6G and beyond will fulfill the requirements of a fully connected world and provide ubiquitous wireless connectivity for all. Transformative solutions are expected to drive the surge for accommodating a rapidly growing number of intelligent devices and services. Major technological breakthroughs to achieve connectivity goals within 6G include: (i) a network operating at the THz band with much wider spectrum resources, (ii) intelligent communication environments that enable a wireless propagation environment with active signal transmission and reception, (iii) pervasive artificial intelligence, (iv) large-scale network automation, (v) an all-spectrum reconfigurable front-end for dynamic spectrum access, (vi) ambient backscatter communications for energy savings, (vii) the Internet of Space Things enabled by CubeSats and UAVs, and (viii) cell-free massive MIMO communication networks. In this roadmap paper, use cases for these enabling techniques as well as recent advancements on related topics are highlighted, and open problems with possible solutions are discussed, followed by a development timeline outlining the worldwide efforts in the realization of 6G. Going beyond 6G, promising early-stage technologies such as the Internet of NanoThings, the Internet of BioNanoThings, and quantum communications, which are expected to have a far-reaching impact on wireless communications, have also been discussed at length in this paper.

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6G and Beyond: The Future of Wireless Communications Systems

The field of terahertz integrated technology has undergone significant development in the past ten years. This has included work on different substrate technologies such as III–V semiconductors and silicon, work on field-effect transistor devices and heterojunction bipolar devices, and work on both fully electronic and hybrid electronic–photonic systems. While approaches in electronic and photonics can often seem distinct, techniques have blended in the terahertz frequency range and many emerging systems can be classified as photonics-inspired or hybrid. Here, we review the development of terahertz integrated electronic and hybrid electronic–photonic systems, examining, in particular, advances that deliver important functionalities for applications in communication, sensing and imaging. Many of the advances in integrated systems have emerged, not from improvements in single devices, but rather from new architectures that are multifunctional and reconfigurable and break the trade-offs of classical approaches to electronic system design. We thus focus on these approaches to capture the diversity of techniques and methodologies in the field. This Review Article examines the development of terahertz integrated electronic and hybrid electronic–photonic systems, considering, in particular, advances that deliver important functionalities for applications in communication, sensing and imaging.

Terahertz integrated electronic and hybrid electronic–photonic systems

Terahertz (THz) science and technology have greatly progressed over the past two decades to a point where the THz region of the electromagnetic spectrum is now a mature research area with many fundamental and practical applications. Furthermore, THz imaging is positioned to play a key role in many industrial applications, as THz technology is steadily shifting from university-grade instrumentation to commercial systems. In this context, the objective of this review is to discuss recent advances in THz imaging with an emphasis on the modalities that could enable real-time high-resolution imaging. To this end, we first discuss several key imaging modalities developed over the years: THz transmission, reflection, and conductivity imaging; THz pulsed imaging; THz computed tomography; and THz near-field imaging. Then, we discuss several enabling technologies for real-time THz imaging within the time-domain spectroscopy paradigm: fast optical delay lines, photoconductive antenna arrays, and electro-optic sampling with cameras. Next, we discuss the advances in THz cameras, particularly THz thermal cameras and THz field-effect transistor cameras. Finally, we overview the most recent techniques that enable fast THz imaging with single-pixel detectors: mechanical beam-steering, compressive sensing, spectral encoding, and fast Fourier optics. We believe that this critical and comprehensive review of enabling hardware, instrumentation, algorithms, and potential applications in real-time high-resolution THz imaging can serve a diverse community of fundamental and applied scientists.

Toward real-time terahertz imaging

The field of terahertz science and technology has been an active and thriving research area for several decades. However, the field has recently experienced an inflection point, as several exciting breakthroughs have enabled new opportunities for both fundamental and applied research. These events are reshaping the field, and will impact research directions for years to come. In this Perspective article, I discuss a few important examples: the development of methods to access nonlinear optical effects in the terahertz range; methods to probe nanoscale phenomena; and, the growing likelihood that terahertz technologies will be a critical player in future wireless networks. Here, a few examples of research in each of these areas are discussed, followed by some speculation about where these exciting breakthroughs may lead in the near future.

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Perspective: Terahertz science and technology

A large increase in oscillation frequency was achieved in resonant-tunneling-diode (RTD) terahertz oscillators by reducing the conduction loss. An n+-InGaAs layer under the air-bridge electrode connected to the RTD was observed to cause a large conduction loss for high-frequency current due to the skin effect. By introducing a new fabrication process removing the InGaAs layer, we obtained 1.92-THz oscillation, which extended the highest frequency of room-temperature electronic single oscillators. Theoretical calculations reasonably agreed with the experiment, and an oscillation above 2 THz is further expected with an improved structure of the slot antenna used as a resonator and radiator.

Oscillation up to 1.92 THz in resonant tunneling diode by reduced conduction loss

This paper presents a fully integrated direct-conversion quadrature transmitter and receiver chipset at 240 GHz. It is implemented in a 0.13- $\mu{\hbox{m}}$ SiGe bipolar-CMOS technology. A wideband frequency multiplier ( $\times$ 16) based local-oscillator (LO) signal source and a wideband on-chip antenna designed to be used with an external replaceable silicon lens makes this chipset suited for applications requiring fixed and tunable LO. The chipset is packaged in a low-cost FR4 printed circuit board resulting in a complete solution with compact form-factor. At 236 GHz, the effective-isotropic-radiated-power is 21.86 dBm and the minimum single-sideband noise figure is 15 dB. The usable RF bandwidth for this chipset is 65 GHz and the 6-dB bandwidth is 17 GHz. At the system level, we demonstrate a high data-rate communication system where an external modem is operated in its two IF-bandwidth modes (250 MHz and 1 GHz). For the quadrature phase-shift keying modulation scheme, the measured data rate is 2.73 Gb/s (modem 1-GHz IF) with bit-error rate of ${\hbox{10}}^{-9}$ for a 15-cm link. The estimated data rate over the 17-GHz RF bandwidth is, hence, 23.025 Gb/s. Also, higher order modulation schemes like 16 quadrature amplitude modulation (QAM) with a data rate of 0.677 Gb/s and 64-QAM with a data rate of 1.0154 Gb/s (modem 250-MHz IF) is demonstrated. A second application demonstrator is presented where the wide tunable RF bandwidth of the chipset is used for material characterization. It is used to characterize an FR4 material (DE104) over the 215–260-GHz range.

A Fully Integrated 240-GHz Direct-Conversion Quadrature Transmitter and Receiver Chipset in SiGe Technology

This paper presents a high-power 0.53 THz source module with programmable diversity to adjust the brightness and the direction of light to obtain the desired diffuse lighting conditions in THz imaging applications. The source module consists of a single SiGe BiCMOS chip which operates an array of 16 source-pixel incoherently. Each source pixel consists of a primary on-chip ring-antenna and two triple-push oscillators locked 180° out-of-phase. The module provides a total radiated power of up to 1 mW (0 dBm) with 62.5 μW (-12 dBm) per source pixel on average and an EIRP per pixel of 25 dBm. The circuit layout is scalable in size and output power. The chip consumes up to 2.5 W from a 2.4 V supply and 3.2 mW from a digital 1.2 V supply respectively. The module includes a secondary silicon lens, is programmable through a CPLD, and supplied from a USB port. The THz radiation can be recorded with a CMOS 1 k-pixel THz video camera and represent an all silicon solution for real-time active THz imaging.

A 0.53 THz Reconfigurable Source Module With Up to 1 mW Radiated Power for Diffuse Illumination in Terahertz Imaging Applications

A complete circularly polarized 210–270-GHz frequency-modulated continuous-wave radar with a monostatic homodyne architecture is presented. It consists of a highly integrated radio-frequency transceiver module, an in-house developed linear-frequency chirp generator, and a data acquisition chain. The radar front end featuring a fundamentally operated $\times$ 16 multiplier-chain architecture is realized as a single chip in 0.13- $\mu$ m SiGe heterojunction bipolar transistor technology with a lens-coupled circularly polarized on-chip antenna and wire-bonded on a low-cost printed circuit board. In combination with a 9-mm-diameter silicon lens, the module achieves an average in-band directivity of 26.6 dB. The measured peak radiated power from the packaged radar module is $+$ 5 dBm and the noise figure is 21 dB. For a 60-GHz frequency sweep, the radar achieves a range resolution of 2.57 mm after calibration, which is close to the theoretical bandwidth-limited resolution of 2.5 mm. With a simple scanning optical setup, this paper further demonstrates the 3-D imaging capability of the radar for detection of hidden objects with a remarkable dynamic range of around 50 dB.

A 210–270-GHz Circularly Polarized FMCW Radar With a Single-Lens-Coupled SiGe HBT Chip

Wideband x16 multiplier chains targeted for high speed communication and radar applications above 200 GHz are presented in this paper. The x16 topology is based on 4 cascaded Gilbert-cell-based frequency doublers without any hybrids and intermediate drive amplifiers. The low external input frequency (14-17.5 GHz), enables the use of very low-power frequency dividers and also interfacing with commercial synthesisers below 20 GHz. Two versions of the circuit were implemented. Version 1 is a standalone x16 chain and Version 2 is Version 1 plus wideband 3-stage PA. For Version 1, the measured peak output power is -8.5 dBm at 255 GHz with 40 GHz bandwidth (3 dB) and dc power consumption of 0.3 W. For Version 2, the peak output power is 0 dBm at 245 GHz with 30 GHz bandwidth (3 dB) and 0.7 W dc power consumption. These results are single ended on-wafer measurements without dembeding the 2.5 dB loss due to the output pad and balun. The available differential output power, to drive on-chip components like mixers, antennas etc, is -6 dBm for Version 1 and 2.5 dBm for Version 2. Additionally, break out sructures for wideband PAs operating at 240 GHz were characterised. The peak power gain at 240 GHz was 7 and 10 dB for the 3 and 4 stages respectively over a 30 GHz bandwidth. The measured P
 sat
 was 5 dBm at 240 GHz.

235–275 GHz (x16) frequency multiplier chains with up to 0 dBm peak output power and low DC power consumption

Recently, silicon-based THz video cameras have been demonstrated for industrial, surveillance, scientific, and medical applications in the THz range (300GHz to 3THz) [1]. Such camera implementations favor pixels with antenna-coupled direct detectors for a low power dissipation and a high pixel count. Despite this progress, they lack the required sensitivity for passive imaging and imagers are in the need of artificial illumination to provide the required image quality. The choice has been to use expensive high-power focused illumination with a single direction for the incoming beam, which seriously limits the image quality due to its specular nature. Additionally, detectors in a focal-plane array configuration share the available source power and the image SNR further drops with the camera resolution. Like imaging at visible light, where shades or reflectors are commonly used, active THz imaging would greatly benefit from incoherent artificial light sources to adjust brightness, phase/frequency, and the direction of light to obtain the desired lighting conditions.

Neelanjan Sarmah

Papers

A Fully Integrated 240-GHz Direct-Conversion Quadrature Transmitter and Receiver Chipset in SiGe Technology

A 0.53 THz Reconfigurable Source Module With Up to 1 mW Radiated Power for Diffuse Illumination in Terahertz Imaging Applications

A 210–270-GHz Circularly Polarized FMCW Radar With a Single-Lens-Coupled SiGe HBT Chip

235–275 GHz (x16) frequency multiplier chains with up to 0 dBm peak output power and low DC power consumption

14.5 A 0.53THz reconfigurable source array with up to 1mW radiated power for terahertz imaging applications in 0.13μm SiGe BiCMOS