The world of information technology and communications is in a stage of continuous improvement. These new developments are accompanied by a very rapid data traffic growth, particularly for mobile wireless networks. An important effect is the increase of the consumed energy by the network, particularly the radio access section, i.e. the base station systems. This latter issue causes that operational costs associated to energy consumption become more significant for telecommunication operators. In addition to the question of the availability of energy, this problem has an ecological aspect, due to the CO2 emissions associated with the operation of the network but also an issue related to public health given the effects of electromagnetic radiation from wireless transmission. For these reasons, great research efforts in the so-called field of Green Radio are being done in the recent years. The aim of this thesis consisted of finding ways to reduce the energy consumption in wireless and mobile networks respecting the QoS levels assigned to users. After a study of the state of the art analysing the possible approaches, we proposed a set of energy efficient techniques supported on the cell breathing and hybrid cellular networks (macro/femtocells). Our proposed decision making mechanisms include distributed strategies and other features such as the use of a novel multimetric approach. In our work, we also take into account the energy consumption of mobile devices, an issue that to the best of our knowledge was not quite analysed so far. The use of renewable energy sources is also considered into our proposals. Our thesis work includes the study of the consequences of our radio resource management algorithms on the electromagnetic radiation taking into account the acceptable EM exposure thresholds. In the perspectives of future work, we can mention the improvement of our techniques with learning mechanisms and cognitive radio.

In the past few years, we have witnessed the continuous growth in popularity of delay-sensitive applications. Applications like live video streaming, multimedia conferencing, VoIP and online gaming account for a major part of Internet traffic these days. It is also predicted that this trend will continue in the coming years. This emphasizes the significance of developing efficient scheduling algorithms in communication networks with guaranteed low delay performance. In our work, we try to address the delay issue in some major instances of wireless communication networks. First, we study a wireless content distribution network (CDN), in which the requests for the content may have service deadlines. Our wireless CDN consists of a media vault that hosts all the content in the system and a number of local servers (base stations), each having a cache for temporarily storing a subset of the content. There are two major questions associated with this framework: (i) content caching: which content should be loaded in each cache? and (ii) wireless network scheduling: how to appropriately schedule the transmissions from wireless servers? Using ideas from queuing theory, we develop provably optimal algorithms to jointly solve the caching and scheduling problems. Next, we focus on wireless relay networks. It is well accepted that network coding can enhance the performance of these networks by exploiting the broadcast nature of the wireless medium. This improvement is usually evaluated in terms of the number of required transmissions for delivering flow packets to their destinations. In this work, we study the effect of delay on the performance of network coding by characterizing a trade-off between latency and the performance gain achieved by employing network coding. More specifically, we associate a holding cost for delaying packets before delivery and a transmission cost for each broadcast transmission made by the relay node. Using a Markov decision process (MDP) argument, we prove a simple threshold-based policy is optimal in the sense of minimum long-run average cost. Finally, we analyze delay-sensitive applications in wireless peer-to-peer (P2P) networks. We consider a hybrid network which consists of (i) an expensive base station-to-peer (B2P) network with unicast transmissions, and (ii) a free broadcast P2P network. In such a framework, we study two popular applications: (a) a content distribution application with service deadlines, and (b) a multimedia live streaming application. In both problems, we utilize random linear network coding over finite fields to simplify the coordination of the transmissions. For these applications, we provide efficient algorithms to schedule the transmissions such that some quality of service (QoS) requirements are satisfied with the minimum cost of B2P usage. The algorithms are proven to be throughput optimal for sufficiently large field sizes and perform reasonably well for finite fields.

Η παρούσα διδακτορική διατριβή ασχολείται με τη μελέτη της τεχνικής μετάδοσης OFDM. Η τεχνική OFDM είναι πια μια ώριμη τεχνολογία με αποδεδειγμένη ικανότητα να προσφέρει υψηλούς ρυθμούς μετάδοσης δεδομένων με ιδιαίτερα μεγάλη αποτελεσματικότητα ακόμα και σε δύσκολα περιβάλλοντα μετάδοσης. Αυτό αντικατοπτρίζεται και από την πολύ μεγάλη διείσδυσή της στην αγορά με ενσωμάτωσή της σε πολλά εμπορικά προϊόντα. Όμως οι σύγχρονες ανάγκες επικοινωνιών επιτάσσουν ακόμα πιο μεγάλη αποτελεσματικότητα στην μετάδοση δεδομένων και πλέον συστήματα και λύσεις που προτάθηκαν πριν 2 με 3 χρόνια θεωρούνται αναποτελεσματικά. Κύριο σημείο για την αύξηση της αποτελεσματικότητας αποτελεί η δυνατότητα προσαρμογής του συστήματος στις συνθήκες μετάδοσης. Οι συνθήκες μετάδοσης χαρακτηρίζονται κυρίως από το ασύρματο κανάλι. Για να επιτευχθεί η μεγαλύτερη δυνατή αποτελεσματικότητα το σύστημα θα πρέπει να είναι ικανό να εκτιμά τα κύρια χαρακτηριστικά του ασύρματου καναλιού και στην συνέχεια να προσαρμόζει ανάλογα τα χαρακτηριστικά μετάδοσης. Ένα τέτοιο προσαρμοστικό σύστημα έχει σημαντικό πλεονέκτημα απέναντι σε ένα μη προσαρμοστικό σύστημα το οποίο αντιμετωπίζει την κάθε περίπτωση με έναν ενιαίο μη βέλτιστο τρόπο. Ένα άλλο σημαντικό πρόβλημα, το οποίο συναντάται σε όλα τα συστήματα και στο OFDM, είναι η διαφορά της απόδοσης των αλγορίθμων ανάμεσα στην εξομοίωση τους σε λογισμικό και στην υλοποίηση τους σε υλικό. Η παρούσα διατριβή έχει τρεις στόχους. Ο πρώτος στόχος είναι να μελετηθεί το κανάλι μετάδοσης και να διερευνηθούν τα κύρια χαρακτηριστικά του. Η εκτίμηση των χαρακτηριστικών αυτών θα αποτελέσει την προσεγγιστική εκτίμηση καναλιού. Στην συνέχεια το σύστημα βασιζόμενο σε αυτά τα χαρακτηριστικά θα μπορεί να προσαρμοστεί κάθε φορά με τον βέλτιστο τρόπο. Αποτέλεσμα αυτής της μελέτης είναι να αποφασιστεί ότι οι συντελεστές που περιγράφουν προσεγγιστικά το κανάλι είναι ο λόγος σήματος προς θόρυβο, η συχνότητα Doppler, η εξάπλωση καθυστέρησης και ο συντελεστής Κ του καναλιού Ricean και να αναπτυχθούν αλγόριθμοι εκτίμησης για κάθε έναν. Μέσω των προτεινόμενων αλγορίθμων το σύστημα θα μπορεί να εκτιμήσει τις συνθήκες μετάδοσης και να προσαρμοστεί κατάλληλα. Ο δεύτερος στόχος της διατριβής είναι η μελέτη της ακριβούς εκτίμησης καναλιού και η ενίσχυση της απόδοσης του συστήματος μέσω αυτής. Για την μελέτη αυτή επιλέχθηκαν δύο διαφορετικοί τρόποι προσέγγισής της. Η πρώτη προσέγγιση ασχολείται με την λεπτομερή εκτίμηση χαρακτηριστικών του καναλιού όπως ο αριθμός των διαδρομών του και η χρονική τοποθέτησή τους. Στην συνέχεια αυτά τα στοιχεία χρησιμοποιούνται για την εκτίμηση των συντελεστών του καναλιού. Η δεύτερη προσέγγιση ασχολείται με την εκτίμηση καναλιού με την χρήση τεχνικών παρεμβολής. Η μελέτη των τεχνικών παρεμβολής οδήγησε στον προσδιορισμό των βασικών προβλημάτων τους και στην πρόταση νέων τεχνικών εκτίμησης καναλιού. Αρχικά προτείνεται μία νέα μέθοδος εκτίμησης μέσω αντικατάστασης των εικονικών υποφορέων του συστήματος με πιλότους. Για την μέθοδο αυτή δίνεται μια νέα μαθηματική ανάλυση η οποία προσδιορίζει τα χαρακτηριστικά της απόδοσής του. Τέλος η απόδοση της εξετάζεται σε πρακτικό OFDM σύστημα σε ιδιαίτερα δύσκολες συνθήκες. Στην συνέχεια προτείνεται μία νέα μέθοδος εκτίμησης καναλιού μέσω παρεμβολής η οποία χρησιμοποιεί παραπάνω από ένα σύμβολα OFDM για την εκτίμηση του καναλιού. Για την μέθοδο αυτή δίνεται μία νέα μαθηματική ανάλυση η οποία εμπεριέχει την επίδραση της χρονικής μεταβολής του καναλιού στην απόδοση του συστήματος. Η απόδοση και αυτής της μεθόδου εξετάζεται σε πρακτικό OFDM σύστημα σε δύσκολες συνθήκες. Τέλος ο τρίτος στόχος της διατριβής είναι η μελέτη της διαδικασίας υλοποίησης και βελτιστοποίησης των προτεινόμενων αλγορίθμων εκτίμησης ρεαλιστικά συστήματα υλικού. Για τον λόγο αυτό επιλέχθηκε η υλοποίηση μέρους των προηγούμενων προτεινόμενων αλγορίθμων σε ένα σύστημα ψηφιακής επεξεργασίας, το οποίο είναι πολύ κοντά στο να θεωρείται μια ρεαλιστική υλοποίηση. Έτσι είναι εφικτή η μελέτη των επιλογών της υλοποίησης και των συμβιβασμών της για την βέλτιστη απόδοση του συστήματος. / This dissertation deals with the study of Orthogonal Frequency Division Multiplexing (OFDM) transmission. OFDM is a mature technology with a proven ability to offer high data transmission rates with particularly big effectiveness even in difficult transmission environments. This is also reflected by its very big infiltration in the market with its incorporation in a lot of commercial products. However the modern needs of communications ordain even bigger effectiveness in data transmission and henceforth systems and solutions that have been proposed 2 or 3 years before are considered ineffective. Main point for the achievement of increase in the effectiveness of the system constitutes the ability of adaptation of the system in the transmission conditions. The transmission conditions are mainly characterized by the wireless channel. In order to achieve the highest effectiveness the system must be able to estimate the channel characteristics and to adjust its transmission characteristics accordingly. Such an adaptive system has a significant advantage compared to a non adaptive system which faces each case with a uniform and not optimal way. Another important problem that all communication systems face, including OFDM, is the difference in the performance of an algorithm between its software simulation and it hardware implementation. The present dissertation has three objectives. The first objective is to study the transmission channel and investigate its main characteristics. The estimation of these characteristics will constitute the coarse channel estimation. Then the system will be able to adapt itself optimally. The result of this study is to decide that the factors that coarsely describe the channel are the signal to noise ratio, the Doppler frequency, delay spread and the Ricean channel K factor. Then new algorithms were developed to estimate each one of these factors. Through the proposed algorithms the system will be able to estimate the transmission conditions and be adapted suitably. The second objective of this dissertation is to study the exact channel estimation and the way that the system performance can be enhanced through it. For this study two different approaches were selected. The first approach deals with the detailed estimation of the channel characteristics such as the number of the paths and their placement in time. Then these elements are used to get the channel estimation coefficients. The second approach deals with the channel estimation through interpolation. The study of interpolation methods led to the determination of their basic problems and the proposal of new channel estimation techniques. Firstly a novel channel estimation is proposed through the replacement of virtual subcarriers with pilots. For this method a novel mathematic analysis is given that determines its performance characteristics. Finally its performance is examined in a practical OFDM system in particularly difficult conditions. Then another novel channel estimation method through interpolation is proposed that uses more than one OFDM symbol to estimate the channel. For this novel method a novel mathematic analysis is given which includes the effect of the time variant channel. The performance of this method is also examined in a practical OFDM system in particularly difficult conditions. Finally the third objective is to study the implementation and optimization process of the proposed estimation algorithms in realistic hardware systems. For this it was selected to implement a part of the previously proposed algorithms in a digital signal processing system that is very close to be considered a realistic implementation. This way it is feasible to study the implementation choices that have to be made and the trade offs for the optimum performance of the system.

abstract: To establish reliable wireless communication links it is critical to devise schemes to mitigate the effects of the fading channel. In this regard, this dissertation analyzes two types of systems: point-to-point, and multiuser systems. For point-to-point systems with multiple antennas, switch and stay diversity combining offers a substantial complexity reduction for a modest loss in performance as compared to systems that implement selection diversity. For the first time, the design and performance of space-time coded multiple antenna systems that employ switch and stay combining at the receiver is considered. Novel switching algorithms are proposed and upper bounds on the pairwise error probability are derived for different assumptions on channel availability at the receiver. It is proved that full spatial diversity is achieved when the optimal switching threshold is used. Power distribution between training and data codewords is optimized to minimize the loss suffered due to channel estimation error. Further, code design criteria are developed for differential systems. Also, for the special case of two transmit antennas, new codes are designed for the differential scheme. These proposed codes are shown to perform significantly better than existing codes. For multiuser systems, unlike the models analyzed in literature, multiuser diversity is studied when the number of users in the system is random. The error rate is proved to be a completely monotone function of the number of users, while the throughput is shown to have a completely monotone derivative. Using this it is shown that randomization of the number of users always leads to deterioration of performance. Further, using Laplace transform ordering of random variables, a method for comparison of system performance for different user distributions is provided. For Poisson users, the error rates of the fixed and random number of users are shown to asymptotically approach each other for large average number of users. In contrast, for a finite average number of users and high SNR, it is found that randomization of the number of users deteriorates performance significantly. / Dissertation/Thesis / Ph.D. Electrical Engineering 2010

A melhora na eficiência do uso do espectro de radiofrequência é fundamental para permitir um desempenho adequado dos diversos sistemas sem fio cuja complexidade e requerimentos aumentam a cada dia. O panorama atual de atribuição de canais é estático, tolerando assim, que o espectro de radiofrequência seja usado de forma desequilibrada, gerando com isso, problemas de coexistência em algumas faixas e subutilização de recursos em outras. Para contornar esse problema, tem sido proposta a ideia de introduzir algoritmos de cognição nos dispositivos sem fio, a fim de permitir um modelo de alocação dinâmico adicional. Neste, os usuários secundários equipados com rádios cognitivos podem utilizar de forma dinâmica os canais subutilizados de usuários primários. Um usuário primário tem prioridade de uso do canal como consequência da atribuição estática, porém, o uso do canal por um usuário secundário qualquer é oportunista e limitando ao tempo de inatividade do usuário primário em um determinado local. As redes de sensores sem fio trabalham em uma banda concorrida e são sistemas que podem melhorar seu desempenho utilizando um mecanismo de acesso dinâmico aos canais, possibilitando o aproveitamento dos períodos de inatividade de usuários primários ou aumentando sua capacidade de coexistência na banda de operação atual. Segundo a literatura pesquisada são vários os desafios existentes para conseguir um método distribuído de acesso dinâmico aos canais que considere as restrições de trocas de sinalizações, consumo de energia e complexidade dos dispositivos de uma rede de sensores sem fio. Neste contexto, propõe-se uma metodologia de sensoriamento e acesso dinâmico aos canais para uma rede de sensores sem fio considerando a simplicidade dos dispositivos. Como fatores de inovação, optou-se pela definição de uma política de sensoriamento por clusters que permite realizar uma aprendizagem cooperativa por reforço da situação dos canais de operação. Além disso, a definição de um mecanismo de acesso dinâmico aos canais fundamentado no padrão IEEE 802.15.4 permite comunicação e coordenação distribuída de forma assíncrona. O funcionamento da metodologia proposta é avaliado e comparado usando simulações e experimentos mediante um estudo de caso específico. As comparações são realizadas com métodos de seleção de canal: fixa, cega ou baseada em recompensas por acesso. Os resultados mostram a eficiência no acesso dinâmico aos canais com aumentos na taxa de entrega de mensagens e na capacidade de coexistir com as redes primárias. / The efficiency improvement of the use of radiofrequency spectrum is fundamental to allow more complex and more optimal wireless systems. The current channel allocation is static. It tolerates unbalanced use of the radiofrequency spectrum generating coexistence problems in some bands and underutilization of resources in other bands. The introduction of cognitive algorithms into wireless devices has been proposed to overcome that problem, in order to allow an additional dynamic allocation model. In this, the secondary users equipped with cognitive radios will be able to use dynamically the underutilized channels of primary users. A primary user has channel usage priority related to the static allocation, on the other hand, the use of the channel by any secondary user is opportunistic and limited to the inactivity time of the primary user in a specific place. Wireless sensor networks work in a competitive band. These systems can improve their performance using a dynamic access to the channels and consequently to enable the utilization of inactivity periods of primary users or to increase the coexistence capability at their current operation band. According to the researched literature, several challenges exist to find a distributed method for dynamic access to the channels considering the restrictions on control signaling, energy consumption and computational complexity of wireless sensor network devices. In this context, a methodology of sensing and dynamic access to the channels in a wireless sensor network considering the restrictions of the devices is proposed. As innovation, we opted for the definition of a sensing policy by clusters that allows the cooperative reinforcement learning of the situation of channels. In addition, a mechanism for dynamic access to the channels based on the IEEE 802.15.4 standard is defined to allow asynchronous and distributed coordination. The behavior of the proposed methodology is evaluated and compared using simulations and experiments through a specific case study. The comparisons are performed with channel selection methods: fixed, blind and access based. The results show good efficiency in the dynamic allocation of the channels, increasing the message delivery rate and the coexistence capability.

This thesis provides novel contribution to the field of body-centric wireless communications (BCWC) with the development of a measurement methodology for wearable antenna characterisation on the human body, the implementation of fully-textile wearable antennas and the on-body channel modelling considering different antenna types and user's dynamic effects. More specifically, a measurement methodology is developed for characterising wearable antennas on different locations of the human body. A cylindrical near-field (CNF) technique is employed, which facilitates wearable antenna measurements on a full-body solid anthropomorphic mannequin (SAM) phantom. This technique allows the fast extraction of the full spherical radiation pattern and the corresponding radiation efficiency, which is an important parameter for optimising wearable system design. It appears as a cost- effective and easy to implement solution that does not require expensive positioning systems to rotate the phantom, in contrast to conventional roll-over-azimuth far-field systems. Furthermore, a flexible fully-textile wearable antenna is designed, fabricated and measured at 2.4 GHz that can be easily integrated in smart clothing. It supports surface wave propagation and exhibits an omni-directional radiation pattern that makes it suitable for on-body communications. It is based on a multilayer low-profile higher-mode patch antenna (HMMPA) design with embroidered shorting vias. Emphasis is given to the fabrication process of the textile vias with conductive sewing thread that play an important role in generating the optimal mode for on-body radiation. The radiation pattern shape of the proposed fully-textile antenna was found to be similar to a copper rigid antenna, exhibiting a high on-body radiation efficiency of 50 %. The potential of the embroidery technique for creating wearable antennas is also demonstrated with the fabrication of a circularly polarised spiral antenna that achieves a broadband performance from 0.9-3 GHz, which is suitable for off-body communications. By testing the textile spiral antenna on the SAM phantom, the antenna-body interaction is examined in a wide frequency range. Finally, a statistical characterisation of on-body communication channels is undertaken both with EM simulations and channel measurements including user's dynamic movement (walking and running). By using antenna types of different polarisation, the on-body channels are examined for different propagation conditions. Four on-body channels are examined with the one part fixed on the waist of the human body while the other part located on the chest, back, wrist and foot. Channel path gain is derived, while large-scale and small-scale fading are modelled by best-fit statistical distributions.

A melhora na eficiência do uso do espectro de radiofrequência é fundamental para permitir um desempenho adequado dos diversos sistemas sem fio cuja complexidade e requerimentos aumentam a cada dia. O panorama atual de atribuição de canais é estático, tolerando assim, que o espectro de radiofrequência seja usado de forma desequilibrada, gerando com isso, problemas de coexistência em algumas faixas e subutilização de recursos em outras. Para contornar esse problema, tem sido proposta a ideia de introduzir algoritmos de cognição nos dispositivos sem fio, a fim de permitir um modelo de alocação dinâmico adicional. Neste, os usuários secundários equipados com rádios cognitivos podem utilizar de forma dinâmica os canais subutilizados de usuários primários. Um usuário primário tem prioridade de uso do canal como consequência da atribuição estática, porém, o uso do canal por um usuário secundário qualquer é oportunista e limitando ao tempo de inatividade do usuário primário em um determinado local. As redes de sensores sem fio trabalham em uma banda concorrida e são sistemas que podem melhorar seu desempenho utilizando um mecanismo de acesso dinâmico aos canais, possibilitando o aproveitamento dos períodos de inatividade de usuários primários ou aumentando sua capacidade de coexistência na banda de operação atual. Segundo a literatura pesquisada são vários os desafios existentes para conseguir um método distribuído de acesso dinâmico aos canais que considere as restrições de trocas de sinalizações, consumo de energia e complexidade dos dispositivos de uma rede de sensores sem fio. Neste contexto, propõe-se uma metodologia de sensoriamento e acesso dinâmico aos canais para uma rede de sensores sem fio considerando a simplicidade dos dispositivos. Como fatores de inovação, optou-se pela definição de uma política de sensoriamento por clusters que permite realizar uma aprendizagem cooperativa por reforço da situação dos canais de operação. Além disso, a definição de um mecanismo de acesso dinâmico aos canais fundamentado no padrão IEEE 802.15.4 permite comunicação e coordenação distribuída de forma assíncrona. O funcionamento da metodologia proposta é avaliado e comparado usando simulações e experimentos mediante um estudo de caso específico. As comparações são realizadas com métodos de seleção de canal: fixa, cega ou baseada em recompensas por acesso. Os resultados mostram a eficiência no acesso dinâmico aos canais com aumentos na taxa de entrega de mensagens e na capacidade de coexistir com as redes primárias. / The efficiency improvement of the use of radiofrequency spectrum is fundamental to allow more complex and more optimal wireless systems. The current channel allocation is static. It tolerates unbalanced use of the radiofrequency spectrum generating coexistence problems in some bands and underutilization of resources in other bands. The introduction of cognitive algorithms into wireless devices has been proposed to overcome that problem, in order to allow an additional dynamic allocation model. In this, the secondary users equipped with cognitive radios will be able to use dynamically the underutilized channels of primary users. A primary user has channel usage priority related to the static allocation, on the other hand, the use of the channel by any secondary user is opportunistic and limited to the inactivity time of the primary user in a specific place. Wireless sensor networks work in a competitive band. These systems can improve their performance using a dynamic access to the channels and consequently to enable the utilization of inactivity periods of primary users or to increase the coexistence capability at their current operation band. According to the researched literature, several challenges exist to find a distributed method for dynamic access to the channels considering the restrictions on control signaling, energy consumption and computational complexity of wireless sensor network devices. In this context, a methodology of sensing and dynamic access to the channels in a wireless sensor network considering the restrictions of the devices is proposed. As innovation, we opted for the definition of a sensing policy by clusters that allows the cooperative reinforcement learning of the situation of channels. In addition, a mechanism for dynamic access to the channels based on the IEEE 802.15.4 standard is defined to allow asynchronous and distributed coordination. The behavior of the proposed methodology is evaluated and compared using simulations and experiments through a specific case study. The comparisons are performed with channel selection methods: fixed, blind and access based. The results show good efficiency in the dynamic allocation of the channels, increasing the message delivery rate and the coexistence capability.

Orientadores: Michel Daoud Yacoub, Ugo Silva Dias / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação / Made available in DSpace on 2018-08-20T07:39:19Z (GMT). No. of bitstreams: 1 Krishnan_Aravind_M.pdf: 14799172 bytes, checksum: ef1f7ee0181d66e5163ba0926643ca9e (MD5) Previous issue date: 2011 / Resumo: Esta tese apresenta os resultados de medidas de campo conduzidas na freqüência de 5.5 GHz, com o objetivo de caracterizar os parâmetros do modelo de desvanecimento alpha-mu. Mais especificamente, uma quantidade de situações é investigada para se determinar a densidade de probabilidade e a função de autocorrelação destes parâmetros. O range de valores possíveis são então sugeridos baseado nos dados empíricos. Adicionalmente, as variações instantâneas da magnitude dos parâmetros correspondentes são mostradas em função do deslocamento do receptor ao longo do percurso. Os resultados provêem informações importantes sobre a utilidade prática do modelo alpha-mu mostrando, em seguida, que as estatísticas do sinal recebido se distanciam bem daquelas dos modelos conhecidos / Abstract: This thesis presents the results of field trial measurements conducted at a frequency of 5.5 GHz in order to characterize the parameters of the !-? fading model. More specifically, a number of situations are investigated in which the probability density function and the autocorrelation function of these fading parameters are described. The ranges of possible practical values of the parameters are then suggested as an outcome of the empirical data. In addition, the instantaneous magnitude variations of corresponding parameters are shown as a function of the receiver position along the path. The results provide important information about the practical usefulness of the !-? fading model, showing, in addition, that the received signal statistics greatly departs from the well known statistical models / Mestrado / Telecomunicações e Telemática / Mestre em Engenharia Elétrica

Les futurs réseaux sans fil devront être conçus pour répondre aux besoins hétérogènes de systèmes entièrement différents. De nouveaux services soumis à des contraintes variées coexisteront avec les utilisateurs actuels sur la même bande de fréquences. L'OFDM, la couche physique utilisée par les systèmes actuels, souffre d’un mauvais confinement spectral et ne permet pas cette coexistence. De nombreuses nouvelles formes d'onde avec une localisation spectrale améliorée ont donc été proposées. Nous étudions la coexistence de nouveaux systèmes basés sur ces formes d'onde avec des utilisateurs OFDM préexistant. Nous fournissons la première analyse théorique et expérimentale de l'interférence inter-système qui se produit dans ces scenarios. Nous appliquons ensuite cette analyse pour évaluer les performances de différentes formes d'ondes avancées et nous étudions finalement les performances d'un réseau où des utilisateurs cellulaires OFDM coexistent avec des paires D2D utilisant l'une des formes d'ondes améliorées étudiées. / Future wireless networks are envisioned to accommodate the heterogeneous needs of entirely different systems. New services obeying various constraints will coexist with legacy cellular users in the same frequency band. This coexistence is hardly achievable with OFDM, the physical layer used by current systems, because of its poor spectral containment. Thus, a myriad of multi-carrier waveforms with enhanced spectral localization have been proposed for future wireless devices. In this thesis, we investigate the coexistence of new systems based on these waveforms with legacy OFDM users. We provide the first theoretical and experimental analysis of the inter-system interference that arises in those scenarii. Then, we apply this analysis to evaluate the merits of different enhanced waveforms and we finally investigate the performance achievable by a network composed of legacy OFDM cellular users and D2D pairs using one of the studied enhanced waveforms.

The electromagnetic wave absorption characteristics of composite cement-based building material have attracted much interest in recent times. Researchers have mainly focused on the 2 GHz to 12 GHz frequency range. Mobile and wireless communication systems use frequencies from 800 MHz upwards. The determination of characteristics such as reflection loss, absorption, attenuation and shielding effectiveness are crucial in the evaluation and development of these materials for the building industry. Absorption is an indication of how much of the EMW energy enters the material. Attenuation indicates how much of the absorbed energy is converted into other forms of energy by the material. Shielding effectiveness (SE) is a combination of reflection loss, attenuation and multiple internal reflections and attenuations. This research determined these characteristics by measuring the S11 and S21 parameters of the composite cement-based material in the GSM and WiFi frequency bands. The time domain gating function of a vector network analyser is applied to measure the reflection from the material. The data was then used to obtain the reflection and absorption losses in the frequency bands. The transmission loss was measured by placing the sample in the propagation path between two antennas. MnZn-ferrite and electrolytic manganese dioxide in powder form were evaluated as absorber material to increase the permeability of the cement-based material to improve absorption and attenuation capabilities to create a cost-effective practical electromagnetic wave absorber. The compound of the cement-based material was cement, sifted river sand and filler powder. The results achieved in the research showed the uniqueness of electrolytic manganese dioxide as filler in composite cement based material for electromagnetic wave shielding effectiveness improvement. The combined measurement techniques used in this research were uniquely used to determine the required electromagnetic wave absorption characteristics and shielding effectiveness of 10 dB was measured in the GSM850 and GSM900 frequency bands. / Dissertation (MEng)--University of Pretoria, 2013. / gm2014 / Electrical, Electronic and Computer Engineering / unrestricted