947 research outputs found
Joint optimization of power and data transfer in multiuser MIMO systems
We present an approach to solve the nonconvex optimization problem that arises when designing the transmit covariance matrices in multiuser multiple-input multiple-output (MIMO) broadcast networks implementing simultaneous wireless information and power transfer (SWIPT). The MIMO SWIPT problem is formulated as a general multiobjective optimization problem, in which data rates and harvested powers are optimized simultaneously. Two different approaches are applied to reformulate the (nonconvex) multiobjective problem. In the first approach, the transmitter can control the specific amount of power to be harvested by power transfer whereas in the second approach the transmitter can only control the proportion of power to be harvested among the different harvesting users. We solve the resulting formulations using the majorization-minimization (MM) approach. The solution obtained from the MM approach is compared to the classical block-diagonalization (BD) strategy, typically used to solve the nonconvex multiuser MIMO network by forcing no interference among users. Simulation results show that the proposed approach improves over the BD approach both the system sum rate and the power harvested by users. Additionally, the computational times needed for convergence of the proposed methods are much lower than the ones required for classical gradient-based approaches.Peer ReviewedPostprint (author's final draft
El centro alfarero de Sobrevilla. Badarán, La Rioja
After a brief review of the bibliography on the site of Sobrevilla at Badarán (La Rioja), we vill introduce new evidence, which has led us to a new and different interpretation. In this paper we try to explain the activity developed in this place in the Late Roman Empire. Here there was an important pottery center devoted to TSHT manufacturing. We also analyze some pottery proceeding from this workshop.Tras exponer brevemente las publicaciones y formulaciones existentes sobre el yacimiento de Sobrevilla en Badarán (La Rioja), daremos a conocer nuevos datos acerca del mismo, los cuales nos han llevado a una interpretación nueva y diferente. En este trabajo exponemos el tipo de actividad que desarrolló este enclave a lo largo del bajo imperio romano, donde existió un importante centro alfarero dedicado a la fabricación de TSHT. También describimos alguna de las producciones cerámicas procedentes de estos alfares
El centro alfarero de Sobrevilla. Badarán, La Rioja
Tras exponer brevemente las publicaciones y formulaciones existentes sobre el yacimiento de Sobrevilla en Badarán (La Rioja), daremos a conocer nuevos datos acerca del mismo, los cuales nos han llevado a una interpretación nueva y diferente. En este trabajo exponemos el tipo de actividad que desarrolló este enclave a lo largo del bajo imperio romano, donde existió un importante centro alfarero dedicado a la fabricación de TSHT. También describimos alguna de las producciones cerámicas procedentes de estos alfares
Chemical recycling of plastics assisted by microwave multi-frequency heating
Handling plastic waste through recycling allows extending the life of polymeric materials, avoiding recurrence to incineration or landfilling. In contrast with traditional mechanical recycling technologies, chemical recycling enables the obtention of the virgin monomers by means of depolymerisation to create new polymers with the same mechanical and thermal properties as the originals. Research presented in this paper is part of the polynSPIRE project (Horizon 2020 European funding programme) and develops and scales-up a heated reactor to carry out the depolymerisation of polyamide-6 (PA6), polyamide-6, 6 (PA66) and polyurethane (PU) using microwave (MW) technology as the heating source. The purpose is to design and optimize a MW reactor using up to eight ports emitting electromagnetic waves. Finite element method (FEM) simulation and optimisation are used to design the reactor, considering as parameters the data obtained from experimental dielectric testing and lab-scale characterisation of the processes and materials studied. Two different COMSOL Multiphysics modules are involved in this work: Radio Frequency (RF) and Chemical Reaction Engineering (RE), to simulate the reactor cavity using two frequency levels (915 MHz and 2.45 GHz) with a power level of 46 kW, and the chemical depolymerisation process, respectively. A sensitivity study has been performed on key parameters such as the frequency, the number of ports, and position inside the reactor to consolidate the final design. It is expected that these results assist in the design and scale-up of microwave technology for the chemical recycling of plastics, and for the large-scale deployment of this sustainable recovery alternative. © 2021 The Author
Phonon and plasmon excitation in inelastic electron tunneling spectroscopy of graphite
The inelastic electron tunneling spectrum (IETS)of highly oriented pyrolitic
graphite (HOPG) has been measured with scanning tunneling spectroscopy (STS) at
6K. The observed spectral features are in very good agreement with the
vibrational density of states (vDOS) of graphite calculated from first
principles. We discuss the enhancement of certain phonon modes by
phonon-assisted tunneling in STS based on the restrictions imposed by the
electronic structure of graphite. We also demonstrate for the first time the
local excitation of surface-plasmons in IETS which are detected at an energy of
40 meV.Comment: PRB rapid communication, submitte
On the Contribution of Dense Multipath Components in an Intrawagon Environment for 5G mmW Massive MIMO Channels
© 2019 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] In this letter, the dependence of the specular multipath components (SMC) and dense multipath components (DMC) to the interuser spatial correlation and sum-rate capacity of a massive multiuser multiple-input--multiple-output (MU-MIMO) setup is evaluated from measurements conducted inside an underground subway wagon at the 25-40 GHz candidate frequency bands for 5G systems. The radio channel consists in a 7 7 uniform rectangular array (URA) acting as access point for eight users uniformly distributed in the wagon. The DMC power ratio is observed to be distance- and frequency-dependent as the SMC and DMC exhibit different propagation mechanisms. Remarkably, it is reported that the interuser spatial correlation computed with DMC offers the best favorable propagation for a massive MIMO setup, whereas SMC contribute to the users correlation. Hence, correlation is found to be strongly dependent on the DMC ratio frequency characteristics. In addition, better interuser correlation and sum-rate capacity values are obtained as the frequency is increased. These results highlight the need to include DMC in 5G massive MIMO channel models and emulators to improve their accuracy at the system level.This work was supported in part by MINECO, Spain, under the National Projects TEC2016-78028-C3-2-P and TEC2017-86779-C2-2-R and in part by the ELSAT2020 OS4 SMARTIES program co-financed by the European Union with the European Regional Development Fund, the French state, and the Hauts de France Region Council.Challita, F.; Rodrigo Peñarrocha, VM.; Rubio Arjona, L.; Reig, J.; Juan-Llácer, L.; Pascual-García, J.; Molina-García-Pardo, J.... (2019). On the Contribution of Dense Multipath Components in an Intrawagon Environment for 5G mmW Massive MIMO Channels. IEEE Antennas and Wireless Propagation Letters. 18(12):2483-2487. https://doi.org/10.1109/LAWP.2019.2940831S24832487181
Vibron-assisted spin excitation in a magnetically anisotropic nickelocene complex
The ability to electrically-drive spin excitations in molecules with magnetic anisotropy is key for high-density storage and quantum-information technology. Electrons, however, also tunnel via the vibrational excitations unique to a molecule. The interplay of spin and vibrational excitations offers novel routes to study and, ultimately, electrically manipulate molecular magnetism. Here we use a scanning tunneling microscope to electrically induce spin and vibrational excitations in a single molecule consisting of a nickelocene magnetically coupled to a Ni atom. We evidence a vibron-assisted spin excitation at an energy one order of magnitude higher compared to the usual spin excitations of nickelocene and explain it using first-principles calculations that include electron correlations. Furthermore, we observe that spin excitations can be quenched by modifying the Ni-nickelocene coupling. Our study suggests that nickelocene-based complexes constitute a model playground for exploring the interaction of spin and vibrations in the electron transport through single magnetic molecules
Pentagonal nanowires: a first-principles study of atomic and electronic structure
We performed an extensive first-principles study of nanowires in various
pentagonal structures by using pseudopotential plane wave method within the
density functional theory. Our results show that nanowires of different types
of elements, such as alkali, simple, transition and noble metals and inert gas
atoms, have a stable structure made from staggered pentagons with a linear
chain perpendicular to the planes of the pentagons and passing through their
centers. This structure exhibits bond angles close to those in the icosahedral
structure. However, silicon is found to be energetically more favorable in the
eclipsed pentagonal structure. These quasi one dimensional pentagonal nanowires
have higher cohesive energies than many other one dimensional structures and
hence may be realized experimentally. The effect of magnetic state are examined
by spin-polarized calculations. The origin of the stability are discussed by
examining optimized structural parameters, charge density and electronic band
structure, and by using analysis based on the empirical Lennard-Jones type
interaction. Electronic band structure of pentagonal wires of different
elements are discussed and their effects on quantum ballistic conductance are
mentioned. It is found that the pentagonal wire of silicon exhibits metallic
band structure.Comment: 4 figures, accepted for publication in Phys. Rev.
A first-principles approach to electrical transport in atomic-scale nanostructures
We present a first-principles numerical implementation of Landauer formalism
for electrical transport in nanostructures characterized down to the atomic
level. The novelty and interest of our method lies essentially on two facts.
First of all, it makes use of the versatile Gaussian98 code, which is widely
used within the quantum chemistry community. Secondly, it incorporates the
semi-infinite electrodes in a very generic and efficient way by means of Bethe
lattices. We name this method the Gaussian Embedded Cluster Method (GECM). In
order to make contact with other proposed implementations, we illustrate our
technique by calculating the conductance in some well-studied systems such as
metallic (Al and Au) nanocontacts and C-atom chains connected to metallic (Al
and Au) electrodes. In the case of Al nanocontacts the conductance turns out to
be quite dependent on the detailed atomic arrangement. On the contrary, the
conductance in Au nanocontacts presents quite universal features. In the case
of C chains, where the self-consistency guarantees the local charge transfer
and the correct alignment of the molecular and electrode levels, we find that
the conductance oscillates with the number of atoms in the chain regardless of
the type of electrode. However, for short chains and Al electrodes the even-odd
periodicity is reversed at equilibrium bond distances.Comment: 14 pages, two-column format, submitted to PR
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