Telmo R. Fernandes

A discrete RET model for millimeter-wave propagation in isolated tree formations

In this paper, a method based on the Radiative Energy Transfer theory (RET) to estimate the scattered radio signals from isolated groups of trees is presented. The proposed method consists of measuring the re-radiation function of each tree in the group, parameterising the function and subsequently using these in a discrete algorithm to estimate the overall attenuation at any location within the forest scenario. The discrete algorithm (dRET) presented here has some major improvements over previously published ones, offering substantially enhanced applicability. These improvements allow the use of larger vegetation cells, the enhancement of angular resolution of predicted results and the consideration of the receiving antenna radiation pattern. The estimated received signals using the re-radiation function, on the one hand, and its parameterised counterpart, on the other hand, are compared with measurements performed inside an anechoic chamber on Ficus Benjamina indoor plants at 20 and 62.4 GHz. The overall model performance was assessed in terms of RMS error between measured and predicted results.

Square Loop and Slot Frequency Selective Surfaces Study for Equivalent Circuit Model Optimization

This paper presents a parametric study of square loop and square slot frequency selective surfaces (FSSs) aimed at their equivalent circuit (EC) model optimization. Consideration was given to their physical attributes, i.e., the unit cell dimensions and spacing, substrate thickness and dielectric properties, for several frequencies and plane wave incident angles. Correlation analysis and evaluation of the influence of physical related input parameters on the FSS performance are presented. Subsequent optimization factor for the square loop classical EC model is analyzed, and a novel EC model formulation for the square slot FSS is proposed. The performance of the proposed EC model was assessed against results obtained from appropriate electromagnetic (EM) simulations, based on a root-mean-square error (RMSE) criteria. Results demonstrate the validity of the optimized EC model, in which good estimations of the frequency response of FSS structures were obtained. Significant reduction of the resonant frequency offsets, in the order of 650 (from 910 to 260) and 460 (770 to 310) MHz, was obtained for square loops and square slots, respectively. The models were further validated against measurements performed on two physical FSS prototypes inside an anechoic chamber at 2.4 GHz. Relatively good agreement was obtained between measurements of real FSS prototypes and results obtained with the EC model. Finally, this work is sought to provide the necessary refinement of elementary models for further studies with more complex and novel FSS structures.

A 2D Ray-Tracing Based Model for Micro- and Millimeter-Wave Propagation Through Vegetation

A novel two dimensional model to characterize the electromagnetic behavior of trees has been developed with the purpose of being used in ray-tracing based simulation platforms. This model uses various point scatterers with specific radiation characteristics to describe the effect of the trees present in the radiowave propagation path. The method to extract the parameters of the point scatterers from measurements is presented. The performance of this novel formulation is assessed in a tree formation scenario against measurements results obtained in a controlled environment, inside an anechoic chamber, at 20 and 62.4 GHz. Additionally, a comparison analysis with a discretized radiative energy transfer (dRET) approach is conducted, where a relatively good agreement has been found. The absence of readily plug-in models considering propagation through and/or around vegetation makes this new tool interesting for radio planning purposes.

Time-Variant Radio Channel Characterization and Modelling of Vegetation Media at Millimeter-Wave Frequency

Results of an extensive measurement campaign to characterize and model dynamic effects on the radio channel in vegetation media at 40 GHz, are presented. Using two small trees in an anechoic chamber, narrowband fast-fading measurements, utilizing co-polarized signals, were conducted to obtain radio channel characterization. This was performed as a function of the bi-static scattering angle, wind speed, wind incidence, and tree species. Furthermore, a modelling methodology is investigated and assessed as to its feasibility as a means to model vegetation dynamics effects on propagation. A radio channel model based on a simplified set of Markovian parameters, is proposed. The model is representative of the radio signal in the three main identified propagation regions around a vegetation volume as a mean to satisfactorily model highly time-variant radio signals. The propagation regions considered are receiver angular sections (in the azimuth plane) around the tree, where the received signal behaves distinctly for each region. Model validation results are presented using one of the tree species.

Hybrid FSS and Rectenna Design for Wireless Power Harvesting

This communication presents a hybrid frequency selective surface (FSS) and rectenna design for wireless power harvesting. To this extent, the center of the original FSS structure acts as a probe-fed patch antenna and an additional layer was added to the rear of FSS structure to mimic the required antenna ground plane, while minimizing its impact on FSS performance. The proposed hybrid solution yields a bandpass frequency response at GSM 900-MHz frequency band, with band reject behavior at 2.4 GHz and, on the other hand, act as a energy harvesting system through the antenna element at the 2.4-GHz ISM band, e.g., for recycling ambient Wi-Fi energy. The developed prototype, consisting of an array of 12 unit cells, was able to power a 56-mW LED at a 2-Hz rate from an incident power level of -10 dBm, resulting in an average output power of 300 μW and an unit cell RF-DC conversion efficiency of 50%.

Performance and evaluation of OFDM and SC - FDE over an AWGN propagation channel under RF impairments using simulink at 60GHz

This paper presents an implementation of the IEEE 802.15.3c standard, in Simulink, for high data-rate applications. The recent standard IEEE 802.15.3c provides the implementation of such wireless communication system using the 60GHz frequency band. OFDM and SC-FDE are the two transmission schemes adopted by the standard. Performance of these technologies at mm-wave signals are severely affected by nonlinearity of the RF front-ends. This paper presents the impact of RF impairments for each transmission scheme along with comparative analysis, in terms of BER and PSNR.

FSS-Inspired Transmitarray for Two-Dimensional Antenna Beamsteering

A novel two-dimensional (2-D) beamsteering transmitarray inspired on frequency selective surfaces is proposed in this paper. Individual phase range is applied to each transmitarray element and thus enabling the steering of the main lobe of an original antenna pattern in both elevation and azimuth planes (2-D-beamsteering). This has been demonstrated on a 5 × 5 unit-cell stacked structure, loaded with surface mount capacitors, coupled to the aperture of a standard gain horn antenna. Additionally, a complete parametric study based on electromagnetic (EM) simulations using CST Microwave Studio is presented to evaluate and characterize unit-cell parameters and the behavior of the array with various numbers of stacked layers and separation distances. Antenna beamsteering with ranges up to Az = 25° and El = 25° are achieved by means of EM simulations and validated against experimental results performed on a physical prototype at 5.3 GHz. The comparisons yield positive results and clearly demonstrate the potential and limitations of using such structures for beamsteering purposes.

A Simple Model for Average Reradiation Patterns of Single Trees Based on Weighted Regression at 60 GHz

Due to their complex and inhomogeneous characteristics, the propagation and radiation parameters of trees and vegetation areas are very difficult and time consuming to obtain. This communication proposes a statistical method, using robust weighted local regression, to minimize the influence of the effect of the tree inhomogeneity on its reradiation pattern, allowing the evaluation of averaged reradiation functions from simple measurements. The proposed method was successfully applied to six tree specimens of both conifer and ficus species, at 60 GHz. Furthermore, once this empirical averaged function is obtained, first-order statistics can be applied to generate several simple reradiation functions, statistically identical to those obtained from measurements. Thus, allowing the full characterization of the tree under study.

3-D Mechanically Tunable Square Slot FSS

We introduce an innovative 3-D mechanically tunable frequency selective surface (FSS), which is inspired by the classical flat square slot FSS. The proposal improves the performance of classical 2-D FSS designs, and it also represents a novel method of achieving mechanical frequency tuning, despite other 3-D designs that consist of a collection of stacked 3-D layers exist. In our proposal, the rotation of an inner element provides tuning capability to the squared cell structure, consisting of metallic grids with a movable inner element. An aluminum prototype was built, which can be tuned from 2.4 to 4 GHz, and also compared its measured performance and numerical simulations. Some characteristics of the proposed structure are the rejection level at main polarization, up to 20 dB, and the maximum frequency sweep of approximately 50% of the fundamental frequency. The prototype showed a stable frequency response for angles of incidence up to 45°. Since results are in good agreement with simulations, we provide parametric equations to design 3-D structures at desired frequencies.

Retrieving Vegetation Reradiation Patterns by Means of Artificial Neural Networks

Modeling vegetation usually implies obtaining experimental data by means of measurement campaigns. In general, the most accurate models need the reradiation pattern of the vegetation volume under study, or some of its related parameters. Obtaining this function might not be easy, and the measurement procedure may introduce errors depending on the location of the vegetation mass. An accurate tool to simplify this process is presented, based on the ability of artificial neural networks to infer behavioral patterns. This proposal reduces the acquisition of the experimental data to a scarce set of angles around the tree or bush, which will then be used to train a neural network capable of retrieving the desired reradiation function desired.