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Combating the Distance Problem in the Millimeter Wave and Terahertz Frequency Bands

Ian F. Akyildiz, Chong Han, Shuai Nie

arXiv:1902.04682v1eess.SP

TL;DR

Mm-wave and THz systems face severe spreading and molecular-absorption losses that limit transmission distance. The paper analyzes four distance-extension technologies and their joint design, with simulations showing links beyond 90 meters in some 60-GHz cases and up to 85 meters at 300 GHz.

  • Problem

    High spreading loss and molecular absorption in mm-wave and THz bands limit communication distance and coverage.

  • Method

    The paper studies distance-aware physical-layer design, ultra-massive MIMO, reflectarrays, HyperSurfaces, and their joint design through analysis and indoor ray-tracing simulations.

  • Results

    More than 90 m is reached at a 10 dB SNR threshold in 60-GHz LOS after applying UM-MIMO, HyperSurface, or joint design, compared with around 18 m baseline distance.

  • Takeaways & Limitations

    The four directions, particularly their joint design, provide a path toward extending mm-wave and THz communication distance in LOS and NLOS environments.

Abstract

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In the millimeter wave (30-300 GHz) and Terahertz (0.1-10 THz) frequency bands, high spreading loss and molecular absorption often limit the signal transmission distance and coverage range. In this paper, four directions to tackle the crucial problem of distance limitation are investigated, namely, a physical layer distance-aware design, ultra-massive MIMO communication, reflectarrays, and intelligent surfaces. Additionally, the potential joint design of these technologies is proposed to combine the benefits and possibly further extend the communication distance. Qualitative analyses and quantitative simulations are provided to illustrate the benefits of the proposed techniques and demonstrate the feasibility of mm-wave and THz band communications up to 100 meters in both line-of-sight and non-line-of-sight areas.

I. INTRODUCTION

Mm-wave and THz communications offer expanded bandwidth and new applications, but severe propagation and molecular absorption losses sharply constrain distance. The paper investigates four technologies and their joint design to extend coverage, with evaluations reaching up to 100 meters.

  • Mm-wave systems provide licensed bands, but their consecutive bandwidth remains below 10 GHz, motivating THz communications for future high-capacity demands.
  • 10-meter THz links can exceed 100 dB free-space path loss because spreading loss grows with f^2 and molecular absorption adds attenuation.
  • The paper investigates distance-aware physical-layer design, ultra-massive MIMO, reflectarrays, and HyperSurfaces as four approaches to the distance problem.
  • Ultra-massive MIMO focuses signals in space and frequency, reflectarrays address line-of-sight blockage, and HyperSurfaces dynamically control electromagnetic behavior.
  • Numerical evaluations examine the four technologies and their joint design in line-of-sight and non-line-of-sight indoor areas, demonstrating distances up to 100 meters.

II. DISTANCE-ADAPTIVE DESIGN

Distance-adaptive communication exploits the relationship between propagation distance, path loss, and molecular spectral windows. The approach uses adaptive waveforms and resource allocation across broadband sub-windows.

  • The strong relationship between distance and spectral windows motivates distance-adaptive communication techniques.
  • Each spectral window offers multi-GHz-to-THz bandwidth that can support parallel transmissions through narrower broadband sub-windows.
  • Distance-adaptive design includes multi-wideband waveform adaptation and distance-aware bandwidth-adaptive resource allocation.

A. Distance-adaptive Multi-wideband Pulse Waveform

The distance-adaptive multi-wideband pulse waveform scheme matches transmitted waveforms to channel bandwidth shaped by molecular absorption. It jointly varies sub-window rate and transmit power to maximize distance under constraints.

  • The scheme dynamically adapts transmitted waveforms to the channel bandwidth available under molecular absorption.
  • It varies the rate and transmit power on each sub-window while optimizing waveform characteristics and repetition rate.
  • The optimization maximizes communication distance subject to rate and transmit-power constraints.

B. Distance-aware Bandwidth-adaptive Resource Allocation

Distance-aware resource allocation treats distance maximization as the objective and assigns spectral sub-windows according to link distance. The supplied passages also identify hardware complexity and converter bandwidth as practical constraints.

  • The resource allocation scheme accounts for spectrum allocation, modulation techniques, and transmit power when allocating mm-wave and THz resources.
  • Distance maximization replaces the traditional objectives of minimizing energy consumption or maximizing data rate.
  • Center sub-windows are assigned to long-distance links, while side sub-windows serve short-distance transmissions.
  • The proposed solutions increase communication distance at the cost of hardware complexity, while current demonstrations remain single-band.
  • 100 Giga-samples-per-second converters support up to 50 GHz transmission bandwidth, below the channel bandwidth and motivating sub-Nyquist sampling.

A. Plasmonic Nano-antenna Arrays

Plasmonic materials enable compact antenna arrays for mm-wave and THz communications. These arrays support ultra-massive MIMO configurations that can focus signals in space and frequency to increase communication distance.

  • A. Plasmonic Nano-antenna Arrays: A 1024×1024 UM-MIMO scheme equips 1024 antenna elements at both transmitter and receiver.The concept uses plasmonic nano-antenna arrays and focuses transmitted signals simultaneously in space and frequency.
  • A. Plasmonic Nano-antenna Arrays: Plasmonic materials support surface plasmon polariton waves at mm-wave and THz frequencies.Graphene supports SPP waves at THz frequencies, while metamaterials can support plasmonic waves at mm-wave frequencies.
  • A. Plasmonic Nano-antenna Arrays: Slower SPP waves produce shorter wavelengths, enabling smaller antennas than metallic antennas.The reduced antenna size facilitates integration of large two-dimensional arrays with dynamic azimuth and elevation control.
  • A. Plasmonic Nano-antenna Arrays: UM-MIMO uses transmitter and receiver antenna subarrays, each composed of P × Q tightly packed elements and driven by an individual baseband-to-RF chain.Each antenna element is connected to a wideband THz analog phase shifter.

B. Dynamic UM-MIMO Modes

Dynamic UM-MIMO modes group antenna subarrays to trade off beamforming, spatial multiplexing, and their combination. Multi-band operation adds frequency as another dimension for using THz spectral windows.

  • B. Dynamic UM-MIMO Modes: Beamforming groups subarrays to steer high-gain narrow beams toward the strongest propagation path.This design overcomes high mm-wave and THz attenuation and enhances communication distance.
  • B. Dynamic UM-MIMO Modes: Spatial multiplexing uses multiple streams on a single carrier to increase user data rates.The combined beamforming and spatial-multiplexing approach requires sufficiently uncorrelated THz propagation paths.
  • B. Dynamic UM-MIMO Modes: Fig. 2 depicts a hybrid-beamforming UM-MIMO architecture whose subarrays contain P × Q tightly packed antenna elements.The caption identifies the subarray composition but does not specify performance results.
  • B. Dynamic UM-MIMO Modes: The beamforming–spatial multiplexing tradeoff affects capacity, spatial degrees of freedom, and distance improvement.The objective is to support multiple ultra-high-speed and long-distance links simultaneously.
  • B. Dynamic UM-MIMO Modes: Multi-band UM-MIMO simultaneously uses different frequency bands by independently modifying individual array-element responses.Different antenna lengths or frequency-tunable graphene nano-antennas can enable this frequency dimension.

IV. REFLECTARRAYS

Reflectarrays use electronically controlled element phases to shape radiation patterns and extend coverage, including around blocked paths. Their application is constrained by channel-estimation demands and THz material limitations.

  • IV. REFLECTARRAYS: 37 Reflectarrays can enhance signal coverage in the propagation medium.The passage presents reflectarrays as an additional approach alongside transmitter- and receiver-side large antenna arrays.
  • IV. REFLECTARRAYS: Reflectarrays use electronically controlled element phase shifts to form adjustable radiation patterns toward desired directions.Their operation combines phased-array principles with geometrical optics.
  • IV. REFLECTARRAYS: In blocked indoor links, a reflectarray near the THz source can tune element phases to bounce signals toward the user equipment.Reflectarrays can serve single or multiple transmitter–receiver pairs and extend transmission range in dense multipath environments.
  • IV. REFLECTARRAYS: Reflectarray tuning depends strongly on array size and environment, making channel-estimation time efficiency and accuracy critical.Small environmental movements can distort mm-wave and THz transmission paths.
  • IV. REFLECTARRAYS: At THz frequencies, reflectarray materials require reconsideration because MEMS are reported to have a 120 GHz upper limit.This boundary is below the THz band.

V. HYPERSURFACES

HyperSurfaces are software-controlled planar metasurfaces that alter indoor electromagnetic behavior at sub-wavelength resolution. Networked tiles can optimize reflection paths to reduce loss, mitigate multipath, and extend distance.

  • V. HYPERSURFACES: A HyperSurface is a planar metasurface that can be coated on indoor surfaces and software-controlled to change its electromagnetic behavior.It uses hundreds of sub-half-wavelength meta-atoms connected to miniaturized controllers and a gateway.
  • V. HYPERSURFACES: Unlike conventional reflectarrays, metasurfaces can steer, absorb, and polarize waves through sub-wavelength interactions.This provides electromagnetic manipulations unavailable to conventional reflectarrays.
  • V. HYPERSURFACES: HyperSurface functionality is organized into metasurface, intra-tile control, and tile gateway layers.These layers respectively implement adjustable meta-atoms, controller-based configuration, and communication with the external network.
  • V. HYPERSURFACES: Networked wall tiles change azimuth and elevation angles to create reflection paths between transmitter and receiver.Optimization selects angles that maintain paths, minimize path loss, mitigate undesired multipaths, and extend transmission distance.
  • V. HYPERSURFACES: HyperSurfaces provide flexible, high-resolution control while remaining embedded behind common construction materials.This contrasts with reflectarrays, which may occupy more space and affect environmental appearance.
  • V. HYPERSURFACES: Indoor NLOS areas have lower SNR, worse coverage, lower data rates, and dominant attenuated multipath fading than LOS areas.These conditions motivate controlling the propagation environment with intelligent surfaces.

VI. JOINT DESIGN OF THE FOUR DIRECTIONS

The paper proposes jointly combining distance-adaptive modulation, UM-MIMO, reflectarrays, and HyperSurfaces to maximize transmission-distance improvement. These technologies can be coordinated to accommodate different user-density and resolution requirements while supporting dynamic links.

  • VI. JOINT DESIGN OF THE FOUR DIRECTIONS: The joint design combines the four proposed directions to target maximum communication-distance improvement.It exploits the complementary strengths of distance-adaptive design, UM-MIMO, reflectarrays, and HyperSurfaces.
  • VI. JOINT DESIGN OF THE FOUR DIRECTIONS: Reflectarrays and HyperSurfaces can form a hybrid intelligent surface because both use ray optics and act as reflectors.The hybrid can accommodate different user-density and resolution requirements.
  • VI. JOINT DESIGN OF THE FOUR DIRECTIONS: UM-MIMO can operate at both the access point and user equipment alongside HyperSurface technology to ease signal-processing overhead and increase time efficiency.The paper also describes HyperSurfaces as dynamically controlled metasurfaces for directing signals through reflections.
  • VI. JOINT DESIGN OF THE FOUR DIRECTIONS: Distance-adaptive design can combine with UM-MIMO or HyperSurface to increase spectral efficiency in mm-wave and THz networks.UM-MIMO beamforming and spatial multiplexing make adaptively modulated signals more robust in complicated indoor and outdoor environments.

VII. PERFORMANCE EVALUATION

The evaluation uses three-dimensional ray tracing in an E-shaped indoor hallway with LOS and NLOS receiver locations. Across 60 GHz, 300 GHz, and 1 THz, the proposed techniques substantially extend the distance at which reliable links are obtained.

  • VII. PERFORMANCE EVALUATION: The simulation uses one ceiling-level transmitter and 15 receiver locations, including nine LOS and six NLOS positions spanning 10–100 m.The receiver distances are selected to compare corresponding LOS and NLOS ranges where the layout permits.
  • VII. PERFORMANCE EVALUATION: At 60 GHz, the 10 dB SNR threshold extends from around 18 m without techniques to more than 90 m with UM-MIMO, HyperSurface, or the joint design.The distance improvement is also significant for receivers in NLOS areas.
  • VII. PERFORMANCE EVALUATION: At 1 THz, the joint design offers robust transmission until 60 m for both LOS and NLOS cases.The figure compares SNR versus distance with and without UM-MIMO, HyperSurface, and the joint design.
  • VII. PERFORMANCE EVALUATION: The average gains are approximately 17 dB for UM-MIMO, 15 dB for HyperSurface, and 32 dB for the joint design across the evaluated bands.The results are obtained from three-dimensional ray-tracing simulations at 0.06, 0.3, and 1 THz.

VIII. CONCLUSION

The paper studies distance limitations in mm-wave and THz communications and evaluates four solution directions together with their joint design. Simulations indicate substantial distance improvements, including at least five times for individual technologies and ten times for the joint design.

  • VIII. CONCLUSION: The distance limitation problem is attributed to atmospheric attenuation and molecular absorption in mm-wave and THz wireless communication systems.The paper analyzes distance-adaptive physical-layer design, ultra-massive MIMO, reflectarrays, and HyperSurfaces as solution directions.
  • VIII. CONCLUSION: At least five times distance improvement is demonstrated with an individual technology, while the joint design achieves ten times improvement in real-world communication scenarios.The conclusion presents the joint design as addressing limited transmission range and blockage effects.
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