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In this article, the experimental investigations on a 263-GHz gyrotron are presented. With the change of the operating magnetic field and the beam energy, the operating frequency is adjusted and the output power is also varied. It is found that when the operating magnetic field changes from 9.59 to 9.82 T at a fixed operating voltage of 20 kV and a beam current of 0.8 A, the operating frequency increases from 263.39 to 264.84 GHz, the frequency-tuning range is about 1.45 GHz, and the output power is from 26 to 463 W. When the operating voltage increases from 14.4 to 22.2 kV at a fixed operating magnetic field of 9.65 T, the operating frequency decreases from 263.68 to 263.40 GHz, the frequency-tuning range is 0.28 GHz, and the output power changes from 52 to 424 W. When the operating magnetic field is fixed at 9.65 T and the operating voltage is fixed at 20 kV, but the beam current increases from 120 to 880 mA, the output power increases from 21 to 277 W, the operating frequency increases from 263.396 to 263.425 GHz, and the frequency-tunable range is 0.029 GHz. The effects of the axial position of the gyrotron in the superconducting magnet system on the operating frequency, the frequency-tuning range, and the output power are also investigated experimentally. When the beam current is increased, the nonstationary phenomenon is observed. [ABSTRACT FROM AUTHOR]

A 500 GHz TE85 mode continuously frequency-tunable gyrotron has been designed, fabricated and investigated experimentally. In the experiment, the gyrotron operates at the second cyclotron harmonic stably. The operating frequency is tuned by means of adjusting the operating magnetic field or the beam energy. The variation of the output power is also investigated. It is found that when the operating magnetic field is changed, the frequency-tuning range is much bigger than that when the beam energy is changed. [ABSTRACT FROM AUTHOR]

This article presents an ultrasensitive open-ended hollow coaxial cable resonator (OE-HCCR) that was trained to detect impact events that occurred in the surrounding space through machine learning (ML) techniques. First, inspired by the well-known mass-based accelerometer, for the first time, we propose an OE-HCCR-based accelerometer with the addition of an inertial mass block as the element responsive to acceleration. The OE-HCCR is formed by a metal shorting post welded at the RF input end and the open end of the cable. The instantaneous displacements of the mass block mounted at the endface of the cable due to accelerations lead to variations of the gap distance between the mass block and the endface of the coaxial line, which can be accurately metered by the coaxial resonating structure. Impact events that occur in the surrounding space of the OE-HCCR result in vibrations transmitted to the OE-HCCR system and thereby measured by the OE-HCCR. Second, we demonstrated that employing ML algorithms to analyze the transient responses from the OE-HCCR makes unambiguous mappings between the transient responses of the OE-HCCR and the sources of vibrations possible, due to the ultrahigh sensitivity of the OE-HCCR and the spatial anisotropy of matter in the surrounding space. The preliminary results demonstrate the feasibility of using ML-assisted OE-HCCR system for locating impact events and recognizing different impactor implements that caused the impact events. [ABSTRACT FROM AUTHOR]

In this article, we present our vision of the application scenarios, performance metrics, and potential key technologies of 6G wireless communication networks. We then comprehensively survey 6G wireless channel measurements, characteristics, and models for all frequency bands and all scenarios, focusing on millimeter-wave (mm-wave), terahertz, and optical wireless communication channels under all spectra; satellite, unmanned aerial vehicle (UAV), maritime, and underwater acoustic communication channels under global coverage scenarios; and high-speed train (HST), vehicle-to-vehicle (V2V), ultra-massive multiple-input, multiple-output (MIMO), orbital angular momentum (OAM), and industry Internet of Things (IoT) communication channels under full application scenarios. We also provide future research challenges of 6G channel measurements, a general standard 6G channel model framework, and models for intelligent reflection surface (IRS)-based 6G technologies and artificial intelligence (AI)-enabled channel measurements and models. [ABSTRACT FROM AUTHOR]

Millimeter wave (mmWave) bands have been utilized for the fifth generation (5G) communication systems and will no doubt continue to be deployed for beyond 5G (B5G). However, the underlying channels are not fully investigated at multi-frequency bands and in multi-scenarios by using the same channel sounder, especially for the outdoor, multiple-input multiple-output (MIMO), and vehicle-to-vehicle (V2V) conditions. In this paper, we conduct multi-frequency multi-scenario mmWave MIMO channel measurements with $4\times 4$ antennas at 28, 32, and 39 GHz bands for three cases, i.e., the human body and vehicle blockage measurements, outdoor path loss measurements, and V2V measurements. The channel characteristics, including blockage effect, path loss and coverage range, and non-stationarity and spatial consistency, are thoroughly studied. The blockage model, path loss model, and time-varying channel model are proposed for mmWave MIMO channels. The channel measurement and modeling results will be of great importance for further mmWave communication system deployments in indoor hotspot, outdoor, and vehicular network scenarios for B5G. [ABSTRACT FROM AUTHOR]

The fast developments of high-speed train (HST) and growing communication demands of users pose new challenges for HST communication systems. To provide reliable communication services, key 5G communication technologies, such as millimeter-wave (mmWave) and massive multiple-input multiple-output (MIMO), and potential architectures, such as coordinated multipoint and mobile relay station, are considered in future railway communications. New technologies and architectures will bring new channel propagation characteristics. Accurate channel models that can capture these channel characteristics play a critical role in the design and test of future railway communication systems. This article reviews the HST channel measurements conducted in different scenarios and frequency bands, and the advanced HST channel models. Then a novel HST channel model incorporating the mmWave and massive MIMO propagation characteristics is presented, and its frequency non-stationarity is analyzed. Finally, some future directions are discussed. [ABSTRACT FROM AUTHOR]

A GaAs monolithic K-band left-handed frequency multiplier based on a composite right/left-handed coplanar-waveguide transmission line (TL), periodically distributed series planar Schottky varactor diode, and shunt rectangle spiral inductor has been successfully designed and fabricated. The anomalous dispersion of the left-handed TL and strong nonlinearity of the Schottky varactor are combined to effectively generate harmonics when a large signal is applied. The maximum output power of the second harmonic is 6.7 dBm (including 3.85-dBm loss from the measurement system), which was achieved when a 20-dBm fundamental frequency of 13.8 GHz is fed. High conversion efficiency for the second harmonic over a broad frequency range of 16.4–35.2 GHz has been obtained experimentally. [ABSTRACT FROM AUTHOR]