Conventional Microwave Frequencies Research Articles

Terahertz-Band Near-Space Communications: From a Physical-Layer Perspective

Facilitated by the rapid technological development of the near-space platform stations (NSPS), near-space communication (NS-COM) is envisioned to play a pivotal role in the space-air-ground integrated network (SAGIN) for the sixth generation (6G) communications and beyond. In NS-COM, ultrabroadband wireless connectivities between NSPSs and various airborne/spaceborne platforms are required for a plethora of bandwidth-consuming applications, such as NSPS-based ad hoc networking, the in-flight Internet and the relaying technology. However, such requirement seem to contradict the scarcity of spectrum resources at conventional microwave frequencies, which motivates the exploitation of terahertz (THz) band ranging from 0.1 to 10 THz. Due to huge available bandwidth, THz signals are capable of supporting ultra-high-rate data transmission for NS-COM over 100 Gb/s, which are naturally suitable for the near-space environment with marginal path loss. Against this background, this article provides an extensive investigation on THz-band NS-COM (THz-NS-COM) from a physical-layer perspective. Firstly, we summarize the potential applications of THz communications in the near-space environment, where the corresponding technical barriers are analyzed. Secondly, the channel characteristics of THz-NS-COM and the corresponding modeling strategies are discussed. Thirdly, three essential research directions are investigated to surpass the technical challenges of THz-NS-COM, i.e., robust beamforming for ultramassive antenna array, signal processing algorithms against hybrid distortions, and integrated sensing and communications. Several open problems are also provided to unleash the full potential of THz-NS-COM.

Measurement of Dielectric Multipactor Thresholds at 110GHz.

We report experimental measurements of the threshold for multipactor discharges on dielectric surfaces at 110GHz. Multipactor was studied in two geometries: electric field polarized parallel to or perpendicular to the sample surface. Measured multipactor thresholds ranged from 15 to 34 MV/m, more than 10 times higher than those found at conventional microwave frequencies. Measured thresholds were compared with prior data at lower frequencies, showing agreement with theoretical predictions that thresholds increase linearly with frequency. Measurements of the rf power dissipated in the multipactor show low dissipation (≤1%) for the parallel electric field case, but very strong dissipation for the perpendicular case, also in agreement with theoretical predictions. The agreement between experiment and theory over a wide range of frequencies provides a strong basis for the understanding of dielectric multipactor discharges.

What will 5G Antennas and Propagation Be?

Fifth-generation wireless technology, denoted as “5G” by the global wireless communications industry, seeks to accomplish multi-fold increases in key aspects of wireless communication. In conjunction with conventional microwave frequencies, millimeter-wave (mmWave) and THz frequencies and technologies are being investigated by major research institutions and industry. Deliberations are underway by regulatory officials around the world to authorize mmWave and sub-THz spectrum for 5G fixed and mobile applications. In the development of 5G communication systems, new propagation phenomena, novel multi-antenna transmitting architectures, and new mmWave operating frequency bands using much wider channel bandwidths than ever before are proving to be extremely effective ways to dramatically increase the communication data rate of future mobile communication networks. The papers in this special issue offer extensive insights and promising results that address many of the technical challenges that must be solved to usher in the 5G era.

Study of the Kramers rare earth ions ground multiplet with a large orbital contribution by multifrequency EPR spectroscopy: in scintillator

Abstract A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce 3 + impurities in PbWO 4 single-crystals at the conventional microwave frequency (CMF) (X-band: 9.43 GHz) and at the high frequencies/fields (HF) 95, 190 and 285 GHz was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective spin one-half system in a tetragonal site symmetry. The diagonal values of the effective g matrix of the lowest doublet of the ground multiplet, g ‖ and g ⊥ , are frequency dependent at high fields. For the magnetic field perpendicular to the tetragonal axis, the g ⊥ -parameter exhibits also a small azimuthal angular dependence, which is frequency dependent, corresponding to the tetragonal S 4 symmetry. These HF effects are associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. The CMF and multifrequency HF-EPR analysis gives a good description of the magnetic properties and allows an estimation of the crystal field splitting of the ground multiplet of Ce 3 + ions with tetragonal symmetry S 4 in the PbWO 4 scintillator.

Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals

A multifrequency electron paramagnetic resonance (EPR) investigation of Nd(3+) impurities in PbWO(4) single-crystals at the conventional microwave frequency (MF) 9.43 GHz, and at the 95, 190, and 285 GHz high frequencies was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective electron spin of one-half and to a tetragonal symmetry. For the magnetic field along the tetragonal axis, the g(parallel)-factor and the hyperfine constant A(parallel) of the lowest doublet of the ground multiplet decreases with frequency increase. For the magnetic field perpendicular to the tetragonal axis, the g(perpendicular)-factor exhibits a small azimuthal angular dependence that increases with increasing the frequency due to the S(4) site symmetry. The azimuthal angular dependence allows to clearly distinguish between different local axial symmetries. These properties are interpreted as high field/frequency (HF) effects associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. We show that from the combined analysis of the multifrequency MF- and HF-EPR spectra and of the optical data, an accurate description of the ground multiplet of the Kramers rare earth ions in solid matrices can be derived.

Extra-high frequency line-of-sight propagation for future urban communications

Spectrum congestion at conventional microwave frequencies has forced communication system designers to explore and investigate higher and higher frequencies, well into the millimetric band. This part of the radio spectrum is currently underutilized and offers a very wide bandwidth and permits higher data rates as well as more channels. This paper presents measurements, analysis and modeling of propagation losses for a 97-GHz line-of-sight terrestrial link. The link was established in an urban environment over a path length of 6.5 km and operated over a period of more than a year. An overview of path geometry, experimental equipment and data acquisition, and data processing is first given. The annual distribution of the measured rain attenuation at 97 GHz is discussed in connection with that of the rainfall rate at the receiving point, and compared with the current ITU-R rain attenuation prediction model. Extensive experimental results on rain fade durations, long-term fade statistics and amplitude scintillations carried out using data recorded from this link are also presented. The experimental results concerning rain attenuation modeling indicate that the agreement between the measured rain attenuation values and those computed on the basis of the ITU-R rain attenuation prediction model is not entirely satisfactory. However, the results presented in this paper are only based on data collected over a period of one year, thus the difference between reported and modeled rain attenuation distributions may be attributed to expected year-to-year variations in rain fall rate and its associated attenuation distributions. Therefore, the experimental results appear to indicate that further research is necessary to fully validate the ITU-R rain attenuation model at higher millimeter wave frequency band over longer period of time. Nevertheless, the experimental results have established a considerable degree of confidence in the feasibility of millimeter wave link systems with short distance and low power requirements.

A Short History of Microwave Acoustics

Microwave acoustics may be defined as the subject embodying the propagation of acoustic waves in solid-state materials at-micron-order wavelengths where analysis, design, and componentry realizations are similar to those used by the microwave engineer exploiting electromagnetic waves. Microwave acoustics has a short history, being about 25 years old, but is underpinned by the theory of sound propagation due to Lord Rayleigh of 100 years ago. Microwave acoustic components inherently have several distinct physical origins including volume acoustic waves in solids excited by piezoelectric thin-film transducers and magnetic propagating modes in yttrium iron garnet, both at conventional microwave frequencies; the later surface acoustic-wave (SAW) technology for operation at VHF/UHF and the realm of acoustooptics, which can embody any of the earlier three. Over its 25-year history, microwave acoustics has matured to become a necessary building-block in many radar and communication systems for efficiently carrying out real-time analog signal processing. Contributions to microwave acoustics have been truly international and have spanned many diverse disciplines. The growth of this subject has led to the formation of several companies dedicated to its application.

Fast-sampling frequency meter

A multi-sampling system having sampling frequencies much below the Nyquist rate is proposed for use in frequency counters. Its measurement range extends into the microwave frequency region without the long acquisition times associated with the conventional microwave frequency translation techniques. Because the system handles mainly low frequencies it can be made cheaply