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Christou I.T.,Athens Information Technology | Christou I.T.,Carnegie Mellon University
IEEE Transactions on Pattern Analysis and Machine Intelligence

We present a novel optimization-based method for the combination of cluster ensembles for the class of problems with intracluster criteria, such as Minimum-Sum-of-Squares-Clustering (MSSC). We propose a simple and efficient algorithmcalled EXAMCEfor this class of problems that is inspired from a Set-Partitioning formulation of the original clustering problem. We prove some theoretical properties of the solutions produced by our algorithm, and in particular that, under general assumptions, though the algorithm recombines solution fragments so as to find the solution of a Set-Covering relaxation of the original formulation, it is guaranteed to find better solutions than the ones in the ensemble. For the MSSC problem in particular, a prototype implementation of our algorithm found a new better solution than the previously best known for 21 of the test instances of the 40-instance TSPLIB benchmark data sets used in [CHECK END OF SENTENCE], [CHECK END OF SENTENCE], and [CHECK END OF SENTENCE], and found a worse-quality solution than the best known only five times. For other published benchmark data sets where the optimal MSSC solution is known, we match them. The algorithm is particularly effective when the number of clusters is large, in which case it is able to escape the local minima found by K-means type algorithms by recombining the solutions in a Set-Covering context. We also establish the stability of the algorithm with extensive computational experiments, by showing that multiple runs of EXAMCE for the same clustering problem instance produce high-quality solutions whose Adjusted Rand Index is consistently above 0.95. Finally, in experiments utilizing external criteria to compute the validity of clustering, EXAMCE is capable of producing high-quality results that are comparable in quality to those of the best known clustering algorithms. © 2011 IEEE. Source

In this paper, a novel cross-layer Adaptive Modulation and Coding scheme that optimizes the overall packet loss (by both transmission errors and excessive delays) probability under a given arrival process is developed. To this end, an improved Large Deviations approximation for the fraction of packets that suffer from excessive queuing delay is proposed. This approximation is valid for G/G/1 queues with infinite buffers that are driven by stationary arrival and service processes which satisfy certain conditions. Such models can capture the time correlations in the amount of traffic generated by streaming media sources and the time varying service capacity of a wireless link. Through numerical examples, the proposed AMC policy is shown to achieve a significant reduction in the overall packet loss rate compared to previously proposed schemes. This algorithmic performance gain can be translated into a sizeable decrease in the required transmit power or an analogous increase in the rate of the arrival process, subject to a given maximum packet loss rate Quality of Service constraint. Furthermore, the proposed AMC policy can be combined with ARQ in order to achieve an even lower overall packet loss probability. © 2009 Springer Science+Business Media, LLC. Source

Alrabadi O.N.,University of Aalborg | Perruisseau-Carrier J.,Ecole Polytechnique Federale de Lausanne | Kalis A.,Athens Information Technology
IEEE Transactions on Antennas and Propagation

An approach for transmitting multiple signals using a single switched parasitic antenna (SPA) has been recently reported. The idea there is to map the signals to be transmitted onto a set of basis functions that serve as "virtual antennas" in the beamspace (i.e., wavevector) domain. In this paper, we generalize the derivation of the antenna pattern basis functions regarding a three-element SPA of arbitrary radiating elements, within a symmetric array topology, for multiplexing signals in the wavevector domain (using different beampatterns) rather than in the hardware antenna domain with multiple feeding ports. A fully operational antenna system example is modeled, optimized regarding its return loss and the power imbalance between the basis functions, and finally realized. The measurements of the SPA show good agreement with the simulated target values, revealing an accurate design approach to be adopted as a fast SPA prototyping methodology. The SPA has been successfully employed for multiplexing two binary phase-shift-keying (BPSK) datastreams over-the-air, thus paving the way for practically compact and highly efficient MIMO transceiver designs. © 2011 IEEE. Source

Du H.,Queens University of Belfast | Ratnarajah T.,Queens University of Belfast | Pesavento M.,TU Darmstadt | Papadias C.B.,Athens Information Technology
IEEE Transactions on Signal Processing

This paper considers the spectrum sharing multiple-input-multiple-output (MIMO) cognitive radio network, in which multiple primary users (PUs) coexist with multiple secondary users (SUs). Joint transceiver cognitive beam former design is introduced to minimize the transmit power of the SU base station (SBS) while simultaneously targeting lower bounds on the received signal-to-interference-plus-noise ratio (SINR) for the SUs and imposing upper limits on the interference temperature to the PUs. With the perfect knowledge of all links, the optimal secondary transceiver beam former is achieved iteratively. Due to the limited cooperation between SBS and PUs, perfect information of primary links may not be available at SBS which could lead to severe interference to the PUs. Robust designs are developed against the uncertainties in the primary links by keeping the interference to the PU below a prespecified threshold with high probability. Simulation results are presented to validate the effectiveness of the proposed algorithms that minimizes the total transmit power and simultaneously guarantees quality-of-service (QoS) of both SUs and PUs. © 2011 IEEE. Source

Christodoulopoulos K.,Trinity College Dublin | Tomkos I.,Athens Information Technology | Varvarigos E.,University of Patras
IEEE Journal on Selected Areas in Communications

We consider the problem of serving traffic in a spectrum-flexible optical network, where the spectrum allocated to an end-to-end connection can change so as to adapt to the time-varying required transmission rate. In the proposed framework, each connection is assigned a route and is allocated a reference frequency over that route, using an appropriate Routing and Spectrum Allocation (RSA) algorithm, but the spectrum it utilizes around the reference frequency is allowed to expand and contract to match source rate fluctuations. We propose and analyze three spectrum expansion/contraction (SEC) policies for modifying the spectrum allocated to each connection. The first policy, named the Constant Spectrum Allocation (CSA) policy, allocates a number of spectrum slots for exclusive use by each connection. We also present two policies that enable the dynamic sharing of spectrum slots among connections, named the Dynamic High Expansion-Low Contraction (DHL) and the Dynamic Alternate Direction (DAD) policy. We give exact formulas for calculating the blocking probability for a connection and for the whole network under the CSA policy and provide corresponding approximate analyses under the DHL and DAD policies. We also present a simple iterative RSA algorithm that uses the developed blocking models so as to minimize the average blocking of the network. © 2012 IEEE. Source

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