Showing papers in "IEEE Transactions on Antennas and Propagation in 2022"

Dual-Polarized Highly Folded Bowtie Antenna With Slotted Self-Grounded Structure for Sub-6 GHz 5G Applications

Abstract: In this communication, a novel dual-polarized highly folded self-grounded Bowtie antenna that is excited through I-shaped slots is proposed for applications in sub-6 GHz 5G multiple-input-multiple-output (MIMO) antenna systems. The antenna consists of two pairs of folded radiation petals whose base is embedded in a double layer of FR-4 substrate with a common ground-plane which is sandwiched between the two substrate layers. The ground-plane is defected with two I-shaped slots located under the radiation elements. Each pair of radiation elements are excited through a microstrip line on the top layer with RF signal that is 180° out of phase with respect to each other. The RF signal is coupled to the pair of feedlines on the top layer through the I-shaped slots from the two microstrip feedlines on the underside of the second substrate. The proposed feed mechanism gets rid of the otherwise bulky balun. The Bowtie antenna is a compact solution with dimensions of 32 $\times $ 32 $\times\,\,33.8$ mm³. Measured results have verified that the antenna operates over a frequency range of 3.1–5 GHz and exhibits an average gain and antenna efficiency in the vertical and horizontal polarizations of 7.5 dBi and 82.6%, respectively.

All-Metal Wideband Frequency-Selective Surface Bandpass Filter for TE and TM Polarizations

Abstract: A novel technique to design a low-cost frequency-selective surface (FSS) bandpass filter is presented in this article. Wideband polarization-independent FSS bandpass filters are predominantly made of multiple microwave dielectric substrates or noncommercially available composite materials with or without active components, contributing to a very high manufacturing cost. The presented FSS filter has neither microwave substrates, nor any active devices, while it has a large controllable operational frequency band, which can support all polarizations, due to its symmetrical configuration. To the best of our knowledge, such a polarization-independent wideband bandpass response has never been achieved by any low-cost fully metallic FSS filter. The proposed FSS filter is made of three thin metal sheets composed of an engineered metallic substrate (EMS) and a metallic orthogonal dipole resonator (ODR). The EMS is responsible for ensuring the mechanical integrity of the filter without imposing electromagnetic (EM) restrictions throughout the desired frequency band. The integration of EMS and ODRs realizes a fully controllable wideband bandpass verified thorough circuital and modal analyses. According to the predicted and measured results, the FSS filter has a large bandwidth of around 31%, extending from 8.76 to 11.96 GHz with sharp roll-offs for the normal incidence. Simulated and measured results show a low sensitivity of the FSS filter response to oblique angles of incidence for both TM and TE polarizations.

Artificial Intelligence: New Frontiers in Real–Time Inverse Scattering and Electromagnetic Imaging

Abstract: In recent years, artificial intelligence (AI) techniques have been developed rapidly. With the help of big data, massive parallel computing, and optimization algorithms, machine learning (ML) and (more recently) deep learning (DL) strategies have been equipped with enhanced learning and generalization capabilities. Besides becoming an essential framework in image and speech signal processing, AI has been also widely applied to solve several electromagnetic (EM) problems with unprecedented computational efficiency, including inverse scattering and EM imaging. In this paper, a review of the most recent progresses in the application of ML and DL for such problems is given. We humbly hope a brief summary could help us to better understand the pros and cons of this research topic and foster future research in using AI to address paramount challenges in the field of EM vision.

All-Metal Wideband Frequency-Selective Surface Bandpass Filter for TE and TM Polarizations

Abstract: A novel technique to design a low-cost frequency-selective surface (FSS) bandpass filter is presented in this article. Wideband polarization-independent FSS bandpass filters are predominantly made of multiple microwave dielectric substrates or noncommercially available composite materials with or without active components, contributing to a very high manufacturing cost. The presented FSS filter has neither microwave substrates, nor any active devices, while it has a large controllable operational frequency band, which can support all polarizations, due to its symmetrical configuration. To the best of our knowledge, such a polarization-independent wideband bandpass response has never been achieved by any low-cost fully metallic FSS filter. The proposed FSS filter is made of three thin metal sheets composed of an engineered metallic substrate (EMS) and a metallic orthogonal dipole resonator (ODR). The EMS is responsible for ensuring the mechanical integrity of the filter without imposing electromagnetic (EM) restrictions throughout the desired frequency band. The integration of EMS and ODRs realizes a fully controllable wideband bandpass verified thorough circuital and modal analyses. According to the predicted and measured results, the FSS filter has a large bandwidth of around 31%, extending from 8.76 to 11.96 GHz with sharp roll-offs for the normal incidence. Simulated and measured results show a low sensitivity of the FSS filter response to oblique angles of incidence for both TM and TE polarizations.

Artificial Intelligence Enabled Radio Propagation for Communications—Part II: Scenario Identification and Channel Modeling

Abstract: This two-part paper investigates the application of artificial intelligence (AI) and, in particular, machine learning (ML) to the study of wireless propagation channels. In Part I of this article, we introduced AI and ML and provided a comprehensive survey on ML-enabled channel characterization and antenna-channel optimization, and in this part (Part II), we review the state-of-the-art literature on scenario identification and channel modeling here. In particular, the key ideas of ML for scenario identification and channel modeling/prediction are presented, and the widely used ML methods for propagation scenario identification and channel modeling and prediction are analyzed and compared. Based on the state of the art, the future challenges of AI-/ML-based channel data processing techniques are given as well.

Highly Selective Frequency Selective Surface With Ultrawideband Rejection

Abstract: Frequency selective surface (FSS) with ultrawide out-of-band rejection is proposed in this article. For achieving an ultrawide and high-intensity stopband, a common second-order bandpass FSS is designed and then improved by an equivalent circuit model (ECM) design. The hybrid resonator pole separation (HRPS) decoupling method is proposed and applied during the design process. The modified ECM could realize the suppress effect on the unexcepted out-of-band transmission pole caused by coupling between layers and achieve the desired ultrawideband rejection. Furthermore, the corresponding FSS structure is established from the modified ECM through the mapping method. In addition, we conducted a detailed analysis of equivalent circuit design and the required frequency response achieve method. The measured results show that a second-order passband from 3.12 to 4.64 GHz with 0.3 dB insertion loss (IL) in the band is obtained by the proposed FSS. Meanwhile, transition bands from the passband to the stopband are narrow, and the transmission coefficients out of the passband could be suppressed under −20 dB from 5.5 to over 40 GHz. All the results demonstrate that the proposed FSS can realize an excellent out-of-band rejection while maintaining a well passband.

Design of Broadband Circularly Polarized All-Textile Antenna and Its Conformal Array for Wearable Devices

Abstract: A broadband circularly polarized (BCP) all-textile antenna and its wearable conformal antenna array (WCAA) are investigated for body-centric communications. Initially, a circularly polarized (CP) microstrip patch antenna loaded with a piece of the modified metasurface is designed to achieve wide impedance and axial ratio (AR) bandwidths. The characteristic mode analysis is employed to understand the operating mechanism, which provides clear physical insight into each mode at various frequencies. Due to the adoption of textile (i.e., felt) as substrate and nylon conductive fabric as conductor, the proposed antenna is flexible, totally conforming to the curve-shaped human body. The antenna with broadside radiation is suitable for off-body communications. For verification, the prototype operating at 5 GHz (5.15–5.825 GHz) band was fabricated. The measured −10 dB impedance and 3 dB AR bandwidths of 35.1% and 17.5% with a peak gain of 8.5 dBi are achieved. Furthermore, the BCP antenna is used as an element to constitute the WCAA, and the omnidirectional radiation pattern in the azimuth plane is achieved for on- and off-body communications. The considerations about how to achieve omnidirectional radiation and avoid radiation nulls are studied in detail. Finally, the WCAA was fabricated and tested; the measured results guarantee its potential for wearable applications.

Single-Fed Triple-Mode Wideband Circularly Polarized Microstrip Antennas Using Characteristic Mode Analysis

Abstract: A method of triple-mode operation by capacitive slot loading is proposed for bandwidth enhancement of single-fed circularly polarized (CP) patch antennas. Instead of using even-numbered linearly polarized (LP) modes with quadrature phase, three orthogonal LP modes are used for CP bandwidth enhancement, where the middle mode is shared by two cross-polarized modes with the same polarization. The advantages include reduced constraints, lower complexity, and a higher degree of freedom for antenna design. Guided by the method, a U-slot antenna and an E-shaped antenna are proposed and designed with characteristic mode analysis (CMA). Both antennas work with a TM ₁₀ -like mode and a TM ₀₁ -like mode. Differently, the U-slot antenna works with an additional slot mode and the E-shaped antenna works with an additional TM ₁₁ -like mode. The operating modes are manipulated by the slot loadings for creating phase difference. As a result, wideband CP radiation is achieved with a single feeding. CMA-based empirical formulas are derived for fast design. The proposed method and antennas are experimentally validated. Both antennas measure a bandwidth exceeding 21% for 10 dB return loss and 3 dB axial-ratio (AR), a significant improvement compared with conventional corner-truncated U-slot patch antennas of similar thickness or volume.

Dual Polarized Wide-Angle Scanning Phased Array Antenna for 5G Communication System

Abstract: A dual-polarized phased array antenna is presented with wide-angle scanning capability in this paper. To improve the mutual coupling in the array, a current cancellation method (CCM) is proposed by changing the current distribution on the excited unit to induce a pair of the canceled currents on the adjacent unit. Meanwhile, this current distribution broadens the beam-width of the unit in the array. Besides, the proposed method can also optimize the array unit size and achieve a small inter-unit distance for wide-angle scanning capability. A low-profile dual-polarization antenna operating in the bandwidth from 4.4 GHz to 5.0 GHz is designed as a linear array and a planar array to verify the proposed method. Regardless of the linear array or planar array, the mutual coupling in the array is below -19 dB, which is better than that in conventional arrays. Meanwhile, the antenna unit in the array can radiate a wide-beam pattern. Two arrays can scan over ±60° for both polarizations. Within the scanning range, the realized gain reduction is less than 3 dB and the side-lobe level is lower than -7.5 dB. To verify the performance, two array antenna prototypes are fabricated and tested. The experimental results agree well with the simulation.

On the Design of Complex EM Devices and Systems Through the System-by-Design Paradigm: A Framework for Dealing With the Computational Complexity

Abstract: The system-by-design (SbD) is an emerging engineering framework for the optimization-driven design of complex electromagnetic (EM) devices and systems. More specifically, the computational complexity of the design problem at hand is addressed by means of a suitable selection and integration of functional blocks comprising problem-dependent and computationally efficient modeling and analysis tools as well as reliable prediction and optimization strategies. Due to the suitable reformulation of the problem at hand as an optimization one, the profitable minimum-size coding of the degrees of freedom (DoFs), and the “smart” replacement of expensive full-wave (FW) simulators with proper surrogate models (SMs), which yield fast yet accurate predictions starting from minimum size/reduced CPU-costs training sets, a favorable “environment” for optimal exploitation of the features of global optimization tools in sampling wide/complex/nonlinear solution spaces is built. This research summary is then aimed at: 1) providing a comprehensive description of the SbD framework and of its pillar concepts and strategies; 2) giving useful guidelines for its successful customization and application to different EM design problems characterized by different levels of computational complexity; and 3) envisaging future trends and advances in this fascinating and high-interest (because of its relevant and topical industrial and commercial implications) topic. Representative benchmarks concerned with the synthesis of complex EM systems are presented to highlight advantages and potentialities as well as current limitations of the SbD paradigm.

A High Data Rate Implantable MIMO Antenna for Deep Implanted Biomedical Devices

Abstract: In this article, a compact multiple-input multiple-output (MIMO) implantable antenna based on meandered resonators is proposed for use in medical implantable devices. Such devices require compactness and high data rates. Regarding compactness, the antenna has a size of $5.35\times 6.2 \times 0.12$ mm³ (operating frequency of 2.45 GHz) which is one of the smallest sizes reported in the literature for this class of antennas. This compactness is achieved using the meandered resonators and slots in the ground plane. For high data rates, the proposed antenna is of MIMO configuration and consists of two probe-fed elements that share the same ground plane and are placed at a distance (edge-to-edge) of 0.4 mm from each other. This configuration reduces multipath distortion and achieves seamless communications with high data rates as evident from conducted link budget analysis and MIMO channel studies. The ex vivo measurements (S-parameters, gain, and radiation patterns) are performed on the proposed antenna when placed inside a capsule and implanted in minced meat. A bandwidth of 320 MHz and isolation of 28 dB are achieved at the design frequency along with omni-directional radiation patterns with a peak realized gain of −20.5 dBi. Specific absorption rate (SAR) analysis and input power studies are performed to ensure patient’s safety. The result shows that the antenna has a safe 10 g SAR of 402.8 W/kg at an input power of 1 W. Also, the capsule device can be operated at a safe power level of 3.97 mW, which is far greater than the maximum allowable power limit ( $25~\mu \text{W}$ ).

Self-Decoupled Dual-Band Dual-Polarized Aperture-Shared Antenna Array

Abstract: This communication presents a self-decoupled dual-band dual-polarized aperture-shared antenna array. The proposed dual-band array consists of a low-band (LB) antenna element interleaved into a $2\times 2$ high-band (HB) antenna array with compact elements spacing. To mitigate the blockage effect resulting from the LB element, shorted patches are loaded on the LB radiator for suppressing its scattering in the HB frequency range. In addition, high cross-band isolation between the closely spaced LB and HB elements is obtained due to their intrinsic filtering response. Moreover, the in-band mutual coupling between the HB elements is reduced by adding cross-shaped strips on them. As a result, the cross-band and in-band couplings and the antenna blockage effect can be simultaneously reduced without adding an extra decoupling component. A prototype of the proposed array operating in 1.71–2.17 GHz (LB) and 3.3–3.8 GHz (HB) is fabricated and measured. The radiation patterns of the LB and HB elements are almost unaffected by each other. Furthermore, all of the port-to-port isolations are higher than 20 dB. These merits make the proposed design suitable for the multiband base station.

Entire-Structure-Oriented Work-Energy Theorem Based Characteristic Mode Theory for Material Scattering Objects

Abstract: Entire-structure-oriented work-energy theorem (WET) framework is used to build the characteristic mode theory (CMT) for material scatterers. Orthogonalizing driving power operator (DPO) method is proposed to construct characteristic modes (CMs). Solution domain compression (SDC) scheme is introduced to suppress spurious modes. Electromagnetic energy characteristic equation similar to the energy eigen-equation in quantum mechanics is derived for unifying the modal analysis theories of classical electrodynamics and quantum mechanics. Employing the concept of driving power in WET framework, the physical meaning of characteristic values is revealed; the reason leading to the non-orthogonal characteristic far fields of lossy scatterers is provided; it is explained why the conventional CMT fails to analyze some classical transmitting antennas; the physical interpretation for normalizing modal real power to 1 is given; the Parseval’s identity in CMT is derived; the classical concept of quality factor (Q-factor) is generalized to a novel concept of field-current phase-mismatching factor (Θ-factor). Employing the orthogonalizing DPO method with SDC scheme, the spurious modes outputted from the conventional orthogonalizing impedance matrix operator method are suppressed. Both the novel SDC and conventional dependent variable elimination schemes confirm the same conclusion that the spurious modes originate from overlooking the dependence relationships among the currents in DPO.

Metasurfaces 3.0: A New Paradigm for Enabling Smart Electromagnetic Environments

Abstract: So far, the environment has been considered as a source of fading, clutter, blockage, etc., with detrimental consequences for the efficiency and robustness of communication systems. However, the intense research developed toward beyond-5G communications is leading to a paradigm change, in which the environment is exploited as a new degree of freedom and plays an active role in achieving unprecedented system performances. For implementing this challenging paradigm, it has been recently proposed the use of intelligent surfaces able to control almost at will the propagation of electromagnetic waves. In this framework, metasurfaces have emerged as a promising solution, thanks to their field manipulation capabilities achieved through low-cost, lightweight, and planar structures. The aim of this paper is to review some recent applications of metasurfaces and cast them in the scenario of next-generation wireless systems. In particular, we show their potentialities in overcoming some detrimental effects presented by the environment in wireless communications, and discuss their crucial role towards the practical implementation of a smart electromagnetic environment.

A Beam-Steering Solution With Highly Transmitting Hybrid Metasurfaces and Circularly Polarized High-Gain Radial-Line Slot Array Antennas

Abstract: A continuous beam-steering solution for circularly polarized (CP) radial-line slot array (RLSA) antennas is presented and demonstrated with a fabricated prototype. The solution is based on the near-field meta-steering (NFMS) method, which is implemented through a pair of highly transmitting hybrid metasurfaces (HMs) that are placed above a high-gain CP RLSA antenna in the near-field region. Cells in HMs, unlike conventional metasurfaces (CMs), can provide exact or close to exact phase shift with transmission magnitude larger than −1 dB. This is achieved by combining the nonoverlapping phase ranges of two dramatically different phase-shifting cells. The RLSA is stationary, while the two HMs are rotated to steer the beam in a 2-D (azimuth and elevation) space. The measured results of the prototype demonstrate that the system can steer its beam to a maximum elevation angle of 40.6° with a maximum gain of 30.9 dBic. The axial ratio remained less than 3 dB in the 3 dB beamwidth even when the beam is steered. The height of the system is only 4.5 cm, which is much less than mechanically steered reflector antennas. Unlike electronically steered high-gain antenna arrays, this system requires only few watts of power only when rotating HMs and has zero power consumption when the beam is stationary.

Prior-Knowledge-Guided Deep-Learning-Enabled Synthesis for Broadband and Large Phase Shift Range Metacells in Metalens Antenna

Abstract: A prior-knowledge-guided deep-learning-enabled (PK-DL) synthesis method is proposed for enhancing the transmission bandwidth and phase shift range of metacells used for the design of metalens antennas. The algorithm of conditional deep convolutional generative adversarial network (cDCGAN) is utilized in the proposed deep-learning (DL) method. Prior knowledge, including well-known fundamental electromagnetic theorems and experience in antenna design, is purposely applied at the early stage of the proposed method to strategically guide and speed up the synthesis. The proposed intelligent method provides the design of pixelated metacells with high degrees of freedom so that the key performance of the synthesized metacells exceeds the existing limit of conventional design methods by generating a rich profusion of cell patterns. For example, the synthesized triple-layer metacell achieves the −1 dB phase shift range of 330° breaking the limit of 308° derived by existing techniques. The proposed synthesis method also provides the additional capability to flexibly control the phase shift not only at the center frequency but also over a frequency range of interest. A Ku-band metalens antenna formed with the synthesized metacells demonstrates the achieved 1 and 3 dB gain bandwidths increase by 52.2% and 42.6%, respectively, compared to the metalens antenna using the well-known Jerusalem cross (JC) metacells. The proposed method extends the capability for the synthesis of metacells and metalens antennas with enhanced performance.

A Back-fire to Forward Wide-Angle Beam Steering Leaky-wave Antenna Based on SSPPs

Abstract: We demonstrate an efficient back-fire to forward wide-angle beam steering leaky-wave antenna (LWA) based on a large dispersion gradient planar spoof surface plasmon polariton (SSPP) transmission line (TL) with split-ring units. To convert the highly confined SSPP waveguiding mode to a radiation mode, two arrays of metallic patches are periodically arranged on both sides of the SSPP TL. The patches are designed as two combined half-elliptical shapes to mitigate the open stopband effect and enhance the radiation performance. We propose a theoretical method to interpret the mechanism and predict the directional pattern of the LWA. To verify the proposed method, the SSPP TL and LWA prototypes are fabricated and measured. The calculation, simulation and experimental results match well and show that the main beam of the LWA can steer a wide range of 112° from back-fire (-90°) to the forward quadrant of 22° passing through the broadside in 6.3 - 11 GHz, demonstrating a large scanning rate of 2.07°%. Furthermore, the LWA also possesses the advantages of low profile, high average gain (12 dBi), high average efficiency (95%), and low sidelobe level (< -13 dB). This work provides an alternative route to achieving wide-angle LWAs for promising microwave radar and wireless communication applications.

Artificial Intelligence Enabled Radio Propagation for Communications—Part I: Channel Characterization and Antenna-Channel Optimization

Abstract: To provide higher data rates, as well as better coverage, cost efficiency, security, adaptability, and scalability, the 5G and beyond 5G networks are developed with various artificial intelligence (AI) techniques. In this two-part article, we investigate the application of AI and, in particular, machine learning (ML) to the study of wireless propagation channels. It first provides a comprehensive overview of ML for channel characterization and ML-based antenna–channel optimization in this first part, and then, it gives a state-of-the-art literature review of channel scenario identification and channel modeling in Part II. Fundamental results and key concepts of ML for communication networks are presented, and widely used ML methods for channel data processing, propagation channel estimation, and characterization are analyzed and compared. A discussion of challenges and future research directions for ML-enabled next-generation networks of the topics covered in this part rounds off this article.

Active Control of THz Waves in Wireless Environments using Graphene-based RIS

Abstract: The active and dynamic control of the TeraHertz (THz) waves is highly demanded due to the rapid development of wireless communication systems. Recently, reconfigurable intelligent surface (RIS) has gained significant attention due to their tremendous potential in controlling the electromagnetic waves. Particularly, RIS-based wireless communications are promising for improving the system’s performance by properly designing the reflection coefficient of the unit cell. In this work, we investigate a graphene-based RIS for active and dynamic control of THz waves. The RIS design consists of rectangular graphene meta-atoms periodic array placed over a metallic grounded silicon substrate. An equivalent circuit modeling of the RIS design and its solution is provided. The RIS performance is numerically analyzed. The graphene-based RIS achieves nearly 100% reflection in the operational frequency ranging from 0.1 to 4 THz. The perfect reflection is insensitive to the polarization and the incident angles. Moreover, 100% absorption in the graphene-based RIS is achieved by electrically reconfiguring the meta-atom response via the chemical potential of the graphene. The graphene RIS also achieves the anomalous reflection performance by controlling the number of unit cells and phase gradient of RIS. In view of the effective THz wireless communication environment, this paper finally presents a simple RIS-aided communication model to study the impact of the proposed graphene-based RIS on the signal-to-noise ratio. The results reveal that the proposed THz RIS with graphene meta-atoms is promising for efficient and intelligent THz wireless communications.

Flexible and Wearable Hybrid RF and Solar Energy Harvesting System

Abstract: In this article, we demonstrate a flexible and wearable hybrid radio frequency (RF) and solar energy harvesting system for powering wearable electronic devices. The system consists of a flexible transparent antenna, a flexible transparent rectifying circuit, and an amorphous silicon solar cell. By utilizing transparent stacked structure, the rectenna and the solar cell can provide more hybrid output power to accommodate more application scenarios. The flexible transparent antenna shows two impedance matching bandwidths of 3.5–3.578 and 4.79–5.09 GHz, covering n78/n79 fifth-generation (5G) communication frequency band and 5-GHz WiFi frequency band. At the frequency of 3.5 GHz, the flexible transparent rectifying circuit achieves a high RF-to-dc conversion efficiency of 54.67% at 13 dBm RF input power. In the room with a light intensity of 210 lux, compared with a single solar cell, the hybrid energy harvesting system can obtain an additional 35.6%–769.5% output power when the RF source power is varied from 4 to 10 dBm. These results demonstrate that the proposed flexible and wearable RF-solar energy harvester can provide reliable electric power supply, which can be a potential candidate for powering wearable electronic devices, distributed sensors and IOT devices.

Accurate Modeling of Antenna Structures by Means of Domain Confinement and Pyramidal Deep Neural Networks

Abstract: The importance of surrogate modeling techniques has been gradually increasing in the design of antenna structures over the recent years. Perhaps the most important reason is a high cost of full-wave electromagnetic (EM) analysis of antenna systems. Although imperative in ensuring evaluation reliability, it entails considerable computational expenses. These are especially pronounced when carrying out EM-driven design tasks such as geometry parameter tuning or uncertainty quantification, both requiring repetitive simulations. Conducting some of the design procedures, e.g., global search or yield optimization, directly at the level of simulation models may be prohibitive. The use of fast replacement models (or surrogates) may alleviate these difficulties; yet, accurate modeling of antenna structures faces its own challenges. The two major obstacles are the curse of dimensionality, manifesting itself in a rapid growth of the number of training data samples necessary to render a reliable model (as a function of the number of antenna parameters) and high nonlinearity of antenna characteristics. Recently, the concept of performance-driven modeling has been introduced, where the modeling process is focused on a small region of the parameters’ space, which contains high-quality designs with respect to the considered performance figures. The most advanced variation in this class of methods is nested kriging, where both the model domain and the surrogate itself are constructed through kriging interpolation. Domain confinement is realized using a set of preoptimized reference designs and allows for significant improvement of the model predictive power while using a limited number of training data samples. In this work, the constrained modeling concept is coupled with a novel pyramidal deep regression network (PDRN) surrogate, which offers improved handling of highly nonlinear antenna responses. Three examples of microstrip antennas are used to demonstrate the advantages of constrained PDRN metamodels over the nested kriging surrogates with the (average) accuracy improved by a factor of 2 without increasing the training dataset cardinality.

Toward a Heterogeneous Smart Electromagnetic Environment for Millimeter-Wave Communications: An Industrial Viewpoint

Abstract: Fifth generation (5G) and beyond communication systems open the door to millimeter wave (mmWave) frequency bands to leverage the extremely large operating bandwidths and deliver unprecedented network capacity. These frequency bands are affected by high propagation losses that severely limit the achievable coverage. The simplest way to address this problem would be to increase the number of installed mmWave base stations (BSs), at the same time augmenting the overall network cost, power consumption, and electromagnetic field (EMF) levels. As alternative direction, here we propose to complement the macro-layer of mmWave BSs with a heterogeneous deployment of smart electromagnetic (Smart EM) entities—namely IAB nodes, smart repeaters, reconfigurable intelligent surfaces (RISs) and passive surfaces—that is judiciously planned to minimize the total installation costs while at the same time optimizing the network spectral efficiency. Initial network planning results underline the effectiveness of the proposed approach. The available technologies and the key research directions for achieving this view are thoroughly discussed by accounting for issues ranging from system-level design to the development of new materials.

Entire-Structure-Oriented Work-Energy Theorem (ES-WET)-Based Characteristic Mode Theory for Material Scattering Objects

Abstract: Entire-structure-oriented work-energy theorem (WET) framework is used to establish the characteristic mode theory (CMT) for material scatterers. Diagonalizing driving power operator (DPO) method is proposed to calculate characteristic modes (CMs). Solution domain compression (SDC) scheme is introduced to suppress spurious modes. An electromagnetic energy characteristic equation similar to the energy eigen-equation in quantum mechanics is derived for unifying the modal analysis theories of classical electrodynamics and quantum mechanics. Using the concept of driving power in the WET framework, the physical meaning of characteristic values is revealed; the reason leading to the nonorthogonal characteristic far fields of lossy scatterers is provided; it is explained why the conventional CMT fails to analyze some classical transmitting antennas; the physical interpretation for normalizing modal real power to 1 is given; the Parseval’s identity in CMT is derived; the classical concept of quality factor ( $Q$ -factor) is generalized to a novel concept of field-current phase-mismatching factor ( $\Theta $ -factor). Using the diagonalizing DPO method with the SDC scheme, the spurious modes outputted from the conventional diagonalizing impedance matrix operator method are suppressed. Both the novel SDC and conventional dependent variable elimination schemes confirm the same conclusion that the spurious modes originate from overlooking the dependence relationships among the currents in CM-generating operator.

Highly Selective Frequency Selective Surface With Ultrawideband Rejection

Abstract: Frequency selective surface (FSS) with ultrawide out-of-band rejection is proposed in this article. For achieving an ultrawide and high-intensity stopband, a common second-order bandpass FSS is designed and then improved by an equivalent circuit model (ECM) design. The hybrid resonator pole separation (HRPS) decoupling method is proposed and applied during the design process. The modified ECM could realize the suppress effect on the unexcepted out-of-band transmission pole caused by coupling between layers and achieve the desired ultrawideband rejection. Furthermore, the corresponding FSS structure is established from the modified ECM through the mapping method. In addition, we conducted a detailed analysis of equivalent circuit design and the required frequency response achieve method. The measured results show that a second-order passband from 3.12 to 4.64 GHz with 0.3 dB insertion loss (IL) in the band is obtained by the proposed FSS. Meanwhile, transition bands from the passband to the stopband are narrow, and the transmission coefficients out of the passband could be suppressed under −20 dB from 5.5 to over 40 GHz. All the results demonstrate that the proposed FSS can realize an excellent out-of-band rejection while maintaining a well passband.

Dual-Band Textile Antenna With Dual Circular Polarizations Using Polarization Rotation AMC for Off-Body Communications

Abstract: A dual-circular polarization (CP) textile antenna is investigated for wearable applications at 3.5 GHz WiMAX and 5.8 GHz industrial, scientific, and medical bands. The proposed antenna is composed of a dual-band monopole antenna and a polarization rotation artificial magnetic conductor (PRAMC). Through the PRAMC, the dual-CP radiation is enabled and the backward radiation is alleviated, in which the left-handed CP is generated in the lower band and the right-handed CP is realized in the upper band. Notably, dyadic reflection coefficient analysis is employed to understand the operating mechanism. Both monopole and PRAMC are printed on textile substrate, achieving conformability and comfortability. In addition, numerical results indicate that the performance of the antenna is robust to lossy human body and structural deformations. For verification, the prototype of the antenna was fabricated by two types of manufacturing process (i.e., laser cutting and screen printing), and the advantages and drawbacks of each fabricating approach are discussed for referring. The antenna made by laser cutting was tested on the phantom with the -10 dB impedance bandwidths of 11.7% and 9.1%, the 3 dB axial ratio bandwidths of 2.0% and 8.2%, and the peak gains of 6.6 and 7.2 dBic, in dual bands, respectively.

A Low-Profile Dual-Band Dual-Circularly Polarized Folded Transmitarray Antenna With Independent Beam Control

Abstract: This communication presents a low-profile, compact, high gain dual-band and dual-circularly polarized (CP) folded transmitarray antenna (FTA) with independent beam control at Ku-band. The element with independent phase control capability in two bands is designed to achieve the functionality of linear polarization to different circular polarization conversion. To reduce the profile of the whole FTA, a dual-band and dual-linearly polarized patch antenna is adopted as the feed source. By controlling the phase distribution and circular polarization of the transmitarray (TA) according to the phase compensation principle, the proposed antenna can radiate left-hand CP (LCP) waves and right-hand CP (RCP) waves at 12 and 15 GHz, respectively. On the basis, three typical FTA prototypes with different beam directions are simulated to validate the capability of independent beam control. Two of the prototypes are fabricated and measured, and the results are shown in reasonable agreement with the simulation. The proposed antenna not only reduces the profile by 2/3 compared to the traditional TA antenna, but also has the advantages of compact planar structure, low cost, high gain, and easy integration, which presents great application potential in two-way satellite communication systems with limited volume and space.

An Angle-Insensitive 3-Bit Reconfigurable Intelligent Surface

Abstract: Recently, reconfigurable intelligent surface (RIS) has attracted continuous attention in wireless communications. Because the normal incidence condition of electromagnetic waves cannot always be satisfied in practical applications, the angular insensitivity in RISs has become an important issue. The angular sensitivity in RISs may lead to the failure of RIS-assisted wireless communication networks that rely on the reciprocity of wireless channels. In this work, an angle-insensitive 3-bit RIS meta-atom is proposed. By introducing metallic vias in the substrate of programmable meta-atoms, the interference induced by multiple reflections in the multilayered structure is restrained, and the decoupling between adjacent meta-atoms makes the metasurface insensitive to the incident angle. Hence the angular insensitivity can be achieved. An RIS is fabricated using the proposed angle-insensitive programmable meta-atom with metallic vias, which is capable of maintaining stable phase and amplitude responses at oblique incidences. Both simulated and measured results show stable angle performance. A maximum of 315° phase range and eight digital coding states with 45° stable interval are obtained for wide incidence angles from 0° to 60°. Finally, numerical simulations and experimental results of anomalous reflections are performed to verify the angular reciprocity of the proposed RIS, which is important in wireless communications.

A Broadside Shared Aperture Antenna for (3.5, 26) GHz Mobile Terminals With Steerable Beam in Millimeter-Waveband

Abstract: The shared-aperture antenna is regarded as one of the promising approaches to support new frequencies with very efficient space utilization. To the best of authors’ knowledge, there are few shared-aperture antennas that include both the sub-6 GHz antenna and the mm-wave beam steering array for broadside applications. In this paper, a broadside sharing-aperture technique is developed so that a 2×4 26 GHz beam-steering substrate-integrated DRA (SIDRA) array can be integrated into a 3.5 GHz bandwidth enhanced perforated patch antenna in a coplanar and aperture-shared way. The proposed solution benefits from several aspects. First, the 3.5 GHz antenna features a compact size as this part is built on the substrate (where the mm-wave SIDRA is constructed) with a high permittivity. Second, the mm-wave SIDRA is a 3D-type device, and in the case of coplanar integration, its height can be freely adapted to the thickness of 3.5 GHz antenna without concerning the impact from surface waves as many 2D-type antennas have to do. Third, the antenna can be implemented with multi-layer printed circuit board (PCB) process, yielding a high integrity level. The dual-frequency antenna was designed, fabricated, and measured. The performances of the antenna are reported with reasonable agreement between the measured and simulated results observed.

Dual-Band Dual-Rotational-Direction Angular Stable Linear-to-Circular Polarization Converter

Abstract: A dual-band dual-rotational-direction reflective linear-to-circular polarization converter based on metasurface is designed, fabricated, and measured. The unit cell consists of an open ring and a square patch. The open ring creates two resonances, and the square patch is used to improve the axial ratio (AR). It is shown that this design can be working at two frequency bands, i.e., 29.0–41.5 GHz and 52.5–61.5 GHz. Interestingly, it is found that the linearly polarized wave in $x(y)$ -direction can be converted into right(left)-handed circularly polarized wave at the former band, and into left(right)-handed circularly polarized wave at the later band. Compared to other designs in the literature, this design demonstrates 45° angular stability for 3 dB AR over two operational bands. In addition, this design is realized on a single substrate, making it easier to be fabricated. Furthermore, the insertion loss can be as low as 0.5 dB, showing a very low-loss property. Lastly, the unit cell is less than 0.2 wavelength at the lower frequency band. The measured results show good agreement with simulation. Potential applications can be envisaged in dual-band and dual-polarization communication.

3-D Manipulation of Dual-Helical Electromagnetic Wavefronts With a Noninterleaved Metasurface

Abstract: While electromagnetic (EM) metasurfaces have been extensively studied for wavefront manipulations, the real 3-D manipulation of wavefront remains challenging. In this article, an advanced strategy of noninterleaved multitasked metaplexer for 3-D manipulation of dual-helical EM wavefronts is reported. We first develop a general method to derive the metasurface phase distribution required to form the desired 3-D electric field patterns. Then, the spin-decoupled meta-atoms are designed to achieve this goal and further increase the degrees of freedom in EM manipulation. As a proof of concept, cascaded multiplane digital images in the propagation direction are employed as the 3-D wavefronts, which are encoded into the spin-decoupled meta-atoms array, making the outputting circularly polarized waves carrying different image information at different planes. Both the simulations and experiments verify the desirable 3-D wavefront manipulation capacities of our strategy for dual-helical states. Besides, this scheme can be further extended to achieve arbitrary energy allocation in 3-D space for dual-helical EM waves. Our strategy paves an alternative route for applications such as multiple-input multiple-output (MIMO) communications, multitarget radar detection, information storage, as well as 3-D microwave imaging.