Time filter

Source Type

Saghezchi F.,University of Tabriz | Saghezchi F.,University of Minho | Radwan A.,Queen's University | Radwan A.,TELL Wireless Communications Research Group | And 4 more authors.
IEEE Wireless Communications | Year: 2013

Multi-standard mobile terminals (MTs) are the trend of current and future mobile devices for taking advantage of heterogeneous integration of wireless access networks and providing ubiquitous connectivity and better quality of service. Holding multiple active interfaces, however, incurs significant power consumption burdens to MTs. This not only increases the carbon footprint of MTs but also makes the batteries of MTs deplete rapidly. Consequently, mobile users may have to relentlessly look for power outlets to charge their devices, which may threaten their true mobility freedom. To this end, in this article, we propose a promising approach based on coalition formation game and inter-terminal cooperation. Our innovative approach motivates MTs to cooperate, while addressing the issue of isolating selfish players. MTs assess radio channels and disseminate the acquired information as well as their available resources to sketch a global view of the radio environment. Based on this view, coalitions are formed whenever energy saving is foreseeable. Within a coalition, MTs pool their resources and perform their tasks cooperatively to maximize their energy efficiency. Simulation results validate that the proposed approach can effectively double the battery lifetimes of MTs, while successfully eliminating selfish players from cooperative groups. © 2013 IEEE.

Dinis R.,Institute Telecomunicac Oes | Montezuma P.,UNINOVA
IEEE Transactions on Communications | Year: 2013

In this paper we consider the optimum detection of OFDM (Orthogonal Frequency Division Multiplexing) signals with strong nonlinear distortion effects. It is shown that the optimum performance with strong nonlinear distortion effects is not as bad as one might expect and can even be better than the performance with conventional, linear transmitters. To achieve these excellent performances we should employ receivers able to take advantage of the information associated to transmitted data symbols that is inherent to the nonlinear distortion component, in opposition to traditional OFDM implementations where nonlinear distortion effects are regarded as an undesirable noise-like component. We study the achievable gains of the optimum receiver both analytically and by simulation. Since the complexity of optimum receivers is extremely high when we have nonlinear distortion effects, even for OFDM signals with a small number of subcarriers, we propose several sub-optimum receivers and evaluate their performance. Our sub-optimal receivers allow remarkable performance improvements, being able to reduce significantly the gap between the optimum performance and the performance of typical OFDM receivers. © 1972-2012 IEEE.

Beko M.,University of Lisbon | Tomic S.,UNINOVA Campus FCT UNL | Dinis R.,New University of Lisbon | Dinis R.,Institute Telecomunicac Oes | Lipovac V.,University of Dubrovnik
IEEE Vehicular Technology Conference | Year: 2012

A novel algorithm for transmit beamforming to single cochannel multicast group is presented in this paper. We consider the max-min fairness (MMF) based beamforming problem where the maximization of the smallest receiver signal-to-noise ratio (SNR) over the secondary users subject to constraints on the transmit power and interference caused to the primary users. It is shown that this problem, which is nonconvex NP-hard, can be approximated by a convex second-order cone programming (SOCP) problem. Then, an iterative algorithm which successively improves the SOCP approximation is presented. Simulation results show the superior performance of the proposed approach, together with a reduced computational complexity, as compared to the state-of-the-art approach. © 2012 IEEE.

Zaric A.,University of Lisbon | Costa J.R.,Institute Telecomunicac oes | Costa J.R.,Instituto Universitario Of Lisbon Iscte Iul | Fernandes C.A.,University of Lisbon
IEEE Transactions on Antennas and Propagation | Year: 2014

A very compact low-profile (37.6 mm × 27 mm × 3.1 mm) unidirectional UWB antenna is proposed for wireless body area network (WBAN) localization applications by focusing on its ranging performance and impulse fidelity in time domain in addition to frequency domain characteristics. The antenna is immune to direct skin contact, and also demonstrates very good frequency and time domain properties in free space or at any distance to a body: reflection coefficient and radiation pattern resilience to body influence; flat transfer function amplitude and linear phase over the desired frequency band from 6 GHz to 9 GHz. Superior time domain performance is demonstrated in simulation and measurements with average impulse fidelity of 97% in both free space and when placed at 0mmor 3mmover the body. Off-body ranging measurements show accuracies of the order of 5 cmand achievable range higher than 5 m for almost all frontal directions (16 m being the maximum), with transmitting power complying with the international regulation. © 2014 IEEE.

Bory B.F.,TU Eindhoven | Wang J.,TU Eindhoven | Gomes H.L.,Institute Telecomunicac oes | Gomes H.L.,University of Algarve | And 4 more authors.
Applied Physics Letters | Year: 2014

Electroforming of indium-tin-oxide/alkali halide/poly(spirofluorene)/Ba/Al diodes has been investigated by bias dependent reflectivity measurements. The threshold voltages for electrocoloration and electroforming are independent of layer thickness and correlate with the bandgap of the alkali halide. We argue that the origin is voltage induced defect formation. Frenkel defect pairs are formed by electron-hole recombination in the alkali halide. This self-accelerating process mitigates injection barriers. The dynamic junction formation is compared to that of a light emitting electrochemical cell. A critical defect density for electroforming is 1025/m3. The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics. © 2014 Author(s).

De Oliveira A.V.C.,Dee Instituto Militar Of Engineering | Rodrigues N.M.M.,Institute Telecomunicac Oes | Rodrigues N.M.M.,Polytechnic Institute of Leiria | De Faria S.M.M.,Institute Telecomunicac Oes | And 3 more authors.
2014 International Telecommunications Symposium, ITS 2014 - Proceedings | Year: 2014

In this work, we present the first results of a new algorithm for image compression, called Variable Size Transform Coder (VSTC). This algorithm uses spatial transforms and recursive optimization functions combined with a highly adaptive segmentation scheme, similar to the one proposed by the MMP algorithm (Multi-dimensional Multiscale Parser), that uses pattern matching. We replaced the pattern matching by spatial multiscale transforms, which include symmetric and asymmetric blocks, and we investigated and implemented some of the required tools for encoding these new blocks. The proposed method uses a much more dynamic adaptive-size transforms than the H.264/AVC standard and compares favorably with this encoder. It also consistently outperforms the pattern matching scheme of the MMP algorithm. Moreover, we have considerably reduced the computational complexity when compared to the MMP algorithm. © 2014 IEEE.

Ribeiro E.,University of Coimbra | Ribeiro E.,Institute Telecomunicac oes | Cardoso A.J.M.,University of Beira Interior | Boccaletti C.,University of Rome La Sapienza
IEEE Transactions on Power Electronics | Year: 2014

Interleaved dc-dc converters have been widely applied, because of their benefits related to efficiency, size, thermal management, modularity, and output current ripple cancellation. These converters present an enhanced fault tolerance capability, but an open-circuit fault can leads to ripple beyond load requirements. This paper presents a fault-diagnostic method for interleaved dc-dc converters using only the dc-link current derivative sign features. The dc-link current derivative is thoroughly studied for both healthy and faulty modes. Its sign variation during different time intervals defined by the number of switches in conduction mode contains important information for open-circuit fault detection. The presented method is robust to transients and current imbalance between phases and no additional sensors are required. A photovoltaic system application is presented to validate this method. © 2013 IEEE.

Ferreira F.M.,Coriant | Ferreira F.M.,University of Coimbra | Fonseca D.,Coriant | Fonseca D.,Institute Telecomunicac oes | Da Silva H.J.A.,University of Coimbra
Journal of Lightwave Technology | Year: 2014

In this paper, we investigate the design of few-mode fibers (FMFs) guiding 2 to 12 linearly polarized (LP) modes with low differential mode delay (DMD) over the C-band, suitable for long-haul transmission. Two different types of refractive index profile have been considered: a graded-core with a cladding trench (GCCT) profile and a multi-step-index (MSI) profile. The profiles parameters are optimized in order to achieve: the lowest possible DMD and macro-bend losses (MBL) lower than the ITU-T standard recommendation. The optimization results show that the MSI profiles present lower DMD than the minimum achieved with a GCCT profile. Moreover, it is shown that the optimum DMD and the MBL scale with the number of modes for both profiles. The optimum DMD obtained for 12 LP modes is lower than 3 ps/km using a GCCT profile and lower than 2.5 ps/km using a MSI profile. The optimization results reveal that the most preponderant parameter of the GCCT profile is the refractive index relative difference at the core center, Δnco. Reducing Δn co, the DMD is reduced at the expense of increasing the MBL. Regarding the MSI profiles, it is shown that 64 steps are required to obtain a DMD improvement considering 12 LP modes. Finally, the impact of the fabrication margins on the optimum DMD is analyzed. The probability of having a manufactured FMF with 12 LP modes and DMD lower than 12 ps/km is approximately 68% using a GCCT profile and 16% using a MSI profile. © 2013 IEEE.

Jeon S.,Institute Telecomunicac oes | Kang N.,Duksung Womens University | Corujo D.,Institute Telecomunicac oes | Corujo D.,University of Aveiro | And 2 more authors.
Computer Networks | Year: 2015

In this paper, we conduct a comprehensive performance study of distributed and dynamic mobility management (DDMM). DDMM presents a new architectural paradigm for a sustainable mobile networking against an ever-increasing amount of Internet data traffic, providing IP mobility management with distributed deployment of mobility anchors and dynamic activation when mobility is needed. Such a distributed mobility management concept is generally and intuitively accepted in terms of effective distribution of mobile traffic when compared with centralized mobility management (CMM) approaches. Nevertheless, the routing strategy of DDMM has not yet been properly examined through performance studies, and especially the impact of potential mobility routing strategies on the user plane is an open question. We perform a mathematical analysis of DDMM and present numerical results aiming to identify in which conditions, by which factors, and how much, DDMM improves mobility performance. For comparison, Mobile IPv6, Proxy Mobile IPv6 (PMIPv6), and PMIPv6 localized routing (PMIPv6-LR) were considered as representative IP mobility protocols following CMM approaches. Analytical results demonstrate that DDMM generally achieves higher performance when compared with CMM-based protocols in terms of packet delivery cost, tunneling overhead, and throughput, but specific performance varies in function of multiple input parameters. © 2015 Elsevier B.V. All rights reserved.

Guerreiro J.,New University of Lisbon | Dinis R.,New University of Lisbon | Dinis R.,Institute Telecomunicac Oes | Carvalho P.,New University of Lisbon
IEEE Vehicular Technology Conference | Year: 2012

In this paper we consider the optimum ML (Maximum-Likelihood) detection for OFDM signals (Orthogonal Frequency Division Multiplexing) with iterative clipping and filtering. It is shown that the nonlinear distortion does not necessarily mean significant performance degradation and, in fact, the optimum performance could even better than the performance with ideal, linear transmitters. We also present sub-optimum receivers that allow remarkable performance improvements, being able to reduce significantly the gap between the optimum performance and the performance of typical OFDM receivers. © 2012 IEEE.

Loading Institute Telecomunicac oes collaborators
Loading Institute Telecomunicac oes collaborators