Terahertz Communication Systems Research Articles

High-speed independent polarization modulation based on dual channel 2DEG anisotropic terahertz metasurfaces

In communication and imaging systems, anisotropic metasurfaces bring new degrees of freedom for the independent control of electromagnetic waves with different polarizations. This paper proposes a dual-polarization modulation metasurface (DPMM) operating in the terahertz (THz) frequency range, which can effectively and independently modulate two orthogonally linearly polarized electromagnetic waves. The metasurface unit adopts an anisotropic structure, and by integrating multiple GaN-based high electron mobility transistors (GaN-HEMTs) into the anisotropic structure, independent control of the two-dimension electron gas (2DEG) carrier concentration in the dual-polarization channel is achieved, enabling selective modulation of x-polarization and y-polarization waves. Simulation and static experimental results demonstrate the excellent modulation isolation performance of the DPMM, with a maximum isolation exceeding 45 dB. Dynamic experiments further highlight the high-speed modulation effect, with modulation speeds exceeding 1 GHz for a single channel and an overall modulation speed of 2 GHz for dual polarization, highlighting its potential for enhancing channel capacity and information throughput in THz communication and imaging systems.

Full-space energy-controlled multi-wavefront generators enabled by hybrid graphene-VO2 terahertz metasurfaces

In terahertz communication, multi-wavefront reconfigurable metasurfaces allowing for energy control are highly desirable for multiple channels and high capacity. However, existing metasurfaces can only produce limited wavefronts in either transmission or reflection space, severely hindering their practical application ranges. Herein, a dual-controlled reconfigurable metasurface integrated with graphene and VO2 is demonstrated for an energy-controlled multi-wavefront generator in full-space. By altering the geometry sizes and rotation angles of the meta-atoms as well as external excitations, the polarization independent phase responses are achieved to actively generate energy-controlled multiple wavefronts in full-space. As a proof-of-concept demonstration, three types of full-space metasurfaces are constructed to respectively realize various meta-deflectors, metalens/meta-deflector/orbital angular momentum (OAM) generator, and multimode OAM generator in full-space by utilizing the spin-decoupling method. Moreover, the intensities of these wavefronts can be actively modulated by adjusting the Fermi energy of graphene. Therefore, such diversified metasurface will possess a promising application prospect in terahertz multifunctional devices, and communication and imaging systems.

High birefringence low loss nearly zero flat dispersion similar to slotted core photonic crystal fibers

Abstract Studying high-performance photonic crystal fibers (PCF) is of significant scientific importance for terahertz (THz) waveguide systems. This study introduces a novel PCF design with a core composed of the smallest sub-wavelength units resembling a slotted structure, aiming to achieve high birefringence and low loss. The optical properties of the proposed PCF are analyzed through simulations, yielding impressive results. The PCF exhibits an ultra-high birefringence of 0.07848, a minimum limiting loss of 10−17 dB/cm, and an effective material loss as low as 0.04251 cm−1. Moreover, it demonstrates near-zero flat dispersion of −0.012 ± 0.074 ps/THz/cm over a broad frequency range of 1.2–2.2 THz. This fiber stands out by not only providing high birefringence but also by striking an optimal balance among birefringence, transmission loss, and dispersion for THz waveguides. The implications of this work are profound for the development of THz communication systems, THz polarization-maintaining transmission, and sensing applications. Furthermore, it established an important benchmark for the design of THz-PCFs that prioritize high birefringence, low loss, and near-zero flat dispersion, offering an essential reference for future research and development in this field.

Channel Modeling Based on Transformer Symbolic Regression for Inter-Satellite Terahertz Communication

Channel modeling is crucial for inter-satellite terahertz communication system design. The conventional method involves manually constructing a mathematical channel model, which is labor-intensive, and using a neural network directly as a channel model lacks interpretability. This paper introduces a channel modeling approach based on symbolic regression. It is the first time that using transformer neural networks as the implementation tool of symbolic regression to generate the mathematical channel model from the channel data directly. It can save manpower and avoid the interpretability issue of using neural networks as a channel model. The feasibility of the proposed method is verified by generating a free space path loss model from simulation data in the terahertz frequency band.

Highly birefringent photonic crystal fibers with near-zero flattened dispersion for polarization-maintaining terahertz transmission

In this work, we present a novel hexagonal lattice porous-core photonic crystal fiber (PCF) exhibiting high birefringence, low loss, and near-zero flattened dispersion in the terahertz regime. The core design incorporates asymmetric rectangular and triangular air holes to achieve enhanced birefringence performance. The background material, TOPAS, is carefully selected to minimize dispersion and transmission losses. Furthermore, by employing the finite element method, the guiding properties of the proposed PCF are comprehensively investigated. Our simulation results reveal a near-zero flattened dispersion value of ±0.2ps⋅THz−1⋅cm−1 within a broad frequency range of 0.8−1.6THz, accompanied by an impressive birefringence value of 0.072. Additionally, the PCF demonstrates outstanding characteristics in terms of confinement loss, effective material loss, effective mode area, core power fraction, and bending loss. Considering its exceptional attributes, the proposed PCF holds significant promise for polarization-maintaining applications and terahertz communication systems.

Graphene Devices for Aerial Wireless Communications at THz

This paper investigates the state-of-the-art of graphene-based technologies for the prospective use cases of end-to-end terahertz (THz) communication systems, such as industrial Internet of Things (IoT) applications and unmanned aerial vehicles (UAVs). THz communications offer ultra-high throughput and enhanced sensing capabilities, enabling advanced applications like UAV swarms and integrated sensing, localization, and mapping. The potential of wireless THz communication can be unlocked by graphene technology. Graphene, owing to its remarkable electrical, thermal, and mechanical properties, emerges as a promising candidate for a multitude of applications in aerial wireless communications in the THz band, including high-speed electronic devices, tunable metamaterials, and innovative antennas. However, reliable tools for the simulation-based design of graphene components, able to account for the fabrication-related uncertainties, are still missing. This paper presents the envisaged possibilities of wireless communications in THz bands, overviews graphene devices for RF applications at THz, and discusses the open issues of modelling THz devices and THz channel.

Experimental and theoretical exploration of terahertz channel performance through glass doors

In the evolving landscape of terahertz communication, the behavior of channels within indoor environments, particularly through glass doors, has garnered significant attention. This paper comprehensively investigates terahertz channel performance under such conditions, employing a measurement setup operational between 113 and 170 GHz. Analyzing scenarios frequently induced by human activity and environmental factors, like door movements, we established a comprehensive theoretical model. This model seamlessly integrates transmission, reflection, absorption, and diffraction mechanisms, leveraging the Fresnel formula, multi-layer transmission paradigm, and knife-edge diffraction theory. Our experimental results and theoretical predictions harmoniously align, revealing intricate dependencies, such as increased power loss at higher frequencies and larger incident angles. Furthermore, door interactions, whether opening or oscillations, significantly impact the terahertz channel. Notably, door edges lead to a power blockage surpassing the transmission loss of the glass itself but remaining inferior to metallic handle interferences. This paper's insights are pivotal for the design and fabrication of terahertz communication systems within indoor settings, pushing the boundaries of efficient and reliable communication.

Ray Tracing Meets Terahertz: Challenges and Opportunities

It is very likely that the terahertz (THz) spectrum will be used for the operation of the upcoming 6th-generation (6G) wireless communication systems. This article presents a study for the application of ray tracing (RT) for THz channel modeling. Because of the extremely short wavelengths of THz signals, we argue that signal propagation in the THz spectrum can be precisely modeled by employing RT techniques and thus accurate channel models can be obtained in an efficient manner. We first review recent research and standardization activities related with RT-assisted channel modeling which mainly deal with millimeter-wave (mmWave) applications and to a much lesser extent have been applied the THz spectrum. Then, we present the basic THz wave propagation characteristics, including reflection, scattering, penetration, shadowing as well as signal absorption by atmospheric molecules and tiny particles under different weather conditions. The methodologies for the appropriate construction of target scenarios are presented as well as efficient computational methods for the effective RT operation. Finally, various research and development (R&D) challenges related to RT-based channels for future THz communication systems are identified and possible solutions are highlighted.

All-Dielectric Integrated Meta-Antenna Operating in 6G Terahertz Communication Window.

Efficient transceivers and antennas at terahertz frequencies are leading the development of 6G terahertz communication systems. The antenna design for high-resolution terahertz spatial sensing and communication remains challenging, while emergent metallic metasurface antennas can address this issue but often suffer from low efficiency and complex manufacturing. Here, an all-dielectric integrated meta-antenna operating in 6G terahertz communication window for high-efficiency beam focusing in the sub-wavelength scale is reported. With the antenna surface functionalized by metagrating arrays with asymmetric scattering patterns, the design and optimization methods are demonstrated with a physical size constraint. The highest manipulation and diffraction efficiencies achieve 84.1% and 48.1%. The commercially accessible fabrication method with low cost and easy to implement has been demonstrated for the meta-antenna by photocuring 3D printing. A filamentous focal spot is measured as 0.86λ with a long depth of focus of 25.3λ. Its application for integrated imaging and communication has been demonstrated. The proposed technical roadmap provides a general pathway for creating high-efficiency integrated meta-antennas with great potential in high-resolution 6G terahertz spatial sensing and communication applications.

Sensing and Computer Vision-Aided Mobility Management for 6G Millimeter and Terahertz Communication Systems

Sensing and Computer Vision-Aided Mobility Management for 6G Millimeter and Terahertz Communication Systems

Two-dimensional topological semimetals: an emerging candidate for terahertz detectors and on-chip integration.

The importance of terahertz (THz) detection lies in its ability to provide detailed information in a non-destructive manner, making it a valuable tool across various domains including spectroscopy, communication, and security. The ongoing development of THz detectors aims to enhance their sensitivity, resolution and integration into compact and portable devices such as handheld scanners or integrated communication chips. Generally, two-dimensional (2D) materials are considered potential candidates for device miniaturization but detecting THz radiation using 2D semiconductors is generally difficult due to the ultra-small photon energy. However, this challenge is being addressed by the advent of topological semimetals (TSM) with zero-bandgap characteristics. These semimetals offer low-energy excitations in proximity to the Dirac point, which is particularly important for applications requiring a broad detection range. Their distinctive band structures with linear energy-momentum dispersion near the Fermi level also lead to high electron mobility and low effective mass. The presence of topologically protected dissipationless conducting channels and self-powered response provides a basis for low-energy integration. In order to establish paradigms for semimetal-based THz detectors, this review initially offers an analytical summary of THz detection principles. Then, the review demonstrates the distinct design of devices, the excellent performance derived from the topological surface state and unique band structures in TSM. Finally, we outline the prospective avenues for on-chip integration of TSM-based THz detectors. We believe this review can promote further research on the new generation of THz detectors and facilitate advancements in THz imaging, spectroscopy, and communication systems.

Joint Multiband Linear Interferometric Receiver for Integrated Microwave and Terahertz Sensing and Communication Systems

Joint Multiband Linear Interferometric Receiver for Integrated Microwave and Terahertz Sensing and Communication Systems

Comprehensive Link-Level Simulator for Terahertz MIMO Integrated Sensing and Communication Systems with TDD Framework

Comprehensive Link-Level Simulator for Terahertz MIMO Integrated Sensing and Communication Systems with TDD Framework

A Survey on the Impact of Intelligent Surfaces in the Terahertz Communication Channel Models.

Terahertz (THz) band will play an important role in enabling sixth generation (6G) envisioned applications. Compared with lower frequency signals, THz waves are severely attenuated by the atmosphere temperature, pressure, and humidity. Thus, designing a THz communication system must take into account how to circumvent or diminish those issues to achieve a sufficient quality of service. Different solutions are being analyzed: intelligent communication environments, ubiquitous artificial intelligence, extensive network automation, and dynamic spectrum access, among others. This survey focuses on the benefits of integrating intelligent surfaces (ISs) and THz communication systems by providing an overview of IS in wireless communications with the scanning of the recent developments, a description of the architecture, and an explanation of the operation. The survey also covers THz channel models, differentiating them based on deterministic and statistical channel modeling. The IS-aided THz channels are elucidated at the end of the survey. Finally, discussions and research directions are given to help enrich the IS field of research and guide the reader through open issues.

Circularly polarized terahertz wave independently controlled tunable spin-decoupled metasurface

The research and development of multifunctional metasurfaces has attracted much attention, and manipulating multifunctional devices with greater degrees of freedom is a continuing trend. In this study, we propose a tunable multifunctional metasurface with terahertz (THz) frequency and demonstrate it theoretically. By introducing the propagation phase decoupling conjugate response, we realize the independent control of beam deflection(deflection angle is 8.1°), beam splitting（split angle is 16.5°）, and vortex beam mode switching under different incident conditions. At the same time, by changing the Fermi level of the Dirac semimetal, the switching between different states under the same incident condition is realized. The results of this study are expected to contribute to promising applications in future for improving the capacity of THz communication systems, independently regulating multi-functional devices.

A 110-170 GHz Wideband LNA Design Using the InP Technology for Terahertz Communication Applications.

This paper proposes a low-noise amplifier (LNA) for terahertz communication systems. The amplifier is designed based on 90 nm InP high-electron-mobility transistor (HEMT) technology. In order to achieve high gain of LNA, the proposed amplifier adopts a five-stage amplification structure. At the same time, the use of staggered tuning technology has achieved a large bandwidth of terahertz low-noise amplification. In addition, capacitors are used for interstage isolation, sector lines are used for RF bypass, and Microstrip is used to design matching circuits. The entire LNA circuit was validated using accurate electromagnetic simulation. The simulation results show that at 140 GHz, the small signal gain is 25 dB, the noise figure is 4.4 dB, the input 1 dB compression point is -19 dBm, and the 3 dB bandwidth reaches 60 GHz (110-170 GHz), which validates the effectiveness of the design.

Ultrabroadband terahertz-band communications with self-healing bessel beams

The large available bandwidth at sub-terahertz and terahertz frequencies has the potential to enable very high data rates for wireless communications. Moreover, given the large electrical size of terahertz antenna apertures, many future terahertz communication systems will likely operate in the near field. However, due to their reliance on highly directional beams, terahertz systems are susceptible to blockage. Here, we propose using Bessel beams to overcome issues caused by blockage due to their diffraction-free nature and self-healing properties in the near field. We compare the performance of information-bearing Bessel beams and Gaussian beams with and without an obstacle. We later discuss the use of reconfigurable intelligent surfaces to construct terahertz Bessel beams. Finally, we propose a metric to quantify the quality of imperfectly generated terahertz Bessel beams and explore their ability to self-heal. The results demonstrate that Bessel beams are an attractive option for near-field terahertz communications, especially when mitigating the effects of partial blockage.

UAV-Assisted Wideband Terahertz Wireless Communications with Time-Delay Phased UPA under Beam Squint

Future Unmanned Aerial Vehicle (UAV)-assisted wireless communication systems are expected to utilize wide bandwidths available at terahertz (THz) frequencies to enhance system throughput. To compensate for the severe path loss in the THz band, it is essential to have a multitude of antennas in the UAV to generate narrow beams for directional transmission. However, narrow beams severely limit its spatial coverage, which greatly affects the efficiency of large-scale access UAV-assisted THz systems. Moreover, the combination of massive antennas and large bandwidth at THz makes the misalignment of the beams caused by beam squint non-negligible and also high energy consumption. UAV-assisted communication technology can effectively increase spatial coverage and provide reliable LoS communication links. In addition, reducing the number of radio frequency (RF) chains while ensuring the number of transmitted data streams and space division multiplexing capability is also an effective way to reduce energy consumption in the UAV communication. In this paper, a single RF chain uniform planar array (UPA) with true-time-delays (TTDs) is equipped on the UAV to achieve two dimensional (2D) beams and split spatial beams to improve transmission efficiency. We analyze the 2D beam squint of the UPA and design a time-delay phased UPA for UAV-assisted THz communication systems. By introducing TTDs between the single RF chain and phase shifters, the beam squint can be controlled flexibly by introducing the delay between each antenna. When TTDs are arranged in both the horizontal and vertical dimensions, the coverage of the beams becomes more complicated compared to uniform linear arrays (ULA). Simulation results show that the proposed time-delay phased UPA can achieve better performance in both single-beam and multi-beam modes for single user and multi-user scenarios compared with conventional phased UPA, respectively. In addition, we propose frequency division beam multiple access (FDBMA) multi access technology, which achieves more efficient multi access by reusing resources from different frequency beam pairs. Finally, the results also show that the enlargement of the beamwidth through the proposed FDBMA strategy can also increase the performance in multi-user scenarios.

Design of Orthogonal Pulse Waveforms With Tunable Frequency and Bandwidth for Carrier-Free THz Communications

Carrier-free pulse-based waveforms are desired in terahertz (THz) communications since pulse-based systems have simpler transceiver architectures than carrier-based systems. For such a pulse-based THz communication system, there still lacks pulse waveforms that can support high-order modulation and flexible multiple access. In this paper, a pulse-based waveform with continuously tunable center frequencies and bandwidths is designed for carrier-free THz communications. More specifically, a Gaussian pulse is utilized as the basic pulse, and then the weighted sum of its high-order derivatives is used to generate waveforms with tunable frequencies and bandwidths according to the probability density function of Rice distribution. Such a pulse-based waveform is called frequency and bandwidth continuously tunable (FBCT) pulse. By making the derivative orders of all weighted terms odd or even, a pair of orthogonal FBCT pulses can be generated. Based on the pair of orthogonal FBCT pulses, a basic pulse-based <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary quadrature amplitude modulation (P <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> QAM) scheme can be readily achieved. Moreover, with frequency and bandwidth tunability, FBCT pulse can support pulse division multiple access (PDMA) with tunable bandwidth, through which frequencies with high molecular absorption loss can be avoided. Numerical results demonstrate that FBCT pulse is significantly effective and flexible in supporting carrier-free THz communications.

Applying Intelligent Reflector Surfaces for Detecting Violent Expiratory Aerosol Cloud Using Terahertz Signals

The recent COVID-19 pandemic has driven researchers from different spectrum to develop novel solutions that can improve detection and understanding of SARS-CoV- 2 virus. In this article we propose the use of Intelligent Reflector Surface (IRS) and terahertz communication systems to detect violent expiratory aerosol cloud that are secreted from people. Our proposed approach makes use of future IRS infrastructure to extend beyond communication functionality by adding environmental scanning for aerosol clouds. Simulations have also been conducted to analyze the accuracy of aerosol cloud detection based on a signal scanning and path optimization algorithm. Utilizing IRS for detecting violent expiratory aerosol cloud can lead to new added value of telecommunication infrastructures for sensor monitoring data that can be used for public health.