One of the fundamental limitations of mobile radio communications is their time-varying fading channel. This thesis addresses the efficient use of channel state information to improve the communication systems, with a particular emphasis on practical issues such as compatibility with the existing wireless systems and low complexity implementation. The closed-loop transmit diversity technique is used to improve the performance of the downlink channel in MIMO communication systems. For example, the WCDMA standard endorsed by 3GPP adopts a mode of downlink closed-loop scheme based on partial channel state information known as mode 1 of 3GPP. Channel state information is fed back from the mobile unit to the base station through a low-rate uncoded feedback bit stream. In these closed-loop systems, feedback error and feedback delay, as well as the sub-optimum reconstruction of the quantized feedback data, are the usual sources of deficiency. In this thesis, we address the efficient reconstruction of the beamforming weights in the presence of the feedback imperfections, by exploiting the residual redundancies in the feedback stream. We propose a number of algorithms for reconstruction of beamforming weights at the base-station, with the constraint of a constant transmit power. The issue of the decoding at the receiver is also addressed. In one of the proposed algorithms, channel fading prediction is utilized to combat the feedback delay. We introduce the concept of Blind Antenna Verification which can substitute the conventional Antenna Weight Verification process without the need for any training data. The closed-loop mode 1 of 3GPP is used as a benchmark, and the performance is examined within a WCDMA simulation framework. It is demonstrated that the proposed algorithms have substantial gain over the conventional method at all mobile speeds, and are suitable for the implementation in practice. The proposed approach is applicable to other closed-loop schemes as well. The problem of (long-range) prediction of the fading channel is also considered, which is a key element for many fading-compensation techniques. A linear approach, usually used to model the time evolution of the fading process, does not perform well for long-range prediction applications. We propose an adaptive algorithm using a state-space approach for the fading process based on the sum-sinusoidal model. Also to enhance the widely-used linear approach, we propose a tracking method for a multi-step linear predictor. Comparing the two methods in our simulations shows that the proposed algorithm significantly outperforms the linear method, for both stationary and non-stationary fading processes, especially for long-range predictions. The robust structure, as well as the reasonable computational complexity, makes the proposed algorithm appealing for practical applications.

One of the fundamental limitations of mobile radio communications is their time-varying fading channel. This thesis addresses the efficient use of channel state information to improve the communication systems, with a particular emphasis on practical issues such as compatibility with the existing wireless systems and low complexity implementation. The closed-loop transmit diversity technique is used to improve the performance of the downlink channel in MIMO communication systems. For example, the WCDMA standard endorsed by 3GPP adopts a mode of downlink closed-loop scheme based on partial channel state information known as mode 1 of 3GPP. Channel state information is fed back from the mobile unit to the base station through a low-rate uncoded feedback bit stream. In these closed-loop systems, feedback error and feedback delay, as well as the sub-optimum reconstruction of the quantized feedback data, are the usual sources of deficiency. In this thesis, we address the efficient reconstruction of the beamforming weights in the presence of the feedback imperfections, by exploiting the residual redundancies in the feedback stream. We propose a number of algorithms for reconstruction of beamforming weights at the base-station, with the constraint of a constant transmit power. The issue of the decoding at the receiver is also addressed. In one of the proposed algorithms, channel fading prediction is utilized to combat the feedback delay. We introduce the concept of Blind Antenna Verification which can substitute the conventional Antenna Weight Verification process without the need for any training data. The closed-loop mode 1 of 3GPP is used as a benchmark, and the performance is examined within a WCDMA simulation framework. It is demonstrated that the proposed algorithms have substantial gain over the conventional method at all mobile speeds, and are suitable for the implementation in practice. The proposed approach is applicable to other closed-loop schemes as well. The problem of (long-range) prediction of the fading channel is also considered, which is a key element for many fading-compensation techniques. A linear approach, usually used to model the time evolution of the fading process, does not perform well for long-range prediction applications. We propose an adaptive algorithm using a state-space approach for the fading process based on the sum-sinusoidal model. Also to enhance the widely-used linear approach, we propose a tracking method for a multi-step linear predictor. Comparing the two methods in our simulations shows that the proposed algorithm significantly outperforms the linear method, for both stationary and non-stationary fading processes, especially for long-range predictions. The robust structure, as well as the reasonable computational complexity, makes the proposed algorithm appealing for practical applications.

Signal processing has been playing a key role in providing solutions to key problems encountered in communications, in general, and in wireless communications, in particular. Time-Frequency Signal Processing (TFSP) provides eective tools for analyzing nonstationary signals where the frequency content of signals varies in time as well as for analyzing linear time-varying systems. This research aimed at exploiting the advantages of TFSP, in dealing with nonstationary signals, into the fundamental issues of signal processing, namely the signal estimation and signal separation. In particular, it has investigated the problems of (i) the Instantaneous Frequency (IF) estimation of Linear Frequency-Modulated (LFM) signals corrupted in complex-valued zero-mean Multiplicative Noise (MN), and (ii) the Underdetermined Blind Source Separation (UBSS) of LFM signals, while focusing onto the fast-growing area of Wireless Communications (WCom). A common problem in the issue of signal estimation is the estimation of the frequency of Frequency-Modulated signals which are seen in many engineering and real-life applications. Accurate frequency estimation leads to accurate recovery of the true information. In some applications, the random amplitude modulation shows up when the medium is dispersive and/or when the assumption of point target is not valid; the original signal is considered to be corrupted by an MN process thus seriously aecting the recovery of the information-bearing frequency. The IF estimation of nonstationary signals corrupted by complex-valued zero-mean MN was investigated in this research. We have proposed a Second-Order Statistics approach, rather than a Higher-Order Statistics approach, for IF estimation using Time-Frequency Distributions (TFDs). The main assumption was that the autocorrelation function of the MN is real-valued but not necessarily positive (i.e. the spectrum of the MN is symmetric but does not necessary has the highest peak at zero frequency). The estimation performance was analyzed in terms of bias and variance, and compared between four dierent TFDs: Wigner-Ville Distribution, Spectrogram, Choi-Williams Distribution and Modified B Distribution. To further improve the estimation, we proposed to use the Multiple Signal Classification algorithm and showed its better performance. It was shown that the Modified B Distribution performance was the best for Signal-to-Noise Ratio less than 10dB. In the issue of signal separation, a new research direction called Blind Source Separation (BSS) has emerged over the last decade. BSS is a fundamental technique in array signal processing aiming at recovering unobserved signals or sources from observed mixtures exploiting only the assumption of mutual independence between the signals. The term "blind" indicates that neither the structure of the mixtures nor the source signals are known to the receivers. Applications of BSS are seen in, for example, radar and sonar, communications, speech processing, biomedical signal processing. In the case of nonstationary signals, a TF structure forcing approach was introduced by Belouchrani and Amin by defining the Spatial Time- Frequency Distribution (STFD), which combines both TF diversity and spatial diversity. The benefit of STFD in an environment of nonstationary signals is the direct exploitation of the information brought by the nonstationarity of the signals. A drawback of most BSS algorithms is that they fail to separate sources in situations where there are more sources than sensors, referred to as UBSS. The UBSS of nonstationary signals was investigated in this research. We have presented a new approach for blind separation of nonstationary sources using their TFDs. The separation algorithm is based on a vector clustering procedure that estimates the source TFDs by grouping together the TF points corresponding to "closely spaced" spatial directions. Simulations illustrate the performances of the proposed method for the underdetermined blind separation of FM signals. The method developed in this research represents a new research direction for solving the UBSS problem. The successful results obtained in the research development of the above two problems has led to a conclusion that TFSP is useful for WCom. Future research directions were also proposed.

Approved for public release, distribution is unlimited / Wide local area networks (WLAN) are increasingly important in meeting the needs of next generation broadband wireless communications systems for both commercial and military applications. Under IEEE 802.11a 5GHz WLAN standard, OFDM was chosen as the modulation scheme for transmission because of its well-known ability to avoid multi-path effects while achieving high data rates. The objective of this thesis is to investigate the performance of the IEEE 802.11a WLAN standard receiver over flat fading Nakagami channels in a worst case, pulse-noise jamming environment, for the different combinations of modulation type (binary and non-binary modulation) and code rate specified by the WLAN standard. Receiver performance with Viterbi soft decision decoding (SDD) will be analyzed for additive white Gaussian noise (AWGN) alone and for AWGN plus pulse-noise jamming. Moreover, the performance of the IEEE 802.11a WLAN standard receiver will be examined both in the scenario where perfect side information is considered to be available (optimum receiver) and when it is not (sub-optimum receiver). In the sub-optimum receiver scenario, the receiver performance is examined both when noise-normalization is utilized and when it is not. The receiver performance is severely affected by the pulse-noise jamming environment, especially in the suboptimum receiver scenario. However, the sub-optimum receiver performance is significantly improved when noise-normalization is implemented. / Lieutenant, Hellenic Navy

A modelagem computacional tem se tornado uma poderosa ferramenta utilizada mundialmente em pesquisas sobre sistemas de comunicação móvel. Em tais sistemas, a modelagem do canal é fator indispensável, pois as características de mobilidade e propagação presentes nesse tipo de sistema são responsáveis por distorções ocorridas sobre o sinal transmitido. O presente trabalho reúne os principais modelos matemáticos e computacionais de canal de comunicação móvel. Realiza-se uma abordagem histórica sobre o assunto, apresentado a natureza do problema do desvanecimento e a forma como foi modelado inicialmente. Por outro lado, apresentam-se modelos recentes, considerados eficazes do ponto de vista matemático e eficientes do ponto de vista computacional. O trabalho traz como contribuição a proposta de um novo modelo de canal com desvanecimento. Os modelos apresentados foram analisados do ponto de vista matemático por meio de suas propriedades estatísticas. Do ponto de vista numérico e computacional a análise se deu por meio de simulações. A principal conclusão obtida mostra que dois modelos com filtragem no domínio da freqüência, sendo um deles o novo modelo proposto, são os mais indicados para simular formas de onda que representem um canal com desvanecimento Rayleigh. Este trabalho tem a pretensão de ser útil para pesquisadores atuantes na área de modelagem e simulação de sistemas atuais de comunicação móvel. / Computational modelling is a powerful and widely used tool for research in mobile communication systems. In such systems, the communication channel modelling is an indispensable factor, because its mobility and propagation characteristics can cause distortion over the transmitted waveform. This work contains the main mathematical and computing communication channel models. A historical cover is provided, which presents the nature of the problem and the initial modelling of the fading phenomenon. On the other hand, recent models are presented, which are mathematically effective and computationally efficient. One contribution of this work is the proposal of a new fading channel computational model, which was suitably tested and validated. The presented models were mathematically analyzed through its statistical properties. From numerical and computational point of view, the models were analyzed through several simulations. The main conclusion, from analytical and simulated results, shows that two models using frequency filtering are the best choice for the generation of multiple uncorrelated Rayleigh fading waveforms, where one of them is the new proposed model. This work can help mobile communications researchers to suitably model the communication channel in a computer simulation.

A modelagem computacional tem se tornado uma poderosa ferramenta utilizada mundialmente em pesquisas sobre sistemas de comunicação móvel. Em tais sistemas, a modelagem do canal é fator indispensável, pois as características de mobilidade e propagação presentes nesse tipo de sistema são responsáveis por distorções ocorridas sobre o sinal transmitido. O presente trabalho reúne os principais modelos matemáticos e computacionais de canal de comunicação móvel. Realiza-se uma abordagem histórica sobre o assunto, apresentado a natureza do problema do desvanecimento e a forma como foi modelado inicialmente. Por outro lado, apresentam-se modelos recentes, considerados eficazes do ponto de vista matemático e eficientes do ponto de vista computacional. O trabalho traz como contribuição a proposta de um novo modelo de canal com desvanecimento. Os modelos apresentados foram analisados do ponto de vista matemático por meio de suas propriedades estatísticas. Do ponto de vista numérico e computacional a análise se deu por meio de simulações. A principal conclusão obtida mostra que dois modelos com filtragem no domínio da freqüência, sendo um deles o novo modelo proposto, são os mais indicados para simular formas de onda que representem um canal com desvanecimento Rayleigh. Este trabalho tem a pretensão de ser útil para pesquisadores atuantes na área de modelagem e simulação de sistemas atuais de comunicação móvel. / Computational modelling is a powerful and widely used tool for research in mobile communication systems. In such systems, the communication channel modelling is an indispensable factor, because its mobility and propagation characteristics can cause distortion over the transmitted waveform. This work contains the main mathematical and computing communication channel models. A historical cover is provided, which presents the nature of the problem and the initial modelling of the fading phenomenon. On the other hand, recent models are presented, which are mathematically effective and computationally efficient. One contribution of this work is the proposal of a new fading channel computational model, which was suitably tested and validated. The presented models were mathematically analyzed through its statistical properties. From numerical and computational point of view, the models were analyzed through several simulations. The main conclusion, from analytical and simulated results, shows that two models using frequency filtering are the best choice for the generation of multiple uncorrelated Rayleigh fading waveforms, where one of them is the new proposed model. This work can help mobile communications researchers to suitably model the communication channel in a computer simulation.

In general, PN code synchronization consists of two steps: PN code acquisition (coarse alignment) and PN code tracking (fine alignment), to estimate the delay offset between received and locally generated codes. Recently, the schemes with a joint adaptive process of PN code acquisition and the weight coefficients of smart antenna have been proposed for improving the received signal-to-interference-plus-noise ratio (SINR) and simultaneously achieving better mean-acquisition-time (MAT) performance in direct-sequence code-division multiple access (DS-CDMA) systems. In which, the setting of the threshold plays an important role on the MAT performance. Often, the received SINR is varying, using the fixed threshold acquisition algorithms may result in undesirable performance. To improve the above problem, in this thesis, a new adaptive threshold scheme is devised in a joint adaptive code acquisition and beam-forming DS-CDMA receiver for code acquisition under a fading multipath and additive white Gaussian-noise (AWGN) channels. The basic idea of this new adaptive threshold scheme is to estimate the averaged output power of smart antenna to scale a reference threshold for each observation interval, such that it can approximately achieve a constant false alarm rate (CFAR) criteria. The system probabilities of the proposed scheme are derived for evaluating MAT under a slowly fading two-paths channels. Numerical analyses and simulation results demonstrate that the proposed adaptive threshold scheme does achieve better performance, in terms of the output SINR, the detection probability and the MAT, compared to a fixed threshold method.

Multipath propagation and mobility in wireless communication systems give rise to variations in the amplitude and phase of the transmitted signal, commonly referred to as fading. Many wireless applications are affected by slowly varying fading, where the channel is non-ergodic, leading to non-reliable transmission during bad channel realizations. These communication scenarios are well modeled by the block-fading channel, where the reliability is quantatively characterized by the outage probability. This thesis focuses on the analysis and design of adaptive transmission schemes to improve the outage performance of both single- and multiple-antenna transmission over the block-fading channel, especially for the cases where discrete input constellations are used. Firstly, a new lower bound on the outage probability of non-adaptive transmission is proposed, providing an efficient tool for evaluating the performance of non-adaptive transmission. The lower bound, together with its asymptotic analysis, is essential for efficiently designing the adaptive transmission schemes considered in the thesis. Secondly, new power allocation rules are derived to minimize the outage probability of fixed-rate transmission over block-fading channels. Asymptotic outage analysis for the resulting schemes is performed, revealing important system design criteria. Furthermore, the thesis proposes novel suboptimal power allocation rules, which enjoy low-complexity while suffering minimal losses as compared to the optimal solution. Thus, these schemes facilitate power adaptation in low-cost devices. Thirdly, the thesis considers incremental-redundancy automatic-repeat-request (INR-ARQ) strategies, which perform adaptive transmission based on receiver feedback. In particular, the thesis concentrates on multi-bit feedback, which has been shown to yield significant gains in performance compared to conventional single-bit ARQ schemes. The thesis proposes a new information-theoretic framework for multi-bit feedback INR-ARQ, whereby the receiver feeds back a quantized version of the accumulated mutual information. Within this framework, the thesis presents an asymptotic analysis which yields the large gains in outage performance offered by multi-bit feedback. Furthermore, the thesis proposes practical design rules, which further illustrates the benefits of multi-bit feedback in INR-ARQ systems. In short, the thesis studies the outage performance of transmission over block-fading channels. Outage analysis is performed for non-adaptive and adaptive transmission. Improvements for the existing adaptive schemes are also proposed, leading to either lower complexity requirements or better outage performance. Still, further research is needed to bring the benefits offered by adaptive transmission into practical systems. / Thesis (PhD)--University of South Australia, 2010

Multipath propagation and mobility in wireless communication systems give rise to variations in the amplitude and phase of the transmitted signal, commonly referred to as fading. Many wireless applications are affected by slowly varying fading, where the channel is non-ergodic, leading to non-reliable transmission during bad channel realizations. These communication scenarios are well modeled by the block-fading channel, where the reliability is quantatively characterized by the outage probability. This thesis focuses on the analysis and design of adaptive transmission schemes to improve the outage performance of both single- and multiple-antenna transmission over the block-fading channel, especially for the cases where discrete input constellations are used. Firstly, a new lower bound on the outage probability of non-adaptive transmission is proposed, providing an efficient tool for evaluating the performance of non-adaptive transmission. The lower bound, together with its asymptotic analysis, is essential for efficiently designing the adaptive transmission schemes considered in the thesis. Secondly, new power allocation rules are derived to minimize the outage probability of fixed-rate transmission over block-fading channels. Asymptotic outage analysis for the resulting schemes is performed, revealing important system design criteria. Furthermore, the thesis proposes novel suboptimal power allocation rules, which enjoy low-complexity while suffering minimal losses as compared to the optimal solution. Thus, these schemes facilitate power adaptation in low-cost devices. Thirdly, the thesis considers incremental-redundancy automatic-repeat-request (INR-ARQ) strategies, which perform adaptive transmission based on receiver feedback. In particular, the thesis concentrates on multi-bit feedback, which has been shown to yield significant gains in performance compared to conventional single-bit ARQ schemes. The thesis proposes a new information-theoretic framework for multi-bit feedback INR-ARQ, whereby the receiver feeds back a quantized version of the accumulated mutual information. Within this framework, the thesis presents an asymptotic analysis which yields the large gains in outage performance offered by multi-bit feedback. Furthermore, the thesis proposes practical design rules, which further illustrates the benefits of multi-bit feedback in INR-ARQ systems. In short, the thesis studies the outage performance of transmission over block-fading channels. Outage analysis is performed for non-adaptive and adaptive transmission. Improvements for the existing adaptive schemes are also proposed, leading to either lower complexity requirements or better outage performance. Still, further research is needed to bring the benefits offered by adaptive transmission into practical systems. / Thesis (PhD)--University of South Australia, 2010

Τα ασύρματα συστήματα τέταρτης γενιάς (4G) στοχεύουν σε πολύ υψηλές ταχύτητες μετάδοσης δεδομένων, 100 Mbps (Mega bits per second) για ταχέως κινούμενους πομποδέκτες και έως 1 Gbps για ακίνητους. Αυτός ο στόχος μπορεί να επιτευχθεί με τα συστήματα Πολλαπλής Εισόδου-Πολλαπλής Εξόδου (Multiple Input-Multiple Output, MIMO) τα οποία χρησιμοποιούν πολλές κεραίες στον πομπό και στο δέκτη. Ο στόχος της παρούσας διδακτορικής διατριβής (ΔΔ) εστιάζεται στην ανάλυση και βελτιστοποίηση αυτών των συστημάτων, υπό το πρίσμα των φαινομένων της σκέδασης και των διαλείψεων μικρής κλίμακας. Το αντικείμενο μελέτης συνοψίζεται στις ακόλουθες θεματικές ενότητες: α) μοντελοποίηση των ασυρμάτων καναλιών με διαλείψεις, β) απόδοση ακριβών και εύχρηστων μαθηματικών εκφράσεων της εργοδικής (μέσου όρου) χωρητικότητας των ασύρματων συστημάτων που χρησιμοποιούν πολλές κεραίες στο δέκτη, γ) αύξηση της εργοδικής χωρητικότητας του συστήματος ΜΙΜΟ χρησιμοποιώντας πληροφορία από το μέσο διάδοσης. Αρχικά περιγράφεται η γενική μοντελοποίηση του ασύρματου καναλιού που είναι αναγκαία για την κατανόηση βασικών εννοιών για την ανάλυση που θα ακολουθήσει. Αυτό έχει ως στόχο μία σύντομη περιγραφή των βασικών χαρακτηριστικών ενός οποιουδήποτε ασύρματου καναλιού και να γίνουν κατανοητές κάποιες σημαντικές έννοιες που προκύπτουν και χρησιμοποιούνται κατά κόρον στις ασύρματες επικοινωνίες. Πιο συγκεκριμένα, παρατίθενται βασικές θεωρητικές γνώσεις όπου περιγράφονται τα διάφορα προβλήματα διάδοσης, δίνοντας μια σύντομη περιγραφή των φυσικών φαινομένων που εμπλέκονται, χωρίς να εμβαθύνουμε σε πολύπλοκες μαθηματικές σχέσεις. Στη συνέχεια, γίνεται προσπάθεια ακριβέστερης μοντελοποίησης, με χρήση στοχαστικών διαδικασιών, των ασύρματων μη επιλεκτικών στη συχνότητα καναλιών με διαλείψεις (frequency non-selective fading channels) σε περιβάλλον τρισδιάστατης ανισοτροπικής σκέδασης καναλιού Rice. Με τον όρο ανισοτροπική εννοείται ότι η λήψη των διαφόρων συνιστωσών για το αζιμούθιο επίπεδο γίνεται από κάποιους τομείς γωνιών και όχι από όλες τις κατευθύνσεις, ενώ στο επίπεδο της ανύψωσης θεωρούμε την ύπαρξη ενός τομέα άφιξης των συνιστωσών στον οποίο η ισχύς δεν κατανέμεται ομοιόμορφα αλλά βάσει μιας κατανομής. Επιπλέον λόγω της θεώρησης καναλιού Rice, συμπεριλαμβάνεται η ύπαρξη μιας δεσπόζουσας συνιστώσας με σταθερό πλάτος η οποία συνήθως προέρχεται από οπτική επαφή του πομπού με το δέκτη. Θεωρώντας συγκεκριμένες κατανομές για την άφιξη των συνιστωσών σε αυτούς τους τομείς από τη διεθνή βιβλιογραφία, εξάγεται αναλυτικά η συνάρτηση της αυτοσυσχέτισης και το φάσμα της ολίσθησης των συχνοτήτων σε αναλυτική μορφή και υπολογίζονται σημαντικά μεγέθη που εκφράζουν την ταχύτητα αυξομείωσης του σήματος και τη διάρκεια των διαλείψεων. Επιπλέον με αυτό τον τρόπο είναι δυνατόν να καθοριστεί η απόσταση μεταξύ των κεραιών που πρέπει να τηρείται ώστε να εξασφαλίζονται οι υψηλές επιδόσεις. Σε αστικό περιβάλλον, αποδεικνύεται ότι η ελάχιστη απόσταση μεταξύ των κεραιών ενός πομποδέκτη θα πρέπει να είναι μεγαλύτερη από ότι σε ένα υπαίθριο περιβάλλον. Στην επόμενη ενότητα επιτυγχάνεται η απόδοση ακριβών και εύχρηστων μαθηματικών εκφράσεων της εργοδικής (μέσου όρου) χωρητικότητας των ασύρματων συστημάτων που χρησιμοποιούν πολλές κεραίες στο δέκτη σε περιβάλλον Nakagami (που θεωρείται από τα πιο αντιπροσωπευτικά για την περιγραφή της ασύρματης διάδοσης σε κλειστούς χώρους) με όσο το δυνατό απλούστερες μαθηματικές συναρτήσεις. Με αυτό τον τρόπο, η ταχύτητα μετάδοσης δεδομένων εκφράζεται συναρτήσει των φυσικών παραμέτρων του συστήματος, δηλαδή το κανάλι, τον αριθμό των κεραιών κτλ. Ήδη έχουν γίνει πολλές δημοσιεύεις σε αυτό τον τομέα για διάφορες περιπτώσεις μοντελοποίησης των καναλιών (Rayleigh, Rice κτλ) και για διάφορες τεχνικές λήψης. Όμως υπάρχουν αρκετές περιπτώσεις όπου υπάρχουν κενά στη διεθνή βιβλιογραφία ή η έκφραση της χωρητικότητας δεν γίνεται με κλειστές μαθηματικές μορφές. Έτσι παρουσιάζονται αναλυτικές μαθηματικές εκφράσεις της εργοδικής χωρητικότητας των συστημάτων SIMO που δεν υπήρχαν έως τώρα στη διεθνή βιβλιογραφία, για διάφορες περιπτώσεις γνώσης του καναλιού. Αυτό γίνεται κάνοντας τον άμεσο παραλληλισμό των συστημάτων SIMO με τις διάφορες τεχνικές διαφορισμού. Εξετάζεται η εργοδική χωρητικότητα ενός συστήματος SIMO το οποίο λειτουργεί σε κανάλι διαλείψεων Nakagami-m στο οποίο όλες οι ζεύξεις είναι ανεξάρτητες αλλά δεν είναι κατά ανάγκη όμοιες. Συγκεκριμένα εξάγονται μαθηματικές εκφράσεις κλειστού τύπου για την εργοδική χωρητικότητα συστημάτων Equal Gain Combining και Selection Combining και Switch and Stay Combining δύο κλάδων. Επιπλέον, παρουσιάζεται για πρώτη φορά, η εργοδική χωρητικότητα ενός συστήματος SIMO στο οποίο δεν εφαρμόζεται καμία τεχνική διαφορικής λήψης και εξάγονται πολύ διδακτικά συμπεράσματα. Αυτό σημαίνει ότι ο δέκτης δεν έχει καμία πληροφορία για το κανάλι (no channel state information CSI) και απλά προσθέτει τα λαμβανόμενα σήματα από κάθε κλάδο-ζεύξη. Επιπλέον γίνεται προσπάθεια οι μαθηματικοί τύποι να είναι εύχρηστοι και υλοποιήσιμοι χωρίς την χρήση ιδιαίτερων μαθηματικών λογισμικών. Ουσιαστικά η μαθηματική έκφραση της χωρητικότητας των συστημάτων SIMO σε κανάλι διαλείψεων Nakagami-m, ανάγεται στην επίλυση ενός είδους ολοκληρώματος που περιέχει ταυτόχρονα τη λογαριθμική συνάρτηση, την εκθετική συνάρτηση και πολυώνυμα νιοστής δύναμης. Αυτός ο τύπος ολοκληρωμάτων είναι δυσεπίλυτος και προκύπτει συχνά στις ασύρματες επικοινωνίες. Στην τελευταία ενότητα, γίνεται προσπάθεια αύξησης του μέσου όρου της χωρητικότητας του συστήματος ΜΙΜΟ χρησιμοποιώντας πληροφορία από το μέσο διάδοσης. Πιο συγκεκριμένα μελετάται η πολιτική εκπομπής, αν ο πομπός γνωρίζει τις παραμέτρους του καναλιού οι οποίες είναι δυνατό να γνωστοποιηθούν στον πομπό σε ρεαλιστικό επίπεδο. Ως παράμετροι του καναλιού οι οποίες είναι απαραίτητο να είναι γνωστές, θεωρούνται ο μέσος όρος και η διασπορά του καναλιού που είναι δυνατό να μετρηθούν στην πράξη ιδιαίτερα για κανάλια που δε μεταβάλλονται πάρα πολύ γρήγορα στο χρόνο. Το πρόβλημα της μεγιστοποίησης της εργοδικής χωρητικότητας, στην γενική του μορφή έως τώρα αντιμετωπίζεται μόνο με χρονοβόρες υπολογιστικές μεθόδους που απαιτούν αρκετή υπολογιστική ισχύ, καθιστώντας τη λύση μη εφαρμόσιμη σε πραγματικό χρόνο και επομένως μη ρεαλιστική. Το πρόβλημα είναι δυσεπίλυτο και οι μόνες αναλυτικές λύσεις που υπάρχουν αναφέρονται σε ιδιαίτερες περιπτώσεις. Η παρούσα ΔΔ ασχολείται με τη μεγιστοποίηση της εργοδικής χωρητικότητας του συστήματος MISO (Multiple Input-Single Output) το οποίο χρησιμοποιεί την τεχνική beamforming στην εκπομπή. Το πρόβλημα επιλύεται και η λύση του ανάγεται στη λύση ενός συστήματος δύο εξισώσεων το οποίο λύνεται αριθμητικά. Έτσι είναι δυνατή η μεγιστοποίηση της χωρητικότητας σε πραγματικό χρόνο χωρίς ιδιαίτερη υπολογιστική ισχύ. Έως τώρα η προσέγγιση αυτού του προβλήματος γίνεται αποκλειστικά με αλγορίθμους μεγιστοποίησης μη γραμμικού προγραμματισμού. Επιπλέον εξετάζοντας τη λύση του απλού συστήματος , εξάγονται καθολικά συμπεράσματα που εκφράζουν το γενικό πρόβλημα. Για τη μεγιστοποίηση του συστήματος MISO beamforming, απαιτήθηκε η διανυσματική ανάλυση του μέσου όρου του καναλιού και του διανύσματος beamforming του πομπού σε μία κατάλληλη ορθοκανονική βάση. Έτσι το πρόβλημα ανάγεται στην εύρεση των γωνιών που σχηματίζει το διάνυσμα beamforming με την ορθοκανονική βάση ώστε να μεγιστοποιείται η χωρητικότητα για δεδομένες παραμέτρους του καναλιού. Με αυτή τη μέθοδο το πρόβλημα επιλύεται πολύ εύκολα με αριθμητικές μεθόδους. Αυτό δίνει, πέρα από την ίδια τη λύση, τη δυνατότητα να γίνει σύγκριση και με υπάρχουσες μεθόδους που προσέγγιζαν τη λύση, όπως η μεγιστοποίηση του σηματοθορυβικού λόγου (Signal to Noise Ratio, SNR). Επίσης αποδεικνύεται ότι το λαμβανόμενο SNR στο δέκτη επηρεάζει το διάνυσμα beamforming που μεγιστοποιεί την χωρητικότητα. Λαμβάνοντας υπόψη όλα αυτά, προτείνεται ένας κανόνας για την πολιτική εκπομπής του πομπού. Η μεθοδολογία που αναπτύχθηκε μπορεί να βοηθήσει σημαντικά στην επίλυση του γενικότερου προβλήματος της μεγιστοποίησης της χωρητικότητας σε συστήματα ΜΙΜΟ. / 4G Wireless Communication Systems aim at high data rates, 100 Mbps (Mega bits per second) for high speed transceivers and up to 1 Gbps for stationary transceivers. This target can be accomplished with Multiple Input Multiple Output (MIMO) Systems which use multiple antennas at both the transmitter and the receiver. The subject of this Philosophy Diploma (PhD) dissertation focuses on analysis and optimization of these systems, taking into account the effects of small scale fading and scattering which occur in a wireless channel. The subject of this study is summarized in the following thematic units: a) Fading channel modelling b) Closed-form mathematical expressions for the ergodic capacity of wireless systems which use multiple antennas at the receiver c) increase MISO ergodic capacity through channel state information. Initially, the general wireless fading channel model is described which is necessary for the better understanding of the analysis used in this dissertation. This aims at a brief description of the basic characteristics of the wireless channel. Specifically, general theoretical knowledge of propagation channel is presented, giving a description of the phenomena occurring in the channel without presenting complex mathematical expressions. Next, using stochastic procedures, an accurate model of frequency non-selective Rician fading channel with 3 dimensional anisotropic scattering is presented. The term anisotropic means that the arrival of the multipath components comes from some specific sectors and not from any direction. In the elevation plane, we assume a sector for the arrival of the multipath components in which power does not arrive uniformly but follows a specific distribution. In addition, assuming a communication system operating in a Rice fading channel, a dominant component is included which usually represents the Line of Sight (LOS) component between the transmitter and the receiver. Taking into account international literature and assuming specific probability density functions for the angle of arrivals in these sectors, analytical mathematical expressions of the auto-correlation function and the power spectral density of the received signal are derived. Moreover important measures of the level crossing rate and the average duration of fades are calculated. By this analysis, the system designer is able to estimate the optimal distance between antennas in order to assure high performance of the communication system. It is proved that the distance between antennas should be greater in rural than in urban environments. In the next section, accurate closed-form mathematical expressions for the ergodic capacity of SIMO (Single Input Multiple Output) systems in Nakagami fading channel are derived with the help of known and easy to use mathematical functions (Nakagami fading is appropriate for indoor channel modelling). Thus channel capacity is expressed with respect to the physical system parameters such as: amount of fading, number of antennas etc. Many studies have been published for different cases of fading channel models (Rayleigh, Rice, etc) and diversity techniques. But for some cases there are no mathematical expressions for the ergodic capacity or it is expressed in a no closed form way. Thus in this study, new analytical mathematical expressions for the ergodic capacity of SIMO systems with different channel knowledge cases are derived. Also the relation between diversity techniques and SIMO systems is taken into account. We assume that the SIMO system operates in a Nakagami fading channel where each branch is statistically independent but not identically distributed. More precisely, new ergodic capacity formulas for dual Equal Gain Combining, Selection Combining and Switch and Stay Combining techniques are presented. In addition, a new mathematical formula for the ergodic capacity of a SIMO system with no channel knowledge is presented, resulting in useful conclusions. All these mathematical expressions are calculated with mathematical functions that are included in any mathematical software. Essentially, the calculation of the ergodic capacity of SIMO systems in Nakagami fading channels entails the calculation of an integral which contains the logarithmic function, the exponential function and n power polynomials. This type of integral is intractable and arises frequently in wireless communications. In the last section, the ergodic capacity of a MIMO channel using channel state information is studied. In particular, this dissertation studies the transmit strategy if the transmitter knows the statistical parameters of the channel which is feasible in a realistic scenario. The statistical parameters of the channel that have to be transferred to the transmitter are channel mean and covariance. These parameters can be measured in practice especially for low time variant channels. Transmitter optimization problem, in its general form, is tackled only with hard optimization methods which are not feasible for real time applications due to large processing time. The problem is intractable and the only analytical solutions in literature are referred to special cases. The current dissertation studies the ergodic capacity optimization problem of a MISO (Multiple Input-Single Output) system which uses beamforming as its transmit strategy. The problem is solved through a system of two equations which is solved numerically. Thus the problem is extremely simplified and beamforming capacity optimization is feasible even for real time applications. So far this problem was tackled with non linear programming optimization methods. Also examining the solution for the MISO system, it is provided intuition into the problem. Also general results are presented which express the general problem. Beamforming capacity optimization solution was achieved by following an analytical approach that projects the beamforming vector on an appropriate orthonormal basis defined by the eigenvectors of the channel covariance matrix. Thus the problem reduces to calculation of the angles between the beamforming vector and the orthonormal basis which maximize capacity for given channel parameters. Following this method, the problem is solved very easily through numerical root finding algorithms. Besides the solution itself, a comparison against existing approximate solutions is possible, e.g. SNR (Signal to Noise Ratio) maximization solution. It is proved that the optimal beamforming vector is dependent on the received SNR. Taking into account all the arising results, a rule of thumb for the transmit policy is proposed. In addition, the used method can help significantly towards the solution of the MIMO transmitter optimization problem.