Terahertz (THz)-band communications are a key enabler for future-generation wireless communication systems that promise to integrate a wide range of data-demanding applications. Recent advancements in photonic, electronic, and plasmonic technologies are closing the gap in THz transceiver design. Consequently, prospect THz signal generation, modulation, and radiation methods are converging, and the corresponding channel model, noise, and hardware-impairment notions are emerging. Such progress paves the way for well-grounded research into THz-specific signal processing techniques for wireless communications. This tutorial overviews these techniques with an emphasis on ultra-massive multiple-input multiple-output (UM-MIMO) systems and reconfigurable intelligent surfaces, which are vital to overcoming the distance problem at very high frequencies. We focus on the classical problems of waveform design and modulation, beamforming and precoding, index modulation, channel estimation, channel coding, and data detection. We also motivate signal processing techniques for THz sensing and localization.

An Overview of Signal Processing Techniques for Terahertz Communications

SUMMARY This contribution presents a full MMIC chip set, transmit and receive RF frontend and data transmission experiments at a carrier frequency of 300GHz and with data rates of up to 64Gbit/s. The radio is dedicated to future high data rate indoor wireless communication, serving application scenarios such as smart offices, data centers and home theaters. The paper reviews the underlying high speed transistor and MMIC process, the performance of the quadrature transmitter and receiver, as well as the local oscillator generation by means of frequency multiplication. Initial transmission experiments in a single-input single-output setup and zero-IF transmit and receive scheme achieve up to 64Gbit/s data rates with QPSK modulation. The paper discusses the current performance limitations of the RF frontend and will outline paths for improvements in view of achieving 100Gbit/s capability.

Towards MMIC-Based 300GHz Indoor Wireless Communication Systems

Terahertz (THz)-band communications are a key enabler for future-generation wireless communication systems that promise to integrate a wide range of data-demanding applications. Recent advances in photonic, electronic, and plasmonic technologies are closing the gap in THz transceiver design. Consequently, prospect THz signal generation, modulation, and radiation methods are converging, and corresponding channel model, noise, and hardware-impairment notions are emerging. Such progress establishes a foundation for well-grounded research into THz-specific signal processing techniques for wireless communications. This tutorial overviews these techniques, emphasizing ultramassive multiple-input–multiple-output (UM-MIMO) systems and reconfigurable intelligent surfaces, vital for overcoming the distance problem at very high frequencies. We focus on the classical problems of waveform design and modulation, beamforming and precoding, index modulation, channel estimation, channel coding, and data detection. We also motivate signal processing techniques for THz sensing and localization.

In this roadmap article, we have focused on the most recent advances in terahertz (THz) imaging with particular attention paid to the optimization and miniaturization of the THz imaging systems. Such systems entail enhanced functionality, reduced power consumption, and increased convenience, thus being geared toward the implementation of THz imaging systems in real operational conditions. The article will touch upon the advanced solid-state-based THz imaging systems, including room temperature THz sensors and arrays, as well as their on-chip integration with diffractive THz optical components. We will cover the current-state of compact room temperature THz emission sources, both optolectronic and electrically driven; particular emphasis is attributed to the beam-forming role in THz imaging, THz holography and spatial filtering, THz nano-imaging, and computational imaging. A number of advanced THz techniques, such as light-field THz imaging, homodyne spectroscopy, and phase sensitive spectrometry, THz modulated continuous wave imaging, room temperature THz frequency combs, and passive THz imaging, as well as the use of artificial intelligence in THz data processing and optics development, will be reviewed. This roadmap presents a structured snapshot of current advances in THz imaging as of 2021 and provides an opinion on contemporary scientific and technological challenges in this field, as well as extrapolations of possible further evolution in THz imaging.

Roadmap of Terahertz Imaging 2021.

Terahertz (THz) links will play a major role in high data rate communication over a distance of few meters. In order to achieve this task, antenna designs with high gain and wideband characteristics will spearhead these links. In this contribution, we present different antenna designs that offer characteristics better suited to THz communication over short distances. Firstly, a single-element antenna having a dipole and reflector is designed to operate at 300 GHz, which is considered as a sub-terahertz band. That antenna achieves a wide impedance bandwidth of 38.6% from 294 GHz to 410 GHz with a gain of 5.14 dBi. Secondly, two designs based on the same dipole structure but with added directors are introduced to increase the gain while maintaining almost the same bandwidth. The gains achieved are 8.01 dBi and 9.6 dBi, respectively. Finally, an array of elements is used to achieve the highest possible gain of 13.6 dBi with good efficiency about 89% and with limited director elements for a planar compact structure to state-of-the-art literature. All the results achieved make the proposed designs viable candidates for high-speed and short-distance wireless communication systems.

Sub-THz Antenna for High-Speed Wireless Communication Systems

A metamorphic high electron mobility transistor (mHEMT) technology with 20 nm gate length for manufacturing of terahertz monolithic integrated circuits (TMICs) is presented. The passive elements include up to four interconnection metallization layers separated by low-k dielectrics (BCB), SiN and air which can be used to realize front side signal lines. Shielding the substrate from the electromagnetic field on the wafer front side eliminates the need of a costly back side process including wafer thinning, through substrate via etching and back side metallization. The semiconductor heterostructure of the mHEMT comprises a strained pure InAs channel with high electron mobility and high electron density for proper device scaling. The realized mHEMTs achieve a source resistance rS of 0.12 Ωmm which is required to minimize resistive losses in combination with an extrinsic maximum transconductance gm_max of 2850 mS/mm. Elaborated on wafer calibration procedures and optimized test transistor layouts were used to improve the precision of the S-parameter measurements up to a frequency of 450 GHz which than could be used for model extraction. The presented 20 nm mHEMT technology was employed for the design of a compact eight stage low-noise amplifier (LNA) using miniaturized microstrip lines on BCB. The measured small signal gain of the LNA exceeds 15 dB from 500-635 GHz.

20 nm Metamorphic HEMT technology for terahertz monolithic integrated circuits

In this paper we demonstrate an antenna for 300 GHz. The antenna is processed on quartz and consists of 2 quartz layers with 2 metal layers each, which are separated by benzocyclobutene (BCB). The edge length is 2 millimeters. Design, simulation and the analysis is done in CST Microwave Studio. Measurement is done after packaging on a brass disk. This work shows the design details, simulated results and measurement data of the presented antenna.

Hermann Masler

Papers

20 nm Metamorphic HEMT technology for terahertz monolithic integrated circuits

A 300 GHz microstrip multilayered antenna on quartz substrate