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Paschos G.S.,Informatics and Telematics Institute | Fragiadakis C.,University of Thessaly | Georgiadis L.,Aristotle University of Thessaloniki | Tassiulas L.,University of Thessaly
Proceedings - IEEE INFOCOM | Year: 2013

We study an 1-hop broadcast channel with two receivers. Due to overhearing channels, the receivers have side information which can be leveraged by interflow network coding techniques to provide throughput increase. In this setup, we consider two different control mechanisms, the deterministic system, where the contents of the receivers' buffers are announced to the coding node via overhearing reports and the stochastic system, where the coding node makes stochastic control decisions based on statistics and the performance is improved via NACK messages. We study the minimal evacuation times for the two systems and obtain analytical expressions of the throughput region for the deterministic and the code-constrained region for the stochastic. We show that maximum performance is achieved by simple XOR policies. For equal transmission rates r1 = r2, the two regions are equal. If r1 ≠ r2, we showcase the tradeoff between throughput and overhead. © 2013 IEEE.

Tsiotsios C.,Imperial College London | Petrou M.,Informatics and Telematics Institute
Pattern Recognition | Year: 2013

Anisotropic diffusion filtering is highly dependent on some crucial parameters, such as the conductance function, the gradient threshold parameter and the stopping time of the iterative process. The various alternative options at each stage of the algorithm are examined and evaluated and the best choice is selected. An automatic stopping criterion is proposed, that takes into consideration the quality of the preserved edges as opposed to just the level of smoothing achieved. The proposed scheme is evaluated with the help of real and simulated images, and compared with other state of the art schemes using objective criteria. © 2012 Elsevier Ltd.

Gitzenis S.,Informatics and Telematics Institute | Paschos G.S.,Informatics and Telematics Institute | Tassiulas L.,University of Thessaly
Proceedings - IEEE INFOCOM | Year: 2012

A key consideration in novel communication paradigms in multihop wireless networks regards the scalability of the network. We investigate the case of nodes making random requests on content stored in multiple replicas over the wireless network. We show that, in contrast to the conventional paradigm of random communicating pairs, multihop communication is a sustainable scheme for certain values of file popularity, cache and network size. In particular, we formulate the joint problem of replication and routing and compute an order optimal solution. Assuming a Zipf file popularity distribution, we vary the number of files M in the system as a function of the nodes N, let both go to infinity and identify the scaling regimes of the required link capacity, from O(√N) down to O(1). © 2012 IEEE.

Kordelas G.,Informatics and Telematics Institute | Daras P.,Informatics and Telematics Institute
Pattern Recognition | Year: 2010

Viewpoint independent recognition of free-form objects and estimation of their exact position are a complex procedure with applications in robotics, artificial intelligence, computer vision and many other scientific fields. In this paper a novel approach is presented that addresses recognition of objects lying in highly cluttered and occluded scenes. The proposed procedure relies on distance maps, which are extracted and stored off-line for each of the 3D objects that might be contained in the scene. During the on-line recognition procedure distance maps are extracted from the scene. Greyscale images, derived from scene's distance maps, are matched with those of the object under recognition by applying similarity measures to the descriptors that are extracted from the images. The similarity is then estimated from image patches, which are defined using the SIFT descriptor in an appropriate way. After finding the best similarities the position of the object in the scene is estimated. This process is repeated until all objects are successfully recognized. Multiple experiments, which were performed on both 2.5D synthetic and real scenes, proved that the proposed method is robust and highly efficient to a satisfactory degree of occlusion and clutter. © 2010 Elsevier Ltd. All rights reserved.

Kalfas G.,Informatics and Telematics Institute | Pleros N.,Informatics and Telematics Institute
Journal of Lightwave Technology | Year: 2010

We present an agile and medium-transparent Medium Access Control (MT-MAC) protocol for seamless and dynamic capacity allocation over both optical and wireless transmission media in 60 GHz broadband Radio-Over-Fiber (RoF) networks. Medium transparency is achieved by means of parallelism between two simultaneously running contention periods and through nesting of wireless user-specific dataframes within Remote Antenna Unit (RAU)-specific optical Superframes. The first contention period reports on the traffic requesting RAUs and decides about the wavelength assignments, whereas the second contention period arbitrates traffic between wireless clients served by the same RAU. Seamless service delivery is completed by RAU-dedicated optical Superframes, each one incorporating multiple user-specific and time-division multiplexed dataframes that are opto-electronically converted at the RAU site and get transmitted wirelessly down to each end-user. The proposed MAC protocol is demonstrated to operate successfully both in RoF-over-bus as well as in RoF-over-Passive Optical Network (PON) architectures requiring only minor variations for getting adapted to the network topology. Its performance for both network topologies is evaluated through simulations for different number of end-users, different loads and network node densities and for bit-rates up to 3 Gb/s, both for a Poisson and for a burst-mode traffic model. Successful operation is demonstrated for all different cases, confirming its agility and showing that extended range 60 GHz LAN areas between wireless users even without line of sight conditions can be obtained. Moreover, the high throughput and low latency values for non-saturated network conditions reveal its potential for transforming broadband 60 GHz picocellular networks into highly effective RoF-enabled 60 GHz Wireless LANs even for high-bandwidth and time-sensitive applications like High-Definition video streaming. © 2010 IEEE.

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