New approaches and algorithms are developed for the identification and estimation of low order models that represent multipath channel effects in Code Division Multiple Access (CDMA) communication systems. Based on these parsimonious channel models, low complexity receivers such as RAKE receivers are considered to exploit these propagation effects and enhance the system performance. We consider the scenario where multipath is frequency selective slowly fading and where the channel components including delays and attenuation coefficients are assumed to be constant over one or few signalling intervals. We model the channel as a long FIR-like filter (or a tapped delay line filter) with the number of taps related to the ratio between the channel delay-spread and the chip duration. Due to the high data rate of new CDMA systems, the channel length in terms of the chip duration will be very large. With classical channel estimation techniques this will result in poor estimates of many of the channel parameters where most of them are zero leading to a reduction in the system performance. Unlike classical techniques which estimate directly the channel response given the number of taps or given an estimate of the channel length, the proposed techniques in this work will firstly identify the significant multipath parameters using model selection techniques, then estimate these identified parameters. Statistical tests are proposed to determine whether or not each individual parameter is significant. A low complexity RAKE receiver is then considered based on estimates of these identified parameters only. The level of significance with which we will make this assertion will be controlled based on statistical tests such as multiple hypothesis tests. Frequency and time domain based approaches and model selection techniques are proposed to achieve the above proposed objectives. / The frequency domain approach for parsimonious channel estimation results in an efficient implementation of RAKE receivers in DS-CDMA systems. In this approach, we consider a training based strategy and estimate the channel delays and attenuation using the averaged periodogram and modified time delay estimation techniques. We then use model selection techniques such as the sphericity test and multiple hypotheses tests based on F-Statistics to identify the model order and select the significant channel paths. Simulations show that for a pre-defined level of significance, the proposed technique correctly identifies the significant channel parameters and the parsimonious RAKE receiver shows improved statistical as well as computational performance over classical methods. The time domain approach is based on the Bootstrap which is appropriate for the case when the distribution of the test statistics required by the multiple hypothesis tests is unknown. In this approach we also use short training data and model the channel response as an FIR filter with unknown length. Model parameters are then estimated using low complexity algorithms in the time domain. Based on these estimates, bootstrap based multiple hypotheses tests are applied to identify the non-zero coefficients of the FIR filter. Simulation results demonstrate the power of this technique for RAKE receivers in unknown noise environments. Finally we propose adaptive blind channel estimation algorithms for CDMA systems. Using only the spreading code of the user of interest and the received data sequence, four different adaptive blind estimation algorithms are proposed to estimate the impulse response of frequency selective and frequency non-selective fading channels. Also the idea is based on minimum variance receiver techniques. Tracking of a frequency selective varying fading channel is also considered. / A blind based hierarchical MDL model selection method is also proposed to select non-zero parameters of the channel response. Simulation results show that the proposed algorithms perform better than previously proposed algorithms. They have lower complexity and have a faster convergence rate. The proposed algorithms can also be applied to the design of adaptive blind channel estimation based RAKE receivers.

Τα τελευταία χρόνια παρατηρείται μια ραγδαία ανάπτυξη στο κλάδο των τηλεπικοινωνιών και αυτό είναι ορατό στην καθημερινή ζωή μας. Η ψηφιακή μετάδοση και επεξεργασία των σημάτων σε συνδυασμό με την βελτίωση των μέσων μετάδοσης, της υποδομής και των τεχνικών διαμόρφωσης, δίνουν την δυνατότητα στους επιστήμονες να επικεντρωθούν όλο και περισσότερο στα θέματα τηλεπικοινωνιών, ειδικά των ασύρματων τηλεπικοινωνιών. Στις ασύρματες τηλεπικοινωνίες οι τεχνικές πολλαπλής πρόσβασης χρησιμοποιούνται για να επιτρέψουν σε πολλούς χρήστες να μοιραστούν ταυτόχρονα ένα περιορισμένο φάσμα συχνοτήτων. Το μοίρασμα του φάσματος είναι απαραίτητο προκειμένου να αυξήσουμε την χωρητικότητα ενός συστήματος, ενώ θα πρέπει να γίνει με τέτοιο τρόπο, ώστε να μην υπάρχει σημαντική υποβάθμιση της απόδοσης του συστήματος. Για να αντιμετωπιστεί αυτή η υποβάθμιση έχουν προταθεί διάφορες τεχνικές πολλαπλής πρόσβασης. Σε μια από αυτές τις τεχνικές το συνολικό φάσμα συχνοτήτων διαιρείται σε συχνοτικά μη-επικαλυπτόμενες υποζώνες, οι οποίες ανατίθενται στους επιμέρους χρήστες (πολλαπλή πρόσβαση με διαίρεση συχνότητας – FDMA). Σε αυτή την περίπτωση, όμως, έχουμε σπατάλη του εύρους ζώνης του καναλιού, αφού οι χρήστες δεν μπορούν να χρησιμοποιούν όλο το κανάλι, όλη την ώρα. Μια τεχνική που δίνει λύση στο παραπάνω πρόβλημα (πολλαπλή πρόσβαση με διαίρεση κωδικών - CDMA), χρησιμοποιεί ορθογώνιους κώδικες που ανατίθενται στους χρήστες, δίνοντας τους την δυνατότητα ταυτόχρονης μετάδοσης στο κανάλι για όση ώρα απαιτείται. Το βασικότερο πρόβλημα αυτής της τεχνικής είναι ότι η ιδιότητα της ορθογωνιότητας των κωδίκων χάνεται όταν χρησιμοποιούνται συχνοτικά επιλεκτικά κανάλια. Μια νέα τεχνική πολλαπλής πρόσβασης που προτάθηκε η οποία αποφεύγει τα παραπάνω προβλήματα είναι η πολλαπλή πρόσβαση με διαίρεση συχνότητας και αναδιάταξη δεδομένων (Interleaved FDMA). Η συγκεκριμένη τεχνική συνδυάζει χαρακτηριστικά από το FDMA και το CDMA. Συγκεκριμένα, συμπιέζει χρονικά την ακολουθία συμβόλων που πρόκειται να αποσταλεί, την επαναλαμβάνει (παρόμοια λειτουργία με το CDMA) και την μετατοπίζει συχνοτικά βασιζόμενη στον εκάστοτε χρήστη (παρόμοια λειτουργία με το OFDMA). Στον δέκτη οι ακολουθίες των χρηστών διαχωρίζονται χωρίς πρόβλημα ακόμα και όταν το κανάλι είναι συχνοτικά επιλεκτικό. Σε αυτή την περίπτωση εισάγεται διασυμβολική παρεμβολή με αποτέλεσμα να απαιτείται κάποια διαδικασία ισοστάθμισης. Η δομή με βάση την οποία έγινε η μελέτη της τεχνικής πολλαπλής πρόσβασης IFDMA είναι η εξής: 1. Αναφορά στις τρέχουσες τεχνικές πολλαπλής πρόσβασης και στα πλεονεκτήματα και τα μειονεκτήματα της κάθε τεχνικής. 2. Μελέτη της τεχνικής IFDMA και αναφορά στα προβλήματα που εμφανίζει πχ ισοστάθμιση. 3. Παρουσίαση της υλοποίησης της τεχνικής IFDMA και των λύσεων για τα προβλήματα που παρουσιάζει, τα πειραματικά αποτελέσματα που σχετίζονται με την υλοποίηση, καθώς και συγκριτικά αποτελέσματα με τις πιο γνωστές τεχνικές πολλαπλής πρόσβασης. 4. Συμπεράσματα της μελέτης και της σύγκρισης της τεχνικής IFDMA με τις γνωστές τεχνικές πολλαπλής πρόσβασης, καθώς και πιθανές μελλοντικές κατευθύνσεις. / In recent years, we have seen a rapid growth in the telecom industry and this is evident in our everyday lives. The digital transmission and processing of signals in conjunction with the improvement of the transmission, infrastructure and technical configuration means, gives scientists the opportunity to focus increasingly on telecommunications issues, specifically on wireless telecommunications. In wireless telecommunications, multiple access techniques are used, in order to allow multiple users to share simultaneously a limited range of frequencies. Spectrum splitting is necessary, in order to increase the capacity of a system and should be done in a way that results in no significant degradation of system performance. To overcome this degradation various multiple access techniques have been proposed. According to one of these techniques, the total frequency spectrum is divided in frequency of non-overlapping sub-bands which are assigned to individual users (multiple access frequency division - FDMA). In this case, however, we are wasting the bandwidth of the channel, since users can not use all the channel, all the time. A technique that solves the above problem (multiple access with codes - CDMA) uses orthogonal codes, assigned to users, allowing them to simulcast in the channel for as long as needed. The main problem of this technique is that the status of orthogonality is lost when frequency selective channels are used. A new multiple access technique proposed, which avoids these problems, is the multiple access with frequency division and reorganization of data (Interleaved FDMA). This technique combines features from FDMA and CDMA. Specifically, it compresses timewise the sequence of symbols to be sent, repeats it (an operation similar to CDMA) and shifts it, in terms of frequency range, based on each user (a function similar to OFDMA). In the receptor’s side, the sequences of users separate without problem, even when the channel is frequency selective. In this case, intersymbol interference is introduced, which, therefore, requires an equalization process. The structure, upon which the study of the multiple access technique IFDMA made, was: 1. Reference to the current multiple-access techniques and the advantages and disadvantages of each one of them. 2. Study of the IFDMA technique and reference to the problems that it presents e.g. equalization. 3. Presentation of the implementation of a) the IFDMA technique, b) solutions for the problems it displays, c) experimental results and d) the results from the comparison against the most popular multiple-access techniques. 4. Conclusions, stemming from the study and the comparison of the IFDMA technique with the known multiple-access techniques; possible future prospects.

The low-voltage power distribution network is considered today as a serious candidate to provide residential customers with a high-speed access to communication services such as Internet. Outdoor Power-Line Communications (PLC) systems represent an alternative to the other classical 'last-mile solutions' such as ADSL, cable modems, or wireless access systems. We developed an accurate powerline channel simulation tool based on the Multiconductor Transmission Line theory. This tool is able to predict the end-to-end channel responses on the basis of the multiconductor cable structure and the network topology. Then the issue of optimal resource allocation in a multiuser environment was addressed in the light of the Multiuser Information Theory. Simultaneously active users are in competition for the limited resources that are the power (constrained by electro-magnetic compatibility restrictions) and the bandwidth (in the range of 1 to 10 MHz for outdoor PLC). The concept of multiuser balanced capacity was introduced to characterize the optimal resource allocation providing the maximum data rates with fairness constraints among the subscribers. The optimal PLC system was shown to require the shaping of the signal spectrum in the transmitters, and successive decoding in the receiver. A generic multiple access scheme based on Filter Banks (FB) was proposed, which offers the required spectral shaping with limited degrees of freedom. Classical multiple-access techniques (TDMA, CDMA, OFDMA) can be obtained by selecting the appropriate FB. The Minimum-Mean-Square-Error Decision-Feedback Joint Detector was shown to approach the performance of the optimal successive decoding receiver. Finally, the robustness of the proposed system against channel estimation and timing synchronization errors was addressed. The problem of multiuser timing synchronization was introduced, and practical multiuser timing error detectors were proposed.

[pt] As comunicações sem fio são um dos pilares do desenvolvimento das novas gerações de comunicações móveis. Cada geração tem usado alguma técnica de acesso múltiplo para aproveitar os recursos do canal. Esta dissertação apresenta uma análise de duas técnicas de multiplexação ortogonal. Ambas técnicas implementam a transmissão em blocos, onde uma delas se combina com a abordagem Code Division Multiple Access (CDMA), enquanto a outra usa a técnica de transmissão multiportadora Orthogonal Frequency Division Multiplexing (OFDM). O desempenho e a ocupação espectral de ambas técnicas e as suas vantagens são analisados neste trabalho. Expressões analíticas para a Densidade Espectral de Potência dos sinais foram obtidas e permitiram estabelecer comparações entre os dois métodos. O estudo dessas técnicas de multiplexação é realizado em diferentes canais de propagação para avaliar o comportamento de ambos sistemas de uma forma geral. Os três tipos de canais avaliados neste trabalho são: linear e invariante no tempo, linear e variante no tempo e, finalmente, não linear e invariante no tempo. Cada tipo de canal foi modelado em forma matricial para ambos sistemas de forma independente. As simulações consideram os equalizadores Zero Forcing e Minimum Mean Square Error, assumindo um canal conhecido. / [en] Wireless communications are one of the pillars of the development of the new generations of mobile communications. Each generation has used some multiple access technique to take advantage of the channel s resources. This dissertation presents an analysis of two orthogonal multiplexing techniques. Both techniques implement block transmission, where one is combined with the Code Division Multiple Access (CDMA) approach, while the other uses the multicarrier transmission technique Orthogonal Frequency Division Multiplexing (OFDM). The performance and spectral occupation of both techniques and their advantages are analyzed. Analytical expressions for the Power Spectral Density of the signals were obtained, which allowed establishing comparisons between both methods. The study of these multiplexing techniques is carried out in different propagation channels to evaluate the behavior of both systems in general. The three types of channels evaluated in this work are linear and time-invariant, linear and time-variant and, finally, non-linear and time-invariant. Each type of channel was modeled in matrix form for both systems independently. The simulations consider the Zero Forcing and Minimum Mean Squared Error equalizers, assuming a known channel.

Les futures générations de réseaux sans fil posent beaucoup de défis pour la communauté de recherche. Notamment, ces réseaux doivent être en mesure de répondre, avec une certaine qualité de service, aux demandes d'un nombre important de personnes et d'objets connectés. Ce qui se traduit par des exigences assez importantes en termes de capacité. C'est dans ce cadre que les méthodes d'accès multiple non orthogonaux (NOMA) ont été introduit. Dans cette thèse, nous avons étudié et proposé une méthodes d'accès multiple basé sur la technique compute and forawrd et sur les réseaux de point (Lattice codes) tout en considérant différentes constructions de lattice. Nous avons également proposé des amélioration de l'algorithme de décodage de la méthode SCMA (Sparse code multiple access) basé sur les réseaux de points. Afin de simplifier les décodeurs multi-niveaux utilisés, nous avons proposé des expressions simplifiées de LLRs ainsi que des approximations. Finalement, nous avons étudié la construction D des lattices en utilisant les codes polaires. Cette thèse était en collaboration avec le centre de recherche de Huawei France. / Future generations of wireless networks pose many challenges for the research community. In particular, these networks must be able to respond, with a certain quality of service, to the demands of a large number of connected people and objects. This drives us into quite important requirements in terms of capacity. It is within this framework that non-orthogonal multiple access methods (NOMA) have been introduced. In this thesis, we have studied and proposed a multiple access method based on the compute and forward technique and on Lattice codes while considering different lattice constructions. We have also proposed improvements to the algorithm for decoding the Sparse code multiple access (SCMA) method based on Lattice codes. In order to simplify the multi-stage decoders used in here, we have proposed simplified expressions of LLRs as well as approximations. Finally, we studied the construction D of lattices using polar codes. This thesis was in collaboration with the research center of Huawei France.