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<h2 id="x-x-x-bp_categories-h"> </h2> / <p>The discrete-time Poisson (DTP) channel models a wide range of optical communication channels. The channel capacity and capacity-achieving distributions are generally unknown. This thesis addresses system design of DTP channels and presents novel contributions to the capacity of DTP channel, properties and closed-form expression of the capacity-achieving distribution under peak and average constraints, signalling design, and sum-capacity-achieving distributions of DTP multiple access channel (MAC) with peak amplitude constraints.</p> <p>Two algorithms are developed to compute the channel capacity of DTP channel as well as the capacity-achieving distribution with average and peak amplitude constraints. Tight lower bounds based on input distributions with simple forms are presented. Non-uniform signalling algorithms to achieve the channel capacity are also demonstrated. Fundamental properties of capacity-achieving distributions for DTP channels are established. Furthermore, necessary and sufficient conditions on the optimality of binary distributions are presented. Analytical expressions for the capacity-achieving distributions of the DTP channel are derived when there is no dark current and when the dark current is large enough. A two-user DTP multiple access channel model is proposed and it is shown that the sum-capacity-achieving distributions under peak amplitude constraints are discrete with a finite number of mass points.</p> / Doctor of Philosophy (PhD)

This dissertation addresses multi-layer optimization aspects of multiple input multiple output (MIMO) and deep learning-based communication systems. The initial focus is on the rate optimization for multi-user MIMO (MU-MIMO) configurations; specifically, multiple access channel (MAC) and interference channel (IC). First, the ergodic sum rates of MIMO MAC and IC configurations are determined by jointly integrating the error and overhead effects due to channel estimation (training) and feedback into the rate optimization. Then, we investigated methods that will increase the achievable rate for parallel Gaussian IC (PGIC) which is a special case of MIMO IC where there is no interference between multiple antenna elements. We derive a generalized iterative waterfilling algorithm for power allocation that maximizes the ergodic achievable rate. We verified the sum rate improvement with our proposed scheme through extensive simulation tests. Next, we introduce a novel physical layer scheme for single user MIMO spatial multiplexing systems based on unsupervised deep learning using an autoencoder. Both transmitter and receiver are designed as feedforward neural networks (FNN) and constellation diagrams are optimized to minimize the symbol error rate (SER) based on the channel characteristics. We first evaluate the SER in the presence of a constant Rayleigh-fading channel as a performance upper bound. Then, we quantize the Gaussian distribution and train the autoencoder with multiple quantized channel matrices. The channel is provided as an input to both the transmitter and the receiver. The performance exceeds that of conventional communication systems both when the autoencoder is trained and tested with single and multiple channels and the performance gain is sustained after accounting for the channel estimation error. Moreover, we evaluate the performance with increasing number of quantization points and when there is a difference between training and test channels. We show that the performance loss is minimal when training is performed with sufficiently large number of quantization points and number of channels. Finally, we develop a distributed and decentralized MU-MIMO link selection and activation protocol that enables MU-MIMO operation in wireless networks. We verified the performance gains with the proposed protocol in terms of average network throughput. / Doctor of Philosophy / Multiple Input Multiple Output (MIMO) wireless systems include multiple antennas both at the transmitter and receiver and they are widely used today in cellular and wireless local area network systems to increase robustness, reliability and data rate. Multi-user MIMO (MU-MIMO) configurations include multiple access channel (MAC) where multiple transmitters communicate simultaneously with a single receiver; interference channel (IC) where multiple transmitters communicate simultaneously with their intended receivers; and broadcast channel (BC) where a single transmitter communicates simultaneously with multiple receivers. Channel state information (CSI) is required at the transmitter to precode the signal and mitigate interference effects. This requires CSI to be estimated at the receiver and transmitted back to the transmitter in a feedback loop. Errors occur during both channel estimation and feedback processes. We initially analyze the achievable rate of MAC and IC configurations when both channel estimation and feedback errors are taken into account in the capacity formulations. We treat the errors associated with channel estimation and feedback as additional noise. Next, we develop methods to maximize the achievable rate for IC by using interference cancellation techniques at the receivers when the interference is very strong. We consider parallel Gaussian IC (PGIC) which is a special case of MIMO IC where there is no interference between multiple antenna elements. We develop a power allocation scheme which maximizes the ergodic achievable rate of the communication systems. We verify the performance improvement with our proposed scheme through simulation tests. Standard optimization techniques are used to determine the fundamental limits of MIMO communications systems. However, there is still a gap between current operational systems and these limits due to complexity of these solutions and limitations in their assumptions. Next, we introduce a novel physical layer scheme for MIMO systems based on machine learning; specifically, unsupervised deep learning using an autoencoder. An autoencoder consists of an encoder and a decoder that compresses and decompresses data, respectively. We designed both the encoder and the decoder as feedforward neural networks (FNNs). In our case, encoder performs transmitter functionalities such as modulation and error correction coding and decoder performs receiver functionalities such as demodulation and decoding as part of the communication system. Channel is included as an additional layer between the encoder and decoder. By incorporating the channel effects in the design process of the autoencoder and jointly optimizing the transmitter and receiver, we demonstrate the performance gains over conventional MIMO communication schemes. Finally, we develop a distributed and decentralized MU-MIMO link selection and activation protocol that enables MU-MIMO operation in wireless networks. We verified the performance gains with the proposed protocol in terms of average network throughput.

[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.

La compagnie Sigfox est reconnue comme un acteur prometteur pour des transmissions de longue-distance et faible consommation, dans le contexte de l'IoT. La modulation à bande ultra étroite (Ultra Narrow Band (UNB)), la technologie de communication choisie par Sigfox, permet de transmettre des informations dans des bandes de signal très étroites (typiquement 100 Hz). A cause de l'imprécision fréquentielle causée par les oscillateurs générateurs de fréquence, il n'est pas réaliste de transmettre des signaux UNB dans des canaux parfaitement orthogonaux. L'accès naturel au canal radio pour le système de UNB est de type ALOHA, avec un aspect aléatoire à la fois en en temps et en fréquence. Cet accès aléatoire peut introduire des collisions qui dégradent la performance du réseau. Le but de cette thèse est de caractériser la capacité des réseaux basés sur UNB, ainsi que d’améliorer la performance en considérant l'aspect aléatoire en temps et en fréquence. La première contribution de cette thèse, est une évaluation de la capacité en théorie et en simulation pour une seule station de base (BS), sous des conditions de canal idéaliste ou réaliste. En conditions idéalistes, nous avons exprimé la capacité pour le cas de l'ALOHA généralisé, et l'avons étendu aux cas de réplications. Pour les conditions réalistes, nous avons pris en compte l'interférence spectrale d'UNB et le path loss (sans et avec Rayleigh fading) afin de caractériser la performance des réseaux UNB, avec l'outil géométrie stochastique. La deuxième contribution est d'appliquer l’annulation successive d'interférence (SIC), qui nous permet d'atténuer les interférences, dans des réseaux de UNB. Nous avons fourni une analyse théorique de la performance des réseaux en considérant le SIC et l'interférence spectrale de UNB, pour le cas de mono-BS. La troisième contribution est l'amélioration de la performance des réseaux UNB, en exploitant la diversité de multi-BS. Nous avons fait une analyse théorique de performance en considérant multi-BS et selection combining (SC). En particulier, nous avons considéré que l’interférence vue par chaque BS est corrélée. Nous avons ainsi démontré mathématiquement que cette corrélation ne peut pas être supprimée dans des systèmes UNB. Ensuite, nous avons appliqué les technologies de la combinaison des signaux plus complexes comme MRC (max ratio combining) et EGC (equal gain combining), ainsi que le SIC à travers multi-BS. Nous avons évalué l'amélioration de performance que chaque technologie apporte, et les avons comparées. Nous avons souligné l'efficacité de ces technologies qui nous permettent d’obtenir des gains importants comparés au cas mono-BS (e.x. 125 fois plus de réduction d'erreur avec SIC globale). La dernière contribution est une validation expérimentale du modèle d'interférence spectrale de UNB, ainsi que la capacité des réseaux UNB, sur un testbed de radio FIT/Cortexlab. / Sigfox rises as a promising candidate dedicated for long-distance and low-power transmissions in the IoT backgrounds. Ultra Narrow Band (UNB), being the communication technology chosen by Sigfox, allows to transmit information through signals whose bandwidth is very limited, typically 100 Hz. Due to the imprecision restraint on electronic devices, it is impossible to transmit UNB signals in orthogonal channels. The natural radio access for this kind of system is thus random ALOHA, in both time and frequency domain. This random access can induce collisions which degrades the networks performance. The aim of this thesis is to characterize the capacity of UNB based networks, as well as to enhance its performance, by considering the randomness in time and frequency. The first contribution of the thesis, is the theoretical and numerical capacity evaluation under idealized and realistic channel conditions, for mono base station (BS) case. Under idealized conditions, we have quantified this capacity for generalized ALOHA case and extended for replications. We highlight the time-frequency duality in UNB systems, and that there exists an optimum replication number for a given network parameter set. Under realistic conditions, we have taken into account the specific spectral interference of UNB systems and propagation path loss (without and with Rayleigh fading) to characterize the performance, with the aid of stochastic geometry. The second contribution is the enhancement of UNB network performance in single BS case. We propose to use successive interference cancellation (SIC) in UNB networks, which allows to mitigate the interference. We have provided a theoretical analysis by considering both SIC and the spectral interference, for mono-BS case. We bring to light the efficiency of SIC in enhancing UNB system performance. The third contribution is the improvement of UNB systems, by exploiting the multiple BS diversity. An analytical performance evaluation considering the simplest selection combining is conducted. In particular, we consider the interference viewed by all the BSs are correlated. Then we apply more complex signal combining technologies such as MRC (max ratio combining) and EGC (equal gain combining), and even interference cancellation across multi-BS in UNB networks. We evaluate the performance improvement that each technology can bring, and compare them with each other. We highlight the efficiency of these multi-BS technologies which allow us to achieve significant performance enhancement compared to mono-BS (e.x. 125 times better performance with global SIC). Last but not least, we experimentally verify the the spectral interference model and network capacity on a cognitive radio testbed.

Asynchronous multiplexing schemes are efficient than synchronous schemes, in a bursty traffic environment of multiple access local area network (LAN), as fixed bandwidth is not allocated among the users and there is no access delay. Fiber- Optic Code-Division Multiple Access (FO-CDMA) is one such asynchronous multiplexing scheme suitable for high speed LAN networks. While FO-CDMA offers potential benefits it also faces challenges in three diverse areas which are 1) coding algorithms and schemes 2) advanced encoding and decoding hardware and 3) network architecture. In this thesis, as a solution to the first challenge, we propose the design and construction of a new family of codes, wavelength/time multiple-pulses-per-row (W/T MPR) codes. These codes have good cardinality, spectral efficiency and minimal cross-correlation values. Performance analysis of the W/T MPR codes is carried out and found to be superior to other codes. In unipolar 1-D Optical Orthogonal Codes (OOCs) proposed by Salehi et al., the ratio of code length/code weight grows rapidly as the number of users is increased for a reasonable weight. Hence, for a given pulse width, the data rate decreases or in other words for a given data rate very narrow pulses have to be used, because of which dispersion effects will be dominant. To overcome the drawbacks of non-linear effects in large spread sequences of 1-D unipolar codes in FO-CDMA networks, several two-dimensional codes have been proposed. Wavelength-time (W/T) encoding of the two-dimensional codes is practical in FO-CDMA networks. W/T codes reported so far can be classified mainly into two types: 1) hybrid sequences, where one type of sequence is crossed with another to improve the cardinality and correlation properties and 2) matrix codes, 1-D sequences converted to 2-D codes or 2-D codes by construc- tion, to reduce the ’time’ spread of the sequences/codes. Prime-hop and eqc/prime W/T hybrid codes have been proposed where one type of sequence is crossed with another to improve the cardinality and correlation properties. Other constructions deal with conversion of 1-D sequences to 2-D codes either by using Chinese remainder theorem or folding GoLomb rulers. W/T single-pulse-per-row (W/T SPR) codes are 2-D codes constructed using algebraic method Addition Modulo Group operation. Motivation for this work: To design a family of 2-D codes which have the design choice of length of one dimension over the other, and also have better cardinality, spectral efficiency and also low cross-correlation values (thereby have low BER) than that of the reported unipolar 2-D codes. In this thesis, we describe the design principles of W/T MPR codes, for in- coherent FO-CDMA networks, which have good cardinality, spectral efficiency and minimal cross-correlation values. Another feature of the W/T MPR codes is that the aspect ratio can be varied by trade off between wavelength and temporal lengths. We lay down the necessary conditions to be satisfied by W/T MPR codes to have minimal correlation values of unity. We analytically prove the correlation results and also verify by simulation (of the codes) using Matlab software tool. We also discuss the physical implementation of the W/T MPR FO-CDMA network with optical encoding and decoding. We show analytically that when distinct 1-D OOCs of a family are used as the row vectors of a W/T MPR code, it will have off-peak autocorrelation equal to ‘1’. An expression for the upper bound on the cardinality of W/T MPR codes is derived. We also show that 1-D OOCs and W/T SPR codes are the limiting cases of W/T MPR codes. Starting with distinct 1-D OOCs, of a family, as row vectors, we propose a greedy algorithm, for the construction of W/T MPR codes and present the repre- sentations of the results. An entire W/T MPR code family, generated using greedy algorithm, is simulated for various number of interfering users. Performance analysis of the W/T MPR codes and their limiting cases is carried out for various parameter variations such as the dimensions of wavelength, time and weight of the code. We evaluate the performance in terms of BER, capacities of the networks, temporal lengths needed (to achieve a given BER). Multiple access interference (MAI) signal can be reduced, by using a bistable optical hard-limiter device in the W/T MPR code receiver, by eliminating those signal levels which exceed a certain preset level. Performance analysis of the W/T MPR codes and their limiting cases is studied for various parameter variations. For given wavelength × time dimensions, we compare various W/T codes, whose cardinalities are known, and show that W/T MPR family of codes have better cardinality and spectral efficiency than the other (reported) W/T codes. As W/T MPR codes are superior to other W/T codes in terms of cardinality, spectral efficiency, low peak cross-correlation values and at the same time have good performance, makes it a suitable coding scheme for incoherent FO-CDMA access networks.

Τα μελλοντικά ασύρματα δίκτυα και συστήματα επικοινωνιών αναμένεται να παρέχουν αξιόπιστα υπηρεσίες δεδομένων με απαιτήσεις ρυθμού μετάδοσης δεδομένων οι οποίες κυμαίνονται από λίγα kbps μέχρι μερικά Mbps και εξαιτίας του υψηλού κόστους του φάσματος συχνοτήτων, αυτά τα συστήματα χρειάζεται να είναι εξαιρετικά αποτελεσματικά όσον αφορά στη χρησιμοποίηση του φάσματος. Συγκεκριμένα, η εφαρμογή τεχνικών μετάδοσης δεδομένων οι οποίες βασίζονται σε MIMO και OFDMA θεωρείται ως μια πολλά υποσχόμενη λύση για να ικανοποιήσει αυτές τις απαιτήσεις. Από την άλλη μεριά, τα συστήματα MIMO-OFDMA είναι εύκαμπτα και φασματικά αποτελεσματικά αλλά ο αξιοσημείωτα μεγάλος αριθμός υποφορέων και ο συνυπολογισμός της διάστασης χώρου καθιστούν την κατανομή ραδιοπόρων πολύ πολύπλοκη. Στην πραγματικότητα, η βέλτιστη κατανομή ραδιοπόρων η οποία μεγιστοποιεί το συνολικό ρυθμό μετάδοσης δεδομένων των χρηστών είναι συχνά πάρα πολύ πολύπλοκη για πρακτικές εφαρμογές. Συνεπώς, απαιτούνται υποβέλτιστες σχετικά αποτελεσματικές και χαμηλής πολυπλοκότητας στρατηγικές κατανομής ραδιοπόρων ώστε να κατανείμουν τους ραδιοπόρους συχνότητας, ισχύος και χώρου του συστήματος στους χρήστες του συστήματος. Η παρούσα ΔΔ διαπραγματεύεται στρατηγικές κατανομής ραδιοπόρων στην κατερχόμενη και στην ανερχόμενη ζεύξη συστημάτων OFDMA, στην κατερχόμενη ζεύξη συστημάτων MISO-OFDMA και στην κατερχόμενη ζεύξη συστημάτων MIMO-OFDMA στοχεύοντας στη μεγιστοποίηση του συνολικού ρυθμού μετάδοσης δεδομένων των χρηστών εγγυώντας οι ρυθμοί μετάδοσης δεδομένων των χρηστών να τηρούν μια προκαθορισμένη αναλογία μεταξύ τους ή να ξεπερνούν προκαθορισμένους ελάχιστους ρυθμούς μετάδοσης δεδομένων. Στο πλαίσιο της επίλυσης του προβλήματος της μεγιστοποίησης του συνολικού ρυθμού μετάδοσης δεδομένων των χρηστών με ανεκτή πολυπλοκότητα για κάθε μία από τις προαναφερθείσες περιπτώσεις, προτείνονται νέοι υποβέλτιστοι αλγόριθμοι. Στην κατερχόμενη ζεύξη των συστημάτων SISO, στόχος είναι η μεγιστοποίση του συνολικού ρυθμού μετάδοσης δεδομένων των χρηστών με περιορισμό στη συνολική διαθέσιμη ισχύ και με αναλογικούς ρυθμούς μετάδοσης δεδομένων μεταξύ των χρηστών. Η προτεινόμενη μέθοδος, η οποία είναι αποτελεσματική όσον αφορά στην πολυπλοκότητα, αποτελείται από τρεις αλγόριθμους: έναν αλγόριθμο ο οποίος προσδιορίζει τον αριθμό των υποφορέων για κάθε χρήστη, έναν αλγόριθμο κατανομής υποφορέων διαιρώντας τους χρήστες σε δύο ομάδες και τον αλγόριθμο water-filling. Οι πρώτοι δύο αλγόριθμοι αναθέτουν τους διαθέσιμους υποφορείς στους χρήστες του συστήματος και ο τρίτος αλγόριθμος κατανέμει τη διαθέσιμη ισχύ με βέλτιστο τρόπο για μεγιστοποίηση του συνολικού ρυθμού μετάδοσης δεδομένων. Στην ανερχόμενη ζεύξη των συστημάτων SISO, στόχος είναι η μεγιστοποίηση του συνολικού ρυθμού μετάδοσης δεδομένων των χρηστών με περιορισμό στην ισχύ κάθε χρήστη και σε ελάχιστους ρυθμούς μετάδοσης δεδομένων μεταξύ των χρηστών. Η προτεινόμενη τεχνική, η οποία είναι αποτελεσματική όσον αφορά στην πολυπλοκότητα, αποτελείται από τρεις αλγόριθμους: έναν αλγόριθμο ο οποίος προσδιορίζει τον αριθμό των υποφορέων για κάθε χρήστη, έναν αλγόριθμο κατανομής υποφορέων διαιρώντας τους χρήστες σε δύο ομάδες και τον αλγόριθμο water-filling. Οι πρώτοι δύο αλγόριθμοι αναθέτουν τους διαθέσιμους υποφορείς στους χρήστες του συστήματος και ο τρίτος αλγόριθμος κατανέμει τη διαθέσιμη ισχύ. Στην κατερχόμενη ζεύξη των συστημάτων MISO αναπτύσσονται τρεις αλγόριθμοι επιλογής χρηστών και κατανομής πόρων για πολυχρηστικά συστήματα κατερχόμενης ζεύξης οι οποίοι είναι λιγότερο πολύπλοκοι από άλλες προσεγγίσεις και ενσωματώνουν τη δικαιοσύνη. Στους πρώτους δύο αλγόριθμους επιβάλλονται αναλογικοί περιορισμοί μεταξύ των ρυθμών μετάδοσης δεδομένων των χρηστών και στον τρίτο αλγόριθμο περιορισμοί στους ελάχιστους ρυθμούς μετάδοσης δεδομένων λαμβάνονται υπόψη. Επίσης, πραγματοποιείται επέκταση του αλγόριθμου μεγιστοποίησης του συνολικού ρυθμού μετάδοσης δεδομένων με αναλογικούς περιορισμούς δικαιοσύνης σε ΣΚΚ και για μείωση της πολυπλοκότητας οι υποφορείς ομαδοποιούνται σε τεμάχια. Τα αποτελέσματα της προσομοίωσης επιβεβαιώνουν την αποτελεσματικότητα τους στη διανομή του συνολικού ρυθμού μετάδοσης δεδομένων δίκαια μεταξύ των χρηστών αλλά και ότι σε ΣΚΚ επιτυγχάνονται μεγαλύτεροι συνολικοί ρυθμοί μετάδοσης δεδομένων. Τέλος, στην κατερχόμενη ζεύξη των συστημάτων MIMO, το πρόβλημα διατυπώνεται με στόχο τη μεγιστοποίηση του συνολικού ρυθμού μετάδοσης δεδομένων των χρηστών με περιορισμό στη συνολική διαθέσιμη ισχύ και ελέγξιμο εύρος ζώνης στο σύστημα εισάγοντας την παράμετρο α. Αφού αυτό το πρόβλημα βελτιστοποίησης πρέπει να εκτελεστεί σε πραγματικό χρόνο, προτείνεται ένας αλγόριθμος αποδοτικός, υποβέλτιστος και αποτελεματικός όσον αφορά στην πολυπλοκότητα ο οποίος παρουσιάζει λογική απώλεια όσον αφορά στην περίπτωση χωρίς περιορισμούς όπου ο μόνος στόχος είναι η μεγιστοποίηση του συνολικού ρυθμού μετάδοσης δεδομένων και εντυπωσιακό όφελος συγκρινόμενος με τη στατική τεχνική TDMA. Πέραν της θεωρητικής ανάλυσης των παραπάνω αλγόριθμων, ο προσομοιωτικός κώδικας που δημιουργήθηκε βασισμένος σε ρεαλιστικές υποθέσεις και απλουστεύσεις, μάς έδωσε τα αποτελέσματα εκείνα τα οποία μετρούν το συνολικό ρυθμό μετάδοσης δεδομένων των χρηστών ο οποίος παρέχεται από κάθε έναν από τους προαναφερθέντες αλγόριθμους και εξετάζουν την πιθανή καταλληλότητα για χρήση τους σε συγκεκριμένα περιβάλλοντα. Τα τελικά συμπεράσματα είναι ότι τα συστήματα MIMO-OFDMA είναι ικανά να προσφέρουν πραγματικές ευρυζωνικές υπηρεσίες πάνω από το ασύρματο κανάλι επικοινωνίας. / Future wireless communication networks and systems are expected to reliably provide data services with data rate requirements ranging from a few kbps up to some Mbps and, due to the high costs of frequency spectrum, these systems also need to be extremely efficient in terms of the spectrum usage. In particular, the application of transmission schemes based on OFDMA and on MIMO is considered as a promising solution to meet these requirements. On the one hand, MIMO-OFDMA systems are flexible and spectrally efficient but the considerably large number of subcarriers and the inclusion of the space dimension make the RRA in such systems very complex. In fact, the optimum RRA that maximizes the sum of the users' data rates is often too complex for practical application. Consequently, suboptimal rather efficient and low-complexity RRA strategies are required in order to allocate the frequency, power, and space radio resources of the system to the users of the system. This doctoral thesis deals with RRA strategies in the downlink and uplink of OFDMA systems, the downlink of MISO-OFDMA systems, and the downlink of MIMO-OFDMA systems aiming at the maximization of the sum of the users' data rates guaranteeing proportional data rates or minimum data rates among users. In order to solve the problem of maximizing the sum of the users' data rates with affordable complexity in each one of the aforementioned cases, new suboptimal algorithms are proposed. In the SISO downlink the objective is to maximize the sum of the users' data rates subject to constraints on the total available power and proportional data rates among users. The proposed method, which is also complexity effective, consists of three algorithms; an algorithm that determines the number of subcarriers for each user, a subcarrier allocation algorithm by dividing the users in two groups and the water-filling algorithm. The first two algorithms assign the available subcarriers to the users of the system and the third one allocates the available power optimally in order to maximize the sum of the users' data rates. In the SISO uplink the objective is to maximize the sum of the users' data rates subject to constraints on per user power and minimum data rates among users. The proposed scheme, which is also complexity effective, consists of three algorithms; an algorithm that determines the number of subcarriers for each user, a subcarrier allocation algorithm by dividing the users in two groups and the water-filling algorithm. The first two algorithms assign the available subcarriers to the users of the system and the third one allocates the available power. In the MISO downlink three user selection and resource allocation algorithms for multiuser downlink systems are developed that are less complex than other approaches and incorporate fairness. In the first two algorithms proportional constraints among the users' data rates are imposed and in the third algorithm minimum data rate constraints are taken into account. The proposed algorithm that maximizes the sum of the users' data rates with proportional data rate constraints is also applied to DAS and subcarriers are grouped to chunks. Simulation results sustain their effectiveness in distributing the sum data rate fairly and flexibly among users and that in DAS higher sum of the users' data rates are obtained. Finally, in the MIMO downlink the problem is formulated in order to maximize the sum of the users' data rates subject to total available power constraint with controllable bandwidth introducing system parameter α. Since this optimization should be performed in real time, an efficient, suboptimal and complexity effective algorithm is proposed which shows reasonable loss with respect to the unconstrained case where the only target is the maximization of the sum data rate and impressive profit compared to static TDMA scheme. Apart from the theoretical analysis of the above algorithms, simulation code, which was created based on realistic assumptions and simplifications, gave us results which measure the sum of the users' data rates that provide each one of the aforementioned algorithms and examine the possible appropriateness for use in specific environments. The final concluding results are that MIMO-OFDMA systems are able to offer real broadband services over the wireless communication channel.

L'objectif général de cette thèse était de proposer une solution de localisation en intérieur à la fois simple et capable de surmonter les défis de la propagation dans les environnements en intérieur. Pour ce faire, un système de localisation basé sur la méthode des signatures et adoptant le temps d'arrivée du signal de l'émetteur au récepteur comme signature, a été proposé. Le système présente deux architectures différentes, une première orientée privée utilisant la méthode d'accès multiple à répartition par code et une deuxième centralisée basée sur la méthode d'accès multiple à répartition dans le temps. Le système calcule la position de l'objet d'intérêt par la méthode de noyau. Une comparaison expérimentale entre le système à architecture orientée privée et un système de localisation sonore déjà existant et basé sur la méthode de trilatération, a permis de confirmer les résultats trouvés dans le cas de la localisation par ondes radiofréquences. Cependant, nos expérimentations étaient les premières à montrer l'effet de la réverbération sur les approches de la localisation acoustique. Dans un second lieu, un système de localisation basé sur la technique de retournement temporel, permettant une localisation simultanée de sources avec différentes précisions, a été testé par simulations en faisant varier le nombre de sources. Ce système a été ensuite validé par expérimentations. Dans la dernière partie de notre étude, nous nous sommes intéressés à la réduction de l'audibilité du signal utile à la localisation par recours à la psycho-acoustique. Un filtre défini à partir du seuil d'audition absolu a été appliqué au signal de localisation. Nos résultats ont montré une amélioration de la précision de localisation comparé au système de localisation sans modèle psycho-acoustique et ce grâce à l'utilisation d'un filtre adapté au modèle psycho-acoustique à la réception. Par ailleurs, l'écoute du signal après application du modèle psycho-acoustique a montré une réduction significative de son audibilité comparée à celle du signal original. / The objective of this PhD is to propose a location solution that should be simple and robust to multipath that characterizes the indoor environments. First, a location system that exploits the time domain of channel parameters has been proposed. The system adopts the time of arrival of the path of maximum amplitude as a signature and estimates the target position through nonparametric kernel regression. The system was evaluated in experiments for two main configurations: a privacy-oriented configuration with code-division multiple-access operation and a centralized configuration with time-division multiple-access operation. A comparison between our privacy-oriented system and another acoustic location system based on code-division multiple-access operation and lateration method confirms the results found in radiofrequency-based localization. However, our experiments are the first to demonstrate the detrimental effect that reverberation has on acoustic localization approaches. Second, a location system based on time reversal technique and able to localize simultaneously sources with different location precisions has been tested through simulations for different values of the number of sources. The system has then been validated by experiments. Finally, we have been interested in reducing the audibility of the localization signal through psycho-acoustics. A filter, set from the absolute threshold of hearing, is then applied to the signal. Our results showed an improvement in precision, when compared to the location system without psychoacoustic model, thanks to the use of matched filter at the receiver. Moreover, we have noticed a significant reduction in the audibility of the filtered signal compared to that of the original signal.

Opportunistic selection algorithms have grown in importance as next generation wireless systems strive towards higher data rates and spectral efficiencies. For example, in orthogonal frequency division multiple access(OFDMA), the system bandwidth is divided into many sub channels. For each sub channel, the user with the highest channel gain is opportunistically assigned to it. .Likewise, in a multi-source, multi-destination (MSD) cooperative relay system, a relay node must be assigned for every source-destination (SD) pair. The assignment decisions are based on local channel knowledge and must be fast so as to maximize the time available for data transmission. We develop novel multiple access based splitting-based selection algorithms for OFDMA and MSD systems. These systems are unique in that the same user and relay can be the most suitable one for multiple sub channels and multiple SD pairs, respectively. For OFDMA systems, we propose an algorithm called Split Select that assigns for every sub channel the user with the highest channel gain over it. For MSD systems, we propose a contention-based en masse assignment (CBEA) algorithm that assigns to each SD pair a relay that is capable of aiding it. Both Split Select and CBEA are fast and scale well with the number of nodes. For example, Split Select requires just 2.2 slots, on average, to assign a sub channel to its best user even when there are an asymptotically large number of contending users. Likewise, CBEA often takes far less than one slot, on average, to assign a relay to each SD pair.

This thesis addresses the design of low-complexity coding schemes for wireless relay networks and multiple access channels. The first part of the thesis is on wireless relay networks and the second part is on multiple access channels. Distributed space-time coding is a well known technique to achieve spatial diversity in wireless networks wherein, several geographically separated nodes assist a source node to distributively transmit a space-time block code (STBC) to the destination. Such STBCs are referred to as Distributed STBCs (DSTBCs). In the first part of the thesis, we focus on designing full diversity DSTBCs with some nice properties which make them amenable for implementation in practice. Towards that end, a class of full diversity DST-BCs referred to as Co-ordinate Interleaved DSTBCs (CIDSTBCs) are proposed for relay networks with two-antenna relays. To construct CIDSTBCs, a technique called co-ordinate vector interleaving is introduced wherein, the received signals at different antennas of the relay are processed in a combined fashion. Compared to the schemes where the received signals at different antennas of the relay are processed independently, we show that CIDSTBCs provide coding gain which comes in with negligible increase in the processing complexity at the relays. Subsequently, we design single-symbol ML decodable (SSD) DSTBCs for relay networks with single-antenna nodes. In particular, two classes of SSD DSTBCs referred to as (i) Semi-orthogonal SSD Precoded DSTBCs and (ii) Training-Symbol Embedded (TSE) SSD DSTBCs are proposed. A detailed analysis on the maximal rate of such DSTBCs is presented and explicit DSTBCs achieving the maximal rate are proposed. It is shown that the proposed codes have higher rates than the existing SSD DSTBCs. In the second part, we study two-user Gaussian Multiple Access Channels (GMAC). Capacity regions of two-user GMAC are well known. Though, capacity regions of such channels provide insights into the achievable rate pairs in an information theoretic sense, they fail to provide information on the achievable rate pairs when we consider finitary restrictions on the input alphabets and analyze some real world practical signal constellations like QAM and PSK signal sets. Hence, we study the capacity aspects of two-user GMAC with finite input alphabets. In particular, Constellation Constrained (CC) capacity regions of two-user SISO-GMAC are computed for several orthogonal and non-orthogonal multiple access schemes (abbreviated as O-MA and NO-MA schemes respectively). It is first shown that NO-MA schemes strictly offer larger capacity regions than the O-MA schemes for finite input alphabets. Subsequently, for NO-MA schemes, code pairs based on Trellis Coded Modulation (TCM) are proposed such that any rate pair on the CC capacity region can be approached. Finally, we consider a two-user Multiple-Input Multiple-Output (MIMO) fading MAC and design STBC pairs such that ML decoding complexity is reduced.