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Kofidis E.,University of Piraeus | Kofidis E.,Computer Technology Institute and Press Diophantus CTI
Proceedings of the International Symposium on Wireless Communication Systems | Year: 2016

Recently, there has been a significant progress in the study of preamble-based channel estimation for fiLTEr bankbased multicarrier systems using offset QAM (FBMC/OQAM), addressing the more demanding highly frequency selective scenario. In this context, analytical optimization of the preamble has been only considered for short preambles consisting of a single pilot FBMC symbol. Optimization of longer preambles with the FBMC/OQAM modulation is considerably more challenging in view of its well-known intrinsic interference effect, and may allow for the estimation of very high dispersion channels. However, this problem has only been studied for per-subchannel estimation via iterative techniques as well as for the low frequency selective case. This paper is concerned with the multi-symbol optimal preamble design for highly frequency selective channels, exhibiting the challenges and difficulties of the problem for preambles of two pilot symbols. For this case, preliminary design examples are provided and simulation results involving highly frequency selective channels are reported and discussed. © 2015 IEEE. Source

Di Luna G.A.,University of Rome La Sapienza | Baldoni R.,University of Rome La Sapienza | Bonomi S.,University of Rome La Sapienza | Chatzigiannakis I.,Computer Technology Institute and Press Diophantus CTI
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2014

This paper addresses the problem of counting the size of a network where (i) processes have the same identifiers (anonymous nodes) and (ii) the network topology constantly changes (dynamic network). Changes are driven by a powerful adversary that can look at internal process states and add and remove edges in order to contrast the convergence of the algorithm to the correct count. The paper proposes two leader-based counting algorithms. Such algorithms are based on a technique that mimics an energy-transfer between network nodes. The first algorithm assumes that the adversary cannot generate either disconnected network graphs or network graphs where nodes have degree greater than D. In such algorithm, the leader can count the size of the network and detect the counting termination in a finite time (i.e., conscious counting algorithm). The second algorithm assumes that the adversary only keeps the network graph connected at any time and we prove that the leader can still converge to a correct count in a finite number of rounds, but it is not conscious when this convergence happens. © 2014 Springer-Verlag. Source

Kofidis E.,University of Piraeus | Kofidis E.,Computer Technology Institute and Press Diophantus CTI
ISCCSP 2014 - 2014 6th International Symposium on Communications, Control and Signal Processing, Proceedings | Year: 2014

Filter bank-based multicarrier (FBMC) techniques have attracted a lot of interest in the last decade as a competitive alternative to the long established OFDM, particularly in wireless applications. Their potential stems from their increased ability to carrying a flexible spectrum shaping together with a major increase in spectral efficiency and robustness to synchronization requirements, features of fundamental importance in future mobile networks. A particular type of FBMC, the so-called FBMC/OQAM system, consisting of pulse shaped OFDM carrying offset QAM (OQAM) symbols, has received increasing attention due to, among other features, its potential for maximum spectral efficiency. It suffers, however, from an inherent inter-carrier/inter-symbol interference that complicates signal processing tasks such as channel estimation. The goal of this paper is to concisely present the state-of-the-art in channel estimation for FBMC/OQAM, putting emphasis on the problems that are still open and outlining related on-going research. Preamble-based channel estimation is given special attention in the realistic scenario of highly frequency/time selective channels and some new results are reported in this context. © 2014 IEEE. Source

Kofidis E.,University of Piraeus | Kofidis E.,Computer Technology Institute and Press Diophantus CTI
ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings | Year: 2014

Channel estimation in multicarrier systems employing filter banks with offset quadrature amplitude modulation (FBMC/ OQAM) is known to face challenges related to the interference effect intrinsic to these systems. Most of the methods reported thus far rely on the assumption of (almost) flat subchannels - implying a low frequency selective channel - to more easily tackle this problem. Others, more recent ones, relax this assumption but they either depend on a model for the channel or they concentrate on per-subchannel estimation and often require long training sequences. This paper revisits this problem by proposing a method that makes no assumption on the channel frequency selectivity, while using a very short preamble for training. The latter is optimized with respect to the channel estimation mean squared error. The superiority of the proposed method over classical ones is demonstrated via simulation results for both mildly and highly frequency selective channels. © 2014 IEEE. Source

Gavalas D.,University of Aegean | Gavalas D.,Computer Technology Institute and Press Diophantus CTI | Konstantopoulos C.,Computer Technology Institute and Press Diophantus CTI | Konstantopoulos C.,University of Piraeus | And 4 more authors.
Journal of Heuristics | Year: 2014

The tourist trip design problem (TTDP) refers to a route-planning problem for tourists interested in visiting multiple points of interest (POIs). TTDP solvers derive daily tourist tours, i.e.; ordered visits to POIs, which respect tourist constraints and POIs attributes. The main objective of the problem discussed is to select POIs that match tourist preferences, thereby maximizing tourist satisfaction, while taking into account a multitude of parameters and constraints (e.g.; distances among POIs, visiting time required for each POI, POIs visiting days/hours, entrance fees, weather conditions) and respecting the time available for sightseeing on a daily basis. The aim of this work is to survey models, algorithmic approaches and methodologies concerning tourist trip design problems. Recent approaches are examined, focusing on problem models that best capture a multitude of realistic POIs attributes and user constraints; further, several interesting TTDP variants are investigated. Open issues and promising prospects in tourist trip planning research are also discussed. © 2014 Springer Science+Business Media New York. Source

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