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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]

Indoor coverage is a major challenge for 5G millimeter waves (mmWaves). In this article, we address this problem through a novel theoretical framework that combines stochastic indoor environment modeling with advanced physical propagation simulation. This approach is particularly adapted to investigate indoor-to-indoor 5G mmWave propagation. Its system implementation, so-called iGeoStat, generates parameterized typical environments that account for the indoor spatial variations, then simulates radio propagation based on the physical interaction between electromagnetic waves and material properties. This framework is not dedicated to a particular environment, material, frequency, or use case, and aims to statistically understand the influence of indoor environment parameters on mmWave propagation properties, especially coverage and path loss. Its implementation raises numerous computational challenges that we solve by formulating an adapted link budget and designing new memory optimization algorithms. The first simulation results for two major 5G applications are validated with measurement data and show the efficiency of iGeoStat to simulate multiple diffusion in realistic environments, within a reasonable amount of time and memory resources. Generated output maps confirm that diffusion has a critical impact on indoor mmWave propagation and that proper physical modeling is of the utmost importance to generate relevant propagation models. [ABSTRACT FROM AUTHOR]

The primary purpose of this article is to demonstrate experimentally and explain theoretically (based on physics-based literature and empirical-based industry-standard models) that, in the limit where the receiver height vanishes above an imperfectly conducting plane such as the surface of the Earth, the propagation characteristics of 4G (800 MHz) signals in a typical cellular wireless setup behave very simply as the two-ray-plane Earth model, with approximately 40 dB/decade of signal loss, for almost the entire cell, in contrast to the more common case of finite height receivers with more complex path loss scaling with distance from the transmitter to the receiver. The simple reason behind this is that the “breakpoint” distance, beyond which all models predict this power law, vanishes as the height of the receive antenna vanishes, leading to the unified theory of signal propagation for ground-level receivers in the microwave regime. [ABSTRACT FROM AUTHOR]

Extremely low-frequency (ELF: below 30 Hz) wave penetration theory in the layered oceanic lithosphere is essential for studying submarine communications. In this article, we investigate the mode theory of ELF wave propagation in a layered oceanic lithosphere waveguide. Based on the real situation of the oceanic lithosphere approximated by the stratified prototype, in which a low-conductivity layer lies between two overlying and underlying high-conductivity regions, a theoretically sound physical model of oceanic lithosphere waveguide is built. The modal equations for transverse magnetic (TM) and transverse electric (TE) polarized guided modes are delivered using surface impedance boundary conditions. By evaluating the reflection coefficients of the upper and lower waveguide boundaries, formulas for the modal equation’s roots are derived. Analytical expressions for propagable mode parameters, such as the phase velocity, attenuation rate, excitation factor, and the field strength for different components, are obtained, and the cutoff frequency for high-order modes is given. Computations show that the field component curves exhibit typical peaks and nulls associated with multiple-mode propagation in the waveguide region, with each mode traveling at different phase velocities and attenuation rates. Moreover, it is found that all the high-order modes are evanescent below the cutoff frequency. [ABSTRACT FROM AUTHOR]

When designing and maintaining a wireless network, engineers need to first evaluate the impact of a specific propagation environment on radio wave propagation. Currently, although many popular propagation models (PMs) are available for reference, their analysis and validation in ultrarugged terrain areas (uRTAs) are still underexplored. This article aims to implement an uRTA PM (uRTAPM), which consists of two modes, i.e., the point-to-point (uRTAPM-P2P) and the point-to-area (uRTAPM-P2A) modes, to meet the requirements of practical applications. First, we consider the digital elevation model (DEM) data sets and three types of propagation, i.e., line of sight (LOS), non-LOS (NLOS), and multiobstacle propagation. Second, we propose a cone multipath model to simulate the NLOS diffraction behavior caused by mountains. Third, we utilize a triangulated irregular network (TIN) to finely divide the terrestrial surface of uRTAs. Finally, based on numerical calculations, compared with the existing commonly used PMs, the uRTAPM is found to reveal more details about the propagation loss (PL) caused by the terrain in uRTAs. The measurements demonstrate that the mean absolute error (MAE), root-mean-square error (RMSE), and standard deviation (SD) between the uRTAPM predicted values and the actual measured values are lower than those of other methods. [ABSTRACT FROM AUTHOR]

At millimeter-wave (mm-wave) frequencies, diffuse scattering (DS) from rough surfaces is an important propagation mechanism. Including this mechanism in radio propagation modeling tools, such as ray tracing (RT), is a key step toward realizing accurate propagation models for 5G and beyond systems. We propose a two-stage solution to this problem. First, we model reflection and transmission through rough slabs, such as doors, walls, and windows with the FDTD method. Our results indicate the influence of roughness and whether this influence is measurable, either reducing the magnitude of the reflected and transmitted waves or (most importantly) generating DS components. In the latter case, the surface effectively acts as a secondary source, whose pattern is computed by full-wave analysis. Then, this pattern is embedded in a ray tracer, enabling the computation and tracing of DS field components. We demonstrate this approach in the RT analysis of a 28 GHz indoor environment. [ABSTRACT FROM AUTHOR]

Wireless technologies are essential components of wearable devices for applications ranging from connected healthcare to human–machine interfaces. Their performance, however, is hindered by the human body, which obstructs the propagation of wireless signals over a wide range of directions. Here, we demonstrate conformal propagation and near-omnidirectional radiation of wireless signals near the body using clothing integrated with spoof surface plasmonic (SSP) structures. These structures, fabricated entirely from conductive textiles, induce wireless signals emitted by nearby antennas to propagate around the body as surface waves and radiate as propagating waves in directions otherwise obstructed by the body. We describe the procedure for designing textile-based spoof surface plasmon waveguides, radiating elements, and impedance matching sections for operation in the 2.4–2.5 GHz industrial, scientific, and medical (ISM) band. Using a tissue phantom model of the human torso, we experimentally demonstrate 2.62 greater angular coverage by a dipole placed near the body compared to without the clothing. [ABSTRACT FROM AUTHOR]

This article presents a two-way parabolic equation (2WPE) method based on the Pade(2,2) approximation form, which can model bidirectional electromagnetic wave (EW) propagation in a sea environment. Compared with other common approximation forms, the Pade(2,2) approximation has better convergence and accuracy. To accurately simulate the reflection and refraction effects of EW propagation near the sea surface, an improved fractal sea surface model is used to construct the geometric rough sea surface. The results of the Pade(2,2) approximation PE model are compared with those of the Miller–Brown model, and good agreement is observed. Numerous experiments prove that the 2WPE model is more accurate than the one-way parabolic equation model for predicting large-scale radio wave propagation in marine environments. Furthermore, we investigate the relationship between the backward-scattered EWs and the wind speed. The polynomial fitting method is adopted to process a large amount of sampled data for backward-scattered EWs with different wind speeds and frequencies. Through accurate judgment of the points of the polynomial fitting curve in a certain frequency range, the wind speed over the sea surface can be precisely determined. Thus, this article provides a novel theoretical method for marine remote sensing. [ABSTRACT FROM AUTHOR]

The objective of this special issue is to publish papers related to the basic physics of Electromagnetism in thecontext of radio wave propagation, see editorial of Part I. [ABSTRACT FROM AUTHOR]

The amplitude and phase distortions of radar echo signal will cause the emergence of undesirable ghost scattering points, which degrade the quality of terahertz (THz) radar images. In this paper, a physics-based procedure is presented to predict the atmospheric attenuation and dispersion characteristics at THz frequencies, which is mainly based on the line-by-line calculation method with a specific modification in phase-shift prediction. The line-by-line parameters provided by the high-resolution transmission spectroscopic database and atmospheric condition parameters obtained from the Air Force Geophysics Laboratory reference atmospheric constituent profiles are adopted to predict the atmospheric transmittance and the phase shift for specific transfer paths. The results are compared with measured data to demonstrate the accuracy, while the proposed procedure is used to analyze the impacts of atmospheric transfer characteristics on radar imaging via the high-resolution range profile simulation. The signal distortion is interpreted in terms of paired echoes to illustrate the importance of frequency band selection for high-resolution imagery at THz frequencies. [ABSTRACT FROM AUTHOR]

A new double-reflection free-space method is presented in this paper for measuring the actual wall material permittivity in indoor scenes, especially for the narrow wall corners which cannot accommodate test settings used in the traditional free-space method. The method is nondestructive, accurate, and easy to implement. In addition, it does not require a priori knowledge of the thickness of the walls under test. Two experimental setups, namely, double-polarization and double-parameter free-space measurements, were conducted according to the proposed method. The presented technique acquires the accurate material permittivity on site, which is the key parameter in the modeling and simulation of the radio wave propagation (RWP) prediction. The permittivity of the walls in the buildings measured by the proposed method, which considers the influences from the aging, machining, and practical usage of the wall materials, is more accurate than that by sample test in the laboratory. The method was examined by measuring the permittivity of the designed wall corner with two perpendicular plates and validated by the waveguide measurement of the sample chosen from the tested plates. Finally, the explicit uncertainty and error analyses were derived. The proposed method is applicable for accurate modeling of RWP prediction in practice. [ABSTRACT FROM AUTHOR]

This paper summarizes the lessons learned using a physics-based macromodel in studying electromagnetic wave propagation over an imperfectly conducting ground in cellular wireless communication. First, it has been observed that the path loss exponent is independent of the nature of the ground parameters inside the cell of interest. Second, the electrical parameters of the environment have little effect on the path loss exponent in the cellular band. Third, it is observed that lowering the base station antenna toward the ground provides a stronger signal in the near field within the cell of interest. Furthermore, tilting the transmitting antenna toward the sky enhances the signal strength. Tilting the antenna toward the ground increases the signal strength but, in addition, enhances the interference pattern and, hence, is not a good solution. A typical path loss inside the cell is 30 dB per decade of distance, and outside the cell, it increases to 40 dB per decade. By bringing the antenna closer to the ground and then tilting it toward the sky, a good nonintuitive solution is provided. In such scenarios, a path loss of 20 dB per decade for some components of the field, the lowest possible, can be achieved for certain orientations and deployment of the base station antenna. In addition, it is shown that operating an antenna inside a metallic box eliminates its radiation capabilities and, hence, has no physical meaning even though it is claimed in the contemporary literature that it simulates a rich multipath environment. Finally, a note on the proper interpretation of the term channel capacity and its implications is delineated. [ABSTRACT FROM AUTHOR]

This paper describes 3-D numerical simulations and radio frequency (RF) measurements of wave propagation in microwave-heated magnetized plasmas of ion sources. Full-wave solution of Maxwell’s equations has been addressed through the finite-element method commercial software COMSOL. Our numerical model takes into account the strongly inhomogeneous and anisotropic magnetized “cold” plasma medium. The simulations reproduce the main features of the wave–plasma interaction of the flexible plasma trap (FPT) that recently came into operations at INFN-LNS. A two-pin RF probe has been ad hoc developed and used as a plasma-immersed antenna for measuring local wave electric fields in the FPT device. The measurements of plasma electron density and RF electric field, performed for different external magnetic field configurations, allowed direct comparison with the assumed simulation model. [ABSTRACT FROM AUTHOR]

In satellite communications, the adverse propagation effects produced by the atmosphere to the signal are compensated by the systems using propagation impairment mitigation techniques, protection by diversity being one of their main categories. Two slant-path propagation experiments have been carried out in Madrid in the Ka- and $Q$ -bands, with the Eutelsat KA-SAT and Alphasat satellites, respectively, with three concurrent years of measurements available: from March 2014 to February 2017. In this paper, results on time diversity and orbital diversity—the latter obtained from a frequency scaling approach— are presented, the first one for both frequency bands and the second for the $Q$ -band. An acceptable overall behavior is obtained by comparing the results with the available models. For both diversity techniques, possible gains are moderate but not negligible due to the climatological conditions of the evaluated site. [ABSTRACT FROM AUTHOR]

This paper proposes a hybrid modeling approach for the prediction of the room-to-room radio propagation channel. The model combines ray tracing (RT) with the propagation graph (PG). The PG vertices are obtained at each room by RT with the assumption that the receive antenna (or transmit antenna) virtually locates on the surface of the separating wall between the two rooms. Rays transmitted from one room to the other through the separating wall are deterministically calculated by Snell’s law of refraction. Predictions by the proposed model are compared with the measurement data from an office-to-office scenario. The results show that the proposed modeling works with the simplest parameter settings, i.e., assuming no propagation from the room containing receive antenna to the room containing transmit antenna, RT applied separately in each room only involves mechanism of line of sight and first-order specular reflection. [ABSTRACT FROM AUTHOR]

A novel 3-D hybrid model is presented for the prediction of propagation of radio waves in the presence of complex obstacles located on regular perfectly conducting terrain. The finite-difference time-domain (FDTD) method is implemented to model the source and obstacle regions, and the wave is then propagated by using the parabolic equation (PE) method-based algorithm. The source region contains all the complex obstacles and takes into consideration all the possible wave interactions in that region. The transition region between the FDTD domain and the PE domain is carefully treated in order to avoid numerical incompatibilities and to realize the time-/frequency-domain transition. The method is applied for different scenarios, starting with relatively simple obstacles like knife-edges for the verification and validation of the approach and then including more complex obstacles to the propagation medium. The results are compared to those obtained from full FDTD implementations of the same scenarios. [ABSTRACT FROM AUTHOR]

In his seminal article [1], Gabor observed thatCommunication theory has up to now been developed mainly on mathematical lines, taking for granted the physical significance of the quantities which figure in its formalism. But communication is the transmission of physical effects from one system to another, hence communication theory should be considered as a branch of physics. Thus it is necessary to embody in its foundations such fundamental physical data as the quantum of action, and the discreteness of electric charges $\ldots$ We observe first that all electric signals are conveyed by radiation. Even if lines or cables are used in the transmission, by the Maxwell-Poynting theory the energy can be located in empty space. Hence we can apply to our problem the well known results of the theory of radiation. The point here is that in the design of the physical layer one has to apply other concepts besides the principles of communication theory. More recently, this point was further investigated by Ivrlac and Nossek [2], [3], where they state thatElectromagnetic field theory provides the physics of radio communications, while information theory approaches the problem from a purely mathematical point of view. While there is a law of conservation of energy in physics, there is no such law in information theory. Consequently, when, in information theory, reference is made (as it frequently is) to terms like energy, power, noise, or antennas, it is by no means guaranteed that their use is consistent with the physics of the communication system. Information theory serves well as the mathematical theory of communication. However, it contains no provision that makes sure its theorems are consistent with the physical laws that govern any existing realization of a communication system. Therefore, it may not be surprising that applications of information theory or signal processing, as currently practiced, easily turn out to be inconsistent with fundamental principles of physics, such as the law of conservation of energy. Ivrlac and Nossek further elaborate [3] thatthere exist a number of fundamental principles in physics which can be stated as conservation laws, meaning that there are quantities which can be calculated for a physical system at one time, and when recalculated at a later time come out the same [4]. They further illustrate that an example is the law of conservation of energy $\ldots$ The movement of the planets around the sun can be obtained solely by following the implications of the laws of conservation of energy and angular momentum [5]. Moreover, the conservation laws are also deep principles for they relate to symmetry in physics [6]. For instance, conservation of energy implies that the laws of Nature are time-invariant, and vice versa. Also, in signal processing and information theory, the concept of energy is a prominent one. It appears as the energy required to transfer one bit of information, or one symbol of the signal alphabet, or sometimes in form of transmit power, i.e., the rate at which energy must be supplied per unit of time to maintain communication. Yet, interestingly, the fact that energy is conserved, which is of such fundamental importance in physics, apparently plays no role in standard textbooks on information theory [7], signal processing [8], communication theory [9] or signal theory [10]. The authors are also not aware of any research work in these areas where the remarkable fact that energy is conserved is explored or discussed. The reason for this strange absence of conservation laws in signal processing, information theory and related disciplines seems to be related to inputs and outputs which are each described by single variables, instead of by a pair of conjugated variables, like position and momentum in Hamiltonian mechanics [6], or voltage and current in circuit theory [11].Therefore, to address the development of the physical layer adequately and to ensure that systems perform according to design criteria, it is necessary to merge the principles of electromagnetics, which are primarily related to antennas and maximum power transfer, to the issues of channel capacity and how it can be quantified using the principles of physics and the radiation efficiency of antennas rather than the use of the maximum power transfer theorem [12]. [ABSTRACT FROM AUTHOR]

Microwave over-the-horizon propagation via troposcatter mechanism is a valuable candidate for passive sensing of remote radiation source applications, e.g., electronic reconnaissance. The result from the misalignment between target emitting and the system receiving antenna, the additional deviation losses in azimuthal and elevation angle are indispensable to model the receiving antenna pattern as well as the arrival scattering signal level. Based on the scatter transfer function, the deviation losses are derived assuming Gaussian antenna patterns and verified by existing measured data from the 1950s. Finally, potential application values of the deviation losses in electronic reconnaissance are provided. [ABSTRACT FROM AUTHOR]

This paper presents a physics-based macro model that can predict with a high degree of accuracy various experimental data available for the propagation path loss of radio waves in a cellular wireless environment. A theoretical macro model based on the classical Sommerfeld formulation can duplicate various experimental data including that of Okumuraet al.carried out in 1968. It is important to point out that there are also many statistical models but they do not conform to the results of the available experimental data. Specifically, there are separate path loss propagation models available in the literature for waves traveling in urban, suburban, rural environments, and the like. However, no such distinction is made in the results obtained from the theoretical analysis and measured experimental data. Based on the analysis using the macro model developed after Sommerfeld’s classic century-old analytical formulation, one can also explain the origin of slow fading which is due to the interference between the direct wave from the base station antenna and the ground wave emanating from the reflections of the direct wave and occurs only in the near field of the transmitting antenna. The so-called height gain occurs in the far field of a base station antenna deployment which falls generally outside the cell of interest, while in the near field, within the cell, there is a height loss of the field strength for observation points near the ground. A physical realization of the propagation mechanism is illustrated through Vander Pol’s exact transformation of the Sommerfeld integrals for the potential to a spatial semiinfinite volume integral and thus illustrates why buildings, trees, and the like have little effects on the propagation mechanism. [ABSTRACT FROM AUTHOR]

This paper provides a survey of various experimental data available on the value of the propagation path loss of radio waves in a cellular wireless environment. It is shown starting with the Okumuraet al.’s paper on propagation measurements and other available published experimental data that they all exhibit that the electric field varies as $\rho ^{-1.5}$ within a cellular radius of a few kilometers, where $\rho $ is the radial distance of the receiving antenna from the transmitting one. This decay in the fields is equivalent to a propagation loss of −9 dB/octave or −30 dB for a decade of the distance. This value is independent of the nature of the ground, whether it be composed of rural, urban, suburban, or water. This is the first time it is stated that the propagation path loss due to the presence of ground generates a path loss of 90 dB when the signals travel a distance of 1 km. This value is rather large when compared to a loss of 30–50 dB produced by buildings, trees, and similar artifacts. Therefore, the experimental data indicate that the effect of trees and buildings have a secondary influence on the decay of the electric field with distance, the dominant one is the propagation loss over an imperfect ground. Contemporary propagation models do not acknowledge this fact. Outside the cellular radius of a few kilometers, the path loss appears to be 12 dB/octave or 40 dB/decade of distance. In a companion paper, it will be demonstrated that the values for the path loss can be explained from an analytical standpoint without taking recourse to statistics which involves a lot of assumptions on the functional variation of the variables of interest. [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]

Physics-based wireless propagation modeling and network protocol design have evolved over decades as orthogonal areas in communication systems research. This fragmented approach does not exploit available efficiencies when planning and deploying communication systems. In an attempt to integrate the two areas, we harness the understanding of the underlying physics of electromagnetic propagation to enhance the robustness of network protocol design by deriving physics-based network-level performance metrics. We use ray-tracing and parabolic equation models of 2.4 GHz propagation along tunnel and open-air sections of London Underground to evaluate the performance of a communications-based train control system. For comparison, we consider existing path loss models for tunnel environments and investigate whether they can provide sufficient accuracy to be used for network protocol design. We show that physics-based models lead to reliable predictions at the network level, similar in fidelity to using measured data and unlike using simplified channel models of the path loss exponent type. [ABSTRACT FROM AUTHOR]

A test setup is deployed to study the influence of base station antenna height on the cellular network coverage. Both the numerical and experimental investigations are undertaken to evaluate the received electric field levels for different heights of the transmitting (TX) antenna and several distances between the transmitter and the receiver. The numerical simulation results demonstrate that when the TX antenna is mounted high above the ground, more fluctuations are induced in the electric field levels, resulting in possible dead zones for mobile communications. The measured received levels and data rates confirm the numerical simulation results. [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]

It is found that for very low-frequency (VLF) signals, the mode-conversion interference produces a cycle slip during the sunrise or sunset transition at certain distances far away from the transmitter. In this paper, an anisotropic terrestrial waveguide with abrupt effective height change of 20 km geometry has been adopted to investigate the diurnal behaviors of amplitude and phase caused by the night-to-day transition. At the waveguide junction, sequential determinations of transmission and reflection coefficients are determined by the resulting linear equations from field matching. Field calculations across the day-to-night transition are carried out by using a reciprocity theorem correspondingly. This paper illustrates that VLF wave propagation experiences multimode interference in periodic variations in amplitude during sunrise and sunset over a long propagation path. [ABSTRACT FROM AUTHOR]

Fixed performing antennas are ineffective in providing optimum performance in a changing multipath environment, such as underground mines where channel conditions quickly change as the antenna is moved. Hence, we investigated the performance of a pattern reconfigurable antenna (RA) in improving the wireless communications link in a hard rock underground mine. An RA and a dipole were characterized and compared in line-of-sight (LOS) and nonline-of-sight (NLOS) propagation scenarios. As a proof of concept, experiments were performed at the University of Queensland Experimental Mine using a 1 GHz frequency band centered at 2.45 GHz. Analysis of complex channel impulses captured through the frequency-domain channel sounding is presented. Also, the results show that reconfigurability in the rich multipath environment achieves a 20% and 34% improvement in path loss and delay spread, respectively, compared to a fixed-beam antenna in NLOS propagation. Furthermore, more than 34% improvement in coherence bandwidth is observed in LOS and NLOS. Therefore, reliable high-data-rate communication is achievable in underground mines by the effective management of multipath effects using the RA. [ABSTRACT FROM AUTHOR]

We present an exact mathematical framework for electromagnetic wave propagation in periodically time-modulated media, in which the permittivity is homogenous and modulated in a step-varying fashion. By using Hill’s equation theory, we show that this problem has analytical solutions. We connect the dispersion relation, which exhibits $k-$ gaps, with the Hill stability analysis, providing an alternative mathematical description for wave propagation in temporal crystals. Our analysis, which is exact and transposable to other kinds of waves or modulation schemes, provides general useful physical and mathematical insights, complementing the use of numerical techniques such as finite differences in the time domain, or harmonic balance schemes, with a more transparent and practical design tool. The present analytical transient mathematical analysis, in contrast with the existing frequency-domain numerical approaches, can exhibit the parametric properties of electromagnetic waves inside a time periodic medium. For this reason, it can be a useful tool for the design of active microwave and optical devices, which employ time periodic wave medium modulation to filter or parametrically amplify wave signals. [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]

When reverberation chambers are loaded to increase the coherence bandwidth for modulated-signal measurements, a secondary effect is decreased spatial uniformity. We show that an appropriate choice of stirring sequence, consisting of a combination of mode-stirring mechanisms such as paddle and antenna-platform stirring, can mitigate the potential for increased uncertainty. We develop a new mode-stirring sample correlation model for uncertainty due to the stirring sequence. In a comparison with an empirical uncertainty analysis, the model is found to have an agreement within 2.5%. Our analysis is demonstrated for four loading cases in each of three reverberation chambers. The model is used to determine an optimal stirring sequence for a given chamber setup directly from correlations associated with each stirring mechanism. The model can also be understood in terms of the entropy of a measurement and it is shown that maximizing the entropy corresponds to a minimized uncertainty. The method presented here not only provides insight into sources of uncertainty but also allows users to determine an optimal mode-stirring sequence with minimized uncertainty for a given chamber setup. [ABSTRACT FROM AUTHOR]

In the Earth-space link communication system design, rain attenuation must be considered, especially with a frequency above 10 GHz. Therefore, a reliable rain attenuation prediction model is necessary for the design of these systems. Recently, two anomalous behaviors have been reported in existing rain attenuation prediction models. First, predicted rain attenuation results from many models vary nonmonotonically with elevation angles. Second, the rain attenuation predicted by the ITU-R model varies nonmonotonically with a percentage of time or rain rate in low latitudes (between 36° S and 36° N) and low elevation angles (below 25°). To tackle these problems and improve the prediction accuracy, a new rain attenuation model is developed based on the exponential rain cell profile, and a concept of rain rate adjustment factor is proposed. Evaluating performance by utilizing the Recommendation ITU-R P.311-15, the new model provides better prediction results than other existing models. Moreover, the anomalous behaviors are also improved in this model. [ABSTRACT FROM AUTHOR]

Accurate site-specific radio propagation simulations provide an important basis for cellular coverage analysis. The quality of these simulations relies on the accuracy of environmental description and electrical properties of constituent materials. This paper presents a novel method of on-site permittivity estimation. The method utilizes an accurate geometrical database of the environment for identifying flat and smooth surfaces producing reflections. The method exploits a limited number of on-site channel sounding to extract reflected multipaths and compare them with ray tracing based on the environmental database. The permittivity of the identified reflecting surfaces is estimated by solving an inverse reflection problem. The method was experimentally tested with limited radio channel measurements at 60 GHz in a large empty office room. The identified reflecting surfaces are classified according to their mean permittivity estimates, showing their consistency with physical material evidence and the permittivity database in the International Telecommunication Union Radiocommunication Recommendation (ITU-R P.2040-1). The estimated permittivity values are visualized as a 3-D map, giving an intuitive understanding of materials constituting the environment. This paper demonstrates on-site permittivity estimation and material classification without the need for isolated measurements of composite materials in an anechoic chamber or in situ measurements of built environments. [ABSTRACT FROM AUTHOR]

A finite-difference time-domain-based technique for evaluating envelope correlation coefficient (ECC) and mean effective gain (MEG) of multiple-input multiple-output (MIMO) antennas operating in real-life nonuniform propagation environment is introduced. The proposed algorithm is based on the formulation of a modified cross correlation Green’s function (CGF), which directly incorporates the specific angular distribution functions and cross-polarization discrimination parameters. Thus, the proposed technique based on modified CGF is able to calculate ECC/MEG for MIMO antennas in nonuniform propagation scenario, directly from the time-domain antenna currents. This avoids the troublesome determination of far-field antenna pattern at each working frequency and the consequent postprocessing, as required in the conventional approach. Numerical examples using thin-wire half-wavelength dipole antennas and printed microstrip-fed slot antennas are provided to show the effectiveness of the proposed technique for characterizing MIMO antennas in nonuniform propagation environment. [ABSTRACT FROM AUTHOR]

Large gateways of future satellite communications (Satcom) systems will likely operate at Q-/V-band frequencies (40–50 GHz) to take advantage of the larger available bandwidth. On the other hand, Q-/V-band Satcom systems suffer from highly variable propagation losses, which must be accurately assessed and modeled. The Alphasat Aldo Paraboni propagation experiment, conceived and realized by the Italian Space Agency (ASI), Rome, Italy, has been running since 2014 to carefully investigate the propagation effects in the Ka- and Q-band and, thus, to support the design of high-throughput systems and propagation impairment mitigation techniques. In the framework of this experiment, ASI deployed two large ground terminals (Cassegrain antenna with 4.2 m diameter) in Tito Scalo (South of Italy) and Spino d’Adda (North of Italy). This contribution investigates the effect of antenna strut elongation on propagation measurements, which causes an additional reduction of 1–2 dB in the received signal during sunny days. [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]

In this paper, the channel characteristics and cell coverage of the 28 GHz millimeter wave (mmWave) band outdoors are investigated by developing an efficient 3-D ray-tracing simulation. First, the accuracy of the simulation is verified by comparing its results with actual measurements. The path loss (PL) agrees well in both line-of-sight (LOS) and non-LOS (NLOS) regions, whereas the shadowing factor exhibits differences in the NLOS regions. Additional simulations are conducted for downtown Gangnam, a representative high-rise urban area in Seoul, South Korea. The simulated and measured coverages of 300 base stations operating in 900 MHz long-term evolution band are compared and analyzed. For the LOS regions, PL of 28 GHz is 30 dB higher because of the free-space PL gaps, whereas PL varied in the NLOS regions because of multipath fading. In addition, the outage probability is determined to evaluate the validity of mmWave cell deployment and coverage for high-rise urban microcell environments. The contributions of this paper include LOS and outage probability models with low root-mean-square errors, indicating improvement over previously developed models. [ABSTRACT FROM PUBLISHER]

Diffuse scattering (DS), mostly caused by macroscopically rough surfaces, can be modeled by an effective roughness (ER) approach. In the ER approach, the object surface is divided into tiles, and the DS field amplitude associated with each tile is given. Assuming that the transmit antenna (or emitted field) is fixed and the phases associated with different tiles are independent, this paper proposes a phase evolution model based on the relative spatial locations of the receive antenna (Rx) to each tile. The proposed model contains two parts: the deterministic part that depends on the variation of the distance from Rx to tile center, and the correlated random part that depends on the variation of the angle between the Rx and the normal vector of tile. The correlation of the tile field phases resulting from the angular variation is modeled by the exponentially damped cosine correlation function. The proposed phase evolution model is evaluated by being applied to the ER approach to predict the DS component of radio channel transfer function (CTF). The predicted DS-CTF is compared with the reference (simulated data by physical optics and measured data in well-controlled environment) in terms of the spatial autocorrelation as well as the spatial Doppler spectrum. [ABSTRACT FROM PUBLISHER]

Compressive sensing is used to reconstruct the time-harmonic electric field created by multipath fading from a limited number of measurement points over a planar region. The multipath fading signal is shown to be the superposition of multiple plane wave components and thus has a sparse representation in the spatial-frequency domain. The discrete Fourier basis used as the dictionary for orthogonal matching pursuit is oversampled to ensure sufficient resolution in the spatial-frequency domain. Experimental results at 10 GHz using an arbitrary plane wave expansion show that the signal-to-error ratio of the compressive sensing reconstruction is approximately 16 dB when randomly selecting only 7.5% of the total points. [ABSTRACT FROM PUBLISHER]

The design, realization, and measurement results of a high-accuracy multiyear 3.5 GHz trans-horizon radio propagation measurement system are discussed, with both emphasis on the results and implemented technical measures to enhance the accuracy and overall reliability of the measurements. The propagation measurements have been performed on two different paths of 253 and 234 km length, using two transmitters and one receiver in the period September 2013 till November 2016. One of the paths travels over wetland; the other path can be considered as a land path. On each path, an additional transmitter is placed at 107 km (in the 253 km path) and 84 km (in the 234 km path) from the receiver. With this arrangement, the correlation between two nonaligned paths of comparable length, and two aligned paths of dissimilar length, was studied. The measurements show that for the land path, the predicted ITU-R P.452-16 cumulative distribution function (CDF) typically shows 5 dB higher path loss than the actual measured CDF for the region of interest; anomalous propagation. This means that the measured signal is on average weaker than predicted (a higher path loss). For the wetland path, the actual CDF is very close to the predicted CDF. Also, the measurements reveal that typically 30% of the anomalous propagation occurrences are correlated with other paths. [ABSTRACT FROM PUBLISHER]

The radiowave propagation through hollow clay brick walls, which are common in southern European construction, is analyzed. The brick walls have thicknesses of 11, 15, and 20 cm, common in both interior and exterior walls, and are bound with a portland cement, water, and sand mixture. For each brick dimension, three prototypes were assembled, varying in the type of wall finish, i.e., exposed brick, smooth painted plaster, and rough painted plaster. A 10 cm concrete wall was also included for comparison purposes. Penetration loss metrics were evaluated in an anechoic chamber at frequencies ranging from 680 MHz to 10 GHz. Results demonstrate that the brick wall internal heterogeneity, as well as the type of finish, significantly influences the propagation phenomena and thus the frequency response of the walls, with relatively high penetration losses observed at some relevant commercial frequency bands. Finally, an alternative brick design, with reduced penetration losses, is also proposed and evaluated under simulation environment only. [ABSTRACT FROM AUTHOR]

Analysis of two years of measurements from a three-site satellite beacon diversity experiment in Norway is presented. The operating frequency was 19.68 GHz, elevation angles were between 21.7° and 22°, and site separations 23, 29.7, and 50.1 km. Due to the spatial decorrelation of rain, simultaneous attenuation at all sites occurs much less frequently than at a single site. All three two-site diversity combinations and a three-site diversity combination were investigated. Measured values for diversity gain at 0.01% exceedance level were 8.8, 10.7, and 12.1 dB for the two-site pairs and 12.5 dB for three sites, given a single-site attenuation level between 13 and 16 dB. Comparison of the measured data showed excellent agreement with the current ITU-R model. Fade duration statistics for single site and for the combinations are analyzed, and it is shown that the statistics for the diversity combinations can be modeled using a double-lognormal function as is the case for single-site statistics. Analysis of the effects of a nonperfect switch-over scheme showed that using moderate switch-over threshold levels and time delays, the number of switchovers can be significantly reduced while most of the diversity gain and reduction in number of fades are retained. [ABSTRACT FROM AUTHOR]

This paper presents the Greene second-order parabolic equation (PE) solution to model radio propagation over irregular terrain. This solution is implemented by finite-difference and shift-map technique. The second-order finite-difference approach performs well for slopes up to about 15°, and discontinuous slope changes up to about 30°, which is better than can be achieved using the split-step/Fourier approach. The results prove that the Greene PE solution has greater propagation angles than does the Claerbout PE solution. Thus, the Greene second-order solution can certainly give better results for large angle propagation over complicated terrain boundaries. Besides, we treat the important problem of accuracy of the different approximations of the PE-based propagation models, and derive the general PE solutions with respect to the terrain slope for the different approximations. It is useful to have collected in one paper, the most important PE approximations for radio propagation. [ABSTRACT FROM AUTHOR]

Wireless communications or navigation is heavily influenced by electromagnetic wave propagation. The power-delay profile (PDP) characterizes the propagation channel by simple statistics. In this communication, we propose a channel model to simulate a given wideband averaged PDP obtained by measurements or theoretical considerations, enabling to test, develop, and design radio links. The channel model is based on a geometrical stochastic approach, allowing to simulate the space-variant channel impulse response for a moving receiver. Furthermore, the appearance and disappearance of multipath components are geometrically taken into account by an angular pattern. We verify the proposed methodology by simulations. [ABSTRACT FROM AUTHOR]

ATMospheric simulator for PROPagation applications (ATM PROP), a comprehensive methodology to assess tropospheric effects affecting high-frequency earth–space communication systems, is presented. The model takes advantage of physically based approaches aimed at synthesizing high-resolution (1 km $\times1$ km horizontal, 100-m vertical) 3-D fields of rain, clouds, and water vapor (dimension: 200 km $\times200$ km horizontal, 20-km vertical), which are all merged so as to maintain their mutual correlation. This, in turn, enables a more realistic combination of the attenuation due to single constituents, if compared to the statistical approach currently recommended by the ITU-R. The accuracy of ATM PROP in predicting tropospheric effects on earth–space systems is initially validated against the large propagation data set collected at the experimental station of Spino d’Adda, Italy, during the ITALSAT propagation campaign. The preliminary results obtained suggest that ATM PROP can be used to predict, with a reasonable level of complexity and limited coarse-resolution Numerical Weather Prediction-derived inputs, the tropospheric fade affecting complex communication systems (e.g., site diversity schemes), especially those involving low elevation links (e.g., low earth orbit or geostationary orbit at high latitude), for which the spatial distribution of the relevant tropospheric constituents needs to be taken in due account. [ABSTRACT FROM AUTHOR]

Alternate direction implicit (ADI) method has been widely used to analyze the tunnel propagation or electromagnetic scattering problem for the finite difference method. In this paper, a novel alternate direction decomposition (ADD) scheme is proposed to simplify the split-step Fourier transform (SSFT) algorithm in parabolic equation (PE) model. The spatial matrix of electric field is decoupled into two subarrays by ADD-SSFT algorithm, which is used to decrease the computational burden for 3-D PE problems in a large scale environment after the integration order exchanged. Also, the phase error of this approximation is analyzed to show the algorithm performance, along with several numerical results, including the tunnel cases with traditional modal theory, and they are provided to validate and calibrate the PE model, and demonstrate the efficiency and accuracy of the proposed algorithm. Moreover, the scattering computation is investigated and compared with multilevel fast multipole method to show that ADD-SSFT is appropriate for solving the scattering problems for the electrical large target. [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]

This paper addresses the problem of nonreciprocity of propagation conditions in meteor-burst communication systems (MBCSs). In prior publications, this problem has not gained appropriate attention. The channel nonreciprocity may have a significant impact on such advanced communication systems as meteor synchronization systems with nanosecond precision and meteor key distribution systems intended to securely create two identical copies of a shared secret key at both channel sides. These systems are based on the processing of phase characteristics of meteor radio reflections that need to be accurately modeled. We propose a new MBCS simulation model that is based on a rigorous solution to the problem of oblique diffraction of radio waves on ionized meteor trails. Our diffraction approach allows more accurate simulation of the amplitude and phase characteristics of oppositely propagating signals that makes possible detailed investigation of the channel nonreciprocity effects. Using this approach, we present some preliminary simulation results on the channel nonreciprocity at meteor-burst propagation that prove satisfactory immunity of MBCS to ionospheric disturbances even while operating in severe conditions of polar regions. [ABSTRACT FROM PUBLISHER]

Presents the table of contents for this issue of this magazine. [ABSTRACT FROM AUTHOR]

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]

A theoretical second-order monostatic high-frequency radar power model is derived for the case of mixed-path ionosphere-ocean propagation. First, the general form of the received electric field is investigated by considering the ionospheric reflection and the second-order sea scattering, which involves both electromagnetic and hydrodynamic contributions. Then, this field is inversely Fourier transformed to the time domain, and a pulsed source is incorporated. Subsequently, the second-order received power model is developed by assuming that the ionospheric reflection coefficient and the ocean surface can be described as Fourier series whose coefficients are random variables. In order to investigate the power spectrum of the mixed-path propagation and its relative intensity to that of the surface propagation under a variety of ionospheric conditions and sea states, a normalized power is simulated. [ABSTRACT FROM PUBLISHER]

The next generation of intelligent vehicle systems will use wireless communication to connect a car and its passengers. It is therefore important to understand the in-cabin propagation characteristics of an automobile. This paper investigates the wireless channel native to these cabin enclosures. We present detailed, polarization dependent field measurements over a plane of interest in the cabin, and show that a simple analytical model gives reasonable agreement with our measurements. [ABSTRACT FROM PUBLISHER]