PaperlayerView exact revision on arXiv

Source-linked AI summary

Terahertz Band: The Last Piece of RF Spectrum Puzzle for Communication Systems

Hadeel Elayan, Osama Amin, Basem Shihada, Raed M. Shubair, Mohamed-Slim Alouini

arXiv:1907.05043v1eess.SPcs.IT

TL;DR

Rapid wireless-traffic growth motivates exploration of the relatively underprobed THz band for high-speed communication. The paper provides a comprehensive review of THz communication fundamentals, technologies, channels, applications, and deployment opportunities, reporting progress including 5.8 km 10 Gbps transmission and coherent sources above 1 THz. It concludes that continuing device advances and emerging THz systems could support applications such as virtual reality, HD streaming, and IoT automation.

  • Problem

    The THz band is relatively underexplored, while generating and detecting signals across 0.1–10 THz remains difficult because existing technologies have limited maturity.

  • Method

    The paper presents a comprehensive review of THz wireless communication fundamentals, including devices, channel models, comparisons, applications, and standardization.

  • Results

    5.8 km 10 Gbps transmission was achieved using 120 GHz signals, and quantum cascade lasers can generate over 10 mW of coherent average power above 1 THz.

  • Takeaways & Limitations

    Continuing progress in THz devices and systems supports prospective beyond-5G applications including virtual reality, HD streaming, and Internet-of-Things automation.

Abstract

from arXiv · show

Ultra-high bandwidth, negligible latency and seamless communication for devices and applications are envisioned as major milestones that will revolutionize the way by which societies create, distribute and consume information. The remarkable expansion of wireless data traffic that we are witnessing recently has advocated the investigation of suitable regimes in the radio spectrum to satisfy users' escalating requirements and allow the development and exploitation of both massive capacity and massive connectivity of heterogeneous infrastructures. To this end, the Terahertz (THz) frequency band (0.1-10 THz) has received noticeable attention in the research community as an ideal choice for scenarios involving high-speed transmission. Particularly, with the evolution of technologies and devices, advancements in THz communication is bridging the gap between the millimeter wave (mmW) and optical frequency ranges. Moreover, the IEEE 802.15 suite of standards has been issued to shape regulatory frameworks that will enable innovation and provide a complete solution that crosses between wired and wireless boundaries at 100 Gbps. Nonetheless, despite the expediting progress witnessed in THz wireless research, the THz band is still considered one of the least probed frequency bands. As such, in this work, we present an up-to-date review paper to analyze the fundamental elements and mechanisms associated with the THz system architecture.

I. INTRODUCTION

The THz band has emerged as a promising response to expanding wireless traffic and demand for higher capacity, while remaining comparatively underexplored. This review synthesizes its fundamental communication mechanisms, applications, technologies, and standardization activities.

  • Wireless data traffic is expanding rapidly, intensifying the need for higher data rates and bandwidth.
  • THz communication can theoretically provide capacities on the order of terabits per second because of its extensive available bandwidth.
  • Prior literature has addressed THz sources, sensors, applications, materials, channel modeling, signal generation, and wireless communication systems.
  • The paper reviews THz generation, channel models, comparisons with other technologies, applications, and standardization activities.

II. TERAHERTZ FREQUENCY GENERATION METHODS

THz generation is difficult because the band lies between microwave and infrared regimes where existing electronic and photonic technologies have limitations. The review organizes prior surveys and discusses recent progress toward more established THz technology.

  • The 0.1–10 THz range is called the THz Gap because signal-generation and detection technologies remain relatively immature.
  • Electron-transport devices are limited to about 300 GHz, while operation at higher frequencies becomes inefficient for semiconductor technologies.
  • The review discusses electronics, photonics, and other techniques for generating THz waves, alongside a timeline of technological progress.

A. Solid-State Electronics

Advances in semiconductor components and manufacturing are making THz systems increasingly feasible, affordable, and compact. Architectural innovations and new device structures have helped overcome earlier technology limitations.

  • Recent semiconductor and manufacturing advances are making THz systems feasible, affordable, and compact.
  • Architectural innovations and new device structures have helped overcome technology limitations in THz systems.
  • The resulting progress supports continued development of practical THz communication devices.

1) Complementary Metal-Oxide-Semiconductor (CMOS):

CMOS offers an integrated and potentially low-cost route toward the lower THz spectrum, while HBT and HEMT technologies provide stronger high-frequency and power performance. The section also describes device-scaling approaches and remaining power constraints.

  • Complementary Metal-Oxide-Semiconductor (CMOS): CMOS sources use VCOs or active multiplier chains to reach the lower THz spectrum while offering integration, small form factor, and potential low cost.
  • Complementary Metal-Oxide-Semiconductor (CMOS): 10 Gbps transmission over 2 m was demonstrated using the 120 GHz band.
  • Complementary Metal-Oxide-Semiconductor (CMOS): High-speed transistor development relies on gate scaling to reduce parasitics and epitaxial materials to improve electron transport.
  • Complementary Metal-Oxide-Semiconductor (CMOS): HBT and HEMT technologies have produced higher-frequency sources with higher output powers than CMOS, although CMOS remains attractive for cost and integration density.
  • Complementary Metal-Oxide-Semiconductor (CMOS): Drastic power reduction in solid-state electronics remains a major bottleneck despite ongoing technological progress.

3) Resonant Tunneling Diodes (RTD):

RTDs provide compact, controllable THz oscillators suitable for wireless transmission, while THz electronics continue to face a major power-decrement bottleneck and limited speed and bandwidth.

  • 3) Resonant Tunneling Diodes (RTD):: RTDs use resonant tunneling through semiconductor double-barrier quantum wells to exhibit wideband negative differential conductance.Their operation relies on electrons passing through resonant energy states.
  • 3) Resonant Tunneling Diodes (RTD):: Over the last decade, RTD output power increased by almost two orders of magnitude while operation frequencies extended from 0.7 THz to nearly 2 THz.
  • 3) Resonant Tunneling Diodes (RTD):: RTD output power can be modulated by bias voltage, and oscillations can be controlled electrically or optically for wireless data transmission.
  • 3) Resonant Tunneling Diodes (RTD):: 34 Gbps transmission was achieved with RTD oscillators, while frequency- and polarization-division multiplexing reached data rates up to 56 Gbps.
  • 3) Resonant Tunneling Diodes (RTD):: The major bottleneck for solid-state THz electronics is the drastic decrement in power, while electronic devices may also have limited speed and bandwidth for wideband high-speed measurements.

1) Unitravelling Carrier Photodiode (UTC-PD):

Photonics technologies support wideband, high-speed THz operation through optical-to-THz conversion, advanced photodiodes, QCLs, and emerging generation techniques, but practical systems require power, distance, and rate trade-offs.

  • 1) Unitravelling Carrier Photodiode (UTC-PD):: Photodiodes and related photonic components provide high-speed, high-saturation operation that supports large-capacity communication systems.Combining a high-saturation-power photodiode with an optical amplifier can extend bandwidth and simplify receiver configuration.
  • 1) Unitravelling Carrier Photodiode (UTC-PD):: UTC-PD operation exceeded 1 THz in 2003, and antenna-integrated UTC photomixers later exceeded 2 THz.
  • 1) Unitravelling Carrier Photodiode (UTC-PD):: A 160 Gbps wireless link was achieved in the 300–500 GHz band using a single UTC-PD-based transmitter.
  • 2) Quantum Cascade Lasers (QCLs):: QCLs bypass semiconductor band-gap limitations through heterostructure engineering and are the only sources capable of generating over 10 mW of coherent average power above 1 THz.
  • 2) Quantum Cascade Lasers (QCLs):: Selecting a feasible THz wireless scenario requires trading off power, propagation distance, and data rate across modulation and electronics–photonics configurations.

III. CHANNEL MODELING IN THE THZ FREQUENCY BAND

THz channel modeling must account for strong attenuation, distinctive reflection and scattering, spatially varying paths, and directive antennas. Outdoor models remain sparse and mainly address point-to-point links.

  • III. CHANNEL MODELING IN THE THZ FREQUENCY BAND: THz propagation is characterized by high attenuation, distinctive reflection and scattering, and both specular and non-specular path distributions.
  • A. Outdoor Channel Models: Outdoor channel models are scarce and currently address only point-to-point cases because few experimental measurements have been reported.
  • III. CHANNEL MODELING IN THE THZ FREQUENCY BAND: Existing channel-model classifications include outdoor, indoor, and nanoscale models, with deterministic and stochastic modeling approaches.
  • A. Outdoor Channel Models: A 5.8 km outdoor link achieved 10 Gbps, and QPSK increased the transmission rate to 22.2 Gbps after earlier 120 GHz experiments.
  • A. Outdoor Channel Models: Unintentional NLoS interference can limit BER performance, while atmospheric weather effects and frequency-selective attenuation constrain long-distance THz links.

B. Indoor Channel Models

Indoor THz channel models include deterministic, statistical, and hybrid approaches, each balancing propagation fidelity, statistical representation, and modeling efficiency.

  • Deterministic models: Ray-tracing models analyze indoor THz propagation using site-specific geometrical optics to capture wave transmission precisely.Their accuracy depends heavily on complete knowledge of material properties.
  • Statistical models: Statistical models represent large- and small-scale channel statistics, including path loss, shadowing, and multipath propagation, using empirical measurements.A first statistical model covered 275–325 GHz, while later geometrical statistical models addressed sub-THz D2D scattering and reflection.
  • Channel characteristics: Indoor THz modeling must account for distinctive reflection, scattering, and spatial path distributions, alongside frequent antenna misalignment from small-scale mobility.Highly directive antennas used to overcome path loss make beam alignment especially important.
  • Hybrid models: Hybrid channel models combine stochastic scatterer placement with deterministic ray tracing to balance accuracy and efficiency.These approaches establish geometry-based stochastic channel models for indoor propagation.
  • Measurements: Channel measurements and multipath models have also been examined at carrier frequencies of 350 GHz and 650 GHz in typical indoor environments.The reported models describe available paths together with their respective power levels.

IV. WILL THE TERAHERTZ BAND SURPASS ITS RIVALS ?

THz communication is presented as a bridge between bandwidth-limited mmW systems and weather- and alignment-sensitive optical links, while retaining high directionality and NLoS capability.

  • Millimeter wave versus Terahertz: 7–9 GHz of allocated mmW bandwidth can limit throughput, while 100 Gbps requires a challenging spectral efficiency of 14 bps/Hz.The passage contrasts this with THz links that could attain Tbps capacity using moderate spectral efficiencies of a few bits per second per Hz.
  • Millimeter wave versus Terahertz: THz links provide greater directionality than mmW at the same transmitter aperture, reducing transmitted power and interference between antennas.The shorter THz wavelength causes less free-space diffraction than mmW.
  • Infrared versus Terahertz: Infrared links can reach 10 Gbps and resist eavesdropping through wall confinement, but require wired interconnections between access points.Infrared transmissions cannot penetrate walls or other opaque barriers.
  • Infrared versus Terahertz: Atmospheric turbulence, scintillation, ambient light, and pointing, acquisition, and tracking requirements constrain infrared communication across indoor and outdoor environments.These effects can degrade link performance, availability, and reliability.
  • Infrared versus Terahertz: THz experiences lower fog attenuation than infrared, is almost unaffected by atmospheric turbulence and cloud dust, and avoids noise from ambient optical sources.At THz frequencies, thermal noise contributes to the total noise because photon energies are low.
  • Visible light versus Terahertz: VLC depends primarily on aligned LoS transmitter and receiver fields of view, and blockage or ambient-light interference can sharply reduce communication quality.Research also commonly addresses downlink traffic without specifying how the uplink operates.
  • Visible light versus Terahertz: THz NLoS propagation can supplement unavailable LoS paths using strategically placed dielectric mirrors with low reflection loss.For distances up to 1 meter and 1 Watt transmit power, the NLoS component’s capacity is reported at around 100 Gbps.

D. Ultra-Violet versus Terahertz

THz is positioned as a suitable competitor to UV communication because it supports short-range NLoS applications without UV’s stated health restrictions and modeling difficulties.

  • Ultra-violet communication: UV communication offers short-range NLoS capability, but ozone absorption limits LoS range and scattering, turbulence, and fading degrade long-range NLoS links.These effects can distort the received wavefront and deteriorate link quality.
  • Ultra-violet versus Terahertz: THz is considered a suitable contender to UV communication for anticipated applications.The comparison emphasizes THz’s non-ionizing nature and lack of associated health risks.
  • Ultra-violet versus Terahertz: THz is a non-ionizing band, so the passage states that no health risks are associated with these frequencies.It further states that THz data rates are not vulnerable to UV-related health constraints.
  • Application scope: THz applications span nano, micro, and macro scales, supported by Tbps data rates, reliable transmission, and minimal latency.The application ranges are illustrated in Fig. 8.
  • Nanoscale applications: At nanoscale, IoNT connects objects, sensors, and devices for device-to-device communication and data extraction from hard-to-access areas.The passage identifies typical nanoscale communication ranges of a few centimeters, including chip-to-chip, board-to-board, and device-to-device links.
  • Nanoscale applications: THz nanosensors, detectors, and cameras are described for security, environmental monitoring, food processing, and imaging applications.Examples include detecting weapons, explosives, chemical and biological agents, and pollutants.

B. Terahertz Microscale Applications

THz microscale applications target high-rate local connectivity, while broader standardization efforts address 100 Gbps links and related wireless networking deployments.

  • Terahertz Microscale Applications: THz WLAN and WPAN applications include HDTV distribution, wireless displays, seamless file transfer, and THz access points.These applications are aimed at meeting demands for higher data rates at the microscale.
  • Terahertz Microscale Applications: A demonstrated THz link streamed and recorded uncompressed HD and 4K video, with BER below the FEC limit of 10^-3 at 175 cm.The passage also reports 8K video trial experiments using proprietary devices.
  • Terahertz Macroscale Applications: At macroscale, THz wireless communication is envisioned for outdoor links spanning a few meters to kilometers, including 100 Gbps backhauling and fronthauling.Point-to-point backhaul links can serve base stations where optical fiber is unavailable.
  • Terahertz Macroscale Applications: Wireless data centers are considered a promising macroscale application as cloud services increase demand for servers and bandwidth.Wireless networking is described as adaptable for traffic bursts and finite network interfaces.
  • Standardization: IEEE 802.15 formed a THz Interest Group in 2008 to investigate operation in frequency bands up to 3000 GHz and coordinate with external organizations.Subsequent work covered channel models, technology trends, modulation, infrastructure, and MAC requirements.
  • Standardization: The 2014 TG3d task group targeted 100 Gbps switched point-to-point links for wireless data centers, backhauling, fronthauling, kiosk downloading, and D2D communication.IEEE also contacted the ITU about allocating 275–325 GHz for mobile and fixed services.

VII. FUTURE RESEARCH DIRECTIONS

Future THz research focuses on enabling high-rate, low-latency links despite severe atmospheric loss, while supporting cellular, vehicular, and heterogeneous-network applications.

  • High-gain directional antennas are needed because atmospheric losses limit THz communication distances to more than a few meters.Smaller THz antennas permit more elements within the same footprint.
  • 1024 × 1024 UM-MIMO systems at 0.3 THz and 1 THz can achieve multi-Tbps links over distances up to 20 m.The result depends on the plasmonic nanoantenna prospects and THz channel characteristics.
  • THz cellular networks target high-capacity, dense-coverage support for interactive high-dynamic-range, high-resolution, high-framerate, and 6DoF video.Such video can require 10 times the bit-rate of 4K video; reported rates reach up to 16.4 Gbps with a 30 ms delay threshold.
  • THz HetNets can improve transmission rate and capacity toward Tbps throughput by using femtocells to shorten links and increase frequency reuse.The proposed deployment includes in-home services, automation, metro stations, shopping malls, and traffic lights.
  • AI-based network frameworks are proposed to improve energy efficiency by learning from big data for base-station operation, proactive caching, and interference-aware resource allocation.

D. Terahertz 3D Beamforming Technology

THz 3D beamforming and related deployment strategies address path loss, interference, mobility, and high-bandwidth communication requirements across urban and vehicular environments.

  • D. Terahertz 3D Beamforming Technology: 3D beamforming constructs directional beams, extends communication range, and lowers interference to mitigate THz channel path loss.It is motivated by real-world three-dimensional channels for which 2D MIMO can be suboptimal.
  • D. Terahertz 3D Beamforming Technology: Active beam steering enables tunable transmitter and receiver modules for THz imaging and sensing.
  • D. Terahertz 3D Beamforming Technology: Per-beam time- and frequency-synchronization reduces channel delay spread, Doppler frequency spread, and overall system overhead in simulations.The technique is designed for severe Doppler effects in mmW/THz massive MIMO systems.
  • E. Terahertz Communication for Urban Environments: THz links are presented as an alternative for urban wireless access, where multiple backup links can help avoid outages associated with mmW mesh networks.The cited obstacles for mmW include future crowding and rain attenuation.

2) Unmanned autonomous vehicles (UAVs):

THz communication is presented as a high-capacity option for UAV applications, including backhaul and secure exchange of safety-critical information, while high-frequency eavesdropping remains possible.

  • UAV applications include weather monitoring, forest-fire detection, traffic control, cargo transport, emergency search and rescue, and communication relaying.These applications require reliable communication links that remain accessible.
  • THz can support high-capacity UAV-UAV wireless backhaul and links between dynamic locations in high-mobility environments.The passage contrasts THz with free-space optical communication and notes reduced Doppler effects at higher carrier frequencies.
  • THz links can exchange safety-critical information between UAVs and ground control stations before autonomous or remotely controlled missions.The large channel bandwidth supports protection measures against jamming and can hide information exchange.
  • High-frequency narrow-beam line-of-sight transmissions can still be intercepted when an object scatters radiation toward an eavesdropper.
  • The paper surveys THz devices, channel models, applications, generation techniques, propagation phenomena, and comparisons with existing technologies.
Loading 1907.05043v1…