Thèses sur le sujet « Sequenze pseudorandom »

Random numbers (in one sense or another) have applications in computer simulation, Monte Carlo integration, cryptography, randomized computation, radar ranging, and other areas. It is impractical to generate random numbers in real life, instead sequences of numbers (or of bits) that appear to be ``random" yet repeatable are used in real life applications. These sequences are called pseudorandom sequences. To determine the suitability of pseudorandom sequences for applications, we need to study their properties, in particular, their statistical properties. The simplest property is the minimal period of the sequence. That is, the shortest number of steps until the sequence repeats. One important type of pseudorandom sequences is the sequences generated by feedback with carry shift registers (FCSRs). In this dissertation, we study statistical properties of N-ary FCSR sequences with odd prime connection integer q and least period (q-1)/2. These are called half-ℓ-sequences. More precisely, our work includes: The number of occurrences of one symbol within one period of a half-ℓ-sequence; The number of pairs of symbols with a fixed distance between them within one period of a half-ℓ-sequence; The number of triples of consecutive symbols within one period of a half-ℓ-sequence. In particular we give a bound on the number of occurrences of one symbol within one period of a binary half-ℓ-sequence and also the autocorrelation value in binary case. The results show that the distributions of half-ℓ-sequences are fairly flat. However, these sequences in the binary case also have some undesirable features as high autocorrelation values. We give bounds on the number of occurrences of two symbols with a fixed distance between them in an ℓ-sequence, whose period reaches the maximum and obtain conditions on the connection integer that guarantee the distribution is highly uniform. In another study of a cryptographically important statistical property, we study a generalization of correlation immunity (CI). CI is a measure of resistance to Siegenthaler's divide and conquer attack on nonlinear combiners. In this dissertation, we present results on correlation immune functions with regard to the q-transform, a generalization of the Walsh-Hadamard transform, to measure the proximity of two functions. We give two definitions of q-correlation immune functions and the relationship between them. Certain properties and constructions for q-correlation immune functions are discussed. We examine the connection between correlation immune functions and q-correlation immune functions.

A new application of chaos to electrical engineering is developed. By creating chaotic binary sequences, and using these as pulse compression codes for radar, many advantages are realized over codes that are presently used. These chaotic codes are generated by quantizing the iterates of one-dimensional chaotic difference equations to two levels. Being chaotic, these binary sequences are unpredictable in the long term, and are nonperiodic or can have periods of billions of bits. The autocorrelations of the sequences are derived, then the sequences are applied to radar, where the unpredictable and nonperiodic nature of these sequences is exploited. It is shown that chaotic codes offer advantages in maximum range performance, range resolution, low probability of intercept, and probability of false alarm compared to codes presently used.

Un dels majors problemes experimentats pels sistemes de detecció de gasos basats en sensors d'òxids metàl·lics és la seva manca de reproduibilitat, estabilitat i selectivitat. A fi i a efecte d'intentar resoldre aquest problemes, diferents estratègies han estat desenvolupades en paral·lel. Algunes es relacionen a la millora dels materials i d'altres impliquen el condicionament o el pre-tractament de les mostres. Les més emprades han consistit en aprofitar que els sensors presenten sensibilitats solapades per construir matrius de sensors i emprar tècniques de processament del senyal o bé utilitzar característiques de la resposta dinàmica dels sensors. En els darrers anys, modular la temperatura de treball del sensors d'òxids metàl·lics s'ha convertit en un dels mètodes més utilitzats per incrementar-ne la selectivitat. Això s'esdevé així donat que la resposta del sensor varia amb la seva temperatura de treball. Per això, en determinats casos, mesurant la resposta d'un sensor a n temperatures de treball diferents pot ser equivalent a tenir una matriu de n sensors diferents. Això permet obtenir informació multivariant de cada sensor individualment i ajuda a mantenir baixa la dimensionalitat del sistema de mesura per resoldre una determinada aplicació. Malgrat que molts i bons resultats han estat publicats dins aquest àmbit, la tria de les freqüències emprades en la modulació de la temperatura de treball dels sensor ha consistit fins ara en un procés empíric que no garanteix la obtenció dels millors resultats per una determinada aplicació. En aquest context, el principal objectiu d'aquesta tesi doctoral ha consistit en desenvolupar un mètode sistemàtic que permeti determinar quines són les freqüències de modulació òptimes que caldria emprar per resoldre un determinat problema d'anàlisi de gasos. Aquest mètode, extret del camp d'identificació de sistemes, ha esta desenvolupat i implementat per primer cop dins l'àmbit dels sensors de gasos. Aquest consisteix en estudiar la resposta dels sensors en presència de gasos mentre la temperatura de treball dels sensors és modulada per un senyal pseudo-aleatori de longitud màxima. Aquest senyals comparteixen algunes propietats amb el soroll blanc, i per tant poden ajudar a estimar la resposta lineal d'un sistema amb no-linealitats (per exemple, la resposta impulsional d'un sistema sensor-gas). El procés d'optimització es duu a terme mitjançant la selecció entre els components espectrals de les estimacions de la resposta impulsional, d'aquells que millor ajuden bé a discriminar o a quantificar els gasos objectiu dins una aplicació d'anàlisi de gasos donada. Tenint en compte que els components espectrals estan directament relacionats amb les xvii Improving the performance of micro-machined metal oxide gas sensors: Optimization of the temperature modulation mode via pseudo-random sequences. freqüències de modulació, la tria d'uns pocs components espectrals resulta en la determinació de les freqüències òptimes de modulació. En els primer experiments, senyals binaris pseudo-aleatoris van ser emprats per modular la temperatura de treball de sensors de gasos basats en òxids metàl·lics micro-mecanitzats dins d'un rang comprès entre 0 i 112,5 Hz. La freqüència superior és lleugerament superior a la frequència de tall de les membranes dels sensors. El resultat principal derivat d'aques estudi va ser que les freqüències de modulació interessants es trobaven en un rang comprès entre 0 i 1 Hz. Això és comprensible donat que la cinètica de les reaccions i dels processos d'adsorció que es produeixen en la superfície dels sensors són lentes i si aquestes s'han de veure modificades per la modulació térmica, llavors caldran senyals de modulació de baixa freqüència. Això explica perquè s'han vingut emprant senyals moduladores de temperatura en el rang dels mHz, malgrat que les membranes d'un dispositiu micromecanitzat presenten respostes tèrmiques molts més ràpides (típicament de l'ordre de 100 Hz). En els experiments que continuaren els primers, un mètode evolucionat per determinar les freqüències de modulació tèrmica òptimes va ser implementat. Aquest es basa en l'ús de seqüències pseudo-aleatòries multi-nivell de longitud màxima. Els senyals de tipus multi-nivell van ser considerats en substitució dels senyals binaris ja que els primers permeten obtenir una millor estimació que els segons de la dinàmica lineal d'un sistema amb no linealitats. I és ben conegut que els sensors de gasos basats en òxids metàl·lics presenten no linealitat en la seva resposta. Aquests estudis sistemàtics van ser completament validats mitjançant la síntesi de senyals multi-sinusoïdals amb les freqüències prèviament identificades emprant sequències pseudo-aleatòries. Quan la temperatura de treball dels sensors va ser modulada amb un senyal, el contingut freqüencial del qual era l'òptim, els gasos i les mescles de gasos considerades van poder ser discriminades perfectament i es va mostrar la possibilitat d'obtenir models de calibració acurats per predir la concentració dels gasos. En alguns casos, aquest procés de validació es va portar a terme emprant sensors que no havien estat utilitzats durant el procés d'optimització (per exemple, una agrupació de sensors diferent però del mateix lot de fabricació). En resum, el nou mètode desenvolupat en aquesta tesi per seleccionar les freqüències de modulació òptimes s'ha mostrat consistent i efectiu. El mètode és d'aplicació general i podria ser emprat en qualsevol problema d'anàlisi de gasos o bé estès a altres tipus de sensors (per exemple sensors polimèrics). Les contribucions científiques d'aquesta tesi s'han recollit en quatre articles en revistes internacionals i 13 llibres d'actes de conferències.
Uno de los mayores problemas experimentados en los sistemas de detección de gases basados en dispositivos de óxidos metálicos es su falta de reproducibilidad, estabilidad y selectividad. Con el fin de intentar resolver estos problemas, diferentes estrategias han sido desarrolladas en paralelo. Algunas de ellas se relacionan con la mejora de los materiales y otras implican acondicionamiento o pre-tratamiento de las muestras. Otras estrategias ampliamente empleadas consisten en aprovechar que los sensores presentan sensibilidades solapadas para construir matrices de sensores y emplear técnicas de procesamiento de señal o bien utilizar características de la respuesta dinámica de los sensores.En los últimos años, modular la temperatura de trabajo de los sensores de óxidos metálicos se ha convertido en uno de los métodos más utilizados para incrementar su selectividad. Esto se debe a, dado que la respuesta del sensor varía con su propia temperatura de trabajo, entonces, en determinados casos, midiendo la respuesta de un sensor a n temperaturas de trabajo diferentes, es equivalente a tener una matriz de n sensores diferentes. Esto permite obtener información multivariante de cada sensor individualmente y ayuda a mantener baja la dimensionalidad del sistema de medida para resolver una determinada aplicación. A pesar de los buenos resultados que han sido publicados dentro de este ámbito, la selección de las frecuencias empleadas en la modulación de la temperatura de trabajo de los sensores ha consistido, hasta el momento, en un proceso empírico lo que no garantiza la obtención de los mejores resultados para una determinada aplicación.En este contexto, el principal objetivo de esta tesis doctoral ha consistido en desarrollar un método sistemático que permita determinar cuales son las frecuencias de modulación óptimas que podrían emplearse para resolver un determinado problema de análisis de gases. Este método, extraído del campo de identificación de sistemas, ha sido desarrollado e implementado por primera vez dentro del ámbito de los sensores de gases. Éste consiste en estudiar la respuesta de los sensores en presencia de gases mientras la temperatura de trabajo de los sensores es modulada mediante una señal pseudo-aleatoria de longitud máxima. Estas señales comparten algunas propiedades con el ruido blanco, y por tanto pueden ayudar a estimar la respuesta lineal de un sistema con no-linealidades (por ejemplo, la respuesta impulsional de un sistema sensor-gas).El proceso de optimización es llevado a cabo mediante la selección entre las componentes espectrales de las estimaciones de la respuesta impulsional, de aquellas que más ayudan ya sea a discriminar o a cuantificar los gases objetivo dentro de una aplicación de análisis de gases dada. Teniendo en cuenta que las componentes espectrales están directamente relacionadas con las frecuencias de modulación, la selección de unas pocas componentes espectrales resulta en la determinación de las frecuencias optimas de modulación.En los primeres experimentos, señales binarias pseudo-aleatorias fueron utilizadas para modular la temperatura de trabajo de los sensores de gases basados en óxidos metálicos micro-mecanizados en un rango comprendido entre 0 a 112.5 Hz. La frecuencia superior es ligeramente mayor a la frecuencia de corte de las membranas de los sensores. El resultado principal derivado de estos estudios fue que las frecuencias de modulación interesantes se encuentran en un rango comprendido entre 0 y 1 Hz. Esto es comprensible dado que la cinética de las reacciones y de los procesos de adsorción que se producen en la superficie del sensor son lentos y si estos se han de alterar mediante la modulación térmica, se habrá de elaborar señales de modulación a bajas frecuencias. Esto explica por que se han venido empleado señales moduladoras de temperatura en el rango de los mHz, a pesar que las membranas de un dispositivo micro-mecanizado presentan respuestas mucho más rápidas (típicamente en el orden de los 100 Hz).En los experimentos posteriores a los primeros, un método evolucionado para determinar las frecuencias de modulación óptimas de los sensores micro-mecanizados fue implementado, el cual se basa en el uso de secuencias pseudo-aleatorias multi-nivel de longitud máxima (MLPRS). Las señales de tipo multi-nivel fueron consideradas en lugar de las binarias ya que las primeras permiten obtener una mejor estimación que las segundas de la dinámica lineal de un sistema con no linealidades. Y es bien conocido que los sensores de gases basados en óxidos metálicos presentan no-linealidades en su respuesta.Estos estudios sistemáticos fueron completamente validados mediante la síntesis de señales multi-senoidales con las frecuencias previamente identificadas utilizando secuencias pseudo-aleatorias. Cuando la temperatura de trabajo de los sensores fue modulada por una señal, el contenido frecuencial de la cual es el óptimo, los gases y mezclas de gases considerados pudieron ser discriminados perfectamente y se verificó la posibilidad de obtener modelos de calibración precisos para predecir la concentración de los gases. En algunos casos, estos procesos de validación se llevaron a cabo con sensores que no habían sido utilizados durante el proceso de optimización (por ejemplo, una agrupación de sensores diferentes pero del mismo lote de fabricación).En resumen, El nuevo método desarrollado in esta tesis para seleccionar las frecuencias de modulación optimas se a mostrado consistente y efectivo. El método es de aplicación general y podría ser utilizado en cualquier problema de análisis de gases o bien extendido a otro tipo de sensores (por ejemplo sensores poliméricos).Las contribuciones científicas de esta tesis se han recogido en 4 artículos en revistas internacionales y trece actas de conferencias.
One of the major problems in gas sensing systems that use metal oxide devices is the lack of reproducibility, stability and selectivity. In order to tackle these troubles experienced with metal oxide gas sensors, different strategies have been developed in parallel. Some of these are related to the improvement of materials, or the use of sample conditioning and pre-treating methods. Other widely used techniques include taking benefit of the unavoidable partially overlapping sensitivities by using sensor arrays and pattern recognition techniques or the use of dynamic features from the gas sensor response.In the last years, modulating the working temperature of metal oxide gas sensors has been one of the most used methods to enhance sensor selectivity. This occurs because, since, the sensor response is different at different working temperatures, and therefore, measuring the sensor response at n different temperatures is, in some cases, similar to the use of an array comprising n different sensors. This allows for measuring multivariate information from every single sensor and helps in keeping low the dimensionality of the measurement system needed to solve a specific application. Although the good results reported, until now, the selection of the frequencies used to modulate the working temperature remained an empirical process and that is not an accurate method to ensure that the best results are reached for a given application.In view of this context, the principal objective of this doctoral thesis was to develop a systematic method to determine which are the optimal temperature modulation frequencies to solve a given gas analysis problem. This method, which is borrowed from the field of system identification, has been developed and introduced for the first time in the area of gas sensors. It consists of studying the sensor response to gases when the operating temperature is modulated via maximum-length pseudo-random sequences. Such signals share some properties with white noise and, therefore, can be of help to estimate the linear response of a system with non-linearity (e.g., the impulse response of a sensor-gas system).The optimization process is conducted by selecting among the spectral components of the impulse response estimates, the few that better help either discriminating or quantifying the target gases of a given gas analysis application. Since spectral components are directly related to modulating frequencies, the selection of spectral components results in the determination of the optimal temperature modulating frequencies.In the first experiments, pseudo-random binary signals (PRBS) were employed to modulate the working temperature of micro-machined metal oxide gas sensors in a frequency range from 0 up to 112.5 Hz. The upper frequency is slightly higher than the cutoff frequency of the sensor membranes. The outcome of this initial study was that the important modulating frequencies were in the range between 0 and 1 Hz. This is understandable, since the kinetics of reaction and adsorption processes taking place at the sensor surface (i.e., physisorption/chemisorption/ionosorption) are slow and if these are to be altered by the thermal modulation, low frequency modulating signals need to be devised. This explains why low-frequency temperature-modulating signals (i.e. in the mHz range) have been used with micro-hotplate gas sensors, even though the thermal response of their membranes is much faster (typically, near 100 Hz).In the experiments that followed the first ones, an evolved method to determine the optimal temperature modulating frequencies for micro-hotplate gas sensors was introduced, which was based on the use of maximum length multilevel pseudo-random sequences (MLPRS). Multilevel signals were considered instead of the binary ones because the former can provide a better estimate than the latter of the linear dynamics of a process with non-linearity. And it is well known that temperature-modulated metal oxide gas sensors present non-linearity in their response.These systematic studies were fully validated by synthesizing multi-sinusoidal signals at the optimal frequencies previously identified using pseudo-random sequences. When the sensors had their operating temperatures modulated by a signal with a frequency content that corresponded to the optimal, the gases and gas mixtures considered could be perfectly discriminated and the building of accurate calibration models to predict gas concentration was found to be possible. In some cases, the validation process was conducted on sensors that had not been used for optimization purposes (e.g. a different sensor array from the same fabrication batch).Summarizing, the new method developed in this thesis for selecting the optimal modulating frequencies is shown to be consistent and effective. The method applies generally and could be used in any gas analysis problem or extended to other type of sensors (e.g. conducting polymer sensors).The scientific contributions of this thesis are collected in four journal papers and thirteen conference proceedings.

The constant demand for capacity increase, together with the foreseen saturation of the single-mode optical fiber, paved the way to technological breakthroughs that have completely changed the landscape of fiber-optic telecommunications. The most important advance was, undeniably, the practical implementation of a coherent detection with the help of high-speed electronics. This has, first, enabled the use of advanced modulation formats that allowed for a more efficient use of the fiber bandwidth, compared to the classical On-Off Keying, while adapted algorithms could not be used in order to mitigate the optical signal degradation. This thesis began a little after the advent of coherent detection and its main objective was to revisit the propagation effects in optical transmission systems using "Quadrature phase shift keying" (QPSK) modulation in the context of terrestrial systems, i.e. for transmission distances of up to about 2000 km. The manuscript is divided into two parts. The first part is dedicated to a study on the data sequences that need to be used in numerical simulations, when advanced modulation is involved. Fiber propagation, and in particular the interplay between chromatic dispersion and nonlinearities, usually introduce a nonlinear inter-symbol interference (ISI) to the transmitted signal. Since this ISI depends on the actual transmitted data pattern, it is obvious that the choice of the sequence used in our numerical simulations will have a direct influence on the estimated channel quality. Since, an infinite length, random sequence is impractical; we very commonly use pseudorandom" (PR) sequences, i.e. finite-length, deterministic sequences with balanced pattern statistics that seem to be random. In the first part we describe the method of generating M-level (with M>2) pseudorandom sequences and we detail their properties. In addition, we propose numerical tools to characterize the non-pseudorandom sequences that we use in numerical simulations, or we are sometimes forced to use in laboratory experiments. Finally, we present results of numerical simulations that quantify the necessity to use PR sequences as a function of our system parameters. After having established the "fairest possible" finite sequences, in the second part of the manuscript, we focus on the study of the nonlinear propagation, in the context of a transmission system using QPSK modulation and assuming a variable dispersion management and fiber type. Specifically, we numerically study the signal statistics due to the interplay of chromatic dispersion and nonlinear effects, neglecting all polarization or multi-wavelength effects and the amplifier noise. In this context, we were first interested in determining whether some empirical laws developed for OOK systems, can be also used in the case of QPSK modulation, such as the criterion of cumulative nonlinear phase (ΦNL) or laws that allow for a quick optimization of the dispersion management. Next we reveal the importance of a global phase rotation added to the initial signal constellation, as a parameter that can provide interesting information for the post-optimization of our system. We also discuss the fact that the constellation shape critically depends on the applied dispersion management, while there are generally 3 types of constellations, concerning the complex signal statistics: (1) the phase variance is higher than the amplitude variance (2) the amplitude variance is higher than the phase variance and (3) the received signal constellation resembles to a constellation of a signal under the influence of just an Additive White Gaussian Noise. Finally, we provide a phenomenological explanation of the constellations shapes revealing the fact that different data sub-sequences suffer from a different kind of signal degradation, while we also use this information to define a parameter that quantifies the potential benefit from a MAP (Maximum A Posteriori probability) correction algorithm

This thesis deals with a method of spectrum-spreading of the data signals transmitted in the radio channel. There exist three different ways of spreading application, they uses pseudorandom sequences and also orthogonal sequences. In these systems we can find various kinds of modulations, one of them (Quadrature Phase Shift Keying) is shown at the end of the thesis. Spectrum spreading orthogonal multiplex OFDM is based on this method. The thesis wants to present the principle of the spectrum spreading method and the description of systems, transmitters and receivers that are using this method.

The subject of this work deals with the problem of channel estimation for CDMA systems. This method of multiple access when individual users share the same full bandwidth simultaneously and are differentiated with any of pseudorandom sequences, is now the most perspective method. That is proved by its wide implementation in mobile networks of the third generation and higher systems. This work describes basic theory principles of spread spectrum, above all DS-CDMA (Direct Sequence-CDMA) and furthermore some phenomena of radio wireless channel that affect changes in transmitted signal in its way from transmitter to receiver. Terms of fading, multipath propagation, loss, refraction, scattering of the wave and Rice and Rayleigh probability density functions are mentioned. The third chapter deals with yet known and used capabilities of channel estimation. Differences, advantages and disadvantages of so-called blind estimation or training-based estimation are discussed. Two algorithms: LS method and sliding correlator are analyzed in more detail. There is also description of their simulations in Matlab and some results of these simulations are discussed. The last chapter deals with comparison of main characteristics and achievable accuracy of wireless channel impulse response estimation by both methods, and their possible utilization in real live.

Il lavoro descrive il processo di messa a punto di una metodologia d’ispezione non distruttiva di componenti meccanici, basata sull’eccitazione non a contatto di ultrasuoni (US) tramite diodi laser modulabili. Il fenomeno fisico a cui tale eccitazione risulta riconducibile è denominato “fotoacustica”: un riscaldamento localizzato del materiale a partire da impulsi luminosi ed il suo successivo raffreddamento comportano la sua dilatazione e contrazione, con conseguente propagazione di onde elastiche nel componente. Poiché il diodo laser rappresenta una tecnologia non convenzionale di eccitazione ultrasonora, requisito fondamentale è comprendere il fenomeno della fotoacustica nel suo complesso al fine di ottimizzare il sistema d’ispezione. L’indagine ha dapprima coinvolto alcuni provini, per essere successivamente traslata verso componenti meccanici e, nello specifico, assili ferroviari; per l’assile, un’ispezione di tipo contactless risulta particolarmente desiderabile, in quanto permette di ispezionare il componente mentre si trova in rotazione, durante l’esercizio. Nondimeno, l’impiego di diodi laser in luogo di soluzioni come i laser impulsati apre le porte alla creazione di un layout d’ispezione non a contatto a basso costo, adatto all’applicazione su vasta scala. La prima fase del progetto di ricerca ha coinvolto lo studio e la messa a punto di un sistema per la rilevazione di onde sulla base dei risultati di una preliminare campagna di prove: impiegando una sonda piezoelettrica a banda larga a contatto, sono stati identificati US di tipo longitudinale e superficiale propaganti in provini ed eccitati tramite diodo da 0.15 W. L’analisi ha permesso di compiere passi in avanti rispetto alla letteratura tecnica in termini di qualità del segnale ultrasonoro rilevato; parametro di particolare rilevanza è la durata (delta) dell’impulso laser impiegato, che influenza direttamente sia il rapporto segnale-rumore (signal-to-noise ratio, S/N ) che la frequenza centrale f0 delle oscillazioni ultrasonore. L’uso di una sorgente dalla maggior potenza (5W) ha permesso di spingere l’applicazione del sistema d’ispezione ad un ambito dal maggior interesse ingegneristico, inerente all’eccitazione di US in componenti meccanici completi come gli assili ferroviari. Stante la maggior criticità delle cricche sulla superficie in conseguenza del momento flettente rotante cui l’assile è continuativamente sottoposto, la campagna sperimentale ha mirato alla rilevazione di tali difettosità tramite onde di tipo superficiale: è stato determinato come una cricca profonda 3 mm sul corpo dell’assile comporti una riduzione del 65% nel valore di S/N, eccitando US con frequenza centrale di 200 kHz. Analoghe considerazioni possono essere estese a cricche posizionate sui raggi di raccordo, in corrispondenza dei quali si vengono sovente a concentrare le tensioni massime. Una successiva sperimentazione tramite sonde non a contatto a banda stretta ha fatto emergere l’impossibilità di rilevare ultrasuoni, richiedendo un’ulteriore messa a punto del sistema d’indagine per concentrare maggiormente la banda dell’US attorno alla frequenza centrale della sonda stessa. Avvalendosi del fenomeno fisico di doppia eccitazione di US tramite un singolo impulso laser (il primo quando ha luogo il riscaldamento, il secondo all’atto del raffreddamento), è stato proposto l’impiego di una sequenza in input al diodo costituita da impulsi dalla medesima delta e relativamente distanziati nel tempo di 2(delta). Tale soluzione permette di concentrare la banda intorno a f0=1/(2(delta)), con un’amplificazione controllata del segnale fino al 210% rispetto al caso di singolo impulso. Nonostante gli ulteriori sforzi profusi nell’ottimizzazione della ricezione, non è stato possibile evidenziare la presenza di US tramite sonde non a contatto. Conseguentemente, il lavoro si conclude con la proposta di metodologie alternative per la rilevazione non a contatto degli US eccitati da diodo: requisito fondamentale per tali alternative è un prezzo limitato tale da non confliggere con uno degli obiettivi del progetto, ovvero la messa a punto di un sistema d’ispezione dal basso costo complessivo. Preme tuttavia sottolineare come la metodologia di eccitazione sviluppata per la rilevazione a contatto di US propaganti in componenti meccanici risulti estremamente performante: la ricerca ha dunque raggiunto il principale scopo previsto di comprendere il complesso fenomeno di eccitazione di US tramite diodo laser, illustrandone le prestazioni effettive in una concreta applicazione ingegneristica ed andando ad arricchire quanto disponibile allo stato dell’arte.

Pseudorandom sequences have many applications in fields such as wireless communication, cryptography and built-in self test of integrated circuits. Maximal length sequences (m-sequences) are commonly employed pseudorandom sequences because they have ideal randomness properties like balance, run and autocorrelation. However, the linear complexity of m-sequences is poor. This thesis considers the use of one-dimensional Cellular Automata (CA) to generate pseudorandom sequences that have high linear complexity and good randomness. The properties of these sequences are compared with those of the corresponding m-sequences to determine their suitability.
Graduate

Maximum linear feedback shift registers (LFSRs) based on primitive polynomials are commonly used to generate maximum length sequences (m-sequences). An m-sequence is a pseudorandom sequence that exhibits ideal randomness properties like balance, run and autocorrelation but has low linear complexity. One-dimensional Cellular Automata (1D CA) have been used to generate m-sequences and pseudorandom sequences that have high linear complexity and good randomness. This thesis considers the use of two-dimensional Cellular Automata (2D CA) to generate m-sequences and psuedorandom sequences that have high linear complexity and good randomness. The properties of these sequences are compared with those of the corresponding m-sequences and the best sequences generated by 1D CAs.
Graduate

This thesis is concerned with the design and analysis of strong de Bruijn sequences and span n sequences, and nonlinear feedback shift register (NLFSR) based pseudorandom number generators for radio frequency identification (RFID) tags. We study the generation of span n sequences using structured searching in which an NLFSR with a class of feedback functions is employed to find span n sequences. Some properties of the recurrence relation for the structured search are discovered. We use five classes of functions in this structured search, and present the number of span n sequences for 6 <= n <= 20. The linear span of a new span n sequence lies between near-optimal and optimal. According to our empirical studies, a span n sequence can be found in the structured search with a better probability of success. Newly found span n sequences can be used in the composited construction and in designing lightweight pseudorandom number generators. We first refine the composited construction based on a span n sequence for generating long de Bruijn sequences. A de Bruijn sequence produced by the composited construction is referred to as a composited de Bruijn sequence. The linear complexity of a composited de Bruijn sequence is determined. We analyze the feedback function of the composited construction from an approximation point of view for producing strong de Bruijn sequences. The cycle structure of an approximated feedback function and the linear complexity of a sequence produced by an approximated feedback function are determined. A few examples of strong de Bruijn sequences with the implementation issues of the feedback functions of an (n+16)-stage NLFSR are presented. We propose a new lightweight pseudorandom number generator family, named Warbler family based on NLFSRs for smart devices. Warbler family is comprised of a combination of modified de Bruijn blocks (CMDB) and a nonlinear feedback Welch-Gong (WG) generator. We derive the randomness properties such as period and linear complexity of an output sequence produced by the Warbler family. Two instances, Warbler-I and Warbler-II, of the Warbler family are proposed for passive RFID tags. The CMDBs of both Warbler-I and Warbler-II contain span n sequences that are produced by the structured search. We analyze the security properties of Warbler-I and Warbler-II by considering the statistical tests and several cryptanalytic attacks. Hardware implementations of both instances in VHDL show that Warbler-I and Warbler-II require 46 slices and 58 slices, respectively. Warbler-I can be used to generate 16-bit random numbers in the tag identification protocol of the EPC Class 1 Generation 2 standard, and Warbler-II can be employed as a random number generator in the tag identification as well as an authentication protocol for RFID systems.

碩士
中原大學
電機工程研究所
90
Abstract In this thesis, we propose three new pseudorandom sequence generators based on modular operation (PSGMO). In the first scheme, the basic pseudorandom sequence is generated from a simple modular equation by changing its initial values and equation constants. Then, through a nonlinear function transfer module, the desired practical pseudorandom sequence is obtained. In the second scheme, a new idea, dynamic transformation table, is proposed and two modular equations are used. The basic pseudorandom sequence is generated by one of the two modular equations as the first scheme to be used as the dynamic transformation table. The other modular equation is used to generate a primary PN sequence. Then, the desired PN sequence is obtained by transforming the primary PN sequence via the dynamic transformation table. In the third scheme, the nonlinear function transfer module is appended to the second scheme to generate the practical pseudorandom sequence. In addition, in order to promote the linear complexity (LC) of these three new pseudorandom sequence generators, we combine the Skip and Latch controller circuit and k-layer Latch-Register in our schemes. As result, the LC may increase from N to N^k*(N-1) provided that the bit number of the seed is N. The features of PSGMO are as follows: (1). Computational speed is faster then the traditional Linear Congruential Generator (LCG), since only the modular congruence operation is used. Furthermore, the security of our schemes is guaranteed. In contrast, the LCG is proven to be unsecured. (2). A new idea called dynamic transfer table is propose. It provides not only the security of the scheme but also the sequence complexity. (3). The PSGMO has high LC with exponential function growth. Hence it can shorten the length of seed to provide the reasonable high LC of the PN sequences. Furthermore, it can be implemented by software easily. Consequently, the propose PSGMO is useful and easily implemented for pseudorandom sequence generators and practical applications.

Generators of pseudorandom sequences are widely used objects, not in the least place because of their application in stream ciphers. One of the ways to improve resistance to different types of attack is to use compression on the generated sequence in order to remove redundant information, that might lead to an attack against the generator. In this work we try to explore from a wider perspective the theoretical foundations for compressing pseudorandom sequences created thus far. Using this general view we will examine some known attacks against the PRN generators and look for a way to resist such attacks.

The focus of this PhD research is to develop non-invasive and cost-effective techniques for assessing the structural condition of pressurised pipelines using fluid transient pressure waves. The specific objectives include the detection of leaks and localised deterioration that is distributed along a pipeline, such as extended sections of corrosion or the spalling of cement lining. The latter is described by pipeline condition assessment in this thesis. The transient behaviour of a leak is studied in the frequency domain. Numerical studies conducted in this research demonstrate that two leak-induced patterns (on the resonant and the anti-resonant responses) can exist in a frequency response diagram (FRD). The amplitudes of the responses are related to the impedance of the valve in a reservoir-pipeline-valve (RPV) system. A new leak detection technique has been developed in this research based on the further understanding of the leak-induced patterns. This technique uses the relative sizes of the first three resonant responses to determine the location and size of a single leak in RPV systems. In reservoir-pipeline-dead end systems, the information required for single event leak detection is further reduced to the first two resonant responses. A new measurement strategy for the extraction of the FRD of single pipelines is proposed in this research. The boundary valve loss is used to adjust the amplitude of the leak-induced pattern on the resonant responses and also the sharpness of the resonant peaks. A specific type of pseudo-random binary sequence (PRBS) termed the inverse repeat sequence (IRS), is used as the excitation signal. The antisymmetric property of IRS enables part of the nonlinear responses of the system under excitation to be cancelled out, yielding a measured FRD close to the theoretical linear system response. A side-discharge valve based transient generator is designed and fabricated in this research to implement the new FRD measurement strategy. Laboratory experiments are conducted on an intact pipeline and a pipeline with a leak. This research also conducts analysis of the characteristics of distributed pipe wall deterioration and develops new detection techniques. In a measured pressure trace, the size of the reflection resulting from a section of pipeline with a change in wall thickness is indicative of the characteristic impedance of this section. Once the impedance of this section is determined, the wave speed and wall thickness can be estimated. A technique for the detection of a single deteriorated section in pipelines is developed based on the above analysis. Two other condition assessment techniques are developed to deal with the complexities induced by multiple deteriorated sections. The first technique is termed reconstructive MOC (method of characteristics) analysis, which uses the pressure trace measured at the upstream face of the valve in a RPV system to determine the distribution of the impedance along the pipeline. The algorithm reconstructs a MOC grid by calculating the MOC compatibility equations backwards in time, estimating the properties of the pipeline (impedance, wave speed) and the length of each pipe reach as discretised by the MOC grid from the valve towards the reservoir. Preliminary experimental verification is conducted to verify the applicability of the new technique. The second technique is reconstructive transient analysis (RTA), which can be conducted at any interior accessible points along a pipeline, and does not require a RPV boundary condition. The RTA uses two pressure transducers in close proximity to measure two transient pressure traces in one test. A signal processing algorithm is developed to extract the directional transient waves (traveling upstream and downstream). The use of the directional transient waves enables the step response function (SRF) of the section of pipe upstream or downstream of the paired pressure transducers to be obtained. The reconstructive MOC analysis is then adapted to interpret the SRF to yield the distribution of the impedance, from which the location and severity of distributed deterioration can be identified.
Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental and Mining Engineering, 2014.

Η παρούσα διπλωματική εργασία έγινε στα πλαίσια του Προγράμματος Μεταπτυχιακών Σπουδών Ηλεκτρονικής και Υπολογιστών, στο Τμήμα Φυσικής του Πανεπιστημίου Πατρών. Αντικείμενό της αποτελεί η μελέτη του φυσικού επιπέδου συστημάτων κινητής τηλεφωνίας τρίτης γενιάς και η εξομοίωση χαρακτηριστικού καναλιού του φυσικού επιπέδου σύμφωνα με την παγκόσμια προτυποποίηση 3GPP. Στο πρώτο κεφάλαιο γίνεται ιστορική ανασκόπηση των συστημάτων προηγούμενων γενεών. Αναφέρονται βασικά προβλήματα που πρέπει να αντιμετωπίσει ένα τηλεπικοινωνιακό σύστημα καθώς και η βασική δομή των κυψελοειδών συστημάτων. Γίνεται εισαγωγή στα συστήματα τρίτης γενιάς ως προς τις απαιτήσεις, τις υπηρεσίες που προσφέρουν, την προτυποποίηση και την αρχιτεκτονική τους. Στο δεύτερο κεφάλαιο μελετώνται οι τρόποι πολλαπλής πρόσβασης με ιδιαίτερη έμφαση στην προσπέλαση με διαίρεση κωδικών. Αναλύονται τα συστήματα διάχυσης φάσματος. Γίνεται εκτενής αναφορά και υλοποίηση προγραμμάτων για τις ψευδοτυχαίες ακολουθίες και τις ακολουθίες Gold. Στο τρίτο κεφάλαιο μελετάται η δομή δικτύων επικοινωνίας κατά το μοντέλο OSI. Αναφέρονται τα επίπεδα των τηλεπικοινωνιακών συστημάτων με διεξοδική μελέτη του φυσικού επιπέδου και στις διεπαφές που χρησιμοποιεί για επικοινωνία καθώς και στις υπηρεσίες που προσφέρει. Αναλύεται η δομή όλων των φυσικών καναλιών και εξομοιώνεται η διαδικασία ενθυλάκωσης του καναλιού PRACH. Στο τέταρτο κεφάλαιο αναλύονται οι κώδικες διάχυσης των φυσικών καναλιών ανερχόμενης ζεύξης καθώς και η διαδικασία διάχυσης τους. Δημιουργούνται προγράμματα που παράγουν αυτούς τους κώδικες και γίνεται εξομοίωση της διάχυσης και της αποδιάχυσης του PRACH καναλιού. Στο πέμπτο κεφάλαιο αναφέρεται η διαδικασία κωδικοποίησης πηγής και μελετάται η PSK διαμόρφωση και αποδιαμόρφωση που χρησιμοποιούν τα συστήματα τρίτης γενιάς. Υλοποιούνται προγράμματα που εξομοιώνουν τη συνολική διαδικασία σε ενθόρυβο κανάλι AWGN μελετώντας την επίδραση του θορύβου σε μεταδιδόμενο σήμα. Όλες οι εξομοιώσεις υλοποιήθηκαν με το πρόγραμμα Matlab 7.1.
This master thesis is a part of the post-graduate course of the Physics department of the University of Patras, on Electronics and Computers. Its main objective is the study of the physical layer of 3rd generation telecommunication systems and the simulation of a particular channel, in accordance to 3GPP specifications. In the first chapter we review telecommunication systems of previous generations. We also refer to major problems that need to be dealt with and also the basic structure of cellular networks. 3rd generation telecommunication systems, their requirements, offered services, standardisation and architecture are all introduced. In the second chapter we study multiple access methods, emphasising CDMA methods. Moreover, spread spectrum systems are analysed. Finally we refer to pseudorandom and Gold sequences. Programs are created which generate these sequences. In the third chapter we look into the structure of communication networks according to the OSI model. The layers of telecommunication systems are mentioned, laying emphasis on the physical layer, the interfaces used for communication as well as the offered services. We report all physical channels and we simulate the encapsulation of the PRACH channel. In the fourth chapter we analyse the spreading and scrambling codes referring to the uplink channels. Next we point out the code allocation process. Programs are created which generate these spreading and scrambling codes and we finally simulate the spreading and de-spreading process of the PRACH channel. In the fifth chapter we outline the source coding process using PCM, and we study PSK modulation and demodulation which are used by 3rd generation telecommunication systems. We create programs that simulate the whole procedure in a noisy channel, and we study the effect of AWGN. The programs were created and the simulations were run using Matlab version 7.1.