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- Simplified RET model derived from path loss and directional spectrum measurements in vegetation media at 11.2 and 20 GHzPublication . Caldeirinha, Rafael F. S.; Fernandes, Telmo; Richter, Jürgen; Al-Nuaimi, Miqdad O.This study presents a simplified radiative energy transfer (RET) model derived from path loss and directional spectrum measurements in vegetation media at 11 and 20 GHz. Thus, a simplified extraction procedure for the inputs parameters based on the method foreseen by the international telecommunications union - recommendation (ITU-R) P.833-8, however for isolated vegetation volumes, is proposed. Results for specific measurements aimed at the extraction of the RET model parameters are subsequently presented and discussed. The extraction method relies on a limited set of propagation measurements to find and optimise the RET input parameters. Specific measurements of the received signal directional profile are used to show the applicability of the method. This parameter evaluation method is tested and assessed based on the received signal directional profile measurements performed in Populous trees at 11.2 and 20 GHz.
- 3-D Mechanically Tunable Square Slot FSSPublication . Ferreira, David; Cuinas, Inigo; Caldeirinha, Rafael F. S.; Fernandes, TelmoWe 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.
- Input parameter extraction method for point scatterer formulation in vegetation media at millimetre-wave frequenciesPublication . Leonor, Nuno R.; Caldeirinha, Rafael F. S.; Sánchez, Manuel García; Fernandes, TelmoAn empirical method to extract the relevant point scatterer formulation input parameters for appropriate radio coverage simulation in vegetation areas, is proposed. While the original propagation model would require the prior knowledge of the complete tree(s) re-radiation function(s) to extract the relevant propagation input parameters, the empirical method proposed herein is based only on two simple measurements. Re-radiation measurements of several specimens of Thuja pelicata and Ficus benjamina trees at two frequencies have been used to validate the proposed simplified extraction method. The proposed empirical model was then used to characterise the propagation phenomena of various trees arranged in four different scenarios. Subsequently, directional spectra measurements performed inside an anechoic chamber at both 20 and 62.4 GHz were used to assess the empirical model performance, while predicting the received signal level arriving from various directions. A relatively good overall model performance when applied to tree formations was obtained which, coupled with the simplified parameter extraction proposed, makes it suitable to be integrated in commercial simulation platforms and/or as input data to other propagation models that may require the prior knowledge of the individual tree re-radiation functions.
- Conjugate impedance matched metamaterials: physical modeling and wave interaction effectsPublication . Maslovski, Stanislav I.; Ferreira, Hugo R. L.; Medvedev, Iurii O.; Brás, Nuno G. B.; Abraray, Abdelghafour; Fernandes, Telmo; Silva, Henrique J. A.; Topa, António L.Conjugate impedance matched metamaterials are shown to be effective traps for electromagnetic waves. Objects made of such materials are able to receive radiation even when it is not directly incident on their surface. Here, we develop methods of physical modeling of such objects and investigate interactions of conjugate impedance matched superabsorbers with passing electromagnetic radiation. We study realizations of such metamaterials with meshes of loaded transmission lines and develop a theory of electromagnetic wave propagation and absorption in such media. Peculiar wave propagation, wave trapping and absorption effects in metamaterial black holes and wormholes are demonstrated. Possible modifications under the goal of optimizing absorption while minimizing complexity of the involved metamaterials are discussed. Conjugate-impedance matched superabsorbers may find applications as efficient harvesters of electromagnetic radiation, and as novel antennas and sensors.
- Towards 5G : performance evaluation of 60 GHz UWB OFDM communications under both channel and RF impairmentsPublication . Gomes, Rodolfo; Hammoudeh, Akram; Caldeirinha, Rafael F. S.; Al-Daher, Zaid; Fernandes, Telmo; Reis, JoãoDetailed analysis on the impact of RF and channel impairments on the performance of Ultra-Wideband (UWB) wireless Orthogonal Frequency Division Multiplexing (OFDM) systems based on the IEEE 802.15.3c standard, for high data-rate applications using the 60 GHz millimetre frequency band is presented in this paper. This frequency band, due to the large available bandwidth is very attractive for future and 5G wireless communication systems. The usage of OFDM at millimetre-wave (mmWaves) frequencies is severely affected by non-linearities of the Radio Frequency (RF) front-ends. The impact of impairments is evaluated, in terms of some of the most important key performance indicators, including spectral efficiency, power efficiency, required coding overhead and system complexity, Out-Of-Band Emissions (OOBEs), Bit Error Rate (BER) target and Peak Signal-to-Noise Ratio (PSNR). Additionally, joint distortion effects of coexisting Phase-Noise (PN), mixer IQ imbalances and Power Amplifier (PA) non-linearities, on the performance degradation of a mmWave radio transceiver, combined with various multipath fading channels, are investigated. Subsequently, the power efficiency of the system is evaluated by estimating values of the PA Output-Power-Backoff (OBO) needed to meet the requirements for the Transmit Spectrum Mask (TSM) and BER target. Finally, a comparison of the system overall performance between uncoded and coded OFDM systems combined with Quadrature Amplitude Modulations (16 and 64 QAM) and its maximum operable range are evaluated by transmitting a Full HD uncompressed video frame under five different RF impairment conditions over a typical LOS kiosk 60 GHz IEEE channel model.
- A 2D ray-tracing based model for wave propagation through forests at micro-and millimeter wave frequenciesPublication . Leonor, Nuno R.; Sanchez, Manuel Garcia; Fernandes, Telmo; Caldeirinha, Rafael F. S.This paper proposes the extension of a 2-D ray-tracing-based model for radiowave propagation in the presence of trees and vegetation areas to include real-sized trees and outdoor forest scenarios. The original propagation model proved to be suitable to characterize the electromagnetic behavior in the presence of indoor tree formation scenarios, despite some limitations found when applied to real-sized trees. In addition, the original propagation model requires the prior knowledge of the trees’ re-radiation function to extract the relevant propagation input parameters, which is not always possible to obtain in outdoor scenarios. Therefore, an empirical method to extract the relevant input propagation parameters based on simple measurements is proposed. The performance of the proposed propagation model extension is extensively assessed in both the line-of-trees and tree formation scenarios, including various (and mixed) species, both in- and out-of-leaf foliation states, and at three signal frequencies. Finally, depending on the type of scenario, a benchmark between the proposed propagation model and both the radiative energy transfer (RET) and discrete RET (dRET) models, for line-of-trees and tree formation, respectively, is presented.
- A Multilayer EM Simulation Tool to Assess RF Transparency Control of Building Wall StructuresPublication . Caldeirinha, Rafael; Ferreira, David; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.; Cuinas, InigoThis paper presents a multilayer electromagnetic (EM) simulation tool to assess the radiofrequency transparency control of building wall structures, developed in Matlab environment, that allows one to explore different layer arrangements and determine the overall frequency response. In particular, the deployment of frequency selective surfaces, either passive or active (electronically controlled), as well as the assessment of EM properties of different types of building walls commonly used in building construction, combined into one complete EM tool, is proposed.
- Will COTS RF Front-Ends Really Cope With 5G Requirements at mmWave?Publication . Gomes, Rodolfo; Sismeiro, Luis; Ribeiro, Carlos; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.; Garcia Sanchez, Manuel; Hammoudeh, Akram; Caldeirinha, RafaelIn this paper, the real-time over-the-air multi-gigabit wideband wireless orthogonal frequency division multiplexing (OFDM) system performance, using two distinct radio frequency (RF) front-ends designed for the millimetre frequency band (mmWave), is evaluated. A thorough comparative analysis between 60-GHz cutting-edge commercial off-the-shelf and reference RF front-ends, taking into account their intrinsic RF impairments and their impact on the overall system performance degradation, is presented. With this, it is possible to verify whether system on chip technology for RF circuits is ready for 5G systems. Due to the large available bandwidth, the mmWave band is very attractive for future 5G wireless communication systems, which might provide transmission data rates over 10 Gb/s and network latency below 1 ms. However, non-linearities of RF front-ends, at these frequencies, severely affect the performance of transmission schemes. In fact, this is the main cause for performance degradation on OFDM systems. It causes significant limitation on both maximum spectral efficiency and data rate of the wireless communication link. Therefore, the OFDM performance degradation due to the non-linearities from both analogue stages of upconversion/downconversion is evaluated considering important key performance indicators for quality of service in digital communications considering 4-, 16-, 64- and 256-quadrature amplitude modulation. Furthermore, based on the results obtained from appropriate RF measurements, a practical EVM link budget formulation for mmWave systems, which takes into account both PA and PN impairment effects, is proposed.
- Using artificial neural networks to scale and infer vegetation media phase functionsPublication . Gómez-Pérez, Paula; Caldeirinha, Rafael; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.; Cuiñas, IñigoAccurate vegetation models usually rely on experimental data obtained by means of measurement campaigns. Nowadays, RET and dRET models provide a realistic characterization of vegetation volumes, including not only in-excess attenuation, but also scattering, diffraction and depolarization. Nevertheless, both approaches imply the characterization of the forest media by means of a range of parameters, and thus, the construction of a simple parameter extraction method based on propagation measurements is required. Moreover, when dealing with experimental data, two common problems must be usually overcome: the scaling of the vegetation mass parameters into different dimensions, and the scarce number of frequencies available within the experimental data set. This paper proposes the use of Artificial Neural Networks as accurate and reliable tools able to scale vegetation parameters for varying physical dimensions and to predict them for new frequencies. This proposal provides a RMS error lower than 1 dB when compared to unbiased measured data, leading to an accurate parameter extracting method, while being simple enough for not to increase the computational cost of the model.
- A Discrete RET Model for Millimeter-Wave Propagation Through VegetationPublication . Leonor, Nuno R.; Caldeirinha, Rafael; Fernandes, Telmo, Telmo Rui Carvalhinho Cunha, Telmo R.; Richter, Jurgen; Al-Nuaimi, MiqdadA modified discrete radiative energy transfer (dRET) model has been developed envisaging to overcome some of the limitations found in the original dRET formulation, namely in terms of angular resolution, oblique and nonuniform incidence, and antenna patterns. In addition, the discretized squared nature of the dRET model can lead to some amount of staircase error. Hence, this paper also proposes an input parameter scaling model, enabling the characterization of tree cells with reduced size. The performance of the modified dRET model is assessed not only against the current ITU-R P.833-8 recommendation for propagation in vegetation but also against directional spectra measurements conducted in an outdoor inhomogeneous forest at 20 and 62.4 GHz.