Terahertz Wireless CommunicationS

Zhi Chen

Chong Han

Xianbin Yu

Guangjian Wang

Nan Yang

Mugen Peng

With Wireless communications in the 0.1-10

TeraHertz (THz) band are envisioned as one of the key enablers towards ubiquitous wireless communications beyond 5G accommodating a massive number of connected devices and ultra-high user data rates in the order of Tera-bit-per-second. The THz band is located between the millimeter-wave (mmWave) and the far infrared (IR) band and still considered as one of the least investigated and exploited regions in the electromagnetic spectrum, although it offers much higher bandwidth than the mmWave range and more favorable propagation conditions than the IR band. Recently, significant progress has been made with respect to THz devices based on different technologies, and commercial THz systems are anticipated to become a reality in the near future.

Recently, THz communications have been identified by IEEE COMSOC as one of the nine communication technology trends to follow. As 5G technology becomes commercial, THz communications are where fundamental scientific and engineering breakthroughs will occur for 6G and beyond wireless systems. THz communications are envisioned as a key wireless technology to meet future demands of Tera-bit-per-second wireless communications, by alleviating the spectrum scarcity and capacity limitations of current wireless systems. Moreover, THz systems are expected to be beneficial not only for traditional macroscale wireless networks but also emerging paradigms such as wireless intra- and inter-chip communications, nanocommunications and the Internet of Bio-Nano Things.

Inspired of these, this feature topic invites submissions of high-quality original research papers capturing the state-of-the-art advances in the theoretical foundations and practical implementation of THz communications and networking. The Call for Papers generated considerable interest in the research community, and 16 out of in total 20 submissions were accepted after a rigorous review process.

The feature topic begins with the article by Guan et al., “Towards 6G: Paradigm of Realistic Terahertz Channel Modeling”. This article 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. New concerns on channel modeling resulting from distinguishing features of THz wave are discussed regarding propagation, antenna array, and device aspects, respectively.

The article by Tang et al., “Channel Measurement and Path Loss Modeling from 220 GHz to 330 GHz for 6G Wireless Communications,” presents an extensive THz channel measurement campaign for 6G wireless communications from 220 GHz to 330 GHz. Moreover, 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, while 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. Furthermore, the cellular performance of THz communication systems is investigated by using the obtained path loss model. This work gives an insight into the design and optimization of THz communication systems for 6G.

By surveying the latest literature findings, the article by Liu et al., “THz Channel Modeling: Consolidating the Road to THz Communications,” reviews the problems 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 modeling. Then, the authors focus on simulators and experiments in the THz band, as well as applications of THz channel models with respects to capacity, security, and sensing as examples. Finally, some key issues in the future THz channel modeling are highlighted.

The article by He et al., “A 3-D Hybrid Dynamic Channel Model for Indoor THz Communications”, proposes a three-dimensional (3-D) dynamic indoor THz channel model 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, the dynamic generation procedure of the channel impulse responses (CIRs) is presented. 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.

The article by Tan et al., “Wideband Channel Estimation for THz Massive MIMO”, proposes a beam split pattern detection-based channel estimation scheme to realize reliable wideband channel estimation in THz massive MIMO systems. A comprehensive analysis of the angle-domain sparse structure of the wideband channel is provided by considering the beam split effect. Based on the analysis, a series of index sets called beam split patterns are proved to have a one-to-one match to different physical channel directions. The sparse channel supports at different subcarriers can be obtained by utilizing a support detection window, which is generated by expanding the beam split pattern. Finally, the wideband channel can be recovered by calculating the elements on the total sparse channel support at all subcarriers. Simulation results show that the proposed scheme is able to achieve higher accuracy than existing schemes.

The article by Yang et al., “Hybrid Precoding for Cluster-Based Multi-Carrier Beam Division Multiple Access in Terahertz Wireless Communications” proposes a hybrid precoding algorithm 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. A three-step hybrid precoding algorithm with low complexity is proposed, 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 the proposed algorithm, especially in the massive-con nection scenarios.

The article by Ma et al., “Towards Intelligent Reflecting Surface Empowered 6G Terahertz Communications: A Survey”, analyzes an intelligent reflecting surface (IRS) for 6G THz wireless communications. In order to mitigate blockage vulnerability caused by serious propagation attenuation and poor diffraction of THz waves, the 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. This paper mainly presents a comprehensive survey on the combination of IRS and THz communications. Concretely, the driving applications, the key technologies as well as the emerging challenges brought by IRS empowered 6G THz communications are investigated thoroughly.

The article by Gao et al., “Coverage and Area Spectral Efficiency Analysis of Dense Terahertz Networks in Finite Region”, develops a general and tractable framework for the finite-sized downlink THz network. Specifically, the molecular absorption loss, receiver locations, directional antennas, and dynamic blockage are taken into account. 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. Numerical results validate the theoretical analysis and suggest that two or more dominant interferers are required to provide sufficiently tight approximations for the SINR coverage probability.

The article by Yang et al., “Terahertz Orbital Angular Momentum: Generation, Detection and Communication”, provides an overview on 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 have become the key engine to drive this direction forward. THz-OAM beams are envisioned to have a great potential in delivering very large capacity in next-generation wireless communications.

The article by Zhang et al., “Terahertz Band: Lighting up Next-Generation Wireless Communications”, provides an overview on the state-of-the-art THz communications, 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 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.

The article by Lu et al., “A Review on Applications of Integrated Terahertz Systems”, presents a review on THz end-to-end systems with an emphasis on integrated approaches. 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. In particular, the development on the microwave side towards the lower portion of the THz spectrum, namely from 100 GHz to 2 THz, is investigated. The distinctive properties of THz waves are examined, including high directivity, high-frequency bandwidth, unique interaction with water, atmosphere, and molecules, and the ability to penetrate optically-opaque materials. The performance of integrated systems is compared with non-integrated solutions, followed by a discussion on the trend in future research avenues and applications. The flourish of THz applications will, in turn, promote the researches of THz integrated devices and systems as well.In the article by Liu et al., “A Semi-Blind Method to Estimate the I/Q Imbalance of THz Orthogonal Modulator”, a semi-blind I/Q imbalance estimation method based on predefined preamble and pilot sequence is proposed, which can be used to evaluate the I/Q unbalance damage of the Y band orthogonal modulator. In particular, a transmitter with a Y band quadrature mixer and a 20Gbps base-band signal is tested. The bandwidth of the baseband signal is GHz, and the modulation type is 16 quadrature amplitude modulation (QAM). By this method, 7dB improvement of the system’s symbol mean square error (MSE) is observed experimentally, which can be used to eliminate the I/Q imbalance effectively.

The article by Zhang et al., “A 20.8-Gbps Dual-Carrier Wireless Communication Link in 220-GHz Band”, describes a 220-GHz solid-state dual-carrier wireless link whose maximum transmission real-time data rates are 20.8Gbps (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 proposed 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 8.6×〖10〗^(-7) and 3.8×〖10〗^(-7), respectively. 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.

The article by Zhou et al., “Terahertz Direct Modulation Techniques for High Speed Communication System”, presents review and outlook of THz direct modulation technology. Specifically, this article describes the key technologies of THz direct modulation by reviewing different types of high-speed THz direct modulation technology including high-electron-mobility-transistor-based THz direct modulation, switches-in-parallel THz direct modulation, diode-based THz direct modulation, quantum-cascade-laser-based THz direct modulation, and new-material-based THz direct modulation. Moreover, the difficulties in THz high-speed direct modulation technology are also examined. The developing direction of THz direct modulation technology is discussed based on the development of present THz direct modulation technology and future demand for communication.

The article by Cheng et al., “A Wideband E-plane Crossover Coupler for Terahertz Applications”, reports a new approach to design THz E-plane crossover coupler. 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 the 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 lower than -15 dB and transmission coefficient higher than -2 dB is achieved.

Finally, the article by Li et al., “A 16-QAM 45-Gbps 7-m Wireless Link Using InP HEMT LNA and GaAs SBD Mixers at 220-GHz-Band”, presents a 220-GHz-band 7-m wireless link with a 45-Gbps transmission data rate by using 16QAM. Super-heterodyne transceiver modules are developed for transmission and reception of the modulated signals, which consist of a Schottky barrier diode (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 varied from 5 GHz to 20 GHz. The measured results indicate that the transceiver modules enable the data transmission of a 45-Gbps 16-QAM signal with the SNR from 11.59 dB to 15.36 dB in a 7-m line-of-sight channel.

In conclusion, the Guest Editors of this feature topic would like to thank all the authors for their contributions, and the anonymous reviewers for their constructive comments and suggestions. We also would like to acknowledge the guidance from Ms. Fan, Ms. Nie and the editorial team of China Communications.

Biographies

Zhi Chenreceived B.Eng, M.Eng., and Ph.D. degree in Electrical Engineering from University of Electronic Science and Technology of China (UESTC), in 1997, 2000, 2006, respectively. On April 2006, he joined the National Key Lab of Science and Technology on Communications (NCL), UESTC, and worked as a professor in this lab from August 2013. He was a visiting scholar at University of California, Riverside during 2010-2011. He is also the deputy director of Key Laboratory of Terahertz Technology, Ministry of Education. His current research interests include Terahertz communication, 5G mobile communications and tactile internet.

Chong Hanreceived Ph.D. degree in Electrical and Computer Engineering from Georgia Institute of Technology, USA in 2016. He is currently an Associate Professor with the Terahertz Wireless Communications (TWC) Laboratory, University of Michigan-Shanghai Jiao Tong University (UM-SJTU) Joint Institute, Shanghai Jiao Tong University, China. He is the recipient of 2018 Elsevier NanoComNet (Nano Communication Network Journal) Young Investigator Award, 2017 Shanghai Sailing Program 2017, and 2018 Shanghai ChenGuang Program. He is an editor with IEEE Open Journal of Vehicular Technology since 2020, an associate editor with IEEE Access since 2017, an editor with Elsevier Nano Communication Network journal since 2016, and is a TPC chair to organize multiple IEEE and ACM conferences and workshops. His research interests include Terahertz band and millimeter-wave communication networks, and electromagnetic nanonetworks. He is a member of the IEEE and ACM.

Xianbin Yuis currently a research professor in the College of Information Science and Electronic Engineering at Zhejiang University. He received his PhD degree in 2005 from Zhejiang University in China. From October 2005 to October 2007, he was a postdoctoral researcher in Tsinghua University, China. Since November 2007, he was employed at the Technical University of Denmark (DTU), where he became an assistant professor in 2009 and was promoted to be a tenured Senior Researcher in 2013. He served as a session chair/TPC member for a number of international conferences, has given 30+ invited talks in prestigious international conferences, and has (co-)authored 3 book chapters and 200+ peer-reviewed international journal and conference papers. His research interests are in the areas of ultrafast millimeter-wave/THz photonic information processing, RF photonic wireless communication systems, as well as emerging new applications of millimeter-wave/THz technologies.

Guangjian Wangreceived M.Eng in Communication and Information Engineering from Dalian Maritime University in 2007. On April 2007, he joined Huawei Technologies Co., Ltd., where he is currently a senior research expert in 2012 lab. His current research interests include advanced wireless technologies, millimeter-wave and Terahertz communication and sensing technologies. He is a member of the IEEE.

Nan Yangreceived the Ph.D. degree in electronic engineering from Beijing Institute of Technology, China, in 2011. He has been with the School of Engineering at the Australian National University since July 2014, where he currently works as an Associate Professor. He received the IEEE ComSoc Asia-Pacific Outstanding Young Researcher Award in 2014 and the Best Paper Awards from the IEEE GlobeCom 2016 and the IEEE VTC 2013-Spring. He also received the Top Editor Award from the Transactions on Emerging Telecommunications Technologies, the Exemplary Reviewer Awards from the IEEE Transactions on Communications, IEEE Wireless Communications Letters, and IEEE Communications Letters, and the Top Reviewer Award from the IEEE Transactions on Vehicular Technology from 2012 to 2019. He is currently serving in the Editorial Board of the IEEE Transactions on Wireless Communications, IEEE Transactions on Molecular, Biological, and Multi-Scale Communications, and IEEE Transactions on Vehicular Technology. His research interests include terahertz communications, ultra-reliable low latency communications, cyber-physical security, next generation multiple access, and molecular communications.

Mugen Peng(IEEE Fellow) received the Ph.D. degree in communication and information systems from the Beijing University of Posts and Telecommunications (BUPT), Beijing, China, in 2005. Afterward, he joined BUPT, where he has been the dean of the School of Information and Communication Engineering since Jun. 2020, and the deputy director of State Key Laboratory of Networking and Switching Technology since Oct. 2018. During 2014 he was also an academic visiting fellow at Princeton University, USA. He has authored and co-authored over 150 refereed IEEE journal papers and over 250 conference proceeding papers. His main research areas include wireless communication theory, radio signal processing, cooperative communication, self-organization networking, heterogeneous networking, cloud communication, and Internet of Things. Dr. Peng was a recipient of the 2018 Heinrich Hertz Prize Paper Award, the 2014 IEEE ComSoc AP Outstanding Young Researcher Award, and the Best Paper Award in the JCN 2016, IEEE WCNC 2015, IEEE GameNets 2014, IEEE CIT 2014, ICCTA 2011, IC-BNMT 2010, and IET CCWMC 2009. He is currently or have been on the Editorial/Associate Editorial Board of the IEEE Communications Magazine, the IEEE Internet of Things Journal, the IEEE Transactions on Vehicular Technology, the Intelligent and Converged Networks, and the Digital Communications and Networks (DCN).