Guest Editorial: Antennas and propagation at millimetre, sub-millimetre wave and terahertz bands

A revolutionary enhancement of transmission data rates is required to meet the challenge of an exponential capacity increase for the sixth generation (6G) and beyond mobile communication and sensing systems in the era of 2030 and beyond. This generates a strong motivation for exploring the underutilised millimetre wave (30–300 GHz), sub-millimetre wave (300–3000 GHz), and terahertz (0.1–10 THz) bands. These frequencies offer great opportunities for device miniaturisation thanks to short wavelengths fostering compact, high gain antennas and an abundance of spectrum for a variety of future applications. However, preserving good performance with ever-decreasing form factor and under ever-increasing interference scenarios is making the complicated task of antenna design even more complex. Moreover, the synergism of susceptibility of molecular absorption, the changed relationships between wavelengths and dimensions of objects, and the ultra-broadband bandwidths, results in propagation at millimetre wave (mmWave), sub-mmWave, and terahertz (THz) bands distinguished from microwave frequencies. The current Special Issue is focused on research ideas, articles, and experimental studies on antennas and propagation at mmWave, sub-mmWave, and THz bands, with the priority of considering the contributions with experimental results between 90 and 100 GHz and higher. In this Special Issue, we have received 22 papers, all of which underwent peer review. Ten high-quality contributions to this Special Issue have been selected in a strict peer review process supported by reputed international experts. They can be clustered into three main categories, namely antennas at the THz band, antennas at the mmWave band, and propagation at mmWave and THz bands. The paper laying in the first category exhibit novelties in the design of antennas for THz applications. The paper in this category is by Xu et al. The second category of papers offers the design of advanced mmWave antennas with cutting-edge technologies, such as shared slotted waveguide arrays and metasurfaces. These papers are by Wang et al., Huang et al., Zhang et al, and Peng et al. The last category investigates the propagation and channel characteristics of mmWave and sub-THz bands and explore the potential applications in these bands. The papers in this category are by Doeker et al., Lee et al., Zhang et al., Wang et al., and Yue et al. A brief presentation of each of the papers in this Special Issue follows. Doeker et al. present a device discovery approach using results from ray tracing combined with an iterative fine tuning. By evaluating the impact of antenna characteristics, it is shown that the sidelobes lead to an error in the final alignment of THz highly directive antennas due to the superposition of the received power via multipath components, and equations to calculate the maximum error are given. Besides, the impact is evaluated with respect to sidelobe suppression. Here, equations to calculate the maximum error are given and it is shown that the sidelobes do not affect the alignment if the gain of the sidelobe is more than 30 dB lower than the gain of the mainlobe. Xu et al. design different on-chip antennas in Complementary metal-oxide-semiconductor (CMOS) technology for THz detector and source. For the THz detector, multi-band antenna with dual-ring structure and miniaturised antennas with circular and diamond structures are designed. At the frequency of 300 GHz, 600 GHz, and 800 GHz, the gain of the ring antennas is 5.7 dBi, 7.2 dBi, and 7.2 dBi respectively. The gain of the circular and diamond antenna is 5.3 dBi and 5.96 dBi at 300 GHz. For the THz source, rectangular-slot and T-slot antennas are designed at the frequency of 300 GHz, the gain of the rectangular-slot and T-slot antenna is 2.92 dBi and 3.87 dBi respectively. The effectiveness of antennas is verified by the measurement of the detector and source, which indicates that the proposed on-chip antennas have potential for CMOS THz applications. Wang et al. design a W-band dual-beam low side-lobe level antenna with a shared slotted waveguide radiating layer. The proposed antenna consists of a symmetrical dual ports feed network layer located on two sides of the antenna, a novel shared symmetrical 4 × 32 slotted waveguide array radiation layer, and a bottom layer. Based on measurement results, the designed antenna has a 24.8 dBi far-field gain, −17/−25.5 dB E/H plane side lobe level, and 19.8° beam deflection at 94 GHz. The performances of the two ports were in good agreement with each other, and the measured isolation was above 14.5 dB at 94 GHz. Huang et al. design a novel 24 GHz circularly polarised (CP) metasurface rectenna for wireless power transmission. Measurements show that this metasurface antenna has a gain of 11.3 dBic and an axial ratio of 2.5 dB at 24 GHz. The proposed CP metasurface rectenna has the characteristics of high conversion efficiency and low profile, and can be conformal to the electrical equipment. It is suitable for mmWave power transmission systems. Zhang et al. present a novel design of multiple lens-based compact antenna test ranges (CATRs) to simulate multiple angles of arrival from different base-stations in radio resource management (RRM) measurement. Composed of a metasurface lens for phase alignment and a 2 × 2 feed array antenna for the emission of electromagnetic signals, only four lens-based CATRs can create plane wave conditions at the device under test with six different angles of arrival. Validated through simulations and measurements, the worst-case amplitude variation in the quiet zone region is lower than ±0.8 dB at 26 GHz, and phase variation is lower than ±7.5° in the designed quiet zone, which could be used in the fifth-generation (5G) mmWave RRM measurement. Peng et al. propose a printed ridge gap (PRGW)-based 2 × 2 mmWave antenna array with broad bandwidth and compact aperture. To validate the reflection and radiation performance, an array prototype is fabricated followed by the experimental verification. Measurement results show that a −10-dB impedance bandwidth of 14.3% (26.0–30.0 GHz) is realised for the prototype. Furthermore, the cross polarisation (XP) radiation of the array is quite low, and an average realised gain of 9.8 dBi is obtained. Lee et al. provide analyses of multi-frequency propagation characteristics based on wideband measurements at 28, 38, 71, 82, and 159 GHz, with an emphasis on the sub-THz (i.e. the 159 GHz) propagation characteristics. All the frequency measurements were conducted in the same cubicle office room environment by locating the transmitter and the receiver at the same positions. The measurement results show that the path loss and the shadow fading characteristics do not exhibit significant differences depending on the frequency range from 28 to 159 GHz. However, the delay spread shows a generally decreasing trend as the frequency increases. These observations and findings will be useful for the development of 5G beyond and/or 6G networks in the sub-THz bands. Zhang et al. propose a rain attenuation prediction model for terrestrial line-of-sight links, particularly for mmWave frequency. The total rain attenuation described in this model accounts for the contributions of wet antenna attenuation and path rain attenuation. In addition, a rainfall rate adjustment factor is proposed. The proposed model has a root-mean-square error of 21.54% in prediction, which is an improvement compared to other existing models. Because the rainfall rate adjustment factor used in the model approaches 1 at short distances, the model can be applied to rain attenuation prediction for short-distance links. The results will assist designers to design mmWave link margins more accurately. Wang et al. analyse the wireless channels characteristics of Hyperloops at mmWave bands based on the ray-tracing method. The simulation results show that the mmWave frequency mainly affects large-scale fading parameters, but has little influence on small-scale fading parameters. Consequently, the research of channel characteristics in vacuum tube scenarios is of great significance for Hyperloop train-ground communication system design. Yue et al. introduce the basic principle of hybrid beamforming design in THz by considering its unique channel features. First, angular orthogonality is obtained by exploiting the spatially sparse nature of the THz channel. Then, typical THz systems are studied, which includes a single-user/multi-user THz communication system, a THz integrated data and energy transfer system and an intelligent reflecting surface aided THz system. Some exemplary simulation results demonstrate the advantage of our hybrid beamforming design in THz. At last, open challenges are provided for stimulating future research interest. All of the papers selected for this Special Issue provide the first-hand information and the state of the art of advanced antenna technologies and propagation modelling at mmWave, sub-mmWave, and THz bands. A great understanding of the behaviour of propagation and wireless channels and corresponding advanced antenna designs will pave the way towards the realisation of 6G and beyond at these challenging frequencies. The Guest Editors would like to thank all the authors for their submissions and the reviewers for their high-quality reviews and helpful suggestions to the authors on improving the content and presentation of the papers. We would also like to extend our sincere thanks to the journal's Editors-in-Chief and the Editorial Office for providing us the opportunity to organise this Special Issue and supporting us throughout the whole process. Finally, the guest editors wish you an enjoyable reading of the contributions to this Special Issue. Ke Guan (Senior Member, IEEE) received B.E. degree and Ph.D. degree from Beijing Jiaotong University in 2006 and 2014 respectively. He is a Full Professor in State Key Laboratory of Rail Traffic Control and Safety & School of Electronic and Information Engineering, Beijing Jiaotong University. In 2016, he has been awarded a Humboldt Research Fellowship for Postdoctoral Researchers. He was a Visiting Scholar with Universidad Politécnica de Madrid, Spain in 2009 and 2013. From 2011 to 2013 and from 2016 to 2018, he was a Research Scholar with the Institut für Nachrichtentechnik (IfN) at Technische Universität Braunschweig, Germany. From February 2023 to July 2023, he was a Guest Professor at Technische Universität Wien, Austria. He has authored/coauthored two books and five book chapter, more than 200 journal and conference papers, and ten patents. His current research interests include measurement and modelling of wireless propagation channels, high-speed railway communications, ray-tracing and machine learning based digital twin of electromagnetic environments in various complex scenarios, such as vehicle-to-x communications, terahertz communication systems, integrated sensing and communications, and space-air-ground integrated networks. Dr. Guan is the pole leader of EURNEX (European Railway Research Network of Excellence). He was the recipient of a 2014 International Union of Radio Science (URSI) Young Scientist Award. His papers received 14 Best Paper Awards, including IEEE vehicular technology society Neal Shepherd memorial best propagation paper award in 2019 and 2022. He is an Editor of IEEE Vehicular Technology Magazine and IET Microwave, Antenna and Propagation, and a Guest Editor of the IEEE Transactions on Vehicular Technology and IEEE Communication Magazine. He serves as a Publicity Chair in PIMRC 2016, the Publicity Co-Chair in ITST 2018, the Track Co-Chair in EuCNC, the International Liaison of EUSIPCO 2019, the Session Convener of EuCAP 2015-2022, and a TPC Member for many IEEE conferences, such as Globecom, ICC and VTC. He has been a delegate in 3GPP and a member of the IC1004, CA15104, and CA20120 initiatives. Wilhelm Keusgen received his Dipl.-Ing. and Dr.-Ing. degrees from RWTH Aachen University in 1999 and 2005 respectively. From 1999 to 2004, he was with the Institute of High Frequency Technology, RWTH Aachen University, where he worked on microwave frontend technologies, millimetre-wave and sub-millimetre wave antennas, and reciprocity of MIMO systems. From 2004 to 2021, he has headed the research group for millimetre-wave and advanced transceiver technologies at Fraunhofer HHI. Since 2021, he has been a Professor at Technische Universität Berlin. His main research areas are millimetre-wave and THz communications for 5G and beyond, measurement and modelling of wireless propagation channels, MIMO, full duplex, and transceiver impairments compensation. He has published more than 130 scientific papers. Wei Fan obtained his Ph.D. degree in wireless communication from Aalborg University (AAU), Denmark in 2014. He was directly promoted to assistant professor in 2014 after his Ph.D. graduation and then promoted to tenured associate professor in August 2017. He was admitted to the professor promotion program in Aalborg University in March 2023. He also holds a Docentship on ‘Radio Propagation Measurement and OTA Testing Methodologies’ with University of Oulu, Finland since Jan 2023. He currently leads the research group ‘wireless propagation and over-the-air (OTA) testing’ at AAU. Cesar Briso is full professor and director of the Radiocommunications Group at the Technical University of Madrid, SPAIN. He has a 30-year research trajectory, initially focused on the study and design of circuits and systems of high frequency and radar, and in the last 20 years he has focused on the design and development of wireless communications for transportation systems, especially focused on high speed trains, metropolitan railways and Unmanned aerial Vehicles. On 2010 he started working on wideband channel critical communications using 5G. On this topic he has done a relevant research on the last years, making several scientific publications and collaborations with international experts of Europe, China and USA. He has managed 23 national and international research projects and hold two patents on critical communications for transportation systems. Now he is working on the project: ‘Next Generation Train Communications Systems’, inside the Chinese program ‘The Belt and the Road’. He is also author of 40 journal papers and has participated on more than 60 international congress. He has been editor of 6 Special Issue and 2 books on wireless communications for transportation. He has received 4 National prizes for his research. Bo Sun is a senior specialist on wireless communication technology and standardisation in Sanechips Technology Co., Ltd. His research interests include wireless communication system design and implementation, IoT and next generation wireless technologies etc. He is proactive in IEEE wireless standard development and was honored with contribution awards for his contribution to several IEEE 802.11 standards. He was the PHY adhoc co-chair of IEEE 802.11 TGax which has successfully developed IEEE 802.11ax standard (a.k.a. Wi-Fi 6). Now, he is the chair of IEEE 802.11 TGbd (a.k.a. Next Generation V2X) and the chair of IEEE 802.11 AMP SG. He is also the chair of China NITS SC41 (IoT) WG2 (Networking and Communications).