sub-THz Frequency Band Research Articles

Wafer-scale silicon microfabrication technology toward realization of low-cost sub-THz waveguide devices

This paper presents a wafer-scale silicon microfabrication technology for the sub-terahertz (sub-THz) waveguide device mass production. Based on the effective scheme, a WR-5 (140–220 GHz) straight rectangular waveguide and a WR-2.8 (260–400 GHz) rectangular waveguide bandpass filter are implemented as demonstrated examples. The silicon deep reactive ion etching (DRIE) process is employed to etch through the total thickness of the silicon wafer and form the main waveguide channels. Then, a low-temperature thermal compression process is used to bond the trough-etched wafer with the top and bottom metallised silicon wafers to form the closed waveguide structures without any precise alignment process. The fabricated waveguide has the benefit of low transmission loss (0.03–0.05 dB mm−1) at the whole G band. Besides, to measure the fabricated WR-2.8 waveguide filter and solve the measuring equipment standard waveguide difference, silicon micromachined waveguide transitions are explored and fabricated to match two different frequency-band modules for measuring the waveguide filters in the desired full frequency band, which also has a potential application for the different size waveguide conversion. The measured results agree well with the simulated ones. The measured 3 dB bandwidth is 9.3%, with a central frequency of 343 GHz; the average insertion loss (IL) is about 1.6 dB in the pass band, including two extra straight waveguides of 8 mm length on input/output ends and two external waveguide-to-waveguide transitions. The proposed method provides a feasible and cost-effective solution for the mass production of high-performance waveguide devices and integrated systems in sub-THz frequency bands and beyond.

Measurement-based channel characterization in a large hall scenario at 300 GHz

Sub-terahertz (Sub-THz), defined as the frequency bands in 100-300 GHz, is promising for future generation communications and sensing applications. Accurate channel measurement and modeling are essential for development and performance evaluation of the future communication systems. Accurate channel modeling relies on realistic channel data, which should be collected by high-fidelity channel sounder. This paper presents the measurement-based channel characterization in a large indoor scenario at 299-301 GHz. We firstly review the state-of-the-art channel measurements at sub-THz frequency bands. We then presented a VNA-based channel sounder for long-range measurements, which uses the radio-over-fiber techniques. Channel measurements using this channel sounder are conducted in a large hall scenario. Based on the measurement data, we calculated and analyzed key propagation channel parameters, e.g., path loss, delay spread, and angular spread. The results are also analyzed both in the line-of-sight (LoS) and none-LoS (NLoS) cases. The large delay components in the measurements demonstrate the possibility of the longrange channel measurement campaign at 300 GHz.

Long-range VNA-based channel sounder: Design and measurement validation at MmWave and sub-THz frequency bands

With the increasing demand for high bandwidth wireless communication systems, and with a congested spectrum in the sub-6 GHz frequency bands, researchers have been looking into exploration of millimeter wave (mmWave) and sub-terahertz (sub-THz) frequency bands. Channel modeling is essential for system design and performance evaluation of new wireless communication systems. Accurate channel modeling relies on reliable measured channel data, which is collected by high-fidelity channel sounders. Furthermore, it is of importance to understand to which extent channel parameters are frequency dependent in typical deployment scenario (including both indoor short-range and outdoor long-range scenarios). To achieve this purpose, this paper presents a state-of-art long-range 28 GHz and 300 GHz VNA-based channel sounder using optical cable solutions, which can support a measurement range up to 300 m and 600 m in principle, respectively. The design, development and validation of the long-range channel sounders at mmWave and sub-THz bands are reported, with a focus on their system principle, link budget, and back-to-back measurements. Furthermore, a measurement campaign in an indoor corridor is performed using the developed 300 GHz system and 28 GHz channel sounding systems. Both measured channels at the 28 GHz and 300 GHz channels are shown to be highly sparse and specular. A higher number of Multi Path Components (MPC) are observed for the 28 GHz system, while the same main MPC are observed for both systems.

Multishape radio: new approach to utilizing the physical properties of electromagnetic waves

Usage of higher frequency bands enable a new possibility for the exploitation of the physical properties of electromagnetic waves such as bended propagation, nondiffractive propagation, and orbital angular momentum. We characterize new concept that takes full advantage of such physical properties a “multishape radio.” As a representative case, we explore the Airy wave that has the unique feature of bended propagation with asymmetric sidelobe distribution. We experiment using sub-THz frequency bands to demonstrate that Airy waves can be used for parallel transmission of multiple streams without causing harmful interference.

Channel Estimation Techniques for RIS-Assisted Communication: Millimeter-Wave and Sub-THz Systems

Millimeter-wave (mm-wave) and sub-terahertz (sub-THz) communications are expected to be one of the biggest beneficiaries of the emerging reconfigurable intelligent surface (RIS) technology. RISs can compensate for large path loss and blockage inherent to the mm-wave and sub-THz frequency bands to yield enhanced communication performance in these bands. To achieve high beamforming gain and realize the enhanced performance in RIS-assisted wireless communication, the acquisition of accurate channel state information (CSI) is critical. In this article, we provide an overview of channel estimation for RIS-assisted mm-wave/sub-THz communication to address technical challenges, tradeoffs, channel estimation frameworks, and training signal design. We summarize the recent RIS-related sparse channel estimation approaches based on beam-space, sparse recovery, array signal processing, and data-driven techniques, highlighting several challenges for future research.

The Sub-THz Emission of the Human Body Under Physiological Stress

We present evidence that in the sub-THz frequency band, human skin can be considered as an electromagnetic bio-metamaterial in which its natural emission is a product of skin tissue geometry and embedded structures. Radiometry was performed on 32 human subjects from 480 to 700 GHz. Concurrently, the subjects were exposed to stress, while heart pulse rate (PS) and galvanic skin response (GSR) were also measured. The results are substantially different from the expected blackbody radiation signal of the skin surface. PS and GSR correlate to the emissivity. Using a simulation model for the skin, we find that the sweat duct is a critical element. The simulated frequency spectra qualitatively match the measured emission spectra and show that our sub-THz emission is modulated by our level of mental stress. This opens avenues for the remote monitoring of the human state.

The AC conductivity of human sweat ducts as the dominant factor in the sub-THz reflection coefficient of skin.

The helical nature of human sweat ducts, combined with the morphological and dielectric properties of skin, suggests electromagnetic activity in the sub-THz frequency band. A detailed electromagnetic simulation model of the skin, with embedded sweat ducts, was created. The model includes realistic dielectric properties based on the measured water content of each layer of skin, derived from Raman Spectroscopy. The model was verified by comparing it to measurements of the reflection coefficient of the palms of 13 volunteers in the frequency band 350-410 GHz. They were subjected to a measurement protocol intended to induce mental stress, thereby also activating the sweat glands. The Galvanic Skin Response was concurrently measured. Using the simulation model the optimal ac-conductivity for each measurement was found. The range of variation for all subjects was found to be from 100 S/m to a maximum value of 6000 S/m with averages of 1000 S/m. These are one order of magnitude increase from the accepted values for water at these frequencies (~100 s/m at 100 GHz). Considering the known biochemical mechanism for inducing perspiration, we conclude that these ac-conductivity levels are probably valid, even though the real time measurements of sweat ac-conductivity levels inside the duct are inaccessible.

Terahertz Reflectivity of YBa2Cu3O7-δ at Cryogenic Temperatures

Modern space-based telescopes operating in the terahertz (THz) spectral range require highly reflecting and cryogenically cooled optics ensuring extremely low noise. The idea of using high-temperature superconductor YBa 2 Cu 3 O 7-δ as a telescope's mirror coating for providing low dissipation and high reflectivity of the surface for the subTHz frequency band has been explored. We are presenting here some results of measurements of the reflection losses in YBa 2 Cu 3 O 7-δ at cryogenic and room temperatures (4.2-300 K) in the frequency band 0.166-0.23 THz. High-accuracy measurements of the absolute value of reflectivity have been performed with the help of the resonance technique. The measured reflection losses in the given frequency band were below 1%. For the same frequency range, these values are well above the reflection losses in normal metals such as copper or aluminum, which are normally used to cover mirrors of telescopes. We, therefore, conclude that the use of high-Tc superconductors YBa 2 Cu 3 O 7-δ as a highly reflecting coating for the THz telescope's mirror is not advisable.

IEEE 802.15.3d: First Standardization Efforts for Sub-Terahertz Band Communications toward 6G

With the ratification of the IEEE 802.15.3d amendment to 802.15.3, a first step has been made to standardize consumer wireless communications in the sub-THz frequency band. IEEE 802.15.3d offers switched point-to-point connectivity with data rates of 100 Gb/s and higher at distances ranging from tens of centimeters up to a few hundred meters. In this article, we provide a detailed introduction to IEEE 802.15.3d and the key design principles beyond the developed standard. We particularly describe the target applications and usage scenarios, as well as the specifics of the IEEE 802.15.3d physical and medium access layers. Later, we present the results of the initial performance evaluation of IEEE 802.15.3d wireless communications. The obtained first-order performance predictions show non-incremental benefits compared to the characteristics of the fifth generation wireless systems, thus paving the way toward the sixth generation THz networks. We conclude the article by outlining the further standardization and regulatory activities on wireless networking in the THz frequency band.

Inverted E shaped dipole antenna with corrugated substrate for mm-wave applications

Abstract This paper presents a novel E armed dipole antenna for System On Chip (SOC) applications in the Sub-THz frequency band. The antenna is printed on a corrugated silicon substrate which suppresses surface waves and provides high efficiency and gain. The corrugated substrate also provides a better impedance matching. The antenna has a simulated gain of 5.4 dBi and 3 dB beam-width of 83.5 degree. The proposed antenna also has a very high radiation efficiency of 90%. Antenna resonates at 141.8 GHz and has an approximate −10 dB bandwidth of 1 GHz. To investigate the effects of different design parameters, some parametric analysis has been carried out and results have been shown. An equivalent circuit model has been reported. The equivalent circuit has been used to find the resonant frequency of the proposed dipole. The deviation of simulated and calculated resonant frequency is less than 0.5% which is well within the acceptable limits for antennas operating in mm-wave and sub-THz frequency band. This antenna is well suited for SOC, radar on-chip, high gain arrays, etc.

Microwave Imaging Customized on Demand Under Random Field Illumination

Recently, microwave imaging under random field illuminations has attracted many interests. This paper theoretically analyzes and experimentally implements a qualitative microwave radar imaging system suitable for 2-D object. It can be customized on the demands of the desired object size and imaging resolution. Based on random illuminations generated by a minimum number of antenna elements, an analytical equation can be used to reconstruct the image of the object in near real time. The simple-structured and cost-effective imaging system can be implemented in microwave, millimeter-wave, and even sub-THz frequency bands, having promising potentials for various microwave imaging applications.

High-<italic>T<sub>c</sub> </italic> Superconducting Fourth-Harmonic Mixer Using a Dual-Band Terahertz On-Chip Antenna of High Coupling Efficiency

This paper presents a dual-band on-chip antenna-coupled high- Tc superconducting (HTS) Josephson-junction subterahertz (THz) fourth-harmonic mixer. The antenna utilizes a couple of different structured twin slots to enable the resonant radiations at two frequencies, and integrates a well-designed coplanar waveguide network for achieving good radiation coupling and signal isolation characteristics. The electromagnetic simulations show that coupling efficiencies as high as −4 and −3.5 dB are achieved for the 160- and 640-GHz operating frequency bands, respectively. Based on this dual-band antenna, a 640-GHz HTS fourth-harmonic mixer is developed and characterized in a range of operating temperatures. The mixer exhibits a measured conversion gain of around −18 dB at 20 K and −22 dB at 40 K, respectively. The achieved intermediate frequency bandwidth is larger than 23 GHz. These are the best results reported for HTS harmonic mixers at comparable sub-THz frequency bands to date.

Analysis of substrate shielding methods for sub‐THz on‐chip inductors

An analysis of the influence of three substrate shielding methods, unshielded, patterned-ground shield and floating shield, on a sub-terahertz (sub-THz) inductor is presented. In the sub-THz frequency band, the unshielded inductor showed the best quality-factor (Q-factor) and self-resonance frequency (ƒSR) since the inserted substrate shield increases the parasitic capacitance. The simulation results using an electromagnetic simulator showed good agreement with the analysis.

TERAHERTZ INTERFEROMETER FOR INTEGRATED GOUBAU-LINE WAVEGUIDES

An integrated Terahertz Mach-Zehnder interferometer is presented in order to perform difierential measurements in a chip. Both simulation and experiment are performed for validating the interferometer structure. Destructive interference peaks are observed, and destructive frequencies are predicted by a mathematical model with a good agreement. The structure is then used to characterize dielectric constant of materials. Simulation results enable to quantify the device sensitivity. An experimental validation is given with the characterization of a thermosensitive polymer (Cyclotene BCB) in the sub-THz frequency band. Perspectives to increase investigated frequencies are discussed.