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Intercarrier interference reduction and channel estimation in OFDM systemsZhang, Yihai 16 August 2011 (has links)
With the increasing demand for more wireless multimedia applications,
it is desired to design a wireless system with higher data rate.
Furthermore, the frequency spectrum has become
a limited and valuable resource, making
it necessary to utilize the available spectrum efficiently
and coexist with other wireless systems.
Orthogonal frequency division multiplexing (OFDM)
modulation is widely used in communication systems to meet the
demand for ever increasing data rates. The major advantage of OFDM
over single-carrier transmission is its ability to deal with severe
channel conditions without complex equalization. However, OFDM systems suffer from
a high peak to average power ratio, and they are sensitive to carrier frequency offset and Doppler spread.
This dissertation first focuses on the development of intercarrier interference (ICI) reduction and signal detection
algorithms for OFDM systems over time-varying channels.
Several ICI reduction algorithms are proposed for OFDM systems over
doubly-selective channels. The OFDM ICI reduction problem over time-varying channels
is formulated as a combinatorial optimization problem based on the maximum likelihood (ML)
criterion. First, two
relaxation methods are utilized to convert the ICI reduction problem
into convex quadratic programming (QP) problems. Next, a low
complexity ICI reduction algorithm applicable to $M$-QAM signal
constellations for OFDM systems is proposed.
This formulates the ICI reduction problem as a QP problem with non-convex constraints. A
successive method is then utilized to deduce a sequence of
reduced-size QP problems. For the proposed algorithms, the QP
problems are solved by limiting the search in the 2-dimensional
subspace spanned by its steepest-descent and Newton directions to
reduce the computational complexity. Furthermore, a low-bit descent
search (LBDS) is employed to improve the system performance.
Performance results are given to demonstrate that the proposed
ICI reduction algorithms provide excellent performance with
reasonable computational complexity.
A low complexity joint semiblind
detection algorithm based on the channel correlation and noise
variance is proposed which does not require channel state information.
The detection problem is relaxed
to a continuous non-convex quadratic programming problem. Then an
iterative method is utilized to deduce a sequence of reduced-size
quadratic programming problems.
A LBDS method is also employed
to improve the solution of the derived QP problems. Results are
given which demonstrate that the proposed algorithm provides
similar performance with lower computational complexity compared to
that of a sphere decoder.
A major challenge to OFDM systems is how to obtain accurate
channel state information for coherent detection of the transmitted signals. Thus
several channel estimation algorithms are proposed for OFDM systems
over time-invariant channels.
A channel estimation method is developed to utilize
the noncircularity of the input signals to
obtain an estimate of the channel coefficients.
It takes advantage of the nonzero cyclostationary
statistics of the transmitted signals,
which in turn allows blind polynomial channel estimation using
second-order statistics of the OFDM symbol.
A set of polynomial equations are formulated based on the correlation
of the received signal which can be used to obtain an
estimate of the time domain channel coefficients.
Performance results are presented which show that the proposed algorithm
provides better performance than the least minimum mean-square error (LMMSE)
algorithm at high signal to noise ratios (SNRs), with low
computational complexity.
Near-optimal performance can be achieved with large OFDM systems.
Finally, a CS-based time-domain channel estimation method is presented
for OFDM systems over sparse channels. The channel estimation
problem under consideration is formulated as a small-scale
$l_1$-minimization problem which is convex and admits fast and
reliable solvers for the globally optimal solution. It is
demonstrated that the magnitudes as well as delays of the
significant taps of a sparse channel model can be estimated with
satisfactory accuracy by using fewer pilot tones than the channel
length. Moreover, it is shown that a fast Fourier transform (FFT)
matrix of extended size can be used as a set of appropriate basis
vectors to enhance the channel sparsity. This
technique allows the proposed method to be applicable to
less-sparse OFDM channels. In addition, a total-variation (TV)
minimization based method is introduced to provide an alternative
way to solve the original sparse channel estimation problem. The
performance of the proposed method is compared to several
established channel estimation algorithms. / Graduate
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Synchronisation en fréquence pour l'allocation de porteuses des systèmes OFDMA en liaison montante / Synchronozation en frequence pour l'allocation de porteuses des systemes ofdma en liaison montanteAziz, Babar 15 December 2011 (has links)
La mise en œuvre de systèmes basés OFDMA nécessite une synchronisation de la fréquence très fine en raison de l'extrême sensibilité de l'OFDMA aux décalages en fréquence porteuse (CFO). La synchronisation en fréquence devient plus difficile dans les systèmes OFDMA en liaison montante. Nos objectifs comprennent l'étude et l'analyse des problèmes résultant de décalages de fréquence et la proposition de solutions pour lutter contre ces problèmes. Nous examinons d'abord les interférences résultant de décalages de fréquence porteuse présents dans l'oscillateur du terminal utilisateur. Nous démontrons que l'on doit prendre en compte le préfixe cyclique tout en analysant les interférences résultant du CFO. Ensuite, nous montrons qu'il existe une contradiction entre la diversité de fréquence de canal et la robustesse contre le CFO. Nous proposons un compromis sous la forme d'une taille de bloc de seuil, afin de permettre un bon compromis entre la diversité des canaux et de robustesse pour les CFO pour le cas où aucune connaissances du canal n'est disponible. Quand le canal est connu, nous proposons une allocation optimale par bloc grâce à laquelle la robustesse aux CFO et à la diversité de fréquence de canal peut être réalisée en utilisant une petite taille de bloc petit pour des CFO de petite valeur. Nous proposons également une valeur CFO critique, en dessous de laquelle l'allocation optimal par bloc est très performant.Ensuite, nous proposons des solutions pour deux problèmes importants rencontrés dans un système OFDMA en liaison montante.Premièrement, nous proposons une méthode efficace pour l'estimation conjointe des réponses impulsionnelles des canaux et fréquences porteuses basée sur l'approximation polynomiale.Notre méthode d'estimation conjointe est plus simple que les méthodes existantes, sans aucune dégradation de performance. Ensuite, nous proposons une méthode de compensation de CFO basée sur l'annulation des interférences successives (SIC). La méthode d'annulation proposée réduit la complexité de mise en oeuvre quand le nombre de porteuses est important. / One of the most prominent issues in the design and implementation of OFDMA based systems is the need for a very fine frequency synchronization due to the fact that OFDMA, like OFDM, is extremely sensitive to carrier frequency offsets (CFO). The task of frequency synchronization becomes more challenging in the uplink OFDMA-based systems where one OFDMA symbol is generated by the contribution of many different users. Our goals include the study and analysis of problems resulting from frequency mismatches provide solution to combat these problems. We first look at the interference resulting from CFOs, resulting from user terminal oscillator mismatch. We demonstrate that one must take into account the cyclic prefix while analyzing interference resulting from CFO. A new analytical expression of the ICI that takes into account the effect of CFO on the cyclic prefix is proposed. Then we focus our attention on analysis of the trade-off between channel frequency diversity and robustness against CFO and show that there exists a contradiction between the two. We propose a trade-off in the form of a Threshold blocksize, to allow a good compromise between the channel diversity and robustness for CFO for the case when no CSI is available. For system where CSI is available, we propose an optimal block carrier allocation scheme through which both robustness to CFO and channel frequency diversity can be achieved with small blocksize for small CFO. We also propose a Critical CFO value, above which the performance of the optimal block carrier allocation loses interest.Next we propose solutions for two important issues encountered in an uplink OFDMA system. First, we propose an efficient method for joint estimation of channel impulse responses and carrier frequency at the receiver based on polynomial approximation. Our proposed joint estimation method is simpler than the existing methods without any performance degradation. Next we propose a CFO compensation method based on successive interference cancellation. The proposed cancellation method reduces the implementation complexity faced in case of large DFT matrices.
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An Improved ICI Self-Cancellation Scheme for Distributed MISO-OFDM SystemsLi, Pei-Hsun 24 August 2011 (has links)
One of the challenges of distributed cooperative orthogonal frequency division multiplexing systems is that the multiple carrier frequency offsets (CFOs) simultaneously present at the receiver. According to our knowledge up to now, even the CFOs are known at the receiver, the way to perfectly eliminate the effect of CFOs is still an open problem. This thesis proposes a scheme to mitigate the effect due to multiple CFOs by using the concept of intercarrier interference self-cancellation in transitional OFDM systems, a scheme where the data are simultaneously modulated on symmetric subcarriers between two transmit antennas. Before processing FFT, two values related to CFOs are used to adjust the time-domain signal resulting in better signal-to-interference ratio in even and odd subcarriers respectively. After that, the data are combined by applying maximum ratio combining and then decoded. Simulation results are given to demonstrate the effectiveness of the proposed scheme as compared to previous scheme.
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Comparison Of The Intercarrier Interference Cancellation Methods In Ofdm SystemsEtiler, Burkay 01 September 2003 (has links) (PDF)
In OFDM systems carrier frequency offset is observed due to Doppler shift
and transmitter-receiver frequency mismatches. This offset induces ICI (Intercarrier
Interference). In this thesis, repeated data methods and pilot-aided carrier frequency
offset(CFO) estimation methods and windowing techniques are used to mitigate the
frequency offset problem and a performance comparison is made between these ICI
cancellation techniques.
Repeated data methods use only half of the bandwidth for information
transmission to eliminate the ICI at the receiver. We have implemented repeated
data methods including Self cancellation scheme and Symmetric Symbol Repetition
(SSR) schemes to overcome ICI problem. We have also implemented Adjacent
Conjugate Symbol Repetiton (ACSR) and Symmetric Conjugate Symbol Repetiton
(SCSR) methods to mitigate both phase rotations and ICI.
CFO estimation and correction methods generally use pilot sequences. We
implemented the &ldquo / Conventional Pilots&rdquo / and &ldquo / Clustered Pilots&rdquo / pilot-aided CFO
estimation techniques for ICI cancellation. Furthermore, we also implemented a new
scheme by using the odd symmetry between pilot symbols.
Nyquist windowing techniques apply windowing at the receiver side. We have
implemented second order polynomial class of Nyquist windows and Nyquist
window with Franks pulse used to mitigate ICI.
These ICI cancellation methods are compared in AWGN and multipath
Rayleigh fading channel models in terms of BER and carrier to interference ratio. It
is shown that repeated data methods shows better performances than pilot-aided
CFO estimation methods with a cost of increased bandwidth usage especially in
high SNR&rsquo / s.
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VLSI αρχιτεκτονική χαμηλής κατανάλωσης για συγχρονισμό σε Multi-band UWB ασύρματα δίκτυαΠούλος, Αθανάσιος 30 July 2007 (has links)
Η ΒΥΠ διαθέτει αντίτυπο της διατριβής σε έντυπη μορφή στο βιβλιοστάσιο διδακτορικών διατριβών που βρίσκεται στο ισόγειο του κτιρίου της. / Τα ψηφιακά συστήματα UWB (Ultra Wide-Band) παρέχουν τη δυνατότητα ασύρματης μετάδοσης σε πολύ υψηλό ρυθμό. Λόγω του μεγάλου εύρους ζώνης, το κανάλι εισάγει πολλαπλές ανακλάσεις οι οποίες φέρουν μεγάλο ποσοστό της ωφέλιμης ενέργειας του μεταδιδόμενου σήματος. Η ικανότητα του δέκτη για σύλληψη όσο το δυνατόν περισσότερης ωφέλιμης ενέργειας έχει αντίκτυπο στη συνολική απόδοση του συστήματος. Η χρήση της τεχνικής διαμόρφωσης με πολύπλεξη συχνότητας ορθογωνίων φερουσών (OFDM), που στην συγκεκριμένη περίπτωση (UWB) συνδυάζεται με πολυζωνική (Multi-band) μετάδοση, απλοποιεί τη διαχείριση του συνολικού φάσματος συχνοτήτων. Όμως η διαμόρφωση OFDM παρουσιάζει ιδιαίτερη ευαισθησία σε προβλήματα τόσο διασυμβολικής παρεμβολής (ISI) όσο και διακαναλικής παρεμβολής (ICI), λόγω του έντονου διασκορπιστικού χαρακτήρα του καναλιού καθώς επίσης και τυχόν αποκλίσεων που εμφανίζονται στους ταλαντωτές πομπού-δέκτη. Τα παραπάνω επιβάλλουν τη χρήση σύνθετων αλγορίθμων συγχρονισμού και συντονισμού (time and frequency synchronization) μεταξύ πομπού και δέκτη για την ομαλή λειτουργία.
Στα πλαίσια της διπλωματικής αυτής θα πραγματοποιηθεί επιλογή κατάλληλων αλγορίθμων για τα προαναφερθέντα προβλήματα, οι οποίοι θα πρέπει να πληρούν τις προδιαγραφές του υπό διαμόρφωση διεθνούς προτύπου 802.15.3α και θα αναπτυχθούν βέλτιστες αρχιτεκτονικές VLSI, με στόχο τόσο το χαμηλό κόστος υλοποίησης όσο και την χαμηλή κατανάλωση ισχύος. / In this project have been studied the low power VLSI architecture for synchronization algorithms in Multi-band UWB Wireless systems. The main issues are timing and frequency synchronization algorithms.
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Multi-Carrier Radar for Target Detection and CommunicationsEllinger, John David 01 August 2016 (has links)
No description available.
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