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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.
- Disruptive Future of Radar Based on All-Digital PN Signal ProcessingPublication . Ribeiro, Carlos; Ferreira Gil, João; Caldeirinha, Rafael; Reis, Joao R.; Sardo, Andre; Duarte, Luis; Leonor, NunoThis paper presents the first results on the design and implementation of a real-time and high resolution monostatic radar at 24 GHz, based on the sliding correlation of pseudonoise (PN) sequences. The real-time radar, with a high time resolution better than 4ns, is used for moving target identification (MTI) in the presence of highly dense clutter, under harsh environments and severe weather conditions (fog, snow and fire smoke or plume). A radar signal processing based on alldigital PN sequences is proposed, which represents a quantum leap in radar future front-end architecture. Results obtained in a controlled environment, inside an anechoic chamber, are presented and a benchmark with a commercially-of-the-shelf solution is presented. © 2019 IEEE.
- 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.
- Electronically Reconfigurable FSS-Inspired Transmitarray for 2-D BeamsteeringPublication . Reis, João R.; Caldeirinha, Rafael F. S.; Hammoudeh, Akram; Copner, NigelA novel electronically reconfigurable transmitarray (TA) with 2-D beamsteering capability is presented in this communication. The proposed structure, inspired on frequency selective surfaces loaded with varactor diodes, allows the phase range of each TA element to be individually controlled enabling an automated 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 with active feeding, coupled to the aperture of a standard gain horn antenna. A complete electromagnetic study using CST Microwave Studio is presented to evaluate and characterize the TA elements and the effect the proposed feeding network has on the structure's behavior. Following initial simulations, a prototype of the active TA has been characterized. Automated antenna beamsteering with ranges up to Az = 28° and El = 26° and 1° of angular resolution, is achieved by means of electromagnetic simulations and validated against experimental results at 5.2 GHz.
- Wearable Textile Antennas: examining the effect of bending on their performancePublication . Ferreira, David; Pires, Pedro; Rodrigues, Ruben; Caldeirinha, Rafael F. S.This article presents a study on the effect of bending on the performance of a rectangular textile-patch antenna operating at a 2.4-GHz industrial, scientific, and medical (ISM) band. The substrate of the antenna was made from denim textile, and the conducting layers were made from a copper and nickel plated polyester fabric. A parametric study was made to determine the influence of an antenna bending around its length and width on its performance parameters in chest, leg, arm, or wrist integration for wireless body-area network (WBAN) scenarios. Results were obtained from bench and anechoic chamber measurements and compared with simulation results. The prototype presents a maximum gain of approximately 4 dBi and 70° of half-power beamwidth (HPBW) in the flat position. When subjected to a wrist equivalent bending, the gain decreases by 2 dB, HPBW has an increase of about 25°, and front-to-back radiation ratio decreases. Mean and standard deviation parameters as a function of bending curvature were calculated from parametric simulations.
- Depolarisation studies of single trees at 20 GHzPublication . Caldeirinha, Rafael F.S.; Al-Nuaimi, Miqdad O.This paper addresses the characterization of the polarization states of the re-radiated signal from a natural tree performed in the vicinity around deep signal fades, at 20 GHz. The polarization pattern method has been used to investigate the effects of depolarization and to further characterize the changes in the polarization state of the re-radiated signal occurring as a result of the currents induced in the trunk, branches and leaves, which are significantly influenced by the complex permittivities of the various materials making up the tree. Results presented in the paper will explain the signal behavior observed in the various regions around the tree. The results also show that depolarization, in addition to absorption and scatter, is an important factor formulating accurate models for propagation in vegetation.
- 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.