According to Edholm’s law, the demand for point-to-point bandwidth in wireless short-range communications has doubled every 18 months over the last 25 years It can be predicted that data rates of around 5–10 Gb/s will be required in ten years In order to achieve 10 Gb/s data rates, the carrier frequencies need to be increased beyond 100 GHz Over the past ten years, several groups have considered the prospects of using sub-terahertz (THz) and THz waves (100–2000 GHz) as a means to transmit data wirelessly Some of the reported advantages of THz communications links are inherently higher bandwidth compared to millimeter wave links, less susceptibility to scintillation effects than infrared wireless links, and the ability to use THz links for secure communications Our goal of this paper is to provide a comprehensive review of wireless sub-THz and THz communications

Review of terahertz and subterahertz wireless communications

Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from ; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques.

Vacuum Electronic High Power Terahertz Sources

The techniques and technologies currently being investigated to detect weapons and contraband concealed on persons under clothing are reviewed. The basic phenomenology of the atmosphere and materials that must be understood in order to realize such a system are discussed. The component issues and architectural designs needed to realize systems are outlined. Some conclusions with respect to further technology developments are presented.

Standoff Detection of Weapons and Contraband in the 100 GHz to 1 THz Region

Mobile communications have been undergoing a generational change every ten years or so. However, the time difference between the so-called “G’s” is also decreasing. While fifth-generation (5G) systems are becoming a commercial reality, there is already significant interest in systems beyond 5G, which we refer to as the sixth generation (6G) of wireless systems. In contrast to the already published papers on the topic, we take a top-down approach to 6G. More precisely, we present a holistic discussion of 6G systems beginning with lifestyle and societal changes driving the need for next-generation networks. This is followed by a discussion into the technical requirements needed to enable 6G applications, based on which we dissect key challenges and possibilities for practically realizable system solutions across all layers of the Open Systems Interconnection stack (i.e., from applications to the physical layer). Since many of the 6G applications will need access to an order-of-magnitude more spectrum, utilization of frequencies between 100 GHz and 1 THz becomes of paramount importance. As such, the 6G ecosystem will feature a diverse range of frequency bands, ranging from below 6 GHz up to 1 THz. We comprehensively characterize the limitations that must be overcome to realize working systems in these bands and provide a unique perspective on the physical and higher layer challenges relating to the design of next-generation core networks, new modulation and coding methods, novel multiple-access techniques, antenna arrays, wave propagation, radio frequency transceiver design, and real-time signal processing. We rigorously discuss the fundamental changes required in the core networks of the future, such as the redesign or significant reduction of the transport architecture that serves as a major source of latency for time-sensitive applications. This is in sharp contrast to the present hierarchical network architectures that are not suitable to realize many of the anticipated 6G services. While evaluating the strengths and weaknesses of key candidate 6G technologies, we differentiate what may be practically achievable over the next decade, relative to what is possible in theory. Keeping this in mind, we present concrete research challenges for each of the discussed system aspects, providing inspiration for what follows.

6G Wireless Systems: Vision, Requirements, Challenges, Insights, and Opportunities

The outstanding phase-noise performance of optical frequency combs has led to a revolution in optical synthesis and metrology, covering a myriad of applications, from molecular spectroscopy to laser ranging and optical communications However, the ideal characteristics of an optical frequency comb are application dependent In this review, the different techniques for the generation and processing of high-repetition-rate (>10GHz) optical frequency combs with technologies compatible with optical communication equipment are covered Particular emphasis is put on the benefits and prospects of this technology in the general field of radio-frequency photonics, including applications in high-performance microwave photonic filtering, ultra-broadband coherent communications, and radio-frequency arbitrary waveform generation

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201300126

Optical frequency comb technology for ultra‐broadband radio‐frequency photonics

Revolutionary THz transmitter and receiver demonstrations are the ongoing focus of a portfolio of programs within the DARPA. Through the sponsorship of the Terahertz Electronics and related programs, a technology base is being established to effectively generate, detect, process, and radiate sub-MMW frequencies to exploit this practically inaccessible frequency domain for imaging, radar, spectroscopy, and communications applications. Transistors, integration technologies, power amplification, and their precision metrology are the key elements under active investigation. THz InP transistors have been achieved that have enabled the world's fastest 0.48 THz monolithic integrated circuits. Compact THz high power amplifiers using micro-machined vacuum electronic devices will enable radiation sources at 1.03 THz with 15 GHz of instantaneous bandwidth. Ultimately, low-loss interconnects and integration techniques will couple TMICs with HPAs to enable THz coherent heterodyne transceivers.

THz electronics projects at DARPA: Transistors, TMICs, and amplifiers

There has been significant recent interest in the application of millimeter wave (MMW), sub-MMW, and THz technology to develop imaging systems that can "see" though either adverse weather environments or through materials such as clothing or baggage. There are numerous challenges to realizing systems operating in these frequency bands which have direct impact on the requirements on the sensor components. The atmosphere and materials exhibit attenuating characteristics that can vary markedly with frequency. These phenomenological issues must be traded off with component performance, architectural designs, as well as resolution requirements. This paper will investigate our current understanding of the phenomenological concerns and there impact on requirements for electronic components and imaging architectures that will produce sensors capable of detecting, and perhaps identifying, concealed objects that may form a security risk. Some conclusions with respect to further technology developments are presented.

Imaging Through the Atmosphere at Terahertz Frequencies

Dramatic progress has been achieved, during Phase II of the Wide Band Gap Semiconductors for RF Applications (WBGS-RF) program, sponsored by the Defense Advanced Research Projects Agency (DARPA), in extending the lifetime of high performance gallium nitride high electron mobility transistors, operating at frequencies up to 40 GHz. This paper summarizes the significant progress made by the contractor teams participating in the Phase II portion of the program.

The DARPA Wide Band Gap Semiconductors for RF Applications (WBGS-RF) Program: Phase II Results

The COmpound Semiconductor Materials On Silicon (COSMOS) program of the US Defense Advanced Research Projects Agency (DARPA) focuses on developing transistor-scale heterogeneous integration processes to intimately combine advanced compound semiconductor (CS) devices with high-density silicon circuits The technical approaches being explored in this program include high-density micro assembly, monolithic epitaxial growth, and epitaxial layer printing processes In Phase I of the program, performers successfully demonstrated world-record differential amplifiers through heterogeneous integration of InP HBTs with commercially fabricated CMOS circuits In the current Phase II, complex wideband, large dynamic range, high-speed digital-to-analog convertors (DACs) are under development based on the above heterogeneous integration approaches These DAC designs will utilize InP HBTs in the critical high-speed, high-voltage swing circuit blocks and will employ sophisticated in situ digital correction techniques enabled by CMOS transistors This paper will also discuss the Phase III program plan as well as future directions for heterogeneous integration technology that will benefit mixed signal circuit applications

The DARPA COSMOS program: The convergence of InP and Silicon CMOS technologies for high-performance mixed-signal

DARPA/MTO has sponsored electronics programs to exploit the unique combination of high intrinsic breakdown voltages, high electron saturation velocities, and large sheet carrier densities of the nitride material system. The NEXT program is pushing III-N-based HEMTs toward its operating frequency (size) scaling limits by simultaneously minimizing carrier transit time, maximizing electron density, reducing access resistances and optimizing parasitic capacitances with innovative epitaxial structures and dielectric heterointerfaces. The Phase I goals of the NEXT program are to demonstrate 300 GHz D-mode and 200 GHz E-mode HEMTs while maintaining the breakdown voltage and transistor cutoff frequency product of more than 5 THz•Volt. The final goal of the NEXT program is to enable a 1000-transistor, high-yield, 500 GHz E/D-mode GaN technology for mixed signal applications.

Mark J. Rosker

Papers

THz electronics projects at DARPA: Transistors, TMICs, and amplifiers

Imaging Through the Atmosphere at Terahertz Frequencies

The DARPA Wide Band Gap Semiconductors for RF Applications (WBGS-RF) Program: Phase II Results

The DARPA COSMOS program: The convergence of InP and Silicon CMOS technologies for high-performance mixed-signal

DARPA's Nitride Electronic NeXt Generation Technology Program