Zugl.: Karlsruhe, Universiẗat, Diss., 2007.

Zugl.: Stuttgart, Univ., Diss., 2009

Zugl.: Zürich, Techn. Hochsch., Diss., 2009 / Fälschl. als Bd. 6 der Schriftenreihe bezeichnet

Zugl.: Zürich, Techn. Hochsch., Diss., 2009

Techn. University, Diss., 2004--Cottbus.

Techn. University, Diss., 2005--Darmstadt.

Στόχος της παρούσας διπλωματικής εργασίας είναι η μελέτη και η παρουσίαση της μεθόδου πολύπλεξης OFDM. Στο Κεφάλαιο 2 γίνεται αναφορά στα ασύρματα κανάλια επικοινωνίας. Παρουσιάζεται το κανάλι πολλαπλής διόδευσης με διαλείψεις (multipath fading), καθώς και το μοντέλο βασικής ζώνης του ασύρματου συστήματος. Η αναφορά σε φυσικές παραμέτρους του καναλιού, όπως η συνοχή στο χρόνο (coherence time) και η συνοχή εύρους ζώνης (coherence bandwidth), καθώς και σε έννοιες όπως η εξάπλωση Doppler (Doppler spread) και η εξάπλωση καθυστέρησης (delay spread), οδηγεί στην κατηγοριοποίηση των ασύρματων καναλιών. Επίσης παρουσιάζονται διάφορα στατιστικά μοντέλα για το κανάλι πολλαπλής διόδευσης με διαλείψεις. Στη συνέχεια, στο Κεφάλαιο 3 παρουσιάζονται μέθοδοι ανίχνευσης και διαφοροποίησης (diversity). Η διαφοροποίηση είναι ένας τρόπος βελτίωσης της απόδοσης της μετάδοσης μέσω καναλιού. Παρουσιάζονται δυο είδη ανίχνευσης: Η σύμφωνη ανίχνευση, όπου ο δέκτης έχει γνώση του καναλιού και η μη σύμφωνη ανίχνευση, όπου ο δέκτης δε γνωρίζει το κανάλι και συγκρίνονται οι πιθανότητες σφάλματος κάθε περίπτωσης. Επίσης μελετώνται τρία είδη διαφοροποίησης: η διαφοροποίηση στο χρόνο, στο χώρο και στη συχνότητα. Βασική αρχή της μεθόδου OFDM, η οποία αναλύεται εκτενώς στο 4ο κεφάλαιο, είναι η ταυτόχρονη μετάδοση πολλών ροών σε χαμηλό ρυθμό σε περισσότερες από μια υποφέρουσες αντί για μετάδοση μιας ροής με γρήγορο ρυθμό σε όλο το φάσμα. Η OFDM αποτελεί μέθοδο διαμόρφωσης, αλλά και πολυπλεξίας. Γίνεται εκτενής αναφορά στο μαθηματικό υπόβαθρο της διαμόρφωσης OFDM και παρουσιάζονται τρόποι αντιμετώπισης τόσο της διασυμβολικής όσο και της διακαναλικής παρεμβολής. Η διασυμβολική παρεμβολή αντιμετωπίζεται με την κυκλική επέκταση του συμβόλου OFDM, ενώ η διακαναλική παρεμβολή με τη χρήση ορθογώνιων υποφερουσών. Επίσης παρουσιάζεται η τεχνική windowing. Τέλος, αναφέρονται βασικές παράμετροι που λαμβάνονται υπόψη για τη σχεδίαση συστημάτων OFDM, όπως ο αριθμός υποφερουσών, ο χρόνος φύλαξης (guard time), η διάρκεια συμβόλου, το εύρος ζώνης (bandwidth), και ο ρυθμός μετάδοσης (bit rate). Στο Κεφάλαιο 5 προσομοιώνεται ένα σύστημα OFDM σε διάφορα περιβάλλοντα μετάδοσης (διάφορα είδη καναλιών). Παρατίθενται τα αποτελέσματα των προσομοιώσεων και γίνεται αποτίμηση και σύγκριση, για την εξαγωγή συμπερασμάτων. / The main objective of this Diploma thesis is a comprehensive presentation of the OFDM method. In Chapter 2, the wireless channel is presented, multipath fading is described, and the baseband equivalent of the wireless system is introduced. The consideration of physical parameters such as the coherence time, the coherence bandwidth, the Doppler spread and the delay spread leads to a categorization of wireless channels. Several statistical models for multipath fading are also surveyed. Chapter 3 introduces detection and diversity techniques. Diversity can be used to improve the performance of transmission over the channel. There are two types of detection: coherent and non-coherent. In coherent detection, in contrast to non- coherent, the receiver has full knowledge of the channel. Moreover, three diversity techniques are studied: time, space and frequency diversity. In Chapter 4, the basics of OFDM are presented. OFDM is a special case of multicarrier transmission, where a single data stream is transmitted over a number of lower-rate subcarriers. It is also a combination of modulation and multiplexing. It is explained how an OFDM signal is formed, how cyclic extension helps mitigate Inter-Symbol Interference (ISI), and how orthogonality between the modulated subcarriers eliminates Inter Channel Interference (ICI) in the absence of frequency offsets. Windowing is also presented. Basic design rules are given on how to choose the OFDM parameters, such as the guard time, the symbol duration, the bandwidth, and the bit rate. Finally, an OFDM system is simulated in Chapter 5. The simulations take place in different channel environments. The results of the simulations are compared, in order to draw conclusions.

Η παρούσα διπλωματική εργασία ασχολείται με την μελέτη αλγόριθμου συγχρονισμού σε OFDM συστήματα. Στα πρώτα δύο κεφάλαια αναλύονται χαρακτηριστικά και τεχνικές της OFDM διαμόρφωσης καθώς και η επίδραση του προσθετικού θορύβου. Στο τρίτο κεφάλαιο αναλύεται η ακολουθία συγχρονισμού. Στη συνέχεια παρουσιάζεται η τεχνική της αυτοσυσχέτισης και ο υπολογισμός του κατωφλίου που σηματοδοτεί το σημείο συγχρονισμού στο δέκτη. Στο πέμπτο κεφάλαιο γίνεται η παρουσίαση των τεχνικών που χρησιμοποιήθηκαν υλοποιημένες με τεχνικές VLSI. Αναφορά στα αποτελέσματα γίνεται στο έκτο κεφάλαιο, ενώ στο αμέσως επόμενο αναλύεται η πολυπλοκότητα, οι επιδόσεις του συστήματος και τα αποτελέσματα που προέκυψαν από την δοκιμή του συστήματος σε FPGA. Στο όγδοο και τελευταίο κεφάλαιο καταγράφονται τα συμπεράσματα που προέκυψαν από την μελέτη και εξομοίωση των τεχνικών που χρησιμοποιήθηκαν. / This thesis deals with the study of synchronization algorithm in OFDM systems. In the first two chapters analyze characteristics and techniques of OFDM modulation and the effect of additive noise. The third chapter discusses the synchronization sequence. Then, the autocorrelation technique and calculation of the threshold that marks the point of synchronization at the receiver. The fifth chapter is to present the techniques used implemented with VLSI techinques. Report on the results is the sixth chapter, while the next deals with the complexity, the system performance and the results obtained from testing the system in FPGA. In the eighth and final chapter, the conclusions drawn from the study and simulation techniques used.

VIGELIS, R. F. Estratégias de estimação para canais variantes no tempo em sistemas OFDM. 2006. 100 f. Dissertação (Mestrado em Engenharia de Teleinformática) – Centro de Tecnologia, Universidade Federal do Ceará, Fortaleza, 2006. / Submitted by Marlene Sousa (mmarlene@ufc.br) on 2016-04-05T12:33:35Z No. of bitstreams: 1 2006_dis_rfvigelis.pdf: 1183326 bytes, checksum: fd832c69a9a0562fe9f20f4707c90f94 (MD5) / Approved for entry into archive by Marlene Sousa(mmarlene@ufc.br) on 2016-04-06T17:57:59Z (GMT) No. of bitstreams: 1 2006_dis_rfvigelis.pdf: 1183326 bytes, checksum: fd832c69a9a0562fe9f20f4707c90f94 (MD5) / Made available in DSpace on 2016-04-06T17:57:59Z (GMT). No. of bitstreams: 1 2006_dis_rfvigelis.pdf: 1183326 bytes, checksum: fd832c69a9a0562fe9f20f4707c90f94 (MD5) Previous issue date: 2006-08-31 / This dissertation deals with the estimation of time-selective fading channels in OFDM (Orthogonal Frequency Division Multiplexing) coding systems. We considered that the channel obeys the WSS-US (Wide Sense Stationary–Uncorrelated Scattering) model, with an integral number of multi-paths. In presence of time-selective fading, we showed that the sub-carriers are the DFT (Discrete Fourier Transform) of the average of Nc channel samples in time-domain, where Nc is the number of employed sub-carriers. Under the assumption that the channel obeys the TDL (Tapped Delay Line) model, and the transmitted symbols at each sub-carrier have the same power, we also showed that the channel power is divided between the sub-carrier and ICI (Inter-Carrier Interference) power. For a grid placement of the pilot sub-carriers, we derived a MMSE (Minimum Mean Square Error) estimator that is able to exploit both time and frequency correlations. Supposing the sub-carrier correlations can be separated as the product between the time and frequency correlations, the filtering structure of the IIR (Infinite Impulse Response) estimator can be separated as the estimation at the pilot positions and the interpolation over time and frequency domain. Just taking into account that the maximum Doppler frequency and the channel and sub-carrier power are known at the receiver, we proposed a robust estimator that does not require the estimation of channel correlations and noise variance. We considered an adaptive estimator derived from the filtering structure obtained in the MMSE estimator. The filtering section that exploits the frequency-domain channel correlations is estimated by the LORAF (Low-Rank Adaptive Filter ) and OPAST(Orthogonal Projection Approximation Subspace Tracking) algorithms. The filters that exploits the time-domain channel correlations are estimated adaptively by an algorithm based on QR decomposition, which supports a fast version, whose computational complexity has the order of the filter length. All the analyzed and proposed estimation techniques are compared by computer simulation. / Nesta dissertação é abordado o problema de estimação de canais variantes no tempo em sistemas de modulação OFDM (Orthogonal Frequency Division Multiplexing). Foi escolhido o modelo de canal WSS-US (Wide Sense Stationary–Uncorrelated Scattering) com um número de percursos bem definido. Mostrou-se que, quando o canal é seletivo no tempo, as sub-portadoras consistem na DFT (Discrete Fourier Transform) da média conjunta de Nc amostras do canal no tempo, em que Nc é o número de sub-portadoras empregadas. Com a suposição de que o canal obedece o modelo TDL (Tapped Delay Line) e os símbolos de cada sub-portadora têm mesma potência, também foi mostrado que a potência do canal se divide entre a potência da sub-portadora e a potência da ICI (Inter-Carrier Interference). Para uma disposição em grid das sub-portadoras piloto, é derivado um estimador MMSE (Minimum Mean Square Error) que explora as correlações do canal tanto no tempo como na freqüência. Da suposição de que as correlações das sub-portadoras podem ser separadas como o produto entre as correlações do canal no tempo e na freqüência, obtêve-se que a estrutura de filtragem do estimador IIR (Infinite Impulse Response) pode ser separada entre as estimativas somente sobre as sub-portadoras piloto e o processo de interpolação, seja no tempo como na freqüência. É proposto um estimador robusto, que não requer as estimativas das correlações do canal e a variância do ruído, apenas considerando que a freqüência Doppler máxima, a potência do canal e a potência das sub portadoras são conhecidas. Também foi considerado um estimador adaptativo derivado a partir da estrutura de filtragem do estimador MMSE. A seção do filtro que explora as correlações do canal na freqüência é estimada pelos algoritmos LORAF (Low-Rank Adaptive Filter ) e OPAST (Orthogonal Projection Approximation Subspace Tracking). Já os filtros que exploram as correlações temporais do canal são estimados adaptativamente por meio de um algoritmo baseado em decomposição QR, que suporta uma versão rápida de complexidade computacional da ordem do comprimento filtro empregado. Todas as técnicas de estimação analisadas ou propostas são comparadas utilizando simulação em computador.

Made available in DSpace on 2018-08-02T00:00:53Z (GMT). No. of bitstreams: 1 tese_3859_DissertacaoMestradoJesseGomesdosSantos.pdf: 1635307 bytes, checksum: 4b377fb82afb8122f9903a637214c4c9 (MD5) Previous issue date: 2011-05-04 / Embora centenária, a tecnologia óptica para transmissão em espaço livre ainda pode ser considerada bastante desconhecida, pois durante décadas as pesquisas nesta área do conhecimento permaneceram restritas aos meios militares. Esta restrição é justificada em virtude de o sistema óptico no espaço livre depender intensamente do ambiente atmosférico e, portanto, está sujeito às intempéries existentes neste meio. Porém, a partir do século XXI, o desenvolvimento da tecnologia voltada à transmissão por fibra óptica, similar a em discussão, vem contribuindo, ainda que de forma lenta, para que o sistema óptico no espaço livre expanda seus domínios. A utilização da técnica de transmissão Ortoghonal Frequency Division Multiplexing (OFDM) é mais um instrumento capaz de ajudar este sistema de comunicação a enfrentar as adversidades existentes no ambiente atmosférico. Diante desta constatação, esta dissertação apresenta os resultados de testes experimentais realizados em laboratório que mostram o desempenho obtido pela utilização em conjunto destas duas tecnologias para interligar ponto a ponto duas estações sem fio em meio a ambientes com e sem turbulência. Os resultados apresentados neste trabalho de pesquisa mostram que a utilização do OFDM no sistema de comunicação óptico em espaço livre é possível e possibilita em parte do tempo manter o enlace de comunicação ativo, mesmo durante a ocorrência de fenômenos adversos como a turbulência óptica.