1,721,012 research outputs found
Performance analysis of a cognitive radio contention-aware channel selection algorithm
No full text©2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.In cognitive radio (CR) networks, due to the ever increasing traffic demands and the limited spectrum resources, it is very likely for several secondary networks (SNs) to coexist and opportunistically use the same primary user (PU) resources. In such scenarios, the ability to distinguish whether a licensed channel is occupied by a PU or by other SNs can significantly improve the spectrum efficiency of the network, while the contention among the SNs already operating on licensed channels with no PU activity may further affect its throughput and energy efficiency. Therefore, the proper selection of licensed channels could result in notable performance gains. In this paper, we propose a novel contention-aware channel selection algorithm, where the SN under study 1) detects the licensed channels with no PU activity by exploiting cooperative spectrum sensing, 2) estimates the probability of collision in each one, and 3) selects the less contended to access. We provide a detailed analytical model for the throughput and the energy efficiency of the SN, and we validate it by means of simulation. We also show the significant performance gains of our proposal in comparison with other relevant state-of-the-art algorithms.Peer ReviewedPostprint (author's final draft
Performance analysis of a cognitive radio contention-aware channel selection algorithm
No full text©2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.In cognitive radio (CR) networks, due to the ever increasing traffic demands and the limited spectrum resources, it is very likely for several secondary networks (SNs) to coexist and opportunistically use the same primary user (PU) resources. In such scenarios, the ability to distinguish whether a licensed channel is occupied by a PU or by other SNs can significantly improve the spectrum efficiency of the network, while the contention among the SNs already operating on licensed channels with no PU activity may further affect its throughput and energy efficiency. Therefore, the proper selection of licensed channels could result in notable performance gains. In this paper, we propose a novel contention-aware channel selection algorithm, where the SN under study 1) detects the licensed channels with no PU activity by exploiting cooperative spectrum sensing, 2) estimates the probability of collision in each one, and 3) selects the less contended to access. We provide a detailed analytical model for the throughput and the energy efficiency of the SN, and we validate it by means of simulation. We also show the significant performance gains of our proposal in comparison with other relevant state-of-the-art algorithms.Peer ReviewedPostprint (author's final draft
Performance analysis of a cognitive radio contention-aware channel selection algorithm
No full text©2014 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.In cognitive radio (CR) networks, due to the ever increasing traffic demands and the limited spectrum resources, it is very likely for several secondary networks (SNs) to coexist and opportunistically use the same primary user (PU) resources. In such scenarios, the ability to distinguish whether a licensed channel is occupied by a PU or by other SNs can significantly improve the spectrum efficiency of the network, while the contention among the SNs already operating on licensed channels with no PU activity may further affect its throughput and energy efficiency. Therefore, the proper selection of licensed channels could result in notable performance gains. In this paper, we propose a novel contention-aware channel selection algorithm, where the SN under study 1) detects the licensed channels with no PU activity by exploiting cooperative spectrum sensing, 2) estimates the probability of collision in each one, and 3) selects the less contended to access. We provide a detailed analytical model for the throughput and the energy efficiency of the SN, and we validate it by means of simulation. We also show the significant performance gains of our proposal in comparison with other relevant state-of-the-art algorithms.Peer Reviewe
Exploiting cognition for energy and spectrum efficient wireless networks
Get PDFThe main objective of this thesis is to propose and evaluate medium access layer algorithms, that will exploit different types of cognition to provide energy and spectrum efficiency enhancement. In particular, two main research directions are followed: i) the first focuses on spectrum-awareness in cognitive radio (CR) networks inspired by the pioneering work conducted by Mitola in 1999, and ii) the second on the context-aware self-adaptation of cellular heterogeneous networks (HetNets) (i.e., self-organizing networks, SONs). In both contexts, the exploitation of the available information plays a key role on providing sustainable wireless networks. However, given the different needs and features that characterize each of these networks, they are studied separately.
Hence, the first part of this thesis focuses on the coexistence of secondary networks (SNs) that share the same primary user (PU) resources. A coexistence scheme is proposed as well as a novel energy-efficient contention-aware channel selection algorithm, which: i) exploits cooperative spectrum sensing to detect the free from PU activity licensed channels, ii) for each one of them, it estimates the probability of collision, and iii) selects the less contended (i.e., with the lowest probability of collision) to access first. An analytical model for the throughput and the energy efficiency of the SN under study is provided, which is validated by means of simulation. The proposed channel selection algorithm is shown to outperform its counterparts both in terms of throughput and energy efficiency. The proposed SN coexistence scheme is also shown to achieve throughput and energy efficiency gains, while maintaining fairness among the coexisting SNs.
The second part of the thesis focuses on cognitive HetNets with multi-hop small cell (SC) backhaul (BH) links. The multi-hop BH is a promising solution since i) not all SCs are expected to have a direct connection to the core network and consequently they are likely to forward their traffic to the neighboring SCs to reach it and ii) the expected short length of BH links enables the use of millimeter wave (mmWave) frequencies to provide high capacity BH. In this context, this thesis studies the role of BH aiming to answer to whether or not it could constitute an energy bottleneck for the HetNet. In particular, the BH energy impact is compared to the access network (AN), i.e., the links between the users and their serving cells, under different traffic distribution scenarios and BH technologies.
Moreover, the user association problem is studied aiming at the joint maximization of energy and spectrum efficiency of the network, without compromising the user equipment (UE) quality of service (QoS) requirements. To that end, analytical user association frameworks are provided, which can be used as benchmarks for the performance evaluation of different user association solutions. Given the need for low complexity solutions, we also propose efficient heuristic algorithms which exploit context-aware information (i.e., UE measurements and requirements, the HetNet architecture knowledge and the available spectrum resources of each base station (BS)) to associate the UEs in an energy and spectrum efficient way, while considering both the AN and BH energy consumption. The proposed algorithms as well as the derived optimal solutions are compared with reference approaches for different BH technologies and traffic distribution scenarios. Insights are gained into the energy and spectrum efficiency trade-off. Furthermore, our results indicate that i) mmWave BH is a promising solution to provide low consumption high capacity BH, and ii) that the proposed algorithms can achieve notable performance gains compared to the state-of-the-art, while achieving near-optimal performance.El objetivo principal de esta tesis es proponer y evaluar algoritmos de la capa de acceso al medio que exploten distintos tipos de cognición para ofrecer mejoras en la eficiencia espectral y eléctrica. En concreto, se han seguido dos direcciones de investigación principales: i) la primera se centra en redes de radio cognitiva inspiradas en el trabajo pionero desarrollado por Mitola, y ii) la segunda se dedica al estudio de la adaptación dinámica de las redes celulares heterogéneas en función del contexto. En ambos contextos, la utilización de la información disponible juega un papel crucial en la provisión de redes inalámbricas sostenibles. A pesar de ello, debido a las diferentes necesidades y características de cada una de las redes, se estudian por separado. La primera parte de la tesis se centra en la coexistencia de redes secundarias (SNs) que comparten los recursos de los usuarios primarios (PUs). Se propone un esquema que permite dicha coexistencia, así como un nuevo algoritmo de selección de canal energéticamente eficiente basado en la detección de congestión que: i) explota el concepto de cooperative sensing para detectar los canales sin actividad de PUs, ii) para cada uno de ellos, estima la probabilidad de colisión, y finalmente iii) prioriza el acceso a aquellos canales que tiene menor contención (es decir, que tienen la menor probabilidad de colisión). Se desarrolla un modelo analítico tanto para el throughput como para la eficiencia energética de la SN, que posteriormente se valida mediante simulaciones. El algoritmo de selección de canal propuesto mejora el comportamiento del estado del arte en términos de throughput y de eficiencia energética. Asimismo, el esquema de coexistencia propuesto también ofrece mejoras de throughput y de eficiencia energética y, a su vez, mantiene la equidad entre las SNs que coexisten. La segunda parte de la tesis se centra en las redes heterogéneas cognitivas (cognitive HetNets) con un backhaul (BH) formado por enlaces multi-hop entre small cells (SC). El multi-hop BH es una solución prometedora debido a que i) se prevé que no todas las SCs tengan conexión directa con la core network y, por consiguiente, deberán encaminar el tráfico a través de las SCs vecinas, y ii) la corta distancia de los enlaces del BH permitirán el uso de las bandas milimétricas (mmWave) para ofrecer un BH de alta capacidad. En este contexto, la tesis tiene el objetivo de responder si el BH puede suponer un cuello de botella para las HetNets. Concretamente, se compara el impacto del BH respecto a la red de acceso (los enlaces entre los usuarios y sus serving cells) en términos de energía para distintas distribuciones de usuarios y tecnologías de BH. Además, se estudia la user association como un problema de maximización conjunta de la eficiencia espectral y energética de la red, siempre sin comprometer los requisitos de Calidad de Servicio (QoS) de los usuarios. Con ese fin, se desarrolla un marco analítico para el problema de user association, que supone un benchmark para el análisis del comportamiento de distintas soluciones. Debido a la necesidad de soluciones de baja complejidad, la tesis también propone algoritmos heurísticos eficientes que utilizan la información de contexto disponible para asociar los usuarios de un modo eficiente espectral y energéticamente, siempre teniendo en cuenta el consumo energético del BH y de la red de acceso. Los algoritmos propuestos, así como las soluciones óptimas derivadas, son comparados para distintas distribuciones de usuarios y tecnologías de BH. Ello permite comprender el compromiso existente entre eficiencia espectral y eficiencia energética. Los resultados obtenidos muestran que i) las ondas milimétricas son una solución prometedora para conseguir un BH de bajo consumo de energía y alta capacidad, y ii) que los algoritmos propuestos permiten obtener mejoras notables en comparación con el estado del arte, así como conseguir soluciones cuasi-óptimas.DOCTORAT EN TEORIA DEL SENYAL I COMUNICACIONS (Pla 2007
Εκμετάλλευση της γνώσης για ενεργειακά και φασματικά αποδοτικά ασύρματα δίκτυα
No full textNowadays, there is an increased need for anywhere, anytime connectivity, which is expected to increase significantly during the next few years. In order to meet the consequent high data traffic demands, dramatic expansion of network infrastructures as well as fast escalation of energy demands are expected. As a result, it becomes urgent for mobile operators not only to maintain sustainable capacity growth to meet these new demands, but also to limit their electric bill. In parallel, the fact that the spectrum resources are limited has led to another important problem, known as spectrum scarcity, which stresses the need for spectral efficient solutions. The aforementioned goals can be summarized into the joint maximization of energy and spectrum efficiency, which constitutes a fundamental design objective for next generation networks. To that end, exploiting cognition is expected to play a key role. In general, a cognitive network is able to sense its environment and dynamically adapt to it. In particular, cognitive networks, which could be alternatively characterized as context-aware or self-organizing networks (SONs), have the ability to perceive current network conditions, plan, decide, act based on those conditions, learn from the consequences of their actions, all while following end-to-end goals. This loop, the cognition loop, senses the environment, plans actions according to input from sensors and network policies, decides which scenario ts best its end-to-end purpose using a reasoning engine, and finally acts on the chosen scenario. The system learns from the past (situations, plans, decisions, actions) and uses this knowledge to improve the decisions in the future. Thus, the main objective of this thesis is to propose and evaluate medium-access layer algorithms that will exploit different types of cognition to provide energy and spectrum efficiency enhancement. In particular, two main research directions are followed: i) the first focuses on spectrum-awareness in cognitive radio (CR) networks inspired by the pioneering work conducted by Mitola in 1999, and ii) the second focuses on the context-aware self-adaptation of cellular heterogeneous networks (HetNets) (i.e., SONs). In both contexts, the exploitation of the available information plays a key role in providing sustainable wireless networks. However, given the different needs and features that characterize each of these networks, they are studied separately. Hence, the first part of this thesis focuses on the coexistence of secondary networks (SNs) that share the same primary user (PU) resources. A coexistence scheme is proposed as well as a novel energy-efficient contention-aware channel selection algorithm, which: i) exploits cooperative spectrum sensing to detect the free from PU activity licensed channels; ii) for each one of them, it estimates the probability of collision; and iii) selects the less contended (i.e., with the lowest probability of collision) to access first. An analytical model for the throughput and the energy efficiency of the SN under study is provided, which is validated by means of simulation. The proposed channel selection algorithm is shown to outperform its counterparts both in terms of throughput and energy efficiency. The proposed SN coexistence scheme is also shown to achieve throughput and energy efficiency gains, while maintaining fairness among the coexisting SNs. The second part of the thesis focuses on cognitive HetNets with multi-hop small cell (SC) backhaul (BH) links. The multi-hop BH is a promising solution since i) not all SCs are expected to have a direct connection to the core network, and consequently they are likely to forward their traffic to the neighboring SCs to reach it, and ii) the expected short length of BH links enables the use of millimeter wave (mmWave) frequencies to provide high-capacity BH. In this context, this thesis studies the role of BH aiming to answer to whether or not it could constitute an energy bottleneck for the HetNet. In particular, the BH energy impact is compared to the access network (AN), i.e., the links between the users and their serving cells, under different traffic distribution scenarios and BH technologies. Moreover, the user association problem is studied, aiming at the joint maximization of energy and spectrum efficiency of the network without compromising the user equipment (UE) quality of service (QoS) requirements. To that end, analytical user association frameworks are provided, which can be used as benchmarks for the performance evaluation of different user association solutions. Given the need for low-complexity solutions, we also propose efficient heuristic algorithms which exploit context-aware information (i.e., UE measurements and requirements, the HetNet architecture knowledge and the available spectrum resources of each base station (BS)) to associate the UEs in an energy- and spectrum-efficient way, while considering both the AN and BH energy consumption. The proposed algorithms as well as the derived optimal solutions are compared with reference approaches for different BH technologies and traffic distribution scenarios. Insights are gained into the energy and spectrum efficiency trade-off. Furthermore, our results indicate that i) mmWave BH is a promising solution to provide low-consumption, high-capacity BH, and ii) that the proposed algorithms can achieve notable performance gains compared to the state-of-the-art while achieving near-optimal performance.O κύριος στόχος αυτής της διπλωματικής εργασίας είναι να προτείνει και να αξιολογήσει αλγορίθμους πρόσβασης μέσου (επιπέδου MAC), που θα εκμεταλλεύονται διαφορετικούς τύπους γνώσης (context-aware) για την επίτευξη φασματικής και ενεργειακής αποδοτικότητας (energy and spectrum efficiency). Συγκεκριμένα, ακολουθήθηκαν δύο βασικές κατευθύνσεις έρευνας: i) η πρώτη επικεντρώνεται στην “επίγνωση του φάσματος” (spectrum-awareness) σε δίκτυα γνωστικού ραδιοφώνου (cognitive radio, CR) εμπνευσμένο από το πρωτοποριακό έργο που πραγματοποίησε ο Mitola το 1999, και ii) το δεύτερο στην αυτοπροσαρμογή (self-adaptation) των κυψελωτών ετερογενών δικτύων (HetNets) με επίγνωση του πλαισίου (context-awareness) (δηλ. self-organzing networks, SONs). Και στα δύο μέρη, η εκμετάλλευση των διαθέσιμων πληροφοριών παίζει καθοριστικό ρόλο στην παροχή βιώσιμων ασύρματων δικτύων (sustainable wireless networks). Ωστόσο, δεδομένων των διαφορετικών αναγκών και χαρακτηριστικών που χαρακτηρίζει το καθένα από αυτά τα δίκτυα, μελετώνται χωριστά σε αυτή την διπλωματική.Ως εκ τούτου, το πρώτο μέρος αυτής της διατριβής εστιάζει στη συνύπαρξη δευτερευόντων δικτύων (Secondary Networks, SNs) που μοιράζονται τους ίδιους πόρους κύριου χρήστη (Primary User, PU resources). Προτείνεται ένα σχέδιο συνύπαρξης (coexistence scheme) καθώς και ένας καινοτόμος ενεργειακά αποδοτικός αλγόριθμος επιλογής καναλιού με επίγνωση διεκδίκησης (contention-aware), που: i) εκμεταλλεύεται την συνεργατική ανίχνευση φάσματος (cooperative spectrum sensing) για να ανιχνεύσει τα αδειοδοτημένα κανάλια που είναι ελεύθερα δραστηριότητας κυρίου χρήστη (licensed channels free from PU activity), ii) για κάθε ένα από αυτά, εκτιμά την πιθανότητα της σύγκρουσης (probability of collision), και iii) επιλέγει το λιγότερο διεκδικούμενο (δηλαδή, με τη μικρότερη πιθανότητα σύγκρουσης) για να μεταβεί πρώτα. Παρέχεται και ένα αναλυτικό μοντέλο για τη ρυθμαπόδοση και την ενεργειακή αποδοτικότητα του δευτερεύοντος δικτύου υπό μελέτη (SN under study), το οποία επικυρώνεται μέσω προσομοιώσεων. Ο προτεινόμενος αλγόριθμος επιλογής καναλιού αποδεικνύεται ότι επιτυγχάνει μεγαλύτερη ρυθμαπόδοση και ενεργειακή αποδοτικότητά από τους αντίστοιχους αλγορίθμους διαθέσιμους στη βιβλιογραφία. Η προτεινόμενη τεχνική συνύπαρξης δευτερεύοντος δικτύου (SN coexistence scheme) αποδεικνύεται επίσης ότι επιτυγχάνει υψηλή ρυθμαπόδοση και ενεργειακή αποδοτικότητα, διατηρώντας παράλληλα και δικαιοσύνη στην πρόσβαση μεταξύ των συνυπαρχόντων δευτερευόντων δικτύων.Το δεύτερο μέρος της διατριβής εστιάζει στα «γνωστικά» (cognitive) ετερογενή δίκτυα (HetNets) με backhaul ζεύξεις μικροκυψελών πολλαπλών αλμάτων multi-hop small cell (SC) backhaul (BH) links. Το backhaul πολλαπλών αλμάτων (multi-hop BH) είναι μια πολλά υποσχόμενη λύση επειδή: i) δεν αναμένεται να έχουν όλες οι μικροκυψέλες (SC) απευθείας σύνδεση με το δίκτυο κορμού (core network) και κατά συνέπεια, είναι πιθανό να χρειάζεται να προωθήσουν την κίνηση τους στις γειτονικές μικροκυψέλες για να φτάσουν το δίκτυο κορμού, και ii) το αναμενόμενο μικρό μήκος των συνδέσεων backhaul επιτρέπει τη χρήση χιλιοστομετρικών κυμάτων (millimeter wave, mmWave) για παροχή συνδέσεων backhaul υψηλής χωρητικότητας. Στο πλαίσιο αυτό, η παρούσα διατριβή μελετά το ρόλο του backhaul με στόχο να απαντήσει στο εάν θα μπορούσε ή όχι να αποτελέσει ένα ενεργειακό σημείο συμφόρησης (energy bottleneck) για το ετερογενές δίκτυο. Συγκεκριμένα, ο αντίκτυπος της κατανάλωσης ενέργειας στο BH συγκρίνεται με εκείνο του δικτύου ραδιοπρόσβασης (Access Network, AN), δηλαδή με τις συνδέσεις μεταξύ των χρηστών και των κυψελών που τους εξυπηρετούν, για μια πληθώρα από διαφορετικά σενάρια κατανομής κίνησης (traffic distribution scenarios) και τεχνολογίες για το backhaul
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Robust and energy-efficient user association and traffic routing in B5G HetNets
Get PDFNext-generation cellular networks, i.e., beyond fifth generation (B5G) and sixth generation (6G) networks,aim at significantly increasing capacity by deploying a massive number of small cells (SCs) and leveragingmillimeter wave (mmWave) links that have large bandwidth availability. However, as optical connectionsto a large number of base stations (BSs) are not cost effective, wireless backhaul (BH) links may be usedto connect SCs to the core network using mmWave frequencies and forming multihop mesh BH paths. Whilevery flexible in deployment, the uncertainty in user demand, which varies over time and place, makes networkplanning and management challenging. In this work, we propose novel algorithms to solve the joint problemof energy-efficient user association, BH traffic routing and BS/BH link on/off switching. We first formulatean exact model assuming user traffic demand uncertainty leveraging -robustness theory. As the model isintractable for large-scale instances, we develop a novel greedy robust heuristic (P-HEUR), which includesa robustifying step to effectively cope with demand uncertainty. Our evaluation demonstrates that P-HEURprovides robust and energy-efficient solutions quickly for different scenarios and traffic demand uncertaintylevels, and it significantly outperforms state-of-the-art approaches while achieving up to 83% of the optimalcapacity, even in the most demanding scenarios in terms of traffic load and demand uncertainty, with up to200k times lower complexity
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