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  • Publication
    Robust signaling for bursty interference
    (MDPI, 2018-11-12) Villacrés Estrada, Grace Silvana; Koch, Tobias Mirco; Sezgin, Aydin; Vázquez Vilar, Gonzalo
    This paper studies a bursty interference channel, where the presence/absence of interference is modeled by a block-i.i.d. Bernoulli process that stays constant for a duration of T symbols (referred to as coherence block) and then changes independently to a new state. We consider both a quasi-static setup, where the interference state remains constant during the whole transmission of the codeword, and an ergodic setup, where a codeword spans several coherence blocks. For the quasi-static setup, we study the largest rate of a coding strategy that provides reliable communication at a basic rate and allows an increased (opportunistic) rate when there is no interference. For the ergodic setup, we study the largest achievable rate. We study how non-causal knowledge of the interference state, referred to as channel-state information (CSI), affects the achievable rates. We derive converse and achievability bounds for (i) local CSI at the receiver side only; (ii) local CSI at the transmitter and receiver side; and (iii) global CSI at all nodes. Our bounds allow us to identify when interference burstiness is beneficial and in which scenarios global CSI outperforms local CSI. The joint treatment of the quasi-static and ergodic setup further allows for a thorough comparison of these two setups.
  • Publication
    Stability analysis and design of negative impedance converters: application to circuit and small antennas
    (Radioengineering Society, 2016-09) Segovia Vargas, Daniel; Jimenez Martin, Jose Luis; Parra Cerrada, Ángel; Albarracín Vargas, Luis Fernando; Ugarte Muñoz, Eduardo; González Posadas, Vicente
    Negative impedance converters (NICs) have been proposed as structures to improve the performance of RF circuits and electrically small antennas. However, NICs suffer from stability problems. This paper presents a compact procedure to analyze the stability of NICs. Then, the required and sufficient conditions to predict the stability of a negative impedance converter are given. These conditions can be evaluated using standard computer-aided design software. Finally, a NIC prototype is given to validate and illustrate the presented design procedure, it is also integrated with a printed, blade-type, electrically small monopole in the VHF band.
  • Publication
    An IoT reader for wireless passive electromagnetic sensors
    (MDPI, 2017-04) Galindo Romera, Gabriel; Carnerero Cano, Javier; Martínez Martínez, José Juan; Herraiz Martínez, Francisco Javier
    In the last years, many passive electromagnetic sensors have been reported. Some of these sensors are used for measuring harmful substances. Moreover, the response of these sensors is usually obtained with laboratory equipment. This approach highly increases the total cost and complexity of the sensing system. In this work, a novel low-cost and portable Internet-of-Things (IoT) reader for passive wireless electromagnetic sensors is proposed. The reader is used to interrogate the sensors within a short-range wireless link avoiding the direct contact with the substances under test. The IoT functionalities of the reader allows remote sensing from computers and handheld devices. For that purpose, the proposed design is based on four functional layers: the radiating layer, the RF interface, the IoT mini-computer and the power unit. In this paper a demonstrator of the proposed reader is designed and manufactured. The demonstrator shows, through the remote measurement of different substances, that the proposed system can estimate the dielectric permittivity. It has been demonstrated that a linear approximation with a small error can be extracted from the reader measurements. It is remarkable that the proposed reader can be used with other type of electromagnetic sensors, which transduce the magnitude variations in the frequency domain.
  • Publication
    A Near-Field Split-Ring Resonator-Based Monopole Sensor for Permittivity Characterization
    (MDPI, 2017-12-18) Carnerero Cano, Javier; Galindo Romera, Gabriel; Martínez Martínez, José Juan; Herraiz Martínez, Francisco Javier
  • Publication
    Strategies to parallelize a finite element mesh truncation technique on multi-core and many-core architectures
    (Springer, 2023-05) Badía, José M.; Amor Martín, Adrián; Belloch Rodríguez, José Antonio; García Castillo, Luis Emilio; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España)
    Achieving maximum parallel performance on multi-core CPUs and many-core GPUs is a challenging task depending on multiple factors. These include, for example, the number and granularity of the computations or the use of the memories of the devices. In this paper, we assess those factors by evaluating and comparing different parallelizations of the same problem on a multiprocessor containing a CPU with 40 cores and four P100 GPUs with Pascal architecture. We use, as study case, the convolutional operation behind a non-standard finite element mesh truncation technique in the context of open region electromagnetic wave propagation problems. A total of six parallel algorithms implemented using OpenMP and CUDA have been used to carry out the comparison by leveraging the same levels of parallelism on both types of platforms. Three of the algorithms are presented for the first time in this paper, including a multi-GPU method, and two others are improved versions of algorithms previously developed by some of the authors. This paper presents a thorough experimental evaluation of the parallel algorithms on a radar cross-sectional prediction problem. Results show that performance obtained on the GPU clearly overcomes those obtained in the CPU, much more so if we use multiple GPUs to distribute both data and computations. Accelerations close to 30 have been obtained on the CPU, while with the multi-GPU version accelerations larger than 250 have been achieved.
  • Publication
    A compact flexible circularly polarized implantable antenna for biotelemetry applications
    (Tech Science Press, 2022) Sarosh, Ahmad; Ullah, Shakir; Ghaffar, Adnan; Segovia Vargas, Daniel
    With the help of in-body antennas, the wireless communication among the implantable medical devices (IMDs) and exterior monitoring equipment, the telemetry system has brought us many benefits. Thus, a very thin-profile circularly polarized (CP) in-body antenna, functioning in ISM band at 2.45 GHz, is proposed. A tapered coplanar waveguide (CPW) method is used to excite the antenna. The radiator contains a pentagonal shape with five horizontal slits inside to obtain a circular polarization behavior. A bendable Roger Duroid RT5880 material (εr = 2.2, tanδ = 0.0009) with a typical 0.25 mm-thickness is used as a substrate. The proposed antenna has a total volume of 21 × 13 × 0.25 mm3. The antenna covers up a bandwidth of 2.38 to 2.53 GHz (150 MHz) in vacuum, while in skin tissue it covers 1.56 to 2.72 GHz (1.16 GHz) and in the muscle tissue covers 2.16 to 3.17 GHz (1.01 GHz). GHz). The flexion analysis in the x and y axes was also performed in simulation as the proposed antenna works with a wider bandwidth in the skin and muscle tissue. The simulation and the curved antenna measurements turned out to be in good agreement. The impedance bandwidth of −10 dB and the axis ratio bandwidth of 3 dB (AR) are measured on the skin and imitative gel of the pig at 27.78% and 35.5%, 13.5% and 4.9%, respectively, at a frequency of 2.45 GHz. The simulations revealed that the specific absorption rate (SAR) in the skin is 0.634 and 0.914 W/kg in muscle on 1g-tissue. The recommended SAR values are below the limits set by the federal communications commission (FCC). Finally, the proposed low-profile implantable antenna has achieved very compact size, flexibility, lower SAR values, high gain, higher impedance and axis ratio bandwidths in the skin and muscle tissues of the human body. This antenna is smaller in size and a good applicant for application in medical implants.
  • Publication
    Propagation of light in the presence of gravity generated by static and spherically symmetric curved space-times using Maxwell equations
    (Springer, 2022-12-29) Falcón-Gómez, Enderson; Amor-Martín, Adrián; De la Rubia, Valentín; Santamaría-Botello, Gabriel Arturo; De Falco, Vittorio; García Muñoz, Luis Enrique; Comunidad de Madrid
    In this manuscript, we present an alternative method for calculating null geodesics in General Static Isotropic Metrics in General Relativity and Extended Theories of Gravity. By applying a conformal transformation, we are able to consider an analogue gravity model, where curvature is encoded in the dielectric and magnetic properties of a medium. In other words, we pass from curved to flat space-times, where instead of the Einstein field equations, the Maxwell equations are solved. Within this geometrical background, the photon geodesics are calculated. Then, given different black hole and wormhole metrics, we apply this method obtaining an excellent agreement with respect to the exact solutions in the original gravity framework by committing angular deviations below 3∘ . Finally, we provide the image of a Schwarzschild black hole surrounded by a thin accretion disk, and the apparent image of a Morris and Thorne-like wormhole within an angular discrepancy below 4∘ .
  • Publication
    High-gain, circularly-polarized THz Antenna with proper modeling of structures with thin metallic walls
    (IEEE, 2020-07-06) Warmowska, Dominika; Abdalmalak Dawoud, Kerlos Atia; García Muñoz, Luis Enrique; Raida, Zbynek; Comunidad de Madrid; European Commission
    In the paper, a corporate feed antenna array with slot radiators and a layer of polarizing patches is designed to operate at 350 GHz. The antenna is conceived as a silicon structure fully metalized by gold. Different methods for modeling thin gold layers at terahertz frequencies are compared and the optimum approaches are chosen depending on the thickness of the metal layer and its relation to the skin depth. Two designed antenna arrays are in 2×2 and 4×4 configuration, radiate circularly polarized waves, and have high gains of 13.8 dBic and 18.4 dBic, respectively. Antenna arrays have a low profile of only 1.3 Lambda 0 . Cheaper and more available manufacturing technology is presented and discussed in detail together with the measurement results of three manufactured prototypes. Small deviations between the simulated results and the measured ones are obtained due to a lower surface roughness, which is confirmed by the scanning probe microscope. Comparison with state-of-the-art antenna arrays demonstrates that the proposed arrays excel in easy/low-cost manufacturing, high gain, circular polarization with good axial-ratio bandwidth, compact size, and the possibility to easily extend the array into a larger version.
  • Publication
    Photonic-based integrated sources and antenna arrays for broadband wireless links in terahertz communications
    (IOP Science, 2019-05) García Muñoz, Luis Enrique; Abdalmalak Dawoud, Kerlos Atia; Santamaría Botello, Gabriel Arturo; Rivera Lavado, Alejandro; Segovia Vargas, Daniel; Castillo Aranibar, P.; Van Dijk, F.; Nagatsuma, T.; Brown, E.R.; Guzmán Martínez, Robinson Cruzoe; Lamela Rivera, Horacio; Carpintero del Barrio, Guillermo; Ministerio de Economía y Competitividad (España)
    This paper analyzes integrated components for ultra-broadband millimeter-wave wireless transmitters enabling the 5 G objective to increase the wireless data rates 10x to 100x . We have pursued the photonic-based approach to generate the millimeter-wave carrier (approximate to 97GHz in this paper) through photomixing. We have achieved up to 10 Gb s(-1) data rate using an OOK modulation format (to reduce latency) and either direct detection (DD) or coherent detection. We show that coherent detection enables a sensitivity improvement of 17 dB over DD. We also demonstrate in this work that such improvement can be achieved using as the transmitter a novel integrated antenna array-the self-complementary chessboard array. This avoids the use of complex coherent schemes at the receiver, enabling simple DD for ultra-broadband links.
  • Publication
    Sensitivity limits of millimeter-wave photonic radiometers based on efficient electro-optic upconverters
    (Optica, 2018-10-20) Santamaría Botello, Gabriel Arturo; Sedlmeir, Florian; Rueda, Alfredo; Abdalmalak Dawoud, Kerlos Atia; Brown, Elliott R.; Leuchs, Gerd; Preu, Sascha; Segovia Vargas, Daniel; Strekalov, Dmitry V.; García Muñoz, Luis Enrique; Schwefel, Harald G. L.; Comunidad de Madrid; Ministerio de Economía y Competitividad (España)
    Conventional ultra-high sensitivity detectors in the millimeter-wave range are usually cooled as their own thermal noise at room temperature would mask the weak received radiation. The need for cryogenic systems increases the cost and complexity of the instruments, hindering the development of, among others, airborne and space applications. In this work, the nonlinear parametric upconversion of millimeter-wave radiation to the optical domain inside high-quality (Q) lithium niobate whispering-gallery mode (WGM) resonators is proposed for ultra-low noise detection. We experimentally demonstrate coherent upconversion of millimeter-wave signals to a 1550 nm telecom carrier, with a photon conversion efficiency surpassing the state-of-the-art by 2 orders of magnitude. Moreover, a theoretical model shows that the thermal equilibrium of counterpropagating WGMs is broken by overcoupling the millimeter-wave WGM, effectively cooling the upconverted mode and allowing ultra-low noise detection. By theoretically estimating the sensitivity of a correlation radiometer based on the presented scheme, it is found that room-temperature radiometers with better sensitivity than state-of-the-art high-electron-mobility transistor (HEMT)-based radiometers can be designed. This detection paradigm can be used to develop room-temperature instrumentation for radio astronomy, earth observation, planetary missions, and imaging systems.
  • Publication
    Maximization of the optical intra-cavity power of whispering-gallery mode resonators via coupling prism
    (Optica, 2016-11-14) Santamaria Botello, Gabriel Arturo; García Muñoz, Luis Enrique; Sedlmeir, F.; Preu, S.; Segovia Vargas, Daniel; Abdalmalak Dawoud, Kerlos Atia; Llorente Romano, Sergio; García Lampérez, Alejandro; Malzer, S.; Dohler, D. H.; Schwefel, H. G. L.; Weber, H. B.; Comunidad de Madrid; Ministerio de Economía y Competitividad (España)
    In this paper, a detailed description of the optical coupling into a Whispering Gallery Mode (WGM) resonator through a prism via frustrated total internal reflection (FTIR) is presented. The problem is modeled as three media with planar interfaces and closed expressions for FTIR are given. Then, the curvature of the resonator is taken into account and the mode overlap is theoretically studied. A new analytical expression giving the optimal geometry of a disc-shaped or ring-shaped resonator for maximizing the intra-cavity circulating power is presented. Such expression takes into consideration the spatial distribution of the WGM at the surface of the resonator, thus being more accurate than the currently used expressions. It also takes into account the geometry of the prism. It is shown an improvement in the geometry values used with the current expressions of about 30%. The reason why the pump laser signal can be seen in experiments under critical coupling is explained on this basis. Then, the conditions required for exciting the highest possible optical power inside the resonator are obtained. The aim is to achieve a highly-efficient up-conversion of a THz signal into the optical domain via the second-order nonlinearity of the resonator material.
  • Publication
    Sensitivity and noise in THz electro-optic upconversion radiometers
    (Nature Research, 2020-06-10) Santamaría Botello, Gabriel Arturo; Popovic, Zoya; Abdalmalak Dawoud, Kerlos Atia; Segovia Vargas, Daniel; Brown, Elliott R.; García Muñoz, Luis Enrique; Comunidad de Madrid; Ministerio de Economía y Competitividad (España)
    This paper presents a study of noise in room-temperature THz radiometers that use THz-to-optical upconversion followed by optical detection of thermal radiation. Despite some undesired upconverted thermal noise, no noise is intrinsically introduced by efcient electro-optic modulation via a sumfrequency-generation process in high quality factor (Q) whispering-gallery mode (WGM) resonators. However, coherent and incoherent optical detection results in fundamentally diferent noise characteristics. The analysis shows that the upconversion receiver is quantum limited like conventional amplifers and mixers, only when optical homodyne or heterodyne detection is performed. However, this type of receiver shows advantages as a THz photon counter, where counting is in the optical domain. Theoretical predictions show that upconversion-based room-temperature receivers can outperform state-of-the-art cooled and room-temperature THz receivers based on low-noise amplifers and mixers, provided that a photon conversion efciency greater than 1% is realized. Although the detection bandwidth is naturally narrow due to the highly resonant electro-optic modulator, it is not fundamentally limited and can be broadened by engineering selective optical coupling mechanisms to the resonator.
  • Publication
    A miniaturized triple-band and dual-polarized monopole antenna based on a CSRR perturbed ground plane
    (IEEE, 2021-12-10) El Yousfi, Ahmed; Lamkaddem, Abdenasser; Abdalmalak Dawoud, Kerlos Atia; Segovia Vargas, Daniel; Ministerio de Ciencia e Innovación (España)
    This paper proposes a new triple-band monopole antenna based on Complementary Split Ring Resonators (CSRR) perturbing the ground plane (GND). The antenna consists of an inverted-L-shaped monopole fed by a modified microstrip line with two CSRRs cut out of the ground plane. The operational bands are independently controlled by the CSRR unit cell parameters. In addition, the antenna presents a dual-polarization performance (vertical polarization at 2.4 GHz band and horizontal polarization at both 3.6 and 5.9 GHz bands). The designed antenna is fully planar and low profile avoiding the vias with the ground plane and covering the WLAN, WiMAX, and IEEE 801.11p bands at 2.45, 3.6, and 5.8 GHz. A compact prototype ( 0.32λ0×0.32λ0 being λ0 is the wavelength corresponding to the lowest resonance frequency) has been fabricated and measured showing good agreement between simulations and measurements. The measured impedance bandwidths are 10% (2.38-2.63 GHz), 2.5% (3.54-3.63 GHz), and 20% (5.83-7.12 GHz) whereas the measured gains are 1.34, 0.68, and 2.65 dBi at 2.4, 3.6, and 5.9 GHz respectively.
  • Publication
    Sensitivity Analysis for Active Matched Antennas With Non-Foster Elements
    (IEEE, 2014-12) Albarracín Vargas, Luis Fernando; Ugarte Muñoz, Eduardo; González Posadas, Vicente; Segovia Vargas, Daniel; Ministerio de Asuntos Económicos y Transformación Digital (España)
    During the last years many researchers have been working on the active matching or on non-Foster matching networks for one-and two-port electrically small antennas (ESAs). A new parameter on the sensitivity of the two-port electrically small antenna when loaded with a non-Foster network is presented. This sensitivity analysis will allow us to choose what kind of antennas can be properly matched with non-Foster networks and their position in order to optimize the performance of the design. Then, a typical high Q two-port antenna will be harder to match over a broad bandwidth, since vertical bar s(21)vertical bar is very small and only agrees with vertical bar s(11)vertical bar over very small frequency bands, yielding very large sensitivity values. However, for these two-port antennas, if high levels of coupling can be engineered for a high Q multiple-port antenna, the return and insertion losses can be similar over larger bandwidths and, hence, the sensitivity can be kept low over larger bandwidths, enabling broader impedance matched bandwidths to be achieved, even for highly resonant antennas.
  • Publication
    Meander dipole antenna to increase CW THz photomixing emitted power
    (IEEE, 2014-09-01) Montero de Paz, Javier; Ugarte Muñoz, Eduardo; García Muñoz, Luis Enrique; Cámara Mayorga, Iván; Segovia Vargas, Daniel; Ministerio de Economía y Competitividad (España)
    The success in conquering the terahertz (THz) gap is subject to some facts such as maximizing the emitted power. Traditionally resonant antenna designs for continuous-wave (CW) THz photomixing include a RF choke to compensate the capacitive part of the photomixer and an antenna with a very high input impedance at its resonance to match the low value of the photomixer conductance. This communication considers that the antenna itself can provide this large impedance margin needed to directly match the photomixer, so that the RF choke can be avoided. The meander antenna constitutes an excellent candidate to achieve that goal based on the Active Integrated Antenna concept to improve both matching and radiation efficiencies. The main objective is to maximize the total efficiency and, as a consequence, the THz emitted power. A prototype working at 1.05 THz is designed and manufactured and results show a 6 dB output power improvement when compared with a conventional log-periodic antenna.
  • Publication
    Fully Planar Dual-Polarized Broadband Antenna for 3G, 4G and Sub 6-GHz 5G Base Stations
    (IEEE, 2020-05-13) Martín Antón, Sergio; Segovia Vargas, Daniel; Ministerio de Educación, Cultura y Deporte (España); Ministerio de Ciencia, Innovación y Universidades (España)
    A fully planar dual polarized radiating element with internal embedded coupled dipoles is presented in this paper. The inclusion of these elements allows achieving three different goals: firstly, an extension of the bandwidth towards lower frequencies (from 1.69 GHz to 1.427 GHz) as required in new 5G standards; secondly, a reduction of the antenna size in comparison with other topologies for bandwidth extension and, finally, a reduction of the secondary lobes. The proposed dual polarized broadband antenna is based on two sets of dual polarized dipoles, with a fully planar structure. The objective of this paper is the design of an antenna working in the extended 5G band in order to fulfil the future 5G requirement in the microwave region, in which the new frequency bandwidth goes from 1.427 up to 2.69 GHz with the return losses lower than -14 dB and the crosspolar isolation larger than 28 dB.
  • Publication
    3-D-Printed dielectric resonator antenna arrays based on standing-wave feeding approach
    (IEEE, 2019-10-04) Althuwayb, Ayman A.; Abdalmalak Dawoud, Kerlos Atia; Lee, Choon S.; Santamaría Botello, Gabriel Arturo; García Castillo, Luis Emilio; Segovia Vargas, Daniel; García Muñoz, Luis Enrique; Comunidad de Madrid
    A novel feeding method for a dielectric resonator array antenna is introduced. Unlike in a corporate feed network, power dividers or quarter-wave transformers are not needed in the new feeding scheme as the design is based on the standing-wave concept. Consequently, the feed network is greatly simplified, and undesired spurious radiation in the feeding network is minimized. The simulated and measured results are in good agreement. A 3-D printer is utilized where the entire array structure is fabricated as a single piece with a dielectric material of polylactic acid. The 3-D printer provides a cost-efficient, simple, and rapid manufacturing process.
  • Publication
    Submersible Printed Split-Ring Resonator-Based Sensor for Thin-Film Detection and Permittivity Characterization
    (IEEE, 2016-05-15) Galindo Romera, Gabriel; Herrainz Martínez, Francisco Javier; Gil Barba, Marta; Martínez Martínez, José Juan; Segovia Vargas, Daniel
    A split-ring resonator (SRR)-based sensor for the detection of solid thickness and relative permittivity characterization of solid and liquid materials is proposed. The structure is composed of two SRRs hosted in a microstrip transmission line. The sensing principle is based on the detection of the notch introduced by the resonators in the transmission coefficient. Hence, a frequency shift of the notch is related to a change in the effective permittivity of the structure when the sensor is covered with any solid or liquid material. A complete characterization of the sensor, for the three proposed applications, is performed through simulations. Finally, all simulated results are corroborated with measurements. The proposed sensor is implemented in single-layer printed technology, resulting in a low-cost and low-complexity solution. It presents real-time response and high sensitivity. Moreover, it is fully submersible and reusable.
  • Publication
    A Novel Quartz Clock With Integrated Wireless Energy Harvesting and Sensing Functions
    (IEEE, 2019-05-01) Song, Chaoyun; López Yela, Ana; Huang, Yi; Segovia Vargas, Daniel; Huang, Yuan; Wang, Yansong; Zhou, Jiafeng
    There has been an increasing demand for smart devices and smart furniture for home automation, monitoring, and security applications. In this paper, we present a novel method of integrating the function of wireless energy harvesting from ambient RF signals to a conventional quartz clock for home applications. The most attractive feature is that the clock itself is used as the power receiving device, thus no additional antennas are needed. A simple rectifier is designed to directly match with the clock antenna and rectify the power captured by the clock. As a design example, a clock rectenna using the proposed new idea achieves good energy conversion efficiency (up to 65%) over its operating frequency bands at around 1.4-1.5, 1.9-2.1, and 2.4-2.8 GHz, respectively. Moreover, a wireless environmental sensor is integrated with the clock and powered by using the harvested power from the proposed clock rectenna. This novel design greatly expands the functionality of the quartz clock without affecting its size and appearance. We believe that the proposed energy harvesting quartz clock could be adopted for smart home applications.
  • Publication
    Design method for actively matched antennas with non-foster elements
    (IEEE, 2016-09-22) Albarracín Vargas, Luis Fernando; González Posadas, Vicente; Herraiz Martínez, Francisco Javier; Segovia Vargas, Daniel; Ministerio de Economía y Competitividad (España)
    The design of electrically small antennas (ESAs) loaded with active non-Foster elements is a topic whose interest has grown in the last years. In this communication, a new strategy for the design of actively matched antennas loaded with non-Foster elements is presented. The analysis of different parameters, such as the sensitivity to non-Foster circuit placement, the overall antenna system stability, and current distributions, has to be considered in order to enhance the antenna performance. A design example using an ESA and its realization is presented to validate the proposed strategy.