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[EN] Integrating terrestrial and non-terrestrial networks has the potential of connecting the unconnected and enhancing the user experience for the already-connected, with technological and societal implications of the greatest long-term significance. A convergence of ground, air, and space wireless communications also represents a formidable endeavor for the mobile and satellite communications industries alike, as it entails defining and intelligently orchestrating a new 3D wireless network architecture. In this article, we present the key opportunities and challenges arising from this revolution by presenting some of its disruptive use cases and key building blocks, reviewing the relevant standardization activities, and pointing to open research problems. By considering two multi-operator paradigms, we also showcase how terrestrial networks could be efficiently re-engineered to cater for aerial services, or opportunistically complemented by non-terrestrial infrastructure to augment their current capabilities.

[EN] This work considers a multi-cell, multi-carrier massive MIMO network with carrier aggregation capabilities, and tackles both the rate versus power consumption and the rate versus energy efficiency (EE) trade-offs, by jointly optimizing the number of employed carriers, transmit antennas, base station density, and transmit power. Provably convergent algorithms for both trade-off problems are developed, together with closed-form results for the individual optimization of the considered resources, taking three main carrier aggregation techniques into account, namely inter-carrier, intra-carrier contiguous, and intra-carrier non-contiguous. Numerical results show how the use of carrier aggregation represents an effective way of increasing the network rate and EE, while keeping the power consumption at bay. By using carrier aggregation, it is possible to reduce the number of deployed antennas, without sacrificing the rate performance and increasing the system EE.

[EN] Intelligent reflecting surfaces (IRSs) have been proposed in recent years as a promising technology to enhance signal quality at high frequencies and save energy. In this letter, a Poisson bipolar network model with line segments is used to analyze the energy efficiency (EE) of an IRS-assisted, large-scale network. Specifically, we investigate the performance impact of the IRS configuration, in particular, the number of IRS elements and the phase-shifting resolution of each element. Using customized energy consumption and channel estimation models, we obtain the theoretical trade-off between signal quality and energy consumption as a function of these IRS configurations. The optimal number of elements and phase-shifting resolution of the IRS are also derived. Our results show that IRS technology has great potential for improving the EE of dense networks if their static energy consumption is small enough. Simulation results verify the accuracy of the obtained theoretical results.

[EN] This work considers a multi-cell, multi-carrier massive MIMO network with carrier aggregation capabilities, and tackles both the rate versus power consumption and the rate versus energy efficiency (EE) trade-offs, by jointly optimizing the number of employed carriers, transmit antennas, base station density, and transmit power. Provably convergent algorithms for both trade-off problems are developed, together with closed-form results for the individual optimization of the considered resources, taking three main carrier aggregation techniques into account, namely inter-carrier, intra-carrier contiguous, and intra-carrier non-contiguous. Numerical results show how the use of carrier aggregation represents an effective way of increasing the network rate and EE, while keeping the power consumption at bay. By using carrier aggregation, it is possible to reduce the number of deployed antennas, without sacrificing the rate performance and increasing the system EE.

[EN] Integrating terrestrial and non-terrestrial networks has the potential of connecting the unconnected and enhancing the user experience for the already-connected, with technological and societal implications of the greatest long-term significance. A convergence of ground, air, and space wireless communications also represents a formidable endeavor for the mobile and satellite communications industries alike, as it entails defining and intelligently orchestrating a new 3D wireless network architecture. In this article, we present the key opportunities and challenges arising from this revolution by presenting some of its disruptive use cases and key building blocks, reviewing the relevant standardization activities, and pointing to open research problems. By considering two multi-operator paradigms, we also showcase how terrestrial networks could be efficiently re-engineered to cater for aerial services, or opportunistically complemented by non-terrestrial infrastructure to augment their current capabilities.

[EN] Intelligent reflecting surfaces (IRSs) have been proposed in recent years as a promising technology to enhance signal quality at high frequencies and save energy. In this letter, a Poisson bipolar network model with line segments is used to analyze the energy efficiency (EE) of an IRS-assisted, large-scale network. Specifically, we investigate the performance impact of the IRS configuration, in particular, the number of IRS elements and the phase-shifting resolution of each element. Using customized energy consumption and channel estimation models, we obtain the theoretical trade-off between signal quality and energy consumption as a function of these IRS configurations. The optimal number of elements and phase-shifting resolution of the IRS are also derived. Our results show that IRS technology has great potential for improving the EE of dense networks if their static energy consumption is small enough. Simulation results verify the accuracy of the obtained theoretical results.

[EN] This work considers a multi-cell, multi-carrier massive MIMO network with carrier aggregation capabilities, and tackles both the rate versus power consumption and the rate versus energy efficiency (EE) trade-offs, by jointly optimizing the number of employed carriers, transmit antennas, base station density, and transmit power. Provably convergent algorithms for both trade-off problems are developed, together with closed-form results for the individual optimization of the considered resources, taking three main carrier aggregation techniques into account, namely inter-carrier, intra-carrier contiguous, and intra-carrier non-contiguous. Numerical results show how the use of carrier aggregation represents an effective way of increasing the network rate and EE, while keeping the power consumption at bay. By using carrier aggregation, it is possible to reduce the number of deployed antennas, without sacrificing the rate performance and increasing the system EE.

[EN] Intelligent reflecting surfaces (IRSs) have been proposed in recent years as a promising technology to enhance signal quality at high frequencies and save energy. In this letter, a Poisson bipolar network model with line segments is used to analyze the energy efficiency (EE) of an IRS-assisted, large-scale network. Specifically, we investigate the performance impact of the IRS configuration, in particular, the number of IRS elements and the phase-shifting resolution of each element. Using customized energy consumption and channel estimation models, we obtain the theoretical trade-off between signal quality and energy consumption as a function of these IRS configurations. The optimal number of elements and phase-shifting resolution of the IRS are also derived. Our results show that IRS technology has great potential for improving the EE of dense networks if their static energy consumption is small enough. Simulation results verify the accuracy of the obtained theoretical results.

[EN] Integrating terrestrial and non-terrestrial networks has the potential of connecting the unconnected and enhancing the user experience for the already-connected, with technological and societal implications of the greatest long-term significance. A convergence of ground, air, and space wireless communications also represents a formidable endeavor for the mobile and satellite communications industries alike, as it entails defining and intelligently orchestrating a new 3D wireless network architecture. In this article, we present the key opportunities and challenges arising from this revolution by presenting some of its disruptive use cases and key building blocks, reviewing the relevant standardization activities, and pointing to open research problems. By considering two multi-operator paradigms, we also showcase how terrestrial networks could be efficiently re-engineered to cater for aerial services, or opportunistically complemented by non-terrestrial infrastructure to augment their current capabilities.

[EN] Wireless in-body devices are those in which a medical sensor is introduced-implanted or ingested- inside the human body and communicates with a remote node. Some in-body applications demand high data rates, so ultrawideband (UWB) spectrum has been proposed as a good candidate because of its large bandwidth available. Besides, breathing can lead to internal movement of the torso and consequently, of devices installed in this area. Thus, the radio channel performance can be affected by such movement leading to a malfunction of the radio interface of the medical device. This work aims at analyzing the effect of breathing on the propagation channel by means of in vivo measurements in living animal models. Continuous wave (CW) measurements have been carried out for live different frequencies in the lower part of the UWB band, and the effects of breathing on the relative received power (module and phase) are analyzed and discussed

[EN] Wireless in-body devices are those in which a medical sensor is introduced-implanted or ingested- inside the human body and communicates with a remote node. Some in-body applications demand high data rates, so ultrawideband (UWB) spectrum has been proposed as a good candidate because of its large bandwidth available. Besides, breathing can lead to internal movement of the torso and consequently, of devices installed in this area. Thus, the radio channel performance can be affected by such movement leading to a malfunction of the radio interface of the medical device. This work aims at analyzing the effect of breathing on the propagation channel by means of in vivo measurements in living animal models. Continuous wave (CW) measurements have been carried out for live different frequencies in the lower part of the UWB band, and the effects of breathing on the relative received power (module and phase) are analyzed and discussed

[EN] The COVID-19 pandemic made all of our universities to move to a full-online teaching scheme this March. Online teaching presents different drawbacks compared to classroom teaching, but probably the main one is the difficulty to keep the interactivity with the students throughout the online sessions. This is why a flipped classroom scheme becomes even more relevant in the new normal.A key aspect for a flipped class is that the information transfer, traditionally performed through lectures, is moved out of the classroom to different types of pre-class assignments. Students are expected to fulfill those assignments before the class allowing the instructor to engage in more interactive activities with the students in the classroom. Nevertheless, getting students to read the proposed contents before the class is a very important problem in higher education. In this work, we show the results of the implementation of pre-class reading assignments in a Global Entrepreneurship course through an educational social platform that allows students to discuss the reading online with their classmates: PERUSALL. We use the platform to understand how students are reading and understanding the different materials, their reading behaviors and how the instructor can take advantage of all that information. Considering the type of group (50 students, 65% of them international students from exchange programmes we have also performed a survey to assess the platform and evaluate the perception of the students of their learning process.

[EN] In order to provide high accuracy in shape measurement with multicore optical fibres, characterization and calibration procedures are an important part of sensor preparation. Some procedures can be considered mandatory for adequate shape reconstruction, while others can help to enhance the measurement accuracy. Several of such procedures are discussed and experimentally applied for demonstrating the possible performance enhancement of curvature sensing, a fundamental phase of the shape reconstruction process. The maximum error observed in curvature calculation for a test object has been proved to be almost halved, decreasing from 2.48% to 1.36%, by applying such calibration corrections. The overall average relative accuracy of curvature measurement was improved from 0.89% to 0.5% (an improvement of 44%).

[EN] The COVID-19 pandemic made all of our universities to move to a full-online teaching scheme this March. Online teaching presents different drawbacks compared to classroom teaching, but probably the main one is the difficulty to keep the interactivity with the students throughout the online sessions. This is why a flipped classroom scheme becomes even more relevant in the new normal.A key aspect for a flipped class is that the information transfer, traditionally performed through lectures, is moved out of the classroom to different types of pre-class assignments. Students are expected to fulfill those assignments before the class allowing the instructor to engage in more interactive activities with the students in the classroom. Nevertheless, getting students to read the proposed contents before the class is a very important problem in higher education. In this work, we show the results of the implementation of pre-class reading assignments in a Global Entrepreneurship course through an educational social platform that allows students to discuss the reading online with their classmates: PERUSALL. We use the platform to understand how students are reading and understanding the different materials, their reading behaviors and how the instructor can take advantage of all that information. Considering the type of group (50 students, 65% of them international students from exchange programmes we have also performed a survey to assess the platform and evaluate the perception of the students of their learning process.

[EN] The EU established a framework of digital competencies for citizens aimed at improving their skills in a professional environment. Bearing that in mind, this study observes the implementation of some of the competencies within an academic cross-cultural project. Through the organisation of a Global Virtual Team activity, two Higher Education Institutions, one in Spain and the other one in Belgium, carried out a monitored activity with identical learning objectives in which students had to organise a trip to the partner city. Teachers divided the students (n=127; 72 in Spain and 55 in Belgium) in groups and each group was paired with another one in the partner institutions. The purpose was to support each other in the best manner to reach the goal of preparing the trip for their own peers following some clear-cut requirements. The activity proved to be satisfactory in several areas, demonstrating the convenience of organising Global Virtual Teams as part of the study plan of a course. Previous literature has shown the advantages of putting these activities into practice, as they also facilitate learning required skills within a professional context and allow the creation of international projects without leaving their own institution. Likewise, students in both institutions expressed their satisfaction with the results and performance through the many obstacles encountered. At the same time, this study sheds new light on the importance of monitoring carefully the progress of students and strengthening academic and personal links.

[EN] In order to provide high accuracy in shape measurement with multicore optical fibres, characterization and calibration procedures are an important part of sensor preparation. Some procedures can be considered mandatory for adequate shape reconstruction, while others can help to enhance the measurement accuracy. Several of such procedures are discussed and experimentally applied for demonstrating the possible performance enhancement of curvature sensing, a fundamental phase of the shape reconstruction process. The maximum error observed in curvature calculation for a test object has been proved to be almost halved, decreasing from 2.48% to 1.36%, by applying such calibration corrections. The overall average relative accuracy of curvature measurement was improved from 0.89% to 0.5% (an improvement of 44%).

[EN] The Third Generation Partnership Project (3GPP) has defined a blockage model as an add-on feature to the channel model used in its evaluations of the physical layer in the 0.5-100 GHz frequency range. This letter focuses on the human body blockage losses in the millimeter-wave band proposing: (i) a new criterion to place the diffraction points over the edges of the blockers that considers the precise position of the blocker with regard to the line-of-sight between the transmitter and the receiver, (ii) a specific criterion to determine which potential blockers, from a set of human bodies present in a certain scenario, should be considered effective blockers given the positions of a transmitter, a receiver, and those bodies, and (iii) a modification of the model to provide more accurate estimations in the case in which several blockers are closely located. The validity of our criteria and the accuracy improvement are confirmed by a set of measurements performed at 30 GHz with directional antennas and with multiple human blockers in different positions.

[EN] The Third Generation Partnership Project (3GPP) has defined a blockage model as an add-on feature to the channel model used in its evaluations of the physical layer in the 0.5-100 GHz frequency range. This letter focuses on the human body blockage losses in the millimeter-wave band proposing: (i) a new criterion to place the diffraction points over the edges of the blockers that considers the precise position of the blocker with regard to the line-of-sight between the transmitter and the receiver, (ii) a specific criterion to determine which potential blockers, from a set of human bodies present in a certain scenario, should be considered effective blockers given the positions of a transmitter, a receiver, and those bodies, and (iii) a modification of the model to provide more accurate estimations in the case in which several blockers are closely located. The validity of our criteria and the accuracy improvement are confirmed by a set of measurements performed at 30 GHz with directional antennas and with multiple human blockers in different positions.

[EN] The EU established a framework of digital competencies for citizens aimed at improving their skills in a professional environment. Bearing that in mind, this study observes the implementation of some of the competencies within an academic cross-cultural project. Through the organisation of a Global Virtual Team activity, two Higher Education Institutions, one in Spain and the other one in Belgium, carried out a monitored activity with identical learning objectives in which students had to organise a trip to the partner city. Teachers divided the students (n=127; 72 in Spain and 55 in Belgium) in groups and each group was paired with another one in the partner institutions. The purpose was to support each other in the best manner to reach the goal of preparing the trip for their own peers following some clear-cut requirements. The activity proved to be satisfactory in several areas, demonstrating the convenience of organising Global Virtual Teams as part of the study plan of a course. Previous literature has shown the advantages of putting these activities into practice, as they also facilitate learning required skills within a professional context and allow the creation of international projects without leaving their own institution. Likewise, students in both institutions expressed their satisfaction with the results and performance through the many obstacles encountered. At the same time, this study sheds new light on the importance of monitoring carefully the progress of students and strengthening academic and personal links.

[EN] The EU established a framework of digital competencies for citizens aimed at improving their skills in a professional environment. Bearing that in mind, this study observes the implementation of some of the competencies within an academic cross-cultural project. Through the organisation of a Global Virtual Team activity, two Higher Education Institutions, one in Spain and the other one in Belgium, carried out a monitored activity with identical learning objectives in which students had to organise a trip to the partner city. Teachers divided the students (n=127; 72 in Spain and 55 in Belgium) in groups and each group was paired with another one in the partner institutions. The purpose was to support each other in the best manner to reach the goal of preparing the trip for their own peers following some clear-cut requirements. The activity proved to be satisfactory in several areas, demonstrating the convenience of organising Global Virtual Teams as part of the study plan of a course. Previous literature has shown the advantages of putting these activities into practice, as they also facilitate learning required skills within a professional context and allow the creation of international projects without leaving their own institution. Likewise, students in both institutions expressed their satisfaction with the results and performance through the many obstacles encountered. At the same time, this study sheds new light on the importance of monitoring carefully the progress of students and strengthening academic and personal links.

[EN] The COVID-19 pandemic made all of our universities to move to a full-online teaching scheme this March. Online teaching presents different drawbacks compared to classroom teaching, but probably the main one is the difficulty to keep the interactivity with the students throughout the online sessions. This is why a flipped classroom scheme becomes even more relevant in the new normal.A key aspect for a flipped class is that the information transfer, traditionally performed through lectures, is moved out of the classroom to different types of pre-class assignments. Students are expected to fulfill those assignments before the class allowing the instructor to engage in more interactive activities with the students in the classroom. Nevertheless, getting students to read the proposed contents before the class is a very important problem in higher education. In this work, we show the results of the implementation of pre-class reading assignments in a Global Entrepreneurship course through an educational social platform that allows students to discuss the reading online with their classmates: PERUSALL. We use the platform to understand how students are reading and understanding the different materials, their reading behaviors and how the instructor can take advantage of all that information. Considering the type of group (50 students, 65% of them international students from exchange programmes we have also performed a survey to assess the platform and evaluate the perception of the students of their learning process.

[EN] In order to provide high accuracy in shape measurement with multicore optical fibres, characterization and calibration procedures are an important part of sensor preparation. Some procedures can be considered mandatory for adequate shape reconstruction, while others can help to enhance the measurement accuracy. Several of such procedures are discussed and experimentally applied for demonstrating the possible performance enhancement of curvature sensing, a fundamental phase of the shape reconstruction process. The maximum error observed in curvature calculation for a test object has been proved to be almost halved, decreasing from 2.48% to 1.36%, by applying such calibration corrections. The overall average relative accuracy of curvature measurement was improved from 0.89% to 0.5% (an improvement of 44%).

[EN] The EU established a framework of digital competencies for citizens aimed at improving their skills in a professional environment. Bearing that in mind, this study observes the implementation of some of the competencies within an academic cross-cultural project. Through the organisation of a Global Virtual Team activity, two Higher Education Institutions, one in Spain and the other one in Belgium, carried out a monitored activity with identical learning objectives in which students had to organise a trip to the partner city. Teachers divided the students (n=127; 72 in Spain and 55 in Belgium) in groups and each group was paired with another one in the partner institutions. The purpose was to support each other in the best manner to reach the goal of preparing the trip for their own peers following some clear-cut requirements. The activity proved to be satisfactory in several areas, demonstrating the convenience of organising Global Virtual Teams as part of the study plan of a course. Previous literature has shown the advantages of putting these activities into practice, as they also facilitate learning required skills within a professional context and allow the creation of international projects without leaving their own institution. Likewise, students in both institutions expressed their satisfaction with the results and performance through the many obstacles encountered. At the same time, this study sheds new light on the importance of monitoring carefully the progress of students and strengthening academic and personal links.

[EN] The COVID-19 pandemic made all of our universities to move to a full-online teaching scheme this March. Online teaching presents different drawbacks compared to classroom teaching, but probably the main one is the difficulty to keep the interactivity with the students throughout the online sessions. This is why a flipped classroom scheme becomes even more relevant in the new normal.A key aspect for a flipped class is that the information transfer, traditionally performed through lectures, is moved out of the classroom to different types of pre-class assignments. Students are expected to fulfill those assignments before the class allowing the instructor to engage in more interactive activities with the students in the classroom. Nevertheless, getting students to read the proposed contents before the class is a very important problem in higher education. In this work, we show the results of the implementation of pre-class reading assignments in a Global Entrepreneurship course through an educational social platform that allows students to discuss the reading online with their classmates: PERUSALL. We use the platform to understand how students are reading and understanding the different materials, their reading behaviors and how the instructor can take advantage of all that information. Considering the type of group (50 students, 65% of them international students from exchange programmes we have also performed a survey to assess the platform and evaluate the perception of the students of their learning process.

[EN] The Third Generation Partnership Project (3GPP) has defined a blockage model as an add-on feature to the channel model used in its evaluations of the physical layer in the 0.5-100 GHz frequency range. This letter focuses on the human body blockage losses in the millimeter-wave band proposing: (i) a new criterion to place the diffraction points over the edges of the blockers that considers the precise position of the blocker with regard to the line-of-sight between the transmitter and the receiver, (ii) a specific criterion to determine which potential blockers, from a set of human bodies present in a certain scenario, should be considered effective blockers given the positions of a transmitter, a receiver, and those bodies, and (iii) a modification of the model to provide more accurate estimations in the case in which several blockers are closely located. The validity of our criteria and the accuracy improvement are confirmed by a set of measurements performed at 30 GHz with directional antennas and with multiple human blockers in different positions.

© 2021 IEEE. Personal use of this material is permitted. Permissíon from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertisíng or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., [EN] This letter presents two slotted array antennas working in the Ka-band with switchable circular polarization capability. The first prototype is a series-fed slotted-waveguide linear array composed of ten T-shaped slots. The second antenna is a 2-D array with 2 x 2 T-shaped slots fed by a corporate distribution network. In both cases, a reflection coefficient below -10 dB has been experimentally observed within the targeted frequency band between 29.5 and 30.5 GHz. Good polarization purity is achieved for both polarization senses and in both prototypes. The fundamental contribution of this letter is to propose a simple mechanism to switch the circular polarization sense in a low-cost, low-profile, and high-efficient antenna. The design and experimental results confirm that the solution is suitable for both one- and two-dimensional arrays in the millimeter-wave band.

[EN] Composites have added new dimensions to the design and construction of buildings and struc-tures. One of the wider spread composite applications in the construction industry is composite beams or pillars, which can be manufactured through pultrusion processes. These types of con-struction elements are usually used to withstand the weight of large loads, so their integrity must be guaranteed. Due to optical sensors¿ advantages over their electrical counterparts¿small sizes, low weight, non-conductive, and immunity to electromagnetic interference¿and FBGs having an outstanding position among optical fibre sensors¿due to their multiplexation capability and relatively easy monitoring¿in this study, we propose the integration of FBG sensors for the observation and analysis of the integrity of structures made with composite beams over time. The validation test results showed the successful embedding integration of FBG-based fibre optical sensors in an FRP pultrusion beam and strain transmission up to 7500 µ¿ from the composite test piece to the sensor. Additionally, we were able to anticipate the piece failure by the FBG spectrum deformation.

[EN] A four-port wideband cavity-backed antenna is presented for indoor base stations applications. The antenna is composed of a square open cavity with an X-shaped isolating block and 4 feeding monopoles symmetrically and orthogonally arranged in the aperture of the cavity. A novel methodology based on Characteristic Modes Analysis (CMA) is used for identifying the modes which are contributing to the coupling. As a result of this analysis, an X-shaped isolating block placed at the center of the cavity is proposed for increasing the isolation between ports. A wide-band four-port antenna with unidirectional radiation patterns is obtained with a measured impedance bandwidth (S-11< 10 dB) ranging from 1.55 to 6 GHz (118%), covering most of the sub-6 GHz 5G bands. The proposed antenna provides four independent radiation patterns, with 16 dB of measured minimum isolation between ports and an efficiency higher than 84%. Multiple-input multiple-output (MIMO) compatibility is confirmed with a 4 x 4 MIMO simulated system with an envelope correlation coefficient (ECC)<0.5 in different propagation conditions. The antenna is easy to fabricate and presents a compact size of 129:5 x 129:5 x 28.2 mm(3) (0.68 lambda x0.68 lambda x0.15 lambda, at a frequency f = f(min) = 1.55 GHz). Moreover, the antenna has the advantage of avoiding complex feeding structures with baluns or directional couplers.

[EN] The present paper assesses the performance and characteristics of fiber Bragg grating sensors, with a special interest in their applications in hydraulic machinery and systems. The hydropower industry is turning to this technology with high expectations of obtaining high quality data to validate and calibrate numerical models that could be used as digital twins of key assets, further strengthening the sector's relevant position within industry 4.0. Prior to any validation, fiber Bragg grating sensors' ability to perform well underwater for long periods of time with minimal degradation, and their ease of scalability, drew the authors ' attention. A simplified modal analysis of a partially submerged beam is proposed here as a first step to validate the potential of this type of technology for hydropower applications. Fiber Bragg grating sensors are used to obtain the beam's natural frequencies and to damp vibrations under different conditions. The results are compared with more established waterproof electric strain gauges and a laser vibrometer with good agreement. The presence of several sensors in a single fiber ensures high spatial resolution, fundamental to precisely determine vibration patterns, which is a main concern in this industry. In this work, the beam's vibration patterns have been successfully captured under different excitations and conditions.

[EN] Interferometers usually require long paths for the ever-increasing requirements of high-performance operation, which hinders the miniaturization and integration of photonic circuits into very compact devices. Slow-light based interferometers provide interesting advantages in terms of both compactness and sensitivity, although their optimization is computationally costly and inefficient, due to the large number of parameters to be simultaneously designed. Here we propose the design of slow-light-enhanced bimodal interferometers by using principal component analysis to reduce the high-dimensional design space. A low-dimensional hyperplane containing all optimized designs is provided and investigated for changes in the silicon core and cladding refractive index. As a result, all-dielectric single-channel interferometers as modulators of only 33 mu m(2) footprint and sensors with 19.2 x 10(3) 2 pi rad/RIU.cm sensitivity values are reported and validated by 2 different simulation methods. This work allows the design and optimization of slow light interferometers for different applications by considering several performance criteria, which can be extended to other photonic structures. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

© 2021 IEEE. Personal use of this material is permitted. Permissíon from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertisíng or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., [EN] This letter presents a 4 x 4 single-layer dual-band array antenna operating in the K- and Ka-band using gap waveguide (GW) technology. Radiating elements consist of I-shaped slots located on the top plate of the antenna and backed by a novel coaxial cavity with the dual-frequency operation. The antenna presents two ports, one for each band, and radiates a directive far field pattern with linear polarization. A single-layer wideband corporate-feed network is used to excite the cavities. In addition, a diplexer is integrated as part of the network to separate both working bands. Experimental results show impedance and radiation pattern bandwidths larger than 2 GHz in both bands.

[EN] Waveguide slot arrays are a widely used solution to attain linearly polarized high-gain antennas. In this article, an all-metal T-shaped radiating element is conceived with the aim to replace the usual rectangular slots and attain circularly polarized arrays, preserving their good properties. The polarization conversion is made possible due to the inclusion of a parasitic arm perpendicular to the main active slot. Due to the fact that this second arm is not directly fed by the waveguide but coupled to the active slot, the T-shaped element can be used in any array originally formed by metallic rectangular slots. The design of resonant shunt arrays has been addressed here for validation purposes. The experimental results of two sample linear arrays at 30 GHz demonstrate the design accuracy and manufacturing reliability, reporting an axial ratio below 2 dB within a bandwidth of 1.9 GHz and a peak efficiency around 98%.

[EN] Periodic photonic configurations as photonic crystals (PhCs) and subwavelength grating (SWG) waveguides are gaining a renewed interest for the development of biosensing structures. By performing a proper design, these periodic configurations allow a significant sensitivity increase while keeping a compact footprint, what is achieved by exploiting concepts such as the slow-wave effect, the increase of the light-matter overlap or the interference of dispersion engineered modes.

[EN] This paper presents a technique to acquire measurements of curvature, twist and pose for two multi-core fibers; one with straight cores and the other with helical cores. Both the fibers have multiple fiber Bragg grating (FBG) sensors inscribed in the cores and the fibers are placed in known configurations in order to compare their measurement accuracy. For the curvature measurements both the fibers are placed in constant curvature slots; for the twist measurements, a set of twists are applied to each fiber and for the pose measurements the fibers are placed in molds of different shape. The mean curvature errors are 0.22 and 0.13 m(-1), in the helical and straight core fiber respectively. For the twist measurement the mean errors are 26.57 degrees 1m and 146.50 degrees 1m in the helical and straight core fiber, respectively. Lastly, the pose measurement consists of position and orientation where the orientation is represented in the axis-angle form. The mean position errors are 0.49 and 0.27 mm, the mean axis orientation errors are 0.12 degrees and 0.26 degrees degrees and the mean angle orientation errors are 1.10 degrees and 1.18 degrees, for the helical and straight core fiber, respectively. The results show that the twist measurement error is significantly low with the helical core fiber, thus helical core fiber is better suited than straight core fiber for applications where twist measurements are required.

[EN] Switches are essential components in several optical applications, in which reduced footprints are highly desirable for mass production of densely integrated circuits at low cost. However, most conventional solutions rely on making long switching structures, thus increasing the final device size. Here, we propose and experimentally demonstrate an ultra-compact 2x2 optical switch based on slow-light-enhanced bimodal interferometry in one-dimensional silicon photonic crystals. By properly designing the band structure, the device exhibits a large group index contrast between the fundamental even mode and a higher order odd mode for TE polarization. Thereby, highly dispersive and broadband bimodal regions for high-performance operation are engineered by exploiting the different symmetry of the modes. Two configurations are considered in the experiments to analyze the dimensions influence on the switching efficiency. As a result, a photonic switch based on a bimodal single-channel interferometer with a footprint of only 63 mu m(2), a power consumption of 19.5 mW and a crosstalk of 15 dB is demonstrated for thermo-optic tunability.

[EN] This article proposes a novel all-metal mechanical phase shifter in gap waveguide technology. The phase shifter is aimed at providing beam-scanning capabilities to conventional slot array antennas along the elevation plane. To validate experimentally the beam-steering functionality, a 4x8 slot-array antenna has been designed and fabricated, along with the phase-shifting mechanism. The whole antenna consists of two pieces: a lower rotatable block, which changes the length of concentric groove gap waveguides, and an upper fixed block, where the slot-array antenna is placed. Experimental results validate the proposed concept, having obtained steering angles of up to 25 degrees, with gain levels around 20 dBi with an antenna efficiency close to 90%. A reflection coefficient below -10 dB is achieved for a wide range of rotation angles from 29.5 to 30.5 GHz. The proposed phase shifter is completely scalable to any array size and its true-time-delay nature enables wide steering ranges for closely-spaced slot arrays with wideband radiation performance

[EN] Fiber Optic Shape Sensing is an innovative Optical Fiber Sensing Technology that uses a fiber optic cable to continuously track the 3D shape and position of a dynamic object (with unknown motion) in real-time without visual contact. This technology offers a valid alternative to existing shape sensing methods, thanks to a combination of advantages (including ease of installation, intrinsic safety, compactness, flexibility, electrically passive operation, resistance to harsh environments and corrosion, no need for proximity or computational or numerical models to reconstruct shape) that can bring remarkable improvements to the fields of Civil, Mechanical and Aerospace Engineering, Biomedicine and Medicine. A considerable research effort has been dedicated to this subject in the last twenty years. This paper presents an ambitious review of the current state of the art of Fiber Optic Shape Sensors (FOSS) based on Optical Multicore Fibers (MCF) or multiple optical single-core fibers with embedded strain sensors and provides a comprehensive analysis of a wide range of aspects, comprising: (1) existing alternative technologies; (2) an overview of optical fiber sensors (3) characteristics and advantages of fiber optic shape sensors; (4) historical achievements; (5) applications; (6) performance and error analysis; and (7) present and future perspectives.

[EN] A four-port wideband cavity-backed antenna is presented for indoor base stations applications. The antenna is composed of a square open cavity with an X-shaped isolating block and 4 feeding monopoles symmetrically and orthogonally arranged in the aperture of the cavity. A novel methodology based on Characteristic Modes Analysis (CMA) is used for identifying the modes which are contributing to the coupling. As a result of this analysis, an X-shaped isolating block placed at the center of the cavity is proposed for increasing the isolation between ports. A wide-band four-port antenna with unidirectional radiation patterns is obtained with a measured impedance bandwidth (S-11< 10 dB) ranging from 1.55 to 6 GHz (118%), covering most of the sub-6 GHz 5G bands. The proposed antenna provides four independent radiation patterns, with 16 dB of measured minimum isolation between ports and an efficiency higher than 84%. Multiple-input multiple-output (MIMO) compatibility is confirmed with a 4 x 4 MIMO simulated system with an envelope correlation coefficient (ECC)<0.5 in different propagation conditions. The antenna is easy to fabricate and presents a compact size of 129:5 x 129:5 x 28.2 mm(3) (0.68 lambda x0.68 lambda x0.15 lambda, at a frequency f = f(min) = 1.55 GHz). Moreover, the antenna has the advantage of avoiding complex feeding structures with baluns or directional couplers.

© 2021 IEEE. Personal use of this material is permitted. Permissíon from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertisíng or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., [EN] This letter presents two slotted array antennas working in the Ka-band with switchable circular polarization capability. The first prototype is a series-fed slotted-waveguide linear array composed of ten T-shaped slots. The second antenna is a 2-D array with 2 x 2 T-shaped slots fed by a corporate distribution network. In both cases, a reflection coefficient below -10 dB has been experimentally observed within the targeted frequency band between 29.5 and 30.5 GHz. Good polarization purity is achieved for both polarization senses and in both prototypes. The fundamental contribution of this letter is to propose a simple mechanism to switch the circular polarization sense in a low-cost, low-profile, and high-efficient antenna. The design and experimental results confirm that the solution is suitable for both one- and two-dimensional arrays in the millimeter-wave band.

© 2021 IEEE. Personal use of this material is permitted. Permissíon from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertisíng or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., [EN] This letter presents a 4 x 4 single-layer dual-band array antenna operating in the K- and Ka-band using gap waveguide (GW) technology. Radiating elements consist of I-shaped slots located on the top plate of the antenna and backed by a novel coaxial cavity with the dual-frequency operation. The antenna presents two ports, one for each band, and radiates a directive far field pattern with linear polarization. A single-layer wideband corporate-feed network is used to excite the cavities. In addition, a diplexer is integrated as part of the network to separate both working bands. Experimental results show impedance and radiation pattern bandwidths larger than 2 GHz in both bands.

[EN] This paper presents a technique to acquire measurements of curvature, twist and pose for two multi-core fibers; one with straight cores and the other with helical cores. Both the fibers have multiple fiber Bragg grating (FBG) sensors inscribed in the cores and the fibers are placed in known configurations in order to compare their measurement accuracy. For the curvature measurements both the fibers are placed in constant curvature slots; for the twist measurements, a set of twists are applied to each fiber and for the pose measurements the fibers are placed in molds of different shape. The mean curvature errors are 0.22 and 0.13 m(-1), in the helical and straight core fiber respectively. For the twist measurement the mean errors are 26.57 degrees 1m and 146.50 degrees 1m in the helical and straight core fiber, respectively. Lastly, the pose measurement consists of position and orientation where the orientation is represented in the axis-angle form. The mean position errors are 0.49 and 0.27 mm, the mean axis orientation errors are 0.12 degrees and 0.26 degrees degrees and the mean angle orientation errors are 1.10 degrees and 1.18 degrees, for the helical and straight core fiber, respectively. The results show that the twist measurement error is significantly low with the helical core fiber, thus helical core fiber is better suited than straight core fiber for applications where twist measurements are required.

[EN] Composites have added new dimensions to the design and construction of buildings and struc-tures. One of the wider spread composite applications in the construction industry is composite beams or pillars, which can be manufactured through pultrusion processes. These types of con-struction elements are usually used to withstand the weight of large loads, so their integrity must be guaranteed. Due to optical sensors¿ advantages over their electrical counterparts¿small sizes, low weight, non-conductive, and immunity to electromagnetic interference¿and FBGs having an outstanding position among optical fibre sensors¿due to their multiplexation capability and relatively easy monitoring¿in this study, we propose the integration of FBG sensors for the observation and analysis of the integrity of structures made with composite beams over time. The validation test results showed the successful embedding integration of FBG-based fibre optical sensors in an FRP pultrusion beam and strain transmission up to 7500 µ¿ from the composite test piece to the sensor. Additionally, we were able to anticipate the piece failure by the FBG spectrum deformation.

[EN] In this paper, the capabilities of Selective Laser Sintering metal 3D-Printing technology to achieve a lightweight and cost-effective flat panel antenna in the millimeter wave band are studied. Specifically, two identical dual-band 4 x 4 array antennas working in the K and Ka bands have been manufactured; one with Selective Laser Sintering (SLS) and one using computer numerical control (CNC). Measured performance of the SLS antenna prototype is benchmarked with the performance of the previously measured CNC antenna. Experimental results reveal the strengths and weaknesses of this particular low-cost additive manufacturing technique with respect to traditional subtractive manufacturing techniques.

[EN] Waveguide slot arrays are a widely used solution to attain linearly polarized high-gain antennas. In this article, an all-metal T-shaped radiating element is conceived with the aim to replace the usual rectangular slots and attain circularly polarized arrays, preserving their good properties. The polarization conversion is made possible due to the inclusion of a parasitic arm perpendicular to the main active slot. Due to the fact that this second arm is not directly fed by the waveguide but coupled to the active slot, the T-shaped element can be used in any array originally formed by metallic rectangular slots. The design of resonant shunt arrays has been addressed here for validation purposes. The experimental results of two sample linear arrays at 30 GHz demonstrate the design accuracy and manufacturing reliability, reporting an axial ratio below 2 dB within a bandwidth of 1.9 GHz and a peak efficiency around 98%.

[EN] The present paper assesses the performance and characteristics of fiber Bragg grating sensors, with a special interest in their applications in hydraulic machinery and systems. The hydropower industry is turning to this technology with high expectations of obtaining high quality data to validate and calibrate numerical models that could be used as digital twins of key assets, further strengthening the sector's relevant position within industry 4.0. Prior to any validation, fiber Bragg grating sensors' ability to perform well underwater for long periods of time with minimal degradation, and their ease of scalability, drew the authors ' attention. A simplified modal analysis of a partially submerged beam is proposed here as a first step to validate the potential of this type of technology for hydropower applications. Fiber Bragg grating sensors are used to obtain the beam's natural frequencies and to damp vibrations under different conditions. The results are compared with more established waterproof electric strain gauges and a laser vibrometer with good agreement. The presence of several sensors in a single fiber ensures high spatial resolution, fundamental to precisely determine vibration patterns, which is a main concern in this industry. In this work, the beam's vibration patterns have been successfully captured under different excitations and conditions.

[EN] Switches are essential components in several optical applications, in which reduced footprints are highly desirable for mass production of densely integrated circuits at low cost. However, most conventional solutions rely on making long switching structures, thus increasing the final device size. Here, we propose and experimentally demonstrate an ultra-compact 2x2 optical switch based on slow-light-enhanced bimodal interferometry in one-dimensional silicon photonic crystals. By properly designing the band structure, the device exhibits a large group index contrast between the fundamental even mode and a higher order odd mode for TE polarization. Thereby, highly dispersive and broadband bimodal regions for high-performance operation are engineered by exploiting the different symmetry of the modes. Two configurations are considered in the experiments to analyze the dimensions influence on the switching efficiency. As a result, a photonic switch based on a bimodal single-channel interferometer with a footprint of only 63 mu m(2), a power consumption of 19.5 mW and a crosstalk of 15 dB is demonstrated for thermo-optic tunability.

[EN] Periodic photonic configurations as photonic crystals (PhCs) and subwavelength grating (SWG) waveguides are gaining a renewed interest for the development of biosensing structures. By performing a proper design, these periodic configurations allow a significant sensitivity increase while keeping a compact footprint, what is achieved by exploiting concepts such as the slow-wave effect, the increase of the light-matter overlap or the interference of dispersion engineered modes.

[EN] Interferometers usually require long paths for the ever-increasing requirements of high-performance operation, which hinders the miniaturization and integration of photonic circuits into very compact devices. Slow-light based interferometers provide interesting advantages in terms of both compactness and sensitivity, although their optimization is computationally costly and inefficient, due to the large number of parameters to be simultaneously designed. Here we propose the design of slow-light-enhanced bimodal interferometers by using principal component analysis to reduce the high-dimensional design space. A low-dimensional hyperplane containing all optimized designs is provided and investigated for changes in the silicon core and cladding refractive index. As a result, all-dielectric single-channel interferometers as modulators of only 33 mu m(2) footprint and sensors with 19.2 x 10(3) 2 pi rad/RIU.cm sensitivity values are reported and validated by 2 different simulation methods. This work allows the design and optimization of slow light interferometers for different applications by considering several performance criteria, which can be extended to other photonic structures. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

[EN] This article proposes a novel all-metal mechanical phase shifter in gap waveguide technology. The phase shifter is aimed at providing beam-scanning capabilities to conventional slot array antennas along the elevation plane. To validate experimentally the beam-steering functionality, a 4x8 slot-array antenna has been designed and fabricated, along with the phase-shifting mechanism. The whole antenna consists of two pieces: a lower rotatable block, which changes the length of concentric groove gap waveguides, and an upper fixed block, where the slot-array antenna is placed. Experimental results validate the proposed concept, having obtained steering angles of up to 25 degrees, with gain levels around 20 dBi with an antenna efficiency close to 90%. A reflection coefficient below -10 dB is achieved for a wide range of rotation angles from 29.5 to 30.5 GHz. The proposed phase shifter is completely scalable to any array size and its true-time-delay nature enables wide steering ranges for closely-spaced slot arrays with wideband radiation performance

[EN] Strongly influenced by the advances in the semiconductor industry, the miniaturization and integration of optical circuits into smaller devices has stimulated considerable research efforts in recent decades. Among other structures, integrated interferometers play a prominent role in the development of photonic devices for on-chip applications ranging from optical communication networks to point-of-care analysis instruments. However, it has been a long-standing challenge to design extremely short interferometer schemes, as long interaction lengths are typically required for a complete modulation transition. Several approaches, including novel materials or sophisticated configurations, have been proposed to overcome some of these size limitations but at the expense of increasing fabrication complexity and cost. Here, we demonstrate for the first time slow light bimodal interferometric behaviour in an integrated single-channel one-dimensional photonic crystal. The proposed structure supports two electromagnetic modes of the same polarization that exhibit a large group velocity difference. Specifically, an over 20-fold reduction in the higher-order-mode group velocity is experimentally shown on a straightforward all-dielectric bimodal structure, leading to a remarkable optical path reduction compared to other conventional interferometers. Moreover, we experimentally demonstrate the significant performance improvement provided by the proposed bimodal photonic crystal interferometer in the creation of an ultra-compact optical modulator and a highly sensitive photonic sensor.