In this thesis we address the problem of providing end-to-end quality of service (QoS) to real-time and data connections in a third generation (3G) cellular network based on the Universal Mobile Telecommunication System (UMTS) standard. Data applications usually use TCP (Transmission Control Protocol) and the QoS is a minimum guaranteed mean throughput. For this one first needs to compute the throughput of a TCP connection sending its traffic through the UMTS network (possibly also through the wired part of the Internet). Thus we obtain closed form expressions for a TCP throughput in a UMTS environment. For downloading data at a mobile terminal, the packets of each TCP connection are stored in separate queues at the base station (node B). These are fragmented into Protocol Data Units (PDU). The link layer uses ARQ (Automatic Repeat Request). Thus there can be significant random transmission/queueing delays of TCP packets at the node B. On the other hand the link may not be fully utilized due to the delays of the TCP packets in the rest of the network. In such a scenario the existing models of TCP may not be sufficient. Thus we provide new approximate models for TCP and also obtain new closed form expressions of mean window size. Using these we obtain the throughput of a TCP connection for the scenario where the queueing delays are non-negligible compared to the overall Round Trip Time (RTT) and also the link utilization is less than one. Our approximate models can be useful not only in the UMTS context but also else where. In the second half of the thesis, we use these approximate models of TCP to provide minimum mean throughput to data connections in UMTS. We also consider real-time applications such as voice and video. These can tolerate a little packet loss (~1%) but require an upper Bound on the delay and delay jitter (≤ 150 ms). Thus if the network provides a constant bandwidth and the received SINR is above a specified threshold ( with a certain probability), QoS for the real-time traffic will be satisfied. The 3G cellular systems are interference limited. Thus wise allocation of power is critical in these systems. Hence we consider the problem of providing end-to-end QoS to different users along with the minimization of the downlink power allocation.

Τα ασύρματα συστήματα πολλαπλών κεραιών MIMO αποτελούν ένα από τα βασικά μέτωπα ανάπτυξης των τηλεπικοινωνιών. Ωστόσο η εξαιρετικά τυχαία φύση τους καθώς και η αλληλεπίδραση μεταξύ των πολλαπλών ροών δεδομένων επιβάλει την χρήση σύγχρονων τεχνικών ισοστάθμισης. Η προσαρμοστική ισοστάθμιση στο δέκτη ενός τηλεπικοινωνιακού συστήματος χρησιμοποιείται για την αντιμετώπιση της δυναμικής φύσης του ασύρματου καναλιού και την ανίχνευση των αλλαγών στα χαρακτηριστικά του. Επίσης, μη γραμμικές τεχνικές ισοστάθμισης ανατροφοδότησης συμβόλων είναι απαραίτητες για την απομάκρυνση της διασυμβολικής παρεμβολής που παρουσιάζεται στα συγκεκριμένα συστήματα. Η παρούσα εργασία ασχολείται με μεθόδους προσαρμοστικής ισοστάθμισης στο δέκτη ενός τηλεπικοινωνιακού συστήματος. Διακρίνουμε τις εξής περιπτώσεις προσαρμοστικών αλγορίθμων για την ελαχιστοποίηση του σφάλματος, του αλγορίθμου Αναδρομικών Ελαχίστων Τετραγώνων (RLS), του επαναληπτικού αλγορίθμου Συζυγών Κλίσεων (CG) και του επαναληπτικού αλγορίθμου τροποποιημένων Συζυγών Κλίσεων (MCG). Όπως διαπιστώνουμε, όταν οι παραπάνω αλγόριθμοι χρησιμοποιηθούν με γραμμικές τεχνικές ισοστάθμισης έχουμε πολύ αργή σύγκλιση και γενικά υψηλό όριο σφάλματος. Συμπεραίνουμε λοιπόν ότι, προκειμένου να έχουμε γρήγορη σύγκλιση των προσαρμοστικών αλγορίθμων και αντιμετώπιση της διασυμβολικής παρεμβολής για τα συστήματα MIMO, είναι απαραίτητη η χρήση μη γραμμικών τεχνικών ισοστάθμισης. Αρχικά χρησιμοποιούμε την μέθοδο της γενικευμένης ανατροφοδότησης συμβόλων GDFE ενώ στη συνέχεια μελετάμε μία σύγχρονη τεχνική ανατροφοδότησης συμβόλων που χρησιμοποιεί ένα κριτήριο διάταξης για την ακύρωση των συμβόλων (OSIC ή V-BLAST). Όπως διαπιστώνεται και από τις εξομοιώσεις η συγκεκριμένη τεχνική επιτυγχάνει το χαμηλότερο όριο σφάλματος, αλλά με αυξημένο υπολογιστικό κόστος. Επίσης, διαπιστώνουμε ότι η εφαρμογή της τεχνικής αυτής με χρήση του τροποποιημένου αλγορίθμου Συζυγών Κλίσεων δεν είναι εφικτή. Στα πλαίσια αυτής της εργασίας, περιγράφουμε μια συγκεκριμένη υλοποίηση της τεχνικής διατεταγμένης ακύρωσης που κάνει χρήση του αλγορίθμου Αναδρομικών Ελαχίστων Τετραγώνων με μειωμένη πολυπλοκότητα. Στη συνέχεια γενικεύουμε την εφαρμογή της για την περίπτωση των αλγορίθμων Συζυγών Κλίσεων, και διαπιστώνουμε ότι ο τροποποιημένος αλγόριθμος Συζυγών Κλίσεων δεν μπορεί να χρησιμοποιηθεί ούτε σε αυτήν την περίπτωση. Για την υλοποίηση ενός συστήματος OSIC με χρήση του αλγορίθμου Συζυγών Κλίσεων είναι απαραίτητη η χρήση ενός αλγορίθμου που δεν έχει χρονική εξάρτηση σύγκλισης, όπως είναι ο βασικός αλγόριθμος Συζυγών Κλίσεων. / Wireless systems with multiple antenna configurations has recently emerged as one of the most significant technical breaktroughs in modern communications. However, because of the extremly random nature of the wireless channels, we have to use modern equalization methods in order to defeat the signal degradation. Adaptive equalization at the receiver of the telecommunication system can be used to compete this dynamic nature of the wireless channel and track the changes of its characteristics. Furthermore, nonlinear decision feedback methods are nessesary for the cancellation of the intersymbol interference which occurs with these systems. This work involves with adaptive equalization methods at the receiver of the telecommunication system. We use the following adaptive algorithms so as to minimize the error : the Recursive Least Squares algorithm (RLS), the iterative Conjugate Gradient algorithm (CG) and the iterative Modified Conjugate Gradient algorithm (MCG). When these algorithms are used with linear methods, they give very slow converge and high final error. So, it is neccessary to use nonlinear equalization methods in order to succeed fast converge rate and deal with the increazed intersymbol interference for MIMO systems. Firstly we use the generalized decision feedback method (GDFE), and then the modern method of ordered successive cancellation method (OSIC or V-BLAST). Based on the emulations we conclude that the last method succeed the lower error, but with high computational cost. Furthermore, we can't use OSIC method with Modified Conjugate Gradient algorithm. In this work, we describe a specific implementation of the OSIC method which uses RLS algorithm with low computational complexity. So we generalize its usage with the Conjugate Gradient algorithms. Finaly, we conclude that we can't also use MCG with OSIC method with low computational complexity. In order to construct an OSIC system based on Conjugate Gradient algorithm, the algorithm must not operate on time basis, like basic Conjugate Gradient algorithm does.

An ever increasing demand for instant and reliable information on modern communication networks forces codewords to operate in a non-asymptotic regime. To achieve reliability for imperfect channels in this regime, codewords need to be retransmitted from receiver to the transmit buffer, aided by a fast feedback mechanism. Large occupancy of this buffer results in longer communication delays. Therefore, codewords need to be designed carefully to reduce transmit queue-length and thus the delay experienced in this buffer. We first study the consequences of physical layer decisions on the transmit buffer occupancy. We develop an analytical framework to relate physical layer channel to the transmit buffer occupancy. We compute the optimal code-rate for finite-length codewords operating over a correlated channel, under certain communication service guarantees. We show that channel memory has a significant impact on this optimal code-rate. Next, we study the delay in small ad-hoc networks. In particular, we find out what rates can be supported on a small network, when each flow has a certain end-to-end service guarantee. To this end, service guarantee at each intermediate link is characterized. These results are applied to study the potential benefits of setting up a network suitable for network coding in multicast. In particular, we quantify the gains of network coding over classic routing for service provisioned multicast communication over butterfly networks. In the wireless setting, we study the trade-off between communications gains achieved by network coding and the cost to set-up a network enabling network coding. In particular, we show existence of scenarios where one should not attempt to create a network suitable for coding. Insights obtained from these studies are applied to design a distributed rate control algorithm in a large network. This algorithm maximizes sum-utility of all flows, while satisfying per-flow end-to-end service guarantees. We introduce a notion of effective-capacity per communication link that captures the service requirements of flows sharing this link. Each link maintains a price and effective-capacity, and each flow maintains rate and dissatisfaction. Flows and links update their respective variables locally, and we show that their decisions drive the system to an optimal point. We implemented our algorithm on a network simulator and studied its convergence behavior on few networks of practical interest.

Οι ραγδαία αυξανόμενες απαιτήσεις για ασύρματες ευρείας ζώνης υπηρεσίες και τα πρόσφατα επιτεύγματα στο σχεδιασμό κι υλοποίηση κινητών τερματικών συσκευών με δυνατότητες παροχής υπηρεσιών διαδικτύου επισπεύδουν την εισαγωγή των ασυρμάτων συστημάτων επικοινωνίας τέταρτης γενεάς στην παγκόσμια αγορά. Βασικό ρόλο στην εκπλήρωση των απαιτήσεων για αυξημένο ρυθμό μετάδοσης δεδομένων και ποιότητα υπηρεσιών που έχουν τεθεί από τα συστήματα αυτά, διαδραματίζουν οι χωροχρονικές τεχνικές επεξεργασίας σήματος που εφαρμόζονται στα ασύρματα συστήματα με πολλαπλές κεραίες στον πομπό ή/και στο δέκτη. Ευρέως διαδεδομένα και συνάμα απλά στην υλοποίηση συστήματα πολλαπλών κεραιών είναι οι δέκτες χωρικού διαφορισμού (ΔΧΔ), οι οποίοι παρέχουν τη δυνατότητα αποδοτικής αντιμετώπισης του φαινομένου των διαλείψεων πολυδιόδευσης που εμφανίζονται στο ασύρματο κανάλι, συνδυάζοντας κατάλληλα τα πολλαπλά ληφθέντα αντίγραφα του εκπεμπόμενου σήματος. Η θεωρητικά αναμενόμενη βελτίωση στις επιδόσεις ασυρμάτων συστημάτων που δύνανται να προσφέρουν οι ΔΧΔ σε σύγκριση με τους συμβατικούς δέκτες μονής κεραίας, προϋποθέτει τη στατιστική ανεξαρτησία των διαλείψεων πολυδιόδευσης που εμφανίζονται στις πολλαπλές κεραίες του δέκτη. Σε πρακτικές υλοποιήσεις, όμως, ποικίλες παράμετροι, όπως για παράδειγμα η μικρή απόσταση μεταξύ των πολλαπλών κεραιών του δέκτη, συντελούν ώστε οι διαλείψεις που εμφανίζονται στους κλάδους των ΔΧΔ να είναι αυθαίρετα συσχετισμένες. Η θεωρητική μελέτη επιδόσεων ΔΧΔ που υπόκεινται σε αυθαίρετα συσχετισμένα κανάλια διαλείψεων πολυδιόδευσης, γνωστών ως διαλείψεις μικρής κλίμακας (ΔΜΙΚ), αποτελεί το αντικείμενο έρευνας της παρούσας διδακτορικής διατριβής. Αν και πολυάριθμες ερευνητικές εργασίες ασχολούνται με τη μοντελοποίηση των συσχετισμένων ΔΜΙΚ και της επίδρασής τους στις επιδόσεις ΔΧΔ, η πλειονότητά τους, χρησιμοποιώντας τις στατιστικές ιδιότητες πολυ-μεταβλητών κατανομών, περιορίζεται σε ειδικές μορφές συσχέτισης των διαλείψεων και σε συμβατικές τεχνικές ΔΧΔ. Το γεγονός αυτό οφείλεται, σε μεγάλο βαθμό, στην απουσία απλών στη χρήση και στον υπολογισμό μαθηματικών εκφράσεων για τις στατιστικές ιδιότητες πολυ-μεταβλητών κατανομών με αυθαίρετα συσχετισμένες τυχαίες μεταβλητές (ΤΜ). Στα πλαίσια της διατριβής αυτής επισκοπούνται, αρχικά, οι προταθείσες μαθηματικές εκφράσεις για τις κυριότερες στατιστικές ιδιότητες των πολυ-μεταβλητών κατανομών Rayleigh, Nakagami-m, Weibull και γενικευμένου Γάμα (ΓG) με διάφορες μορφές συσχέτισης και περιγράφονται οι δυνατότητες χρησιμοποίησής τους στη μελέτη επιδόσεων ΔΧΔ που λειτουργούν σε συσχετισμένες ΔΜΙΚ. Κατόπιν, παρουσιάζοντας μια νέα μεθοδολογία δημιουργίας αυθαίρετα συσχετισμένων και μη απαραιτήτως ταυτόσημα κατανεμημένων (ΤΚ) ΤΜ ΓG, η οποία βασίζεται σε αυθαίρετα συσχετισμένες ΤΜ Gauss και στην ειδική κατηγορία των πινάκων Householder για την τριδιαγωνιοποίηση του πίνακα συσχέτισης (ΠΣ) των ΤΜ Gauss, προέκυψαν μια κλειστής μορφής έκφραση άνω φράγματος για την από κοινού συνάρτηση πυκνότητας πιθανότητας (ΣΠΠ) και μια αναλυτική έκφραση άνω φράγματος σε αναπαράσταση απειροσειρών για την από κοινού αθροιστική συνάρτηση κατανομής (ΑΣΚ) αυθαίρετα συσχετισμένων και μη απαραιτήτως ΤΚ ΤΜ ΓG. Τα προταθέντα άνω φράγματα περιέχουν αρκετές γνωστές μαθηματικές εκφράσεις για τις από κοινού ΣΠΠ κι ΑΣΚ ως ειδικές περιπτώσεις. Στη συνέχεια, προσεγγίζοντας τον ΠΣ αυθαίρετα συσχετισμένων ΤΜ Gauss με έναν ειδικής κατηγορίας πίνακα Green, εξάγεται μια κλειστής μορφής έκφραση προσέγγισης για την από κοινού ΣΠΠ αυθαίρετα συσχετισμένων και μη απαραιτήτως ΤΚ ΤΜ ΓG καθώς και μια αναλυτική έκφραση προσέγγισης σε αναπαράσταση απειροσειρών για την από κοινού ΑΣΚ τους. Επίσης, παρουσιάζονται αναλυτικές εκφράσεις σε αναπαραστάσεις απειροσειρών για τις κυριότερες στατιστικές ιδιότητες της τρι-μεταβλητής κατανομής ΓG με αυθαίρετο ΠΣ και μη απαραιτήτως ΤΚ ΤΜ καθώς και της πολυ-μεταβλητής κατανομής ΓG με σταθερό ΠΣ και μη απαραιτήτως ΤΚ ΤΜ. Όλων των μορφών οι προταθείσες αναλυτικές μαθηματικές εκφράσεις για τις ΣΠΠ κι ΑΣΚ της πολυ-μεταβλητής κατανομής ΓG χρησιμοποιούνται για τη μελέτη επιδόσεων δεκτών διαφορισμού επιλογής (ΔΕ), διαφορισμού μέγιστου λόγου (ΔΜΛ) και διαφορισμού μεταγωγής κι εξέτασης (ΔΜκΕ) που υπόκεινται σε ποικίλα περιβάλλοντα αυθαίρετα συσχετισμένων ΔΜΙΚ. Αρχικά, εξάγονται αναλυτικές εκφράσεις άνω φραγμάτων για την πιθανότητα διακοπής επικοινωνίας (ΠΔΕ), τη μέση πιθανότητα σφάλματος συμβόλου (ΜΠΣΣ) διαφόρων σχημάτων διαμόρφωσης και τη μέση χωρητικότητα καναλιού (ΜΧΚ) κατά Shannon δεκτών ΔΕ που λειτουργούν σε περιβάλλον αυθαίρετα συσχετισμένων και μη απαραιτήτως ΤΚ διαλείψεων ΓG. Επίσης, παρουσιάζονται αναλυτικές εκφράσεις για τα ίδια κριτήρια επίδοσης δεκτών ΔΕ με τρεις κεραίες καθώς κι αναλυτικές εκφράσεις προσεγγίσεων για τα κριτήρια επίδοσης δεκτών ΔΕ οποιουδήποτε πλήθους κεραιών. Κατόπιν, εξάγοντας νέες αναλυτικές μαθηματικές εκφράσεις σε αναπαραστάσεις απειροσειρών για τις κυριότερες στατιστικές ιδιότητες του αθροίσματος οποιουδήποτε αριθμού αυθαίρετα συσχετισμένων και ΤΚ ΤΜ Γάμα, προκύπτουν αναλυτικές εκφράσεις για την ΠΔΕ, τη ΜΠΣΣ διαφόρων σχημάτων διαμόρφωσης και τη ΜΧΚ κατά Shannon δεκτών ΔΜΛ που λειτουργούν σε αυθαίρετα συσχετισμένες και ΤΚ διαλείψεις Nakagami-m. Για το ίδιο περιβάλλον διαλείψεων, παρουσιάζονται αναλυτικές εκφράσεις σε αναπαραστάσεις απειροσειρών για την ΠΔΕ και τη ΜΠΣΣ διαφόρων σχημάτων διαμόρφωσης δεκτών ΔΜκΕ οποιουδήποτε πλήθους κεραιών. Η στενότητα των προταθέντων άνω φραγμάτων για τα κριτήρια επίδοσης των δεκτών ΔΕ, ΔΜΛ και ΔΜκΕ που υπόκεινται σε περιβάλλοντα αυθαίρετα συσχετισμένων ΔΜΙΚ, η ορθότητα των αναλυτικών εκφράσεων για τα ίδια κριτήρια κι η ακρίβεια των προταθέντων προσεγγίσεών τους μελετήθηκαν εκτενώς συγκρίνοντας πολυάριθμα αριθμητικά αποτελέσματα των εκφράσεων αυτών με αντίστοιχα αποτελέσματα που προέκυψαν από προσομοιώσεις σε Η/Υ, οι οποίες υλοποιήθηκαν για το σκοπό αυτό. / The rapidly increasing demands for wireless wideband services and the recent advances in the design and implementation of mobile terminal devices with Internet-based service providing capabilities expedite the introduction of fourth generation (4G) wireless communications systems in the international wireless market. These systems are expected to ensure increased data rates and quality of service in an anytime anywhere basis. Wireless systems that utilize multiple antennas at the transmitter and/or receiver as well as space-time signal processing techniques play a fundamental role in accomplishing the demands imposed by 4G wireless communications systems. Well-known multiple-antenna systems that enable simple implementations are space diversity receivers (SDRs). By properly combining the multiple received replicas of the transmitted signal, SDRs are capable of effectively mitigating the detrimental effects of multipath fading, known as small-scale fading (SSF), that is inherent in wireless channels. SDRs are theoretically known to improve wireless system’s performance compared with conventional single-antenna receivers. This improvement requires that the SSF channels among multiple receiver’s antennas are statistically independent. However, in practical implementations, due to several parameters such as for example the small distance among the receiver’s multiple branches, SSF channels are arbitrarily correlated. This doctoral dissertation presents a theoretical performance study of SDRs operating over arbitrarily correlated SSF channels. Although numerous scientific papers deal with correlated SSF channel modeling and the impact of correlated SSF on the performance of SDRs, their vast majority, which utilizes the statistical properties of multivariate distributions for studying SDRs’ performance, is restricted to special forms of fading correlation and conventional SDR techniques. This happens mainly due the fact that there is a lack of simple mathematical expressions for the statistical properties of multivariate distributions with arbitrarily correlated random variables (RVs) in the literature. Within the framework of this dissertation, firstly, the previously proposed mathematical expressions for the most prevalent statistical properties of the multivariate Rayleigh, Nakagami-m, Weibull and generalized Gamma (ΓG) distributions with various forms of correlation are summarized. Moreover, their capabilities of being utilized for the performance study of SDRs operating over correlated SSF are described. Next, by presenting a new methodology for generating arbitrarily correlated and not necessarily identically distributed (ID) ΓG RVs that is based on arbitrarily correlated Gaussian RVs and the special class of Householder matrices for tridiagonalizing the correlation matrix (CM) of Gaussian RVs, a closed-form upper bound expression for the joint probability density function (PDF) and an analytical upper bound expression in infinite series form for the joint cumulative distribution function (CDF) of arbitrarily correlated and not necessarily ID ΓG RVs are derived. The proposed upper bounds contain several known mathematical expressions for the joint PDF and CDF as special cases. In addition, by approximating the CM of arbitrarily correlated Gaussian RVs with the special class of Green’s matrices, a closed-form approximate expression for the joint PDF and an analytical approximate expression in infinite series form for the joint CDF of arbitrarily correlated and not necessarily ID ΓG RVs are obtained. Furthermore, analytical expressions in infinite series form for the most prevalent statistical properties of the trivariate ΓG distribution with an arbitrary CM and not necessarily ID RVs as well as of the multivariate ΓG distribution with a constant CM and not necessarily ID RVs are presented. The proposed analytic mathematical expressions of all forms for the PDF and CDF of the multivariate ΓG distribution are used for the performance study of selection diversity (SD), maximal-ratio diversity (MRD), and switch-and-examine diversity (SED) receivers over various arbitrarily correlated SSF channels. Firstly, analytical upper bound expressions for the outage probability (OP), average symbol error probability (ASEP) for several modulation formats, and average channel capacity (ACC) in Shannon’s sense of SD receivers operating over arbitrarily correlated and not necessarily ID ΓG fading are derived. Moreover, analytical expressions for the same performance criteria of triple-branch SD receivers as well as analytical approximate expressions for the performance criteria of multibranch SD receivers are presented. Next, by obtaining new analytic mathematical expressions in infinite series form for the most prevalent statistical properties of the sum of any number of arbitrarily correlated and ID Gamma RVs, analytical expressions for the OP, ASEP for several modulation formats, and ACC in Shannon’s sense of multibranch MRD receivers operating over arbitrarily correlated and ID Nakagami-m fading are derived. For the same fading conditions, analytical expressions in infinite series form for the OP and ASEP for several modulation formats of multibranch SED receivers are presented. The tightness of the proposed upper bounds for the performance criteria of multibranch SD, MRD, and SED receivers in various arbitrarily correlated SSF environments, the correctness of the analytical expressions for the same criteria, and the accuracy of the proposed approximations for them are studied in depth through comparisons between numerically evaluated results for the expressions and equivalent results obtained by means of computer simulations that were implemented for this purpose.

Submitted by Jaqueline Silva (jtas29@gmail.com) on 2014-12-12T17:31:05Z No. of bitstreams: 1 Dissertação-Victor Hugo Teles Costa-2013.pdf: 20678399 bytes, checksum: a39c778934ebe127bd74f506467fe0a3 (MD5) / Rejected by Jaqueline Silva (jtas29@gmail.com), reason: on 2014-12-12T17:31:57Z (GMT) / Submitted by Jaqueline Silva (jtas29@gmail.com) on 2014-12-12T19:42:30Z No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dissertação-Victor Hugo Teles Costa-2013.pdf: 20678399 bytes, checksum: a39c778934ebe127bd74f506467fe0a3 (MD5) / Approved for entry into archive by Jaqueline Silva (jtas29@gmail.com) on 2014-12-16T09:25:22Z (GMT) No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dissertação-Victor Hugo Teles Costa-2013.pdf: 20678399 bytes, checksum: a39c778934ebe127bd74f506467fe0a3 (MD5) / Made available in DSpace on 2014-12-16T09:25:22Z (GMT). No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Dissertação-Victor Hugo Teles Costa-2013.pdf: 20678399 bytes, checksum: a39c778934ebe127bd74f506467fe0a3 (MD5) Previous issue date: 2013-12-06 / This paper presents a model based on Markov Chains and enhanced with the use of Kernel Density Estimation and of MMFM (Markov Modulated Fluid Model) in order to evaluate the performance of the transmission link in OFDMTDMA systems. For that purpose, traffic models based on the Kernel method and the MMFM with adjusted autocorrelation function are proposed. From the model implemented for the OFDM-TDMA system, it was derived equations for estimation of QoS parameters such as delay and average queue size in the buffer. The obtained results confirm that the proposed model is efficient in describing the link performance indicators. The use of MMFM to model the arrival process improves the QoS parameter estimates of the queueing model making their values very close to those of the simulation results. It was also developed an equation to the OFDMTDMA system’s service curve. Through this equation and the concept of Envelope Process, it was proposed an equation to estimate the probability of buffer overflow in OFDM-TDMA systems. The results show that the estimates of the overflow probability based on the system’s service curve are very close to the ones obtained by simulations and that the computational complexity to obtain them is significantly reduced compared to the model based on Markov Chains due to the absence of matrix computation. / Este trabalho apresenta um modelo baseado em Cadeias de Markov e aprimorado com o uso do método de Kernel de estimação não-paramétrica e de MMFM (Markov Modulated Fluid Model) com o objetivo de avaliar e descrever o desempenho do enlace de transmissão em sistemas OFDM-TDMA. Para tal, modelos de tráfego baseados no Método de Kernel e em MMFM com ajuste da função de autocorrelação são propostos. A partir do modelo implementado para o sistema OFDM-TDMA, foram obtidas equações para estimação de parâmetros de QoS como retardo e tamanho médio da fila no buffer. Os resultados obtidos confirmam que o modelo proposto é bastante eficiente ao descrever os indicadores de desempenho do sistema. O uso de MMFM para modelar o processo de chegada de pacotes aprimora os estimadores de parâmetros de QoS tornando as estimativas bem próximas dos valores obtidos com as simulações. Também deduziu-se uma equação para a curva de serviço de Sistemas OFDM-TDMA. Em seguida, utilizando-se desta curva de serviço e do conceito de Processo Envelope foi proposta uma equação para estimação de probabilidade de transbordo do buffer em sistemas OFDM-TDMA. Os resultados obtidos mostram que as estimativas de probabilidade de transbordo baseadas na curva de serviço do sistema se aproximam bem dos resultados da simulação e a complexidade computacional do cálculo necessário para obtê-los é significativamente reduzida em relação ao modelo definido utilizando Cadeias de Markov.

Este trabalho apresenta o desenvolvimento de um método para garantir a proteção dos dados contra falhas de comunicação, em redes de sensores inteligentes sem fios. A rede sem fios foi empregada nos ensaios de modelos físicos reduzidos em centrífuga para estudar o comportamento das âncoras \"tartaruga\", utilizadas nas linhas de ancoragem de sistemas oceânicos para produção de óleo e gás em águas profundas. O emprego da tecnologia de redes de sensores sem fios tem aumentado em muitas áreas da sociedade, principalmente na engenharia. Dessa forma, muitos problemas estão sendo enfrentados e estudados pelos pesquisadores. Sobretudo no que diz respeito à interferência ambiental, relação de potência e alcance, integridade da informação, miniaturização dos dispositivos, autonomia de energia, limitação de algoritmos e vida útil. A inserção da tecnologia de redes de sensores inteligentes sem fios nos ensaios de modelos físicos reduzidos em centrífuga, além de ser inédita no Brasil, permitirá avanços significativos na determinação de parâmetros que, atualmente, são de difícil obtenção com tecnologias convencionais como os sistemas de aquisição de dados e controle que utilizam cabos e fios. Muitas vezes depara-se com a inconveniência do peso dos cabos e dificuldades de instalação dos dispositivos nos modelos, pela sua pouca portabilidade. / This work present a method development to guarantee of the integrity data in the wireless smart sensor networks, employed in centrifuge physical modeling tests of anchors used as fixing devices of deep water production oil and gas systems. The use of wireless smart sensor networks technology has been increasing in many engineering areas. In spite of this evidence, lots of problems are being faced and studied by the researchers, environmental interference, power and reach relations, integrity of the information, miniaturization of the devices, energy autonomy, algorithms and useful life. The use of the wireless smart sensor networks technology in the reduced physical models test in centrifuge, by first time in Brazil, will allow significant advances in the parameters determination. The conventional instrumentation presents inconvenience of the weight cables and difficulties of the devices in the models by limited portability.

Multi-antenna wireless systems have become very popular due to their theoretically predicted higher spectral efficiencies and improved performance compared to single-antenna systems. Large-scale multiple-input multiple-output (MIMO) systems refer to wireless systems where communication terminals employ tens to hundreds of antennas to achieve in-creased spectral efficiencies/sum rates, reliability, and power efficiency. Large-scale multi-antenna systems are attractive to meet the increasing wireless data rate requirements, without compromising on the bandwidth. This thesis addresses key signal processing issues in large-scale MIMO systems. Specifically, the thesis investigates efficient algorithms for signal detection and channel estimation in large-scale MIMO systems. It also investigates ‘spatial modulation,’ a multi-antenna modulation scheme that can reduce the number of transmit radio frequency (RF) chains, without compromising much on the spectral efficiency. The work reported in this thesis is comprised of the following two parts: 1 investigation of low-complexity receiver algorithms based on Markov chain Monte Carlo (MCMC) technique, tabu search, and belief propagation for large-scale uplink multiuser MIMO systems, and 2 investigation of achievable rates and signal detection in generalized spatial modulation. 1. Receiver algorithms for large-scale multiuser MIMO systems on the uplink In this part of the thesis, we propose low-complexity algorithms based on MCMC techniques, Gaussian sampling based lattice decoding (GSLD), reactive tabu search (RTS), and factor graph based belief propagation (BP) for signal detection on the uplink in large-scale multiuser MIMO systems. We also propose an efficient channel estimation scheme based on Gaussian sampling. Markov chain Monte Carlo (MCMC) sampling: We propose a novel MCMC based detection algorithm, which achieves near-optimal performance in large dimensions at low complexities by the joint use of a mixed Gibbs sampling (MGS) strategy and a multiple restart strategy with an efficient restart criterion. The proposed mixed Gibbs sampling distribution is a weighted mixture of the target distribution and uniform distribution. The presence of the uniform component in the sampling distribution allows the algorithm to exit from local traps quickly and alleviate the stalling problem encountered in conventional Gibbs sampling. We present an analysis for the optimum choice of the mixing ratio. The analysis approach is to define an absorbing Markov chain and use its property regarding the expected number of iterations needed to reach the global minima for the first time. We also propose an MCMC based algorithm which exploits the sparsity in uplink multiuser MIMO transmissions, where not all users are active simultaneously. Gaussian sampling based lattice decoding: Next, we investigate the problem of searching the closest lattice point in large dimensional lattices and its use in signal detection in large-scale MIMO systems. Specifically, we propose a Gaussian sampling based lattice decoding (GSLD) algorithm. The novelty of this algorithm is that, instead of sampling from a discrete distribution as in Gibbs sampling, the algorithm iteratively generates samples from a continuous Gaussian distribution, whose parameters are obtained analytically. This makes the complexity of the proposed algorithm to be independent of the size of the modulation alpha-bet. Also, the algorithm is able to achieve near-optimal performance for different antenna and modulation alphabet settings at low complexities. Random restart reactive tabu search (R3TS): Next, we study receiver algorithms based on reactive tabu search (RTS) technique in large-scale MIMO systems. We propose a multiple random restarts based reactive tabu search (R3TS) algorithm that achieves near-optimal performance in large-scale MIMO systems. A key feature of the proposed R3TS algorithm is its performance based restart criterion, which gives very good performance-complexity tradeoff in large-dimension systems. Lower bound on maximum likelihood (ML) bit error rate (BER) performance: We propose an approach to obtain lower bounds on the ML performance of large-scale MIMO systems using RTS simulation. In the proposed approach, we run the RTS algorithm using the transmitted vector as the initial vector, along with a suitable neighborhood definition, and find a lower bound on number of errors in ML solution. We demonstrate that the proposed bound is tight (within about 0.5 dB of the optimal performance in a 16×16MIMO system) at moderate to high SNRs. Factor graph using Gaussian approximation of interference (FG-GAI): Multiuser MIMO channels can be represented by graphical models that are fully/densely connected (loopy graphs), where conventional belief propagation yields suboptimal performance and requires high complexity. We propose a solution to this problem that uses a simple, yet effective, Gaussian approximation of interference (GAI) approach that carries out a linear per-symbol complexity message passing on a factor graph (FG) based graphical model. The proposed algorithm achieves near-optimal performance in large dimensions in frequency-flat as well as frequency-selective channels. Gaussian sampling based channel estimation: Next, we propose a Gaussian sampling based channel estimation technique for large-scale time-division duplex (TDD) MIMO systems. The proposed algorithm refines the initial estimate of the channel by iteratively detecting the data block and using that knowledge to improve the estimated channel knowledge using a Gaussian sampling based technique. We demonstrate that this algorithm achieves near-optimal performance both in terms of mean square error of the channel estimates and BER of detected data in both frequency-flat and frequency-selective channels. 2. Generalized spatial modulation In the second part of the thesis, we investigate generalized spatial modulation (GSM) in point-to point MIMO systems. GSM is attractive because of its ability to work with less number of transmit RF chains compared to traditional spatial multiplexing, without com-promising much on spectral efficiency. In this work, we show that, by using an optimum combination of number of transmit antennas and number of transmit RF chains, GSM can achieve better throughput and/or BER than spatial multiplexing. We compute tight bounds on the maximum achievable rate in a GSM system, and quantify the percentage savings in the number of transmit RF chains as well as the percentage increase in the rate achieved in GSM compared to spatial multiplexing. We also propose a Gibbs sampling based algorithm suited to detect GSM signals, which yields impressive BER performance and complexity results.

Wireless sensor networks(WSNs) have a diverse set of applications such as military surveillance, health and environmental monitoring, and home automation. Sensor nodes are equipped with pre-charged batteries, which drain out when the nodes sense, process, and communicate data. Eventually, the nodes of the WSN die and the network dies. Energy harvesting(EH) is a green alternative to solve the limited lifetime problem in WSNs. EH nodes recharge their batteries by harvesting ambient energy such as solar, wind, and radio energy. However, due to the randomness in the EH process and the limited amounts of energy that can be harvested, the EH nodes are often intermittently available. Therefore, even though EH nodes live perpetually, they do not cater to the network continuously. We focus on the energy-efficient design of WSNs that incorporate EH, and investigate the new design trade-offs that arise in exploiting the potentially scarce and random energy arrivals and channel fading encountered by the network. To this end, firstly, we compare the performance of conventional, all-EH, and hybrid WSNs, which consist of both conventional and EH nodes. We then study max function computation, which aims at energy-efficient data aggregation, in EH WSNs. We first argue that the conventional performance criteria used for evaluating WSNs, which are motivated by lifetime, and for evaluating EH networks are at odds with each other and are unsuitable for evaluating hybrid WSNs. We propose two new and insightful performance criteria called the k-outage and n-transmission durations to evaluate and compare different WSNs. These criteria capture the effect of the battery energies of the nodes and the channel fading conditions on the network operations. We prove two computationally-efficient bounds for evaluating these criteria, and show their use in a cost-constrained deployment of a WSN involving EH nodes. Next, we study the estimation of maximum of sensor readings in an all-EH WSN. We analyze the mean absolute error(MAE) in estimating the maximum reading when a random subset of the EH nodes periodically transmit their readings to the fusion node. We determine the optimal transmit power and the number of scheduled nodes that minimize the MAE. We weigh the benefits of the availability of channel information at the nodes against the cost of acquiring it. The results are first developed assuming that the readings are transmitted with infinite resolution. The new trade-offs that arise when quantized readings are instead transmitted are then characterized.Our results hold for any distribution of sensor readings, and for any stationary and ergodic EH process.

Orthogonal frequency division multiplexing (OFDM) in next generation wireless systems provides high downlink data rates by employing frequency-domain scheduling and rate adaptation at the base station (BS). However, in order to control the significant feedback overhead required by these techniques, feedback reduction schemes are essential. Best-m feedback is one such scheme that is implemented in OFDM standards such as Long Term Evolution. In it, the sub channel (SC) power gains of only the m strongest SCs and their corresponding indices are fed back to the BS. However, two assumptions pervade most of the literature that analyze best-m feedback in OFDM systems. The first one is that the SC gains are uncorrelated. In practice, however, the SC gains are highly correlated, even for dispersive multipath channels. The second assumption deals with the treatment of unreported SCs, which are not fed back by the best-m scheme. If no user reports an SC, then no data transmission is assumed to occur. In this thesis, we eschew these assumptions and investigate best-m feedback in OFDM systems with correlated SC gains. We, first, characterize the average throughput as a function of correlation and m. A uniform correlation model is assumed, i.e., the SC gains are correlated with each other by the same correlation coefficient. The system model incorporates greedy, modified proportional- fair, and round robin schedulers, discrete rate adaptation, and non-identically distributed SC gains of different users. We, then, generalize the model to account for feedback delay. We show in all these cases that correlation degrades the average throughput. We also show that this effect does not arise when users report all the SC power gains to the BS. In order to mitigate the reduction in the average throughput caused by unreported SCs, we derive a novel, constrained minimum mean square error channel estimator for the best-m scheme to estimate the gains of these unreported SCs. The estimator makes use of the additional information, which is unique to the best-m scheme, that the estimated SC power gains must be less than those that were reported. We, then, study its implications on the downlink average cell throughput, again for different schedulers. We show that our approach reduces the root mean square error and increases the average throughput compared to several approaches pursued in the literature. The more correlated the SC gains, greater is the improvement.

Opportunistic selection and rate adaptation play a vital role in improving the spectral and power efficiency of current multi-node wireless systems. However, time-variations in wireless channels affect the performance of opportunistic selection and rate-adaptation in the following ways. Firstly, the selected node can become sub-optimal by the time data transmission commences. Secondly, the choice of transmission parameters such as rate and power for the selected node become sub-optimal. Lastly, the channel changes during data transmission. In this thesis, we develop a comprehensive and tractable analytical framework that accurately accounts for these effects. It differs from the extensive existing literature that primarily focuses on time-variations until the data transmission starts. Firstly, we develop a novel concept of a time-invariant effective signal-to-noise ratio (TIESNR), which tractably and accurately captures the time-variations during the data transmission phase with partial channel state information available at the receiver. Secondly, we model the joint distribution of the signal-to-noise ratio at the time of selection and TIESNR during the data transmission using generalized bivariate gamma distribution. The above analytical steps facilitate the analysis of the outage probability and average packet error rate (PER) for a given modulation and coding scheme and average throughput with rate adaptation. We also present extensive numerical results to verify the accuracy of each step of our approach and show that ignoring the correlated time variations during the data transmission phase can significantly underestimate the outage probability and average PER, whereas it overestimates the average throughput even for packet durations as low as 1 msec.