Διπλωματικές Εργασίες

Μόνιμο URI για αυτήν τη συλλογήhttps://dspace.library.tuc.gr/handle/123456789/118

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Cache management policies for streaming proxies : the cases of fixed and variable video sizes and collaboration among proxies
(Πολυτεχνείο Κρήτης, 2015) Kyriazis Athanasios; Κυριαζης Αθανασιος; Paterakis Michalis; Πατερακης Μιχαλης; Koutsakis Polychronis; Κουτσακης Πολυχρονης; Bletsas Aggelos; Μπλετσας Αγγελος
The easy access on the Internet, the exponential growth of the Internet traffic and the ever enlarging sizes of files which are transferred over it, often lead to network congestion and significant delays in many applications that require timely data transfer. The most important part of data transfer over the Internet corresponds to media streaming applications which require a large portion of the available bandwidth, while at the same time there is the need of achieving low delays, because the delays in these applications are very annoying to the users and some of them may not be willing to experience them. In order to improve the services provided to the users of these applications, the techniques of video caching and of collaborative caching are used. Video caching embodies proxy servers on the network in order to keep frequently accessed data at locations close to the clients. Collaborative caching uses group of proxies which can collaborate with each other, and achieve better performance than with independent standalone proxies. This Thesis proposes new efficient algorithms for video caching which manage the cache of the proxy servers for video transfer in real time and compares them to existing algorithms. Also, a new effective collaborative scenario is proposed and evaluated. In the first two parts of the Thesis the performance of three existing efficient algorithms in the literature, the Number of Common Clients - Size (NCCS), the Least Recently and Least Frequently Used (LRLFU) and the Least Recently Used (LRU) is evaluated and compared for the cases of fixed and variable video sizes. In each of the first two parts of the Thesis a new cache management algorithm is also designed and proposed, this new algorithm consists of a combination of existing efficient algorithms. In the first part of the Thesis the proposed new algorithm (referred to as the LRLFU_Aggressive) does not perform much better than the existing algorithms, but in the second part the proposed new algorithm (referred to as the LRLFU_Modified) significantly improves the performance of the system in comparison to the existing algorithms. In the cache replacement algorithms we examine a value defined for each video, namely the caching value, which is based on the popularity of the videos, in order to retain in the cache the most popular videos. For each requested video which does not exist into the cache, other videos have to be removed (the ones with the lower caching values) in order for the new video to be cached. The performance metrics we use to evaluate and compare the algorithms in our system are the Video Hit Ratio and the Fraction of the Delayed Starts. In the third part of the Thesis a collaborative scheme of two client communities each associated with one proxy server, is defined and studied. While most of the studies in the area of collaborative proxies use the same cache management algorithm in each proxy and assume that each client community communicates via the corresponding proxy with the same content server, in our collaborative system we have used a different cache management algorithm in each of the two proxies and we assumed that there are two content servers each one associated with one of the client communities. Furthermore, we assumed that each of the client communities communicates via the corresponding proxy only with its corresponding content server. Also, we assumed that there is an overlap between the contents of the two content servers. Each of the two different cache management algorithms used in the collaborative scheme caches different videos in the corresponding proxy, and this helps the overall system performance. From the results of our simulation study we derive important conclusions for the performance of the examined collaborative scheme.
Community RF sensing
(Πολυτεχνείο Κρήτης, 2012) Alimpertis Emmanouil; Αλιμπερτης Εμμανουηλ; Bletsas Aggelos; Μπλετσας Αγγελος; Garofalakis Minos; Γαροφαλακης Μινως; Lagoudakis Michael; Λαγουδακης Μιχαηλ
Dynamic resource allocation in rooftop networking
(Πολυτεχνείο Κρήτης, 2014) Oikonomakos Panagiotis; Οικονομακος Παναγιωτης; Bletsas Aggelos; Μπλετσας Αγγελος; Karystinos Georgios; Καρυστινος Γεωργιος; Deligiannakis Antonios; Δεληγιαννακης Αντωνιος
Τα mesh networks είναι δίκτυα τα οποία επιτρέπουν σε ένα σύνολο από κόμβους, χωρίς συγκεκριμένη τοπολογία, να συνεργάζονται με σκοπό την αποτελεσματική δρομολόγηση πακέτων. Η δρομολόγηση των πακέτων γίνεται με άλματα από κόμβο σε κόμβο και μπορεί να αλλάζει, καθώς νέα βέλτιστα μονοπάτια υπολογίζονται. Οι κόμβοι μπορούν να είναι τοποθετημένοι σε κτήρια ή να είναι χρήστες που κινούνται. Στην πρώτη περίπτωση αναφερόμαστε στα rooftop networks. Είναι πολύ φτηνότερα από DSL ή καλωδιακά δίκτυα και μπορούν να συνεισφέρουν στην αποτελεσματικότερη παροχή υψηλής ταχύτητας δικτύου όταν δεν υπάρχει υποδομή για ενσύρματη πρόσβαση. Για την υλοποίηση της διπλωματικής εργασίας χρησιμοποιήθηκαν Linux Boxes ALIX 2D2 και 3D2 καθώς και WiFi κάρτες συμβατές με οδηγούς λογισμικού (drivers) τύπου atheros. Εγκαταστήσαμε προσωρινά τέσσερις κόμβους στις ταράτσες των κτηρίων A1, A2, K4 και Μ3 της πολυτεχνειούπολης με δύο WiFi κάρτες στον κάθε κόμβο. Σε κάθε κόμβο η μία WiFi κάρτα χρησιμοποίησε μια omni directional κεραία ενώ η δεύτερη μία grid. Δημιουργήθηκε μηχανισμός που ελέγχει όλα τα συχνοτικά κανάλια σε κάθε ζεύξη, υπολογίζει έναν παράγοντα παρεμβολών (interference factor) ανά κανάλι και τέλος γίνεται μια αλλαγή στο “βέλτιστο” κανάλι (σύμφωνα με τον παραπάνω παράγοντα).
Energy harvesting and sensing for backscatter tags
(Πολυτεχνείο Κρήτης, 2014) Daskalakis Spyridon-Nektarios; Δασκαλακης Σπυριδων-Νεκταριος; Bletsas Aggelos; Μπλετσας Αγγελος; Koutroulis Eftychios; Κουτρουλης Ευτυχιος; Deligiannakis Antonios; Δεληγιαννακης Αντωνιος
Nowadays, RF energy harvesting has become an increasingly attractive research topic due to the proliferation of radio frequency emitters. The goal is to collect unused ambient RF energy and supply small sensors with power. In the first part, an efficient, low-cost and low-complexity rectenna-grid is analyzed, fabricated and measured, for low-power RF input and RF density. Despite the fact that a lossy and low-cost FR-4 substrate was used, the RF-to-DC rectification efficiency of 20.5% and 35.3% is achieved at -20 and -10dBm power input, respectively. The rectenna was connected to a custom DC-to-DC converter and the open voltage was increased from 298mV to 1.4V, charging a 1mF capacitor at 37min. Monitoring, environmental variables such as soil moisture, relative humidity, etc. with wireless sensor networks (WSNs) is invaluable for precision agriculture applications. However, the effectiveness of existing conventional (e.g. ZigBee-type) radios in large-scale deployments is limited by power consumption, cost and complexity constraints. While the existing WSN solutions employing non-conventional, scatter-radio principles have been restricted to communication ranges of up to a few meters, a novel joint analog design of wireless transmitter (tag) with scatter radio and extended communication ranges is presented in this thesis. Two sensing elements that can be connected with the tag were implemented, a soil moisture sensor and a plant signal sensor. The network tag with the soil moisture sensor was selected to be evaluated in terms of accuracy and communication range. The design is based on a custom microstrip capacitor, exploits bistatic analog scatter radio principles and is able to wirelessly convey soil moisture percentage by mass (% MP) with RMS error of 1.9%. Power consumption and communication range is on the order of 100 uW and 100 meters, respectively. It is tailored for ultra-low cost (5 Euro per sensor) agricultural sensor network applications for soil moisture.
Equalization and pre-equalization techniques and implementation in software deﬁned radios
(Πολυτεχνείο Κρήτης, 2015) Iliadis Stylianos; Ηλιαδης Στυλιανος; Liavas Athanasios; Λιαβας Αθανασιος; Karystinos Georgios; Καρυστινος Γεωργιος; Bletsas Aggelos; Μπλετσας Αγγελος
The huge expansion of wireless communication the recent decades and the increased demand for high-speed wireless connections points out the importance of eﬃcient use of frequency bandwidth which is generally expensive and limited. Communication links between multiple transmit and receive antennas, the so-called Multiple Input and Multiple Output (MIMO) channels, have been under extensive theoretical studies for their potential to provide extra dimensions for communication, without sacriﬁcing bandwidth resources, by multiplexing various independent data streams over the wireless medium. Optimal detection and separation of the multiplexed messages is provided by the Maximum-Likelihood method, at the cost of high complexity. The sub-optimal linear Zero-Forcing Equalization and Pre-equalization methods can drastically decrease the detection complexity at the cost of performance degradation. Both ML and linear Equalization exploit Channel-State Information (CSI) at the receiver for retrieving the various interfering symbols. Linear pre-equalization in contrast, precancels interference at the transmitter which requires feedback in order to obtain the required CSI. The need for feedback can be avoided in Time-Division-Duplex (TDD) systems, where the transmitter can exploit the reverse-channel estimate, based on the reciprocal property of wireless propagation. Utilizing the reciprocal property in practical transceivers requires some form of calibration in order to compensate for the non-reciprocal relationship of their transmit and receive chains. A suitable testbed for implementing and evaluating the above methods by means of Software Deﬁned Radio is provided by the USRP hardware platform and the GNU radio software toolkit
Implementation of Frequency Division Multiple Access Digital Backscatter Sensor Network
(Πολυτεχνείο Κρήτης, 2014) Tountas Konstantinos; Τουντας Κωνσταντινος; Bletsas Aggelos; Μπλετσας Αγγελος; Karystinos Georgios; Καρυστινος Γεωργιος; Deligiannakis Antonios; Δεληγιαννακης Αντωνιος
Environmental sensing applications require dense sensor deployments for precise monitoring of each plants' micro-climate; For such applications, scatter radio is a promising communication and networking technology. Since modulation is achieved by means of reflection, very low-cost and low-power RF front-ends and sensor nodes can be constructed. However, such technology has demonstrated limited range in commercial scatter radio applications, like radio frequency identification (RFID) systems. In order to alleviate that problem, the bistatic architecture has been proposed, where emitter of illuminating carrier towards the tag/sensor and reader of the modulated reflection (from tag/sensor) are dislocated. Also, frequency shift keying (FSK) modulation has been utilized for simultaneous transmission of many scatter radio sensors, through frequency division multiple access (FDMA). These schemes allow the creation of a scatter sensor network, using flexible topologies with multiple emitters and a single reader, increasing the probability for short emitter-to-tag/sensor distances. This work experimentally demonstrates the creation of such scatter radio network with low-cost 8051 micro-controller units (MCU), where sensed information is converted to digital messages using the internals analog-to-digital (ADC) converters. Furthermore, implementation of short-block length channel encoding based on Reed-Muller codes are tested experimentally. This work is a small step towards the realization of large-scale, scatter radio sensor networks.
Low-density parity-check codes for the relay channel
(Πολυτεχνείο Κρήτης, 2014) Chatzinikolaou Panagiotis; Χατζηνικολαου Παναγιωτης; Liavas Athanasios; Λιαβας Αθανασιος; Karystinos Georgios; Καρυστινος Γεωργιος; Bletsas Aggelos; Μπλετσας Αγγελος
Since 1948, when Claude Shannon introduced the notion of channel capacity, the ultimate goal of coding theory has been to find practical capacity-approaching codes. This thesis is concerned with the analysis, design, construction, and, mainly, the decoding of an extremely powerful and flexible family of error-control codes, called low-density parity-check (LDPC) codes. LDPC codes can be designed to perform close to the capacity of many different types of channels with a practical decoding complexity. The main design tool, Density Evolution, which predicts the asymptotic performance of a belief-propagation decoder is analysed. An effective construction method, called configuration model, for picking LDPC codes at random from LDPC ensembles is presented in depth. In addition, a construction tool for LDPC codes of moderate lengths, namely the Progressive Edge Growth algorithm, which tries to maximize the girth of a code is presented. Furthermore, we present the analysis,design, construction and decoding of bilayer LDPC codes for the half-duplex relay channel. To analyse the performance of bilayer LDPC codes, bilayer Density Evolution is presented as an extension of the standard Density Evolution. Finally, we analyse in depth the construction procedure of bilayer LDPC codes as an extension of single user's configuration model.
Maximally sparse convex estimation and equalization
(Πολυτεχνείο Κρήτης, 2014) Lourakis Georgios; Λουρακης Γεωργιος; Liavas Athanasios; Λιαβας Αθανασιος; Digalakis Vasilis; Διγαλακης Βασιλης; Bletsas Aggelos; Μπλετσας Αγγελος
Sparse multipath channels are wireless links commonly found in communication systems such as High Frequency radio channels, horizontal and vertical underwater acoustic channels and terrestrial broadcasting channels for High Definition Television. Their impulse responses are characterized by a few significant terms that are widely separated in time. With high speed transmission, the length of a sampled sparse channel can reach hundreds of symbol interval. Thus, the amount of Intersymbol Interference (ISI) at the receiver is very high. Consequently, the presence of an ISI mitigating structure at the receiver, such as the Decision Feedback Equalizer (DFE) is essential. Due to the sparse impulse responses of these channels, traditional estimation techniques such as Least Squares (LS) result in over-parameterization and thus poor performance of the estimator. Also, classical equalizers become too complex for tackling these channels. The problem of estimating and equalizing sparse multipath channels is considered in this thesis. We formulate the sparse channel estimation and the computation of the sparse DFE filters as sparse approximation problems. A usual approach in sparse approximation problems is regularization with an l_1 norm penalty term and usage of convex optimization techniques in order to acquire a solution. Other sparsity promoting penalty functions are available, but the l_1 norm has the advantage to be a convex function, making the l_1 norm regularized approximation problem a convex one. When a problem is formulated as a convex optimization problem, it can be solved by very fast, efficient and reliable algorithms. In order to achieve sparser solutions and still gain from the benefits of the convex optimization theory, the Maximally Sparse Convex (MSC) algorithm utilizes a non-convex regularization term, that promotes sparsity more strongly than the l_1 norm, but chosen such that the total cost function remains convex.
Maximization of a rank-4 quadratic form by a binary vector with complexity O(N^3logN)
(Technical University of Crete, 2014) Sklikas Alexandros; Σκληκας Αλεξανδρος; Karystinos Georgios; Καρυστινος Γεωργιος; Bletsas Aggelos; Μπλετσας Αγγελος; Samoladas Vasilis; Σαμολαδας Βασιλης
We consider the problem of maximizing a quadratic form over the binary alphabet. This problem is known as the unconstrained (−1,1)-quadratic maximization problem or binary quadratic programming (in computer science terminology) and is an NP-hard combinatorial problem that can be solved through an exponential-complexity exhaustive search. Recently, it has been shown that the exhaustive search is not necessary and this problem is polynomially solved, if the rank of the quadratic form is constant (which is a case that is met is certain optimization problems in communication theory). A few polynomial-time algorithms have been reported from several research groups that differ in their actual space and/or time complexity. In this thesis, we focus on the case where the rank of the form is 4 and present an optimal algorithm with complexity O(N^3*log(N)) that is based on novel ideas that combine the auxiliary-angle framework developed in TUC and a few elements from computational geometry. For completeness, we present our method for the cases of rank-2 and rank-3 quadratic forms, with complexity O(N*log(N)) and O(N^2*log(N)), respectively. For all three cases, we show that our algorithm is the fastest known implementable one among the several choices in the literature. Finally, we also comment on how our approach can be generalized to any rank-D quadratic form and lead to an algorithm of complexity O(N^(D-1)*log(N)).
A software-defined implementation of an OFDM - adaptive OFDMA system using USRPs 1
(Πολυτεχνείο Κρήτης, 2014) Magounaki Theoni; Μαγουνακη Θεονη; Liavas Athanasios; Λιαβας Αθανασιος; Bletsas Aggelos; Μπλετσας Αγγελος; Karystinos Georgios; Καρυστινος Γεωργιος
Software-defined radio implementation of OFDM
(Πολυτεχνείο Κρήτης, 2014) Leonidakis Stylianos; Λεωνιδακης Στυλιανος; Liavas Athanasios; Λιαβας Αθανασιος; Bletsas Aggelos; Μπλετσας Αγγελος; Karystinos Georgios; Καρυστινος Γεωργιος
Wireless communications is by any measure the fastest growing segment of the communications industry. Cellular phones, Wireless Local Area Networks (WLANs), Smart Homes and telemedicine are some applications that make extensive use of wireless communications. Orthogonal Frequency Division Multiplexing (OFDM) is a modulation that has been applied to a wide variety of applications and is adopted in many recently standardized broadband communication systems. OFDM is an appealing technique for achieving high-bit-rate data transmision, due to its robustness against the multipath fading and Inter Symbol Interference. Since the OFDM transmission is vulnerable to time and frequency offsets, accurate estimation of these parameters is one of the most important tasks of the OFDM receiver. In this thesis, we implement a time synchronization algorithm based on correlation of two identical parts of an OFDM pilot symbol. In addition, knowledge of the channel in the transmitter is very useful for effective transmission and/or interference mitigation (especially in static environments). Thus, channel feedback is utilized throughout the experiments conducted for this thesis. The experiments were conducted on a software-defined-radio (SDR) testbed based on the Universal Software Radio Peripheral (USRP).
Synchronization and detection for Gen2 RFID signals
(Technical University of Crete, 2015) Bamiedakis-Pananos Michail; Μπαμιεδακης-Πανανος Μιχαηλ; Bletsas Aggelos; Μπλετσας Αγγελος; Karystinos Georgios; Καρυστινος Γεωργιος; Deligiannakis Antonios; Δεληγιαννακης Αντωνιος
Κατανεμημένοι αλγόριθμοι για μεγιστοποίηση ρυθμού μετάδοσης σε ευφυή ασύρματα δίκτυα
(Technical University of Crete, 2014) Kaloumenos Nikolaos-Alexandros; Καλουμενος Νικολαος-Αλεξανδρος; Paterakis Michalis; Πατερακης Μιχαλης; Bletsas Aggelos; Μπλετσας Αγγελος; Koutsakis Polychronis; Κουτσακης Πολυχρονης
Κωδικοποίηση διαύλου για την αποφυγή υποκλοπών
(Technical University of Crete, 2015) Kosta Antzela; Κωστα Αντζελα; Karystinos Georgios; Καρυστινος Γεωργιος; Liavas Athanasios; Λιαβας Αθανασιος; Bletsas Aggelos; Μπλετσας Αγγελος
In this work we consider the problem of achieving information-theoretic security with practical coding complexity. Specifically, we consider a transmitter, a receiver, and an eavesdropper where the communication channels between the transmitter and the receiver and between the transmitter and the eavesdropper are both binary erasure channels. The objective is the development of a practical coding scheme that attains maximum transmission rate between the transmitter and the receiver and, simultaneously, complete protection of the transmitted data from the eavesdropper. We utilize polar codes which are known to achieve the capacity of any symmetric binary-input discrete memoryless channel. We focus on the binary erasure channel and present the pertinent polar encoding and decoding algorithms. Finally, we show how polar coding can be used for the wire-tap binary erasure channel, achieving maximum transmission rate and complete protection.
Συγκριτική αξιολόγηση μεθόδων καθολικής βελτιστοποίησης για την παραμετροποίηση και ανάλυση βιο-οπτικών σημάτων διαγνωστικού ενδιαφέροντος
(Πολυτεχνείο Κρήτης, 2014) Klapaki Styliani; Κλαπακη Στυλιανη; Balas Costas; Μπαλας Κωστας; Bletsas Aggelos; Μπλετσας Αγγελος; Stavrakakis Georgios; Σταυρακακης Γεωργιος

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