievable only in extreme cases.
  • Additional Capabilities: Many APs can support low-latency mission-critical applications through nearby content caching and distributed computing that offloads network-intensive tasks.The architecture also uses declining storage and computing costs to support these capabilities.
  • Initial Access: Cell-free networks require redesigned cell search and random access because existing procedures are tailored to cellular systems and affect latency, energy consumption, and supported-user capacity.NOMA-enabled two-step RACH and autonomous grant-free transmission are identified as candidate approaches.
  • Scalability and Deployment: Scalability is constrained by CSI exchange, coordination complexity, and the need for many APs with suitable fronthaul links.Consequently, the technology is mainly of interest for crowded areas with high traffic demand or robustness requirements.
  • Frequency Range: Cell-free architectures can operate from below 6 GHz through THz bands, with spatial diversity particularly useful where high-frequency links are easily blocked.At sub-THz and THz frequencies, APs may provide bandwidths of 100 GHz or higher over short distances and low mobility.

3.4 Integrated Access and Backhaul

Integrated access and backhaul uses wireless links to connect dense high-frequency AP deployments while simultaneously serving mobile devices. It is especially promising at upper mmWave and THz frequencies, where fiber deployment is difficult and expensive.

  • Motivation and Architecture: Dense mmWave-and-above deployments need wireless backhaul because reduced coverage and costly fiber rollout make connecting every AP challenging.IAB allows a few fiber-connected APs to provide wireless backhaul to other APs while those APs provide access to mobile devices.
  • Multi-Hop Operation: IAB differs from conventional relaying because each hop may carry aggregated traffic from multiple mobile devices, changing both information load and interference conditions.These varying loads complicate network design across hops.
  • 6G Role: IAB networks are expected to play a major role beyond 5G, particularly at upper mmWave and THz carrier frequencies.

3.5 Integrated Space and Terrestrial Networks

Integrated space and terrestrial networks combine ground, airborne, and satellite layers to extend coverage and support global wireless connectivity. Their design must balance high-speed links with satellite reliability and airborne-channel uncertainty.

  • Architecture: ISTN architectures integrate spaceborne, airborne, and ground-based networks to provide coverage beyond conventional terrestrial systems.Space networks complement terrestrial coverage over oceans, remote rural areas, and airspace.
  • Airborne layer: High-speed airborne backbones can use mmWave links because they provide wide bandwidth and rely on relatively mature technology.The airborne layer bridges spaceborne and ground-based networks.
  • Satellite constraints: Satellite-layer hardware and software must be well proven because satellites are costly and difficult to access after launch.Satellite processing and transmit power are limited by energy harvested from sunlight.
  • Open challenges: Reliable airborne communication requires aeronautical mmWave channel models that account for carrier frequency and propagation conditions.The paper identifies the current lack of such models as a significant research need.

3.6 Integrated Broadcast and Multicast Networks

6G broadcast and multicast can complement unicast when beamforming capacity is limited and can support convergence between mobile broadband and traditional broadcast networks. Wideband broadcasting is presented as one possible implementation.

  • Scalable delivery: Broadcast and multicast can deliver identical content simultaneously to many devices when unicast capacity or beamforming separation is insufficient.They support large-scale delivery with predefined quality of service.
  • Scalable delivery: 6G could dynamically select broadcasting or multicasting as cost-efficient delivery options when access points can only form wide beams.This extends content delivery across the available coverage area.
  • Network convergence: Mobile broadband and traditional broadcast networks could converge by combining adaptive-beamforming LPLT cellular systems with fixed-pattern HPHT broadcast coverage.A flexible distribution technology would span both network types.
  • Network convergence: Convergence may involve a shared radio physical layer, common transport protocols, or applications that adapt to underlying network capabilities.The approaches can encompass broadcast, multicast, and unicast delivery.
  • Wideband broadcasting: A 6G wideband broadcast system could reuse all RF channels at HPHT sites, unlike traditional reuse-7 broadcasting.The entire wideband signal requires half the transmission power of one traditional digital RF channel.

4 Enablers at the Spectrum Level

6G spectrum enablers extend from enhanced sub-100 GHz systems toward sub-THz, THz, and visible-light bands. These bands require distinct physical-layer choices because bandwidth, propagation, hardware impairments, and complexity trade-offs vary by scenario.

  • Beyond mmWave: 5G NR studies beyond 52.6 GHz must address rising phase noise and declining power-amplifier efficiency as frequencies approach the THz band.Waveforms and subcarrier spacing affect bandwidth, impairment robustness, and coverage.
  • Beyond mmWave: DFT-s-OFDM can scale to at least 90 GHz for lower-order modulations when PTRS and phase-noise estimation are optimized.Higher-order modulations are more difficult and may require increased subcarrier spacing.
  • THz communications: THz communication is envisioned to support data rates up to around 1 Tbps and networks with billions of interconnected devices.Device technologies for generating, modulating, detecting, and demodulating THz signals have historically limited feasibility.
  • Sub-THz systems: Sub-THz systems are expected to emphasize LOS-dominated propagation and simple single-carrier RF and power-amplifier architectures.The first sub-THz deployment step is positioned between 100 and 300 GHz, where technology is more mature.
  • Physical-layer paradigms: Physical-layer designs can prioritize spectral efficiency for high-rate backhaul or minimize complexity and energy for hotspots and short-range links.The high-rate paradigm uses IQ transceivers and high-order modulation, whereas the low-complexity paradigm uses simpler RF and modulation schemes.
  • THz communications: THz systems face distance limitations from spreading loss and low transmitter power, while IRSs and ultra-massive MIMO can support NLOS propagation and beamforming.Hybrid or analog beamforming may dominate before fully digital architectures become practical.
  • Optical wireless: VLC complements rather than replaces existing wireless technologies, with demonstrated laboratory data rates up to hundreds of Gbps.Practical VLC systems use intensity modulation and direct detection, creating positive-amplitude constraints and motivating hybrid optical-radio networks.

5 Enablers at the Protocol/Algorithmic Level

6G protocol and algorithmic enablers target higher throughput, reliability, flexibility, and resource efficiency across diverse use cases. The paper discusses coding, modulation, waveforms, full-duplex, interference management, and machine learning while emphasizing substantial implementation challenges.

  • Coding and waveforms: 6G infrastructure links may require hundreds of Gbps, making code design and encoding/decoding algorithms important alongside integrated-circuit progress.Spatial multiplexing increases aggregate throughput demands.
  • Coding and waveforms: 6G coding must support flexible codeword lengths and coding rates while improving error-floor and waterfall performance for high-reliability scenarios.Polar codes are identified as a possible choice, but serial list decoding does not scale efficiently with throughput.
  • Coding and waveforms: No single waveform can satisfy every 6G scenario because use cases impose different requirements on PAPR, coverage, frequency, reliability, and complexity.Modulation and waveform choices therefore need to be adapted to the application context.
  • Full-duplex: Full-duplex can potentially double spectral efficiency by allowing simultaneous transmission and reception in the same frequency band.Its 6G suitability remains open because hardware, circuit, theoretical, MIMO, and massive-MIMO challenges persist.
  • Interference management: NOMA and rate-splitting can supplement massive-MIMO interference management when antenna-panel spatial resolution is insufficient.Rate-splitting divides messages into private and common parts, while NOMA uses power- or code-domain non-orthogonality.
  • Interference management: Protocol-level interference management remains difficult because coupled coding, modulation, scheduling, and retransmission decisions increase complexity and can cause error propagation.These challenges are especially pronounced under fading, time-varying interference, and beamforming.
  • Machine learning: Machine learning can complement model-driven design when channel models are deficient or conventional algorithms are too complex for latency-critical joint optimization.The paper distinguishes modeling deficiencies from algorithmic deficiencies as two motivations for ML.

5.4 Coded Caching

Coded caching increases data rates by combining local and global caching gains through carefully designed multicast transmissions. Its use in immersive applications is promising, but practical deployment depends on multicast design, subpacketization, channel diversity, and coverage constraints.

  • Coded caching mechanism: Coded caching combines local caching gains with a global caching gain proportional to the total cache size across users.The global gain comes from multicasting codewords that contain useful data for every user in a target group.
  • Coded caching mechanism: Coded caching can add to spatial multiplexing gains in multi-antenna communications through multicast beamforming of parallel codewords.Inter-codeword interference is removed or suppressed using carefully designed beamforming vectors.
  • Application scenario: Immersive extended-reality applications are a potential coded-caching scenario because many users consume heavy multimedia traffic and location-dependent content can become highly popular.The scenario includes educational, industrial, gaming, defense, and social-networking applications using high-end eyewear.
  • Implementation challenges: Practical 6G implementation faces dependence on efficient multicasting, large subpacketization requirements, and poor performance for users with diverse channel conditions.The passage identifies multicast and broadcast strategy design as ongoing research.

6 Summary: New Concepts for 6G from a Broadband Connectivity Point of View

The paper summarizes a shift toward user-centric 6G broadband connectivity, combining wide-area coverage with short-range extreme-rate links. It highlights cell-free massive MIMO, integrated access and backhaul, IRSs, higher-frequency spectrum, and new signal-processing techniques as key directions.

  • Open problems: The summary presents these technologies as novel directions associated with open problems for future 6G broadband connectivity.Table 1 is identified as a summary of key open problems.
  • User-centric connectivity: 6G broadband connectivity is framed as a shift from network-centric deployment for exceptional peak rates toward user-centric support with consistently high rates.Cell-free massive MIMO, IAB, and IRSs are identified as complementary technologies for ubiquitous connectivity.
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