This paper presents a 220-GHz-band 7-m wireless link with a 45-Gbps transmission data rate by using 16 quadrature amplitude modulation (16-QAM). Super-heterodyne transceiver modules are developed for transmission and reception of the modulated signals, which consist of a Schottky barrier diodes (SBD) based sub-harmonic mixer (SHM), an InP HEMT low noise amplifier (LNA), a waveguide band-pass filter (BPF), and a 108-GHz local oscillator (LO) multiplier chain. The transmitter features a peak transmit power of 1.41 dBm, and the IF frequency varies from 5 GHz to 20 GHz. Besides, the receiver features a conversion gain of 9.3 dB in average and a noise temperature of 3052.8 K. The measured results indicate that the transceiver modules enable data transmission of a 45-Gbps 16-QAM signal with Signal-Noise-Ratio (SNR) from 11.59 dB to 15.36 dB in a 7-m line-of-sight channel.

In this paper, a new approach to design terahertz (THz) E-plane crossover coupler is reported. By cascading two symmetrical septum polarizers, a simple structure with wide operating bandwidth and high isolation performance is achieved. The working principle is explained by operating waveguide modes. To simplify the optimization process, the scattering matrix (S-matrix) of the crossover is calculated. Two prototypes loaded and unloaded electromagnetic band gap (EBG) are fabricated and measured. The electrical contact problem at assembly plane is eliminated by the prototype loaded EBG. A measured bandwidth of 17.3% from 92.5 to 110 GHz for reflection and isolation coefficients < -15 dB and transmission coefficient > -2 dB is achieved.

Terahertz communication technology can provide abundant frequency resources, strong confidentiality, antijamming capability, communication tracking capability and the ability to achieve high-speed data transmissions and can serve as an important technical method for high-speed communications in the future. Among these terahertz communication technologies, terahertz direct modulation technology is a key means to achieve low system complexity and power consumption. In this paper, a review and outlook of terahertz direct modulation technology are proposed from the aspects of high-electron-mobility-transistor-based terahertz direct modulation, parallel-switch terahertz direct modulation, diode-based terahertz direct modulation, quantum cascade laser-based terahertz direct modulation and new-material-based terahertz direct modulation. We hope through this paper that more readers can gain knowledge about the development and challenges of terahertz direct modulation technology for high-speed communication systems, thus promoting the development of high-speed terahertz communication technology based on direct modulation.

With the successful demonstration of terahertz (THz) high-speed wireless data transmission, the THz frequencies are now becoming a worth candidate for post-5G wireless communications. On the other hand, to bring THz communications a step closer to real scenario application, solving high data rate real-time transmission is also an important issue. This paper describes a 220-GHz solid-state dual-carrier wireless link whose maximum transmission real-time data rates are 20.8 Gbps (10.4 Gbps per channel). By aggregating two carrier signals in the THz band, the contradiction between high real-time data rate communication and low sampling rate analog-to-digital (ADC) and digital-to-analog converter (DAC) is alleviated. The transmitting and receiving front-ends consist of 220-GHz diplexers, 220-GHz sub-harmonic mixers based on anti-parallel Schottky barrier diodes, G-band low-noise amplifiers (LNA), WR-4.3 band high-gain Cassegrain antennas, high data rates dual-DAC and -ADC baseband platform and other components. The low-density parity-check (LDPC) encoding is also realized to improve the bit error rate (BER) of the received signal. Modulated signals are centered at 214.4 GHz and 220.6 GHz with -11.9 dBm and -13.4 dBm output power for channel 1 and 2, respectively. This link is demonstrated to achieve 20.8-Gbps real-time data transmission using 16-QAM modulation over a distance of 1030 m. The measured signal to noise ratio (SNR) is 17.3 dB and 16.5 dB, the corresponding BER is 8.6e-7 and 3.8e-7, respectively. Furthermore, 4K video transmission is also carried out which is clear and free of stutter. The successful transmission of aggregated channels in this wireless link shows the great potential of THz communication for future wireless high-rate real-time data transmission applications.

In terahertz communication, the direct frequency conversion structure in which orthogonal mixer is the main frequency conversion unit, makes engineers get into trouble of in-phase(I) branch and quadrature(Q) branch imbalance, carrier wave leakage, etc. These damages result in system performance tremendous degrades. We proposed a semi-blind method to estimate the I/Q imbalance of THz orthogonal modulator, based on predefined preamble and pilot symbols for quadrature amplitude modulation (QAM). In this paper, a transmitter with Y band quadrature mixer and 20Gbps base-band signal has been tested. The bandwidth of the baseband signal was 7GHz, and the modulation type was 16QAM. By this method, 7dB improvement of the system's symbol Mean Square Error (MSE) has been got. That means the proposed method can be used to eliminate the I/Q imbalance effectively.

A review on Terahertz end-to-end systems with an emphasis on integrated approaches is presented. Four major catalogs of THz integrated systems, including THz communication systems, THz imaging systems, THz radars, and THz spectroscopy systems, are reviewed in this article. The performance of integrated systems is compared with non-integrated solutions, followed by a discussion on the trend in future research avenues and applications.

With the explosion of wireless data rates, the terahertz (THz) band (0.1-10 THz) is envisioned as a promising candidate to break the existing bandwidth bottleneck and satisfy the ever-increasing capacity demand. The THz wireless communications feature a number of attractive properties, such as potential terabit-per-second capacity and high energy efficiency. In this paper, an overview on the state-of-the-art THz communications is studied, with a special focus on key technologies of THz transceivers and THz communication systems. The recent progress on both electronic and photonic THz transmitters are presented, and then the THz receivers operating in direct- and heterodyne reception modes are individually surveyed. Based on the THz transceiver schemes, three kinds of THz wireless communication systems are reviewed, including solid-state electronic systems, photonics-assisted systems and all-photonics systems. The prospective key enabling technologies, corresponding challenges and research directions for lighting up high-speed THz communication systems are discussed as well.

To accommodate the ever-increasing wireless capacity, the terahertz (THz) orbital angular momentum (OAM) beam that combines THz radiation and OAM technologies has attracted much attention recently, with contributing efforts to explore new dimensions in the THz region. In this paper, we provide an overview of the generation and detection techniques of THz-OAM beams, as well as their applications in communications. The principle and research status of typical generation and detection methods are surveyed, and the advantages and disadvantages of each method are summarized from a viewpoint of wireless communication. It is shown that developing novel THz components in generating, detecting and (de)multiplexing THz-OAM beams has become the key engine to drive this direction forward. Anyway, beneficial from the combination of infinite orthogonal modes and large bandwidth, THz-OAM beams will have great potential in delivering very large capacity in next generation wireless communications.

This paper develops a general and tractable framework for the finite-sized downlink terahertz (THz) network. Specifically, the molecular absorption loss, receiver locations, directional antennas, and dynamic blockage are taken into account. Using the tools from stochastic geometry, the exact expressions of the blind probability, signal-to-interference-plus-noise ratio (SINR) coverage probability, and area spectral efficiency (ASE) for the reference receivers and random receivers are derived. The upper bounds of the SINR coverage probability are also obtained by using the generalized dominant interferers approach. Numerical results validate the accuracy of our theoretical analysis and suggest that two or more dominant interferers are required to provide sufficiently tight approximations for the SINR coverage probability. We also show that densifying the finite terahertz networks over a certain density threshold will degrade the coverage probability while the ASE keeps increasing. Moreover, deploying more obstructions appropriately in ultra-dense THz networks will benefit both the coverage probability and ASE.

Terahertz (THz) communications have been widely envisioned as a promising enabler to provide adequate bandwidth and achieve ultra-high data rates for sixth generation (6G) wireless networks. In order to mitigate blockage vulnerability caused by serious propagation attenuation and poor diffraction of THz waves, an intelligent reflecting surface (IRS), which manipulates the propagation of incident electromagnetic waves in a programmable manner by adjusting the phase shifts of passive reflecting elements, is proposed to create smart radio environments, improve spectrum efficiency and enhance coverage capability. Firstly, some prospective application scenarios driven by the IRS empowered THz communications are introduced, including wireless mobile communications, secure communications, unmanned aerial vehicle (UAV) scenario, mobile edge computing (MEC) scenario and THz localization scenario. Then, we discuss the enabling technologies employed by the IRS empowered THz system, involving hardware design, channel estimation, capacity optimization, beam control, resource allocation and robustness design. Moreover, the arising challenges and open problems encountered in the future IRS empowered THz communications are also highlighted. Concretely, these emerging problems possibly originate from channel modeling, new material exploration, experimental IRS testbeds and intensive deployment. Ultimately, the combination of THz communications and IRS is capable of accelerating the development of 6G wireless networks.

Terahertz (THz) wireless communication has the capability to connect massive devices using its ultra-large spectrum resource. We propose a hybrid precoding scheme for the cluster-based multi-carrier beam division multiple access (MC-BDMA) to enable THz massive connections. Both the inter-beam interference and inter-band power leakage in this system are considered. A mathematical model is established to analyze and reduce their effects on the THz signal transmission. By considering the peculiarities of THz channels and characteristics of THz hardware components, we further propose a three-step hybrid precoding algorithm with low complexity, where the received signal power enhancement, the inter-beam interference elimination, and the inter-band power leakage suppression are conducted in succession. Simulation results are presented to demonstrate the high spectrum efficiency and high energy efficiency of our proposed algorithm, especially in the massive-connection scenarios.

Terahertz (THz) communication is considered to be a promising technology for future 6G network. To overcome the severe attenuation and relieve the high power consumption, massive multiple-input multiple-output (MIMO) with hybrid precoding has been widely considered for THz communication. However, accurate wideband channel estimation, which is essential for hybrid precoding, is challenging in THz massive MIMO systems. The existing wideband channel estimation schemes based on the ideal assumption of common sparse channel support will suffer from a severe performance loss due to the beam split effect. In this paper, we propose a beam split pattern detection based channel estimation scheme to realize reliable wideband channel estimation in THz massive MIMO systems. Specifically, a comprehensive analysis on the angle-domain sparse structure of the wideband channel is provided by considering the beam split effect. Based on the analysis, we define a series of index sets called as beam split patterns, which are proved to have a one-to-one match to different physical channel directions. Inspired by this one-to-one match, we propose to estimate the physical channel direction by exploiting beam split patterns at first. Then, the sparse channel supports at different subcarriers can be obtained by utilizing a support detection window. This support detection window is generated by expanding the beam split pattern which is determined by the obtained physical channel direction. The above estimation procedure will be repeated path by path until all path components are estimated. Finally, the wideband channel can be recovered by calculating the elements on the total sparse channel support at all subcarriers. The proposed scheme exploits the wideband channel property implied by the beam split effect, i.e., beam split pattern, which can significantly improve the channel estimation accuracy. Simulation results show that the proposed scheme is able to achieve higher accuracy than existing schemes.

To meet the demands for the explosive growth of mobile data rates and scarcity of spectrum resources in the near future, the terahertz (THz) band has widely been regarded as a key enabler for the upcoming beyond fifth-generation (B5G) wireless communications. An accurate THz channel model is crucial for the design and deployment of the THz wireless communication systems. In this paper, a three-dimensional (3-D) dynamic indoor THz channel model is proposed by means of combining deterministic and stochastic modeling approaches. Clusters are randomly distributed in the indoor environment and each ray is characterized with consideration of molecular absorption and diffuse scattering. Moreover, we present the dynamic generation procedure of the channel impulse responses (CIRs). Statistical properties are investigated to indicate the non-stationarity and feasibility of the proposed model. Finally, by comparing with delay spread and K-factor results from the measurements, the utility of the proposed channel model is verified.

For the sake of meeting the demand of data rates at terabit (Tbit) per second scale in future networks, the terahertz (THz) band is widely accepted as one of the potential key enabling technologies for next generation wireless communication systemsWith the progressive development of THz devices, regrading THz communications at system level is increasing crucial and captured the interest of plenty of researchersWithin this scope, THz channel modeling serves as an indispensable and fundamental elementBy surveying the latest literature findings, this paper reviews the problem of channel modeling in the THz band, with an emphasis on molecular absorption loss, misalignment fading and multipath fading, which are major influence factors in the THz channel modelingThen, we focus on simulators and experiments in the THz band, after which we give a brief introduction on applications of THz channel models with respects to capacity, security, and sensing as examplesFinally, we discuss some key issues in the future THz channel modeling.

Terahertz (THz) communication has been envisioned as a key enabling technology for sixth-generation (6G). In this paper, we present an extensive THz channel measurement campaign for 6G wireless communications from 220 GHz to 330 GHz. Furthermore, the path loss is analyzed and modeled by using two single-frequency path loss models and a multiple-frequencies path loss model. It is found that at most frequency points, the measured path loss is larger than that in the free space. But at around 310 GHz, the propagation attenuation is relatively weaker compared to that in the free space. Also, the frequency dependence of path loss is observed and the frequency exponent of the multiple-frequencies path loss model is 2.1. Moreover, the cellular performance of THz communication systems is investigated by using the obtained path loss model. Simulation results indicate that the current inter-site distance (ISD) for the indoor scenario is too small for THz communications. Furthermore, the tremendous capacity gain can be obtained by using THz bands compared to using microwave bands and millimeter wave bands. Generally, this work can give an insight into the design and optimization of THz communication systems for 6G.

Terahertz (THz) communications are envisioned as a key technology for the sixth-generation wireless communication system (6G). However, it is not practical to perform large-scale channel measurements with high degrees of freedom at THz frequency band. This makes empirical or stochastic modeling approaches relying on measurements no longer stand. In order to break through the bottleneck of scarce full-dimensional channel sounding measurements, this paper presents a novel paradigm for THz channel modeling towards 6G. With the core of high-performance ray tracing (RT), the presented paradigm requires merely quite limited channel sounding to calibrate the geometry and material electromagnetic (EM) properties of the three-dimensional (3D) environment model in the target scenarios. Then, through extensive RT simulations, the parameters extracted from RT simulations can be fed into either ray-based novel stochastic channel models or cluster-based standard channel model families. Verified by RT simulations, these models can generate realistic channels that are valuable for the design and evaluation of THz systems. Representing two ends of 6G THz use cases from microscopy to macroscopy, case studies are made for close-proximity communications, wireless connections on a desktop, and smart rail mobility, respectively. Last but not least, new concerns on channel modeling resulting from distinguishing features of THz wave are discussed regarding propagation, antenna array, and device aspects, respectively.