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Reconfigurable intelligent surfaces (RISs) have attracted major attention in the last few years, due to their useful characteristics. An RIS is a nearly passive thin surface that can dynamically change the reradiated field and can therefore realize anomalous reflection, refraction, focalization, or other wave transformations for engineering the radio propagation environment or realizing novel surface-type antennas. Evaluating the performance and optimizing the deployment of RISs in wireless networks need physically consistent frameworks that account for the electromagnetic characteristics of dynamic metasurfaces. In this article, we introduce a general macroscopic model for evaluating the scattering from an RIS. The proposed method decomposes the wave reradiated from an RIS into multiple scattering contributions and is aimed at being embedded into ray-based models. Since state-of-the-art ray-based models can already efficiently simulate specular wave reflection, diffraction, and diffuse scattering but not anomalous reradiation, we enhance them with an approach based on Huygens’ principle and propose two possible implementations for it. Multiple reradiation modes can be modeled through the proposed approach using the power conservation principle. We validate the accuracy of the proposed model by benchmarking it against several case studies available in the literature, which are based on analytical models, full-wave simulations, and measurements. [ABSTRACT FROM AUTHOR]

This paper describes two important improvements to a 3-D shooting-and-bouncing-polygon (SBP) ray tracer for radio propagation modeling. It introduces a way to trace diffracted–reflected (D–R) rays in the SBP framework and improves SBP accuracy when the D–R rays play an important role. It also introduces a better space partitioning scheme for the use with SBP and improves the SBP’s computation efficiency by more than $10\times $ in some applications. The numerical results are compared against published measurements and 3-D full-wave solutions. They show that the improved SBP ray tracer is a good multipurpose 3-D ray tracer which can be used to simulate urban street canyon (without double over-rooftop diffraction), indoor, and tunnel propagation environments. [ABSTRACT FROM AUTHOR]

Wireless communications are expected to take place in increasingly complicated scenarios, such as dense urban, forest, tunnel, and other cluttered environments. A key emerging challenge is to understand the physics and characteristics of wave propagation in these environments, which is critical for the analysis, design, and application of advanced mobile and wireless communication systems. In this paper, we present a full-wave field-based computational methodology for radio wave propagation in complex urban environments. Both transmitting/receiving antennas and propagation environments are modeled by first-principles calculations. A system-level, large scene analysis is enabled by the scalable, ultraparallel algorithms on the emerging high-performance computing platforms. The proposed computational framework is verified and validated with semianalytical models and representative measurements. [ABSTRACT FROM AUTHOR]

Fabry–Perot (FP)/leaky-wave antennas fed with multiple sources are analyzed and modeled. A transmission line (TL) approach for a rapid and efficient analysis of such antenna configuration is developed. The method is based on the superposition of the traveling leaky waves excited by each of the applied sources. The field distribution inside the cavity is derived by using a longitudinal TL model which takes into account the interference of the multiple leaky waves excited by the sources. The radiation pattern of the multiple fed antennas is obtained by using the fast Fourier transformation of the field distribution. The propagation constants of the leaky waves, traveling inside the FP cavity, are computed analytically, and the accuracy of the proposed expressions is verified by using the transverse resonance method approach. A novel and accurate closed-form expression for the leaky-wave propagation constant has been derived. A detailed analysis of the antenna properties is carried out by using the model showing how this antenna configuration is suitable for designing very large aperture antennas with very high gain and fairly acceptable bandwidth. A prototype of a multifeed FP antenna has been fabricated by using 3-D printing technology and tested. [ABSTRACT FROM AUTHOR]

Recent experimental campaigns provided the opportunity to measure radio wave propagation and atmospheric conditions with the $X$ -band beacon-receiver (XBBR) array system. The system consists of vertical arrays of transmitters and receivers for measuring the $X$ -band propagation. Measurements near the sea surface can be used to obtain information regarding the refractivity profile of the lower atmosphere. Since ducted propagation acts as a leaky waveguide, the vertical array elements in various transmit and receive height combinations effectively observe differing combinations of the modal components propagating in the duct, the use of multiple combinations improves the estimation of duct properties. The aforementioned measurement campaigns occurred near the coast of southern California; the SoCal 2013 experiment and the Scripps Pier Campaign. During both campaigns, the propagation loss recorded at each of the receivers from each of the transmitters, standardized by the total received power, was compared to variable terrain radio parabolic equation model predictions in order to estimate the evaporation duct height (EDH). Point meteorological data were recorded and used with the Navy Atmospheric Vertical Surface Layer Model to obtain in situ measurements of the EDH. Comparisons show strong correlation between EDH values inferred from XBBR measurements and meteorological information. [ABSTRACT FROM PUBLISHER]

A 3-D bidirectional solution to the parabolic approximation of the wave equation is investigated by using a vector field representation. The backward propagating wave is integrated to the classical parabolic equation approach, which represents the forward propagating wave. Propagation over flat terrain in the presence of knife-edges is considered as well as over irregular terrain consisting of hills modeled by the succession of knife-edges. At each knife-edge, appropriate boundary conditions are enforced, and the wave is partly reflected in the backward direction. The wave is marched in both directions by using the split-step algorithm. Different tests are conducted in order to analyze and validate the results obtained by the proposed algorithm. Comparisons with results from both, 2-D parabolic equation-based algorithm, and 3-D finite-difference time domain-based algorithm, are presented in this paper. [ABSTRACT FROM PUBLISHER]

Physical models describe the radio channel by characterizing the multipath propagation. Also, measurements are frequently employed in the establishment of such models, or for tuning the existing ones. This paper presents a systematic analysis of double-directional radio channel data, obtained by combining two single-input multiple-output measurements (forward and reverse), each, containing the channel frequency responses at the locations of a virtual rectangular antenna array. The measurement setup uses a vector network analyzer, which, together with the adopted data processing, represents an affordable, but very consistent, alternative to more sophisticated equipment, given that, a physical analysis of the identified channel contributions provided sensible results. The study comprised three main steps: 1) the characterization of each single-directional channel by employing a high-resolution algorithm to obtain the parameters of a chosen number of multipath components (MPCs); 2) the classification of the obtained MPCs in a twofold way, first, by grouping MPCs using clustering tools, and, second, by classifying these clusters according to their type of interaction with the propagation scenario; and 3) the extraction of typical values for the number of clusters, the number of MPCs per cluster, rules for power decay, and statistical distributions for inter and intracluster delays and azimuths. [ABSTRACT FROM AUTHOR]

This paper describes an extensive propagation channel measurement campaign in a warehouse environment for line-of-sight (LOS) and nonline-of-sight (NLOS) scenarios. The measurement setup employs a vector network analyzer operating in the 2–8-GHz frequency band combined with an $8 \times 8$ virtual multiple-input multiple-output (MIMO) antenna array. We develop a comprehensive statistical propagation channel model based on high-resolution extraction of multipath components and subsequent spatiotemporal clustering analysis. The intracluster direction of departure (DoD), direction of arrival (DoA), and the time of arrival (ToA) are independent, both for the LOS and NLOS scenarios. The intracluster DoD and DoA can be approximated by the Laplace distribution, and the intracluster ToA can be approximated by an exponential mixture distribution. The intercluster analysis, however, shows a dependency between the cluster DoD, DoA, and ToA. To capture this dependency, we separately model the clusters caused by single and multiple bounce scattering along the aisles in the warehouse. The intercluster DoD distribution follows a Laplace distribution, while the cluster DoA conditioned on the DoD is approximated by a Gaussian mixture distribution. The model was validated using the capacity and delay-spread values. [ABSTRACT FROM AUTHOR]

This paper presents the results of the first propagation measurement campaign conducted in the 2.4 GHz industrial, scientific and medical (ISM) band for low antenna heights in a North American boreal forest. The $2 \times 1$ multiple-input, single output measurements capture propagation conditions for wireless peer-to-peer or sensor networks in the boreal environment. Unlike many other forests, it is shown that ground reflections strongly influence large scale propagation for the low antenna boreal case. Results also indicate that scattering is severe but that both frequency diversity and multiple antenna techniques will be effective for mitigating and/or exploiting the fading caused by this scattering. [ABSTRACT FROM AUTHOR]

The paper discusses the possibility of generating a pseudo-Bessel beam, with a propagation distance of several hundreds of wavelengths in microwave and millimeter frequency band, by using a radial line slot array (RLSA). A specific application for non-contact microwave detection of buried mines has been considered as test case. The design benefits of a holographic approach to assure the required aperture field distribution and makes use of an ad hoc optimization tool to control the antenna slot layout. The predicted and measured antenna behaviors show that high efficiency and polarization purity can be obtained by such a compact and flat antenna, achieving at the same time both manufacturing and setup simplicity. [ABSTRACT FROM PUBLISHER]

A introduction to the special section on antenna systems and propagation for cognitive radio in the March 2014 issue that covers reconfigurable antenna design, phased array applications, and basic design challenges, is presented.

Synthesis, simulation and experiment of unequally spaced resonant slotted-waveguide antenna arrays based on the infinite wavelength propagation property of composite right/left-handed (CRLH) waveguide has been demonstrated in this paper. Both the slot element spacing and excitation amplitude of the antenna array can be adjusted to tailor the radiation pattern. A specially designed shorted CRLH waveguide, as the feed structure of the antenna array, is to work at the infinite wavelength propagation frequency. This ensures that all unequally spaced slot elements along the shorted CRLH waveguide wall can be excited either inphase or antiphase. Four different unequally spaced resonant slotted-waveguide antenna arrays are designed to form pencil, flat-topped and difference beam patterns. Through the synthesis, simulation and experiment, it proves that the proposed arrays are able to exhibit better radiation performances than conventional resonant slotted-waveguide antenna arrays. [ABSTRACT FROM PUBLISHER]

In ultra-wideband (UWB) wireless propagation channels, each multipath component (MPC) can exhibit delay dispersion or, equivalently, frequency dependence; even more, this dispersion can be different for different MPCs. While this effect is understood to have important implications for UWB system design, extraction of the frequency-dependence parameters from measurements has been lacking. This paper will provide both the theoretical tools for the parameter extraction, and provide experimental results of such parameters in UWB indoor channels. We first develop a method that extracts a single MPC. Based on the image principle, the method identifies locations and intensity of radio sources including both original and image sources (corresponding to MPCs) by a distributed antenna array. We show that the method provides spatial resolution better than 0.5 m and is robust against possible measurement errors. Having applied the method to the measured channels in indoor line-of-sight scenarios, we track the detected image sources over (partially overlapping) frequency subbands to obtain frequency dependent properties of their intensity and location. The variation of the source intensity over the frequency was found to be significant. Furthermore, the locations of the image sources can vary as the frequency changes. Comparison of the results with models that do not describe varying per-path frequency dependence revealed noticeable difference in the shape of the CIRs, and as a result, in the delay spread and the number of significant delay bins for Rake reception. [ABSTRACT FROM PUBLISHER]

The time domain (TD) solution for the multiple diffraction of spherical waves after an arbitrary number of obstacles with different shapes such as knife-edges and wedges, is presented. The proposed TD solution is based on the representation of the inverse Laplace transform of the corresponding frequency domain solution in closed form, as it is given by a hybrid of the uniform theory of diffraction (UTD)-physic optics (PO) solution. The proposed formulation, validated with the results from technical literature, does not need to incorporate the TD version of the higher-order diffraction coefficients due to a recursive relation, thus significantly reducing both the mathematical complexity and computation time of the formulation when compared with other TD solutions. [ABSTRACT FROM PUBLISHER]

We investigate the situation where there are obstructing elements present in the near field of a retrodirective array. We describe three scattering cases, (1) by an array of straight wires, (2) by low loss medium density fibre board partially obscuring the array, and (3) by concrete blocks, totally and then partially obscuring the array. For all scenarios retrodirective action was shown to be able to provide various degrees of automatic compensation for loss in gain relative to that which would have occurred for a conventional (non-retrodirective) array in the presence of the same scattering screens. Gain improvements of up to 10 dB were observed when the retrodirective array was used. In addition we show how the induced variation of received and re-transmited amplitudes across the array, caused by the scattering screens, is the principle mechanism causing deterioration of the retrodirective arrays monostatic response. [ABSTRACT FROM AUTHOR]

Conformal scanning arrays are important for scanning ranges beyond ±90 degrees and the retrodirective array (RDA) is known as a promising candidate for this application. We discuss the optimization of a previously realized conformal sparse RDA by means of the genetic algorithm, and the RDA numerical model that is used in this approach. In particular, we improve the numerical model of the RDA by identifying practical factors that previous model failed to account for. The impact of each practical factor on the RDA monostatic pattern is visualized, and the improved model provides much better prediction of the monostatic pattern. [ABSTRACT FROM AUTHOR]

A high-resolution measurement system for the characterization of the delay and angle-of-arrival properties of all significant multipath components on mobile radio channels is presented. The system uses complex impulse response data obtained from a novel 3-D tilted cross switched antenna array as input to an improved version of 3-D Unitary ESPRIT. Results from simulations and measurements show that the approach is capable of accurately characterizing the delay and angle-of-arrival characteristics of the dominant multipath components at moderate urban speeds along a trajectory in mobile propagation environments with high resolution in both azimuth and elevation. [ABSTRACT FROM PUBLISHER]

The design of a novel ultrawideband printed-circuit antenna is presented. By using a stepped-patch in combination with multiple resonating elements, a bandwidth of more than 150% is obtained. The performance is demonstrated through measurements and calculations with a commercially available field solver. A phase center analysis shows very small variations over the entire available bandwidth. [ABSTRACT FROM AUTHOR]

A modified pseudocovariance matrix is described which can improve the performance of Kim et al.'s (‘Fast DOA estimation algorithm using pseudocovariance matrix’ IEEE Trans. Antennas Propagat., vol. 53, no. 4, pp. 1346–1351, Apr. 2005) algorithm to realize the fast direction finding with single snapshot in coherent or uncorrelated case. By adding the backwards formulation of pseudocovariance matrix, the rank of pseudocovariance matrix is enhanced. Then a set of pseudocovariance matrices are constructed by circularly shifting the matrix. By weighted averaging, a modified pseudocovariance matrix is reconstructed whose noise is reduced. The proposed matrix has less array element loss than that of the original matrix. Moreover, this algorithm can be applied in the lower signal-noise-ratio (SNR) through noise preprocessing. The new matrix has expanded the application of high-resolution direction finding to nonuniform linear array (NLA), since traditional methods usually assume the array is uniform linear when handling coherent sources. Simulations are conducted to compare the performance of the modified method and the original method. The results illustrate that the high performance of DOA estimate could be obtained rapidly. [ABSTRACT FROM AUTHOR]

We study models for propagation in circular tunnels. The excitation is a fundamental electric current source. We produce expressions for the fields in terms of a Fourier transform over the axial variable. We perform an asymptotic analysis of the complex wavenumbers for use as initial estimates in a complex root finder. We then produce the modes in the lossy structure by contour integration techniques. We include numerical results for the field intensity as a function of axial distance for several frequencies and constitutive parameters of interest. We comment in particular concerning the modal content and the specific form of the field both near to and far from the antenna source. [ABSTRACT FROM AUTHOR]

We measured the spatial properties of the three-dimensional (3-D) double-directional. radio channel in urban macroceil environments separately at both ends of the link. In this paper, we study propagation conditions pertaining to reception and transmission at the mobile terminal, measured using a wideband channel sounder and a dual-polarized spherical antenna array. We were able to refine the results of the measurements conducted at the base station, and extend the study to full double-directional 3-D channels. Individual propagation paths could be identified precisely, in some cases even considerable scattering from lampposts was observed. Our results show that over-rooftop-dominated propagation often occurs via building roofs with LOS to the base station antenna, acting as strong secondary signal sources. Based on measurements along continuous routes we demonstrate that the dominant propagation mechanisms can vary considerably when the mobile moves in the environment. We also present typical directional properties of the 3-D radio channel at the mobile terminal in urban macroceil environments characterized by street canyons, showing how the angular distribution of energy is correlated with the excess delay. [ABSTRACT FROM AUTHOR]

The mutual impedance between two rectangular patch antennas recessed in an elliptic cylinder is computed using a finite element boundary integral method. Results show that the coupling between the antennas is dependent on the curvature of the cylinder surface. For H-plane coupling, the coupling decreases as the radius of curvature increases. For E-plane coupling, the strongest coupling occurs for a specific curvature. [ABSTRACT FROM AUTHOR]

A two-element time-modulated array system is configured to operate as a direction finding antenna. The signal from each element of the array is time switched to provide a phase modulated output in which the depth of modulation is dependent on the angle of arrival of the received signal. Details of an experimental system designed to operate at 2400 MHz are presented and the results are compared to theoretical predictions. [ABSTRACT FROM AUTHOR]

The article calls for the submission of papers on antenna and propagation that focus on various topics such as sparse and thinned arrays, overlapped and interlaced subarrays, and beamforming and multibeam excitation of irregular arrays.