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In this paper, a novel concept of a three-dimensional full metal system including a Dual-Mode Converter (DMC) network integrated with a high-gain Conical Horn Antenna (CHA) is presented. This system is designed for 5G millimeter wave applications requiring monopulse operation at K-band ( 37.5 – 39   GHz ). The DMC integrates two mode converters. They excite either the  T E 11 c i r  or the  T E 01 c i r  modes of the circular waveguide of the CHA. The input of the mode converters is the  T E 10 r e c  mode of two independent WR-28 standard rectangular waveguide ports. By integrating the DMC with the CHA, the whole system, called a Dual-Mode Conical Horn Antenna (DM-CHA), is formed, radiating the sum (Σ) and difference (Δ) patterns associated to the monopulse operation. To adequately prevent the propagation of higher order modes and mode mutual coupling, this integration procedure is carefully designed and fabricated. To prove the performance of the design, the DMC network was fabricated using subtractive manufacturing by Computer Numerical Control (CNC) technology. The CHA was fabricated using additive manufacturing by Direct Metal Laser Sintering (DLMS) technology. Finally, the simulation and measurement results were exhaustively compared, including return loss, isolation, radiation pattern, and gain of the full DM-CHA structure. It is noteworthy that this system provided up to  ± 11 °  per beam in the angular of arrival detection to support the high data rate operation for 5G satellite communications in the millimeter-wave band. [ABSTRACT FROM AUTHOR]

In this article, a novel three-dimensional (3-D) imaging method based on the range decomposing algorithm (RDA) is proposed for millimeter wave imaging. We combined it with binomial theory and we derive the theoretical formulation of RDA applied to single-input–single-output (SISO)/multiple-input–multiple-output (MIMO) array; meanwhile, its computational complexity and computational error are analyzed. Compared to the classical Fourier algorithm, such as the range migration algorithm (RMA) and the phase shift migration (PSM), the proposed algorithm can replace the time-consuming interpolation and accumulation operations with reasonable approximations and transformations offering a more efficient approach, while maintaining the image quality. In addition, a method based on RDA which is applicable to the transformation between MIMO and SISO, is proposed to further enhance the processing efficiency. Proof-of-principle simulation using echo data collected from a large number of antennas, verifies that the proposed algorithm has higher efficiency. In order to better verify the feasibility of the proposed algorithm, a scanning prototype located in the millimeter wave band is designed. The experimental results of different targets demonstrate that the proposed algorithm achieves significantly higher reconstruction efficiency when compared to the traditional algorithms. [ABSTRACT FROM AUTHOR]

In this letter, a multibeam folded transmitarray antenna based on an improved phase matching method is presented. A Ku‐band magneto‐electric dipole antenna is designed as the primary source for the proposed antenna. Besides, a Fabry–Perot metasurface with high transmission efficiency is proposed. Finally, a multibeam folded transmitarray antenna is designed, simulated, fabricated, and measured. By exciting different ports, the proposed antenna achieves a ±25° scanning range. The impedance bandwidth is 14%, and the measured peak gain is 23.5 dBi, which means the aperture efficiency is about 17.8%. Measured results show that the proposed antenna achieves a good multibeam performance while maintaining a low profile. [ABSTRACT FROM AUTHOR]

Purpose: This paper aims to present the design, fabrication and analysis of a wideband, enhanced gain 1 × 2 patch antenna array with a simple profile structure to meet the desired antenna traits, such as wide bandwidth, high gain and directional patterns expected for the upcoming fifth-generation (5G) wireless applications in the millimeter wave band. To enhance these parameters (bandwidth and gain), a new antenna geometry by using a T-junction power divider is presented. Design/methodology/approach: The theory behind this paper is connected with advancements in the 5G communications related to antennas. The methodology used in this work is to design a high gain array antenna and to identify the best possible power divider to deliver the power in an optimized way. The design methodology adopts several steps like the selection of proper substrate material as per the design specification, size of the antenna as per the frequency of operation and application-specific environment condition. The simulation has been performed on the designed antenna in the electromagnetic simulation tool (high-frequency structure simulator [HFSS]), and optimization has been done with parametric analysis, and then the final array antenna model is proposed. The proposed array contains 2-patch elements excited by one port adapted to 50 Ω through a T-junction power divider. The 1 × 2 array configuration with the suggested geometry helps to improve the overall gain of the antenna, and the implementation of the T-junction power divider provides enhanced bandwidth. The proposed array designed using a 1.6 mm thick flame retardant substrate occupies a compact area of 14 × 12.14 mm2. Findings: The prototype of the array antenna is fabricated and measured to validate the design concept. A good agreement has been reached between the measured and simulated antenna parameters. The measured results confirm its wideband and high gain characteristics, covering 24.77–28.80 GHz for S11= –10 dB with a peak gain of about 15.16 dB at 27.65 GHz. Originality/value: The proposed antenna covers the bandwidth requirements of the 26 GHz n258 band (24.25–27.50 GHz) to be deployed in the UK and Europe. The suggested antenna structure also covers the federal communications commission (FCC)-regulated 28 GHz n261 band (27.5–28.35 GHz) to be deployed in America and Canada. The low profile, compact size, simple structure, wide bandwidth, high gain and desired directional radiation patterns confirm the applicability of the suggested array antenna for the upcoming 5 G wireless systems. [ABSTRACT FROM AUTHOR]

This article describes a fractal-based MIMO antenna for 5G mm-wave mobile applications with micro-strip feeding. The proposed structure is a fractal-based spherical configuration that incorporates spherical slots of different iterations on the patch, as well as rectangular slots on the ground plane. These additions are meant to reduce patch isolation. The two-element MIMO antenna has closely spaced antenna elements that resonate at multiple frequencies, 9.5 GHz, 11.1 GHz, 13.4 GHz, 15.8 GHz, 21.1 GHz, and 26.6 GHz, in the frequency range of 8 to 28 GHz. The antenna's broadest operational frequency range spans from 17.7 GHz to 28 GHz, encompassing a bandwidth of 10,300 MHz. Consequently, it is well-suited for utilization within the millimeter wave (mm wave) application, specifically for the 5G new radio frequency band n258, and partially covers some other bands X (8.9–9.9 GHz, 10.4–11.4 GHz), and Ku (13.1–13.7 GHz, 15.4–16.2 GHz). All the resonating bands have isolation levels below the acceptable range of (|S12| > −16 dB). The proposed antenna utilizes a FR4 material with dimension of 28.22 mm × 44 mm. An investigation is conducted to analyze the effectiveness of parameters of the antenna, including radiation pattern, surface current distributions and S parameters. Furthermore, an examination and assessment are conducted on the efficacy of the diversity system inside the multiple input multiple output (MIMO) framework. This evaluation encompasses the analysis of key performance metrics such as the envelope correlation coefficient (ECC), diversity gain (DG), and mean effective gain (MEG). All antenna characteristics are determined to be within a suitable range for this suggested MIMO arrangement. The antenna design underwent experimental validation and the simulated outcomes were subsequently verified. [ABSTRACT FROM AUTHOR]

Unmanned Aerial Vehicles (UAVs) have become crucial components of building air–ground integrated networks for sixth-generation mobile communications. UAV-related technology has advanced quickly in both military and civilian contexts. Problems with long-distance and dynamic communication need to be fixed in the non-terrestrial transmission channels of high-altitude UAVs. For long-distance UAV dynamic non-terrestrial transmission channel modeling, a spatial statistical channel model–ray tracing (SSCM-RT) hybrid channel model is proposed and verified, and SSCM-RT channel simulations incorporating spatial consistency analysis are carried out to realize the description of the communication process in the dynamic scenario of the non-terrestrial transmission in symmetric and asymmetric situations. The SSCM-RT model can output channel state information continuously, which is helpful for the quantitative description and characterization of the UAV channel in practical settings. [ABSTRACT FROM AUTHOR]

This paper presents a multiband microstrip patch antenna for future 5G Millimeter Wave (mm-wave) wireless technologies. The synthesized antenna is designed to be based on the genetic algorithm (GA) optimization tool in the high frequency simulation software (HFSS). The optimized dimensions of the engineered antenna are 24 × 24 × 0.203 mm3, comprising of a ground plane. The antenna design simulated results have been implemented via (HFSS) and microwave studio (CST) tools. The proposed antenna structure is engineered using Rogers RT-Duroid 4003 substrate as a dielectric medium. The Conformist rectangular and circular patches are modified to get multiband operation in the mm-wave band 25–32 besides 40–60 GHz frequency ranges. The input reflection coefficient ( S 11 ) simulation results were compared with the measurements to check the validity of the design. The fabricated antenna radiation efficiency and gain values are also measured over the desired bandwidths. The measured gain and radiation efficiency values reached up to 9.75 dBi, 0.85%, and 10.75 dBi, 0.84%, for both bandwidths, respectively. [ABSTRACT FROM AUTHOR]

For most applications in the millimeter wave band, corresponding to Ka and higher-frequency bands, relatively high atmospheric absorption necessitates the use of high-power sources. Here, a new approach for projecting an oversized beam tunnel in an overmoded structure by concentrating the axial field is demonstrated to meet the high-frequency and high-power demands of compact devices. Due to the enhanced intense beam loading capability of the interaction circuit, a six-cavity Ka-band extended interaction klystron with a four-coupling-hole disk-loaded structure is designed that can stably obtain high output power. An analysis of optimization tradeoffs from introducing high order modes for allowing the application of more powerful beams to improving high order mode field distribution for enhancing the electron-wave coupling and suppressing mode competition is reported. 3D particle-in-cell simulations show attainable output powers of 1.11 MW at 32.94 GHz with a saturated gain of 57 dB by injecting a 3.3 mm diameter electron beam with a current of 24 A. [ABSTRACT FROM AUTHOR]

Uncrewed aerial vehicles (UAVs) have become a vital element in nonterrestrial networks, especially with respect to 5G communication systems and beyond. The use of UAVs in support of 4G/5G base station (uncrewed aerial vehicle base station [UAV‐BS]) has proven to be a practical solution for extending cellular network services to areas where conventional infrastructures are unavailable. In this study, we introduce a UAV‐BS system that utilizes a high‐capacity wireless backhaul operating in millimeter‐wave frequency bands. This system can achieve a maximum throughput of 1.3 Gbps while delivering data at a rate of 300 Mbps, even at distances of 10 km. We also present the details of our testbed implementation alongside the performance results obtained from field tests. [ABSTRACT FROM AUTHOR]

An approach is presented to enhance the isolation of a two-port Multiple Input Multiple Output (MIMO) antenna using a decoupling structure and a common defected ground structure (DGS) that physically separates the antennas from each other. The antenna operates in the 24 to 40 GHz frequency range. The innovation in the presented MIMO antenna design involves the novel integration of two arc-shaped symmetrical elements with dimensions of 35 × 35 × 1.6 mm3 placed perpendicular to each other. The benefits of employing an antenna with elements arranged perpendicularly are exemplified by the enhancement of its overall performance metrics. These elements incorporate a microstrip feed featuring a quarter-wave transformer (QWT). This concept synergizes with decoupling techniques and a defected ground structure to significantly enhance isolation in a millimeter wave (mm wave) MIMO antenna. These methods collectively achieve an impressively wide bandwidth. Efficient decoupling methodologies have been implemented, yielding a notable increase of 5 dB in isolation performance. The antenna exhibits 10 dB impedance matching, with a 15 GHz (46.87%) wide bandwidth, excellent isolation of more than 28 dB, and a desirable gain of 4.6 dB. Antennas have been analyzed to improve their performance in mm wave applications by evaluating diversity parameters such as envelope correlation coefficient (ECC) and diversity gain (DG), with achieved values of 0.0016 and 9.992 dB, respectively. The simulation is conducted using CST software. To validate the findings, experimental investigations have been conducted, affirming the accuracy of the simulations. [ABSTRACT FROM AUTHOR]

This research aims to introduce an innovative method of constellation modification in MPSK data transmission through utilization of frequency multiplication. This method presents a significant solution for providing efficient data transmission in millimeter wave frequency band, addressing the requisites for high data rates and capacity. However, the application of frequency multiplication to MPSK signal causes problems wherein data recovery becomes unattainable due to nonlinearity. Hence, a modified MPSK modulator is developed. Along with the standard MPSK modulator, back-to-back and line-of-sight (LOS) measurements of the modified MPSK modulator are performed. To conduct the standard and modified MPSK modulators in real-time, the development of transmitter and receiver blocks is undertaken using LabVIEW and experimentation is performed employing the SDRs. Comparative analysis reveals that the modified MPSK modulator, conceived through the proposed method, yields improvement in error performance while mitigating the computational burden from digital processing. Importantly, these enhancements are achieved without necessitating alterations in receiver. BER results for measurements of standard and modified MPSK are obtained. The findings substantiate the effectiveness of the proposed method, manifesting a discernible enhancement in error performance. This method proves appropriate for MPSK data in experimental millimeter wave band investigations utilizing frequency multiplier chains. [ABSTRACT FROM AUTHOR]

The cumulative effects of network transmission link imperfections and realtime limitations of video data result in multiple challenges for video transmission in massive millimeter wave (mmwave) network. The challenges, together with the increasing expectations of users for goodquality videos, further extend the complexity particularly in dense areas. To address the issues this work proposes perceptual quality aware video encoding (PQAVE) scheme in two phases: in phase 1 an effective perceptual quality encoding method leveraging low-rank approximation to reduce overall video size is done. In phase 2, a novel bitstream optimization technique is introduced to improve perceptual quality of videos. Experiment are conducted using standard video dataset show the proposed PQAVE model attain better bit error rate (BER), symbol error rate (SER), error vector magnitude (EVM), and coding gain in comparison with existing perceptual video encoding scheme. [ABSTRACT FROM AUTHOR]

LDS-MIDs (laser direct structured mechatronic integrated devices) are 3D (three-dimensional) circuit carriers that are used in many applications with a focus on antennas. However, thanks to the rising frequencies of HF (high-frequency) systems in 5G and radar applications up to the mmWave (millimeter wave) region, the requirements regarding the geometrical accuracy and minimal wall thicknesses for proper signal propagation in mmWave circuits became more strict. Additionally, interest in combining those with 3D microstructures like trenches or bumps for optimizing transmission lines and subsequent mounting processes is rising. The change from IM (injection molding) to ICM (injection compression molding) could offer a solution for improving the 3D geometries of LDS-MIDs. To enhance the scientific insight into this process variant, this paper reports on the manufacturing of LDS-MIDs for mmWave applications. Measurements of the warpage, homogeneity of local wall thicknesses, and replication accuracy of different trenches and bumps for mounting purposes are presented. Additionally, the effect of a change in the manufacturing process from IM to ICM regarding the dielectric properties of the used thermoplastics is reported as well as the influence of ICM on the properties of LDS metallization—in particular the metallization roughness and adhesion strength. This paper is then concluded by reporting on the HF performance of CPWs (coplanar waveguides) on LDS-MIDs in comparison to an HF-PCB. [ABSTRACT FROM AUTHOR]

The potentialities of improving the penetration of millimeter waves for breast cancer imaging are here explored. Methods: A field shaping technique based on a convex optimization method is proposed, capable of increasing the field level inside a breast-emulating stratification. Results: The theoretical results are numerically validated via the design and simulation of two circularly polarized antennas. The experimental validation of the designed antennas, using tissue-mimicking phantoms, is provided, being in good agreement with the theoretical predictions. Conclusion: The possibility of focusing, within a lossy medium, the electromagnetic power at millimeter-wave frequencies is demonstrated. Significance: Field shaping can be a key for using millimeter waves for breast cancer detection. Index Terms—Breast Cancer, Microwave Imaging, Millimeter Waves, Radial Line Slot Arrays, Near-Field Optimization. A field focusing technique based on a convex optimization method is proposed, capable of increasing the field level inside a breast-emulating stratification. A focusing technique based on convex optimization has been used here to increase the penetration in breast cancer imaging scenarios at millimeter waves. Significant enhancements of the field level inside a breast model have been achieved by employing focused apertures. The key challenge in cancer detection is how to classify tumors into malignant or benign machine learning techniques can dramatically improves the accuracy of diagnosis. As the methods itself show that it’s much complicated in designing and implementation. The data visualization process itself is a complicated process which can generate false results if the image has noise, so the result won’t be correct. It usually contains two major steps but the further steps are much similar to the above methods,which makes it further more complicated as it including converting patched images to real images which makes the result inaccurate in diagnosis. [ABSTRACT FROM AUTHOR]

This paper presents a waveguide–slot antenna to generate a radiation beam with high gain fed by a low-loss feeding network at 110 GHz. The proposed antenna consists of a compact eight-way power divider and a waveguide–slot array. The eight-way power divider provides equal-amplitude and alternative-phase excitation for the slot array, and each of them supports two waveguides. The integral structure is implemented by two layers with a channeled substratum and a slotted superstratum. To verify the proposed slot array, the designed array is fabricated with computer numerical control (CNC) milling and measured. The measured peak gain of the designed antenna is 32 dBi at 110 GHz. The proposed antenna with a simple structure provides a promising solution to develop high gain antenna in upper millimeter-wave and sub-terahertz (THz) applications. [ABSTRACT FROM AUTHOR]

Given the shortage of spectrum resources and the demand for communication systems of diminutive size, multi-function, and adaptive characteristics, this paper proposes an L-slot frequency reconfigurable antenna based on the MEMS switch. The antenna size is 4.07 × 5.27 mm2 and is suitable for the U-band. The antenna structure consists of two RF MEMS switches, a Rogers RT5880 dielectric substrate, an L-slot patch, and a full-coverage ground. The switch is of a series contact structure and is arranged at the corner of an L-slot. By controlling the on and off state of the switch, the antenna can switch between four states of 42.36, 47.65, 53.13, and 56.72 GHz. According to the simulation results in CST STUDIO SUITE 2018, the maximum gain of the antenna is 7.90 dB, the impedance bandwidth of each state is above 1 GHz, and the direction is mainly consistent. The antenna can meet the demand for multi-frequency millimeter wave communication. [ABSTRACT FROM AUTHOR]

Today's 5G wireless communication evolution system demands millimeter wave frequency range antenna for its uses in several communication device applications. A 2-port Asymmetric Flare-Shape Patch Multiple Input Multiple Output (MIMO) antenna for mm-wave communication system is designed and presented. The antenna structure is constructed on a Rogers RT Duroid 5880 dielectric substrate with 1.6mm thickness, 2.2 dielectric constant, and 0.0009 loss tangent. The constructed MIMO structure has an overall size of 26 x 19:2mm2. The proposed MIMO design has 10 dB return loss performance over a frequency range of 20{40 GHz with more than 20 dB isolation between antenna elements, which shows the low mutual coupling between antenna elements. The performance of the suggested MIMO antenna is reported in terms of return loss, gain, ECC, and radiation pattern. The simulated and measured MIMO antenna performance characteristics are in good agreement. The suggested design achieves more than 20 dB isolation and 8.17 dB gain with an ECC value lower than 0.0001, which meets the diversity performance of the MIMO design with two antenna elements. The proposed MIMO design is compact and the best choice for 5G mm-wave applications. [ABSTRACT FROM AUTHOR]

In 5G communications, the acquisition of accurate channel state information (CSI) is of great importance to the hybrid beamforming of millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) system. In classical mmWave MIMO channel estimation methods, the exploitation of inherent sparse or low-rank structures has demonstrated to improve the performance. However, most high-accurate CSI estimators incur a high computational complexity and require the prior channel information, which hence present the major challenges in the practical deployment. In this work, we leverage machine learning to design the low-complexity and high-performance channel estimator. To be specific, we first formulate the CSI estimation, in the case of sparse structure, as one classical least absolute shrinkage and selection operator problem. In order to reduce the time complexity of existing compressed sensing (CS) methods, we then approximate the original optimization problem to another one, by imposing the other low-rank constraint that was barely considered by CS. We thus solve this new approximated problem and attain the near-optimal solution of the original problem. One new method excludes any prior channel information, and greatly improves the estimation performance, which only incurs a low time complexity. Simulation results demonstrate the superiority of our proposed method both in the estimation accuracy and time complexity. [ABSTRACT FROM AUTHOR]

The development of intelligent transport systems, mobile cellular networks, microwave links, and vehicle communications has accelerated with the use of wireless connections as a communication channel in 5G wireless technology. Weather, including rain, fog, snow, sand, and dust, impacts wireless communication channels in various ways. These effects are more pronounced at the high frequencies of millimeter-wave bands. Recently, the 5G network has made it possible to support a variety of applications with fast speeds and high-quality content. To facilitate the use of high-millimeter-wave frequencies, a recent study investigated how sand and dust affect the 5G communication channel. In this paper, we consider the impact of frequent and heavy rainfall on millimeter-wave propagation and cross-polarization of the wave at various points along the propagation path caused by rainfall in urban and highway scenarios in Malaysia. We estimate rainfall attenuation, path loss, and link margin at various millimeter-wave frequencies. From our simulation results, it is evident that rainfall attenuation, path loss, and link margin depend on the operating frequency, path difference, and rainfall rate. In this paper, we estimate and compare the optimal path difference values under urban and highway scenarios both with and without rainfall attenuation. [ABSTRACT FROM AUTHOR]

The eruption of the Hunga Tonga undersea volcano in January 2022 injected water vapor to altitudes as high as 53 km, but also an unprecedented and much larger amount of water vapor into the stratosphere. Several months after the eruption, measurements from the Aura Microwave Limb Sounder (MLS) and from three ground‐based Water Vapor Millimeter Wave Spectrometer instruments began to measure record‐high amounts of water vapor in the mesosphere over a wide range of latitudes. While there are indications that some of this mesospheric increase in water vapor was probably caused by the Hunga Tonga eruption, variations in water vapor mixing ratios also depend on dynamical factors. The phase of the QBO in 2015 was similar to that in 2022, and we make use of this similarity in order to better understand what role dynamics played in establishing the unusually large 2022 water vapor mixing ratios, both in the upper and lower mesosphere. Plain Language Summary: The eruption of the Hunga Tonga undersea volcano in January 2022 injected water vapor to altitudes as high as 53 km. While the direct injection to 53 km was impressive, the quantity was insufficient to significantly affect the global mesospheric (∼50–80 km) water vapor budget. However, the Hunga Tonga eruption can also affect mesospheric water vapor through the gradual ascent of the unprecedented and much larger amount of water vapor that was directly injected into the stratosphere (∼15–50 km). Several months after the eruption, measurements from the Aura Microwave Limb Sounder (MLS) and from three ground‐based Water Vapor Millimeter Wave Spectrometer instruments began to measure record‐high amounts of water vapor in the mesosphere. While some of this increase is probably caused by the rise of unusually wet air from the Hunga Tonga plume, determining the precise contribution of the plume is difficult because there are a number of other factors that also caused an increase in mesospheric water vapor in 2022. Key Points: Aura Microwave Limb Sounder and three ground‐based Water Vapor Millimeter Wave Spectrometer instruments all observed record‐high water vapor in the upper and lower mesosphere in 2022Some of this mesospheric increase in water vapor was probably caused by the Hunga Tonga eruptionDynamics also played a role in establishing record‐high water vapor mixing ratios, both in the upper and lower mesosphere [ABSTRACT FROM AUTHOR]

In this paper, a compact two-port MIMO antenna array system is described. The antenna array spans the range from 27 to 40 GHz, whereas the impedance of the antenna element is matched at 50 Ω. The gain of the antenna element is between 5.5 and 8.5 dBi, and its radiation efficiency is between 65 and 90%. With high impedance matching for 5G MMW (Millimeter Wave), particularly at 28 GHz and 38 GHz, the 2-port antenna array operates in the frequency range of 27–40 GHz. The suggested MIMO array operates effectively with a gain of approximately 10 dBi and a radiation efficiency of approximately 95%. The antenna array's overall dimensions are a length of 55.27 mm, a width of 27.635 mm, and a depth of 1.6 mm with partial ground. A FR-4 substrate is used in the antenna's fabrication, greatly reducing the cost. In the antenna array, a decoupling surface is used between the antennas, with orthogonality being maintained between the ports to reduce mutual coupling. The results of the modelling show a reduction in the measured mutual coupling between array ports of less than − 35 dB. An envelope correlation coefficient (ECC) of less than 1 × 10–4 is preferable. Additionally, the channel loss capacity is less than 0.3 bits/s/Hz, the mean effective gain is approximately − 6 dB, and the total active reflection coefficient is upgraded to be less than − 30 dB. Moreover, a diversity gain of approximately 10 dB is achieved. The proposed construction was created using CST Microwave Studio 2019. When the antennas are constructed and tested, the experimental outcomes surpass those of the simulation. Each antenna element is 27.635 mm long, 27.635 mm wide, and 1.6 mm thick. Slots in the radiating circular patch antenna element could be used to improve the radiation characteristics across the intended bands. The parametric study specifies that the distance between the antenna elements should be 0.5 λ, where λ is the operational wavelength. [ABSTRACT FROM AUTHOR]

This study presents a reliable classification of walking pedestrians and animals using a radar operating in the millimeter waves (mmW) regime. In addition to the defined targets, additional targets were added in an attempt to fool the radar and to present the robustness of the proposed technique. In addition to the classification capabilities, the presented scheme allowed for the ability to detect the range of targets. The classification was achieved using a deep neural network (DNN) architecture, which received the recordings from the radar as an input after the pre-processing procedure. Qualitative detection was made possible due to the radar's operation at extremely high frequencies so that even the tiny movements of limbs influenced the detection, thus enabling the high-quality classification of various targets. The classification results presented a high achievable accuracy even in the case where the targets attempted to fool the radar and mimicked other targets. The combination of the use of high frequencies alongside neural-network-based classification demonstrated the superiority of the proposed scheme in this research over the state of the art. The neural network was analyzed with the help of interpretable tools such as explainable AI (XAI) to achieve a better understanding of the DNN's decision-making process and the mechanisms via which it was able to perform multiple tasks at once. [ABSTRACT FROM AUTHOR]

In this paper, we investigate non-orthogonal multiple access (NOMA) scheme in millimeter wave (mmWave) communication to serve nonclustered multiple users. We explore a low complex design of an analog beamforming weight vector and the power requirement of users aiming to minimize the total power targeting to satisfy the spectral efficiency (SE) requirements of all users. We propose a low complex constant modulus analog beamforming (CMAB) algorithm, where we first reduce the number of signal to interference plus noise ratio (SINR) constraints, which is attained from the order of equivalent channel gain of users. Then, the nonconvex constraint of constant modulus (CM) is relaxed and semi-definite programming (SDP) is used to solve the problem. Obtained weight vector and power for all users are optimal since the rank of positive semi-definite (PSD) matrix is one. Later, CM constraint is included. Simulation results show that the proposed algorithm requires less power with minimum complexity compared to the existing research, digital beamforming NOMA and time division multiple access (TDMA) for the same SE requirements. [ABSTRACT FROM AUTHOR]

In this paper, a novel gamma-shadowed two-ray with diffuse power (GS-TWDP) composite fading model is proposed by assuming that the shadowing-caused mean power variations of the received signal, affected by TWDP multipath fading, are gamma distributed. For the proposed model, the essential statistics such as probability density function (PDF), cumulative distribution function (CDF), moment generating function (MGF) and higher order moments are obtained and used to derive the approximate general order rectangular quadrature amplitude (RQAM) average symbol error probability (ASEP). That expression enabled us to investigate the effect of shadowing on overall error probability, in case when signal is propagating in channels with TWDP multipath fading. The proposed model is verified by comparing analytically obtained results with those measured at 28 GHz and reported in literature, which makes the GS-TWDP model a promising candidate for modeling propagation at millimeter wave (mmWave) frequencies. Analytical results are validated by Monte-Carlo simulation. [ABSTRACT FROM AUTHOR]

An environment-aware millimeter wave radar (EMWR) is one of the most important sensors in an autonomous driving system. With the increasing penetration of an EMWR on an autonomous vehicle, the possibility of radar interference increases accordingly. Interference may seriously affect the detection performance of an EMWR. When the transmitted signals from other EMWRs are received by the EMWR on an autonomous vehicle, the accuracy of target detection will be affected, which may affect the environment-aware performance. Therefore, more and more attention is paid to how to restrain the interference between EMWR signals. Radar interference can be divided into two types: same-frequency interference (SFI) and different-frequency interference (DFI). In this paper, an anti-DFI algorithm of an EMWR is proposed. Firstly, the causes and signal characteristics of DFI are analyzed. Secondly, the signal amplitude is obtained via Hilbert transform to locate the interference according to the signal characteristics. Then, the Lagrange interpolation based on empirical-mode decomposition (EMD) is used to reconstruct the interference signal to reduce the influence of DFI. Finally, the feasibility of the proposed algorithm is verified by the simulation results. The simulation results validate that the interference signal after EMD filtering and Lagrange interpolation can repair the interference region and achieve the purpose of anti-DFI. [ABSTRACT FROM AUTHOR]

In Millimeter-Wave (mm-Wave) massive Multiple-Input Multiple-Output (MIMO) systems, hybrid precoders/combiners must be designed to improve antenna gain and reduce hardware complexity. Sparse Bayesian learning via Expectation Maximization (SBL-EM) algorithm is not practically feasible for high signal dimensions because estimating sparse signals and designing optimal hybrid precoders/combiners using SBL-EM still provide high computational complexity for higher signal dimensions. To overcome the issues of high computational complexity along with making it suitable for larger data sets, in this paper, we propose Learned-Sparse Bayesian Learning with Generalized Approximate Message Passing algorithm (L-SBL-GAMP) algorithm for designing optimal hybrid precoders/combiners suitable for mmWave Massive MIMO systems. The L-SBL-GAMP algorithm is an extension of the SBL-GAMP algorithm that incorporates a Deep Neural Network (DNN) to improve the system performance. Based on the nature of the training data, the L-SBL-GAMP can design the optimal Hybrid precoders/combiners, which enhances the spectral efficiency of mmWave massive MIMO systems. The proposed L-SBL-GAMP algorithm reduces the iterations, training overhead, and computational complexity compared to the SBL-EM algorithm. The simulation results unveil that the proposed L-SBL-GAMP provides higher achievable rates, better accuracy, and low computational complexity compared to the existing algorithm, such as Orthogonal Matching Pursuit (OMP), Simultaneous Orthogonal Matching Pursuit (SOMP), SBL-EM and SBL-GAMP for mmWave massive MIMO architectures. [ABSTRACT FROM AUTHOR]

Effective mobility management is crucial for efficient operation of next-generation cellular systems in the millimeter wave (mmWave) band. Massive multiple-input–multiple-output (MIMO) systems are seen as necessary to overcome the significant path losses in this band, but the highly directional beam makes the channels more susceptible to radio link failures due to blockages. To meet stringent capacity and reliability requirements, multi-connectivity has attracted significant attention. This paper proposes a multiagent distributed Q learning-based mobility management scheme for multi-connectivity in mmWave cellular systems. A hierarchical structure is adopted to address the model complexity and speed up the learning process. The performance is assessed using a realistic measurement data set collected from Wireless Insite in an urban area and compared with independent Q learning and a heuristic scheme in terms of handover probability and spectral efficiency. [ABSTRACT FROM AUTHOR]

As a convenient and natural way of human-computer interaction, gesture recognition technology has broad research and application prospects in many fields, such as intelligent perception and virtual reality. This paper summarized the relevant literature on gesture recognition using Frequency Modulated Continuous Wave (FMCW) millimeter-wave radar from January 2015 to June 2023. In the manuscript, the widely used methods involved in data acquisition, data processing, and classification in gesture recognition were systematically investigated. This paper counts the information related to FMCW millimeter wave radar, gestures, data sets, and the methods and results in feature extraction and classification. Based on the statistical data, we provided analysis and recommendations for other researchers. Key issues in the studies of current gesture recognition, including feature fusion, classification algorithms, and generalization, were summarized and discussed. Finally, this paper discussed the incapability of the current gesture recognition technologies in complex practical scenes and their real-time performance for future development. [ABSTRACT FROM AUTHOR]

Objectives: Millimeter wave radar has been widely used in automotive industry and other fields, but its application is mainly limited to the environmental perception of obstacles or specific tasks. At present, there is little research on the application of millimeter wave radar in the field of navigation and positioning. Methods: This paper first studies the raw data processing principle of millimeter wave radar, and then designs an indoor ego localization method which only depends on a low‐cost millimeter wave radar. The process mainly includes extracting centroid feature points using density based spatial clustering of applications with noise (DBSCAN) algorithm, matching centroid feature point pairs through nearest neighbor criterion, constructing nonlinear optimization function and solving positioning results using levenberg marquardt method. Results and Conclusions: Experiments show that indoor navigation and positioning can be solved in real time by using a low‐cost millimeter wave radar. Under static conditions, the average horizontal positioning accuracy can reach sub centimeter level (mean value is 0.82 cm and standard deviation is 0.47 cm). Under dynamic conditions, the absolute trajectory error can reach 0.66 m and the average heading angle error can reach 4.58°, which shows the feasibility of ego‐localization of low‐cost millimeter wave radar. Finally, this paper discusses the problems and feasible research ideas of low cost millimeter wave radar in navigation and positioning. [ABSTRACT FROM AUTHOR]

Channel state information in millimeter wave (mmWave) and terahertz (THz) communications systems is vital for various tasks ranging from planning the optimal locations of BSs to efficient beam tracking mechanisms to handover design. Due to the use of large-scale phased antenna arrays and high sensitivity to environmental geometry and materials, precise propagation models for these bands are obtained via ray-tracing modeling. However, the propagation conditions in mmWave/THz systems may theoretically change at very small distances, that is, 1 mm–1 μ m, which requires extreme computational effort for modeling. In this paper, we first will assess the effective correlation distances in mmWave/THz systems for different outdoor scenarios, user mobility patterns, and line-of-sight (LoS) and non-LoS (nLoS) conditions. As the metrics of interest, we utilize the angle of arrival/departure (AoA/AoD) and path loss of the first few strongest rays. Then, to reduce the computational efforts required for the ray-tracing procedure, we propose a methodology for the extrapolation and interpolation of these metrics based on the convolutional long short-term memory (ConvLSTM) model. The proposed methodology is based on a special representation of the channel state information in a form suitable for state-of-the-art video enhancement machine learning (ML) techniques, which allows for the use of their powerful prediction capabilities. To assess the prediction performance of the ConvLSTM model, we utilize precision and recall as the main metrics of interest. Our numerical results demonstrate that the channel state correlation in AoA/AoD parameters is preserved up until approximately 0.3–0.6 m, which is 300–600 times larger than the wavelength at 300 GHz. The use of a ConvLSTM model allows us to accurately predict AoA and AoD angles up to the 0.6 m distance with AoA being characterized by a higher mean squared error (MSE). Our results can be utilized to speed up ray-tracing simulations by selecting the grid step size, resulting in the desired trade-off between modeling accuracy and computational time. Additionally, it can also be utilized to improve beam tracking in mmWave/THz systems via a selection of the time step between beam realignment procedures. [ABSTRACT FROM AUTHOR]

An inverted "U" shape patch antenna has been proposed in this research. It is reconfigurable and operates with millimeter waves. Therefore, it is appropriate for 5G wireless networks. Three separate resistors--100, 500, and 750 ohms--that are integrated into the antenna's radiating element have been used to achieve frequency reconfigurability. The proposed antenna is (5mm x 3.2mm) rectangular patch. In this patch, a rectangular slot is made with dimensions (1mmx2.6mm). The proposed antenna is designed on Rogers RT5880 substrate with dielectric constant er= 2.2 and loss tangent = 0.0009 and its thickness h=1.6mm. It produces three frequencies: 46.2 GHz, 50 GHz, and 50.8 GHz with return losses: -35dB, -25dB, -27dB and peak gains: 7.22dB, 7.15dB ,6.1dB respectively. The proposed antenna has been simulated using CST electromagnetic simulator. [ABSTRACT FROM AUTHOR]

The rough structure may affect the flow field and electromagnetic (EM) scattering characteristics of a hypersonic vehicle. In this study, the EM scattering characteristics of the hypersonic vehicle RAM C-II with a microrough surface were investigated in the millimeter wave band. We first simulated the flow field of a smooth RAM C-II vehicle and calculated the radar cross section (RCS) in the wideband using the physical optics method. The calculation results show that many factors contribute to the variation in the RCS; however, it is occasionally difficult to predict. We then used a one-dimensional Gaussian random rough surface to generate a RAM C-II with a microrough surface through a point-coordinate transformation and simulated its flow field. The electron density flow field of the rough surfaced aircraft fluctuates greatly on the wall. Finally, we calculated the RCS of RAM C-II with different values of roughness in the millimeter wave band. The change of surface structure can be reflected in millimeter waves, and the RCS is determined by both plasma and rough structure. [ABSTRACT FROM AUTHOR]

In video surveillance, real-time detection of hidden or concealed objects under the clothes of humans is a challenging task. Very few research works have been carried out on this hidden object detection and recognition with the aid of various coherent technologies namely Millimeter wave (MMW) imaging, Infrared (IR) imaging, and Terahertz (THz) imaging systems. Hidden objects that include weapons like axe, knife, gun, bomb, pistol, etc., are a major threat to security surveillance as it needs to be recognized in a few seconds irrespective of their size. This paper proposes an efficient hidden object recognition method that focuses on the detection and recognition of concealed weapons in humans using Modified Weighted You Only Look Once v5 (MWYOLOv5) model. The probable occurrence of inaccurate forecast results due to the sense of a low confidence score is mitigated by the use of the weighted boxes fusion (WBF) method. As a result, higher-confidence boxes contribute more to fused box coordinates than lower-confidence boxes. Meanwhile, it is significant to select optimal hyperparameter values for training YOLOv5 model with CSPDarknet53 based feature extractor and Path Aggregation Network (PANet) based feature aggregation procedures to predict concealed objects. To achieve this, a new crossover salp swarm algorithm (CSSA) is developed to tune the YOLO hyperparameters such as learning rate, momentum, weight decay and batch size. This provides more accuracy in recognizing hidden objects on using THz and MMW images on comparison with existing methods. The proposed hazardous weapon recognition model is trained and tested on both MMW and THz Imagery datasets and it shows that the proposed methodology is showing good results with high mAP@.5 and mAP@.5:95. [ABSTRACT FROM AUTHOR]

A new microwave sensor is proposed in this study for skin cancer identification. The Co-Planar Waveguide fed sensor is designed on the low-cost Roger substrate with the overall size of (15 × 20 mm2). The sensor is operated in millimeter waves range (0.03–0.1 THz). The skin carcinoma is easily monitored by the sensor, due to good penetration strength of the electromagnetic waves in this frequency range. The sensor performance is verified in terms of the reflection parameters, gain, and directivity. Further, Normal and malignant models of the hand and scalp are developed using Computer Simulation Tool. In order to reflectometry analysis, the sensor is interacted with these skin models. The magnitude and phase angle of the reflection coefficient are evaluated for both normal and malignant case. Sensitivity of the sensor is calculated from these results for hand and scalp models. The sensor sensitivity for hand and scalp models are (60.92%, 69% and 45.1%) and (67.26% and 92.33%) respectively. In the presence of the dielectric contrast between normal and malignant skin, malignancy is easily identified. The SAR analysis of the normal and malignant scalp is also performed. Maximum result of the SAR for normal skin is 196 W/Kg and the malignant skin has maximum SAR of 191 W/Kg. Wide Operating bandwidth (0.03–0.1 THz) stable peak gain more than 7.5 dBi better directivity (more than 8 dBi) are most favorable for early-stage diagnosis of the skin cancer. [ABSTRACT FROM AUTHOR]

Compared to frequency bands below 6 GHz, 5G millimeter waves offer several advantages, including a large bandwidth, minimal null delay, and flexible null port configuration. To comprehend the channel characteristics of 5G millimeter-wave technology, conducting channel measurements on it is essential. Hence, to ensure precise 5G millimeter-wave channel measurements and facilitate channel modelling, this study recommends utilizing a phased array antenna-based method for channel measurement. The experimental outcomes demonstrated that the shadow fading term of the actual measurement data follows a normal distribution in both line-of-sight and non-line-of-sight scenarios. Additionally, the K-S test confirms that all Boolean variables are equal to 1 and all numerical variables are greater than a. The lineof-sight scenario produces a logarithmic mean delay extension of -7.4 and a standard deviation of 0.12, based on actual measured data. Meanwhile, 3GPP shows a logarithmic mean of -7.4 and a standard deviation of 0.15. In the non-line-of-sight scenario, the logarithmic mean delay extension is -7.3 with a standard deviation of 0.17, while 3GPP produces a logarithmic mean of -7.4 and a standard deviation of 0.19. The data presented closely adheres to the 3GPP model. It is evident that the channel measurement method, proposed within the study, effectively measures the parameters within the delay domain. Concerning the prolonged pitch angle and azimuth angle ranges, they measure 14°-31° and 14°-29° in the line-of-sight situation, and 21°-33° and 19°-37° in the corresponding non-line-of-sight situation. Additionally, the logarithmic mean and standard deviation for both the pitch angle and azimuth angle in the line-of-sight scenario are 1.32 and 0.09, respectively. The logarithmic mean and standard deviation of the azimuth angle of arrival are 1.35 and 0.08, respectively. The above results show that the method proposed in the study enables the measurement of 5G millimeter-wave channels and is important for millimeter-wave channel modelling. [ABSTRACT FROM AUTHOR]

The high-precision terrestrial reference frame, as the spatial benchmark for geodesy, is an important national infrastructure. However, due to the influence of nonlinear factors related to geophysical phenomena, the overall maintenance accuracy of the ITRF framework is still at the centimeter level. Therefore, accurately characterizing the true trajectories of linear motion, nonlinear motion, and geocentric motion of the reference station is the key to achieve the construction and maintenance technology of a millimeter level terrestrial reference framework. Based on long-term global and regional GNSS observation data, more Chinese geodesy scientists devoted much efforts to the maintenance of millimeter-level geodetic reference framework. The main contributions of this work included the followings: â' Dynamic maintenance of millimeter-level terrestrial reference frame;â'¡Research progress on the method of maintenance of regional reference frame based on GNSS; â'¢The progress of CGCS2000 frame maintenance in millimeter level accuracy; â'£Reprocessing and reanalysis of two-decade GNSS observation in continental China; â'¤Research on current GNSS velocity field model and deformation in Chinese mainland; â'¥The preliminary realization and evaluation of CTRF2020. [ABSTRACT FROM AUTHOR]

The goal of this paper is the performance evaluation of a deep learning approach when deployed in fifth-generation (5G) millimeter wave (mmWave) multicellular networks. To this end, the optimum beamforming configuration is defined by two neural networks (NNs) that are properly trained, according to mean square error (MSE) minimization. The first network has as input the requested spectral efficiency (SE) per active sector, while the second network has the corresponding energy efficiency (EE). Hence, channel and power variations can now be taken into consideration during adaptive beamforming. The performance of the proposed approach is evaluated with the help of a developed system-level simulator via extensive Monte Carlo simulations. According to the presented results, machine learning (ML)-adaptive beamforming can significantly improve EE compared to the standard non-ML framework. Although this improvement comes at the cost of increased blocking probability (BP) and radiating elements (REs) for high data rate services, the corresponding increase ratios are significantly reduced compared to the EE improvement ratio. In particular, considering 21.6 Mbps per active user and ML adaptive beamforming, the EE can reach up to 5.3 Mbps/W, which is significantly improved compared to the non-ML case (0.9 Mbps/W). In this context, BP does not exceed 2.6%, which is slightly worse compared to 1.7% in the standard non-ML case. Moreover, approximately 20% additional REs are required with respect to the non-ML framework. [ABSTRACT FROM AUTHOR]

At present, it is difficult to obtain the indoor propagation loss quickly and accurately by directly using measurements in the millimeter wave band. To solve this problem, in this paper, a ray tracing method suitable for indoor scenes based on geometric optics theory, the uniform theory of diffraction and image theory is presented; the space-alternation generalized expectation-maximization (SAGE) algorithm is used to analyze the measured data and the multipath information of the wireless channel is analyzed; three deep learning models are used to predict the path loss at different receiving distances based on 1600 sets of path loss data. The results show that the comparison between the ray tracing and experimental results shows a good agreement. Moreover, the root-mean-square error (RMSE) and mean absolute error (MAE) of the long short-term memory (LSTM) network are the smallest, and the LSTM has a better fitting effect on the propagation loss sequences predicted at more distant locations when compared with the recurrent neural network (RNN) and gate recurrent unit (GRU) methods, which can better reflect the propagation trend. This provides theoretical support for the layout of base stations and network optimization in typical open indoor environments. [ABSTRACT FROM AUTHOR]

Concealed object detection in millimeter wave (MMW) images has gained significant attention in the realm of public safety, primarily due to its distinctive advantages of non-hazardous and non-contact operation. However, this undertaking confronts substantial challenges in practical applications, owing to the inherent limitations of low imaging resolution, small concealed object size, intricate environmental noise, and the need for real-time performance. In this study, we propose Swin-YOLO, an innovative single-stage detection model built upon transformer layers. Our approach encompasses several key contributions. Firstly, the integration of Local Perception Swin Transform Layers (LPST Layers) enhanced the network's capability to acquire contextual information and local awareness. Secondly, we introduced a novel feature fusion layer and a specialized prediction head for detecting small targets, effectively leveraging the network's shallow feature information. Lastly, a coordinate attention (CA) module was seamlessly incorporated between the neck network and the detection head, augmenting the network's sensitivity towards critical regions of small objects. To validate the efficacy and feasibility of our proposed method, we created a new MMW dataset containing a large number of small concealed objects and conducted comprehensive experiments to evaluate the effectiveness of overall and partial improvements, as well as computational efficiency. The results demonstrated a remarkable 4.7% improvement in the mean Average Precision (mAP) for Swin-YOLO compared with the YOLOv5 baseline. Moreover, when compared with other enhanced transformer-based models, Swin-YOLO exhibited a superior accuracy and the fastest inference speed. The proposed model showcases enhanced performance and holds promise for advancing the capabilities of real-world applications in public safety domains. [ABSTRACT FROM AUTHOR]

Copyright of Smart Rail Transit is the property of Smart Rail Transit Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Przegląd Elektrotechniczny is the property of Przeglad Elektrotechniczny and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Background/Aim: The widespread use of fifthgeneration 5G millimeter-waves (MMW) generates concern about potential adverse health effects. The latest international guidelines for MMW exposure adopt an absorbed power density (APD) of 200 W/m2 to avoid a local temperature rise of 5˚C in human tissues as an operational adverse health effect threshold. However, because APD is estimated by simulations using human tissue models, it is unknown whether a similar value can be confirmed for living tissues. The aim of this study was to investigate the relationship between APD and skin temperature rise in vivo, and to validate the estimated values. Materials and Methods: The rat dorsal skin was locally exposed to a 26.5 GHz quasiMMW (qMMW) for 18 min using a patch antenna. The qMMW exposure intensities estimated by dosimetry were set to 0-500 W/m2 of APD. The temperatures in the dorsal skin and rectum were simultaneously measured during exposure. Results: The qMMW-induced local temperature increase at different sites. The dorsal skin temperature increased by approximately 11.3˚C at a maximum intensity of 500 W/m2, but the rectal temperature increased by only 0.6˚C, indicating highly localized effects of exposure to rats. A significant correlation was observed between APD and skin temperature rise. The relationship provided a linear regression model, and a temperature rise of less than 5˚C was estimated in the skin exposed to 200 W/m2 of APD. Conclusion: These results suggest that the operational threshold for the MMW exposure guidelines is valid under the present experimental conditions using rats. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Remote Sensing is the property of Editorial Office of Journal of Remote Sensing & Science Publishing Co. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Microplasma generated within a millimeter wave (MMW) photonic crystal (PhC) is analyzed by direct measurement of the electron density and the wave transmission spectra (S21) of the crystal. A continuous wave (cw) drive frequency of 43.66 GHz maintains plasma, while a low power frequency sweep (43.5–44.1 GHz) simultaneously probes the wave transmission in the presence of microplasma. Rotational gas temperature and electron density are measured from the CH emission spectrum and the Stark broadening of the Hβ atomic transition. The permittivity of the plasma depends on the electron density. As higher cw power and argon gas pressure increase the electron density in the PhC vacancy, the resonant frequency of the PhC shifts upward in accordance with the measured electron density and plasma permittivity. As the PhC obscures the central core of the microplasma, we present a simple diffusion model that approximates the electron density distribution within the microplasma. The diffuse electron density is then used in a numerical model for S21(ω). The measured and modeled transmission spectra can only be reconciled using the diffusion density gradient of the microplasma, thus resolving previous discrepancies observed when using simplified, abrupt-boundary plasma models. [ABSTRACT FROM AUTHOR]

We designed, fabricated, and measured anti-reflection coating (ARC) on sapphire that has 116% fractional bandwidth and transmission of at least 97% in the millimeter wave band. The ARC was based on patterning pyramid-like sub-wavelength structures (SWS) using ablation with a 15 W femto-second laser operating at 1030 nm. One side of each of two disks was fabricated with SWS that had a pitch of 0.54 mm and a height of 2 mm. The average ablation volume removal rate was 1.6 mm 3 /min. Measurements of the two-disk sandwich show transmission higher than 97% between 43 and 161 GHz. We characterize instrumental polarization (IP) arising from differential transmission due to asymmetric SWS. We find that with proper alignment of the two-disk sandwich, RMS IP across the band is predicted to be 0.07% at normal incidence and less than 0.6% at incidence angles up to 20 °. These results indicate that laser ablation of SWS on sapphire and on other hard materials such as alumina is an effective way to fabricate broadband ARC. [ABSTRACT FROM AUTHOR]

We present the transmission characteristics of pulsed 44 GHz millimeter waves (MMWs) through a plasma formed within a photonic crystal. Initially, a low density plasma (n e = 0.3 × 10 18 m − 3 ) is sustained in a crystal vacancy defect using a continuous wave (cw) MMW power of 0.06 W. This weak plasma does not much attenuate wave transmission, but it does provide seed electrons such that dense plasma forms in response to incoming MMW pulses. The dense plasma formation limits the transmission of high power pulses through the photonic crystal. A high power pulse increases the plasma electron density and shifts the resonant frequency of the crystal vacancy in accordance with the plasma permittivity. A rapid decrease in the transmitted power is observed by following the time-evolution of the plasma. A numerical electromagnetic model estimates electron density and traces the time-evolution of the plasma during MMW pulsing. The plasma-filled photonic crystal is evaluated as a power limiter. As plasma properties are readily tunable, we observe an improved operational bandwidth for incoming pulse frequencies by controlling the cw power and gas pressure of the initial weak plasma. [ABSTRACT FROM AUTHOR]

This article presents ultrawide bandwidth (BW) and large-angle helical metamaterial absorbers (HMMAs) consisting of a periodic array of coaxially nested magnetic helices. The enhanced properties of these HMMAs, including ultrawide BW, polarization insensitivity, and wide-angle absorption, are confirmed and illustrated experimentally. Large helix diameter (20–30 mm), small coupling constant (close to 1), and small and large helix nesting (5 and 20 mm) help in the enhancement of the absorption properties. Current distributions and coupling property of these HMMAs are provided, and results reveal that the synergistic effect of the LC resonance and dipole responses of the patterned structure is responsible for the broadband absorption mechanism. HMMAs consisting of a 14 × 14 array of a triple-nested helix exhibit optimal performance. The effective BW with reflection loss below −10 dB (90% attenuation) is up to 27.68 GHz at 8–40 GHz. The HMMAs also show polarization insensitivity in the incident range of 0°–315° and angle insensitivity at all incident angles in the range of 35°–70°. Therefore, our findings here provide a valuable guide to the design and fabrication of chiral MMAs, in particular, the ultrawide BW and large-angle chiral MMAs at microwave or millimeter wave frequencies. [ABSTRACT FROM AUTHOR]