1,720,959 research outputs found
Advanced Coding and Modulation for Digital Terrestrial Broadcasting and Cellular Systems
Get PDFA new platform for HDTV transmissions, in the form of a new generation terrestrial digital video broadcasting standard (DVB-T2), has been developed by the DVB Project. A number of technical innovations have been included in DVB-T2 to boost throughput and ruggedness, enhance single-frequency network coverage, and ease both transmitter and receiver implementation. Several strategies have been investigated during the standard definition. This thesis present a research branch of this investigation starting from a multi-antenna overview: a new soft detector for space-time block codes (STBCs) is proposed, which is claim to be more efficient than the state of art techniques. Commercial and technical reasons have suggested the adoption of orthogonal frequency division multiplexing (OFDM). As multiple-antennas techniques are not backward compatible with DVB-T constellations, new modulation methods and iterative demapping schemes, suitable for OFDM in the presence of Rayleigh fading channels, are assessed in terms of diversity gain without spectral or power inefficiencies. A very important role is played by LDPC codes which are one of the principal innovation of the new digital terrestrial standard. We show how all these scenarios change once an LDPC encoding is applied to the systems. We also evaluate the performance of the coded decision directed demodulation (CD3) technique for channel estimation in DVB-T2. Since the insertion of pilot tones in the OFDM signal affects the spectral efficiency, CD3 is able to provide an accurate channel estimation with very few pilots, even in the presence of highly frequency dispersive and time-variant channels, increasing the spectrum efficiency. We give a comparison in terms of achievable throughput between DVB-T2 with CD3, even with multiple antennas, and current DVB-T.
A second part is related to a cellular network scenario. Assume a set of nodes drawn uniformly at random on a finite surface having a time slot where each node broadcasts its identifier. This problem becomes of practical and theoretical interest in a dense scenario where the number of available physical resources is much smaller than the total number of nodes in the system. Here we address link level strategies for single-hop peer discovery assuming that peer identifiers are broadcasted via coded transmission and we are interested on simultaneous detection of peers which share the same physical resource then cause reciprocal interference. Several strategies are investigated through system simulations: it is shown that, in general, joint iterative decoding achieves impressive gains compared with interference cancellation even preserving almost the same complexity. Elementary modifications of broadcasted signals such as symbol permutations permit to further improve the system performance. Finally, in order to extend this work, we introduce some channel estimation procedures for primary channel models which suit with multiple detection goal.Nel progetto DVB è stato recentemente sviluppato uno standard di nuova generazione per la trasmissione terrestre del segnale video, conosciuto come DVB-T2. Sostanzialmente si tratta di una nuova piattaforma per la trasmissione della televisione digitale ad alta dezione. Il DVB-T2 include una serie di novità tecniche per migliorarne il throughput e la robustezza, estendere la copertura delle reti a singola frequenza, e semplicare la realizzazione del trasmettitore e del ricevitore. Per questa ragione, molteplici strategie sono state studiate nella fase di definizione e stesura dei protocolli del nuovo standard. In questa tesi è presentato un lavoro di ricerca inserito in questo contesto, partendo da una prima indagine sulle trasmissioni multiantenna: a tal fine è stato proposto un decodicatore `soft' per i codici a blocco che operano nello spazio-tempo (codici STBC). Viene illustrato come sia maggiormente efficiente rispetto alle procedure presentate no ad oggi. Le strategie commerciali e tecnologiche hanno favorito l'utilizzo della modulazione OFDM. Considerando la non retrocompatibilità dei sistemi ad antenna multipla con le attuali modulazioni del sistema corrente DVB-T, nuovi schemi di modulazione e demappatura iterativa, specifici per gli schemi OFDM in presenza di canali Rayleigh fading, sono stati valutati in termini di guadagno di diversità senza inefficienze spettrali o di potenza. Un ruolo fondamentale è attribuito ai codici LDPC che sono tra le principali innovazioni del nuovo standard per il digitale terrestre. Mostreremo come cambiano le performance di tutti gli scenari analizzati quando ad essi viene accostata una codica LDPC. Inoltre daremo una valutazione del comportamento della tecnica CD3 per la stima di canale, anch'essa adottata nel DVB-T2. Poiché l'inserimento di toni pilota nel segnale OFDM indebolisce l'efficienza spettrale, il CD3 è in grado di garantire una stima di canale molto accura facendo ricorso ad un numero molto esiguo di toni pilota favorendo una maggiore efficienza dello spettro. Anche in presenza di canali tempo-varianti e altamente dispersivi in frequenza, il CD3 offre prestazioni ragguardevoli. Presenteremo un confronto tra DVB-T2 con stima di canale CD3 in singola e multipla antenna e l'attuale DVB-T in termini di throughput raggiungibile.
La seconda parte riguarda uno scenario relativo alle reti cellulari. Consideriamo un insieme di nodi sparsi uniformemente a caso su di un area geografica e ipotiziamo che debbano trasmettere il loro indirizzo identificativo. Il problema assume interesse sia teorico che pratico in uno scenario ad alta densità dove il numero di risorse fisiche è di molto inferiore al numero totale di dispositivi presenti nel sistema. In questa sede vengono proposte strategie a livello di collegamento per il riconoscimento diretto dei dispositivi assumendo che l'indirizzo identificativo sia trasmesso protetto da codica e lo scopo sia di rilevare simultaneamente tutti i dispositivi che condividono la stessa risorsa fisica causando reciproca interferenza. Molteplici strategie sono state analizzate mediante simulazioni del sistema: si darà prova che, in generale, una decodica iterativa congiunta comporta un guadagno rilevante confrontanto con la tecnica classica della cancellazione di interferenza, pur mantenedo una complessità computazionale pressoché simile. Ulteriori semplici modifiche sui segnali trasmessi come la permutazione dei simboli permettono un ulteriore miglioramento delle prestazioni di sistema. Come ultimo passo, per dare un estensione pratica al lavoro, vengono introdotte alcune procedure di stima di canale per modelli di canale elementari che si adattino alla filosofia del rilevamento multiplo
Topology Estimation for Smart Micro Grids via Powerline Communications
No full textAutomated estimation of topology in smart micro grids (SMGs) has been advocated for the optimization of locally generated power. Moreover, when a power line communication (PLC) network is overlaid to the SMG, topology reconstruction is useful also for routing. In this paper, we propose a technique that allows estimating the SMG topology by exploiting PLC signal itself, thus providing an appealing plug-and-play solution for routing optimization. We use measurements of the transmission time taken by the PLC signal to propagate between couples of nodes, and exploit the fact that the communication signal follows the shortest route along the wires to identify the nodes across which it passed. To this end, we cast the topology estimation problem into an hypothesis testing problem, solved by the generalized likelihood ratio test (GLRT). Low complexity solutions based upon a Gaussian approximation of the measurement errors will also be investigated. Performance of the topology estimation technique is assessed on a realistic low voltage outdoor SMG scenario, proving its effectiveness for reliable SMG management
Joint discovery in wireless networks
No full textGiven a wireless network with N nodes located uniformly at random on a finite plane, we consider the problem of distributed peer discovery: all nodes want to discover as many other nodes as possible. We assume that there are a total of K physical resources dedicated for the discovery purpose. Each node can pick one resource to transmit its node identifier on, and can receive on the remaining K -1 resources. We assume that node identifiers are broadcast via coded transmission on one single physical resource using a single hop. This paper addresses link level strategies to maximize the average number of discovered devices when K <;<; N. A classical strategy of decoding one node per resource would lead to a maximum of K -1 nodes being discovered. We observe that typically this number is much lower (~K/2). We focus on n-user joint coding schemes to improve this performance and, more particularly, on joint iterative decoding (JID). It is shown through system simulations that JID may gain by significantly more than 100% over the classical single-user decoding, and by 20-25% over interference cancellation, provided that the number n of jointly decoded identifiers matches the actual interference density
Joint Discovery in Synchronous Wireless Networks
No full textGiven a synchronous wireless network with N nodes uniformly located at random on a finite plane, we consider the problem of distributed peer discovery: all nodes want to discover as many other nodes as possible. We assume that there are a total of K physical resources dedicated for the discovery purpose. Each node can pick one resource to transmit its node identifier on, and can receive on the remaining K-1 resources. We assume that node identifiers are broadcast via coded transmission on one single physical resource. This paper addresses link level strategies to increase the average number of discovered devices when N ≅ aK, where a is the degree of density of the network. A classical strategy of discovering one node per resource would lead to a maximum of K-1 nodes being discovered. We focus on a multiple-access channel (MAC) scernario where multiple interferer users are jointly decoded. We propose a scheme to improve the performance. The method, based on iterative belief propagation on factor graph, is called joint iterative decoding (JID). It is shown, through system simulations, that JID may gain by significantly more than 100% over the classical single-user decoding, and by 20-25% over the successive interference cancellation (SIC)
On hard and soft detection of space-time block codes by a novel soft output sphere decoder
No full textA new soft detector for space-time block codes (STBCs) is proposed, which is claimed to be more efficient than existing techniques. This tool has been used to compare four STBCs: 1) Alamouti, 2) rotated and scaled Alamouti, 3) Golden and 4) tilted QAM. We evaluate the performance in uncoded and low-density parity-check coded (LDPC) scenario and we stress the difference between the two cases
Soft detection of modulation diversity schemes for next generation digital terrestrial television
No full textThe next generation of digital terrestrial television
(DVB-T2) standard requires to improve performance of the
current DVB-T standard. Commercial and technical reasons
suggest that orthogonal frequency division multiplexing (OFDM) could still be adopted. As multiple–input multiple–output antennas techniques are not backward compatible with current constellations, new modulation methods suitable for OFDM in the presence of Rayleigh fading channels could be developed in order to provide diversity gain without spectral or power inefficiencies.
In this paper we compare three modulation methods, namely,
1) re–mapped repetition, 2) rotational multi–carrier and 3)
multidimensional rotated QAM, in a low–density parity–check
(LDPC) block code scenario. As most of the modulation methods were introduced for an uncoded scenario, indeed it is seen that in the presence of coding, the bit error rate deeply differs from that in an uncoded scenario
Coded Decision Directed Demodulation for Second Generation Digital Video Broadcasting Standard
No full textSecond generation terrestrial digital video broadcasting (DVB-T2) standard aims at providing high definition television and high rate services for the same spectrum allocation of current DVB-T standard. This goal is achieved by adopting new physical layer features that increase the spectrum efficiency. In this respect, the insertion of pilot tones in the orthogonal frequency division multiplexed (OFDM) signal, though allowing an efficient channel estimation, decreases the spectral efficiency. In this paper we evaluate performance of the coded decision directed demodulation (CD3) technique for channel estimation in DVB-T2. In fact, by iterating between decoding and channel estimation, with the latter obtained by using previously decoded symbols, CD3 is able to provide an accurate channel estimation with very few pilots, even in the presence of highly frequency dispersive and time-variant channels. Numerical results are provided for a comparison in terms of achievable throughput between DVB-T2 with CD3 and current DVB-T. Moreover, multiple antennas at the base station are also considered to provide spatial diversity and hence further improve system performance
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
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