Οι ομιλίες παρουσιάζονται στην Αίθουσα Τηλεδιασκέψεων Κέντρου Διαχείρισης Δικτύων. Πιθανές εξαιρέσεις θα ανακοινώνονται ανά περίπτωση.
Η πλέον πρόσφατη παρουσίαση δεικνύεται με πράσινο χρώμα στο φόντο.

Η φυσική πρόσβαση στο Τμήμα Πληροφορικής & Tηλεπικοινωνιών, μπορεί να γίνει με: λεωφορείο  ή με μετρό-λεωφορείο (Metro-Buses).

28 Ιουλίου 2008 (Δευτέρα 12:00)
Ομιλητής: Prof. John S. Baras
"Trust and Reputation in Communication, Social and Other Networks"
 

Trust and reputation are critical concepts in networks - communication, control, computer, social, web-based, economic, biological. Trust evaluation leads to the development of relations and collaborations. These evaluations are based either on direct ‘communal’ monitoring and inference by the nodes, or on indirect references and credentials. We describe new fundamental ways for analyzing and evaluating trust in autonomic networks. Due to the dynamic and changing nature of autonomic networks trust evidence and the resulting evaluations may be uncertain and incomplete. The indirect evaluation process is modeled as a path problem on a directed graph, where nodes represent entities, and edges represent trust relations. We develop a novel formulation of trust computation as ‘linear’ iterations on ordered semirings. Using the theory of semirings, we analyze several key problems on the performance of trust algorithms. The direct trust evaluation process is modeled as iterated games on dynamic graphs. We present several explicit examples. We also present results on trust dynamics and propagation, including connections with statistical mechanics of spin glasses and consensus problems. We conclude with new fundamental principles for ‘Network Science’ bringing together algebraic, optimization and game theoretic methods. We conclude with future directions and open problems.

Short CV: John S. Baras, Lockheed Martin Chair in Systems Engineering B.S. in Electrical Eng. from the Nat. Techn. Univ. of Athens, Greece, 1970; M.S. and Ph.D. in Applied Math. from Harvard Univ. 1971, 1973. Since 1973 with the Electrical and Computer Engineering Department, and the Applied Mathematics Faculty, at the University of Maryland College Park. Faculty member in the Department of Computer Science and Fischell Department of Bioengineering. He was the founding Director of the Institute for Systems Research (ISR) from 1985 to 1991. Since 1991, has been the Director of the Maryland Center for Hybrid Networks (HYNET). Fellow of the IEEE and a Foreign Member of the Royal Swedish Academy of Engineering Sciences. Received the 1980 George Axelby Prize from the IEEE Control Systems Society and the 2006 Leonard Abraham Prize from the IEEE Communications Society. Professor Baras' research interests include control, communication and computing systems
Email: baras@isr.umd.edu
Web Page: http://www.isr.umd.edu/~baras

12 Ιουνίου 2008 (Πέμπτη 12:00)
Ομιλητής: Prof. Hari Sundaram
"Making sense of meaning; new computational perspectives"
 

The problem of the semantic gap; how to resolve sensory data to meaning, is a familiar and significant challenge within the media computing and AI communities. Today, a majority of the work in resolving the semantic gap lies in the learning paradigm, where our knowledge of the domain is encoded in terms of identification of effective features and appropriate learning algorithms. This talk will offer two long term research perspectives to address the semantic gap, where computational learning is integral, but which expand the problem space. The first perspective is in integrating multimedia with the physical world. The important consequence of this idea is that we move away from offline analysis of media semantics. In this idea, semantics are jointly constructed by the media system and the user, within a real-time, mediated, feedback control loop. I shall present our work in stroke rehabilitation as a specific example of this class of research. The goal of this research is to transfer the specific movement semantics, to enable stroke patients to perform challenging functional tasks (reach and grasp a cup). The second perspective is recognizing that meaning is an emergent, evolving artifact of collaborative human activity. In this idea, identification of human networks that produce meaning is a critical first step. I will present community discovery in large scale blog networks as a concrete example of this research. Both long term research frameworks provide us with a fresh set of computational problems that are synergistic with the learning paradigm. I shall briefly present other examples; optimal resource constrained real-time decision making, information flow in online networks, and collaborative annotation in data sparse media collections.

Short CV: Hari Sundaram is currently an assistant professor of media arts and computing, with the Arts Media and Engineering program, and Computer Science at Arizona State University. He received his Ph.D. from the Department of Electrical Engineering at Columbia University in 2002. He received his MS degree in Electrical Engineering from SUNY Stony Brook 1995 and a B.Tech in Electrical Engineering from Indian Institute of Technology, Delhi in 1993. His intellectual commitment is towards understanding how meaning emerges through our engagement with the physical online worlds. His research has focused on two complementary (but coupled) directions (a) designing intelligent media environments that exist as part of: our physical world (e.g. mediated environments that assist stroke patients recover) (b) developing new algorithms and systems to understand the media artifacts resulting from human activity (e.g.emails, photos / video). Specific projects include - context models for interpreting human action, understanding communication patterns in media sharing social networks, collaborative annotation, as well discovering emergent groups in online social networks. His research has won several awards; the best student paper award at JCDL 2007, the best ACM Multimedia demo award in 2006. The best student paper award at ACM Multimedia 2002, the 2002 Eliahu I. Jury Award for best Ph.D. dissertation. He has also received a best paper award on video retrieval from IEEE Trans. On Circuits and Systems for Video Technology, 2000. He is an active participant in the Multimedia community; he is an associate editor for ACM Transactions on Multimedia Computing, Communications and Applications (TOMCCAP), as well as the IEEE Signal Processing magazine. He has co-organized workshops at ACM multimedia on experiential telepresence (ETP 2003, ETP 2004), archival of personal experiences (CARPE 2004, CARPE 2005) and a conference of image and video retrieval (CIVR 2006).
Web Page: http://ame.asu.edu/faculty/hs/

28 Μαΐου 2008 (Τετάρτη 13:00)
Ομιλητής: Prof. John G. Proakis
"Equalization of MIMO broadcast channels"
 

This presentation treats multiple-input, multiple-output (MIMO) wireless communication systems that employ multiple transmit and receive antennas to increase the data rate and achieve signal diversity in fading multipath channels. The performance of MIMO systems is affected by two types of interference. One is intersymbol interference due to channel multipath dispersion. The other is interchannel interference due to the simultaneous transmission from the multiple transmit antennas. The focus of the presentation is on equalization and interference mitigation in point-to-multipoint (broadcast) MIMO systems in which the channel characteristics are known at the transmitter. Both linear and nonlinear equalization methods are described.

Short CV: Dr. Proakis is an Adjunct Professor at the University of California at San Diego and a Professor Emeritus at Northeastern University. He was a faculty member at Northeastern University from 1969 through 1998 and held the following academic positions: Associate Professor of Electrical Engineering, 1969-1976; Professor of Electrical Engineering, 1976-1998; Associate Dean of the College of Engineering and Director of the Graduate School of Engineering, 1982-1984; Chairman of the Department of Electrical and Computer Engineering, 1984-1997. His professional experience and interests are in the general areas of digital communications and digital signal processing. He is the co-author of the book Digital Communications (New York: McGraw-Hill, 2008, 5th ed.), Introduction to Digital Signal Processing ( Upper Saddle River, NJ: Prentice Hall, 2007, 4th ed.); Digital Signal Processing Laboratory (Englewood Cliffs: Prentice-Hall, 1991); Advanced Digital Signal Processing (New York: Macmillan, 1992); Digital Processing of Speech Signals (New York: Macmillan, 1992, IEEE Press, 2000); Communication Systems Engineering, (Upper Saddle River, NJ: Prentice Hall, 2002, 2nd ed.); Digital Signal Processing Using MATLAB V.4 (Boston: Brooks Cole-Thomson Learning, 2007, 2nd ed.); Contemporary Communication Systems Using MATLAB (Boston: Brooks Cole-Thomson Learning, 2004, 2nd ed.); Algorithms for Statistical Signal Processing(Upper Saddle River, NJ: Prentice Hall, 2002); Fundamentals of Communication Systems (Upper Saddle River, NJ: Prentice Hall, 2005).

20 Μαΐου 2008 (Τρίτη 11:00)
Ομιλητής: Dr. Roberto Cascella
"Security issues in autonomic systems: the "value" of reputation."
 

In this talk we will provide some basic definitions for an autonomic system, we will discuss security issues that might arise in this new communication paradigm, and we will focus mainly on trust and reputation approaches to thwart selfish behaviour. In this talk we discuss the importance for a node to build and use its reputation value and we present a game-theoretical framework, based on the generalized form of the Iterated Prisoner's Dilemma, to model the interactions of rational and selfish nodes in distributed systems. We study how a node takes into account the change of its reputation when deciding its behaviour in a transaction and discuss the Nash Equilibrium in the system. We finally present simulation results on nodes' interactions under different strategies to show how cooperation and reputation evolve in the system.

Short CV: Roberto G. Cascella received his Doctoral Degree in Information and Communication Technologies from the University of Trento in 2007, the "Laurea" Degree in Telecommunication Engineering from Politecnico di Torino and the Master of Science in Electrical Engineering from the Royal Institute of Technology (KTH) in Stockholm both in 2003. During 2002-2003, he was an intern at the Wireless@Kth, Center for Wireless Systems (Stockholm, Sweden), where he performed a Master of Science thesis on service discovery in reconfigurable networks. Currently, he is a research scientist at University of Trento in the project DAMASCO, founded by the Italian Ministry of Education, University and Research in the framework of international collaboration between Italy and USA. He is also involved in two European Projects, CASCADAS and BIONETS, on autonomic communication systems. His main interests are on trust and reputation management schemes for autonomic communication systems and on network security solutions for distributed and intelligent transport systems.

20 Μαΐου 2008 (Τρίτη 12:00)
Ομιλητής: Ass. Prof. Pietro Michiardi
"On the Impact of Greedy Strategies in BitTorrent Networks: the Case of BitTyrant"
 

The success of BitTorrent has fostered the development of variants to its basic components. Some of the variants adopt greedy approaches aiming at exploiting the intrinsic altruism of the original version of BitTorrent in order to maximize the benefit of participating to a torrent. In this work we study BitTyrant, a recently proposed strategic client. BitTyrant tries to determine the exact amount of contribution necessary to maximize its download rate by dynamically adapting and shaping the upload rate allocated to its neighbors. We evaluate in detail the various mechanisms used by BitTyrant to identify their contribution to the performance of the client. Our findings indicate that the performance gain is due to the increased number of connections established by a BitTyrant client, rather than for its subtle uplink allocation algorithm; surprisingly, BitTyrant reveals to be altruistic and particularly efficient in disseminating the content, especially during the initial phase of the distribution process. The apparent gain of a single BitTyrant client, however, disappears in the case of a widespread adoption: our results indicate a severe loss of efficiency that we analyzed in detail.

Short CV: Pietro Michiardi received his M.S. in Communication Systems in 1999 from Institut Eurecom and his M.S. in Electrical Engineering from "Politecnico di Torino" in 2001. In September 2001 Pietro joined "Ecole Nationale Superieure des Telecommunications" (ENST, Paris) as a Ph.D. student working on network security for mobile, wireless multi-hop networks. During his Ph.D. Pietro worked in the Network Security Team at Institut Eurecom focusing on topics ranging from game theoretic models of multi-hop networks to trust and reputation establishment schemes and identity-based cryptographic techniques. He obtained his Ph.D. in December 2004. Since January 2005, Pietro is an Assistant Professor in the Networking department, working on distributed systems and algorithms.

7 Μαΐου (Τετάρτη, 14:30) ΠΡΟΣΟΧΗ! η διάλεξη θα λάβει μέρος στην ΑΙΘΟΥΣΑ ΣΥΝΕΔΡΙΑΣΕΩΝ του τμήματος.
Ομιλητής: Dr. Jean-Dominique Decotignie
"Low power medium access control (MAC) protocols"
 

Low power medium access control (MAC) protocols have received a lot of attention in the last few years because of their impact on the lifetime of wireless sensor networks. Due to the cost of implementation on real hardware, and sometimes to a lack of detail in the description of these protocols, it is difficult to evaluate them. This talk will describe the latest advances in the field and introduces an ideal protocol as a benchmark. It presents detailed analytical models of the power consumption of the best and latest low power MAC protocols. These models are then used to evaluate how the performance of these protocols evolves when modifying traffic rate and network density. It will be shown that the most recent scheduled protocols are not acceptable because they cannot go below the maximal acceptable mean power consumption for battery operated sensor networks. The synchronous random access protocols S-MAC and SCP-MAC scale better, but are outperformed by asynchronous random access protocols based on preamble sensing such as WiseMAC, CSMA-MPS, X-MAC and SyncWUF. These last protocols offer the lowest power consumption for all considered data rates and for all considered network densities.

Short CV: Jean-Dominique Decotignie is head of the real-time software and networking group at CSEM. He is also adjunct professor at the Ecole Polytechnique Fιdιrale de Lausanne (EPFL), CH. From 1977 to 1982, he has worked at EPFL and the University of Tokyo in the area of optical communications. In 1983, he joined the Industrial Computer Engineering Lab. at EPFL where he became Assistant Professor in 1992. From 1989 to 1992, he has been the head of an interdisciplinary project of Computer Integrated Manufacturing at EPFL. Since January 1997, he is with CSEM. His current research interests include real-time networks and self-organizing wireless sensor networks as well as software engineering and middleware for real-time systems. Dr. Decotignie received his MS in 1977 and PhD in 1982 both in electrical engineering from the EPFL. He has published more than 100 papers in international journals and conferences. He is an IEEE fellow.

16 Απριλίου (Τετάρτη, 15:00)
Ομιλητής: Δρ. Γεώργιος Δ. Μήτσης
"Εξελιγμένες τεχνικές αναγνώρισης μη γραμμικών συστημάτων και εφαρμογές στη φυσιολογία, τις νευροεπιστήμες και τη λειτουργική απεικόνιση μαγνητικού συντονισμού (fMRI) του εγκεφάλου"
 

Η αναγνώριση μη γραμμικών συστημάτων παρουσιάζει σημαντικές δυσκολίες ως προς τον αριθμό και τον τύπο των δεδομένων εισόδου-εξόδου που απαιτούνται, τον αριθμό των ελεύθερων παραμέτρων που πρέπει να υπολογιστούν, ειδικά για συστήματα υψηλής τάξης, καθώς και την επίδραση του θορύβου στις εκτιμητέες ποσότητες. Το μεθοδολογικό υπόβαθρο Volterra-Wiener έχει χρησιμοποιηθεί ευρέως για το σκοπό αυτό και αποτελεί μια καθαρά μη παραμετρική προσέγγιση, δηλ. δεν στηρίζεται σε a priori υποθέσεις για τη δομή του συστήματος, περιγράφοντας τα δυναμικά χαρακτηριστικά τους με μια ιεραρχία γραμμικών και μη γραμμικών πυρήνων Volterra-Wiener. Σε αυτό το πλαίσιο, θα παρουσιάσουμε αρχικά μια νέα μεθοδολογική προσέγγιση (δίκτυα Laguerre-Volterra, LVN) για την αποδοτική εκτίμηση μη γραμμικών συστημάτων, η οποία συνδυάζει συναρτησιακές επεκτάσεις Laguerre με δίκτυα τύπου Volterra, ήτοι δίκτυα με πολυωνυμικές συναρτήσεις ενεργοποίησης. Η προσέγγιση αυτή αντιμετωπίζει επιτυχώς τις προαναφερόμενες δυσκολίες, παρέχοντας ακριβή μοντέλα από δεδομένα εισόδου-εξόδου μικρού μήκους και απαιτώντας πολύ λιγότερες ελεύθερες παραμέτρους συγκριτικά με άλλες μεθόδους. Επιπρόσθετα, μπορεί να χρησιμοποιηθεί για την αποδοτική μοντελοποίηση μη γραμμικών συστημάτων με πολλαπλές εισόδους, καθώς και συστημάτων με αργά και γρήγορα δυναμικά χαρακτηριστικά. Θα παρουσιάσουμε επίσης εφαρμογές της θεωρίας Bayes στην αναγνώριση μη γραμμικών συστημάτων, όπως η χρησιμοποίηση της Μπεϋζιανής τεκμηρίωσης μοντέλου ως κριτήριο για την επιλογή της πολυπλοκότητας των χρησιμοποιούμενων μοντέλων. Οι προαναφερόμενες μέθοδοι, λόγω της γενικότητας τους, είναι κατάλληλες για τη μοντελοποίηση βιολογικών/φυσιολογικών συστημάτων, καθώς τα συστήματα αυτά συχνά χαρακτηρίζονται από δυναμική, μη γραμμική συμπεριφορά και ενδογενή πολυπλοκότητα. Έτσι, θα παρουσιάσουμε διάφορες εφαρμογές τους στη φυσιολογία και τις νευροεπιστήμες. Συγκεκριμένα, θα μελετήσουμε τα αιμοδυναμικά χαρακτηριστικά του εγκεφάλου και την αυτορύθμιση της εγκεφαλικής αιματικής ροής, χρησιμοποιώντας πειραματικά δεδομένα τόσο από υπερηχογραφία Doppler όσο και από λειτουργική μαγνητική απεικόνιση του εγκεφάλου, το μεταβολισμό της γλυκόζης, καθώς και την κωδικοποίηση μηχανικών ερεθισμάτων σε δυναμικά ενέργειας σε νευρώνες-μηχανουποδοχείς.

Short CV: Ο Γεώργιος Μήτσης έλαβε το Δίπλωμα Ηλεκτρολόγου Μηχανικού και Μηχανικού Η/Y από το Ε.Μ.Π. το 1997, το Πτυχίο M.S. Μηχανικού Βιοϊατρικής, το Πτυχίο M.S. Ηλεκτρολόγου Μηχανικού καθώς και το Διδακτορικό Δίπλωμα (Ph.D.) Μηχανικού Βιοϊατρικής το 2000, 2001 και 2002 αντίστοιχα από το University of Southern California, Los Angeles. Έχει εργαστεί ως ερευνητής στο Ερευνητικό Κέντρο Biomedical Simulations Resource, Los Angeles καθώς και στο University of Oxford. Επί του παρόντος εςργάζεται ως ερευνητής στα πλαίσια του προγράμματος ΕΝΤΕΡ της ΓΓΕΤ στο Τμήμα Ηλεκτρολόγων Μηχανικών και Μηχανικών Η/Y του Ε.Μ.Π. Τα ερευνητικά του ενδιαφέροντα περιλαμβάνουν την αναγνώριση μη γραμμικών και χρονικά μεταβλητών συστημάτων, την επεξεργασία και ανάλυση βιολογικών/φυσιολογικών σημάτων και συστημάτων για διαγνωστικούς και κλινικούς σκοπούς, καθώς και τη λειτουργική απεικόνιση μαγνητικού συντονισμού του εγκεφάλου.

2 Απριλίου (Τετάρτη, 15:00)
Ομιλητής: Prof. Constantinos Mavroidis
"Nano-Robotics: From Science Fiction to Reality"
 

This talk will present the state of the art and future challenges in the field of nano-robotics. Nano-robots are controllable machines at the nano (10-9) meter or molecular scale that are composed of nano-scale components. With the modern scientific capabilities, it has become possible to attempt the creation of nanorobotic devices and interface them with the macro world for control. There are countless such machines that exist in nature and there is an opportunity to build more of them by mimicking nature. A roadmap towards the progression of this field from science fiction to reality is described and some design concepts and philosophies are illustrated. There are many applications for nanorobotic systems and its biggest impact would be in the area of medicine. Furthermore, we will present the design hypothesis, architectures and computational results of a novel protein based nanoGripper that could eventually serve as a component of a bionanorobotic system. Our goal was to engineer nanoGrippers that are inspired by nature and could be used in various biological, chemical and nano-manufacturing applications. We first characterized a natural peptide motif that could act as a template for designing nanoscale grippers due to its structure, stability and specificity at various physio-chemical conditions. We then altered and improved the functionality of these natural elements using protein engineering, which leads to protein-based nanoGrippers with optimal performance characteristics.

Short CV: Prof. Constantinos Mavroidis is a Professor of Mechanical and Industrial Engineering at Northeastern University in Boston MA since July 1, 2006. He is also a Visiting Scientist at the Massachusetts General Hospital and Shriners Hospital for Children in Boston, MA since October 2001. He was an Associate Professor in the same department from January 1, 2004 to June 30, 2006, an Associate Professor at the Department of Mechanical and Aerospace Engineering at Rutgers University (2001 - 2004) and an Assistant Professor at the same department (1996 – 2001), He received the Diploma in Mechanical Engineering from the National Technical University of Athens, Greece in 1988 and the M.S. and Ph.D. degrees in Robotics from the University of Paris VI, France, in 1989 and 1993 respectively. From 1993 to 1996 he was a Post-Doctoral Associate at the Department of Mechanical Engineering at the Massachusetts Institute of Technology. Dr. Mavroidis is a Fellow of the ASME and has received numerous prestigious awards including the 2004 Best of What’s New Award in the Personal Health category from the magazine Popular Science for the invention: “Smart Orthotic Device Using Electrorheological Fluids”. He has authored and co-authored more than 150 journal and conference papers and book contributions. He is a Technical Editor of the IEEE/ASME Transactions on Mechatronics, Member of the Editorial Board of the Journal of Computational and Theoretical Nanoscience and of the journal Bionanotechnology.
Email: mavro@coe.neu.edu
Web Page: http://www.bionano.neu.edu

11 Μαρτίου (Τρίτη, 15:30)
Ομιλητής: Dr. Nikolaos Laoutaris
"Can ISPs reclaim their networks from P2P? The dawn of Traffic Engineering 2.0"
 

Traditional routing and traffic engineering enable network providers to stir traffic according to their economic and technical constraints. These solutions are best suited to applications like Web and Email with fixed end-points, but don't interact as well with the newest breed of P2P applications. By selecting selfishly the sources that provide the fastest data rates, such applications affect actively the paths from which data flow in, and thus undermine the value of standard traffic control mechanisms. This can create a state of conflict between ISPs and P2P applications, and lead to drastic measures such as throttling or blocking of P2P connections. In this paper, we study the potential of less radical "traffic engineering 2.0" control mechanisms that intervene on the P2P overlay construction in order to leverage unexploited P2P service capacity that sits in more favorable network locations. Using real traffic and topology data from one of the world's largest ISPs, detailed information from 25,000 swarms with more than one million P2P clients, and a combination of analytic, numeric, and emulation methods, we show that ISPs can reclaim control of their network without severely impacting on their customer's QoS. In particular, we show that access providers can achieve a three-fold reduction of transit costs, while transit providers can balance P2P traffic across peering points in order to delay equipment upgrades caused by expensive or congested links. Finally, we show how to address "traffic hysteresis" phenomena due to P2P, that if left unattended, can seriously prolong the negative effects of transient network anomalies.
based on joint work with G. Siganos, X. Yang, and P. Rodriguez

Short CV:Nikolaos Laoutaris is a research scientist at Telefonica Research in Barcelona. Prior to joining the Barcelona lab he was a postdoc fellow at Harvard University and a Marie Curie postdoc fellow at Boston University. He got his PhD in computer science from the University of Athens in 2004. His main research interests go into algorithmic and performance evaluation issues of content distribution networks, overlay routing systems, peer-to-peer networks, and multimedia streaming systems.
Email: nikos@tid.es
Web Page: http://research.tid.es/nikos/

18 Φεβρουαρίου (Δευτέρα, 13:00)
Ομιλητής: Assistant Prof. Gregory V. Bard
"Open Questions in Boolean Systems of Equations"
 

The properties of boolean functions are of enormous importance in cryptography. Systems of boolean equations (and systems over finite fields of any characteristic for that matter) can be used to model any map from any finite set to any finite set. For this reason, they are of interest to cryptanalysts as well. While the general problem of solving a polynomial system of equations over the boolean field is provably NP-Complete, in practice, small systems can be solved. Research therefore focuses on tools to extend this threshold of solvability. The speaker will highlight a series of open questions, and discuss how they impact the future of cryptanalysis.

Short CV:
Gregory V. Bard received his B.Sc. in Computer and Systems Engineering from the Rensselaer Polytechnic Institute in 1999, and M.Sc. in Electrical and Computer Engineering and Ph.D. in Applied Mathematics and Scientific Computation both from the University of Maryland at College Park in 2002 and 2007 respectively. He is currently an Assistant Professor at the Department of Mathematics, Fordham University. His research interests include Cryptology, Algebra with emphasis on Finite Fields, Security Protocols, and Complexity Theory.

8 Φεβρουαρίου (Παρασκευή, 12:00)
Ομιλητής: Αναπλ. Καθηγητής Αθανάσιος Π. Λιάβας
"Cooperative and Opportunistic Communications: Why and How?"
 

Είναι γνωστό ότι το ασύρματο κανάλι θέτει σημαντικές δυσκολίες στην αξιόπιστη επικοινωνία εξαιτίας, κυρίως, του φαινομένου των διαλείψεων (fading). Όταν ένα ασύρματο κανάλι βρίσκεται σε διάλειψη, το μέτρο του είναι “μικρό” και σαν αποτέλεσμα έχουμε σημαντική εξασθένηση του μεταδιδόμενου σήματος. Δυστυχώς, σε περιβάλλοντα “πλούσια σε ανακλάσεις” (rich scattering) η πιθανότητα το κανάλι να βρίσκεται σε διάλειψη είναι σημαντική, με άμεση συνέπεια τη σημαντική υποβάθμιση της αξιοπιστίας της επικοινωνίας. Ένας τρόπος αντιμετώπισης του φαινομένου των διαλείψεων είναι η χρήση πολλών κατάλληλα τοποθετημένων κεραιών στο δέκτη ή/και στον εκπομπό. Η τεχνική αυτή καλείται διαφοροποίηση στο χώρο (space diversity) και χρησιμοποιείται ευρέως. Όταν η χρήση πολλών κεραιών είναι ανέφικτη λόγω, π.χ., κόστους ή/και μεγέθους της ασύρματης συσκευής, τότε πολλές συσκευές, με μία κεραία η καθεμία, μπορούν να συνεργαστούν και να δημιουργήσουν ένα ιδεατό (virtual) σύστημα πολλών κεραιών. Αυτή η τεχνική επικοινωνίας, που καλείται συνεργατική επικοινωνία (cooperative communication), έχει τις ρίζες της στο μοντέλο της θεωρίας Πληροφορίας Relay-channel. Σημειώνεται ότι, παρόλο που το μοντέλο αυτό έχει μελετηθεί επί 35 χρόνια, η χωρητικότητά του και, συνεπώς, ο βέλτιστος τρόπος επικοινωνίας είναι άγνωστα! Θα παρουσιαστούν πρόσφατα αποδοτικά προτόκολλα συνεργατικής επικοινωνίας και θα αναφερθούν μερικά ανοικτά προβλήματα στην περιοχή. Όταν θεωρούμε το ασύρματο σύστημα σαν ένα σύστημα με έναν εκπομπό (π..χ, σταθμός βάσης) και πολλούς δέκτες (χρήστες) που επιδιώκουν να εξασφαλίσουν πόρους επικοινωνίας (π.χ., χρόνο, φάσμα), τότε, αν ο εκπομπός γνωρίζει το (χρονικά μεταβαλλόμενο) κανάλι του κάθε χρήστη, η ποσότητα του συνολικού μεταδιδόμενου ποσού πληροφορίας μεγιστοποιείται αν σε κάθε χρονική στιγμή (σαν χρονική στιγμή ορίζουμε τη διάρκεια ενός πακέτου πληροφορίας) ο εκπομπός μεταδίδει μόνο στο χρήστη με το δυνατότερο κανάλι. Όταν υπάρχουν πολλοί εν δυνάμει χρήστες, τότε υπάρχει μεγάλη πιθανότητα να υπάρχει τουλάχιστον ένας με “ισχυρό” κανάλι κάθε χρονική στιγμή. Συνεπώς, το σύστημά μας μεταδίδει συνεχώς με το μέγιστο ρυθμό μετάδοσης πληροφορίας. Αυτός ο τρόπος επικοινωνίας καλείται οππορτουνιστική επικοινωνία (opportunistic communication) διότι οι χρήστες λαμβάνουν πληροφορία οππορτουνιστικά (μόνο όταν το κανάλι τους είναι δυνατό). Θα αναφέρουμε πρόσφατα αποτελέσματα και ανοιχτά προβλήματα στην περιοχή.

Short CV:
Ο Αθανάσιος Π. Λιάβας έλαβε το δίπλωμα και το διδακτορικό από το Τμήμα Μηχανικών Η/Υ και Πληροφορικής, Πανεπιστημίου Πατρών το 1989 και 1993, αντίστοιχα. Από το Μάρτιο του 2004, είναι Αναπληρωτής Καθηγητής του Τμήματος Ηλεκτρονικών Μηχανικών και Μηχανικών Υπολογιστών, Πολυτεχνείου Κρήτης. Είναι Associate Editor στο περιοδικό IEEE Transactions on Signal Processing και μέλος του IEEE Signal Processing for Communications and Networking Technical Committee. Τα ερευνητικά του ενδιαφέροντα εστιάζονται στις περιοχές Επεξεργασίας Σήματος για Τηλεπικοινωνίες και Θεωρίας Πληροφορίας.

23 Οκτωβρίου (Τρίτη, 15:00)
Ομιλητής: Prof. Sandeep Kumar S Gupta
"Thermal Management of Data Centers"
 

Current trends in data center growth show a rapid increase in both computing and storage capacity; IBM and Intel have announced integration process plans below 45nm for the next 10 years, while Google continually increases web-based storage space. Moreover, technological trends increase both the operating frequency and the power density which, for data centers, is expected to reach 60 KW/m2 by 2010. These factors combined yield an exponential increase in power needs of data centers which is not sustainable; in 2006, data centers in the U.S. used 59 billion KWh of electricity per year, costing US $4.1 billion and generating 864 million metric tons of carbon dioxide (CO2) emissions; this accounted for 2% of the total USA energy budget, while it is projected that it will reach 3% by 2010. The increasing cost of power and recent legislative efforts press for greener data centers. Half of that power was consumed by the data center cooling systems, which suggests a very inefficient mode of operation. A key effort in making data centers more efficient and environment-friendly is the study of the thermal aspects of data center operation and combating the thermal effects. Work performed at the IMPACT Lab (http://impact.asu.edu/) includes the study of the thermal issues in data centers, namely heat recirculation and hot spots, the derivation of analytical thermal models, the development of thermal-aware scheduling algorithms, and the design of a holistic, thermal-aware management software architecture. Thermal-aware management brings the operation of a data center closer to the optimal efficiency at any utilization pattern, and can save up to 20% in total cost of ownership (TCO).

Short CV:
Sandeep Kumar S Gupta received the BTech degree in computer science and engineering from the Institute of Technology, Banaras Hindu University, Varanasi, India, the MTech degree in computer science and engineering from the Indian Institute of Technology, Kanpur, and the MS and PhD degree in computer and information science from The Ohio State University, Columbus. He is currently an associate professor in the School of Computing and Informatics at the Ira A. Fulton School of Engineering, Arizona State University. Dr. Gupta's research interests include wireless sensor networks, mobile and pervasive computing, middleware, and embedded sensor networks for biomedical applications. He is a senior member of the ACM, the IEEE and a member of the IEEE Computer Society.

5 Οκτωβρίου (Παρασκευή, 11:00)
Ομιλητής: Dr. Gerasimos Potamianos
"Audio-Visual Speech Processing for Robust Human-Computer Interaction"
 

This talk will be structured in two parts. In the first half I will provide an overview of the activities in my group. The main emphasis will be placed on recent work conducted as part of FP6 EU projects, in particular integrated project CHIL – “Computers in the Human Interaction Loop”. CHIL is a technology driven project that aims to develop robust audio-visual perception technologies of human interaction during meetings and lectures inside smart rooms. The second part of the talk will delve more deeply into a specific class of audio-visual perceptual technologies, namely the problem of audio-visual speech processing with emphasis on automatic bimodal speech recognition. This line of work aims to exploit visual speeech information to improve speech recognition robustness in noisy environments, in a process akin to human lipreading. I will discuss in detail my work in this field, with emphasis on visual feature extraction in realistic environments and ongoing research in the area of audio-visual fusion.

Short CV:
Gerasimos (Makis) Potamianos received the Diploma degree in Electrical and Computer Engineering from the National Technical University of Athens, Greece in 1988, and the M.S.E. and Ph.D. degrees in Electrical and Computer Engineering from the Johns Hopkins University, Baltimore, Maryland, in 1990 and 1994, respectively. His thesis work has focused on statistical models for image processing. During 1994-1996 he has been a Postdoctoral Fellow with the Center for Language and Speech Processing, and from 1996 to 1999 a Senior Member of Technical Staff with the Speech and Image Processing Services Laboratory at AT&T Labs-Research. In 1999, he joined the Human Language Technologies department at the IBM Thomas J. Watson Research Center as a Research Staff Member, where he is currently manager of the Multimodal Conversational Technologies Department. Makis' research interests span the areas of multimodal speech processing and human-computer interaction with particular emphasis on audio-visual speech processing, automatic speech recognition, multimedia signal processing and fusion, as well as computer vision for human detection and tracking. Makis has published over 70 articles in these areas that have received over 400 citations and has a number of patents granted. He is a member of IEEE and a member of the Technical Chamber of Greece.

Web Page: http://www.research.ibm.com/people/g/gerasimos.potamianos

7 Σεπτεμβρίου (Παρασκευή, 12:00)
Ομιλητής: Dr. Angela Doufexi
"Random Beamforming in MIMO OFDMA Broadband Wireless Access Systems Exploiting Multiuser Diversity"
 

Recently, there has been considerable interest in Multiple Input Multiple Output (MIMO) systems, and their application to Orthogonal Frequency Division Multiplexing (OFDM) based Wireless Local Area Networks (WLANs) and Wireless Metropolitan Area Networks (WMANs), such as IEEE 802.16-2004 and 802.16e. Multiple antennas can be used to either increase capacity or improve performance due to diversity. Spatial Multiplexing (SM) can be applied to achieve higher data rates. Space Time Coding is another MIMO scheme to achieve both coding and diversity gains. Another form of diversity that can be exploited in wireless systems is multiuser diversity. Multiuser diversity gain arises from the fact that in a system with many users, whose channels vary independently, there is likely to be a user whose channel is near its peak at any one time (frequency or space). This multiuser diversity is best motivated by Knopp and Humblet’s information theoretic result, which shows that overall system throughput, can be maximized by allocating at any time the common channel resource to the user that can best exploit it. This can also be extended to the frequency and spatial domains. Whereas most diversity techniques aim to improve reliability of communications in fading channels, the role of multiuser diversity is to increase the total throughout. Hence, multiuser diversity explores the channel fading since there will be a user with a channel strength much larger than the mean level and by allocating the recourses to that user, the benefit of this strong channel is fully capitalized. Novel techniques that employ multiple antennas to provide spatial multiplexing gain and also exploit multiuser diversity due to user location that results from independent fading for each user in every subcarrier will be described. If we combine transmit beamforming with multiuser diversity, called opportunistic beamforming, the total system throughput can be increased considerably only with a small amount of feedback. Random beamforming is an extension of this concept in MIMO systems. Random beamforming techniques will be discussed that exploit multiuser and spatial diversity and these algorithms can be applied to an OFDMA MIMO wireless system for performance evaluation. Considerations such as QoS and fairness, will be taken into account.

Short CV:
Angela Doufexi is currently Lecturer in Wireless Networks at the University of Bristol. She graduated from the University of Athens with a B.Sc. in Physics in 1996. She received her M.Sc. in Electronic Engineering from Cardiff University in 1998. She then joined the Centre for Communications Research at the University of Bristol, where she received her Ph.D. in 2002. She has worked in a number of projects including the European IST projects SATURN, ROMANTIK, WCAM and ASTRALS. Her research interests include OFDM and OFDMA systems, multiuser diversity and resource allocation, wireless LANs and WiMax (PHY and MAC), space time coding and MIMO, 4th generation communications systems, and multimedia transmission. She has published over 60 journal and conference papers in these areas.

Email: A.Doufexi@bristol.ac.uk

5 Σεπτεμβρίου (Τετάρτη, 14:00)
Ομιλητής: Dr. Dimitris Kalofonos
"Nokia Research Center Cambridge: Nokia-MIT Collaborative Research on Pervasive Computing"
 

Started in 2006 to implement a deeper collaboration with MIT, the Nokia Research Center Cambridge (NRCC) was Nokia's first effort to "embed" a research team in a world class university. NRCC research currently features seven projects, each part of a larger vision where mobile devices become elements of an "ecosystem" of information, services, peripherals, sensors and other devices. The goal is to enhance consumers' lives and business productivity by enabling more intuitive interaction between people, machines and environments. Projects range from developing the underlying computer architecture to leveraging and extending the Semantic Web. This talk will present an overview of NRCC-MIT collaborative research, with a special focus on "MyNet", a project that aims at creating a platform for Peer-to-Peer (P2P) personal and social networking.

Short CV:
Dr. Dimitris N. Kalofonos joined the Nokia Research Center in 2001. He is currently a Principal Scientist at the Pervasive Computing Group of NRC Cambridge, where he is a technical lead of project "MyNet", and a Research Affiliate at MIT. He received his Dipl. Ing. from the National Technical University of Athens (NTUA) in 1994, and the M.Sc. and Ph.D. both in Electrical Engineering from Northeastern University, Boston, MA, in 1996 and 2001 respectively. From 1993 to 1994 he was an R&D Engineer at the Microwave and Optical Systems department of Intracom S.A., Athens, Greece. From 1996 to 2001 he was a Member and later a Senior Member of the Technical Staff with the Network Planning, Wireless Systems and Mobile Networking Systems departments of GTE Laboratories, Verizon Technology Organization and BBN Technologies respectively, all Corporate Research & Technology units of Verizon Inc. In these positions he conducted research on performance modeling of 2G and 3G CDMA cellular networks. Dr. Kalofonos has served in the Editorial Board of the Digital Living Network Alliance (DLNA), an industry standardization body of smart-home technology, and has served in the Technical Program Committees and as a reviewer of numerous IEEE conferences and publications. He was elected a Senior Member of the IEEE in 2005. His current research area is Pervasive Computing. His research interests include distributed computing platforms, pervasive networking, and security, with particular emphasis on making user interaction with such systems intuitive. Other areas where he has conducted research in the past are CDMA cellular systems, proximity (WPAN, WLAN) and ad-hoc networking, and the physical layer of CDMA-based systems.

3 Σεπτεμβρίου (Δευτέρα, 15:00)
Ομιλητής: Dr. Pierre R. Chevillat
"Intelligent Sensor Networks - The role of messaging middleware for end-to end business solutions"
 

This talk will give an overview of research activities in wireless sensor networking at the IBM Zurich Research Laboratory. We develop and prototype wireless network protocols, messaging middleware, and low-power sensor platforms to connect sensors & actuators with IBM enterprise computing. Our goal is to enable 'end-to-end' solutions, e.g. for asset management, logistics applications, or utility metering. In addition, we are working on location-sensing concepts which have application in retail environments (‘Store of the Future’). A particular focus is on publish/subscribe messaging protocols which offer unique advantages, e.g. for resilience and scalability, and which facilitate the deployment of applications across heterogeneous wireless sensor networks. Finally, we participate in EU-funded Project e-SENSE exploring the use of sensor networks to capture ambient intelligence for B3G mobile communication systems.

Short CV:
Pierre R. Chevillat received the Dipl. El-Ing. ETH degree from the Swiss Federal Institute of Technology, Z?rich, Switzerland, and MSEE and Ph.D. degrees from Illinois Institute of Technology, Chicago, USA. He joined the IBM Z?rich Research Laboratory in 1976 where he has done research in signal processing, coding algorithms, and protocols for wireless and wireline communication systems. As a research manager (since 1985) he has led projects on voiceband data modems, detectors for magnetic recording, radio and infrared wireless LANs, wideband CDMA for 3G cellular systems, and sensor networking. Dr. Chevillat received IBM Outstanding Technical Innovation and Achievement Awards. He is an IEEE Fellow ('95) and a recipient of the IEEE Millennium Medal. He has been an Associate Editor for the IEEE Transactions on Information Theory and is currently an editor of the International Journal of Wireless Information Networks. He served on the Senate Committee of the “Deutsches Zentrum f?r Luft- und Raumfahrt” (DLR) and on scientific boards for the "Deutsche Forschungsgemeinschaft". He was a Steering Board member of the “Wireless World Research Forum” and is currently chairing the scientific board of the “Swiss National Competence Center on Mobile Communication”.