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Sagduyu Y.E.,University of Maryland University College | Sagduyu Y.E.,Intelligent Automation Inc. | Georgiadis L.,Aristotle University of Thessaloniki | Tassiulas L.,CERTH ITI | And 2 more authors.
IEEE Transactions on Information Theory | Year: 2013

We consider a source node broadcasting to two receivers over a general erasure channel with receiver feedback. We characterize the capacity region of the channel and construct algorithms based on linear network coding (either randomized or depending on channel dynamics) that achieve this capacity. We then consider stochastic arrivals at the source for the two destinations and characterize the stable throughput region achieved by adapting the same algorithms that achieve capacity. Next, we modify these algorithms to improve their delay performance and characterize their stable throughput regions. Although the capacity and stability regions obtained by the algorithms are not always identical (because of the extra overhead needed for the algorithms to handle stochastic traffic), they are within a few bits of each other and have similar forms. This example exhibits an unusual relationship between capacity and stability regions and extends similar prior studies for multiple access channels. © 1963-2012 IEEE.

Vagenas E.,University of Patras | Paschos G.S.,CERTH ITI | Kotsopoulos S.A.,University of Patras
IEEE Transactions on Wireless Communications | Year: 2011

The beamforming capacity optimization problem in MISO systems, when the transmitter has both mean and covariance feedback of the channel, has been tackled only with the SNR maximization approach, which is known to give a sub-optimal solution. Numerical solutions of the full rank input covariance matrix, presented in the literature, are capable of tracking the beamforming vector only if it is the optimal capacity achieving solution. In this paper, we solve the beamforming capacity optimization problem by following an analytical approach that projects the beamforming vector on an orthonormal basis defined by the eigenvectors of the channel covariance matrix. The proposed formulation reduces the complexity of calculating the solution and provides intuition into the problem itself. In particular, we express the necessary conditions for beamforming capacity maximization as a system of two equations, which can be solved numerically very efficiently using the secant method. Surprisingly, our indicative numerical results for the 2 x 1 and 10 x 1 MISO systems, showed that for some cases the performance gain through beamforming capacity optimization compared to the SNR maximization approach can reach 0.4 bps/Hz. This means that the SNR maximization solution deviates considerably from the optimal beamforming vector. Finally, the optimality of the SNR maximization solution is also examined. © 2006 IEEE.

Mezaris V.,CERTH ITI | Huet B.,Eurecom
MM 2014 - Proceedings of the 2014 ACM Conference on Multimedia | Year: 2014

This is the abstract for the "Video Hyperlinking" tutorial, presented as part of the 2014 ACM Multimedia Conference. Video hyperlinking is the introduction of links that originate from pieces of video material and point to other relevant content, be it video or any other form of digital content. The tutorial presents the state of the art in video hyperlinking approaches and in relevant enabling technologies, such as video analysis and multimedia indexing and retrieval. Several alternative strategies, based on text, visual and/or audio information are introduced, evaluated and discussed, providing the audience with details on what works and what doesn't on real broadcast material.

Vardakas J.S.,CERTH ITI
2014 IEEE 19th International Workshop on Computer Aided Modeling and Design of Communication Links and Networks, CAMAD 2014 | Year: 2014

We present and analyze an Electric Vehicle (EV) charging management model for a fast charging station network in a smart grid environment. The proposed model considers a charging station network that provides service to multiple EV charging-classes. The basic feature of the proposed model is when EVs are blocked by their preferred station due to the unavailability of charging outlets, they are prompted via a communication system to select their next station, which either provides fixed or elastic charging services.We derive an analytical framework for the determination of the arrival procedure to the additional stations, which is then used for the derivation of the distribution of occupied charging outlets in both stations. Simulation results verify the high accuracy of the proposed analysis. The proposed analysis can be used for the determination of the number of stations that should be installed in a specific geographical area, so that EVs do not suffer charging delays. © 2014 IEEE.

Papagiannopoulou C.,CERTH ITI | Mezaris V.,CERTH ITI
HuEvent 2014 - Proceedings of the 2014 Workshop on Human Centered Event Understanding from Multimedia | Year: 2014

In this work we deal with the problem of summarizing image collections that correspond to a single event each. For this, we adopt a clustering-based approach, and we perform a comparative study of different clustering algorithms and image representations. As part of this study, we propose and examine the possibility of using trained concept detectors so as to represent each image with a vector of concept detector responses, which is then used as input to the clustering algorithms. A technique which indicates which concepts are the most informative ones for clustering is also introduced, allowing us to prune the employed concept detectors. Following the clustering, a summary of the collection (thus, also of the event) can be formed by selecting one or more images per cluster, according to different possible criteria. The combination of clustering and concept-based image representation is experimentally shown to result in the formation of clusters and summaries that match well the human expectations. Copyright 2014 ACM.

Drosou A.,Imperial College London | Moustakas K.,CE.R.T.H. I.T.I. | Tzovaras D.,CE.R.T.H. I.T.I.
ARTEMIS'10 - Proceedings of the 1st ACM Workshop on Analysis and Retrieval of Tracked Events and Motion in Imagery Streams, Co-located with ACM Multimedia 2010 | Year: 2010

This paper presents a novel framework for dynamic activity-related user authentication utilizing dynamic and static anthropometric information. The recognition of the performed activity is based on Radon transforms that are applied on spatiotemporal motion templates. User authentication is performed exploiting the behavioural variations between different users. The upper body limb anthropometric information is extracted for each user and an attributed body-related graph structure framework is employed for the detection of static biometric features of substantial discrimination power. Finally, a quality factor based on ergonomic criteria evaluates the recognition capacity of each activity. Experimental validation illustrates that the proposed approach for integrating static anthropometric features and activity-related recognition advances significantly the authentication performance.

Mantziou E.,CERTH ITI | Papadopoulos S.,CERTH ITI | Kompatsiaris Y.,CERTH ITI
MM 2013 - Proceedings of the 2013 ACM Multimedia Conference | Year: 2013

The paper describes the approach, the experimental settings, and the results obtained by the proposed methodology at the ACM Yahoo Multimedia Grand Challenge. Its main contribution is the use of fast and efficient features with a highly scalable semi-supervised learning approach, the Approximate Laplacian Eigenmaps (ALEs), and its extension, by computing the test set incrementally for learning concepts in time linear to the number of images (both labelled and unlabelled). A combination of two local visual features combined with the VLAD feature aggregation method and PCA is used to improve the efficiency and time complexity. Our methodology achieves somewhat better accuracy compared to the baseline (linear SVM) in small training sets, but improves the performance as the training data increase. Performing ALE fusion on a training set of 50K/concept resulted in a MiAP score of 0.4223, which was among the highest scores of the proposed approach. Copyright © 2013 ACM.

Katmada A.,CERTH ITI | Satsiou A.,CERTH ITI | Kompatsiaris I.,CERTH ITI
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2016

Crowdsourcing emerged with the development of Web 2.0 technologies as a distributed online practice that harnesses the collective aptitudes and skills of the crowd in order to reach specific goals. The success of crowdsourcing systems is influenced by the users’ levels of participation and interactions on the platform. Therefore, there is a need for the incorporation of appropriate incentive mechanisms that would lead to sustained user engagement and quality contributions. Accordingly, the aim of the particular paper is threefold: first, to provide an overview of user motives and incentives, second, to present the corresponding incentive mechanisms used to trigger these motives, alongside with some indicative examples of successful crowdsourcing platforms that incorporate these incentive mechanisms, and third, to provide recommendations on their careful design in order to cater to the context and goal of the platform. © Springer International Publishing AG 2016.

Apostolidis K.,CERTH ITI | Mezaris V.,CERTH ITI
HuEvent 2015 - Proceedings of the 2nd International Workshop on Human-Centered Event Understanding from Multimedia, co-located with MM 2015 | Year: 2015

This paper describes a method to temporally align photo collections that have been created during the same event by different users using their own unsynchronized digital photo capture devices. Using multiple similarity measures, we identify pairs of similar photos from different collections. We then temporally align the photo collections by traversing a graph, whose nodes represent the collections, and edges represent the similar photo pairs between collections. Outcome of this process is a set of modified timestamps for the photos, which could be used in applications such as time-based clustering and sub-event detection in multi-user photo collections. We evaluate the proposed synchronization method on benchmark datasets and we compare it to state-of-the-art methods, demonstrating its superiority. © 2015 ACM.

Apostolidis K.,CERTH ITI | Mezaris V.,CERTH ITI
CEUR Workshop Proceedings | Year: 2015

This paper describes the results of our participation to the Synchronization of Multi-User Event Media Task at the MediaEval 2015 challenge. Using multiple similarity measures, we identify pairs of similar media from different galleries. We use a graph-based approach to temporally synchronize user galleries; subsequently we use time information, geolocation information and visual concept detection results to cluster all photos into different sub-events. Our method achieves good accuracy on considerably diverse datasets.

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