Examinando por Autor "Ramos, Javier"

Mostrando 1 - 15 de 15

Blind Adaptive Krylov Subspace Multiuser Detection
(2001-01-01T08:12:44Z) Caamaño, Antonio J.; Segovia Vargas, Daniel; Ramos, Javier
A new method for low-complexity Multiuser Detection (MUD)based in the Fast Subspace Decomposition (FSD) is proposed. The use of FSD allows the estimation of the number of users along with the multiuser detection on line. This leads to a fast multiuser estimation-detection scheme with ultra-low complexity. Furthermore, the method is proved to be strongly consistent and blind. This is applied here to the MMSE.Results included show MMSE performance at a fraction of the computational cost reported until now. The use for UMTS-TDD receivers is also proposed.
Distributed Double-Differential Modulation for Cooperative Communications Under CFO
(Global Telecommunications Conference, 2007. GLOBECOM '07. IEEE, 2007-11-26) Cano, Alfonso; Morgado, Eduardo; Caamaño, Antonio J.; Ramos, Javier
When a terminal is recruited to cooperate with other neighboring terminals, its channel state and carrier frequency offset (CFO) may be unknown to the destination. Under these circumstances, this paper considers the use of distributed double-differential (DD) modulation, which simplifies receiver implementation because it by-passes channel and CFO estimation. Two double-differential codecs are proposed transmitting: i) across orthogonal channels using time-division multiplexing, achieving rate and error performance similar to that of co-located multi-antenna DD systems; or ii) simultaneously, benefiting from the distinct CFOs across terminals and bypassing the need of ordering protocols. Both (i)-(ii) approaches are considered in adaptive- and selective-relaying cooperation protocols demonstrating that maximum spatial diversity is achievable. Simulations corroborate the theoretical error performance claims.
End-to-End Average BER in Multihop Wireless Networks over Fading Channels
(2011-03-10T15:48:29Z) Morgado, Eduardo; Mora Jiménez, Inmaculada; Vinagre, Juan José; Ramos, Javier; Caamaño, Antonio
This paper addresses the problem of finding an analytical expression for the end-to-end Average Bit Error Rate (ABER) in multihop Decode-and-Forward (DAF) routes within the context of wireless networks. We provide an analytical recursive expression for the most generic case of any number of hops and any single-hop ABER for every hop in the route. Then, we solve the recursive relationship in two scenarios to obtain simple expressions for the end-to-end ABER, namely: (a) The simplest case, where all the relay channels have identical statistical behaviour; (b) The most general case, where every relay channel has a different statistical behaviour. Along with the theoretical proofs, we test our results against simulations. We then use the previous results to obtain closed analytical expressions for the end-to-end ABER considering DAF relays over Nakagami-m fading channels and with various modulation schemes. We compare these results with the corresponding expressions for Amplify-and-Forward (AAF) and, after corroborating the theoretical results with simulations, we conclude that DAF strategy is more advantageous than the AAF over Nakagami-m fading channels as both the number of relays and m-index increase.
Enhancing the recyclability of ELV plastic bumpers: Characterization of molecular, morphological, rheological, mechanical properties and ageing degradation
(Elsevier, 2024-12) Juan, Rafael; Expósito, M. Teresa; Paredes, Beatriz; Lechuga, Daniel; Martínez, Cristina; Cardil, Andres; Aguilera-Villegas, Lola; Ramos, Javier; Domínguez, Carlos; Vega, Juan F.; Grieken, Rafael Van; García-Muñoz, Rafael A.
While the automotive industry has traditionally prioritized metal recycling, the increasing use of plastics in vehicles underlines the need for sustainable management of these materials. Recycling rates of plastics from end-of-life vehicles (ELVs) in Europe are low, but recent EU legislation requires 25 % of the plastics in new vehicles to be recycled, forcing innovative designs and strategies to enhance the recovery and quality of recycled resins. This study focuses on post-consumer bumpers, a significant recoverable component of ELVs, by assessing their molecular, morphological, rheological, and mechanical properties to investigate their homogeneity and quality from various sources to assess their suitability for recycling. Since the aging of these materials, caused by thermo-oxidative and thermo-mechanical degradation processes, can compromise the quality of recycled bumpers, we propose a blend of recycled PP from different bumper cars with virgin resin, representing a realistic scenario where the post-consumer ELV bumpers are collected. Interestingly, this blend, which can replace up to 50 % of the virgin resin without additional compatibilizers or additives, mitigates the degradation effects. Furthermore, the study evaluates the degradation resistance of these blends through multiple extrusion cycles and accelerated weathering tests in a temperature-controlled UV chamber to ascertain the number of cycles the material can tolerate without significant quality degradation, and to determine its suitability for long-term applications. Our results not only support the feasibility of using recycled PP for automotive components but also contribute to meeting the EU's recycling targets. This research highlights the potential for significant advances in the circularity of automotive plastics, providing a sustainable pathway for integrating recycled materials into new vehicle production
Hybrid Higher-Order Statistics Learning in Multiuser Detection
(Institute of Electrical and Electronical Engineering, 2004-11) Caamaño, Antonio J.; Boloix-Tortosa, R.; Ramos, Javier; Murillo-Fuentes, Juan J.
In this paper, we explore the significance of second- and higher-order statistics learning in communication systems. The final goal in spread-spectrum communication systems is to receive a signal of interest completely free from interference caused by other concurrent signals. To achieve this end, we exploit the structure of the interference by designing second-order statistics detectors, such as the minimum square error, in conjunction with higher-order statistics (HOS) techniques, such as the blind source separation (BSS). This hybrid higher-order statistics (HyHOS) approach enables us to alleviate BSS algorithms of one of their main problems, that is, their sensitiveness to high levels of noise. In addition, we benefit from remarkable properties of BSS in learning such as fast learning (superefficiency) and independence of the initial settings of the problem (equivariance). We successfully applied the results of this approach to the design of multiuser detectors in code-division multiple access channels.
Melting Temperature Depression of Polymer Single Crystals: Application to the Eco-Design of Tie-Layers in Polyolefinic-Based Multilayered Films
(MDPIST ALBAN-ANLAGE 66, CH-4052 BASEL, SWITZERLAND, 2022-04-17) Vega, Juan F.; Souza-Egipsy, Virginia; Expósito, M. Teresa; Ramos, Javier
In this paper, we describe a method for determining polymer compatibility, which will aid in establishing the requirements of polyolefinic materials for the eco-design of multilayer films for mechanical recycling while avoiding the use of reactive tie layers. Our ultimate goal is to define the molecular characteristics of the polyolefinic structural layer that improve compatibility with the tie layer during mechanical recycling. We have investigated the melting temperature depression of single crystals of various polyethylenes embedded in commercial polymeric matrices with various functionalities (ester, acrylate, acetate and methacrylic acid sodium ionomer), which can be potentially used as tie layers. We demonstrate how the concentration and molecular architecture of the matrices affect the melting temperature of the embedded single crystals differently depending on the latter’s molecular architecture. The main finding indicates that the tie layers are more compatible with linear polyethylene than with branched polyethylenes. Indeed, our results show that the heterogeneous Ziegler–Natta linear low-density polyethylene is incompatible with all of the tie layers tested. The depression of melting temperatures observed are in excellent agreement with the results obtained by investigating the rheological behaviour and morphological features of solution-mixed blends in which segmental interactions between polymeric chains have been, in theory, maximized. Because Ziegler– Natta linear density polyethylene is one of the most commonly used polymers as a structural layer in multi-layer applications, the findings of this study are useful as they clearly show the unsuitability of this type of polyethylene for recycling from an eco-design standpoint. The specific molecular requirements for polyethylene layers (branching content less than 0.5/100 carbon atoms) can be specified for use in packaging, guiding the eco-design and valorisation of recycled multi-layered films containing this material.
Minimizing Power in Wireless OFDMA with Limited Rate Feedback
(2007-03-11T09:52:37Z) Garcia Marques, Antonio; Giannakis, Georgios B.; Digham, Fadel F.; Ramos, Javier
Emerging applications involving low-cost wireless sensor networks motivate well optimization of multi-user orthogonal frequency-division multiple access (OFDMA) in the power-limited regime. In this context, the present paper relies on limited-rate feedback (LRF) sent from the access point to terminals to minimize the total average transmit-power under individual average rate and error probability constraints. The characterization of optimal bit, power and subcarrier allocation policies based on LRF, as well as optimal channel quantization are provided. Numerical examples corroborate the analytical claims and reveal that significant power savings result even with few fed back bits.
Natural Gradient Based Blind Multiuser Detection
(2002) Murillo-Fuentes, Juan J.; González Serrano, F.J.; Ramos, Javier; Caamaño, Antonio J.
In this paper, novel structures for dynamic removal of multiuser interference are proposed. The natural gradient (NG) is used either to compute whitening matrices in linear blind minimum MSE or to develop new structures. As a result, we propose a family of centralized and non-centralized multiuser detectors (MUD's). The NG provides the MUD's with the equivariant and superfficiency properties, making them near-far resistant by construction and their convergence optimal. Moreover, the complexity of these structures is significantly reduced. These novel solutions are successfully applied to the multiuser synchronous CDMA channel.
Near infrared spectroscopy (NIRS) and machine learning as a promising tandem for fast viral detection in serum microsamples: A preclinical proof of concept
(Elsevier, 2024) Gomez, Jose; Barquero-Pérez, Oscar; Gonzalo, Jennifer; Salgüero, Sergio; Riado, Daniel; Casas, Maria Luisa; Gutíerrez, Maria Luisa; Jaime, Elena; Pérez-Martínez, Enrique; García-Carretero, Rafael; Ramos, Javier; Fernández-Rodriguez, Conrado; Catalá, Myriam
Fast detection of viral infections is a key factor in the strategy for the prevention of epidemics expansion and follow-up. Hepatitis C is paradigmatic within viral infectious diseases and major challenges to elimination still remain. Near infrared spectroscopy (NIRS) is an inexpensive, clean, safe method for quickly detecting viral infection in transmission vectors, aiding epidemic prevention. Our objective is to evaluate the combined potential of machine learning and NIRS global molecular fingerprint (GMF) from biobank sera as an efficient method for HCV activity discrimination in serum. GMF of 151 serum biobank microsamples from hepatitis C patients were obtained with a FT-NIR spectrophotometer in reflectance mode. Multiple scatter correction, smoothing and Saviztsky-Golay second derivative were applied. Spectral analysis included Principal Component Analysis (PCA), Bootstrap and L1-penalized classification. Microsamples of 70 μl were sufficient for GMF acquisition. Bootstrap evidenced significant difference between HCV PCR positive and negative sera. PCA renders a neat discrimination between HCV PCR-positive and negative samples. PCA loadings together with L1-penalized classification allow the identification of discriminative bands. Active virus positive sera are associated to free molecular water, whereas water in solvation shells is associated to HCV negative samples. Divergences in the water matrix structure and the lipidome between HCV negative and positive sera, as well as the relevance of prooxidants and glucose metabolism are reported as potential biomarkers of viral activity. Our proof of concept demonstrates that NIRS GMF of hepatitis C patients’ sera aided by machine learning allows for efficient discrimination of viral presence and simultaneous potential biomarker identification.
Optimum Scheduling for Orthogonal Multiple Access over Fading Channels using Quantized Channel State Information
(2008-07-01T10:38:31Z) Garcia Marques, Antonio; Giannakis, Georgios B.; Ramos, Javier
The efficiency of multi-access communications over wireless fading links benefits from channel-adaptive allocation of the available bandwidth and power resources. Different from most existing approaches that allocate resources based on perfect channel state information (P-CSI), this work optimizes channel scheduling and resource allocation over orthogonal fading channels when user terminals and the scheduler rely on quantized channel state information (Q-CSI). The novel unifying approach optimizes an average transmit-performance criterion subject to average quality of service requirements. The resultant optimal policy per fading realization either allocates the entire channel to a single (winner) user, or, to a small group of winner users whose percentage of shared resources is found by solving a linear program. Both alternatives become possible by smoothing the allocation scheme. The smooth policy is asymptotically optimal and incurs reduced computational complexity.
Path Efficiency in Mobile Ad-Hoc Networks
(3rd International Symposium on Wireless Communication Systems, 2006. ISWCS '06, 2006-09) Caamaño, Antonio J.; Vinagre Díaz, Juan José; Mora Jiménez, Inma; Figuera Pozuelo, Carlos; Ramos, Javier
The process of routing in large ad-hoc mobile networks is theoretically analyzed as the capacity of a packet to be directed form a source to a destination. The equivalence between directivity and an effective radius, which represents the actual knowledge of any node of its neighbourhood, is demonstrated. The mobility of the network is modelled as that resulting from the most probable distribution of mobile nodes. The results are conclusive: mobility reduces the throughput and delay performance of any routing algorithm with a finite effective radius.
Patrones Eficientes de Pilotos en Sistemas OFDM para Canales Inalámbricos Selectivos en Tiempo y Frecuencia
(2005-07-01T07:45:41Z) Garcia Marques, Antonio; Morgado, Eduardo; Cano, Alfonso; Caamaño, Antonio J.; Ramos, Javier
In coherent systems, when the channel is not known at the receiver, pilot-assisted techniques are needed to estimate the channel. Using OFDM, this paper overcomes the problem of designing such optimum pilot patterns that efficiently estimate doubly selective (in time and frequency)fading channels. We show that,decoupling time- and frequency- selectivity, the challenging process of estimating such channels can be seen as a two-dimensional sampling problem. We further propose efficient sampling patterns depending on the spreading (multipath and Doppler) function of the channel.
Power-Efficient Wireless OFDMA Using Limited-Rate Feedback
(IEEE, 2008-02-01) Marques, Antonio G.; Giannakis, Georgios B.; Digham, Fadel F.; Ramos, Javier
Emerging applications involving low-cost wireless sensor networks motivate well optimization of multi-user orthogonal frequency-division multiple access (OFDMA) in the power-limited regime. In this context, the present paper relies on limited-rate feedback (LRF) sent from the access point to terminals to minimize the total average transmit-power under individual average rate and error probability constraints. Along with the characterization of optimal bit, power and subcarrier allocation policies based on LRF, suboptimal yet simple schemes are developed for channel quantization. The novel algorithms proceed in two phases: (i) an off-line phase to construct the channel quantizer as well as the rate and power codebooks with moderate complexity; and (ii) an on-line phase to obtain, based on quantized channel state information, the optimum, rate, power and user-subcarrier allocation with linear complexity. Numerical examples corroborate the analytical claims and reveal that significant power savings result even with suboptimal schemes based on practically affordable LRF.
Reduced-Complexity Power-Efficient Wireless OFDMA using an Equally Probable CSI Quantizer
(2007-06-24T11:18:13Z) Garcia Marques, Antonio; Digham, Fadel F.; Giannakis, Georgios B.; Ramos, Javier
Emerging applications involving low-cost wireless sensor networks motivate well optimization of multi-user orthogonal frequency-division multiple access (OFDMA) in the powerlimited regime. In this context, the present paper relies on limitedrate feedback (LRF) sent from the access point to terminals to acquire quantized channel state information (CSI) in order to minimize the total average transmit-power under individual average rate and error probability constraints. Specifically, we introduce two suboptimal reduced-complexity schemes to: (i) allocate power, rate and subcarriers across users; and (ii) design accordingly the channel quantizer. The latter relies on the solution of (i) to design equally probable quantization regions per subcarrier and user. Numerical examples corroborate the analytical claims and reveal that the power savings achieved by our reduced-complexity LRF designs are close to those achieved by the optimal solution.
Stochastic resource allocation for orthogonal access based on quantized CSI: optimality, convergence and delay analysis
(2009-04-01T11:29:29Z) Garcia Marques, Antonio; Giannakis, Georgios B.; Ramos, Javier
Dynamic allocation of power, rate and channel access is a critical task in wireless networks. Capitalizing on convex optimization and stochastic approximation tools, this paper develops a stochastic resource allocation algorithm that minimizes average transmit power under individual average rate constraints. Focus is placed on networks where users transmit orthogonally over a set of parallel channels and transmissions are adapted based on quantized channel state information (CSI) allowing even channel statistics to be unknown. Convergence of the developed stochastic scheme is characterized and the average queue delays are obtained in closed form.