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In this work we present the results of a measurement campaign carried out in the Mediterranean sea aimed at characterizing the underwater acoustic channel in a wideband at ultrasonic frequencies centered at 80 kHz with a width of 96 kHz, covering two octaves from 32 to 128 kHz. So far, these type of wideband measurements are not found in the literature. Periodic orthogonal frequency division multiplexing (OFMD) sounding signals using Zadoff-Chu sequences have been specially designed for this purpose. The collected data has been post-processed to estimate the time-variant impulse and frequency responses and relevant parameters for system design like the time coherence, bandwidth coherence, delay spread and Doppler bandwidth. The statistical behavior of the channel gain random fluctuation has also been analyzed. This information has been extracted for both the global channel and each path separately. The wide bandwidth of the measurements have allowed the characterization of the channel in a scarcely explored ultrasonic band with an accuracy that is far beyond what is reported in previous works., Comment: 13 pages, 15 figures

Web application pentesting is a crucial component in the offensive cybersecurity area, whose aim is to safeguard web applications and web services as the majority of the web applications are mounted in publicly accessible web environments. This method requires that the cybersecurity experts pretend and act as real attackers to identify all the errors and vulnerabilities in web applications with the objective of preventing and reducing damages. As this process may be quite complex and the amount of information pentesters need may be big, being able to automate it will help them to easily discover the vulnerabilities given. This project is the direct continuation of the previous initiative called PThelper: An open source tool to support the Penetration Testing process. This continuation is focused on expanding PThelper with the functionality to detect and later report web vulnerabilities in order to address emerging threats and strengthen the ability of the organizations to protect their web applications against potential cyber-attacks.

We prove that two natural Markov chains on the set of monotone paths in a strip mix slowly. To do so, we make novel use of the theory of non-positively curved (CAT(0)) cubical complexes to detect small bottlenecks in many graphs of combinatorial interest. Along the way, we give a formula for the number c_m(n) of monotone paths of length n in a strip of height m. In particular we compute the exponential growth constant of c_m(n) for arbitrary m, generalizing results of Williams for m=2, 3., Comment: 30 pages, 9 figures

We show that coupling the $\textrm{SU}(2)$-valued Skyrme field to the $\rho$-meson solves the long-standing issue of (in)compressibility in the solitonic Skyrme model. Even by including only one $\rho\pi$ interaction term, motivated by a holographic-like reduction of Yang-Mills action by Sutcliffe, reduces the compression modulus from $K_0 \simeq 1080$ MeV, in the massive Skyrme model, to $K_0\simeq 351$ MeV., Comment: 20 pages, 5 figures

We show that any nonlinear field theory giving rise to static solutions with finite energy like, e.g., topological solitons, allows us to derive an infinite number of integral identities which any such solution has to obey. These integral identities can always be understood as being generated by field transformations and their related Noether currents. We also explain why all integral identities generated by coordinate transformations become trivial for Bogomolnyi-Prasad-Sommerfield (BPS) solitons, i.e., topological solitons which saturate a topological energy bound. Finally, we consider applications of these identities to a broad class of nonlinear scalar theories, including the Skyrme model. More concretely, we find nontrivial integral identities that can be seen as model-independent relations between certain physical properties of the solitons in such theories, and we comment on the possible connection between these new relations and those already found in the context of astrophysical compact objects. We also demonstrate the usefulness of said identities to estimate the precision of the numerical calculation of soliton observables., Comment: 30 pages, LaTex, v4: improved presentation, typos and an error in Section V corrected

FIDO2 authentication is starting to be applied in numerous web authentication services, aiming to replace passwords and their known vulnerabilities. However, this new authentication method has not been integrated yet with network authentication systems. In this paper, we introduce FIDO2CAP: FIDO2 Captive-portal Authentication Protocol. Our proposal describes a novel protocol for captive-portal network authentication using FIDO2 authenticators, as security keys and passkeys. For validating our proposal, we have developed a prototype of FIDO2CAP authentication in a mock scenario. Using this prototype, we performed an usability experiment with 15 real users. This work makes the first systematic approach for adapting network authentication to the new authentication paradigm relying on FIDO2 authentication., Comment: This is an author version. It has not been peer reviewed

This article discusses the development of an IoT system for monitoring and controlling various devices and systems from different vendors. The authors considered key challenges in IoT projects, such as interoperability and integration, scalability, and data storage, processing, and visualization, during the design and deployment phases. In addition to these general challenges, the authors also delve into the specific integration challenges they encountered. Various devices and systems were integrated into the system and five real-world scenarios in a university campus environment are used to illustrate the challenges encountered. The scenarios involve monitoring various aspects of a university campus environment, including air quality, environmental parameters, energy efficiency, solar photovoltaic energy, and energy consumption. The authors analyzed data and CPU usage to ensure that the system could handle the large amount of data generated by the devices. The platform developed uses open source projects such as Home Assistant, InfluxDB, Grafana, and Node-RED. All developments have been published as open source in public repositories. In conclusion, this work highlights the potential and feasibility of IoT systems in various real-world applications, the importance of considering key challenges in IoT projects during the design and deployment phases, and the specific integration challenges that may be encountered., Comment: 21 pages, 16 figures, Published in Internet of Things

In an increasingly demanding marketplace that will put great strain on railway services, research on broadband wireless communication must continue to strive for improvement. Based on the mature narrowband GSM technology, Global System for Mobile Communications-Railways (GSM-R) has been deployed both for operational and voice communications. Although GSM-R fulfills the requirements of current railway services, it imposes limited capacity and high costs that restrict enhancements of operational efficiency, passenger security and transport quality. 4G Long Term Evolution (LTE) is expected to be the natural successor of GSM-R not only for its technical advantages and increasing performance, but also due to the current evolution of general-purpose communication systems. This paper examines the key features of LTE as well as its technical ability to support both the migration of current railway services and the provisioning of future ones., Comment: This is a portion of the ACCEPTED VERSION of the published document in: Kassab, M., Berbineau,M., Vinel, A., Jonsson, M., Garcia, F., Soler, J. (eds) Communication Technologies for Vehicles. Nets4Cars/Nets4Trains/Nets4Aircraft 2015. Lecture Notes in Computer Science, vol 9066. Springer, Cham. https://doi.org/10.1007/978-3-319-17765-6_14

We present an innovative approach that simultaneously enables direct antenna frequency-hopped M-ary frequency shift keying (DAFH-MFSK) modulation and beamsteering through the use of time-modulated arrays (TMAs). The distinctive feature of our approach lies in the modulation of the TMA excitations with binary periodic sequences, which can be easily frequency-adjusted and time-delayed to simultaneously allow for DAFH-MFSK direct antenna modulation and beamsteering. Notably, our TMA proposal offers a distinct advantage over conventional architectures in terms of performance metrics, including reduced insertion losses and enhanced phase resolution for beamsteering, while also simplifying hardware complexity., Comment: 5 pages, 7 figures, published in IEEE Antennas and Wireless Propagation Letters as open access article

In the world of Internet of Things (IoT), obtaining the physical location of devices has always been a task of great interest for developing increasingly complex location-based services (LBS). That is why in recent years wireless communication standards have been incorporating new additions focused on providing localization mechanisms to technologies widely used in the IoT world, such as Wi-Fi or Bluetooth. In particular, the IEEE 802.11-2016 Wi-Fi standard introduced ranging estimation between two devices through the so-called fine time measurement (FTM) protocol, defined by the IEEE 802.11mc. FTM is not yet widespread in the IoT field, but commercial modules capable of offering this functionality at a reasonable price are starting to appear. In early 2021, the most widespread system on a chip (SOC) family among IoT devices, the ESP32-XX series, added support for this Wi-Fi standard, enabling, for the first time, the use of a standard designed for location-based systems. This article analyzes the performance of this FTM implementation by carrying out and studying several measurement campaigns in different indoor and outdoor scenarios. Additionally, this work proposes an alternative real-time implementation for distance estimation inside the ESP32 using an approach based on machine learning. Such an implementation is successfully validated in a scenario totally different than those considered for the training and test sets. Finally, both the measurement sets and the developed software are available to the scientific community., Comment: 14 pages, 21 figures, Published in IEEE Internet of Things Journal

In contemporary society, the pressing challenge of preserving user privacy clashes with the imperative for smart buildings to efficiently manage their resources, particularly in the context of occupancy monitoring for optimized energy utilization. This paper delves into the application of millimiter wave (mmWave) frequency modulated continuous wave (FMCW) radar technology for occupancy monitoring. mmWave FMCW radar, unlike conventional methods that often require the use of identifiable tags or involve image analysis, operates without the need for such identifiers, mitigating privacy concerns. However, challenges arise when attempting to cover extensive indoor spaces due to the limited range of individual mmWave FMCW radar devices. The present work proposes the use of a flexible software architecture to integrate the measurements of several mmWave FMCW radar devices, so that the whole behaves as a single sensor. To validate the proposal, an example of use in a real environment in an indoor space monitored with three mmWave FMCW radar devices is also presented. The example details the whole process, from the physical installation of the devices to the use of the different software modules that allow the integration of the data into a common internet of things (IoT) management platform such as Home Assistant. All the elements, from the measurements captured during the test to the different software implementations, are shared publicly with the scientific community., Comment: 13 pages, 9 figures, submitted to IEEE Internet of Things Journal

We present a single sideband time-modulated multibeam phased array governed by periodic Nyquist pulsed signals. A Nyquist pulse is a physically realizable approach to the ideal sinc function. Hence, its low-pass spectrum suits particularly well for time-modulated arrays (TMAs) to perform harmonic beam steering. Contrarily to switched TMAs and standard solutions based on variable phase shifters, the performance and complexity of the proposed time modulation scheme is rather robust when increasing the number of multibeams., Comment: 4 pages, 4 figures, Published in IEEE Antennas and Wireless Propagation Letters

Time-modulated arrays (TMAs) can effectively perform beamsteering over the first positive harmonic pattern by applying progressively delayed versions of stair-step approximations of a sine waveform to the antenna excitations. In this letter, we consider synthesizing such stair-step sine approximations by means of Haar wavelets. Haar functions constitute a complete orthonormal set of rectangular waveforms, which have the ability to represent a given function with a high degree of accuracy using few constituent terms. Hence, when they are applied to the TMA synthesis, employing single-pole double-throw switches, such a feature leads to an excellent rejection level of the undesired harmonics as well as a bandwidth greater than that supported by conventional TMAs with on-off switches., Comment: 5 pages, 4 figures, Published in IEEE Antennas and Wireless Propagation Letters

Conventional time-modulated arrays are based on the application of variable-width periodical rectangular pulses (easily implemented with radio frequency switches) to the individual antenna excitations. However, a serious bottleneck arises when the number of exploited harmonic beams increases. In this context, the modest windowing features of the rectangular pulses produce an inflexible and ineffective harmonic beamforming. The use of other pulses, such as sum of weighted cosines, partially solves these issues at the expense of introducing additional non-timing variables. We propose the discrete-time preprocessing of rectangular pulses before being applied to the antenna to accomplish an agile, efficient, and accurate harmonic beamforming, while keeping the simplicity of the hardware structure., Comment: 4 pages, 6 figures, Published in IEEE Antennas and Wireless Propagation Letters

Today's needs for monitoring and control of different devices in organizations require an Internet of Things (IoT) platform that can integrate heterogeneous elements provided by multiple vendors and using different protocols, data formats and communication technologies. This article provides a comprehensive review of all the architectures, technologies, protocols and data formats most commonly used by existing IoT platforms. On this basis, a comparative analysis of the most widely used open source IoT platforms is presented. This exhaustive comparison is based on multiple characteristics that will be essential to select the platform that best suits the needs of each organization., Comment: 15 pages, 4 figures, Published in Internet of Things

Time-modulated arrays (TMAs) are able to improve the side-lobe level of the radiation pattern at the fundamental mode but cannot steer the beam at such a mode towards a given direction. Beam-steering is possible in a TMA, but only at the harmonic patterns and at the expense of a severe TMA efficiency reduction. In this work we propose a TMA approach that simultaneously performs both features over the same beam by using two sets of switches: (1) single-pole four-throw switches to generate periodic stair-step pulses suitable for efficiently synthesizing a uniform steerable beam over the first positive harmonic, and (2) single-pole single-throw switches to reconfigure the side-lobe level of the previous beam. Performance, small size, cost-effectiveness, and performance invariability with the carrier frequency are features that make this TMA approach a competitive solution for analog beamforming. Accordingly, the structure is an attractive proposal for the design of multibeam transceivers., Comment: 14 pages, 9 figures, published in Signal Processing

Bipolar (+/-1) sequences with no zero state suit particularly well for safeguarding the switched feeding network efficiency when applied to time-modulated arrays (TMAs). During the zero state of a conventional time-modulating sequence, if a given array element is switched off, a certain amount of energy of the transmitted/received signal is wasted. We propose a novel single sideband time-modulated phased array (TMPA) architecture governed by realistic bipolar squared sequences in which the rise/fall time of the switches is considered. By using single-pole dual-throw switches and non-reconfigurable passive devices, the TMPA exploits, exclusively, the first positive harmonic pattern while exhibiting an excellent performance in terms of efficiency and control level of the undesired harmonics without using synthesis optimization algorithms (software simplicity)., Comment: 5 pages, 5 figures, published in IEEE Antennas and Wireless Propagation Letters

Doubly-selective channels, such as those that occur when the transmitter and the receiver move relative to each other at high speeds, are a key scenario for fifth generation (5G) cellular systems, which are mostly based in the use of the orthogonal frequency-division multiplexing (OFDM) modulation. In this paper, we consider an OFDM system using quadrature amplitude modulation (QAM) symbols and we show that, when transmitting over deterministic doubly-selective channels, the inter-carrier interference (ICI) affecting a symbol can be well approximated by a complex-valued normal distribution. Based on this, we derive a lower bound for the link capacity using the Shannon-Hartley theorem. Finally, we provide an approximation of the bit error probability (BEP) using the well-known BEP expressions for Gray-coded QAM constellations over additive white Gaussian noise (AWGN) channels, and show numerical results that confirm that the proposed BEP expression approximates accurately the bit error ratio (BER) of the OFDM system for standardized channel models. The proposed closed-form analytical expressions for the capacity and the BEP do not only allow for discarding the need of computationally-costly Monte-Carlo system simulations, but also provide a theoretical framework to optimize the system parameters directly impacting on the achievable performance., Comment: 12 pages, 11 figures, published in IEEE Transactions on Vehicular Technology

In classes of Weyl semimetals where the symmetry protects nodes with higher than unit charge, the nematic Weyl liquid appears as interactions destroy this underlying symmetry. In the symmetry-broken phase, the multiple-charge nodes are split into objects of unit charge, the position of which in momentum space is determined by the nematic order parameter. We examine the phenomenology of this phase, focusing on topological edge states and Landau levels. We find that the symmetry-broken phase itself, as well as the orientation of the nematic order, are identifiable from the resulting edge states. We also find that the nematic order couples to an in-plane magnetic field, indicating that it can be controlled in situ via an external field. Finally, we provide an estimate for the critical coupling where spontaneous symmetry-breaking occurs for contact interaction., Comment: 12 pages, 6 figs

This paper shows how Task-Based Learning (TBL) is used to equip second-year Spanish students at Cardiff University for their placements during their year abroad. We present a set of task-based projects embedded within the curriculum to enhance both (1) the competences students have to acquire throughout their degree, and (2) the skills to overcome the challenges involved in tasks of similar nature when studying or working abroad. These activities (including translations, presentations, essay writing and the production of a vlog) put all the knowledge acquired into practice in real-life situations. The impact of these tasks on the learning process, academic, sociocultural, and employability skills, and the students' perceptions of the transition to placements abroad have been assessed through a survey. The sequences designed, the results and feedback collected are presented as evidence in favour of TBL as a suitable approach to qualify Higher Education (HE) students as proficient users of the target language during their time abroad. [For the complete volume, "Innovative Language Teaching and Learning at University: Facilitating Transition from and to Higher Education," see ED619814.]