Studio e implementazione di tecniche di accelerazione dell'algoritmo di ray tracing per la stima di campi elettromagnetici

Questo elaborato affronta una tecnica di accelerazione di un algoritmo di ray-tracing basata sulle informazioni di visibilità piastra-piastra, raccolte sotto forma di una matrice. Vengono inoltre proposti due modelli empirici, COST-Hata e COST-Walfish-Ikegami, e un modello ibrido per la predizione del campo. Si chiude con l’implementazione della possibilità di selezionare porzioni dello scenario mantenendo una adeguata accuratezza nella determinazione del campo. Vengono altresì mostrati i risultati sperimentali delle simulazioni su alcuni scenari per mostrare la validità delle approssimazione e i guadagni temporali delle modifiche introdotte sul software EMvironment 3.0

On the use of electromagnetic asymptotic methods for the estimation of communications propagation channel in complex environments

The ray tracing method and the high frequency theories this method is based on are recapitulated in chapter 1 in the formulation implemented in the software EMviroment-EMv used in this thesis to predict the electromagnetic propagation in complex environment. The estimation of channel parameters by asymptotic propagation model based software could show some drawbacks, such as long simulation times caused by the multipath reconstruction, and a huge memory dimension requirement to store the multipath info that is useful for the frequency analysis of the channel. A lot of energies were spent in optimizing and speeding up the ray tracing algorithms by researchers in the last decades. In the implementation of ray tracing based solver the study of the acceleration of the geometrical algorithm to predict the multipath has a great importance, because the physical coherence of this deterministic approach leads to a boost of the computational time. A method for speeding up a ray tracing based electric field prediction model suitable for urban environments investigation is described in chapter 2. The physical coherence of this high frequency method allows to accurately estimate the wireless communication channel frequency response and the antennas influence on the transmitted and received signals. A frequency analysis of the channel is required, for example, in ultra-wideband applications. In this case the frequency selective behavior of the channel needs to be estimate to correctly predict the communication link impairment. The frequency response definition by multipath prediction is shown in chapter 3 The Doppler frequency shift is caused by the presence of relative movement by the transmitter, the receiver and the complex objects present in the scene. The power distribution in the Doppler frequency shift domain can be derived by the spreading function estimation. By definition the spreading function could be calculated starting from the knowledge of the impulse response of a channel link by means of the Fourier transform. An alternative direct deterministic approach for its estimation avoiding the Fourier transform has been formulated and is presented in chapter 4 applied to the Munich airport complex scenario. The radar cross section evaluation of metal and dielectric objects needs to invoke the asymptotic techniques to overcome the limitations the full-wave techniques present at high frequencies. The method of moments, for example, needs a denser and denser mesh definition of the studied object as the wavelength decreases to respect the applicability constraints of the method. Because the complexity of this method is proportional to the currents on the facets, the number of unknowns can get to saturate the computer memory availability and increase the simulation time. The approximations that can be introduced with the assumption of far-field sources and observers at high frequencies allow to unburden the calculation procedures obtaining good estimations of the scattered field. The Physical Optics - PO theory is the most diffuse solution solving these kind of problems, often together with a ray bouncing analysis to predict the illuminated facets of the object. A PO based solver to predict the scattered field by a metallic or dielectric object and the radar cross section spectrum of rotating objects is presented in chapter 5

A stepwise transmission/reflection multiline-based algorithm for broadband permittivity measurements of dielectric materials

Transmission/reflection (T/R) techniques for measuring dielectric material's complex permittivity are broadband but have usually problems when the length of the tested sample is a multiple of λ/2. In this paper, we apply a stepwise scheme to a multiline T/R measurement method that solve those ambiguity problems thus allowing wideband and accurate permittivity estimation

Indoor channel characterization for future 5G applications

The shortage of frequency band below 6 GHz available for communications and data transfer has recently fostered the interest toward the millimeter wave (mmW) spectrum. In fact, mmW carrier frequencies allow for larger bandwidth allocations thus higher data transfer rates. It is therefore useful to evaluate the channel propagation properties of mmW within an indoor environment. In particular, the statistical parameters such as path loss exponent and shadowing have been examined by using a reliable numerical solver based on a ray-tracing (RT) technique. The results for both line-of-sight (LOS) and non-line-of-sight (NLOS) conditions at 28 GHz and 72 GHz are reported for the case of an office environment

Mortality and pulmonary complications in patients undergoing surgery with perioperative SARS-CoV-2 infection: an international cohort study

Background: The impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on postoperative recovery needs to be understood to inform clinical decision making during and after the COVID-19 pandemic. This study reports 30-day mortality and pulmonary complication rates in patients with perioperative SARS-CoV-2 infection. Methods: This international, multicentre, cohort study at 235 hospitals in 24 countries included all patients undergoing surgery who had SARS-CoV-2 infection confirmed within 7 days before or 30 days after surgery. The primary outcome measure was 30-day postoperative mortality and was assessed in all enrolled patients. The main secondary outcome measure was pulmonary complications, defined as pneumonia, acute respiratory distress syndrome, or unexpected postoperative ventilation. Findings: This analysis includes 1128 patients who had surgery between Jan 1 and March 31, 2020, of whom 835 (74·0%) had emergency surgery and 280 (24·8%) had elective surgery. SARS-CoV-2 infection was confirmed preoperatively in 294 (26·1%) patients. 30-day mortality was 23·8% (268 of 1128). Pulmonary complications occurred in 577 (51·2%) of 1128 patients; 30-day mortality in these patients was 38·0% (219 of 577), accounting for 81·7% (219 of 268) of all deaths. In adjusted analyses, 30-day mortality was associated with male sex (odds ratio 1·75 [95% CI 1·28–2·40], p\textless0·0001), age 70 years or older versus younger than 70 years (2·30 [1·65–3·22], p\textless0·0001), American Society of Anesthesiologists grades 3–5 versus grades 1–2 (2·35 [1·57–3·53], p\textless0·0001), malignant versus benign or obstetric diagnosis (1·55 [1·01–2·39], p=0·046), emergency versus elective surgery (1·67 [1·06–2·63], p=0·026), and major versus minor surgery (1·52 [1·01–2·31], p=0·047). Interpretation: Postoperative pulmonary complications occur in half of patients with perioperative SARS-CoV-2 infection and are associated with high mortality. Thresholds for surgery during the COVID-19 pandemic should be higher than during normal practice, particularly in men aged 70 years and older. Consideration should be given for postponing non-urgent procedures and promoting non-operative treatment to delay or avoid the need for surgery. Funding: National Institute for Health Research (NIHR), Association of Coloproctology of Great Britain and Ireland, Bowel and Cancer Research, Bowel Disease Research Foundation, Association of Upper Gastrointestinal Surgeons, British Association of Surgical Oncology, British Gynaecological Cancer Society, European Society of Coloproctology, NIHR Academy, Sarcoma UK, Vascular Society for Great Britain and Ireland, and Yorkshire Cancer Research

An Accurate Link Budget Estimation of UAV Considering the Degradation of Antenna Patterns

The operating environment introduces distortions on the antenna patterns, that are not considered in the classical application of the link budget formula that approximates the gain values of the antennas with their maximum value. The proposed method considers the antenna patterns modified by the environment as well as the flight conditions of UAV by means of full-wave or ray tracing simulations. The comparison between measurement and simulation shows that the proposed model is accurate and suitable for the design of the radio system of UAV

Fast Optimization Procedure for the Synthesis of Arbitrarily Conformable Magnetic Metasurfaces

In recent times, important research has been carried out within the wireless power transfer (WPT), due to the ever increasing demand for contactless charging systems [1]. The inductive WPT is characterized by the rapid decay of near-field components. The magnetic field decays from the active coil proportionally to the cube of the distance from the passive receiver coil. This is why research in this sector has been oriented towards solutions that improve efficiency and working distance. Many solutions to enhance the performance of an inductive link have been presented in the literature. The three-coil system is probably one of the most common solutions, but at the same time efficient and compact. Instead of using just a driver circuit inductively coupled to a receiver (both resonating at the same frequency), an additional intermediate resonant passive coil is introduced. This transmit coil is strongly coupled to both the driver coil and the receiver. In this way, very small currents in the driver are enough to induce a large amount of current flowing into the transmitter. Our approach is applied to a similar three-coil system, but the intermediate passive coil is substituted by a three-dimensional distribution of small size loops. The system consists of two parallel coils, a transmitter and a receiver, and a planar metasurface placed orthogonally to the plane of the main coils. In particular, the surface is constituted by a planar distribution of unit cells of circular shape, and the size of the unit cell is very small w.r.t the operative wavelength (less than 10 MHz). The aim of our work is to optimize the distribution of currents on the surface by maximizing the magnetic flux in the receiving coil by a new method based on the analytical evaluation of the contribution of the single loop at the total magnetic flux. The best distribution of currents on the surface is obtained by our method in three steps. Firstly, the total magnetic flux from both the driving coil and the metasurface concatenated with the receiver is calculated on the assumption that each coil is radiating as an elementary magnetic dipole fed with unitary source. Secondly, the optimum currents of each unit cell are determined by means of an optimizer whose cost function is the maximization of the total magnetic flux on the receiving coil. Subsequently, in order to achieve the desired currents on each loop of the surface, the loads to be added in series to the ports of the loops are computed by applying the theory developed in [2]. Because this method depends on the matrix of the impedances of the loops present in the scene, evaluable by a full-wave simulation, the real coupling between the unit cells is considered. The proposed procedure allows to design magnetic metasurfaces with arbitrary shapes and able to synthetize the magnetic field distribution required for a given application with a high degree of freedom. A further objective of this work is the application of the presented method to WPT scenarios where an arbitrary distributions of unit cells in space can be the enabling step in order to find the best configuration that maximize the coupling between the transmitting coil and the receiving one

Fast RCS prediction of electrically large targets coated with radar absorbing materials

A method for a fast prediction of the Radar Cross Section (RCS) reduction of large scattering objects coated with radar absorbing materials is presented. The proposed technique is based on the identification of the dominant hotspots of the target from an Inverse Synthetic Aperture Radar (ISAR) image generated over an azimuthal angle of 360°. This ISAR image is the result of the superposition of multiple ISAR images obtained by electromagnetic simulations based on a GO-PO method of the metallic target (without radar absorbing materials). The lowered RCS of the target covered with radar absorbing materials is estimated by a proper attenuation of the hotspots treated with radar absorbing materials in the single ISAR images. The proposed method is applied to some test cases, i.e. a rocket and a ship analysed at the central frequency of 20 GHz

Phase-coded Magnetic Metasurfaces for Interference Free Multi-receiver Wireless Power Transfer System

In this study, the performance of a resonant inductive wireless power transfer system has been enhanced by introducing a phase-coded low-frequency magnetic metasurface in a multi-receiver arrangement. The magnetic metasurface, composed of an array of spirals cylindrically located in 3D space and loaded with a suitable set of capacitors, is able to both control the amplitude and phase of the magnetic field transferred to the receivers. By carrying out accurate full-wave simulations, we proved that it is possible to obtain two spatial regions characterized by a magnetic field distribution constant in amplitude and 180° out-of-phase even by adopting a single driving coil. Thus, two different receivers can be positioned in the aforementioned regions, obtaining two correspondingly out-of phase useful currents. In this way, power transfer can be combined with interference-free telemetry for both the receivers, opening the way for a new generation of wireless power transfer systems