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Estimation Techniques for OFDM with application to Digital Video Broadcasting Standards
Full text linkThis thesis deals with three among the major tasks of a receiver designed for Orthogonal Frequency Division Multiplex (OFDM): i) timing and frequency synchronization, ii) channel estimation and iii) data detection. These problems are very general, since OFDM is used in many different telecommunication environments, however the techniques involved are strongly dependent on the particular scenario. Here we focus on digital video broadcasting standards for terrestrial television.
Terrestrial Digital Video Broadcasting, also known as DVB-T, is a broadcasting standard from the European consortium DVB. It is the most widely deployed Digital Terrestrial Television (DTT) system worldwide. The system is based on a transmission of an audio/video stream from the MPEG-2 family, using an OFDM modulation. Besides, in March 2006, the desire to improve the service quality as well as to increase the number of services and the power of the digital television system, has stimulated the DVB group to study some new options for an improved DVB-T standard. A call for technology has been published in 2007, followed in 2009 by the release of the standard for the new generation DVB-T, called DVB-T2. Deployment tests have been studied, and field tests show that DVB-T2, still based on OFDM, can guarantee a 30% increase of the system capacity with respect to its predecessor.
As already stated, in both DVB-T standards the elected modulation scheme is OFDM. In fact, OFDM is widely used thanks to its low complexity structure allowing for simple demodulation and modulation by means of Fast Fourier ransform (FFT) and inverse FFT, respectively. The FFT algorithm is well optimized for computing blocks of samples with power of two sizes, i.e. 2048, 8096, etc. However, to allow simple equalization procedures and avoid interblock interference, a guard interval (GI) has to be appended to the OFDM block. GI reduces the efficiency of the transmission, because it is discarded at the receiver. The first part of this thesis proposes a receiver structure which takes advantage of the GI to improve data detection exploiting the redundancy of the signal. The performance of this structure is then analyzed comparing it with other two structures in a simulation scenario arisen from DVB-T2 reception configurations. The impact of low-density parity check codes (LDPC) which are part of the standard is also outlined.
The second part of the thesis focuses on the channel estimation which is the basis for most of the signal processing techniques involved in the reception of an OFDM signal. In particular we focus on channel estimation using known pilot symbols. Transmission of pilot symbols on a sub-set of subcarriers in an orthogonal frequency division multiplexing system allows for an efficient channel estimation at the receiver by means of the Least-Square (LS) method. Usually, for these systems channel estimation is performed on pilots and then interpolated over the time and the frequency axis. We propose to improve the LS estimate on pilots by filtering the estimated channel both across the subcarriers and across different OFDM blocks, with adaptive filters in order to exploit the inter-carrier correlation and reduce the noise on the stimate. We consider various adaptive techniques to let the filter track the time-varying channel conditions: least mean square and Kalman filter theory are applied to design the finite impulse response filters. Besides, also infinite impulse response filters have been designed. Aim of the design is to obtain simple structures suitable for large FDM blocks and receivers with limited computational/memory resources, while still being adaptive to time-varying channel conditions. However, while most pproaches consider a single smoothing filter over time-interpolated estimates, i.e. after time interpolation, we propose the innovative use of multiple adaptive filters that take into account the different correlation among pilot estimates and interpolated estimates. In other words, we propose to design multiple adaptive frequency filters to perform frequency filtering in order to decrease the degree of noisiness of the time-interpolated samples. The idea is to leverage the fact that time-interpolated estimates have different reliability with respect to pilot LS estimates, and thus weighting accordingly the estimates during filtering, performance can be improved. Moreover, as a simpler approach we consider also the use of a single adaptive filter updated taking into account the reliability of the various estimates. Performance results are reported with reference to the DVB-T and DVB-T2 standards showing that the proposed multiple filters technique performs similar or better than existing approaches at a much lower complexity. Although these techniques have been developed considering the DVB-T/T2 pilot patterns, they can be applied to whatever system using pilot-patterned OFDM symbols.
The last part of the thesis deals with the initial signal ynchronization for DVB-T2. In fact, the new standard for DTT provides a specific symbol, called P1 symbol, to ease initial signal acquisition and synchronization. We revise a synchronization technique based on the correlation of parts of the received signal showing that it fails to synchronize in practical channel conditions, e.g. single-frequency network (SFN) channels. Hence, we first propose a modified version of the existing technique to better exploit the correlation signal, then we analytically derive the maximum likelihood (ML) time and carrier frequency offset synchronization function. Unfortunately, the maximization of the likelihood function requires numerical methods, therefore we decided to simplify the scheme by considering a ML time (MLT) synchronization scheme, while resorting to a suboptimal estimator for carrier frequency offset (CFO). The proposed schemes are compared with existing solutions on both flat and frequency selective channels typically encountered in DVB-T2 transmissions, showing that both ML and MLT approaches are able to achieve synchronization with probability higher than 99%.
The last chapter collects all the simulation results supporting the topics covered in the previous chapters.La tesi discute tre tra le principali operazioni svolte da un ricevitore per segnali OFDM: i) sincronizzazione di simbolo e di frequenza, ii) stima di canale e iii) demodulazione del segnale. Dal momento che la modulazione OFDM è adottata in molti sistemi diversi, ognuno dei quali fa fronte agli stessi problemi con tecniche caratteristiche, gli argomenti proposti assumono un carattere molto generale. In questo lavoro, perciò, si tratteranno alcune tecniche OFDM applicate agli standard di diffusione terrestre della televisione digitale.
La radiodiffusione terrestre della TV digitale ha come standard più diffuso in ambito mondiale il DVB-T. Il sistema è basato sulla trasmissione di un flusso audio/video della famiglia MPEG-2, mediante modulazione OFDM. Recentemente, nel Marzo 2006, il desiderio di migliorare la qualità del servizio, e migliorare la potenza del sistema televisivo hanno stimolato il gruppo DVB a studiare delle nuove opzioni per il miglioramento dello standard DVB-T. Nel 2007 è iniziata l’opera di standardizzazione del nuovo sistema. Dopo due anni, nel 2009, è stato pubblicato lo standard del sistema DVB-T di nuova generazione, che ha preso il nome di DVB-T2. Test sul campo hanno dimostrato che la nuova tecnologia, ancora basata su OFDM, può garantire un incremento di capacità del 30% in più rispetto al sistema precedente. I primi a trasmettere secondo la modalità DVB-T2 sono state alcune reti inglesi. Tuttavia, benché BBC, ITV1 e Channel 4, trasmettessero già in DVB-T2 su uno o due canali, nell’Ottobre 2010, l’emittente televisiva italiana Europa 7 ha lanciato il suo bouquet di 12 canali che trasmettono con tecnologia DVB-T2, divenendo di fatto il primo broadcaster al mondo a trasmettere esclusivamente in DVB-T2. Nel Novembre 2010, i 14 paesi componenti la comunità dei paesi Sud-Africani ha selezionato DVB-T2 come standard di diffusione televisivo della regione. La comunità prevede di operare lo switch-over digitale indicativamente nel Dicembre 2013.
Come già abbiamo detto, entrambi gli standard DVB-T hanno scelto OFDM come schema di modulazione. Infatti, il largo utilizzo dell’OFDM si deve alla sua praticità di utilizzo, dovuta alla possibilità di realizzare modulatori e demodulatori a ridotta complessità. Le operazioni di demodulazione e modulazione sono realizzate rispettivamente tramite Trasformata di Fourier (FFT) e la sua operazione inversa (IFFT). FFT e IFFT sono entrambe realizzate mediante lo stesso algoritmo che è ottimizzato per operare su blocchi di campioni di lunghezza pari alle potenze di due, per esempio 2048 e 8096. Comunque, per permettere la realizzazione di procedure semplici di equalizzazione e quindi evitare l’interferenza tra simboli successivi, un Intervallo di Guardia (GI) deve essere trasmesso prima del blocco OFDM. Il GI riduce l’efficienza di trasmissione, in quanto è subito scartato dal ricevitore. La prima parte di questa tesi propone una struttura per il ricevitore che permetta di usare il GI per migliorare la decodifica dei dati. Le prestazioni di questa struttura sono analizzate confrontandola con altre due strutture in uno dei possibili scenari per il DVB-T2. Successivamente, è anche valutato l’impatto dei codici LDPC sulle prestazioni della struttura proposta.
La seconda parte della tesi si concentra sulla stima di canale, che in molti casi rappresenta la base per le tecniche di elaborazione di segnale utilizzate nella ricezione di un segnale OFDM generico. In particolare noi consideriamo la stima di canale per sistemi OFDM che utilizzano delle portanti note al ricevitore. La trasmissione di simboli pilota in un sottoinsieme di portanti in un sistema OFDM permette di ottenere una efficiente stima di canale Least Square (LS) al ricevitore. Per questi sistemi la stima di canale è realizzata prima sulle portanti pilota, e successivamente interpolata nel tempo e in frequenza. In questo lavoro si propone di migliorare la stima LS sulle portanti filtrando la stima di canale con dei filtri adattativi. L’operazione di filtraggio è realizzata prima tra simboli OFDM successivi e poi tra portanti di un medesimo simbolo, in modo da sfruttare le correlazioni tra i diversi campioni stimati e ridurre così il livello di rumore. Si sono considerati diverse tecniche adattative per permettere ai filtri di seguire le condizioni di tempo-varianza del canale. Sono state applicate le teorie sui filtri least mean square e Kalman, per progettare filtri a risposta impulsiva limitata o infinita. Lo scopo degli algoritmi adattativi sviluppati è di ottenere delle strutture semplici, in grado di operare su simboli OFDM di grandi dimensioni, e con ricevitori con limitate risorse in termini di potenza di calcolo e memoria. Comunque, mentre molte tecniche considerano un singolo filtro per operare nel dominio della frequenza, cioé dopo l’interpolazione temporale, questa tesi propone l’utilizzo innovativo di più filtri che tengano in considerazione le diverse statistiche di correlazione tra le stime ottenute sulle portanti pilota e quelle ottenute tramite interpolazione temporale. In altre parole, si è proposto di progettare in parallelo più filtri adattativi, ognuno che operi su un sottoinsieme indipendente di portanti. L’idea è quella di sfruttare il fatto che le stime ottenute per interpolazione temporale hanno un’affidabilità diversa rispetto alle stime ottenute con il metodo LS, e quindi pesando le correlazioni in modo appropriato, al momento della procedura di filtraggio adattativo, è possibile un miglioramento delle prestazioni di stima. I risultati sono riportati con riferimento agli standard DVB-T e DVB-T2, dimostrando che la tecnica proposta è competitiva, o addirittura migliore, rispetto alle tecniche utilizzate in precedenza, permettendo una riduzione sostanziale della complessità di calcolo. Sebbene i risultati siano presentati con riferimento agli standard televisivi, queste tecniche possono essere applicate a qualsiasi sitema che utilizzi una modulazione OFDM con portanti pilota note al ricevitore.
L’ultima parte della tesi si occupa di sincronizzazione del segnale DVB-T2. Lo standard DVB-T2 definisce un simbolo specifico, detto simbolo P1, per facilitare l’acqusizione iniziale del segnale e la sincronizzazione. In questa parte della tesi si rivisita la tecnica di sincronizzazione basata su correlazione delle parti ridondanti del segnale ricevuto, definita nelle linee guida dello standard. Dopodiché si mostra che essa fallisce quando certe configurazioni reali di canale si presentano, per esempio nel caso di canale caratteristico di una single-frequency network (SFN). Perciò, prima si propone una versione modificata della tecnica base che sfrutti meglio la correlazione del segnale, successivamente si ricava l’espressione analitica della funzione di massima verosimiglianza (ML) per la sincronizzazione di tempo e frequenza. Sfortunatamente non è possibile trovare una forma analitica dello stimatore ML che massimizza la funzione di massima verosimiglianza, perciò è stato introdotto uno schema semplificato che considera la stima ML solo del tempo (MLT), lasciando la stima dell’offset di frequenza ad uno stimatore sub-ottimo. Gli schemi proposti sono confrontati con soluzioni pre-esistenti sia con canali selettivi in frequenza che non. I risultati mostrano che sia la tecnica ML, che la tecnica MLT, sono in grado di raggiungere una corretta sincronizzazione con una probabilità maggiore del 99%.
Infine, l’ultimo capitolo raccoglie i risultati di tutte le simulazioni delle tecniche trattate nei capitoli precedenti
Comment on the Italian Supreme Court’s ‘‘Rotoloni’’ Decision
No full textComment on the Italian Supreme Court’s ‘‘Rotoloni’’ Decisio
On the analysis of P1 symbol performance for DVB-T2
No full textIn this paper the second generation digital terrestrial distribution system standardized by the digital video broadcasting consortium (DVB-T2) is considered and, in particular, we focus on the new structure of the P1 training symbol. Two algorithms making use of this symbol are proposed and analyzed, one for the recovery of the carrier frequency offset, the other for the channel estimation. In particular, their performances are evaluated, theoretically and via simulations, and finally compared to the respective Cramer Rao Lower Bound (CRLB). The results show the two algorithms perform very close to the respective CRLB
Improved Data Detection Exploiting Full Cyclic Prefix for the evolution of DVB-T
No full textOrthogonal frequency division multiplexing (OFDM) has significantly reduced the complexity of the receivers, however to allow simple equalization procedures and avoid interblock interference, a cyclic prefix (CP) has to be appended to the OFDM block. CP reduces the efficiency of the transmission, because it is discarded at the receiver. This paper proposes a receiver structure which recovers the CP and exploits it to improve data detection. The performance of this structure is then analyzed comparing it with other two structures in a simulation scenario suitable for the evolution of the current Terrestrial Digital Video Broadcasting (DVB-T). The impact of low-density parity check codes (LDPC) is also outlined
Maximum Likelihood Estimation of Time and Carrier Frequency Offset for DVB-T2
No full textThe new terrestrial digital video broadcasting standard DVB-T2 provides a specific symbol - called P1-symbol for the initial time and frequency synchronization. In this paper the maximum likelihood (ML) time and carrier frequency offset (CFO) synchronization scheme, which exploits the structure of the P1 symbol in both time and frequency domains is derived. Two lower-complexity solutions are then proposed: 1) a ML estimator that only exploits the time structure of the P1 symbol and 2) a pseudo ML (PML) scheme that resorts to a suboptimal CFO estimator while still performing ML time synchronization. The proposed schemes are compared in terms of both synchronization capabilities and implementation complexity. The Cramer-Rao bounds for the CFO estimators are also evaluated. Simulation results in a typical DVB-T2 scenario show that both ML and PML schemes have a very close performance while significantly outperforming existing schemes
Software Defect Prediction evaluation: New metrics based on the ROC curve
No full textContext: ROC (Receiver Operating Characteristic) curves are widely used to represent how well fault-proneness models (e.g., probability models) classify software modules as faulty or non-faulty. AUC, the Area Under the ROC Curve, is usually used to quantify the overall discriminating power of a fault-proneness model. Alternative indicators proposed, e.g., RRA (Ratio of Relevant Areas), consider the area under a portion of a ROC curve. Each point of a ROC curve represents a binary classifier, obtained by setting a specified threshold on the fault-proneness model. Several performance metrics (Precision, Recall, the F-score, etc.) are used to assess a binary classifier. Objectives: We investigate the relationships linking ‘‘under the ROC curve area’’ indicators such as AUC and RRA to performance metrics. Methods: We study these relationships analytically. We introduce iso-PM ROC curves, whose points have the same value isoPM for a given performance metric PM. When evaluating a ROC curve, we identify the iso-PM curve with the same value of AUC or RRA. Its isoPM can be seen as a property of the ROC curve and fault-proneness model under evaluation. Results: There is an S-shaped relationship between isoPM and AUC for performance metrics that do not depend on the proportion ρ of faulty modules, i.e., dataset balancedness. φ (Matthews Correlation Coefficient) depends on ρ: with very imbalanced datasets, AUC appears over-optimistic and φ over-pessimistic. RRA defines the region of interest in terms of ρ, so all performance metrics depend on ρ. RRA is related to performance metrics via S-shaped curves. Conclusion: Our proposal helps gain a better quantitative understanding of the goodness of a ROC curve, especially in practically relevant regions of interest. Also, showing a ROC curve and iso-PM curves provides an intuitive perception of the goodness of a fault-proneness model
On the reliability of the area under the ROC curve in empirical software engineering
No full textBinary classifiers are commonly used in software engineering research to estimate several software qualities, e.g., defectiveness or vulnerability. Thus, it is important to adequately evaluate how well binary classifiers perform, before they are used in practice. The Area Under the Curve (AUC) of Receiver Operating Characteristic curves has often been used to this end. However, AUC has been the target of some criticisms, so it is necessary to evaluate under what conditions and to what extent AUC can be a reliable performance metric.
We analyze AUC in relation to φ (also known as Matthews Correlation Coefficient), often considered a more reliable performance metric, by building the lines in the ROC space with constant value of φ, for several values of φ, and computing the corresponding values of AUC.
By their very definitions, AUC and φ depend on the prevalence ρ of a dataset, which is the proportion of its positive instances (e.g., the defective software modules). Hence, so does the relationship between AUC and φ. It turns out that AUC and φ are very well correlated, and therefore provide concordant indications, for balanced datasets (those with ρ ≃ 0.5). Instead, AUC tends to become quite large, and hence provide over-optimistic indications, for very imbalanced datasets (those with ρ ≃ 0 or ρ ≃ 1).
We use examples from the software engineering literature to illustrate the analytical relationship linking AUC, φ, and ρ. We show that, for some values of ρ, the evaluation of performance based exclusively on AUC can be deceiving. In conclusion, this paper provides some guidelines for an informed usage and interpretation of AUC
On correlation-based synchronization for DVB-T2
No full textThe transmission format of the new digital video broadcasting standard DVB-T2 provides a block of samples (the P1 symbol) to ease initial signal acquisition and synchronization. In this paper we revise a synchronization technique based on the correlation of parts of the received signal and we show that it fails to synchronize in practical channel conditions. Then we propose a modified version of the existing technique that better exploits the correlation signal and compare the two techniques
Knowledge-Based Neural Pre-training for Intelligent Document Management
No full textBanks are usually large and complex companies that face a number of challenges to support the rapid and effective sharing of information and content across their organizations. Extracting complex metadata from raw bank documents is therefore central to support intelligent data indexing, information circulation and to promote more complex predictive capabilities, e.g., compliance assessment problems. In this paper, we present a weakly-supervised neural methodology for creating semantic metadata from bank documents. It exploits a neural pre-training method optimized against legacy semantic resources able to minimize the training effort. We studied an application to business process design and management in banks and tested the method on documents from the Italian banking community. The measured impact of the proposed training approach to process-related metadata creation confirms its applicability
RAL: A RESTful M2M communications framework for IoT
No full textThe idea that we are moving towards a future networked society is widely accepted. The increase of network access, in particular to the Internet, and the wide heterogeneity of devices and components that access the network, and occasionally communicate with each other, pose great design challenges, but at the same time open unprecedent opportunities. In order to be able to fully exploit the new opportunities, a uniform and standardized software architecture must be adopted. The aim is to regulate and organize the multidisciplinary nature of the Internet-of-Things (IoT) applications. In this article we first introduce the REpresentational State Transfer (REST) architecture style as one of the de-facto standards in a totally-IP-connected world. Then the main focus will move to a new M2M RESTful framework for the IoT, developed following the REST paradigm, but enhancing its interface to introduce new web functionalities that are still accessed via the usual interface of the Internet Web sites
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