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TeraHertz Band Communication: An Old Problem Revisited and Research Directions for the Next Decade

Ian F. Akyildiz, Chong Han, Zhifeng Hu, Shuai Nie, Josep M. Jornet

arXiv:2112.13187v3eess.SP

TL;DR

THz communications require coordinated progress across devices, channels, communication, networking, and experimental systems to become practical for 6G and Beyond. This paper surveys developments over more than a decade, revisits unresolved problems, and identifies future research directions. It reports substantial progress in THz technology while emphasizing that important hardware, channel, and system challenges remain.

  • Problem

    THz communication must overcome unresolved device, propagation, spectrum, and system challenges before its broad 6G applications can be fully realized.

  • Method

    The paper surveys THz devices, channels, communication techniques, networking, testbeds, experimental progress, and open research directions.

  • Results

    The paper concludes that the THz technology gap is almost filled, while meaningful challenges remain in analog RF, digital hardware, integrated circuits, and higher-frequency channel studies.

  • Takeaways & Limitations

    THz communications remain a candidate infrastructure for 6G applications ranging from Tbps networks to nanoscale and integrated sensing-and-communication systems.

Abstract

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Terahertz (THz) band communications are envisioned as a key technology for 6G and Beyond. As a fundamental wireless infrastructure, THz communication can boost abundant promising applications. In 2014, our team published two comprehensive roadmaps for the development and progress of THz communication networks [1], [2], which helped the research community to start research on this subject afterwards. The topic of THz communications became very important and appealing to the research community due to 6G wireless systems design and development in recent years. Many papers are getting published covering different aspects of wireless systems using the THz band. With this paper, our aim is looking back to the last decade and revisiting the old problems and pointing out what has been achieved in the research community so far. Furthermore, in this paper, open challenges and new research directions still to be investigated for the THz band communication systems are presented, by covering diverse topics ranging from devices, channel behavior, communication and networking, to physical testbeds and demonstration systems. The key aspects presented in this paper will enable THz communications as a pillar of 6G and Beyond wireless systems in the next decade.

I. INTRODUCTION

THz communications are positioned as a 6G technology because their broad spectrum can support demanding performance objectives and diverse applications. This paper reviews a decade of progress across devices, channels, communication, networking, experiments, and remaining challenges.

  • Motivation: 6G objectives include 1 Tbps peak data rate, 60 bps/Hz peak spectral efficiency, 10^−9 packet error rate, and 0.1 ms latency.The passage also states that energy efficiency is expected to improve 100 times relative to 5G.
  • Applications: THz communications address spectrum scarcity through multi-GHz bandwidth and support applications including Tera-WiFi, Tera-IoT, Tera-IAB, and Tera-SpaceCom.The paper also identifies nanoscale applications such as WiNoC and IoNT.
  • Scope and contribution: The paper revisits THz communication problems and achievements from the preceding decade while identifying open challenges and research directions for the next decade.It follows two 2014 roadmap papers that helped stimulate research in this area.
  • Scope and contribution: Its review spans THz device pathways, channel measurement and modeling, physical-layer technologies, higher-layer networking, experimental testbeds, and future directions.Covered physical-layer topics include modulation, coding, beamforming, beam tracking, synchronization, localization, and security; networking topics include MAC, interference, coverage, routing, and scheduling.
  • Challenges: THz systems face a limited communication distance caused by propagation loss, molecular absorption, scattering, LoS blockage, limited transmit power, and reduced receiver sensitivity.The paper treats the distance problem as a central challenge arising from both channel physics and device limitations.

II. RELATED WORK

The paper reviews the expanding THz communications literature across hardware, channels, communication techniques, networking, and experimental or simulation testbeds. It also identifies the THz technology gap as substantially narrowed while organizing hardware systems around analog front-ends, antenna systems, and digital back-ends.

  • Research scope: The literature covers five main aspects: hardware, THz channels, communication techniques, networking, and experimental and simulation testbeds.
  • Research scope: Research on THz communications has expanded substantially, motivating a holistic review of classical and novel research directions.The literature includes technical papers, tutorials, and reviews, with surveying activity rising markedly after 2017.
  • Technology progress: The THz technology gap has significantly narrowed over the last decade through advances in semiconductor technologies and new materials.Historically, compact energy-efficient high-power transmitters and low-noise high-sensitivity receivers limited practical THz communications.
  • Hardware building blocks: A THz wireless system comprises analog front-ends, antenna systems, and digital back-ends, each with distinct functional roles and performance metrics.Front-ends generate, process, detect, and recover signals; antennas couple on-chip signals to free-space radiation; digital back-ends interface computing devices with the wireless channel.
  • Hardware building blocks: THz systems require operation in frequency windows with modulation bandwidths exceeding 10 GHz and link budgets supported by transmission power, antenna gain, and receiver sensitivity.

B. Technology Pathways to Terahertz Front-ends

THz front-ends follow electronic, photonic, or plasmonic technology pathways, each trading off power, integration, spectral properties, maturity, and bandwidth. Electronic frequency-multiplying chains are expected to underpin early commercial systems, while photonic and plasmonic approaches offer distinct longer-term advantages and constraints.

  • Technology pathways: THz systems use three main technology pathways: electronic, photonic, and plasmonic approaches.
  • Electronic approach: Electronic systems extend microwave and millimeter-wave technologies, using devices such as RTDs, IMPATT diodes, TWTs, and frequency-multiplying chains.Frequency-multiplying chains concatenate doublers and/or triplers, generally followed by a broadband mixer in a heterodyne architecture.
  • Electronic approach: Silicon CMOS and SiGe BiCMOS provide high integration, but their maximum transmission power is up to a few milliwatts per element at frequencies up to a few hundred GHz.
  • Electronic approach: III-V semiconductors have demonstrated hundreds of milliwatts at a few hundred GHz and up to a few milliwatts above 1 THz.Electronic arrays are also being explored for higher transmission power and mobile THz networking, including experimentally demonstrated 8-channel transceiver arrays at 140 GHz.
  • Electronic approach: As of 2022, initial commercial THz systems were expected in the 110–300 GHz lower G-band, likely using frequency-multiplying chains.
  • Photonic approach: Photonic THz generation includes frequency-difference generation and photoconductive antennas, while a DFG receiver concept uses an on-chip local oscillator for down-conversion to an electronic intermediate frequency.
  • Photonic approach: Photonic systems offer higher spectral purity, lower amplitude and phase noise, and high bandwidth, but their transmission power is at least one order of magnitude lower than electronic systems.Photonic arrays had not been experimentally demonstrated, and fiber-optical-to-wireless THz backhaul was identified as a potential application.

3) Plasmonic Approach:

The plasmonic approach uses plasma-wave properties and materials such as graphene to create intrinsically THz devices. It offers compactness, broadband operation, and tunability, but remains less mature than electronic and photonic approaches and faces material-integration challenges.

  • Operating principle: Plasmonic devices operate intrinsically at THz frequencies without up-conversion from microwave frequencies or down-conversion from optics.The approach leverages plasma waves and graphene-based surface plasmon polariton waves to generate, radiate, detect, and modulate THz signals.
  • Capabilities: Plasmonic devices naturally operate above hundreds of GHz and at a few THz, with bandwidths exceeding 10% of their carrier frequency.
  • Capabilities: Their dimensions are typically hundreds of nanometers or a few micrometers, enabled by plasmonic confinement below the THz wavelength.
  • Maturity and challenges: The plasmonic approach remains at an early stage compared with electronic and photonic approaches refined over decades.Graphene-based properties can degrade when graphene contacts conventional materials, motivating heterogeneous structures that remain mainly at the material or single-device level.
  • Maturity and challenges: The approach is less mature and more risky but offers potential rewards through true THz operation, ultrabroadband bandwidths, and high reconfigurability and tunability.

C. Intelligent Antenna Systems in Transmission, Reception and Reflection

THz antenna systems span conventional, plasmonic, lens-integrated, metasurface, and beamforming architectures for transmission, reception, and reflection. Their small wavelengths enable compact antennas, but small effective area creates spreading-loss and directionality trade-offs.

  • THz antenna systems combine conventional antennas with electronic or photonic front-ends, plasmonic antennas with plasmonic front-ends, and lenses with any antenna type.
  • A 1 THz graphene-based plasmonic nano-patch antenna can be 1 µm long and 10 nm wide, enabling wireless networks-on-chip, wearable, and implantable devices.
  • Very small antennas have high spreading losses because their effective area is small; increasing area produces highly directional radiation.
  • Directional systems include fixed antennas such as horns and reflectors, and fixed or dynamic beamforming arrays using metals or nanomaterials.
  • Metamaterials and metasurfaces can provide programmable lenses and engineered electromagnetic properties at THz frequencies.
  • THz RISs can control specular and non-specular reflections, but reconfigurable elements remain challenging because graphene is tunable yet less reflective than metals.
  • Orbital angular momentum modes can increase THz system capacity without increasing system bandwidth.

E. Open Challenges

Open THz challenges span hardware integration, AI-driven reconfigurability, extreme-environment operation, and channel measurement. Progress requires coordinating diverse technologies and validating models with measurements.

  • AI-driven Smart Hardware: AI-orchestrated reconfigurable hardware is needed to support dynamic THz communication and networking solutions.Reconfigurability includes tunable signal generators, filters, amplifiers, modulators, and antenna components.
  • Heterogeneous integration and fabrication: Heterogeneous integration must combine materials, co-design interacting blocks, and establish scalable, cost-effective fabrication processes.Candidate technologies include silicon CMOS, III-V semiconductors, graphene, photonics, electronics, and plasmonics.
  • Terahertz devices for extreme environments: THz devices must accommodate operation from nanoscale intra-body and wearable networks to inter-satellite links in space.Different environments impose different operating conditions, including biocompatibility inside the body and space qualification for satellites.
  • Channel measurement systems must provide wideband characteristics, long-distance sounding, and efficient spatial scanning with highly directive antennas.The paper presents VNA, sliding-correlation, and THz-TDS techniques as the three main sounding approaches.

1) Frequency-domain Vector-Network-Analyzer-based Method:

THz channel measurement uses frequency-domain, correlation-based, and direct-pulse techniques, each balancing power, noise, bandwidth, distance, sampling demands, and measurement speed. Measurements support both model validation and empirical channel characterization.

  • Frequency-domain Vector-Network-Analyzer-based Method: VNA-based measurement characterizes wireless-channel frequency response but suffers from low output power, high noise, limited distance, and long measurement times.The approach has supported 140 GHz indoor and 220 GHz line-of-sight and non-line-of-sight measurements.
  • Time-domain Sliding Correlation Method: Correlation-based sounding transmits a signal with Dirac-delta autocorrelation, enabling high transmit power, low PAPR, and real-time measurement.Its drawbacks include uneven power spectral density and the need for ultra-high-speed DACs and ADCs; sliding correlation can increase SNR by extending measurement duration.
  • Direct Pulse Method: THz-TDS sends narrow time-domain pulses separated by more than the maximum excess delay, allowing direct extraction of the channel impulse response.It suits ultra-wideband THz waves but has low power, large equipment size, short distance, and limited reflection, scattering, and diffraction applications.
  • Channel measurements validate physics-driven models and provide empirical data for channel characterization.THz channel characterization can be deterministic, stochastic, or hybrid.
  • Deterministic Channel Modeling: Deterministic modeling uses electromagnetic theory and geometric information, while ray tracing captures reflected, diffracted, scattered, and line-of-sight paths.Ray tracing preserves reasonable computational complexity in very large systems.

2) Statistical Channel Modeling:

Statistical and hybrid THz channel models address the band’s spatial sparsity and the trade-off between deterministic accuracy and computational burden. Hybrid approaches combine detailed local modeling with more efficient statistical or ray-based treatment.

  • Statistical Channel Modeling: Statistical models fit distributions for directions of departure and arrival, time of arrival, and complex amplitudes, or describe channel impulse responses mathematically.The Saleh-Valenzuela tapped-delay-line model can represent multipath clusters, including at 300 GHz.
  • Statistical Channel Modeling: Independent Rayleigh fading is unsuitable for sparse THz channels, while the Kronecker model can produce high bias in ultra-massive MIMO channels.
  • Hybrid Channel Modeling: Hybrid modeling is attractive because deterministic methods provide accuracy while statistical methods reduce computational burden.The paper frames hybridization as a way to resolve complex THz channel models under low latency.
  • Hybrid Channel Modeling: Combining FDTD near scatterers with ray tracing elsewhere can provide accurate modeling with improved time efficiency.The FDTD component handles high-accuracy local modeling, while ray tracing models the remaining wireless channel.
  • Channel and Propagation Characteristics: THz propagation includes spreading loss, molecular absorption, frequency-dependent spectral windows, temporal broadening, and distance-dependent coherence bandwidth.At 30 m, the 0.38–0.4 and 0.62–0.72 THz windows merge into one at 1 m, motivating distance-adaptive and multi-wideband designs.

D. Link Budget Analysis

THz link-budget analysis requires combining theoretical propagation and channel characteristics with practical measurement campaigns. The section also highlights unresolved challenges in measurement systems, channel models, and broader scenario validation.

  • Link-budget requirements: Link-budget evaluation must account for frequency, desired data rate, and noise figure, using both theoretical channel characteristics and practical measurements.These inputs provide the calculus terms and empirical guidance needed for realistic THz link-budget analysis.
  • Scenario analysis: At 287.28 GHz, AR/VR over 10 m reaches 20 Gbps with 10 GHz bandwidth and 10 dBm transmit power.The corresponding backhaul case reaches 200 Gbps with 100 GHz bandwidth and 20 dBm transmit power.
  • Measurement systems: Existing THz channel-measurement methods adapted from mmWave systems require more advanced and flexible designs to handle high path loss and ultra-wide bandwidth.Further campaigns are needed across UAV, VR/AR/MR/XR, and nano-communication scenarios.
  • Channel models: Common deterministic and stochastic channel models remain insufficient, motivating hybrid approaches that balance modeling efficiency and accuracy.Smooth transitions, extensive measurements, and complex parameter extraction remain challenges.
  • Outdoor and mobile channels: Outdoor and mobile THz measurements cover several scenarios, but non-stationary and outdoor channels remain open research issues.Reported scenarios include vehicular, terrestrial-satellite, airplane-satellite, and inter-satellite links.
  • AI-enabled modeling: AI techniques are being explored for THz channel-parameter analysis and path-loss prediction, alongside the need for accurate multipath measurements and models.The paper presents AI as a preferred tool for complex THz systems with distinctive channel properties.

A. Modulation and Coding Schemes

THz modulation, coding, and beamforming must be redesigned around distance-dependent bandwidth, high Doppler spread, large arrays, and hardware constraints. Proposed methods address sensing, reliability, beam squint, and beam-management efficiency.

  • Modulation and coding: Short-range THz links require ultra-low transmission power and low-complexity implementations, while molecular absorption is nearly negligible at very short distances.At macro- and micro-scales, distance-dependent bandwidth caused by molecular absorption must instead be incorporated into waveform design.
  • Modulation and coding: Multiple transmission windows with variable numbers of narrower-band pulses can exploit distance-dependent THz bandwidth while meeting data-rate demands.This approach reframes atmospheric loss as an opportunity to maximize utilization of available transmission windows.
  • Integrated sensing and communication: SI-DFT-s-OFDM achieves ten times higher estimation accuracy than conventional OFDM in simulations for integrated sensing and communication.Other proposed waveforms use non-uniform subcarrier spacing, superposition, or delay-Doppler-domain processing for THz sensing and communication.
  • Modulation and coding: THz forward-error-correction design requires stochastic models of noise, multipath fading, and interference before selecting advanced codes such as turbo, LDPC, or polar codes.The paper emphasizes that channel-error knowledge should precede error-control policy design.
  • Dynamic hybrid beamforming: Hybrid beamforming divides processing between digital baseband and RF phase-shifter networks, with fully connected, AoSA, and dynamic AoSA architectures.Very large arrays provide beamforming and multiplexing gains, while hybrid structures address performance and hardware constraints.
  • Dynamic hybrid beamforming: 36% energy-efficiency enhancement is achieved with quantized phase shifters at the expense of 2% spectral efficiency compared with infinite-resolution phase shifters.Fixed-phase-shifter designs are also considered to reduce cost and hardware complexity.
  • Dynamic hybrid beamforming: True-time-delay beamforming addresses wideband beam squint, while dynamic-subarray fixed-TTD architectures target lower power consumption and hardware complexity.Beam squint occurs because conventional phase shifters cannot generate frequency-proportional weights.
  • Beam estimation and tracking: THzPrism-based hybrid beamforming achieves 124% more data rate, serves 147% more users, and consumes 71% less power than existing approaches.Its frequency-dependent beams also support simultaneous tracking with 90% less overhead than counterpart hybrid beamforming architectures.

D. Synchronization

THz systems require new synchronization, localization, security, sensing, and signal-processing designs because ultra-wide bandwidth, large arrays, mobility, and distinctive propagation create stringent PHY challenges. The paper surveys proposed techniques and identifies several integration trade-offs.

  • Synchronization: Iterative synchronization determines symbol start time while shortening the observation window, and extended sampling theory supports sub-Nyquist THz reception.These designs respond to the difficulty of sampling and digitally processing signals at Tbps rates.
  • Synchronization: Separate synchronization for each beam is developed because beam-domain channel correlation with time and frequency gradually vanishes as transceiver antenna counts increase.This issue is identified for mmWave and THz MIMO systems.
  • Localization: Ultra-wide THz bandwidth is expected to support 50 cm outdoor localization, 1 cm indoor localization, and 1–3 mm imaging resolution.Cooperative base-station AoA tracking and recurrent neural networks using power, 3D AoA, and ToA are among the proposed approaches.
  • Physical-layer security: THz physical-layer security remains necessary because eavesdroppers can lie inside the beam sector or intercept signals scattered by objects on LoS paths.Multipath-based methods can reduce eavesdropping probability and increase secrecy rate.
  • Physical-layer security: Distance-adaptive absorption peak modulation selects frequency bands with suitable molecular absorption coefficients to shrink the eavesdroppable area.The scheme uses the frequency-dependent absorption of the THz spectrum for spectrum-based security.
  • Physical-layer security: Artificial-noise transmission is not applicable to some THz LoS scenarios when nearby eavesdroppers are closer to the transmitter than the legitimate user.The paper motivates alternative security designs for this proximity configuration.
  • Integrated sensing and communication: THz ISAC requires waveform designs that jointly address sensing and communication, beamforming that balances scanning and stable beams, and AI methods for system imperfections.Sensing favors scanning beams, whereas communication requires stable beams toward receivers.
  • Multiplexing: THz UM-MIMO multiplexing is limited by the sparse channel’s spatial degrees of freedom, motivating widely spaced multi-subarray architectures for additional intra-path gain.The WSMS architecture is also described as LoS MIMO.

VI. HIGHER LAYER NETWORKING PROTOCOLS

THz networking protocols must address directional-link deafness, neighbor discovery, interference, coverage, and distance-limited propagation. Proposed MAC, resource-allocation, multi-hop, and RIS-assisted methods target these constraints across network scales.

  • Medium access control: Narrow directional beams make link-layer synchronization and neighbor discovery resource-intensive because transmitter and receiver deafness can occur without a separate control channel.These constraints arise from the need for high-gain directional antennas at both endpoints.
  • Medium access control: Receiver-initiated MAC protocols periodically sweep space according to a predefined pattern so nodes can discover one another more efficiently.Side-lobe information and microwave control radios are additional mechanisms for neighbor discovery and short control signaling.
  • Medium access control: NOMA uses power-domain multiplexing and successive interference cancellation to improve spectral efficiency and fairness, especially for weak users.The approach exploits channel differences between users in each NOMA pair.
  • Resource allocation: LUCW allocates central spectral-window frequencies first to long-distance users and side spectrum to short-distance users, using distance-aware and bandwidth-adaptive allocation.The approach can be extended by dividing spectrum into unequal-width adaptive sub-bands.
  • Interference and coverage: Directional interference is extremely high inside a main beam but unlikely because an interfering user is rarely located within that beam.This combination requires revisiting established multi-user interference and coverage models.
  • Interference and coverage: An access-point density of 0.15/m^2 is recommended for 93% coverage probability and 30 Gbps/m^2 network throughput in the analyzed setting.The recommendation corresponds to one access point per 6.7 m^2.
  • Interference and coverage: With 1 W output power, increasing access points from 1 to 20 raises coverage probability from 25% to 95%.Multiple access points can increase coverage without severe inter-symbol interference in the reported analysis.
  • Multi-hop communications: Multi-hop relays extend THz coverage by repeating or amplifying signals and by circumventing obstacles through multiple LoS segments.Wireless relays can also support integrated access and backhaul while reducing wireline connections among base stations.

D. Routing and Scheduling

THz networks require routing and scheduling mechanisms tailored to distance-dependent bandwidth, fast-varying directional channels, limited buffers, and ultra-high-rate traffic. Cross-layer designs and experimental platforms are developing to support efficient, reliable communication.

  • Routing: Distance-dependent bandwidth and rapidly changing optimal paths make efficient routing essential in multi-hop THz networks.Routing must account for molecular composition, narrow beams, and fast channel variation.
  • Routing: Reinforcement learning increased packet delivery rate and reduced hop count in nano-scale THz networks with limited buffers.
  • Scheduling: Limited memory and ultra-high data rates motivate distributed scheduling that uses future traffic, virtual debts, and channel sensing.The proposed algorithm targets timely optimal throughput in bufferless THz IoNT.
  • Transport Layer Protocols: THz transport protocols must address congestion control, reliable end-to-end delivery, and outage risks from large propagation loss.The TCP congestion-control window mechanism requires redesign for THz traffic dynamics.
  • Cross Layer Design: Cross-layer solutions merging network, transport, link, and physical layers are proposed to improve end-to-end efficiency under ultra-high-rate and low-latency requirements.

B. Simulation Tools

THz research is moving from theory toward practice through experimental platforms, simulation tools, and intermediate emulation approaches. Remaining challenges concern real-time operation, modular hardware, and scalable hardware-in-the-loop testing.

  • Simulation Tools: Simulation tools reduce the cost and time of developing THz communication and networking protocols compared with experimental testing.They include propagation/channel simulators and communication/networking simulators.
  • Simulation Tools: Nano-Sim and TeraSim extend ns-3 with THz-oriented channel models and protocol stacks for nanoscale and macroscale network scenarios.
  • Open Challenges: Existing waveform-generator and oscilloscope platforms are well suited to channel sounding and physical-layer testing but insufficient for real-time THz networking.
  • Open Challenges: Most testbeds are difficult to upgrade because their digital processors, analog front-ends, and antenna systems are tightly specific and integrated.Standard physical and logical plug-and-play interfaces would facilitate component-level upgrades.
  • Open Challenges: The cost of THz experiments limits large-network testing, motivating hardware-in-the-loop emulation that captures device and channel behavior at scale.Higher-frequency and directional-transmission emulation is identified as a needed capability.

VIII. POLICY AND STANDARDIZATION

THz communications offer very large bandwidths for 6G applications, but spectrum policy restricts usable frequencies because passive sensing services require protection. Coexistence technologies, dynamic sharing, experimental zones, and standardization are therefore central research directions.

  • Spectrum Policy: Atmospheric and planetary sensing signals are weak and vulnerable to communication interference, motivating protected and passive frequency bands above 100 GHz.
  • Spectrum Policy: 12.5 GHz is currently the largest legally usable contiguous bandwidth between 100 and 200 GHz, far below the channel’s physical bandwidth.
  • Spectrum Policy: Moving toward 300 GHz offers access to larger bandwidths but encounters lower transmission power, worse noise figure, and higher conversion losses.
  • Spectrum Sharing: Coexistence approaches span the protocol stack, including intelligent radiation, spread-spectrum transmission, and dual-band systems that dynamically allocate frequencies around sensing activity.
  • Standardization: IEEE 802.15.3d and ongoing ITU-R studies establish standardization activity for 300 GHz and above-100-GHz communications.
  • Spectrum Policy: THz spectrum can provide tens of GHz of bandwidth, supporting Tera-WiFi, Tera-IoT, Tera-IAB, and nanoscale networks such as IoNT and WiNoC.
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