Papers by Mário Jorge Leitão
IEE Proceedings - Microwaves, Antennas and Propagation, 1994
A new method for prediction of attenuation on satellite-earth links is presented for systems oper... more A new method for prediction of attenuation on satellite-earth links is presented for systems operating with low fade margins. The method is based on modelling the physical structure of rainfall and includes, in order of importance: (1) an improved approach to combining components of fading; (ii) a non-uniform vertical on-average profile of rain specific-attenuation: (iii) the differing frequency dependencies of melting zone and rain components; and (iv) contributions from widespread and showery rain. The method has been tested using 21 years of data covering the 11-30GHz range and shows good agreement. Finally, contour maps of link availabilities are presented for a hypothetical 20/30 GHz satellite at 6" West.
A flexible technique for source clock recovery of real-time services in ATM networks
This paper addresses source clock recovery in ATM networks for real-time services which require a... more This paper addresses source clock recovery in ATM networks for real-time services which require a timing relation between source and destination. A detailed jitter analysis is performed for the synchronous methods, from which a simple design approach is suggested for the clock recovery system. A new timing information transport protocol is proposed for constant bit rate and variable bit rate real-time services, leading to a unified ATM adaptation layer for both types of services. The method presented is called generic synchronous technique (GST) and exhibits the following properties: (a) good control on its parameters to satisfy jitter objectives; (b) decoupling between timing and service information; (c) efficient coding leading to reduced overhead; (d) good transmission error immunity and (e) simple implementation. As a case study, it is shown that GST provides adequate clock recovery for MPEG video signals, in the most stringent case when the reconstructed system clock is used to derive a chrominance sub-carrier for use in composite video
Web based geographic information platform for tourism
Several developments have been observed recently in areas such as Web development, social network... more Several developments have been observed recently in areas such as Web development, social networks, interface design, recommendation systems and Geographic Information Systems (GIS). The integration of these developments can provide a superior experience, greater than the sum of their individual contributions, regarding user satisfaction. This paper proposes an integration of all these innovations in e-tourism, more specifically by the development of a Web based geographic information platform adaptable to any tourist region. As a case study, we also show how this platform was adapted to the Douro region, in Portugal. The Web platform developed as a proof of concept combines geospatial information from diverse and heterogeneous data sources, encompassing events, news, routes and points of interest (POI). This platform provides also a recommendation engine and features the possibility that users can contribute with content as part of the community, thus emerging a mini social network.
The resource allocation in the Broadband Integrated Services Digital Network (B-ISDN) can be base... more The resource allocation in the Broadband Integrated Services Digital Network (B-ISDN) can be based in an overall network performance function described in this paper and named quality of operation. The quality of operation function is determined itself by bandwidth and quality of service functions. The traffic patterns of the quality of service for each call are predicted by neural networks. The applicability of the quality of operation function to connection admission control and call routing is proposed and supported by simulation results.
This paper discusses a technique for call admission and routing control, based on a global qualit... more This paper discusses a technique for call admission and routing control, based on a global quality function, which is dependent on the allocated bandwidth, the free network capacity and the call rejection rate, and incorporates quality of service functions, predicted by neural networks. The superior capability of this technique to support admission and routing decisions, according to the characteristics of the traffic generated by admitted calls, is demonstrated by simulation results carried out using suitable traffic and network models, which are equally discussed.
The resource allocation in the Broadband Integrated Services Digital Network (B-ISDN) must guaran... more The resource allocation in the Broadband Integrated Services Digital Network (B-ISDN) must guarantee the quality of service negotiated with new and existing calls, taking into account the Asynchronous Transfer Mode (ATM) statistical characteristics. A quality of operation function, characterizing the overall network performance, is proposed, and based on this function, it is introduced a new strategy for the admission control and routing of the ATM call connections. As it is shown by simulation results, feedforword Neural Networks trained with the backpropagation algorithm, can learn the traffic patterns in previous traffic situations, and can be used to predict the quality of operation changes caused by each new call.
IEEE Communications Magazine, 1995
The combination of shared bandwidth and rejection rate parameters, together with the quality of s... more The combination of shared bandwidth and rejection rate parameters, together with the quality of service predicted by neural networks in a novel strategy for connection admission control and call routing.
Iee Proceedings F Communications, Radar and Signal Processing, 1987
A new method for prediction of attenuation on satellite-earth links is presented for systems oper... more A new method for prediction of attenuation on satellite-earth links is presented for systems operating with low fade margins. The method is based on modelling the physical structure of rainfall and includes, in order of importance: (1) an improved approach to combining components of fading; (ii) a non-uniform vertical on-average profile of rain specific-attenuation: (iii) the differing frequency dependencies of melting zone and rain components; and (iv) contributions from widespread and showery rain. The method has been tested using 21 years of data covering the 11-30GHz range and shows good agreement. Finally, contour maps of link availabilities are presented for a hypothetical 20/30 GHz satellite at 6" West.
Iee Proceedings F Communications, Radar and Signal Processing, 1986
A new method for prediction of attenuation on earth-space links is presented, which is more rigor... more A new method for prediction of attenuation on earth-space links is presented, which is more rigorous in relation to modelling the physical characteristics of rainstorms than previously published techniques. It is also more accurate in providing attenuation predictions for the European region than all other published methods. The method has been derived nonempirically, based entirely on the structure of storms as observed on a dual-polarisation radar and interpreted using scattering theory.
Iee Proceedings F Radar and Signal Processing, 1986
A new method for prediction of attenuation on earth-space links is presented, which is more rigor... more A new method for prediction of attenuation on earth-space links is presented, which is more rigorous in relation to modeling the physical characteristics of rainstorms than previously published techniques. It is also more accurate in providing attenuation predictions for the European region than all other published methods. The method has been derived nonempirically, based entirely on the structure of storms as observed on a dual-polarization radar and interpreted using scattering theory.
The combination of shared bandwidth and rejection rate parameters, together with the quality of s... more The combination of shared bandwidth and rejection rate parameters, together with the quality of service predicted by neural networks in a novel strategy for connection admission control and call routing.
It is generally considered that a key component of electronic government in the future will be el... more It is generally considered that a key component of electronic government in the future will be electronic voting, as a means of facilitating the participation of citizens in elections and public debates. However, a long path has to be pursued before electronic voting, particularly if based on Internet, is accepted as a reliable system alternative to conventional methods. In this paper, we propose a new and simple platform, based on open software, which can be used primarily in small to medium sized communities, as a means to build confidence and experience for future larger elections. We try to provide adequate answers to multiple requirements, such as accuracy, democracy, privacy, verifiability and mobility. This can be done by establishing a distributed system which supports the different roles of a voting system and by using cryptography techniques in the interactions between these components.
A Methodology for Auditing eVoting Processes and Systems used at the Elections for the Portuguese Parliament
The 2006 conference on Electronic Voting took place in Castle Hofen near Bregenz at the wonderful... more The 2006 conference on Electronic Voting took place in Castle Hofen near Bregenz at the wonderful Lake Constance from 2nd to 4th of August. This volume contains the twenty papers selected for the presentation at the conference out of more than forty submissions. To assure scientific quality, the selection was based on a strict and anonymous review process. The papers cover the following subjects: e-voting experiences, social, legal, political, democratic and security issues of e-voting, as well as solutions on how to (re)design election workflows, and finally how ...
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Papers by Mário Jorge Leitão