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Millimeter-wave and Terahertz Spectrum for 6G Wireless

Shuchi Tripathi, Nithin V. Sabu, Abhishek K. Gupta, Harpreet S. Dhillon

arXiv:2102.10267v1cs.IT

TL;DR

Emerging 6G applications may require hundreds of gigabits per second to several terabits per second with low latency and high reliability, motivating mmWave and THz communications. The chapter unifies propagation, channel modeling, design, applications, and standardization considerations for these bands. It concludes that their potential is substantial, but commercial deployment still faces major propagation and design challenges.

  • Problem

    Emerging applications may exceed the gigabit-per-second capabilities of 5G, requiring hundreds of gigabits per second to several terabits per second with low latency and high reliability.

  • Method

    The chapter provides a unified treatment of mmWave and THz propagation, channel models, design and implementation considerations, applications, and standardization.

  • Results

    The chapter identifies mmWave and THz bands as promising for high-rate 6G communications while documenting their propagation characteristics and deployment challenges.

  • Takeaways & Limitations

    mmWave and THz communications offer a route toward 6G applications demanding extreme data rates, but require designs that address their unfavorable propagation and implementation conditions.

Abstract

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With the standardization of 5G, commercial millimeter wave (mmWave) communications has become a reality despite all the concerns about the unfavorable propagation characteristics of these frequencies. Even though the 5G systems are still being rolled out, it is argued that their gigabits per second rates may fall short in supporting many emerging applications, such as 3D gaming and extended reality. Such applications will require several hundreds of gigabits per second to several terabits per second data rates with low latency and high reliability, which are expected to be the design goals of the next generation 6G communications systems. Given the potential of terahertz (THz) communications systems to provide such data rates over short distances, they are widely regarded to be the next frontier for the wireless communications research. The primary goal of this chapter is to equip readers with sufficient background about the mmWave and THz bands so that they are able to both appreciate the necessity of using these bands for commercial communications in the current wireless landscape and to reason the key design considerations for the communications systems operating in these bands. Towards this goal, this chapter provides a unified treatment of these bands with particular emphasis on their propagation characteristics, channel models, design and implementation considerations, and potential applications to 6G wireless. A brief summary of the current standardization activities related to the use of these bands for commercial communications applications is also provided.

I. Background and Motivation

The chapter motivates mmWave and THz communications as responses to spectrum and data-rate demands that emerging applications place beyond conventional cellular systems. It presents a unified treatment of propagation, channel models, design considerations, applications, and standardization.

  • I. Background and Motivation: 5G mmWave communication became commercially feasible despite unfavorable propagation, because modern antenna and device technologies enabled use of these bands.
  • I. Background and Motivation: Emerging applications such as extended reality, 3D gaming, and ultra-HD conferencing may require hundreds of gigabits per second to several terabits per second with low latency and high reliability.
  • I. Background and Motivation: The chapter systematically connects frequency-dependent propagation characteristics to channel models, system design considerations, 6G applications, and commercial standardization.
  • I. Background and Motivation: mmWave offers roughly 50–100 times more available spectrum than sub-6 GHz, while THz bands provide tens of gigahertz of bandwidth for Tbps-rate links.
  • I. Background and Motivation: Although these bands offer large bandwidths and potential interference reduction, poor propagation, blockage, penetration, scattering, coverage, and directionality remain deployment challenges.

III. Propagation at the mmWave and THz Frequencies

mmWave and THz propagation differs sharply from sub-6 GHz communication because atmospheric absorption, blockage, penetration, scattering, and directionality strongly shape link behavior. These effects motivate specialized channel characterization and system designs.

  • III. Propagation at the mmWave and THz Frequencies: A single blockage can cause 20–40 dB loss, making mmWave and THz links highly dependent on line-of-sight paths.
  • III. Propagation at the mmWave and THz Frequencies: Large antenna arrays provide high beamforming gain and reduced interference, but narrow beams create deafness and beam-training latency, especially under mobility.
  • III. Propagation at the mmWave and THz Frequencies: Propagation studies measure path loss, spatial, angular, and temporal characteristics, ray mechanisms, penetration, and weather-related attenuation across indoor and outdoor settings.
  • III. Propagation at the mmWave and THz Frequencies: Atmospheric absorption increases at higher frequencies because molecular vibration converts propagating-wave energy into kinetic energy, with oxygen and water vapor as major absorbers.
  • III. Propagation at the mmWave and THz Frequencies: Absorption transmission decays exponentially with distance as τ(r, f) = exp(−κ_a(f)r), where κ_a(f) aggregates gas-specific absorption coefficients.

2) Rainfall Attenuation:

Rainfall attenuation increases with precipitation intensity and carrier frequency, but its impact can be reduced through short-range links and lower mmWave bands.

  • 2) Rainfall Attenuation:: 42 dB/km is the maximum attenuation reported for a 150 mm/hr monsoon downpour at frequencies over 60 GHz.Light rain at 2 mm/hr imposes 2.55 dB/km, while heavy rain at 50 mm/hr imposes 20 dB/km.
  • 2) Rainfall Attenuation:: Foliage attenuation increases across the cited carrier frequencies, reaching 17 dB at 28 GHz, 22 dB at 60 GHz, and 25 dB at 90 GHz.The severity depends on carrier frequency and vegetation depth.
  • 2) Rainfall Attenuation:: Material penetration losses constrain coverage, with 24.4 dB through two walls and 45.1 dB through four doors reported at 28 GHz.These losses limit indoor-to-outdoor and outdoor-to-indoor coverage.

4) Human Shadowing and Self Blockage:

Human and structural blockages make mmWave and THz links strongly dependent on line-of-sight propagation, while surface roughness determines whether energy reflects or scatters.

  • 4) Human Shadowing and Self Blockage:: Human blockages are modeled with Boolean or Poisson-process geometries, while self-blockage is represented by a user-centered cone of blocked base stations.The models include 3D human cylinders, indoor 2D circles, and a cone whose angle depends on equipment width and user distance.
  • 4) Human Shadowing and Self Blockage:: As wavelength decreases, surfaces can transition from smooth to rough when their height variations exceed the Rayleigh critical height.The critical height depends on incident angle and wavelength; h_0 < h_c indicates smoothness, whereas h_0 > h_c indicates roughness.
  • 4) Human Shadowing and Self Blockage:: At THz frequencies, surface roughness makes scattering significant, whereas reflections remain more prominent in lower mmWave bands.Rough surfaces also introduce additional loss into any reflected wave through a scattering loss factor.
  • 4) Human Shadowing and Self Blockage:: The directive scattering model steers the main scattering lobe toward the specular reflection direction and characterizes scattered power by lobe width.It was applied to 60 GHz propagation in a hospital room and validated against rural and suburban building measurements at 1.29 GHz.

6) Diffraction:

Diffraction is weak at mmWave and THz frequencies because of their short wavelengths, leaving non-line-of-sight paths much weaker than line-of-sight paths.

  • 6) Diffraction:: NLOS paths have significantly less power than LOS paths at mmWave and THz frequencies because diffraction is less prominent than at microwave frequencies.Diffraction may nevertheless support THz links in the shadow of objects.

7) Doppler Spread:

Higher carrier frequencies produce substantially larger Doppler spread, while atmospheric absorption and scintillation introduce additional signal impairments whose THz effects remain incompletely understood.

  • 7) Doppler Spread:: Doppler spread at 30 GHz and 60 GHz is respectively 10 and 20 times higher than at 3 GHz.The passage attributes this increase to Doppler spread being directly proportional to frequency and user speed.
  • 7) Doppler Spread:: Molecular absorption adds absorption noise because excited atmospheric molecules emit electromagnetic radiation at the incident frequency.This noise is generally represented through an equivalent noise temperature of the surroundings.
  • 7) Doppler Spread:: Scintillation creates rapid phase and amplitude fluctuations through local refractive-index changes caused by temperature, pressure, or humidity variations.Its impact on practical THz communication is smaller than for infrared beams, but the influence of atmospheric turbulence near Earth’s surface remains insufficiently understood.

1) Analog Beamforming Patterns:

Analog beamforming analyses use tractable antenna-pattern approximations, ranging from sinc and sectorized models to multi-lobe, Gaussian, and cosine patterns. These models trade fidelity to actual gain variation against analytical simplicity and robustness to misalignment.

  • 1) Analog Beamforming Patterns:: Analog beamforming evaluations use antenna patterns to compute effective received-signal gain from transmitter and receiver directional responses.
  • 1) Analog Beamforming Patterns:: The sinc model approximates array gain with a squared sinc function when element spacing is generally set to half the wavelength to avoid grating lobes.
  • 1) Analog Beamforming Patterns:: The sectorized model replaces continuous array gains with fixed main-lobe and side-lobe gains within and outside the half-power beam-width.Its side-lobe gain should not be ignored in highly dense networks because aggregated side-lobe interference can be significant.
  • 1) Analog Beamforming Patterns:: Multi-lobe, Gaussian, and cosine models extend pattern flexibility, with Gaussian patterns capturing roll-off associated with perturbations and transmitter–receiver misalignment.The multi-lobe model uses constant-gain lobes fitted by minimizing error against the actual pattern, but lacks roll-off.

Cosine antenna model:

The antenna-model discussion extends beyond basic patterns to flexible multi-lobe and hybrid-beamforming settings, while THz systems rely on compact, highly directive arrays and face stringent hardware constraints. Channel modeling is essential because THz propagation differs substantially from lower-frequency bands.

  • Cosine antenna model:: Hybrid beamforming produces an effective pattern formed from individual analog beam patterns for each stream or user in multi-stream or multi-user transmission.
  • Cosine antenna model:: THz systems can use ultramassive MIMO arrays to form high-gain pencil beams that partially extend their limited transmission range.The number of antennas fitting in a footprint increases with the square of the wavelength.
  • Cosine antenna model:: Analog beamforming reduces the number of THz RF phase shifters, but digitally controlled quantized phase values constrain practical performance.
  • Cosine antenna model:: THz communications use antenna options including photoconductive, horn, lens, microstrip, and on-chip antennas, with lens-fed and layered-substrate designs addressing radiation-pattern and efficiency issues.
  • Cosine antenna model:: Accurate THz channel and noise models are necessary because molecular absorption makes even free-space propagation nontrivial and differs sharply from lower-frequency channels.Existing work includes ray-tracing and statistical models spanning indoor, outdoor, device-to-device, intra-body, and chip-to-chip scenarios.

1) mmWave Channel:

The mmWave channel model represents received power for LOS or NLOS links through path loss, transmit power, antenna gains, and small-scale fading. Extensions incorporate THz-specific absorption, reflections, scattering, multiple paths, and wideband operation.

  • 1) mmWave Channel:: The analytically tractable mmWave channel model supports system-level and stochastic-geometry analysis for LOS and NLOS links under narrowband analog beamforming.
  • 1) mmWave Channel:: Received power combines path loss, transmit power, transmitter and receiver antenna gains, and small-scale fading, with Nakagami fading parameters differing between LOS and NLOS links.
  • 1) mmWave Channel:: The THz channel adds molecular-absorption loss to free-space path loss, reflecting prominent atmospheric attenuation and scattering and the large LOS–NLOS disparity.
  • 1) mmWave Channel:: The model can include reflected and scattered paths through coefficients dependent on surface orientation and properties, and can extend to multiple paths and wideband communication.
  • 1) mmWave Channel:: mmWave’s abundant spectrum enables multi-gigabit communication, but blockage and foliage losses make highly directional transmission necessary and complicate beam alignment.Directional search creates delay and overhead, with mobility increasing handover occurrences.

A. Key System Design Implications

mmWave system design addresses limited range, interference, mobility overhead, and deployment cost through heterogeneous networks, spectrum sharing, dense deployments, learning-based beamforming, and short-range or wireless-backhaul applications.

  • A. Key System Design Implications: Because mmWave coverage is limited, deployments may use sub-6 GHz control-plane transmissions for load balancing and handovers while mmWave carries data.
  • A. Key System Design Implications: Spectrum sharing can improve mmWave utilization because noise-limited operation reduces the need for sophisticated inter-cell coordination.
  • A. Key System Design Implications: Directional transmission and blockage sensitivity make mmWave suitable for ultra-dense local networks by limiting interference, while self-backhauling can connect densely deployed access points.
  • A. Key System Design Implications: High mobility creates severe beam-training overhead because large-array beamforming vectors require frequent updates, motivating deep-learning beam prediction from pilot-derived environmental signatures.After successful learning, the models are described as requiring negligible training overhead for reliable coverage and low latency.
  • A. Key System Design Implications: mmWave applications include unlicensed 60 GHz WLANs and WPANs, wireless backhaul for dense small cells, and information showers delivering multi-gigabit data over about 10 m.

4) Aerial communications:

Aerial and vehicular communications are emerging mmWave/THz applications, while THz systems face especially short range, molecular absorption, wideband hardware, and beam-alignment challenges.

  • High likelihood of line-of-sight links and high data-rate requirements motivate mmWave and THz spectrum use for vehicular communications.Vehicle communications support autonomous navigation and accident avoidance through alerts and route guidance.
  • THz links may cover only about 10 m in small cells because propagation and molecular absorption losses are high.
  • THz transceivers require wideband components because conventional oscillators and optical photon emitters do not directly cover the operating range.This frequency-generation difficulty is known as the THz gap, and ultra-wideband antennas and amplifiers are also challenging.
  • Beamforming is needed to overcome THz propagation losses, but large arrays make beam switching and codebook design computationally complex.High-resolution beams improve angle estimation, creating a design trade-off between complexity and beam precision.
  • THz communications are especially suited to indoor applications because their limited coverage area favors short-range deployment.Indoor links can remain robust with one or two non-line-of-sight reflection components.

2) Micro/nanoscale THz Communication:

THz communication supports microscale and nanoscale networking, including nanomachine links for sensing and actuation and hybrid molecular–electromagnetic systems connecting biological and artificial components.

  • 2) Micro/nanoscale THz Communication:: THz links can connect nanomachines over distances ranging from a few micrometers to a few meters.Nanomachines support simple computation, data storage, actuation, and sensing, while networks of them can perform more complex tasks.
  • 2) Micro/nanoscale THz Communication:: Nanomachine communication supports health monitoring, defense sensing, Internet of nano-things, Internet of bio-nano-things, and wireless network-on-chip communication.
  • 2) Micro/nanoscale THz Communication:: A nano-scale communication network contains message-carrier, motion, field, perturbation, and specificity components.Perturbation represents transmitted information analogously to modulation, while specificity describes reception at the target.
  • 2) Micro/nanoscale THz Communication:: A hybrid network can combine molecular communication inside the body with electromagnetic communication through an implanted graphene-based nanodevice.The molecular stage benefits from biocompatibility, energy efficiency, and infrastructure-free diffusion-based propagation.

VI. Standardization Efforts

Standardization has progressed from commercial mmWave and 5G specifications to early THz standards for fixed links and nano-network practices.

  • VI. Standardization Efforts: IEEE 802.11ad supports 60 GHz multi-gigabit wireless applications with approximately 14 GHz of unlicensed bandwidth and rates up to 8 Gbps.
  • VI. Standardization Efforts: The enhancement of IEEE 802.11ad supports fixed point-to-point and point-to-multipoint mmWave links up to 100 Gbps through channel bonding and aggregation.
  • VI. Standardization Efforts: 3GPP Release 15 established the first 5G NR standards for eMBB, URLLC, and mMTC, initially targeting non-standalone operation with 4G LTE.
  • VI. Standardization Efforts: IEEE 802.15.3d-2017 is the first THz standard for fixed point-to-point links at 252–321 GHz, with channel bandwidths up to 69.12 GHz.THz standardization remains nascent, while IEEE P1906.1 addresses recommended practices for nano-networks.
  • VI. Standardization Efforts: The chapter frames THz research as a response to 6G applications requiring hundreds of gigabits per second to several terabits per second over short distances.
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