1,721,007 research outputs found
Design and evaluation of technical components building the flexible 5G radio interface supporting heterogeneous use cases
Full text linkNegli ultimi decenni, le comunicazioni mobili si sono evolute da una tecnologia limitata e costosa ad un bene di largo consumo disponibile per miliardi di persone e utilizzata ogni giorno. Anche il loro campo d'azione è cambiato: prima era limitato alle chiamate vocali, poi è stata introdotta la messaggistica e successivamente sono state aggiunte comunicazioni multimediali con immagini e video. A partire dal 2018, l'uso prevalente delle reti cellulari è quello di connettere le persone a Internet, consentendo loro di lavorare, cercare informazioni, comunicare e molte altre attività. Questo è stato possibile grazie alle tecnologie di quarta generazione (4G) come LTE e LTE-Advanced.
Questo scenario è pronto a evolvere ancora una volta: la quinta generazione (5G) di reti cellulari è quasi pronta per il lancio della sua prima iterazione, con l'obiettivo di migliorare ulteriormente le prestazioni dei servizi esistenti e aprire la strada a scenari completamente nuovi. I piani attuali includono l'introduzione della connettività a banda larga nelle aree rurali remote, il supporto di un numero enorme di piccoli sensori alimentati a batteria e dispositivi IoT, fornire Internet veloce su treni ad altissima velocità e molto altro ancora. Ma ancora più importante, il 5G è stato progettato come una tecnologia flessibile, modulare ed estensibile, che può essere migliorata a piacimento in caso di necessità, per essere in grado di supportare nuovi scenari che oggi nemmeno esistono.
Date queste premesse, l'attività di ricerca svolta durante il dottorato è consistita principalmente nell'identificazione, modellazione e valutazione di tecniche promettenti adatte a casi d'uso specifici di future reti 5G. La maggior parte del lavoro è stato condotto nell'ambito del progetto EU H2020 FANTASTIC-5G (Flexible Air iNTerfAce for Scalable service delivery wiThin wIreless Communication networks of the 5th Generation), il cui obiettivo principale è stato la progettazione di una air interface flessibile per il 5G, con particolare attenzione ai livelli più bassi dello stack protocollare e all'integrazione di molti componenti in un framework unificato.
Il capitolo 1 di questa tesi offre una descrizione generale della tecnologia 5G, della visione che ne è alla base, dei principali casi d'uso considerati al momento attuale, insieme alle relative sfide.
Il Capitolo 2 introduce la tecnologia Massive MIMO, che impiega un gran numero di antenne alla stazione radio base e un'elaborazione avanzata del segnale per offrire vantaggi significativi rispetto ai sistemi tradizionali con un numero limitato di antenne. Innanzitutto, viene preso in considerazione il problema dell'implementazione della tecnologia in modalità FDD, in quanto la formulazione originale richiede la modalità TDD, ma è scomoda nella maggior parte delle reti esistenti. La tecnica di precodifica a due stadi JSDM è esplorata come una possibile soluzione, insieme a una tecnica di beam coordination per ridurre l'interferenza, ed entrambe le tecniche hanno dimostrato di offrire un grande vantaggio rispetto alla tecnologia precedente LTE-Advanced negli scenari urbani e rurali. Successivamente, viene indagato un aspetto specifico della tecnica JSDM, ovvero l'identificazione di cluster di utenti adatti per la precodifica. Questa attività è stata perseguita durante un periodo di ricerca all'estero, presso l'Istituto Fraunhofer Heinrich Hertz di Berlino, in Germania. Un approccio di clustering esistente basato sulla densità è stato esteso per coprire una più ampia varietà di condizioni operative, che vanno da piccoli gruppi di utenti isolati a grandi folle che devono essere suddivise. L'approccio risultante mostra prestazioni migliori e conformità con gli obiettivi previsti. L'ultima parte di questo capitolo è dedicata alla descrizione di un algoritmo di allocazione delle risorse distribuito adatto per l'uplink di una rete Massive MIMO, che consente agli utenti di selezionare autonomamente un sottoinsieme di risorse e un profilo di assegnazione di potenza, con l'obiettivo di massimizzare l'efficienza energetica nel rispetto dei vincoli della QoS. Viene mostrato che l'algoritmofunziona come previsto e fornisce miglioramenti significativi rispetto a due diverse strategie di riferimento.
Successivamente, il capitolo 3 affronta il problema della fornitura di accesso a Internet a banda larga su treni ad altissima velocità, nonostante una serie di ostacoli dovuti alla mobilità. In particolare, poiché il feedback sullo stato del canale diventa obsoleto molto rapidamente, il concetto di Predictor Antenna viene sfruttato per fornire una stima del canale più affidabile, con un guadagno fino al 100% a una velocità di 500 km/h.
Il Capitolo 4 si occupa distribuzioni di sensori su larga scala, in cui un gran numero di dispositivi può tentare di accedere alla rete in un breve lasso di tempo, causando numerose collisioni e insuccessi durante la fase iniziale di connessione. Viene descritta una versione estesa del protocollo di accesso LTE, che è in grado di supportare un numero maggiore di dispositivi, come mostrato nella valutazione numerica.
Il Capitolo 5 propone miglioramenti per l'operazione multicast/broadcast disponibile in LTE, che si basano sull'introduzione di un canale di uplink associato alla trasmissione downlink. Con questo presupposto, è possibile implementare un adattamento dinamico dei livelli di modulazione e codifica, nonché richiedere la ritrasmissione di pacchetti di dati che non vengono ricevuti correttamente. Come mostrato nella valutazione, questi strumenti possono rendere la trasmissione più efficace e più affidabile.
Infine, l'Appendice descrive gli aspetti e gli sviluppi più importanti dello strumento di simulazione LTE-Sim, che è stato utilizzato nella maggior parte dei lavori presentati in questa tesi di dottorato.In the last decades, mobile communications evolved from a limited and expensive technology to a commodity that is available to billions of people and is used every day. Their scope changed as well: first it was limited to voice calls, then messaging was introduced, and later multimedia communications with images and video were added. As of 2018, the prevailing use of cellular networks is to connect people to the Internet, allowing them to work, look for information, communicate, and many other activities. This has been possible thanks to fourth generation (4G) technologies such as LTE and LTE-Advanced.
This landscape is ready to evolve once again: the fifth generation (5G) of cellular networks is almost ready for the roll-out of its first iteration, with the goal of further improving the performance of existing services and paving the road for completely new scenarios. Current plans include bringing broadband connectivity to remote rural areas, supporting huge numbers of small battery-powered sensors and IoT devices, providing fast Internet on very high-speed trains, and much more. But more importantly, 5G is being designed as a flexible, modular, and extensible technology, that can be improved at will when the need arises, to be able to support new scenarios that do not even exist today.
Given these premises, the research activity carried out during the Ph.D. consisted mostly in the identification, modeling, and evaluation of promising techniques suitable for specific use cases of future 5G networks. Most of the work has been conducted as part of the EU H2020 FANTASTIC-5G project (Flexible Air iNTerfAce for Scalable service delivery wiThin wIreless Communication networks of the 5th Generation), whose main objective has been the design of a flexible air interface for the 5G, with focus on the lowest layers of the protocol stack and the integration of many components in a unified framework.
Chapter 1 of this thesis offers an overall description of the 5G technology, of the vision behind it, of the main use cases being considered at the present time, together with their corresponding challenges.
Chapter 2 introduces the Massive MIMO technology, which employs a large number of base station antennas and advanced signal processing to offer significant advantages over traditional systems with a limited number of antennas. First, the problem of implementing the technology in FDD mode is considered, as the original formulation requires the TDD mode, but it is inconvenient in most existing networks. The two-stage JSDM precoding technique is explored as a possible solution, together with a beam coordination technique to reduce the interference, and both techniques are shown to offer a large advantage over the legacy LTE- Advanced in urban and rural scenarios. Subsequently, a specific aspect of the JSDM technique is investigated, that is the identification of suitable user clusters for precoding. This activity has been pursued during a research period abroad, at the Fraunhofer Heinrich Hertz Institute in Berlin, Germany. An existing density-based clustering approach has been extended to cover a larger variety of operating conditions, ranging from small isolated user clusters to large crowds that need to be broken down. The resulting approach shows improved performance and compliance with the intended goals. The last part of this chapter is dedicated to the description of a distributed resource allocation algorithm suitable for the uplink of a Massive MIMO network, which allows the users to autonomously select a subset of resources and a power allocation profile, with the goal of maximizing the energy efficiency while respecting QoS constraints. The algorithm is shown to work as intended and provide significant improvements over two different reference strategies.
Next, Chapter 3 tackles the problem of providing broadband Internet access on very high-speed trains, despite a number of obstacles due to the mobility. Specifically, as the feedback about the channel state becomes outdated very quickly, the Predictor Antenna concept is exploited to provide a more reliable channel estimation, resulting in up to 100% gain at a speed of 500 km/h.
Chapter 4 deals with massive sensor deployments, where a large number of devices may try to access the network in a short time span, resulting in many collisions and failures during the initial connection phase. An extended version of the LTE access protocol is described, which is able to support a larger number of devices, as shown in the numerical evaluation.
Chapter 5 proposes enhancements for the multicast/broadcast operation available in LTE, which are based on the introduction of an uplink channel associated with the downlink transmission. With this hypothesis, it is possible to implement a dynamic adaptation of the modulation and coding levels, as well as requesting retransmission of data packets that are not received correctly. As shown in the evaluation, these tools can make the broadcast transmission more effective and more reliable.
Finally, the Appendix describes the most important aspects and developments of the LTE-Sim simulation tool, that has been used in most of the works presented in this Ph.D. thesis
Design of AWG Devices for All-Optical Time-Frequency Packing
No full textWe present the design guidelines for novel passive arrayed waveguide grating (AWG) devices to all-optically implement time-frequency packing techniques in orthogonal frequency division multiplexing (OFDM) systems. The proposed approach can increase the spectral efficiency in long-haul fiber links. We analyze the system performances, evaluating the maximum achievable information rate and spectral efficiency, and we found the optimal AWG layout to maximize the overall system capacity. We demonstrate, through simulations, that an overall 1.4 Tb/s transmission over 200 GHz bandwidth can be reached, using two polarizations, quadrature phase shift keying modulation, and digital feedback equalization
Joint channel decoding in non-cooperative block-faded orthogonal access schemes
No full textIn this paper, we study the performance of non-cooperative wireless multiple access systems with noisy separated channels, where correlated sources communicate to an access point (AP) in presence of block-faded links. Our goal is to explore the potential benefits which can be obtained when source correlation is exploited at the AP, comparing the performance with that obtained by using distributed source coding (DSC) at the nodes. We consider both the average bit error probability and the outage
probability as performance indicators, and we derive a theoretical approach to evaluate their limits. Our results show that the improvement brought by the exploitation of the correlation at the AP is more evident when the correlation becomes sufficiently high. Moreover, some simulation results are presented for two classes of channels codes: serially concatenated convolutional codes (SCCCs) and low-density parity-check (LDPC) codes. Our results show that SCCCs can exploit better the correlation in
scenarios with high values of the correlation coefficient (e.g., 0.999)
Distributed source coding of hyperspectral images
No full textA first attempt to exploit Distributed Source Coding (DSC) principles for the lossless compression of hyperspectral images is presented. The DSC paradigm is exploited to design a very light coder which minimizes the exploitation of the correlation between the image bands. In this way we managed to move the computational complexity from the encoder to the decoder, thus matching the needs of classical acquisition system where compression is achieved on board of the aerial platform and decoding at the ground station. Though the encoder does not explicitly exploit inter-band correlation, the achieved bit rate is about 1 b/p lower than classical 2D schemes such as JPEG-LS or CALID 2D, and only about 1 b/p higher than the best performing, and much more complex, 3D schemes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Design of Optimized Convolutional and Serially Concatenated Convolutional Codes in the Presence of A-priori Information
Full text linkIn this paper, we focus on the design of optimized binary convolutional codes (CCs) and serially concatenated convolutional codes (SCCCs) in the presence of a-priori information (API) at the receiver. For large signal-to-noise ratios (SNRs), we first propose a CC design criterion based on the minimization of a union bound on the bit error probability (BEP). In this case, relevant performance gains, with respect to previously proposed CCs, are obtained. These gains persist even in the presence of estimation errors on the API. Then, we apply the same union bound-based design criterion to SCCCs. Since the BEP of SCCCs is characterized by a typical waterfall shape, the proposed union bound-based design criterion is accurate only at large SNR, to estimate the BEP floor. In order to complement this analysis, we propose a density evolution-based approach to optimize the SCCC design in terms of minimization of the SNR of the "knee" of the BEP curve. The obtained simulation results show substantial gains with respect to previously proposed parallel concatenated convolutional coding (PCCCing) schemes optimized under the assumption of no API at the decoder. Moreover, in the presence of strong API the proposed SCCCs allow to approach the Shannon limit (SL) more than any previously proposed turbo coding scheme
Variations on the Author
Full text link“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
Full text linkWe 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
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