Abstract-Multi-Beam Steering for 6G Communications Based on Graphene Metasurfaces

 

Huifang Ai, Qianlong Kang, Wei Wang, Kai Guo, Zhongyi Guo

 (a) Indoor THz communication scheme for multi-user scenarios. (b) Schematic of a graphene metasurface that consists of a metal substrate, a dielectric layer, and top graphene ribbons. (c) A sectional view of the graphene metasurface. p is the grating period, w is the width of the graphene ribbon and s is the thickness of the SiO2 dielectric layer.

https://www.mdpi.com/1424-8220/21/14/4784/htm

As communication technology is entering the 6G era, a great demand for high-performance devices operating in the terahertz (THz) band has emerged. As an important part of 6G technology, indoor communication requires multi-beam steering and tracking to serve multi-users. In this paper, we have designed a graphene metasurface that can realize multi-beam steering for directional radiations. The designed metasurface consists of graphene ribbons, dielectric spacer, and metal substrate. By designing the graphene ribbons and controlling the applied voltage on them, we have obtained single-, double-, and triple-beam steering. In addition, we have also numerically calculated the far-field distributions of the steered multi-beam with a diffraction distance of 2 m. Our design has potential applications in future indoor directional 6G communications.

Quantum Tunneling in Graphene Advances the Age of High Speed Terahertz Wireless Communications

 

Quantum tunneling. Credit: Daria Sokol/MIPT Press Office

https://scitechdaily.com/quantum-tunneling-in-graphene-advances-the-age-of-high-speed-terahertz-wireless-communications/

Scientists from MIPT, Moscow Pedagogical State University and the University of Manchester have created a highly sensitive terahertz detector based on the effect of quantum-mechanical tunneling in graphene. The sensitivity of the device is already superior to commercially available analogs based on semiconductors and superconductors, which opens up prospects for applications of the graphene detector in wireless communications, security systems, radio astronomy, and medical diagnostics. The research results are published in a high-rank journal Nature Communications.

Information transfer in wireless networks is based on the transformation of a high-frequency continuous electromagnetic wave into a discrete sequence of bits. This technique is known as signal modulation. To transfer the bits faster, one has to increase the modulation frequency. However, this requires a synchronous increase in carrier frequency. A common FM-radio transmits at frequencies of hundred megahertz, a Wi-Fi receiver uses signals of roughly five gigahertz in frequency, while the 5G mobile networks can transmit up to 20 gigahertz signals.

This is far from the limit, and a further increase in carrier frequency admits a proportional increase in data transfer rates. Unfortunately, picking up signals with hundred gigahertz frequencies and higher is an increasingly challenging problem.

A typical receiver used in wireless communications consists of a transistor-based amplifier of weak signals and a demodulator that rectifies the sequence of bits from the modulated signal. This scheme originated in the age of radio and television, and becomes inefficient at frequencies of hundreds of gigahertz desirable for mobile systems. The fact is that most of the existing transistors aren’t fast enough to recharge at such a high frequency.

An evolutionary way to solve this problem is just to increase the maximum operation frequency of a transistor. Most specialists in the area of nanoelectronics work hard in this direction. A revolutionary way to solve the problem was theoretically proposed in the beginning of 1990’s by physicists Michael Dyakonov and Michael Shur, and realized, among others, by the group of authors in 2018. It implies abandoning active amplification by transistor, and abandoning a separate demodulator. What’s left in the circuit is a single transistor, but its role is now different. It transforms a modulated signal into bit sequence or voice signal by itself, due to non-linear relation between its current and voltage drop.

In the present work, the authors have proved that the detection of a terahertz signal is very efficient in the so-called tunneling field-effect transistor. To understand its work, one can just recall the principle of an electromechanical relay, where the passage of current through control contacts leads to a mechanical connection between two conductors and, hence, to the emergence of current. In a tunneling transistor, applying voltage to the control contact (termed as ‘’gate’’) leads to alignment of the energy levels of the source and channel. This also leads to the flow of current. A distinctive feature of a tunneling transistor is its very strong sensitivity to control voltage. Even a small “detuning” of energy levels is enough to interrupt the subtle process of quantum mechanical tunneling. Similarly, a small voltage at the control gate is able to “connect” the levels and initiate the tunneling current.

“The idea of ​​a strong reaction of a tunneling transistor to low voltages is known for about fifteen years,” says Dr. Dmitry Svintsov, one of the authors of the study, head of the Laboratory of 2D Materials for Optoelectronics at the MIPT center for Photonics and 2D materials. “But it’s been known only in the community of low-power electronics. No one realized before us that the same property of a tunneling transistor can be applied in the technology of terahertz detectors. Georgy Alymov (co-author of the study) and I were lucky to work in both areas. We realized then: if the transistor is opened and closed at a low power of the control signal, then it should also be good in picking up weak signals from the ambient surrounding. ”

The created device is based on bilayer graphene, a unique material in which the position of energy levels (more strictly, the band structure) can be controlled using an electric voltage. This allowed the authors to switch between classical transport and quantum tunneling transport within a single device, with just a change in the polarities of the voltage at the control contacts. This possibility is of extreme importance for an accurate comparison of the detecting ability of a classical and quantum tunneling transistor.

The experiment showed that the sensitivity of the device in the tunneling mode is few orders of magnitude higher than that in the classical transport mode. The minimum signal distinguishable by the detector against the noisy background already competes with that of commercially available superconducting and semiconductor bolometers. However, this is not the limit – the sensitivity of the detector can be further increased in “cleaner” devices with a low concentration of residual impurities. The developed detection theory, tested by the experiment, shows that the sensitivity of the “optimal” detector can be a hundred times higher.

“The current characteristics give rise to great hopes for the creation of fast and sensitive detectors for wireless communications,” says the author of the work, Dr. Denis Bandurin. And this area is not limited to graphene and is not limited to tunnel transistors. We expect that, with the same success, a remarkable detector can be created, for example, based on an electrically controlled phase transition. Graphene turned out to be just a good launching pad here, just a door, behind which is a whole world of exciting new research. ”

The results presented in this paper are an example of a successful collaboration between several research groups. The authors note that it is this format of work that allows them to obtain world-class scientific results. For example, earlier, the same team of scientists demonstrated how waves in the electron sea of ​​graphene can contribute to the development of terahertz technology. “In an era of rapidly evolving technology, it is becoming increasingly difficult to achieve competitive results.” – comments Dr. Georgy Fedorov, deputy head of the Laboratory of Nanocarbon Materials, MIPT, “Only by combining the efforts and expertise of several groups can we successfully realize the most difficult tasks and achieve the most ambitious goals, which we will continue to do.”

Reference: “Tunnel field-effect transistors for sensitive terahertz detection” by I. Gayduchenko, S. G. Xu, G. Alymov, M. Moskotin, I. Tretyakov, T. Taniguchi, K. Watanabe, G. Goltsman, A. K. Geim, G. Fedorov, D. Svintsov and D. A. Bandurin, 22 January 2021, Nature Communications.
DOI: 10.1038/s41467-020-20721-z

The work was supported by Russian Science Foundation (grant # 16-19-10557)  and Russian Foundation for Basic Research (grant # 18-29-20116 mk).

Abstract-Terahertz and photonics seamless short‐distance links for future mobile networks

 T. Kawanishi,  K. Inagaki,  A. Kanno,  N. Yamamoto,  T. Aiba,  H. Yasuda,  T. Wakabayashi, 

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020RS007156

High‐speed data transfer and high‐performance imaging can be realized by using radio‐waves in high‐frequency bands, such as millimeter‐waves and THz‐waves, where wide frequency bands are available. However, the cell size would be smaller than a few hundred meters, due to large free space propagation loss and large atmospheric attenuation. Thus, many base stations, which are connected by networks, are required to offer nationwide or global network services by such high‐frequency radio‐bands. The networks would be constructed by various transmission media including optical fibers and fixed wireless links, where many media converters are required. This paper reviews various technologies for seamless bridges between radio and optical links. For the time being, congestion of radio spectrum in THz bands is not significant. However, if we look at the history of radio‐wave technologies, spectral congestion has been high even in newly developed high frequency bands. In active radio services in millimeter‐wave or THz‐wave bands, interference mitigation with passive services such as radio astronomy and Earth observation satellites is an important issue, as of now. This paper describes research trends of THz‐wave technologies from the point of view of a figure of merit defined by a product of the carrier frequency and spectral efficiency, to discuss the significance of spectral efficiency enhancement in the high‐frequency region. Analysis of power consumption of short‐distance radio systems is also shown to discuss expected performance of THz‐wave links.

This article is protected by copyright. All rights reserved.

Abstract-Dispersion-limited versus power-limited terahertz communication links using solid core subwavelength dielectric fibers

                                                               

Kathirvel Nallappan, Yang Cao, Guofu Xu, Hichem Guerboukha, Chahé Nerguizian, and Maksim Skorobogatiy

Schematic of the THz wireless and fiber communication links for reliable and versatile intra-/inter-vehicle communication applications.

https://www.osapublishing.org/prj/fulltext.cfm?uri=prj-8-11-1757&id=441881

Terahertz (THz) band (0.1–10 THz) is the next frontier for ultra-high-speed communication systems. Currently, most of communications research in this spectral range is focused on wireless systems, while waveguide/fiber-based links have been less explored. Although free space communications have several advantages such as convenience in mobility for the end user, as well as easier multi-device interconnectivity in simple environments, fiber-based communications provide superior performance in certain short-range communication applications such as multi-device connectivity in complex geometrical environments (ex., intra-vehicle connectivity) and secure communications with low probability of eavesdropping, as well as secure signal delivery to hard-to-reach or highly protected environments. In this work, we present an in-depth experimental and numerical study of the short-range THz communications links that use subwavelength dielectric fibers for information transmission and define the main challenges and trade-offs in the link implementation. Particularly, we use air or foam-cladded polypropylene-core subwavelength dielectric THz fibers of various diameters (0.57–1.75 mm) to study link performance as a function of the link length of up to ∼10  m$\sim 10\mathrm{m}$, and data bit rates of up to 6 Gbps at the carrier frequency of 128 GHz (2.34 mm wavelength). We find that depending on the fiber diameter, the quality of the transmitted signal is mostly limited either by the modal propagation loss or by the fiber velocity dispersion (GVD). An error-free transmission over 10 m is achieved for the bit rate of 4 Gbps using the fiber of smaller 0.57 mm diameter. Furthermore, since the fields of subwavelength fibers are weakly confined and extend deep into the air cladding, we study the modal field extent outside of the fiber core, as well as fiber bending loss. Finally, the power budget of the rod-in-air subwavelength THz fiber-based links is compared to that of free space communication links, and we demonstrate that fiber links offer an excellent solution for various short-range applications.

© 2020 Chinese Laser Press

Abstract-Additive manufacturing of highly reconfigurable plasmonic circuits for terahertz communications

Yang Cao, Kathirvel Nallappan, Hichem Guerboukha, Guofu Xu, and Maksim Skorobogatiy

(a) Schematic of the two-wire WBG (top) and a photo from the top with half of the cage removed (bottom). (b) (I) Schematic of a single period of the WBG that comprises two sections, one containing a blank paper sheet, and the other containing metallized paper with the corresponding transverse cross-sections shown in (II) and (III). The electric field distributions (|𝐸|$|E|$) of principal modes propagating in Section 1–Mode 1 (IV), and Section 2–Mode 2 (V), and Mode 3 (VI) at 140 GHz. (c) Simulated modal electric field distribution (|𝐸|$|E|$) of a WBG in the symmetry plane in the mid gap between the two wires at 140 GHz. (d) Simulated power transmission |S21|2$|{\mathrm{S}21|}^2$ and reflection |S11|2$|{\mathrm{S}11|}^2$ coefficients for WBGs with different number of periods. (e) Experimentally measured transmittance of the paper/metal WBGs of different lengths inserted into a 10 cm-long two-wire waveguide. Transmittance is computed by dividing the grating transmission spectra (by field) by that of an empty two-wire waveguide (reference).

https://www.osapublishing.org/optica/abstract.cfm?uri=optica-7-9-1112

While in most existing terahertz communications systems, the THz carrier wave is transmitted via free-space channels, the THz waveguide-based integrated solutions can be of great utility at both the transmitter and receiver ends, thus simplifying the miniaturization and mass production of cost-effective THz communications systems. Here we present a new type of modular THz integrated circuits based on the two-wire plasmonic waveguide components fabricated using a combination of stereolithography (SLA) 3D printing, wet chemistry metal deposition, and hot stamping techniques. Particular attention is paid to the design of the optical circuits based on the two-wire waveguides suspended inside a protective micro-sized enclosure. Such waveguides feature low transmission and bending losses, as well as low dispersion. Using such waveguides as basic building blocks, we then demonstrate several key optical subcomponents, such as low-loss broadband 2×1THz$2\times 1\mathrm{T}\mathrm{H}\mathrm{z}$ couplers that use two coalescing two-wire waveguide bends, as well as broadband waveguide Bragg gratings that feature a paper sheet with a periodic sequence of metal strips inserted into the air gap of a two-wire waveguide. Finally, using these developed subcomponents, a two-channel add-drop multiplexer is demonstrated to operate at 140 GHz. We believe that the reported micro-encapsulated two-wire waveguide-based modular platform can have a strong impact on the field of THz signal processing and sensing due to the ease of device fabrication and handling, high degree of reconfigurability, and high potential for real-time tunability.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Researchers say 6G will stream human brain-caliber AI to wireless devices

Image Credit: Natali_Mis/Getty Images

JEREMY HORWITZ

https://venturebeat.com/2019/06/14/researchers-say-6g-will-stream-human-brain-caliber-ai-to-wireless-devices/

As 5G networks continue to expand in cities and countries across the globe, key researchers have already started to lay the foundation for 6G deployments roughly a decade from now. This time, they say, the key selling point won’t be faster phones or wireless home internet service, but rather a range of advanced industrial and scientific applications — including wireless, real-time remote access to human brain-level AI computing.

That’s one of the more interesting takeaways from a new IEEE paper published by NYU Wireless’s pioneering researcher Dr. Ted Rappaport and colleagues, focused on applications for 100 Gigahertz (GHz) to 3 Terahertz (THz) wireless spectrum. As prior cellular generations have continually expanded the use of radio spectrum from microwave frequencies up to millimeter wave frequencies, that “submillimeter wave” range is the last collection of seemingly safe, non-ionizing frequencies that can be used for communications before hitting optical, x-ray, gamma ray, and cosmic ray wavelengths.

Dr. Rappaport’s team says that while 5G networks should eventually be able to deliver 100Gbps speeds, signal densification technology doesn’t yet exist to 

eclipse that rate — even on today’s millimeter wave bands, one of which offers access to bandwidth that’s akin to a 500-lane highway. Consequently, opening up the terahertz frequencies will provide gigantic swaths of new bandwidth for wireless use, enabling unthinkable quantities and types of data to be transferred in only a second.

The most relatable one would enable wireless devices to remotely transfer quantities of computational data comparable to a human brain in real time. As the researchers explain it, “terahertz frequencies will likely be the first wireless spectrum that can provide the real time computations needed for wireless remoting of human cognition.” Put another way, a wireless drone with limited on-board computing could be remotely guided by a server-sized AI as capable as a top human pilot, or a building could be assembled by machinery directed by computers far from the construction site.

Some of that might sound familiar, as similar remote control concepts are already in the works for 5G — but with human operators. The key with 6G is that all this computational heavy lifting would be done by human-class artificial intelligence, pushing vast amounts of observational and response data back and forth. By 2036, the researchers note, Moore’s law suggests that a computer with human brain-class computational power will be purchasable by end users for $1,000, the cost of a premium smartphone today; 6G would enable earlier access to this class of computer from anywhere.

Dr. Rappaport’s team also expects that the submillimeter wave spectra will enable enhancements of existing technologies, such as see-in-the-dark millimeter wave cameras, high-definition radar, and terahertz (rather than millimeter wave) security body scanning. The incredibly high bandwidth will also enable a transition from reliance on fiber cable infrastructure to “wireless fiber” for network backhaul and data center connectivity.

There are, of course, significant practical challenges to overcome before 6G can move from theoretical to real, including miniaturization of the core technologies, and health studies to confirm that terahertz frequencies are as safe as currently believed. Additionally, like millimeter wave transmissions, sub-millimeter wave frequencies will require highly directional antennas, in part because they’re highly susceptible to interference from the atmosphere, particularly above 800 GHz.

But the researchers note that overcoming those challenges, as was successfully accomplished with millimeter wave over the past decade, will lead to great benefits for users. Data transmissions will consume far less energy, and ultra-high gain antennas will be able to be made “extremely small.” That will pave the way for tinier devices, including military-grade secure communications links that are “exceedingly difficult” to intercept or eavesdrop upon.

In March, the FCC unanimously voted to open the 95GHz to 3THz range for “6G, 7G, or whatever is next,” though commissioners suggested the speculative uses of the frequencies at that point made the vote akin to “designating zoning laws for the moon.” Based on past history, Dr. Rappaport and others will be at the forefront of transitioning these concepts from science fiction to science fact — in the foreseeable if not immediate future.

Abstract-Inter-satellite Quantum Key Distribution at Terahertz Frequencies

Ziqing Wang, Robert Malaney, Jonathan Green

https://arxiv.org/abs/1902.07921

Terahertz (THz) communication is a topic of much research in the context of high-capacity next-generation wireless networks. Quantum communication is also a topic of intensive research, most recently in the context of space-based deployments. In this work we explore the use of THz frequencies as a means to achieve quantum communication within a constellation of micro-satellites in Low-Earth-Orbit (LEO). Quantum communication between the micro-satellite constellation and high-altitude terrestrial stations is also investigated. Our work demonstrates that THz quantum entanglement distribution and THz quantum key distribution are viable deployment options in the micro-satellite context. We discuss how such deployment opens up the possibility for simpler integration of global quantum and wireless networks. The possibility of using THz frequencies for quantum-radar applications in the context of LEO deployments is briefly discussed.

Abstract-Simultaneous low-loss and low-dispersion in a photonic-crystal waveguide for terahertz communications

Xiongbin Yu, Masaki Sugeta, Yuichiro Yamagami, Masayuki Fujita and Tadao Nagatsuma

http://iopscience.iop.org/article/10.7567/1882-0786/aaf4b3/meta

Conventional photonic-crystal waveguides make use of an equilateral triangular lattice of through holes, here we develop an isosceles triangular lattice photonic-crystal waveguide based on a silicon slab at 0.3 terahertz (THz) band, for THz high-speed communications. The propagation loss of the proposed waveguide is as small as ~1/10, compared with that of a conventional waveguide under the conditions of broadband bandwidth (>20 GHz) for both the loss and dispersion, due to the broadband low-dispersion below the light line of air, at which low-loss conditions are satisfied. Finally, we demonstrate 36 Gbit s−1 error-free THz communications using the isosceles triangular lattice photonic-crystal waveguide.