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We give an overview of recent developments in the modeling of radiowave propagation, based on machine learning (ML) algorithms. We identify the input and output specification and the architecture of the model as the main challenges associated with ML-driven propagation models. Relevant papers are discussed and categorized based on their approach to each of these challenges. Emphasis is given on presenting the prospects and open problems in this promising and rapidly evolving area. [ABSTRACT FROM AUTHOR]

The derivation of the probability density of irradiance for the received electromagnetic wave propagating through random inhomogeneities has applications in many diverse fields, such as optical wireless communications, radar, and imaging systems, to name few. The conventional approach in this regard is to combine distribution models, with each being found valid under different conditions, in a modulation process, e.g., the extended Rytov theory. However, in these techniques, the nature of the physical processes responsible for the randomness is not considered directly. Moreover, it is not clear how the model parameters are related to the physical environment. Recent research studies found that in random media, such as underwater, the exact physical processes in effect can significantly alter the distribution of the received signal intensity. In this article, a novel intensity distribution model is derived based on Rytov theory of propagation through weak turbulence. The physical parameters of the medium are directly included into the model parameters. Therefore, the resultant model is general enough to describe different physical environments. The utility of the model is validated by studying the effect of the medium parameters on the distribution through simulations. [ABSTRACT FROM AUTHOR]

The “time” factor in time-modulated arrays (TMAs) offers a new degree of freedom for securing wireless communication. However, the widely used periodic time modulation introduces limited randomness for eavesdropping receivers, and thus gives weak security. To meet this challenge, this work proposes a TMA with chaotic-enabled phase modulation (CPM) for secure transmission. The proposed CPM technique offers the chaos features of high randomness, unpredictability, and initial condition sensitive characteristic, to increase the information uncertainty for eavesdropping receivers. An optimization-based time sequence design approach is introduced to design optimal CPM time sequences for secure transmission in a given direction. Thus, a variety of security strategies including beam scanning, directional modulation (DM), and chaotic encryption are simultaneously enabled by the optimized CPM time sequence. Moreover, the CPM is a radio frequency (RF)-domain security technique without requiring any baseband modulation modules, which exhibits a simplified hardware architecture. Numerical analysis and experimental results are presented to illustrate the effectiveness of the proposed CPM technique. The results indicate that the security is improved by lowering the bit error rate (BER) between the legitimate receiver and the transmitter, and reducing the mutual information between eavesdropping receivers and the transmitter. [ABSTRACT FROM AUTHOR]

We present a C-band amplifying reconfigurable intelligent surface (ARIS) integrated with power amplifier (PA) to enhance the reflected electromagnetic (EM) signals. An aperture-coupled patch element with two orthogonal hourglass-shaped slots is elaborately constructed to ensure a broad operation bandwidth. A power combining and dividing network is also introduced to combine the 2 $\times2$ elements into a subarray to reduce the number of PAs and the system cost. The simulation results indicate that the achieved gain of ARIS is 7.7–12.2 dB from 5.0 to 6.0 GHz. A prototype of the proposed ARIS is designed, fabricated, and experimentally verified. The measured results agree well with the simulation. The proposed ARIS is promising for applications in future wireless communication systems for signal enhancement and coverage extension. [ABSTRACT FROM AUTHOR]

A metasurface (MTS)-based direction of arrival (DoA) estimation method is presented. The method exploits the properties of space–time modulated reflective metasurfaces to estimate in real-time the impinging angle of an illuminating monochromatic plane wave. The approach makes use of the amplitude unbalance of the received fields at broadside at the frequencies of the two first-order harmonics generated by the interaction between the incident plane wave and the modulated metasurface. Here, we first describe analytically how to generate the desired higher order harmonics in the reflected spectrum and how to realize the breaking of the spatial symmetry of each order harmonic scattering pattern. Then, the 1-D omnidirectional incident angle can be analytically computed using +1st- and −1st-order harmonics. The approach is also extended to 2-D DoA estimation by using two orthogonally arranged 1-D DoA modulation arrays. The accuracy of 1-D DoA estimation is verified through full-wave numerical simulations. Compared to conventional DoA estimation methods, the proposed approach simplifies the computation and hardware complexity, ensuring at the same time estimation accuracy. The proposed method may have potential applications in wireless communications, target recognition, and identification. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) are of potential use in wireless communication systems due to their ability to perform beam steering, harmonic wave manipulation, and polarization conversion. However, the main challenge in designing RISs is to obtain their electromagnetic (EM) responses, which is time-consuming and computationally prohibitive. In this work, an efficient modeling method is proposed for fast evaluation of the reflection coefficient and far-field patterns of RISs. The proposed method utilizes the microwave network theory to compute the accurate reflection coefficient of RISs elements under the illumination of incident waves from arbitrary directions. Compared to the classical equivalent circuit model, it enables the separate design of meta-atoms and tunable devices at the network level. The proposed method is verified by the full-wave simulation results of two RIS elements and experimental results of a 3-bit RIS. This work forms a fast and feasible route to obtain the responses of RISs, which allows the RISs to find extensive applications in many scenarios. [ABSTRACT FROM AUTHOR]

A novel metamaterial-loaded, miniaturized, broadband, wide-beam dielectric resonator (DR) antenna is presented for low-cost wide-angle beam-scanning applications. The mushroom structure is loaded on the DR structure, so the proposed DR radiator reduces the size reduction (low profile, $0.09\lambda _{0}$) and achieves a broadened bandwidth with a dual-mode response. The realized 3 dB beamwidth of the DR radiator is widened from 90° to 194° by introducing a pair of miniaturized equal-amplitude and out-of-phase Huygens sources. Owing to its compact size and wide beamwidth, a five-element low-cost wide-angle beam-scanning phased array is presented by using the proposed wide-beam DR elements. The measured −10 dB impedance bandwidth covers the 5G-78 band well (3.30–3.80 GHz, 14.1%). The realized scanning angle of the fabricated linear array can be extended from ±36° to ±65°, and its 3 dB beamwidth scanning coverage can up to ±90°. In virtue of miniaturized size, wideband, low cost, and good beam-scanning ability, the presented element, and the resulting wide-angle beam-scanning array are promising candidates for wireless communication and low-cost phased array applications. [ABSTRACT FROM AUTHOR]

The active and dynamic controls of the terahertz (THz) waves are 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 electromagnetic (EM) 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 controls of THz waves. The RIS design consists of a 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 article finally presents a simple RIS-aided communication model to study the impact of the proposed graphene-based RIS on the signal-to-noise ratio (SNR). The results reveal that the proposed THz RIS with graphene meta-atoms is promising for efficient and intelligent THz wireless communications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

So far, the environment has been considered as a source of fading, clutter, and blockage, with detrimental consequences for 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, the use of intelligent surfaces able to control the propagation of electromagnetic (EM) waves has been recently proposed. 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 article 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 toward practical implementation of a smart EM environment. [ABSTRACT FROM AUTHOR]

The independent control of two orthogonal polarizations that possess dual functionalities is highly desirable in antenna and wireless communication technologies. Spin-decoupled metasurfaces with integrated dynamic and geometric phases have been proposed to generate two independent right-handed and left-handed circularly polarized (RCP and LCP, respectively) waves. However, the RCP and LCP phases are correlated by a linear equation of the dynamic and geometric phases. In this article, we propose a meta-atom in which the 1 b phase of two orthogonal polarizations can be controlled independently. Moreover, a high-gain broadband millimeter-wave multidimensional metasurface to generate an $x$ -polarized $l=0$ beam with a 10° beam deflection and a $y$ -polarized $l=+1$ beam independently is simulated, fabricated, and tested. The proposed metasurface operating over the frequency range of 16–28 GHz has the maximum gains of 28.7 and 23.8 dBi for the $l=0\,\,x$ -polarized waves and $l=+1\,\,y$ -polarized waves, respectively. Compared with spin-decoupled metasurfaces, the proposed metasurface provides a simpler design process to generate two independent vortex beams for the two orthogonal polarizations. The proposed vortex metasurface also has the advantages of dual polarization, high gain, wide relative bandwidth, and is broadband. The proposed metasurface has potential application in high-capacity communication systems. [ABSTRACT FROM AUTHOR]

In this study, an ultraminiaturized implantable antenna is developed for biotelemetry applications. It has triple-band characteristics in the industrial, scientific, and medical (ISM) bands of 902–928 MHz for data telemetry and 2400–2483.5 MHz for power-saving applications and midfield band (1824–1980 MHz) for wireless power transfer simultaneously. The size of the proposed antenna is 0.068 $\lambda _{g}\,\,\times0.048\,\,\lambda _{g}\,\,\times0.0036\,\,\lambda _{g}$ and it is the smallest size of any triple-band implantable antenna ever devised without using shortening vias. Such a compact size is achieved by meandering the shape of the radiating patch and adding open-ended ground slots. Unlike most miniaturized implantable antennas reported in the available literature, no shortening pin is used and this makes the fabrication of the antenna very easy. The bandwidths measured in minced pork muscles are 14% and 24% in the lower and upper ISM bands, respectively, and 12% in the midfield band. The maximum values of the realized gain of the proposed antenna obtained in the heterogeneous skin phantom at 915, 1900, and 2450 MHz are −27.2, −22.2, and −19.9 dBi, respectively. In addition, specific absorption rates (SARs) in the heterogeneous skin and muscle tissues at the three resonance frequencies are calculated and show compliance with the IEEE standards for ensuring human safety. Finally, the range of the biotelemetry link is evaluated by calculating the wireless communication link budget at data rates of 7 kb/s, 100 kb/s, and 10 Mb/s at resonance frequencies. [ABSTRACT FROM AUTHOR]

In recent years, there are increasing interests and applications on radar–communication integration for system miniaturization, cost reduction, and so on. Moreover, thanks to the common functions of hardware components including transceiver, antennas, and even basic signal/data processing, the integrated design of radar and communication systems becomes feasible. In this article, through design of proper time sequences, waveforms, and recognition algorithms, radar sensing and wireless communication can be integrated into 4-D antenna arrays simultaneously. In addition, with the high design degrees of freedom of 4-D arrays, the proposed approach can also provide a low probability of intercept capability. Selected numerical and experimental results are provided to validate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

A novel high-efficiency polarization-selective bifunctional metasurface (MS) is proposed, which can realize cross-polarized focusing transmission for left-handed circular polarization (LHCP) incidence and co-polarized specular reflection for right-handed circular polarization (RHCP) incidence. As an example of its practical application, the designed MS is applied to form an RHCP folded transmitarray antenna (FTA) whose feed source (FS) is integrated with the bottom metallic surface. To prove the concept, a prototype of millimeter-wave (MMW) FTA based on the proposed MS is numerically simulated, physically implemented, and experimentally measured. The measured results coincide well with the simulated ones, showing a peak gain of 23.6 dBi at 27.7 GHz with the peak aperture efficiency of 43.1%, a 3 dB gain bandwidth of 15.8% (25–29.3 GHz), and a 3 dB axial-ratio (AR) bandwidth over 29.6%. The proposed high-performance antenna has the advantages of low profile, high efficiency, integration design, and broad bandwidth, which may have promising applications in the fifth-generation MMW communication system. The proposed MS and its application to high-gain antenna may promote the uses of MS to real-world applications in satellite communication, radar detection, and wireless communication. [ABSTRACT FROM AUTHOR]

The Internet-of-Things (IoT) applications require orientation-insensitive wireless devices to maintain stable and reliable communication. For those reasons, antennas providing a wide quasi-isotropic and circular polarization (CP) coverage are very attractive. However, achieving wide quasi-isotropic and CP coverage simultaneously is challenging. In this work, we show that properly designed sloped dipoles on a 3-D structure can maximize the CP coverage (theoretically up to 100%) even with equal-phased feed to the dipole elements. We derive the conditions and present the design graphs for the optimum slope angle for the dipole elements on a 3-D hexagonal-shaped package to achieve a wide quasi-isotropic and CP coverage simultaneously. Based on the proposed theory, a practical antenna has been designed and fabricated using additive manufacturing. The measured results demonstrate a 7 dB isotropy of 92% and a CP coverage of 85%, which matches well with the predicted results from the theoretical analysis and full-wave simulations. [ABSTRACT FROM AUTHOR]

The concept of dual-polarized reconfigurable reflectarray (RRA) featuring dual-polarized beam control is proposed and demonstrated using a dual-channel programmable metasurface. The p-i-n diodes are incorporated into each metasurface element as the active components to provide independent phase control in two orthogonal polarization channels. By altering the working states of each p-i-n diode, well-defined beam scanning can be obtained. Element design, focal diameter ratio selection, and discretized phase distributions are carefully optimized to ensure stable radiation performance of the proposed RRA. As verification, a prototype consisting of 20 $\times20$ elements is fabricated and measured, which is controlled by a field programmable gate array-based controller. Measured results show that wide-angle beam scanning up to ±40° can be obtained around 7.45 GHz, with peak gains of 21.13 and 20.89 dBi in dual channels, corresponding to aperture efficiencies of 16.35% and 15.47%. The proposed dual-polarized RRA brings a new freedom in polarization manipulations for versatile beam scanning, which provides promising applications in wireless communication and tracking/detecting systems. [ABSTRACT FROM AUTHOR]

The magnetic induction (MI) technique enables wireless communications in extreme environments. However, the existing MI systems rely on large antennas and high power, which is not suitable for Internet of Things (IoT) and other mobile devices. Although the new metamaterial-enhanced MI ($\text{M}^{2}\text{I}$) technique can theoretically enhance the MI signal by 30 dB, none of the existing real-world systems achieve the performance due to: 1) the inherent metamaterial loss is significant; 2) the complicated metamaterial structure is not optimized; and 3) an $\text{M}^{2}\text{I}$ system with fixed parameters cannot keep the optimal performance in dynamic environments. In this article, an active and reconfigurable meta-sphere is designed, optimized, and implemented to approach the theoretical $\text{M}^{2}\text{I}$ enhancement bound. Specifically, the new $\text{M}^{2}\text{I}$ meta-sphere is realized by a spherical array of active and reconfigurable coil units. Given fixed meta-sphere size and power constraints, there exist complicated trade-offs among coil number, coil size, and the power consumption. Based on the theoretical design, the prototype of the active and reconfigurable $\text{M}^{2}\text{I}$ meta-sphere system is implemented and tested. Significant power gain and range extension are proved through comprehensive analytical deduction, COMSOL Multiphysics-based simulations, and real-world experiments. [ABSTRACT FROM AUTHOR]

In this article, a single-sheet metasurface (MS) for independently manipulating the transmissive phase and amplitude of linearly polarized electromagnetic (EM) wave is proposed. The amplitude modulation is achieved via imposing a rotation angle to flexibly control the ratio of polarization conversion, while the phase modulation is realized by tuning the physical parameters of the unit cell to change its resonance. By integrating the proposed single-sheet MS, a low-profile metalens antenna operating around 12.2 GHz is designed, fabricated, and experimentally validated. In particular, spatially varying amplitude on the MS aperture is designed to meet the Taylor distribution, thus reducing the sidelobe level (SLL) of the metalens antenna. In addition, special MS elements rotation strategy is performed to maintain good polarization isolation-level (PIL) performance. The measured results show that the proposed metalens antenna achieves the peak gain of 25.3 dBi at 12.6 GHz with 3 dB gain bandwidth of 16.4% (11.2–13.2 GHz). Within the 3 dB gain bandwidth, this metalens antenna achieves SLL lower than −20.5 dB with an average value of −23 dB and PIL lower than −22.2 dB with an average value of −26.5 dB. Compared with metalens antenna with phase-only modulations, the proposed metalens antenna realizes a 5.7 dB average SLL suppression. The proposed metalens antenna is a promising candidate in point-to-point communications for wireless and satellite system applications. [ABSTRACT FROM AUTHOR]

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domains simultaneously. In this article, an analytical design of space-time-modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space-/frequency-domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first-order harmonic can be obtained. While, in other directions, the opposite first-order harmonic can be easily controlled by changing the dimension of I/Q paths, and the objective first-order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first-order harmonic can be used for beam scanning by predesigning the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2 bit unit cells loaded with dynamically switchable pin diodes are used as I/Q modulation. Both the analytical and numerical results demonstrate that space- and frequency-domain manipulations of the reflected fields by the first-order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking. [ABSTRACT FROM AUTHOR]

The era of wireless communications began at the turn of the 20th century, when Guglielmo Marconi used electromagnetic waves to transmit telegraph signals from ships to stations onshore. To understand how radio signals propagate is critical for wireless communication system design. With the development of wireless communications, much effort has been made to characterize radio propagation in different frequency bands and physical environments. Radio propagation and wireless channel modeling are essential for communication system simulation, channel emulator design, wireless system planning and optimization, and the development of regulations and standards in wireless communications ,. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Electromagnetic signals in the ultralow frequency (ULF) range below 3 kHz are well suited for underwater and underground wireless communication thanks to low signal attenuation and high penetration depth. However, it is challenging to design ULF transmitters that are simultaneously compact and energy efficient using traditional approaches, e.g., using coils or dipole antennas. Recent works have considered magneto-mechanical alternatives, in which ULF magnetic fields are generated using the motion of permanent magnets, since they enable extremely compact ULF transmitters that can operate with low energy consumption and are suitable for human-portable applications. Here we explore the design and operating principles of resonant magneto-mechanical transmitters (MMT) that operate over frequencies spanning a few 10 s of Hz up to 1 kHz. We experimentally demonstrate two types of MMT designs using both single-rotor and multirotor architectures. We study the nonlinear electro-mechanical dynamics of MMTs using point dipole approximation and magneto-static simulations. We further experimentally explore techniques to control the operation frequency and demonstrate amplitude modulation up to 10 bits-per-second. We additionally demonstrate how using oppositely polarized MMT modules can permit systems that have low dc-field but do not sacrifice the ac magnetic field produced. [ABSTRACT FROM AUTHOR]

We demonstrate a new approach to implementing directional modulation that leverages distributed array dynamics to change the antenna array pattern over time. With appreciable spacing between the elements, the spatial phase pattern varies rapidly, such that small physical motions result in large phase variations. We define an average radiation pattern that characterizes the space–time modulation of the array pattern and calculate a spatial information filter based on the additional modulation imparted on transmitted data. The concept is demonstrated experimentally using a two-element 1.5 GHz transmit array with relative motion between the elements. The array consists of two nodes, each with one dipole antenna transmitting BPSK data, and each with two log-periodic antennas used to estimate the relative separation of the nodes and correct the beamsteering phase of the transmitters. The node separation is varied dynamically over a range of $0.98\lambda $ – $4.7\lambda $. We investigate the use of linearly and sinusoidally varying internode dynamics, and demonstrate a low bit-error-ratio of approximately 10−5 at the desired transmission direction, indicating high throughput, and a high error ratio of approximately 10−0.3 at angles outside of the mainbeam, indicating corrupted data, at a signal-to-noise ratio of 12 dB. [ABSTRACT FROM AUTHOR]

Polarization reconfigurable antennas at millimeter-wave (mm-wave) frequencies are important technologies for 5G and beyond wireless communications systems. This article presents an mm-wave multi-linear polarization (LP) reconfigurable antenna. It employs a resonant TM510-mode air-filled cavity as the base, and a circular array of slots is uniformly etched on its top surface to provide a rotationally symmetrical “excitation” for the multi-LP antenna. An upper substrate printed with engineered patch layers is introduced above the slots for an in-phase high-gain composition, and it is mechanically rotated to facilitate the multi-LP reconfiguration. Each slot radiation is separately monitored and analyzed by using single-slot cavity models. A final multi-LP antenna that can switch among five LP states at mm-wave band was constructed. To overcome the classic high sidelobe problem associated with circular arrays, a modified bottom layer is introduced to the multi-LP antenna. The measured overlapped bandwidth of the optimized multi-LP antenna ranges from 28.58 to 28.99 GHz. For all of the five LP states, the simulated and measured sidelobe levels are −13 and −10 dB, respectively. The measured realized gains are varied from 15.5 to 16.8 dBi in the overlapped operating bandwidth. This is the first time to report an mm-wave multi-LP reconfigurable antenna. [ABSTRACT FROM AUTHOR]

Transmissive and reflective metasurfaces have been extensively studied for generating an orbital angular momentum (OAM)-carrying beam. However, the existing OAM generators suffer from feed blockage, large profile, OAM mode nonreconfigurable, and narrowband. An OAM generator that integrates multiple functions into a single module is in high demand in OAM-based applications. In this study, we proposed an OAM generator based on a folded reflectarray with high-gain low-profile broadband and programmable characteristics. The proposed multifunctional OAM generator, comprising half the volume of the normal OAM metasurfaces, can generate a high-gain $l=0$ , $l=+1$ , and $l=+2$ OAM-mode-reconfigurable beams with high mode purity over the frequency range of 9.5–10.5 GHz. The proposed OAM generator-integrated multiple functions have potential applications in OAM-based wireless communications and radar detection. [ABSTRACT FROM AUTHOR]

We present a study on beam coupling in radiating structures that support multiple simultaneous beams. The formation of multiple beams is relevant in modern wireless communication applications when diverse data streams can be sent from a single transmitter to users located in different directions. General expressions are provided that relate the coupling between radiated beams to the associated current distributions on the radiating aperture. When expanding the currents in terms of basis functions, the beam coupling can be also written in terms of coupling contributions due to each pair of basis functions. The analysis is applied to antenna arrays with different levels of mutual coupling between the array elements. More specifically, arrays of resonant elements, characterized by very low mutual coupling, are compared with arrays of connected elements, which are strongly coupled. We show that, even for a very high level of mutual coupling between individual array elements, it is possible to obtain orthogonal beams if the total current distributions associated with the beams are uncoupled. [ABSTRACT FROM AUTHOR]

In this article, the theory, design, and demonstration of wideband and low-profile dual-circularly polarized (dual-CP) transmit arrays (TAs) are reported, which support independent beamforming for left-/right-handed CP waves. This is achieved by utilizing an antenna–filter–antenna configuration for the TA unit cells with a profile of only about $0.11\lambda _{0}$ , which consists of receiving and transmitting stacked patches connected by two microstrip delay lines. By utilizing the phase delay lines and unit cell rotation, the dynamic phase and the Berry phase can be simultaneously exploited for providing continuous dual-CP transmissive phase compensation insensitive to the angle of incidence. The diffraction analysis of a linear gradient array of the proposed cells confirms its wideband performance of abnormal refraction. In addition, broadband CP planar feed arrays were designed and verified for functioning as low-profile feeds with symmetrical patterns for the TAs. As proof-of-concept examples, two $K$ -band dual-CP TAs integrated with the planar feeds were synthesized, fabricated, and measured, possessing a total profile of about $7.1\lambda _{0}$. The two prototypes offer either symmetrical or asymmetrical dual-CP beams over a wide bandwidth. Their measured peak gain is higher than 22 dBic, while the joint $S_{11} < -15$ dB, axial ratio < 2 dB, beam squinting <3°, and 2–dB gain bandwidth is wider than 12%. The design methodology and the proposed dual-CP TAs are useful in communication systems for space and wireless applications. [ABSTRACT FROM AUTHOR]

A polarization-dependent transmissive and reflective phase-gradient metasurface (PGMS) is investigated and experimentally verified in the $Ku$ -band. The PGMS is based on a single-layer substrate-integrated element, which consists of a cross-shaped dielectric unit and a couple of metallic strips/patches that are printed on the dielectric unit. Taking advantage of both the dielectric and the metallic components, the proposed element can transmit or reflect the linearly polarized (LP) incident wave depending on its polarization. Moreover, the phase of the transmitted or reflected wave can be independently controlled by different phase-control parameters. As a result, two independent foci can be obtained on the transmission and reflection sides of the PGMS for two orthogonal LP waves. For demonstration, a prototype of the PGMS is fabricated and tested. Both the transmissive and reflective foci are observed at the target positions. Also, the PGMS is shown to be able to manipulate the wavefronts independently on its two sides by adjusting the phase distributions. These features make the proposed PGMS a great choice for use in polarization identification, near-field energy transfer, or far-field wireless communication. [ABSTRACT FROM AUTHOR]

In this article, a compact triple-port dual-band triple-mode antenna enabled by a shielded mushroom structure with operational bands centered at 2.4 and 5.8 GHz is proposed possessing both pattern and polarization diversities. By tailoring the dispersive properties of the artificial transmission line (ATL) cells and structuring them into a multilayer architecture, the integrated antenna is capable of generating vertically polarized, $y$ -horizontally polarized, and $x$ -horizontally polarized radiation patterns simultaneously at two predefined bands with good port isolations and field orthogonality. The design concept and the analysis of the ATL cells in the form of shielded mushrooms are described, illustrating the dual-band operating principle of the antenna. A proof-of-concept antenna is designed with a footprint of $0.48\lambda _{0}^{2}$ and a profile of $0.07\lambda _{0}$ , where $\lambda _{0}$ is the free space wavelength at 2.4 GHz. The proposed antenna was fabricated and characterized, achieving measured $S_{11} < - 10$ dB bandwidths of 35 and 85 MHz in the two targeted bands, respectively, with mutual coupling smaller than −15 dB and envelope correlation coefficients below 0.01 for all three ports. Moreover, it is shown that the two operational bands of the antenna can be independently controlled, which reveals the unprecedented degrees-of-freedom in its design. The demonstrated dual-band triple-mode antenna is highly useful for dual-band multimode wireless devices and dual-band multiple-input multiple-out communication systems. [ABSTRACT FROM AUTHOR]

A novel method of dual-band dual-beam control (DDC) is proposed and applied in metasurface antennas (MSAs). Based on this DDC method, the feeding network employed in MSAs can realize independent forward and/or backward radiation of two bands. Combining the metasurface cells and feeding network, the desired bands and the corresponding beam angles of MSAs can be controlled. The symmetrical two-port feeding network consists of uniformly spaced slots in the ground and a bent microstrip line beneath the slots, so that two identical beams are realized. The radiation direction of the two bands can be implemented independently by adjusting the distance of the slots and length of the bent line simultaneously. To verify the design strategies, two prototypes (MSA A and MSA B) with different performance are designed, respectively. As a result, the proposed MSAs can achieve independent beam angels of two bands with low profile and compact design. The above good features make the proposed MSAs potential candidates in base-station antennas of high-capacity wireless communication system. [ABSTRACT FROM AUTHOR]

In this article, millimeter-wave (mmWave) wireless channel characteristics (Doppler spread and path loss modeling) for unmanned aerial vehicle (UAV)-assisted communication is analyzed and studied by emulating the real UAV motion using a robotic arm. The motion considers the actual turbulence caused by the wind gusts to the UAV in the atmosphere, which is statistically modeled by the widely used Dryden wind model. The frequency under consideration is 28 GHz in an anechoic chamber setting. A total of 11 distance points from 3.5 to 23.5 ft in increments of 2 ft were considered in this experiment. At each distance point, three samples of data were collected for better inference purposes. In this emulated environment, it was found out that the average Doppler spread at these different distances was around −20 and +20 Hz at the noise floor of −60 dB. On the other hand, the path loss exponent was found to be 1.843. This study presents and lays out a novel framework of emulating UAV motion for mmWave communication systems, which will pave the way out for future design and implementation of next-generation UAV-assisted wireless communication systems. [ABSTRACT FROM AUTHOR]