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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]

A communication blackout is one of the major problems that occurs during the reentry of a satellite into the Earth's atmosphere, which is caused by the reentry plasma blocking electromagnetic waves near the satellite for telecommunication. Moreover, it prevents tracking and data transmission, resulting in inaccurate prediction of landing sites and data loss. Therefore, there is a necessity to evaluate the propagation of electromagnetic waves in the reentry plasma and to mitigate the communication blackout. An inflatable aeroshell technology with lightweight and large-area features enables aerodynamic drag at high altitudes to reduce aerodynamic heating and to mitigate communication blackouts. Thus, a nanosatellite mission using such an inflatable aeroshell has been proposed. For the purpose of telecommunication possibilities during the reentry in future nanosatellite missions, a detailed investigation of communication blackout mitigation by inflatable aeroshell is required. In the present article, the plasma flow and electromagnetic wave propagation near the nanosatellite during atmospheric reentry were revealed by using a computational science approach. A low-temperature and low-density wake is formed behind the nanosatellite. Moreover, an electromagnetic wave propagation path is formed in the wake, indicating that this path is maintained during the reentry, and no communication blackout occurs when using the deployable nanosatellite. [ABSTRACT FROM AUTHOR]

Meta-surfaces intend to improve the performance of future wireless networks significantly by controlling the wireless propagation and shaping the radio waves according to the generalized Snell's law. A recent application of meta-surfaces are reconfigurable intelligent surfaces that are mainly proposed for the reflection and steering of the impinging signal. In this article, we introduce randomly reconfigurable surfaces (RRSs) aiming to diffuse the incoming wave and characterize the performance of an RRS-assisted communication network. To facilitate the performance analysis of an RRS-assisted network, first, we present novel closed-form expressions for the probability density function, the cumulative distribution function, the moments, and the characteristic function of the amplitude of the distribution of the sum of double-Nakagami- $m$ random vectors, whose amplitudes follow the double-Nakagami- $m$ distribution, and phases following the circular uniform distribution. We also consider a special case of this distribution, namely the distribution of the sum of Rayleigh-Nakagami-m random vectors. Then, we exploit the obtained expressions to investigate the RRS-assisted composite channel, assuming that the two links undergo Nakagami- $m$ fading. Specifically, closed-form expressions for the outage probability, the average received signal-to-noise ratio, the ergodic capacity, the bit error probability, the amount of fading, and the channel quality estimation index are provided to evaluate the performance of the considered system. Finally, these metrics are also derived for the practical special case where one of the two links undergoes Rayleigh fading, implying that this link is non-line-of-sight. [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]

The fine timing measurement (FTM) protocol is designed to determine precise ranging between Wi-Fi devices using round-trip time (RTT) measurements. However, the multipath propagation of radio waves generates inaccurate timing information, degrading the ranging performance. In this study, we use a neural network (NN) to adaptively learn the unique measurement patterns observed at different indoor environments and produce enhanced ranging outputs from raw FTM measurements. Moreover, the NN is trained based on an unsupervised learning framework, using the naturally accumulated sensor data acquired from users accessing location services. Therefore, the effort involved in collecting training data is significantly minimized. The experimental results verified that the collection of unlabeled data for a short duration is sufficient to learn the pattern in raw FTM measurements and produce improved ranging results. The proposed method reduced the errors in raw distance measurements and well-calibrated ranging results requiring the collection of ground truth data by 47–50% and 17–29%, respectively. Consequently, positioning errors were reduced by 17–30% compared to the well-calibrated ranging scenario. [ABSTRACT FROM AUTHOR]

Vehicular Visible Light Communication (VVLC) is preferred as a vehicle to everything (V2X) communications scheme due to its highly secure, low complexity and radio frequency (RF) interference free characteristics, exploiting the line of sight (LoS) propagation of visible light and usage of already existing vehicle light emitting diodes (LEDs). Current VVLC channel models based on deterministic and stochastic methods provide limited accuracy for path loss prediction since deterministic methods heavily depend on site-specific geometry and stochastic models average out the model parameters without considering environmental effects. Moreover, there exists no wireless channel model that can be adopted for channel frequency response (CFR) prediction. In this paper, we propose a novel framework for the machine learning (ML) based channel modeling of the VVLC with the goal of improving the model accuracy for path loss and building the CFR model through the consideration of multiple input variables related to vehicle mobility and environmental effects. The proposed framework incorporates multiple measurable input variables, e.g., distance, ambient light, receiver inclination angle, and optical turbulence, with the exploitation of feed forward neural network based multilayer perceptron neural network (MLP-NN), radial basis function neural network (RBF-NN) and decision tree based Random Forest learning methods. The framework also includes data pre-processing step, with synthetic minority over sampling technique (SMOTE) data balancing, and hyper-parameter tuning based on iterative grid search, to further improve the accuracy. The accuracy of the proposed ML based channel modeling is demonstrated on the real world VVLC vehicle-to-vehicle (V2V) communication channel data. The proposed MLP-NN, RBF-NN and Random Forest based channel models generate highly accurate path loss predictions with 3.53 dB, 3.81 dB, 3.95 dB root mean square error (RMSE), whereas the best performing stochastic model based on two-term exponential fitting provides prediction accuracy of 7 dB RMSE. Moreover, MLP-NN and RBF-NN models are demonstrated to predict VVLC CFR with 3.78 dB and 3.60 dB RMSE, respectively. [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]

Indoor device-free object detection has many applications, such as assisted living, home security, and occupant-centered building utility control. To accommodate 3-D room layout variations and improve accuracy, multiple observation units with spatial diversity are often required, which will unavoidably cause a significant increase in the component and deployment cost if each unit needs wired power or battery. In this article, we present an indoor device-free object detection system implemented by a commercial radio frequency identification reader and many passive tags around the room. The passive tag as a dispersed observation point offers a cost-effective solution for the necessary spatial diversity. The tag backscattering phase is assembled to generate the reflectivity image inside the capture volume using Fourier-based reconstruction. A new calibration technique is proposed to compensate for multiplicative path losses and subtract the effect of background clutters, such as furniture. Noisy channels and channels with their line-of-sight blocked by the target are also filtered out during postprocessing to improve locating accuracy and mitigate ambiguity. A 1:6 room model was first used to study the effects of the indoor materials and room layout, where occupant recognition and centimeter-level locating were successfully demonstrated. Real-scale rooms were then tested, where decimeter-level 3-D location error was achieved for a single occupant, and useful information for occupant posture can also be evaluated. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RIS) is considered as a revolutionary technique to improve the wireless system performance by reconfiguring the radio wave propagation environment artificially. Motivated by the potential of RIS in vehicular networks, we analyze the secrecy outage performance of RIS-aided vehicular communications in this paper. More specifically, two vehicular communication scenarios are considered, i.e., a vehicular-to-vehicular (V2V) communication where the RIS acts as a relay and a vehicular-to-infrastructure (V2I) scenario where the RIS functions as the receiver. In both scenarios, a passive eavesdropper is present attempting to retrieve the transmitted information. Closed-form expressions for the secrecy outage probability (SOP) are derived and verified. The results demonstrate the potential of improving secrecy with the aid of RIS under both V2V and V2I communications. [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]

Operating remotely piloted aircraft is not imaginable without a continuous data exchange between the air vehicle and the remote pilot. This data exchange requires reliable data links. One approach for such a data link discussed in the community is a terrestrial system deployed in C-band. A good knowledge of the physical conditions of the communication channel, in this case the air-ground/ground-air channel, is indispensable for the development of wireless data links. Therefore we carried out a 50 MHz bandwidth channel sounding campaign with a terrestrial transmitter and an airborne receiver. In this paper we give a detailed description of our campaign setup and the processing of the collected data. The campaign covered several flight scenarios, such as take-off, taxiing, and multiple en-route maneuvers. We furthermore present results on the received power and the amplitude distribution of the dominant component of the received signal for the different flight scenarios. We observed significant drops in reception power during certain maneuvers that need to be considered in the design process of a data link for unmanned aviation. Additionally, we show that the amplitude distribution follows the distributions commonly used in statistical channel modeling of wireless channels to some extent. We finally present parameter sets for multiple flight scenarios for scaling the amplitude distributions to allow a statistical channel modeling of the reception power of the first resolvable signal path. [ABSTRACT FROM AUTHOR]

Autonomous robotic swarms are envisioned for a variety of sensing applications in space exploration, search and rescue, and disaster management. An important capability of a swarm is sensing spatio-temporal processes such as radio wave propagation and seismic activities. The spa-tio-temporal properties of these processes dictate the required sensing position and time accuracy, as well as update rate. A dedicated wireless communication system needs to be jointly designed for swarm information exchange, self-localization, and sensing. In this article, we emphasize the role of time in a robotic swarm. We introduce the system ingredients and dive into realistic clock models. Clock models and channel access schemes decisively influence the swarm self-localization and synchronization accuracy, and consequently the swarm sensing performance. Finally, we discuss practical implementation aspects, introduce our developed swarm radio system, and give an outlook on a moon-analog exploration mission. [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]

Benefiting from modeling human behavior and the multipath propagation of wireless signals in the environment, passive behavior-sensing systems can effectively identify individual behaviors by model-based or pattern-based methods. The existing methods use an individual sample to model or profile behavior feature, which leads to their failure in multiperson scenarios due to time-varying environments. In this paper, an online method on modeling the position-behavior feature of multiperson scene is presented. First, a position-based model of individual behavioral feature transformation is proposed. The behavior features in other positions are generated by the behavior of the individual in a particular position. Second, a multiperson behavior feature generation method based on noise reduction is proposed to generate the same behavior feature in an online pattern. Finally, taking advantage of the models, a multiperson fitness coaching system is designed and implemented, named multiuser fitness coach. The system can identify the irregular behavior of individuals in the multiperson environment. The performance of the system is evaluated in different scenarios, and the results show that the precision of feature generation can be effectively applied to the decision of irregular behavior in multiperson scenarios. [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]

Although there are several methods described in the literature to accurately measure the complex permittivity of solid dielectrics at radio frequencies in a laboratory environment, none of these methods allow for a large-scale accurate characterization of natural ionic dielectrics. The work presented here reports results of the dielectric permittivity retrieved from in situ measurements in a Romanian salt mine. Measurements were performed in the 164–174-MHz bandwidth, over a propagation distance of 100 m. The characteristics of the layers of sedimentary salt are determined from measurements using a least mean square fitting algorithm, based on a detailed propagation model for a heterogeneous medium. The coupling of the instrumentation is also considered. The proposed approach demonstrates great promise for a large number of applications. [ABSTRACT FROM AUTHOR]

The communications between two driving vehicles along a narrow street may be limited by the presence of a third vehicle blocking the transmission. In this work, we investigate radio wave propagation at 28 GHz in an urban street canyon scenario by conducting channel measurements, where the vehicle(s) occlude(s) the line-of-sight path. We quantify the impact of the car blockage and study the alternative propagation paths, which can be used for establishing a data link. Based on the obtained results, we report that besides the low-loss (3.4 dB) reflection from the wall, a radio link through the blocking car may potentially be established for data sharing. Specifically, the attenuation through clear windows is 2 dB, while the attenuation caused by sun protective film is 15 dB. Diffraction over the car and propagation in foliage reduce the multipath power drastically by 21–24 dB and 16–19 dB, respectively, and cannot be associated with reliable links. Finally, measurement results were compared with the ray-based simulation data, which demonstrate agreement to within ± 4.3 dB of measured losses. [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]

Antennas, and more generally any equipment under test (EUT) capable of coupling to impinging electromagnetic waves, are well-known sources of power loss in reverberation chambers (RCs). As such, it is fundamental to model their contribution to an RC quality factor. The standard model available in the literature is based on power dissipated in loads connected to an antenna. This letter argues about the inaccuracy of this description, where internal dissipation within an antenna (or EUT), associated to potentially low radiation efficiencies, is neglected. An alternative model is validated by showing its self-consistency in predicting change in loading effects as opposed to the standard model. It is expected to have practical importance for tests performed in the low-frequency range of RCs, where antenna-related dissipation is the dominant loss mechanism, and in particular for EUTs which can be expected to display poor radiation efficiencies. [ABSTRACT FROM AUTHOR]

We address the uncertainty of reverberation chamber (RC) measurements in presence of both mechanical and frequency stirring (FS). A base-case model is derived for reverberation fields affected by the measurement uncertainty due to the lack of a perfect statistical uniformity of fields in a RC. It is found that the measurement uncertainty associated with the FS depends on both the total uncorrelated samples and the local insertion loss (IL). The local IL depends on the frequency stirring bandwidth (FSB). The model allows us for obtaining separate measurement uncertainty contributions. Measurements support the achieved uncertainty model. In particular, results show that the dependence on the IL is normally rather weak also when very wide FSBs are used. [ABSTRACT FROM AUTHOR]

In the fifth-generation (5G) system, millimeter-wave (mmWave) communication has been recognized as a candidate technique. To overcome the high transmission loss of mmWave, distributed antenna systems that consist of a base station (BS) connecting with multiple remote radio units (RRUs) were studied to improve the energy efficiency. In this letter, by utilizing the propagation characteristics of mmWave, we propose a cooperative multiuser beamforming method for energy-efficient transmissions in mmWave distributed antenna systems. The RRUs first decide the local serving group separately and the BS makes an optimization globally with limited data exchange. From simulation results, compared to the state-of-the-art method, it is shown that higher energy efficiency can be realized by the proposed method for the multiuser mmWave distributed system. [ABSTRACT FROM AUTHOR]

It has been shown that a class of spatially coupled low-density generator-matrix (SC-LDGM) code ensembles displays distortion saturation for the lossy binary symmetric source coding problem with the belief propagation guided decimation (BPGD) algorithm, i.e., the BPGD distortion approaches the optimal expected distortion of the underlying ensemble asymptotically in code length. We investigate the distortion performance of a practical class of protograph-based SC-LDGM code ensembles and demonstrate distortion saturation numerically. Moreover, taking advantage of the convolutional structure of the SC-LDGM codes, we propose an efficient windowed encoding (WE) algorithm with two decimation techniques for lowering the WE complexity that maintain distortion performance close to the rate-distortion bound. [ABSTRACT FROM AUTHOR]

Cell-free (CF) massive multiple-input multiple-output (MIMO) is an alternative topology for future wireless networks, where a large number of single-antenna access points (APs) are distributed over the coverage area. There are no cells but all users are jointly served by the APs using network MIMO methods. Prior works have claimed that the CF massive MIMO inherits the basic properties of cellular massive MIMO, namely, channel hardening and favorable propagation. In this paper, we evaluate if one can rely on these properties when having a realistic stochastic AP deployment. Our results show that channel hardening only appears in special cases, for example, when the pathloss exponent is small. However, by using 5–10 antennas per AP, instead of one, we can substantially improve the hardening. Only spatially well-separated users will exhibit favorable propagation, but when adding more antennas and/or reducing the pathloss exponent, it becomes more likely for favorable propagation to occur. The conclusion is that we cannot rely on the channel hardening and the favorable propagation when analyzing and designing the CF massive MIMO networks, but we need to use achievable rate expressions and resource allocation schemes that work well also in the absence of these properties. Some options are reviewed in this paper. [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]

Reduction of end-to-end network delay is an optimization task with applications in multiple domains. Low delays enable improved information flow in social networks, quick spread of ideas in collaboration networks, low travel times for vehicles on road networks, and increased rate of packets in the case of communication networks. Delay reduction can be achieved by both improving the propagation capabilities of individual nodes and adding additional edges in the network. One of the main challenges in such network design problems is that the effects of local changes are not independent, and as a consequence, there is a combinatorial search-space of possible improvements. Thus, minimizing the cumulative propagation delay requires novel scalable and data-driven approaches. We consider the problem of network delay minimization via node upgrades. We show that the problem is NP-hard and prove strong inapproximability results about it (i.e., APX-hard) even for equal vertex delays. On the positive side, probabilistic approximations for a restricted version of the problem can be obtained. We propose a greedy heuristic to solve the general problem setting which has good quality in practice, but does not scale to very large instances. To enable scalability to real-world networks, we develop approximations for Greedy with probabilistic guarantees for every iteration, tailored to different models of delay distribution and network structures. Our methods scale almost linearly with the graph size and consistently outperform competitors in quality. We evaluate our approaches on several real-world graphs from different genres. We achieve up to two orders of magnitude speed-up compared to alternatives from the literature on moderate size networks, and obtain high-quality results in minutes on large datasets while competitors from the literature require more than four hours. [ABSTRACT FROM AUTHOR]

In this paper, we study the impact of obstacles (such as buildings) in the radio propagation in the urban vehicular network's environment. The impact has been realized not only by the network-level performance metrics, but also by the application-level performance metrics. A majority of the existing works consider perfect physical layer, which ignores the impact of path loss and shadowing effect in radio propagation caused by the urban canyon. First, we show that ignoring the impact of obstacles in radio propagation provides very high superficial network-level performance. Second, under obstacle shadowing, we perform analytical analysis and Ns-3 based realistic urban vehicular simulation of current dedicated short-range communication based safety message broadcasting to study the achievable performance of application-level performance metrics. This study reveals the answer of the key question, whether the achieved network-level performance metric is good enough for the safety-critical applications under the acute attenuation in radio propagation in urban environment. Third, we propose opportunistic vehicle-assisted or dedicated road side unit-assisted network coded relaying for improving reliability at road intersection. Simulation results show that the proposed approach improves more than 65% reliability for delay sensitive and around 25% reliability for less delay sensitive applications at intersections. [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]