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João Barros

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Morteza Taherkhani

Sharif University of Technology

Armando Leite da Silva

Pontifícia Universidade Católica do Rio de Janeiro

Vladimiro Miranda

INESC TEC, formerly INESC PORTO

Mehrdad Tarafdar Hagh

University of Tabriz

University of Tehran

Phillipe Vilaça Gomes

Pontifícia Universidade Católica do Rio de Janeiro

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Papers by João Barros

An approach to the explicit consideration of unreliability costs in transmission expansion planning

International Conference on Probabilistic Methods Applied to Power Systems, 2004

This paper proposes a new methodology to explicitly consider unreliability costs in a multistage ... more This paper proposes a new methodology to explicitly consider unreliability costs in a multistage transmission expansion planning. Starting from a reference plan, alternative expansion plans are derived based on postponement/anticipation of circuit implementations. These plans are then ranked by using an economic index of merit and the associated Pareto-optimal set is obtained through cost/benefit (trade-off) analyses. The cost/benefit is related

On Counteracting Byzantine Attacks in Network Coded Peer-to-Peer Networks

Computing Research Repository, 2009

Random linear network coding can be used in peer- to-peer networks to increase the efficiency of ... more Random linear network coding can be used in peer- to-peer networks to increase the efficiency of content distribution and distributed storage. However, these systems are particularly susceptible to Byzantine attacks. We quantify the impact of Byzantine attacks on the coded system by evaluating the prob- ability that a receiver node fails to correctly recover a file. We show that even

Network-Coded Cooperation Over Time-Varying Channels

IEEE Transactions on Communications, 2014

In this paper, we investigate the optimal design of cooperative network-coded strategies for a th... more In this paper, we investigate the optimal design of cooperative network-coded strategies for a three-node wireless network with time-varying half-duplex erasure channels. To this end, we formulate the problem of minimizing the total cost of transmitting M packets from source to two receivers as a Markov decision process (MDP). The actions of the MDP model include the source and the type of transmission to be used in a given time slot given perfect knowledge of the system state. The cost of packet transmission is defined such that it can incorporate the difference between broadcast and unicast transmissions, e.g., in terms of the rate of packet transmission or the energy consumption. A comprehensive analysis of the MDP solution is carried out under different network conditions to extract optimal rules of packet transmission. Inspired by the extracted rules, we propose two near-optimal heuristics that are suitable for practical systems. We use two wireless channel models to analyze the performance of the proposed heuristics in practical wireless networks, namely, 1) an infrastructure-to-vehicle communication in a highway scenario considering Rayleigh fading and 2) real packet loss measurements for WiFi using Aalborg University's Raspberry Pi testbed. We compare our results with random linear network coding broadcasting schemes showing that our heuristics can provide up to 2× gains in completion time and up to 4× gains in terms of reliably serviced data packets.

Aplicabilidade da escala de Torrington e Henderson

Revista da Associação Médica Brasileira, 2000

RESUMO -OBJETIVO. Validar a escala de Torrington e Henderson na estratificação do risco cirúrgico... more

Towards Usage of Formal methods within Embedded Systems Co-design

2005 IEEE Conference on Emerging Technologies and Factory Automation, 2005

Abstract This paper presents the planned and on-going activities inside the FORDESIGN research pr... more Abstract This paper presents the planned and on-going activities inside the FORDESIGN research project, funded by the Portuguese FCT (Foundation for Science and Technology), whose main objective is to foster the effective use of formal methods for embedded systems co-design. The project proposes a methodology where the use-cases are used as a starting point, not only for capturing requirements but also to support sub-model compositions. The methodology will support several different graphical formalisms, namely state diagrams, ...

On the capacity of the reachback channel in wireless sensor networks

2002 IEEE Workshop on Multimedia Signal Processing., 2002

We consider the problem of reachback communication in wireless sensor networks: multiple sensors ... more We consider the problem of reachback communication in wireless sensor networks: multiple sensors are deployed on a field, and they collect local measurements of some random process which then need to be encoded and reproduced at a remote location. In this paper we present a number of information theoretic bounds on the performance of a distributed transmission array that is formed by a large number of cheap, unreliable sensors. We formulate this problem in terms of classical network information theory concepts, formulation which leads us to consider two important cases: transmission of correlated sources over multiple independent channels, and rate/distortion with separate encoders.

Reachback capacity with non-interfering nodes

IEEE International Symposium on Information Theory, 2003. Proceedings., 2003

We consider a multiaccess communication problem, in which multiple sources want to send correlate... more We consider a multiaccess communication problem, in which multiple sources want to send correlated data to a far receiver, over a multiple access channel that is rendered virtually collision free by a suitable MAC protocol. This problem arises, e.g., in the context of sensor networks, where sensors are deployed on a field, and the goal is to send back to a remote location the signals picked up by all nodes. In this paper, we give an exact characterization of the joint source/channel capacity region, i.e., we give conditions on the sources and the channels under which reliable communication is possible in this context. These conditions generalize in a very meaningful way the condition that ¢ ¡ ¤ £ ¦ ¥ § © (entropy of a source less than channel capacity) for point-to-point channels, and surprisingly, give a separation theorem for this problem as well-we know of no other network information theory problem for which source and channel separation holds.

Effects of nonsinusoidal supply on voltage tolerance of equipment

IEEE Power Engineering Review, 2002

This letter shows the effects of harmonics in the voltage supply on the voltage tolerance of equi... more This letter shows the effects of harmonics in the voltage supply on the voltage tolerance of equipment. Several laboratory tests have been made to obtain the voltage tolerance curves of a personal computer using a pure sinusoidal voltage supply and under different nonsinuoidal conditions. The analysis shows that the voltage tolerance of the equipment used in the test depends on both the voltage harmonic distortion magnitude and the phase difference between harmonics, with the equipment being more sensitive to low order harmonics.

Práticas e Perspectivas dos Professores Face À Inclusão de Alunos com Necessidades Educativas Especiais na sua Disciplina

How to build vehicular networks in the real world

SenseMyCity: Crowdsourcing an Urban Sensor

Trade-off between cost and goodput in wireless: Replacing transmitters with coding

We study the cost of improving the goodput, or the useful data rate, to user in a wireless networ... more We study the cost of improving the goodput, or the useful data rate, to user in a wireless network. We measure the cost in terms of number of base stations, which is highly correlated to the energy cost as well as capital and operational costs of a network provider. We show that increasing the available bandwidth, or throughput, may not necessarily lead to increase in goodput, particularly in lossy wireless networks in which TCP does not perform well. As a result, much of the resources dedicated to the user may not translate to high goodput, resulting in an inefficient use of the network resources. We show that using protocols such as TCP/NC, which are more resilient to erasures and failures in the network, may lead to a goodput commensurate the throughput dedicated to each user. By increasing goodput, users' transactions are completed faster; thus, the resources dedicated to these users can be released to serve other requests or transactions. Consequently, we show that translating efficiently throughput to goodput may bring forth better connection to users while reducing the cost for the network providers.

Spherical codes for the Gaussian wiretap channel with continuous input alphabets

ABSTRACT We address the problem of designing codes for transmission over a Gaussian wiretap chann... more ABSTRACT We address the problem of designing codes for transmission over a Gaussian wiretap channel when the source is continuously-valued. We show that spherical codes based on flat tori can provide a natural constructive method to design codes for secrecy. In particular, their geometrical properties allow to fine tune the code for prescribed levels of reliability and secrecy, while providing a basis for efficient encoding and decoding. We present a suitable parametrization for these codes which enables legitimate users to communicate under a small distortion, while forcing the eavesdropper to operate at large distortions.

Random puncturing for secrecy

ABSTRACT The traditional framework under which error-correcting codes can enable secrecy involves... more ABSTRACT The traditional framework under which error-correcting codes can enable secrecy involves the use of nested codes. While this structure provide us with means to achieve weak secrecy, it is not clear how these codes can be applied to varying wiretap channels. In the spirit of rate-compatible codes, we propose a new framework for coding for the wiretap channel. The proposed scheme is based on random-puncturing, where the puncturing pattern is kept secret from the eavesdropper. Our results indicate that such strategy can indeed achieve high equivocation-rates (even when the channel is degraded) without the need to re-design the underlying codes.

Contextual Query Using Bell Tests

Tests are essential in Information Retrieval and Data Mining in order to evaluate the effectivene... more Tests are essential in Information Retrieval and Data Mining in order to evaluate the effectiveness of a query. An automatic measure tool intended to exhibit the meaning of words in context has been developed and linked with Quantum Theory, particularly entanglement. "Quantum like" experiments were undertaken on semantic space based on the Hyperspace Analogue Language (HAL) method. A quantum HAL model was implemented using state vectors issued from the HAL matrix and query observables, testing a wide range of window sizes. The Bell parameter S, associating measures on two words in a document, was derived showing peaks for specific window sizes. The peaks show maximum quantum violation of the Bell inequalities and are document dependent. This new correlation measure inspired by Quantum Theory could be promising for measuring query relevance.

Network coding for wireless cooperative networks: Simple rules, near-optimal delay

We consider the problem of finding an optimal packet transmission policy that minimizes the total... more We consider the problem of finding an optimal packet transmission policy that minimizes the total cost of transmitting M data packets from a source S to two receivers R1, R2 over half-duplex, erasure channels. The source can either broadcast random linear network coding (RLNC) packets to the receivers or transmit using unicast sessions at each time slot. We assume that the receivers can share their knowledge with each other by sending RLNC packets using unicast transmissions. We model this problem by using a Markov Decision Process (MDP), where the actions include the source of and type of transmission to be used in a given time slot given perfect knowledge of the system state. We study the distribution of actions selected by the MDP in terms of the knowledge at the receivers, the channel erasure probabilities, and the ratio between the cost of broadcast and unicast. This allowed us to learn from the optimal policy and devise two simple, yet powerful heuristics that are useful in practice. Our heuristics rely on different levels of feedback, namely, sending 1 or 2 feedback packets per receiver per M data packets by choosing the right moment to send this feedback. Our numerical results show that our heuristics are able to achieve the same performance of the MDP solution.

Impact of Position Errors on Path Loss Model Estimation for Device-to-Device Channels

Many wireless applications require a propagation model that describes the attenuation of the tran... more Many wireless applications require a propagation model that describes the attenuation of the transmitted signal as a function of the distance between devices. Such channel models are derived commonly from signal strength measurements, and assume that the true distances between wireless terminals are known. In practice, however, the true distances may be unavailable or difficult to obtain, for instance in mobile scenarios or in the absence of line-of-sight. These conditions typically occur in forested environments, urban areas, etc. This paper addresses the problem of path loss model parameter estimation in presence of erroneous distance measurements, such as the ones derived from the GPS positions. We provide a model for the uncertainties, and study the impact of distance errors on the estimation of a log-distance channel model.

Countering Byzantine adversaries with network coding: An overhead analysis

Finding Coverage Bounds for Constrained Random Walks over Sensor Networks

Page 1. 1 Finding Coverage Bounds for Constrained Random Walks over Sensor Networks Luısa Lima Jo... more

Physical-layer security over correlated erasure channels

We explore the additional security obtained by noise at the physical layer in a wiretap channel m... more We explore the additional security obtained by noise at the physical layer in a wiretap channel model setting. Security enhancements at the physical layer have been proposed recently using a secrecy metric based on the degrees of freedom that an attacker has with respect to the sent ciphertext. Prior work focused on cases in which the wiretap channel could be modeled as statistically independent packet erasure channels for the legitimate receiver and an eavesdropper. In this paper, we go beyond the state-of-the-art by addressing correlated erasure events across the two communication channels. The resulting security enhancement is presented as a function of the correlation coefficient and the erasure probabilities for both channels. It is shown that security improvements are achievable by means of judicious physical-layer design even when the eavesdropper has a better channel than the legitimate receiver. The only case in which this assertion may not hold is when erasures are highly correlated across channels. However, we are able to prove that correlation cannot nullify the expected security enhancement if the channel quality of the legitimate receiver is strictly better than that of the eavesdropper.

An approach to the explicit consideration of unreliability costs in transmission expansion planning

International Conference on Probabilistic Methods Applied to Power Systems, 2004

This paper proposes a new methodology to explicitly consider unreliability costs in a multistage ... more This paper proposes a new methodology to explicitly consider unreliability costs in a multistage transmission expansion planning. Starting from a reference plan, alternative expansion plans are derived based on postponement/anticipation of circuit implementations. These plans are then ranked by using an economic index of merit and the associated Pareto-optimal set is obtained through cost/benefit (trade-off) analyses. The cost/benefit is related

On Counteracting Byzantine Attacks in Network Coded Peer-to-Peer Networks

Computing Research Repository, 2009

Random linear network coding can be used in peer- to-peer networks to increase the efficiency of ... more Random linear network coding can be used in peer- to-peer networks to increase the efficiency of content distribution and distributed storage. However, these systems are particularly susceptible to Byzantine attacks. We quantify the impact of Byzantine attacks on the coded system by evaluating the prob- ability that a receiver node fails to correctly recover a file. We show that even

Network-Coded Cooperation Over Time-Varying Channels

IEEE Transactions on Communications, 2014

In this paper, we investigate the optimal design of cooperative network-coded strategies for a th... more In this paper, we investigate the optimal design of cooperative network-coded strategies for a three-node wireless network with time-varying half-duplex erasure channels. To this end, we formulate the problem of minimizing the total cost of transmitting M packets from source to two receivers as a Markov decision process (MDP). The actions of the MDP model include the source and the type of transmission to be used in a given time slot given perfect knowledge of the system state. The cost of packet transmission is defined such that it can incorporate the difference between broadcast and unicast transmissions, e.g., in terms of the rate of packet transmission or the energy consumption. A comprehensive analysis of the MDP solution is carried out under different network conditions to extract optimal rules of packet transmission. Inspired by the extracted rules, we propose two near-optimal heuristics that are suitable for practical systems. We use two wireless channel models to analyze the performance of the proposed heuristics in practical wireless networks, namely, 1) an infrastructure-to-vehicle communication in a highway scenario considering Rayleigh fading and 2) real packet loss measurements for WiFi using Aalborg University's Raspberry Pi testbed. We compare our results with random linear network coding broadcasting schemes showing that our heuristics can provide up to 2× gains in completion time and up to 4× gains in terms of reliably serviced data packets.

Aplicabilidade da escala de Torrington e Henderson

Revista da Associação Médica Brasileira, 2000

RESUMO -OBJETIVO. Validar a escala de Torrington e Henderson na estratificação do risco cirúrgico... more

Towards Usage of Formal methods within Embedded Systems Co-design

2005 IEEE Conference on Emerging Technologies and Factory Automation, 2005

Abstract This paper presents the planned and on-going activities inside the FORDESIGN research pr... more Abstract This paper presents the planned and on-going activities inside the FORDESIGN research project, funded by the Portuguese FCT (Foundation for Science and Technology), whose main objective is to foster the effective use of formal methods for embedded systems co-design. The project proposes a methodology where the use-cases are used as a starting point, not only for capturing requirements but also to support sub-model compositions. The methodology will support several different graphical formalisms, namely state diagrams, ...

On the capacity of the reachback channel in wireless sensor networks

2002 IEEE Workshop on Multimedia Signal Processing., 2002

We consider the problem of reachback communication in wireless sensor networks: multiple sensors ... more We consider the problem of reachback communication in wireless sensor networks: multiple sensors are deployed on a field, and they collect local measurements of some random process which then need to be encoded and reproduced at a remote location. In this paper we present a number of information theoretic bounds on the performance of a distributed transmission array that is formed by a large number of cheap, unreliable sensors. We formulate this problem in terms of classical network information theory concepts, formulation which leads us to consider two important cases: transmission of correlated sources over multiple independent channels, and rate/distortion with separate encoders.

Reachback capacity with non-interfering nodes

IEEE International Symposium on Information Theory, 2003. Proceedings., 2003

We consider a multiaccess communication problem, in which multiple sources want to send correlate... more We consider a multiaccess communication problem, in which multiple sources want to send correlated data to a far receiver, over a multiple access channel that is rendered virtually collision free by a suitable MAC protocol. This problem arises, e.g., in the context of sensor networks, where sensors are deployed on a field, and the goal is to send back to a remote location the signals picked up by all nodes. In this paper, we give an exact characterization of the joint source/channel capacity region, i.e., we give conditions on the sources and the channels under which reliable communication is possible in this context. These conditions generalize in a very meaningful way the condition that ¢ ¡ ¤ £ ¦ ¥ § © (entropy of a source less than channel capacity) for point-to-point channels, and surprisingly, give a separation theorem for this problem as well-we know of no other network information theory problem for which source and channel separation holds.

Effects of nonsinusoidal supply on voltage tolerance of equipment

IEEE Power Engineering Review, 2002

This letter shows the effects of harmonics in the voltage supply on the voltage tolerance of equi... more This letter shows the effects of harmonics in the voltage supply on the voltage tolerance of equipment. Several laboratory tests have been made to obtain the voltage tolerance curves of a personal computer using a pure sinusoidal voltage supply and under different nonsinuoidal conditions. The analysis shows that the voltage tolerance of the equipment used in the test depends on both the voltage harmonic distortion magnitude and the phase difference between harmonics, with the equipment being more sensitive to low order harmonics.

Práticas e Perspectivas dos Professores Face À Inclusão de Alunos com Necessidades Educativas Especiais na sua Disciplina

How to build vehicular networks in the real world

SenseMyCity: Crowdsourcing an Urban Sensor

Trade-off between cost and goodput in wireless: Replacing transmitters with coding

We study the cost of improving the goodput, or the useful data rate, to user in a wireless networ... more We study the cost of improving the goodput, or the useful data rate, to user in a wireless network. We measure the cost in terms of number of base stations, which is highly correlated to the energy cost as well as capital and operational costs of a network provider. We show that increasing the available bandwidth, or throughput, may not necessarily lead to increase in goodput, particularly in lossy wireless networks in which TCP does not perform well. As a result, much of the resources dedicated to the user may not translate to high goodput, resulting in an inefficient use of the network resources. We show that using protocols such as TCP/NC, which are more resilient to erasures and failures in the network, may lead to a goodput commensurate the throughput dedicated to each user. By increasing goodput, users' transactions are completed faster; thus, the resources dedicated to these users can be released to serve other requests or transactions. Consequently, we show that translating efficiently throughput to goodput may bring forth better connection to users while reducing the cost for the network providers.

Spherical codes for the Gaussian wiretap channel with continuous input alphabets

ABSTRACT We address the problem of designing codes for transmission over a Gaussian wiretap chann... more ABSTRACT We address the problem of designing codes for transmission over a Gaussian wiretap channel when the source is continuously-valued. We show that spherical codes based on flat tori can provide a natural constructive method to design codes for secrecy. In particular, their geometrical properties allow to fine tune the code for prescribed levels of reliability and secrecy, while providing a basis for efficient encoding and decoding. We present a suitable parametrization for these codes which enables legitimate users to communicate under a small distortion, while forcing the eavesdropper to operate at large distortions.

Random puncturing for secrecy

ABSTRACT The traditional framework under which error-correcting codes can enable secrecy involves... more ABSTRACT The traditional framework under which error-correcting codes can enable secrecy involves the use of nested codes. While this structure provide us with means to achieve weak secrecy, it is not clear how these codes can be applied to varying wiretap channels. In the spirit of rate-compatible codes, we propose a new framework for coding for the wiretap channel. The proposed scheme is based on random-puncturing, where the puncturing pattern is kept secret from the eavesdropper. Our results indicate that such strategy can indeed achieve high equivocation-rates (even when the channel is degraded) without the need to re-design the underlying codes.

Contextual Query Using Bell Tests

Tests are essential in Information Retrieval and Data Mining in order to evaluate the effectivene... more Tests are essential in Information Retrieval and Data Mining in order to evaluate the effectiveness of a query. An automatic measure tool intended to exhibit the meaning of words in context has been developed and linked with Quantum Theory, particularly entanglement. "Quantum like" experiments were undertaken on semantic space based on the Hyperspace Analogue Language (HAL) method. A quantum HAL model was implemented using state vectors issued from the HAL matrix and query observables, testing a wide range of window sizes. The Bell parameter S, associating measures on two words in a document, was derived showing peaks for specific window sizes. The peaks show maximum quantum violation of the Bell inequalities and are document dependent. This new correlation measure inspired by Quantum Theory could be promising for measuring query relevance.

Network coding for wireless cooperative networks: Simple rules, near-optimal delay

We consider the problem of finding an optimal packet transmission policy that minimizes the total... more We consider the problem of finding an optimal packet transmission policy that minimizes the total cost of transmitting M data packets from a source S to two receivers R1, R2 over half-duplex, erasure channels. The source can either broadcast random linear network coding (RLNC) packets to the receivers or transmit using unicast sessions at each time slot. We assume that the receivers can share their knowledge with each other by sending RLNC packets using unicast transmissions. We model this problem by using a Markov Decision Process (MDP), where the actions include the source of and type of transmission to be used in a given time slot given perfect knowledge of the system state. We study the distribution of actions selected by the MDP in terms of the knowledge at the receivers, the channel erasure probabilities, and the ratio between the cost of broadcast and unicast. This allowed us to learn from the optimal policy and devise two simple, yet powerful heuristics that are useful in practice. Our heuristics rely on different levels of feedback, namely, sending 1 or 2 feedback packets per receiver per M data packets by choosing the right moment to send this feedback. Our numerical results show that our heuristics are able to achieve the same performance of the MDP solution.

Impact of Position Errors on Path Loss Model Estimation for Device-to-Device Channels

Many wireless applications require a propagation model that describes the attenuation of the tran... more Many wireless applications require a propagation model that describes the attenuation of the transmitted signal as a function of the distance between devices. Such channel models are derived commonly from signal strength measurements, and assume that the true distances between wireless terminals are known. In practice, however, the true distances may be unavailable or difficult to obtain, for instance in mobile scenarios or in the absence of line-of-sight. These conditions typically occur in forested environments, urban areas, etc. This paper addresses the problem of path loss model parameter estimation in presence of erroneous distance measurements, such as the ones derived from the GPS positions. We provide a model for the uncertainties, and study the impact of distance errors on the estimation of a log-distance channel model.

Countering Byzantine adversaries with network coding: An overhead analysis

Finding Coverage Bounds for Constrained Random Walks over Sensor Networks

Page 1. 1 Finding Coverage Bounds for Constrained Random Walks over Sensor Networks Luısa Lima Jo... more

Physical-layer security over correlated erasure channels

We explore the additional security obtained by noise at the physical layer in a wiretap channel m... more We explore the additional security obtained by noise at the physical layer in a wiretap channel model setting. Security enhancements at the physical layer have been proposed recently using a secrecy metric based on the degrees of freedom that an attacker has with respect to the sent ciphertext. Prior work focused on cases in which the wiretap channel could be modeled as statistically independent packet erasure channels for the legitimate receiver and an eavesdropper. In this paper, we go beyond the state-of-the-art by addressing correlated erasure events across the two communication channels. The resulting security enhancement is presented as a function of the correlation coefficient and the erasure probabilities for both channels. It is shown that security improvements are achievable by means of judicious physical-layer design even when the eavesdropper has a better channel than the legitimate receiver. The only case in which this assertion may not hold is when erasures are highly correlated across channels. However, we are able to prove that correlation cannot nullify the expected security enhancement if the channel quality of the legitimate receiver is strictly better than that of the eavesdropper.