Terahertz Wireless Communication Systems Research Articles

Plasmonic antennas based on rectangular graphene nanoribbons with controlled polarization of terahertz and infrared radiation

Background. To develop new terahertz wireless communication systems with high throughput and transmission speeds, such as 6G and above, effective control of the polarization direction of emitted terahertz waves is necessary, but most methods are technologically complex and expensive. The implementation of terahertz antennas and devices based on 2D materials such as graphene solves the problem associated with developing effective control. Aim. Study of the possibility of controlling the polarization of terahertz and IR radiation of plasmonic antennas based on rectangular graphene nanoribbons by changing the chemical potential (application of an external electric field). Methods. This important scientific problem related to the design of terahertz antennas can largely be solved by simulation using the electrodynamic simulation program CST MWS 2023. Results. Plasmon terahertz antennas based on rectangular graphene nanoribbons were chosen as the object of analysis and the possibility of emitting waves of two orthogonal polarizations was shown. Methods have been identified for controlling the polarization of terahertz and IR radiation from such antennas, based on the selection of operating frequencies corresponding to the resonances of the modes of surface plasmon-polaritons, and the application of metallization to the dielectric substrate. Conclusion. The ability to control the polarization of terahertz and IR radiation makes it possible to create both new elements of plasmonic antenna arrays and new communication technologies, including future 6G networks.

High-speed 0.22 THz communication system with 84 Gbps for real-time uncompressed 8K video transmission of live events

Terahertz frequency band (0.1 ~ 10 THz) can provide ultra-high transmitting rates for massive emerging applications. However, due to the radio frequency impairments and limited signal processing property of baseband devices, it is difficult for the current terahertz communication systems to be used in real applications. In this work, we report a 0.22 THz communication system, which can provide 84 Gbps air interface rate over 1.26 kilometers distance. Such a system is underpinned by three breakthroughs, namely, integrating the reported terahertz wireless communication system, redesigning the signal processing in baseband devices, and increasing the cut-off frequency and transmitting power of radio frequency component by parameter optimizing. Being applied to realize the real-time transmission of the uncompressed 8K ultra high-definition video in the 31st International University Sports Federation in Chengdu, China, this work is a leaping advancement to enable terahertz systems from experiment to promising applications.

Multi-mode non-diffraction vortex beams enabled by polarization-frequency multiplexing transmissive terahertz metasurfaces

In terahertz (THz) wireless communication systems, non-diffraction vortex beams carrying an orbital angular momentum (OAM) have attracted extensive attention due to their ability to transmit information over long distances with high capacity. However, existing metasurfaces can only generate a single OAM mode non-diffracting vortex beam at reflection space for circular polarization (CP) incidence, limiting practical applications. To address this issue, we propose and design a polarization-frequency multiplexing transmissive THz metasurface to realize multi-mode non-diffracting vortex beams at linear polarization (LP) incidence. The meta-atom of this metasurface is composed of three anisotropic rectangular metallic structures embedded in vanadium dioxide (VO2) square rings, two circular aperture metallic grid layers, and four dielectric layers. By reasonably designing the size of the metal patch and the state of VO2, the designed metasurface can achieve polarization multiplexing and frequency multiplexing for LP incidence. Based on the phase response of the proposed meta-atoms, the transmissive metasurface can implement not only multi-mode non-diffraction vortex beams but also their space separation at two frequency ranges of 0.80–0.90 THz and 1.50–1.80 THz by changing the state of VO2. Therefore, the proposed multiple multiplexing metasurfaces can effectively shape the wavefront of non-diffraction vortex beams, which have broad application prospects in 6G THz communication.

Photonics-aided exceeding 200-Gb/s wireless data transmission over outdoor long-range 2 × 2 MIMO THz links at 300 GHz

Outdoor long-range terahertz (THz) communications often come at the expense of transmission rate. Moreover, the practicability of the single polarization optical/THz link, which is commonly used in the previous long-range THz demonstrations based on photonics, is extremely limited by the following two fatal defects. One is relying on active polarization control, and the other is not supporting the transparent bridging of optical polarization division multiplexed (PDM) signals for mature coherent optical communication networks. In this work, a large-capacity photonics-aided THz wireless communication system based on the outdoor long-range 2 × 2 multiple-input multiple-output (MIMO) links has been successfully demonstrated. We first build the 200-m 2 × 2 MIMO THz wireless links at the 300 GHz band. The cascaded linear and nonlinear equalizers are proposed which can significantly improve the transmission performance of 100- and 200-Gb/s PDM quadrature phase shift keying (QPSK) signals. Then an interesting 2 × 2 MIMO structure which can provide certain diversity reception gain under 200-m long-range wireless delivery using the same polarization scheme is also presented and further compared with the orthogonal polarization scheme. Since each THz receiver simultaneously receives data from both the two THz transmitters for this MIMO links, an improvement over 6 dB in receiving sensitivity and one order of magnitude in bit error ratio performance under low signal-to-noise ratio conditions can be achieved. Finally, based on the proposed cascaded equalizers and novel 2 × 2 MIMO structure, we successfully demonstrate a record-breaking 58-Gbaud (232-Gb/s) PDM-QPSK signal transmission over 200-m 2 × 2 MIMO THz wireless links. This is an important attempt for the photonics-aided THz wireless communication systems to achieve 2 × 2 MIMO transmission over a long wireless distance in the outdoors. Furthermore, attaining over 200 Gb/s at a wireless distance of 200 m also represents a key milestone for the long-range and large-capacity THz wireless communication systems.

Polarization-multiplexing graphene-based coding metasurface for flexible terahertz wavefront control

In terahertz wireless communication systems, flexible wavefront control devices based on various structure metasurfaces have attracted enormous attention for next-generation communication. In general, tunable terahertz metasurfaces integrated with active materials or MEMS technologies are used for dynamic wavefront control. However, most existing metasurfaces suffer from various limitations, including intrinsic properties of active materials, low reliability of MEMS technologies, and single polarization mode of incident waves, which hinders their development and application. To address these challenges, herein, we design two types of reflective graphene-based coding metasurfaces for active wavefront control. The metasurface coding meta-atom is composed of a graphene split-ring resonator, a dielectric layer, and a metal ground plane. By simply rotating the coding meta-atom, independent 2π phase coverage for circularly polarized (CP) or linearly polarized (LP) illumination can be achieved, enabling polarization multiplexing. Thus, a metasurface (MS-1) is constructed based on the vortex phase profile to generate different wavefronts. Moreover, these wavefronts can be actively switched between a vortex beam, a multi-beam, and a specular reflection beam by altering the polarization mode of the incident waves and the Fermi level of the graphene coding regions Additionally, another metasurface (MS-2) is developed according to the parabolic phase profile to create a tunable metalens that allows active control over focal intensity and depth by adjusting the Fermi level of graphene. Such wavefront-controlled metasurfaces have high capacity and integration, making them very promising for potential applications in terahertz communication and imaging systems.

Metallic waveguide transmitarrays for dual-band terahertz antennas with high gain and low sidelobe levels

The antenna is an indispensable device in terahertz (THz) wireless communication systems. Owing to the high propagation attenuation of THz waves in atmospheric, THz antennas with high gain and low sidelobe levels (SLLs) are in an urgent need. Here, a metal waveguide transmitarry antenna (MWTA) is proposed to realize high gain and low SLLs at dual THz bands simultaneously. At the frequencies of 0.14 and 0.0972 THz, the peak gains are 32 and 28 dBi, the SLLs are both 21 dB lower than the main lobe, and the 3 dB beamwidths are 4.4° and 5.7°, respectively. As a proof of concept, a prototype with a diameter of 48 mm is fabricated and the measured results are in good agreement with the simulations. The proposed MWTA with high gain and low SLLs at dual THz bands holds great application potential in 6G wireless communication, non-destructive testing, and biomedicine.

3D‐Printed Photonic Crystal Sub‐Terahertz Leaky‐Wave Antenna

AbstractWireless systems are facing increasing pressure due to the growing demand for data transmission. One potential solution to this problem is to shift communication frequencies toward the terahertz (THz) spectrum. However, this requires the development of new components that can efficiently process signals at these high frequencies and transmit them via highly directional beams. In this study, a novel approach is proposed to achieving efficient THz signal processing by combining two existing technologies: photonic crystals and leaky‐wave antennas. Incorporating a 2D photonic crystal inside a leaky‐wave waveguide allows to manipulate the wave vector of the guided wave in unique ways, which in turn impacts the far‐field radiation pattern emitted through the leaky‐wave aperture. The device fabrication uses 3D printing of alumina and allows for convenient and scalable manufacturing. Through numerical simulations and experiments, free‐space data transmission at rates of few hundred Mbps at a carrier frequency of 101.2 GHz is demonstrated. The findings illustrate the feasibility of photonic crystal‐based leaky‐wave antennas and lay the groundwork for the development of compact and high‐performance components for THz wireless communication systems.

Composite phase modulated beam steering controllable reflective metasurface

Terahertz metasurface functional devices as an effective method to control terahertz waves have attracted extensive attention from researchers. In order to enhance the functionality and flexibility of the metasurface and adapt to diverse application scenarios and demands, a beam-steering controllable reflective metasurface is designed by combining the Pancharatnam-Berry phase principle and the phase change material vanadium dioxide in this work. The metasurface unit consists of five layers, they being the top layer that is a metal patterned layer, the third layer that is made of vanadium dioxide and located between the dielectric layers with different thickness, the dielectric layer that is made of polytetrafluoroethylene (PTFE), and the bottom layer that serves as a metal reflective layer. The metasurface units are rotated based on the Pancharatnam-Berry phase principle to obtain four metasurface units with fixed phase differences in between, after which the metasurface units are arranged in two dimensions based on the generalized Snell reflection law to obtain the desired phase-gradient deflected reflection beam. The insulating state-metallic state transition of the vanadium dioxide layer on the metasurface can change the phase gradient of the preset metasurface, thereby realizing the on/off function of deflection. The simulation results show that when the vanadium dioxide is in the insulating state, the phase gradient of the designed metasurface appears, and the metasurface can deflect the vertically incident circularly polarized wave with specific angle anomalies in a operating band of 1.1–2.0 THz; when the vanadium dioxide is in the metallic state, for the same operating band of the same metasurface, the phase gradient of the metasurface disappears, and the metasurface mirror reflects the vertically incident circularly polarized waves, thereby realizing the function switching. This design provides new possibilities for modulating the terahertz reflected beam, which will have potential applications in terahertz wireless communication and radar systems.

An Overview of Key Machine Learning Technologies in 6G-Oriented Terahertz Wireless Communication Systems (Invited)

An Overview of Key Machine Learning Technologies in 6G-Oriented Terahertz Wireless Communication Systems (Invited)

Graphene-enabled chiral metasurface for terahertz wavefront manipulation and multiplexing holographic imaging

The ability of metasurfaces to simultaneously manipulate both the spin and phase of circularly polarized (CP) incident waves has attracted considerable attention from researchers. Here, we propose a reconfigurable double circular split rings (DCSRs) chiral metasurface. By introducing graphene and changing the Fermi level of graphene, dynamic circular dichroism (CD) can be achieved. The calculation results show that when the Fermi level of graphene is increased from 0.1 eV to 1 eV, the CD decreases from 0.72 to 0.09 at 0.839 THz, which provides a CD-tunable spin-controlled method. In addition, based on the graphene-enabled DCSRs, we design two metasurface arrays combining Pancharatnam-Berry (PB) phase and coding metasurface theory to realize terahertz CP wave switchable focusing and spin-controlled multi-pencil beam generation. Furthermore, we demonstrate the application of reflective holography for multiplexing circularly polarized waves. The results of this paper are expected to play a significant role in the future terahertz wireless communication system, multi-spectral imaging, and information encryption fields.

Gate-readout and a 3D rectification effect for giant responsivity enhancement of asymmetric dual-grating-gate plasmonic terahertz detectors

Abstract We experimentally investigated the asymmetric dual-grating-gate plasmonic terahertz (THz) detector based on an InGaAs-channel high-electron-mobility transistor (HEMT) in the gate-readout configuration. Throughout the THz pulse detection measurement on the fabricated device, we discovered a new detection mechanism called the “3D rectification effect” at the positive gate bias application, which is a cooperative effect of the plasmonic nonlinearities in the channel with the diode nonlinearity in the heterobarrier between the InGaAs channel layer and the InAlAs spacer/carrier-supply/barrier layers, resulting in a giant enhancement of the detector responsivity. We also found that an undesired long-tail waveform observed on the temporal pulse photoresponse of the device is due to trapping of carriers to the donor levels in the silicon δ-doped carrier-supply layer when they tunnel through the barrier to the gate and can be eliminated completely by introducing the so-called inverted-HEMT structure. The internal current responsivity and noise-equivalent power are estimated to be 0.49 A/W (with the equivalent voltage responsivity of 4.9 kV/W with a high output impedance of 10 kΩ) and 196 pW/√Hz at 0.8 THz. These results pave the way towards the application of the plasmonic THz detectors to beyond-5G THz wireless communication systems.

Terahertz wireless communication systems: challenges and solutions for realizations of effective bidirectional links

Wireless communication systems have evolved through a pursuit for broader bandwidths and a drive to higher frequencies. The drive has continued to present day and is now approaching the terahertz (THz) spectrum, where there exists great potential for broadband communication—and equally great challenges. Of note are the challenges of mobility for conventional THz transmitters, which have low transmitted powers, large sizes, and high power consumption. The proposed work recognizes these challenges and introduces the concept of retro-modulation in passive THz transceivers to have them establish passive THz links. Conventional (active) THz transceivers and links are contrasted to the proposed (passive) THz transceivers and links, with experimental and theoretical results given for THz retro-modulators having corner-cube and spherical retroreflectors with optical and electrical modulators. Ultimately, the findings show that passive THz links with high-frequency electronic transmitters and detectors and the proposed THz retro-modulators are capable of operation with signal-to-noise ratios between 10 to 20 dB at 300 GHz. Such findings open the door to future bidirectional THz wireless communication systems with mobile THz transceivers.

Performance Analysis of IRS-Assisted THz Communication Systems over α-μ Fading Channels with Pointing Errors.

In this paper, we analyze the performance of an intelligent reflecting surface (IRS)-aided terahertz (THz) wireless communication system with pointing errors. Specifically, we derive closed-form analytical expressions for the upper bounded ergodic capacity and approximate expression of the outage probability. We adopt an α-μ fading channel model for our analysis that is experimentally demonstrated to be a good fit for THz small-scale fading statistics, especially in indoor communication scenarios. In the proposed analysis, the statistical distribution of the α-μ fading channel is used to derive analytical expressions for the ergodic capacity and outage probability. Our proposed analysis considers not only the IRS reflected channels, but also the direct channel between the communication nodes. The results of the derived analytical expressions are validated through Monte Carlo simulations. Through simulations, it has been noticed that pointing errors degrade the performance of the IRS-assisted THz wireless communication system which can be compensated by deploying an IRS having a large number of reflecting elements.

Active terahertz beam steering based on mechanical deformation of liquid crystal elastomer metasurface

Active metasurfaces are emerging as the core of next-generation optical devices with their tunable optical responses and flat-compact topography. Especially for the terahertz band, active metasurfaces have been developed as fascinating devices for optical chopping and compressive sensing imaging. However, performance regulation by changing the dielectric parameters of the integrated functional materials exhibits severe limitations and parasitic losses. Here, we introduce a C-shape-split-ring-based phase discontinuity metasurface with liquid crystal elastomer as the substrate for infrared modulation of terahertz wavefront. Line-focused infrared light is applied to manipulate the deflection of the liquid crystal elastomer substrate, enabling controllable and broadband wavefront steering with a maximum output angle change of 22° at 0.68 THz. Heating as another control method is also investigated and compared with infrared control. We further demonstrate the performance of liquid crystal elastomer metasurface as a beam steerer, frequency modulator, and tunable beam splitter, which are highly desired in terahertz wireless communication and imaging systems. The proposed scheme demonstrates the promising prospects of mechanically deformable metasurfaces, thereby paving the path for the development of reconfigurable metasurfaces.

6G-oriented ultra-wideband fiber-THz-fiber seamless converged communication system: architecture, key techniques and verification

Terahertz (THz) communication is widely regarded as a key component of the future 6G mobile communication system. This paper proposes a novel ultra-wideband fiber-THz-fiber seamless converged real-time architecture that fully exploits the commercially mature digital coherent optical module (DCO) to realize ultra-high capacity THz real-time wireless communication by conducting a comparative analysis for some of the main existing technical routes of THz up- and down-conversion in THz wireless communication systems. (1) The proposed architecture employs the techniques of dual-polarization photonic up-conversion for THz generation and hybrid optoelectronic down-conversion for THz reception, respectively, to enable seamless integration of optical fiber and THz communications. (2) Due to the limited bandwidth of the optoelectronic devices, multi-dimensional modulation techniques are adopted for ultra-wideband terahertz signals to improve the spectral efficiency and transmission capacity. (3) An intelligent nonlinear joint compensation technique based on deep neural network (DNN) is proposed, which can effectively improve the signal-to-noise ratio of the time-varying hybrid fiber-THz-fiber channel. Based on the above-mentioned investigations, we for the first time realize the photonics-aided record-high 100/200 GbE real-time THz wireless transmission at 360 (∼) 430 GHz band, with a capacity of 10 to 20 times that of 5G. The proposed fiber-THz-fiber architecture can realize the smooth conversion between high-speed THz signal and lightwave signal. Furthermore, the architecture can significantly reduce research complexity and development costs and thus greatly accelerate the commercialization of 6G THz technology by highly reusing commercial DCO modules, which is compatible with the physical layer transmission protocols such as IEEE 802.3 and ITU-T G.798. Finally, this paper discusses some potential research and development directions for higher-capacity, longer-distance, and more-integrated fiber-THz-fiber seamless communication.

An ultra-wideband orthogonal-beam directional graphene-based antenna for THz wireless systems

In terahertz (THz) wireless systems, graphene shows a tremendous promise for the implementation of miniaturized and reconfigurable antennas due to its unique tunable property. This paper presents a wideband beam reconfigurable directional antenna for THz wireless communication systems. The antenna design concept is based on the Yagi-Uda antenna working principle. The proposed antenna consists of a centre-fed graphene-based driven dipole and three graphene-based parasitic elements on either side of the driven element. These graphene-based parasitic elements either act as directors or reflectors by controlling the surface conductivity of these elements. The surface conductivity of the elements is adjusted individually by applying a bias voltage via the chemical potential of the graphene. The main beam direction of the antenna can be reconfigured by controlling the chemical potentials of the graphene-based parasitic elements. Specifically, the proposed graphene THz antenna reconfigures the main beam into four orthogonal directions (0°, 90°, 180° and 270°) at an operational frequency 1.25 THz. The antenna achieves a directional symmetrical radiation pattern with 14 dBi gain and a front-to-back ratio of 15.5 dB. Furthermore, the proposed graphene THz antenna provides a stable reflection coefficient in four reconfigurable cases and an ultra-wideband of 10-dB impedance bandwidth of 120%. Therefore, this novel design technique of graphene-based ultra-wideband high gain beam reconfigurable THz antenna is promising for THz wireless systems.

Performance Analysis of Dual-Hop THz Transmission Systems Over α-μ Fading Channels With Pointing Errors

In this article, the performance of a dual-hop relaying terahertz (THz) wireless communication system is investigated. In particular, the behaviors of the two THz hops are determined by three factors, such as the deterministic path loss, fading effects, and pointing errors. Assuming that both THz links are subject to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> - <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> fading with pointing errors, we derive exact expressions for the cumulative distribution function (CDF) and probability density function (PDF) of the end-to-end signal-to-noise ratio (SNR). With these statistical expressions, some important performance metrics are evaluated, such as the outage probability (OP), average bit error rate (BER), and average channel capacity (ACC). Moreover, the asymptotic analyses are presented to obtain more insights. The results show that the dual-hop relaying scheme has better performance than the single THz link. Furthermore, the system diversity order depends on the fading parameters and pointing errors of both THz links. In addition, we extend the analysis to a multirelay cooperative diversity system and derive the asymptotic symbol error rate (SER) expressions. Finally, the derived analytical expressions are verified by the Monte Carlo simulation.

Graphene-VO2-based-defect-induced tunable multiple narrowband unidirectional photonic terahertz absorber

This work demonstrates a one-dimensional unidirectional terahertz (THz) absorber with thermal switching from broadband to narrowband and tunable multiple narrowband absorption with vanadium dioxide (VO2)-graphene-based defective photonic crystal. The thermal tuning of defect layer switches the phase of VO2 and obtain multiple narrowband optical absorptance with 70-90% peak at 4.12, 4.86 and 5.23THz respectively with a Q factor around 291 for 4.86 THz peak. The thermal dependent Q factor of the stack varies from 19 to 291 with phase transition from metal to the insulating state of the defect. The optical non-magnetic THz unidirectional absorber has switchable propagation functions within the metallic phase from non-absorption to higher peak absorption with forward and backward propagated wave. The asymmetrical dual defect layer with dual absorption peaks can be switched and the wavelength can be changed by changing the distance between the two peaks. Multiple graphene-based VO2 defects have increased peaks of narrowband absorption. This novel phase changing material (PCM) based asymmetric defective photonic layer can tune the defect layer for optimum and adjustable absorption at THz range and non-magnetic reciprocal and unidirectional structure with temperature dependent dual band switchable, which leads to this structure for terahertz wireless communication systems as well as other THz sensing devices.

Error Rate Analysis of Different Modulation Schemes Over Shadowed Beaulieu-Xie Fading Channels

ABSTRACT A new composite model called shadowed Beaulieu-Xie (BX) is used in terahertz and millimeter-wave wireless communication systems. This model characterizes an environment with an arbitrary number of non-line-of-sight and line-of-sight signals. In this paper, an analysis of the average bit error rate (ABER) is done. The analysis is carried out for different coherent and non-coherent modulation techniques. The mathematical expressions of ABER are obtained using the probability density function of the output signal to noise ratio of the shadowed BX fading channel. A new closed-form ABER expression for gray-coded rectangular quadrature amplitude modulation over the shadowed BX channel is also evaluated. The effects of fading and shadowing parameters on the modulation techniques are analyzed and compared.

Graphene-based plasmonic metamaterial for terahertz laser transistors

Abstract This paper reviews recent advances in the research and development of graphene-based plasmonic metamaterials for terahertz (THz) laser transistors. The authors’ theoretical discovery on THz laser transistors in 2007 was realized as a distributed-feedback dual-gate graphene-channel field-effect transistor (DFB-DG-GFET) in 2018, demonstrating ∼0.1 µW single-mode emission at 5.2 THz and ∼80 µW amplified spontaneous 1–7.6 THz emission at 100 K. To realize room-temperature, dry-cell-battery operating intense THz lasing with fast direct modulation, various approaches based on graphene plasmonic metamaterials are investigated and introduced as real device implementations, including (i) replacement of the laser photonic cavity with plasmonic cavity enormously improving the THz photon field confinement with larger gain overlapping, (ii) introduction of THz amplification of stimulated emission via current-driven graphene Dirac plasmons (GDPs), and (iii) controlling the parity and time-reversal symmetry of GDPs enabling ultrafast direct gain-switch modulation. Possible real device structures and design constraints are discussed and addressed toward coherent light sources applicable to future 6G- and 7G-class THz wireless communication systems.