Η μετάδοση δεδομένων µέσω ασύρματων δικτύων αποτελεί ένα από τα σημαντικότερα αντικείμενα μελέτης στον χώρο των τηλεπικοινωνιών τα τελευταία χρόνια. Λόγω του μεγάλου όγκου δεδομένων που απαιτείται για μετάδοση μέσα σε πολύ μικρό χρονικό διάστημα, απαιτείται η εφαρμογή όσο το δυνατόν αποδοτικότερης κωδικοποίησης και διαμόρφωσης, σκοπεύοντας παράλληλα στην επίτευξη μικρής κατανάλωσης ισχύος και ταυτόχρονα υψηλού throughput. Η παρούσα διπλωματική εργασία πραγματεύεται τη μελέτη, σχεδίαση και την υλοποίηση της αρχιτεκτονικής ενός OFDM συστήματος, βασισμένο στις προδιαγραφές που ορίζει το πρότυπο IEEE802.11. Το πρότυπο αυτό επιλέχθηκε, καθώς τυγχάνει ευρείας αποδοχής, όσον αφορά τη μετάδοση δεδομένων σε ασύρματα δίκτυα. Στα πλαίσια της διπλωματικής παρουσιάζονται εξομοιώσεις και μετρήσεις για την εύρεση των κατάλληλων χαρακτηριστικών και του τύπου των φίλτρων που χρησιμοποιούνται σε ένα OFDM σύστημα, ενώ εισάγονται δύο νέες αρχιτεκτονικές του τμήματος κατασκευής του OFDM συμβόλου, καθώς και τα αποτελέσματα της σύνθεσης αυτών. Ιδιαίτερη βαρύτητα δίνεται στη σχεδίαση και την υλοποίηση των αρχιτεκτονικών αυτών με σκοπό την ενσωμάτωσή τους σε συσκευές FPGA. / Transmission of data through wireless networks is one of the most important aspects in the study telecommunications systems. The large volume of data that needs to be transmitted in a very small time interval has resulted in the need to make the system more efficient while increasing the throughput. This has been accomplished through more efficient coding, reduction of the power consumption and through the use of modulation. This masters thesis deals with the study, design and implementation of the typical OFDM system architecture, based on the standard IEEE802.11. This standard was chosen because of its wide acceptance and use regarding transmission of data in wireless networks. The thesis details the adjustments and measurements needed for determining suitable characteristics and the types of filters required in an OFDM system. Additionally two new OFDM system architectures arte introduced aiming at reducing the overall power consumption and the complexity of the formulae used for symbol construction. Particular importance is given in the designing and the implementation of these architectures regarding their incorporation in FPGA devices.

No description available.

When placed between the antenna and receiver electronics of a cellular base transceiver station, a Cryogenic Receiver Front End (CRFE), consisting of a High Temperature Superconducting (HTS) filter and modern Low Noise Amplifier (LNA), can significantly improve the base stations' coverage and capacity. Due to CRFEs being hurried to the telecommunications industry in a competitive market, the development of CRFEs and their performance have been classified. This left it to be pondered whether HTS filters could really have been beneficial or if they were always just of academic interest. It is the main objective of this thesis to investigate if and under what circumstances high temperature RF-superconductivity can prove to be an important technological contribution to current and future wireless communications. This dissertation presents the analysis of an existing CRFE developed by Cryoelectra GmbH and its performance characteristics measured in a field trial held in rural China. With the aid of a CDMA Uplink Model developed by the author, the data was analysed and several novel engineering improvements were made to create an advanced CRFE which was economical to deploy. The analysis of results from a field trial in Beijing city using the CDMA Uplink Model led to the exploration of alternative filter technologies which could achieve similar results to the HTS filter technology. This culminated in the development of dielectric resonators filters which could be used as an alternative and as a supplement to the HTS filters used in the CRFE. The design of two novel dielectric resonator duplexers and two advanced multioperator combiner antenna sharing solutions followed the successful implementation of a high performance dielectric resonator filter. The performed investigation and development described in this thesis suggest that HTS filter technology for terrestrial wireless communications can be beneficial in current cellular networks, but due to its high cost is economical for use only under certain conditions. However, HTS filter technology may be of great importance in the design and implementation of spectrum friendly wireless communications systems in the future.

Τα πλεονεκτήματα που προκύπτουν από τη χρησιμοποίηση πολλαπλών κεραιών εκπομπής και πολλαπλών κεραιών λήψης (Multiple Input Multiple Output, MIMO) στις τηλεπικοινωνίες προκάλεσαν εκτεταμένο ενδιαφέρον μελέτης από την επιστημονική κοινότητα και υιοθετήθηκαν σε πολλές εφαρμογές της καθημερινής μας ζωής. Στο Κεφάλαιο 2 περιγράφονται με συντομία τα πλεονεκτήματα της τεχνολογία MIMO και πώς μπορεί να βελτιώσει τη ποιότητα επικοινωνίας. Όπως σε κάθε τεχνολογία υπάρχουν και οι αντίστοιχοι περιορισμοί που δημιουργούνται λόγω διαφόρων φαινομένων μετάδοσης των ηλεκτρομαγνητικών κυμάτων όπως διαλείψεις (fading), σκίαση (shadowing) και απώλειες διαδρομής (path loss). Τα φαινόμενα αυτά είναι απαραίτητο να μελετηθούν προκειμένου να αναπτυχθούν καλύτεροι αλγόριθμοι επικοινωνίας πομπού και δέκτη βελτιώνοντας τη ποιότητα επικοινωνίας. Για τη μελέτη αυτών των φαινομένων είναι απαραίτητη η ανάπτυξη διάταξης πομπού - δέκτη (test-bed) που υιοθετεί τη τεχνολογία MIMO και επιτρέπει τη περεταίρω μελέτη του συστήματος. Στο Κεφάλαιο 3 αναφέρονται τα βασικά χαρακτηριστικά της σχεδίασης ενός ασύρματου συστήματος. Αναλύεται η δημιουργία των προδιαγραφών βάσει των απαιτήσεων της εφαρμογής και η επιλογή των χαρακτηριστικών στο πομπό και στο δέκτη. Επιπλέον γίνεται εκτεταμένη αναφορά στους περιορισμούς και στα προβλήματα που δημιουργούνται σε αναλογικά συστήματα πομπών – δεκτών και τρόποι αύξησης της απόδοσής τους. Στο Κεφάλαιο 4 περιγράφεται το υλοποιημένο ενσύρματο σύστημα τεχνολογίας ΜΙΜΟ και οι τροποποιήσεις που έγιναν για την ενσωμάτωση του ασύρματου RF υποσυστήματος. Στη συνέχεια αναλύεται η αρχική μελέτη του RF υποσυστήματος και τα αναμενόμενα χαρακτηριστικά του με τη βοήθεια προσομοιώσεων (simulations). Τέλος παρατίθεται η τελική συσκευή που αναπτύχθηκε, τα θέματα που παρουσιάστηκαν κατά τη διάρκεια κατασκευής τους αλλά και τρόποι αντιμετώπισής τους και παραδοχές που έγιναν. Το Κεφάλαιο 5 αναφέρεται στις μετρήσεις που έγιναν σε πραγματικές συνθήκες. Οι μετρήσεις αφορούν αξιολόγηση των αναλογικών συσκευών πομπού – δέκτη ξεχωριστά αλλά και όλο το σύστημα μαζί. Το Κεφάλαιο 6 κάνει αναφορά στο τελικό χαμηλοπερατό ενεργό φίλτρο που αναπτύχτηκε για τη συγκεκριμένη εφαρμογή ενώ το Κεφάλαιο 7 περιγράφει το σχεδιασμό ενός αποκλειστικού, ειδικά σχεδιασμένου, τροφοδοτικού για τις ανάγκες τροφοδοσίας των συσκευών. / The benefits arising from the use of multiple transmitter antennas and multiple receiver antennas (Multiple Input Multiple Output, MIMO) in the field of telecommunications caused extensive interest and became subject of study by the scientific community and adopted in many applications in our daily life. Chapter 2 describes briefly the advantages of MIMO technology and how it can improve the quality of communication. As with any technology there are restrictions caused by various transmission phenomena of electromagnetic waves such as fading, shadowing and path loss. These phenomena need to be studied in order better algorithms can be developped and improve communication quality. A transmitter – receiver set up which adopts MIMO technology (test-bed) is necessary in order to study these phenomena and enable further research in the system. In chapter 3 the basic design concepts of a wireless system are presented. Thorough analysis has been made in many issues such as the creation of specifications based on application requirements, the selection of features in the transmitter and receiver, the limitations and problems arising in analog systems and ways to increase their performance. Chapter 4 describes the implemented wired MIMO system, the changes made to integrate the wireless RF subsystem, the initial study of the RF subsystem and the expected characteristics according the simulations. Finally the device developped is presented along with the issues encountered during construction and the solutions found to resolve them. Chapter 5 refers to measurements made in the field (indoor conditions). The evaluation of the analog devices transmitter and receiver has been done both separately and the whole system. Chapter 6 refers to the final low pass active filter that was developed for the specific application while Chapter 7 describes the design of a unique, specially designed, power supply for the power needs of the devices.

Ces travaux de thèse s'inscrivent dans la thématique des objets connectés, désormais connue sous le nom de Internet of Things (IoT). Elle trouve son origine dans la démocratisation d'Internet depuis le début des années 2000 et la migration vers des appareils hautement mobiles, rendue possible grâce à la miniaturisation des systèmes embarqués. Dans ce contexte, l'efficacité énergétique est primordiale puisque les projections actuelles parlent de dizaines de milliards de composants connectés à l'horizon 2020. Or pour une question de facilité de déploiement et d'usage, une grande partie des échanges de données dans ces réseaux s'effectue via une liaison sans-fil dont l'implémentation représente une part importante de la consommation. Effectivement, la question de l'efficacité énergétique est en général considérée comme un problème de perfectionnement des architectures matérielles, souvent associé à une évolution favorable de la technologie. Toutefois, ce paradigme atteint rapidement ses limites puisqu'il implique nécessairement un dimensionnement fortement contraint pour être compatible avec les pires conditions d'utilisation, même si elles ne sont pas effectives la plupart du temps. C'est typiquement le cas avec les communications sans-fil puisque le canal radio est un milieu caractérisé par une forte variabilité en raison des phénomènes de propagation et de la présence d'interférences. Notre étude a donc porté sur la conception d'une chaîne de transmission dont le budget de liaison peut être dynamiquement modifié en fonction de l'atténuation réelle du signal, afin de réduire la consommation du système. La thèse a notamment contribué à la mise au point d'un récepteur auto-adaptatif spécifique à la norme IEEE 802.15.4, en proposant à la fois une architecture de modem numérique reconfigurable et à la fois une méthode de contrôle automatique du point de fonctionnement. Plus précisément, le travail s'est appuyé sur deux approches, l'échantillonnage compressif et l'échantillonnage partiel, pour réduire la taille des données à traiter, diminuant ainsi l'activité interne des opérateurs arithmétiques. En contrepartie, le processus de démodulation nécessite un SNR supérieur, dégradant la sensibilité du récepteur et donc le budget de liaison. Cette solution, portée sur une technologie STMicroelectronics CMOS 65 nm LP, offre une faible empreinte matérielle vis-à-vis d'une architecture classique avec seulement 23,4 kcellules. Grâce au modèle physique du circuit qui a été développé, la consommation pour la démodulation d'un paquet est estimée à 278 uW lorsque le modem est intégralement utilisé. Elle peut toutefois être abaissée progressivement jusqu'à 119 uW, correspondant à une baisse de la sensibilité de 10 dB. Ainsi, le modem implémenté et sa boucle de contrôle permettent d'économiser en moyenne 30 % d'énergie dans un cas d'utilisation typique. / This thesis work takes part in the connected objects theme, also known as the Internet of Things (IoT). It emerges from the Internet democratization since the early 2000's and the shift to highly mobile devices, made possible by the miniaturization of embedded systems. In this context, the energy efficiency is mandatory since today's projections are around tens of billions of connected devices in 2020. However for ease of deployment and usage, a large part of the data transfers in these networks is wireless, which implementation represents a significant part of the power consumption. Indeed, the energy efficiency question is addressed in general as a fine tuning of hardware architectures, which is often associated with a favorable technology evolution. Nevertheless, this design paradigm quickly reached its limits since it necessary implies a highly constrained sizing to be compatible with the worst operating conditions, even if they are not effective most of the time. It's typically the case with wireless communications since the radio channel is a medium characterized by a strong variability due to propagations effects and interferences. Thus, our study focused on the design of a communication chain whose link budget can be dynamically tuned depending on the actual signal attenuation, in order to reduce the system power consumption. The thesis has contributed to the design of a self-adaptive receiver dedicated to IEEE 802.15.4 standard, by proposing both a reconfigurable digital baseband architecture and an automatic control method of the operating mode. More precisely, the work relied on two approaches, the compressive sampling and the partial sampling, to reduce the data's size to process, decreasing the internal activity of arithmetics operators. In return, the demodulation processing needs a higher SNR, degrading in the same time the receiver sensitivity and thus the link budget. This solution, implemented in an STMicroelectronics CMOS 65 nm LP process, offers a low hardware overhead compared to conventional architecture with only 23,4 kgates. Thanks to the circuit physical model that has been developed, the power consumption for a packet demodulation is estimated to 278 uW when the baseband is fully activated. It can however be gradually decreased down to 119 uW, corresponding to a sensitivity reduction of 10 dB. Thus, the proposed digital baseband and its control loop save 30 % of energy in average in a typical use case.

A evolução dos sistemas de comunicação sem fio ou wireless apresentam até agora três gerações em menos de duas décadas. Fazendo parte desta evolução estão os sistemas de telefonia celular e os emergentes Sistemas de Comunicação Pessoal ou PCS (Personal Communications Systems). Este trabalho visa apresentar duas questões importantes na evolução destes sistemas, primeiro a questão da segurança e segundo a questão da interconexão com redes heterogêneas. No trabalho, abordando a questão de segurança, são estudados, analisados e comparados os mecanismos de autenticação e privacidade implementados nos atuais padrões de telefonia celular digital e analógico, como sistemas AMPS (celular analógico americano), USCD (celular digital americano) e GSM (celular digital europeu). São identificadas a vulnerabilidade e os fraudes mais comuns nestes sistemas. Também são analisados as propostas das recentes pesquisas e o state of art em termos de segurança, para os sistemas PCS emergentes. Como conclusão apresentado um quadro comparativo resumindo as principais características de segurança adotados pelos sistemas abordados neste estudo. Uma vez apresentado os aspectos de segurança em forma isolada para cada um dos sistemas acima mencionados, é apresentado a questão segurança no contexto da interconexão com redes heterogêneas. A interconexão com redes heterogêneas é outro problema a ser resolvido na implementação da terceira geração de sistemas wireless, para fornecer o roaming automático e a mobilidade pessoal e de terminal. Neste trabalho são revisados os protocolos de sinalização SS7 (Signaling System Number 7) e MAP (Mobile Application Part), como requisitos importantes na solução da interconexão e interoperabilidade entre as redes fixas atuais com as futuras redes móveis. Como conclusão deste estudo é apresentado a proposta de uma arquitetura de protocolos de sinalização, representando a interconexão de um sistema PCS baseado no padrão J-STD-007, com a rede de telefonia celular IS-95. Esta interconexão é realizada através de uma interface de rede denominada IIF (Interworking and Interoperability Function) usando protocolos de sinalização por canal comum SS7 e MAP. Finalmente são apresentados as conclusões quanto aos objetivos alcançados, e é proposto um trabalho futuro, tomando como base o desenvolvido neste trabalho. / The evolution of wireless communications systems has developed into the third generation which is mainly represented by the Telephone Cellular Systems and PCS (Personal Communication systems). This work presents an overview about two importante issues in the evolution of the Wireless Communication Systems: Security and Interworking. Initially we carry out our study on the wireless communication security issue by analyzing and comparing some major authentication features and privacy characteristics implemented in the currently standardized cellular phone systems such as AMPS (American Analog Cellular), USCD (American Digital Cellular) and GSM (Euroupean Digital Cellular). For this end. we tentatively determine how vulnerable to the fraudulent attacks these systems are and what are those frauds commonly occurring in these systems. Also we analyze the state of art and some proposals for security problems in PCS systems found in the literature. We conclude our preliminary study with a table summarizing some principal systems' characteristics in the security issue. Next, we extend our discussion to the case of heterogeneous networks. In fact, the interconnection of heterogeneous networks is another important issue that needs to be deeply investigated in order to develop a rebust third generation of wireless communication systems, special those capable of providing automatic "roaming" and personal as well as terminal mobility. In this work we particularly discuss the protocols employed in the signaling system SS7 ( Signaling Systems Number 7) and MAP ( Mobile Application Part) which are important issues in terms of the interworking and interoperability of wireless and wireline networks. As a conclusion of this discussion, we propose a protocol architecture based on the RM-OSI model relating to the interconnection of a PCS system, J-STD-007, with the IS-95 cellular phone networks. Note that the proposed model uses the signaling systems SS7 and MAP. Finally we present some importante conclusion with respect to the objetives achieved in this work, and propose some future research activities based on this work.

A evolução dos sistemas de comunicação sem fio ou wireless apresentam até agora três gerações em menos de duas décadas. Fazendo parte desta evolução estão os sistemas de telefonia celular e os emergentes Sistemas de Comunicação Pessoal ou PCS (Personal Communications Systems). Este trabalho visa apresentar duas questões importantes na evolução destes sistemas, primeiro a questão da segurança e segundo a questão da interconexão com redes heterogêneas. No trabalho, abordando a questão de segurança, são estudados, analisados e comparados os mecanismos de autenticação e privacidade implementados nos atuais padrões de telefonia celular digital e analógico, como sistemas AMPS (celular analógico americano), USCD (celular digital americano) e GSM (celular digital europeu). São identificadas a vulnerabilidade e os fraudes mais comuns nestes sistemas. Também são analisados as propostas das recentes pesquisas e o state of art em termos de segurança, para os sistemas PCS emergentes. Como conclusão apresentado um quadro comparativo resumindo as principais características de segurança adotados pelos sistemas abordados neste estudo. Uma vez apresentado os aspectos de segurança em forma isolada para cada um dos sistemas acima mencionados, é apresentado a questão segurança no contexto da interconexão com redes heterogêneas. A interconexão com redes heterogêneas é outro problema a ser resolvido na implementação da terceira geração de sistemas wireless, para fornecer o roaming automático e a mobilidade pessoal e de terminal. Neste trabalho são revisados os protocolos de sinalização SS7 (Signaling System Number 7) e MAP (Mobile Application Part), como requisitos importantes na solução da interconexão e interoperabilidade entre as redes fixas atuais com as futuras redes móveis. Como conclusão deste estudo é apresentado a proposta de uma arquitetura de protocolos de sinalização, representando a interconexão de um sistema PCS baseado no padrão J-STD-007, com a rede de telefonia celular IS-95. Esta interconexão é realizada através de uma interface de rede denominada IIF (Interworking and Interoperability Function) usando protocolos de sinalização por canal comum SS7 e MAP. Finalmente são apresentados as conclusões quanto aos objetivos alcançados, e é proposto um trabalho futuro, tomando como base o desenvolvido neste trabalho. / The evolution of wireless communications systems has developed into the third generation which is mainly represented by the Telephone Cellular Systems and PCS (Personal Communication systems). This work presents an overview about two importante issues in the evolution of the Wireless Communication Systems: Security and Interworking. Initially we carry out our study on the wireless communication security issue by analyzing and comparing some major authentication features and privacy characteristics implemented in the currently standardized cellular phone systems such as AMPS (American Analog Cellular), USCD (American Digital Cellular) and GSM (Euroupean Digital Cellular). For this end. we tentatively determine how vulnerable to the fraudulent attacks these systems are and what are those frauds commonly occurring in these systems. Also we analyze the state of art and some proposals for security problems in PCS systems found in the literature. We conclude our preliminary study with a table summarizing some principal systems' characteristics in the security issue. Next, we extend our discussion to the case of heterogeneous networks. In fact, the interconnection of heterogeneous networks is another important issue that needs to be deeply investigated in order to develop a rebust third generation of wireless communication systems, special those capable of providing automatic "roaming" and personal as well as terminal mobility. In this work we particularly discuss the protocols employed in the signaling system SS7 ( Signaling Systems Number 7) and MAP ( Mobile Application Part) which are important issues in terms of the interworking and interoperability of wireless and wireline networks. As a conclusion of this discussion, we propose a protocol architecture based on the RM-OSI model relating to the interconnection of a PCS system, J-STD-007, with the IS-95 cellular phone networks. Note that the proposed model uses the signaling systems SS7 and MAP. Finally we present some importante conclusion with respect to the objetives achieved in this work, and propose some future research activities based on this work.

[pt] Os engenheiros e projetistas enfrentam diversos desafios na fase de implantação das redes móveis celulares, sobretudo no planejamento da área de cobertura e da penetração da onda eletromagnética em ambientes indoor, em particular na faixa de 2.5 GHz. Adicionalmente, não se dispõe de muitos resultados sobre as constantes de propagação através das edificações que permitam calcular com precisão a contribuição de campo que atravessa edifícios que são especialmente importantes na predição de cobertura em sistemas microcelulares. A primeira parte desse estudo trata da caracterização do canal em banda larga e banda estreita a partir de resultados experimentais que fornecem informações essenciais na implementação e desenvolvimento de projetos de redes móveis. Em banda larga os resultados de dispersão temporal do canal, tais como, o retardo médio e o espalhamento de retardo, são obtidos através de dois métodos distintos e comparados entre si em condições de LOS e NLOS em ambientes urbano e suburbano. Em banda estreita é realizada a análise estatística do desvanecimento em pequena e larga escala característicos de cada região urbana e suburbana. Na segunda parte desse estudo discute-se a importância para a cobertura em regiões urbanas do campo eletromagnético que atravessa as edificações num ambiente microcelular e formula-se um modelo de perda de potência do sinal transmitido através de edificações características de ambientes urbanos e suburbanos. / [en] The engineers and designers face many challenges in the implementation of cellular mobile networks, especially in planning the coverage and penetration of the signal into indoor environments, in particular in the range of 2.5 GHz. Additionally, they do not have many results on the propagation constants through the buildings that allow the accurate calculation of the contribution of field that crosses buildings and are especially important in the prediction of coverage. The first part of this study deals with the broadband and narrowband characterization of the channel from experimental results that provide essential information to the development and implementation of projects of mobile networks. Broadband results of temporal dispersion of the channel, such as the mean delay and delay spread, are obtained by two distinct methods and compared for LOS and NLOS conditions in urban and suburban environments. The narrowband characterization includes the statistical analysis of the small-scale fading characteristic of urban and suburban region. In the second part of this study the representativeness of the signal going through the buildings is discussed and a model of loss of signal strength by the propagation through buildings in urban and suburban environments is formulated.

Made available in DSpace on 2014-12-17T14:54:55Z (GMT). No. of bitstreams: 1 RanilsonCF_TESE.pdf: 2036029 bytes, checksum: f7ba8fa789f5420decc75fb98f14f807 (MD5) Previous issue date: 2010-07-27 / The main purpose of this work was the development of ceramic dielectric substrates of bismuth niobate (BiNbO4) doped with vanadium pentoxide (V2O5), with high permittivity, used in the construction of microstrip patch antennas with applications in wireless communications systems. The high electrical permittivity of the ceramic substrate provided a reduction of the antenna dimensions. The numerical results obtained in the simulations and the measurements performed with the microstrip patch antennas showed good agreement. These antennas can be used in wireless communication systems in various frequency bands. Results were satisfactory for antennas operating at frequencies in the S band, in the range between 2.5 GHz and 3.0 GHz. / O objetivo principal deste trabalho foi o desenvolvimento de substratos diel?tricos cer?micos de niobato de bismuto (BiNbO4) dopados com pent?xido de van?dio (V2O5), com alta permissividade el?trica, usados na constru??o de antenas patch de microfita com aplica??es em sistemas de comunica??es sem fio. A alta permissividade el?trica do substrato cer?mico proporcionou uma redu??o no tamanho das antenas. Os resultados num?ricos obtidos nas simula??es e medi??es realizadas com as antenas patch de microfita mostraram boa concord?ncia. Essas antenas podem ser usadas em sistemas de comunica??es sem fio em v?rias faixas de freq??ncias. Foram obtidos resultados satisfat?rios em antenas com freq??ncias de opera??o na banda S, na faixa compreendida entre 2,5 GHz e 3,0 GHz.

Made available in DSpace on 2014-12-17T14:55:54Z (GMT). No. of bitstreams: 1 CarlosGM_DISSERT.pdf: 4338321 bytes, checksum: 5834ff97d013cae154b31036fbc6dbe6 (MD5) Previous issue date: 2011-12-29 / Even living in the XXI century are still some difficulties in access to broadband Internet in several Brazilian cities, due to the purchasing power of people and lack of government investment. But even with these difficulties, we seek to encourage the use of wireless technology, which is based on the IEEE 802.11b protocol - also known as Wi-Fi (Wireless Fidelity) Wireless Fidelity Communications, having wide range of commercial applications in the world market, nationally and internationally. In Brazil, this technology is in full operation in major cities and has proved attractive in relation to the access point to multipoint and point-to-point. This paper is a comparative analysis of prediction field, using models based on the prediction of propagation loss. To validate the techniques used here, the Okumura-Hata models, modified Okumura-Hata, Walfisch-Ikegami model, were applied to a wireless computer network, located in the neighborhood of Cajupiranga in the city of Melbourn, in Rio Grande do Norte . They are used for networking wireless 802.11b, using the Mobile Radio to measure signal levels, beyond the heights of the antennas and distances from the transmitter. The performance data versus distance are added to the graphs generated and compared with results obtained through calculations of propagation models / Mesmo vivendo em pleno s?culo XXI ainda encontram-se dificuldades no acesso ? internet banda larga nos diversos munic?pios brasileiros, em decorr?ncia do poder aquisitivo dos habitantes bem como falta de investimento do governo. A tecnologia das redes de acesso sem fio baseadas no protocolo IEEE 802.11b - tamb?m denominada de Wi Fi, (Wireless Fidelity) Fidelidade em Comunica??es sem Fio, possui larga gama de aplica??es atualmente no mercado mundial comercial, nacional e internacional. No Brasil, esta tecnologia encontra-se em plena opera??o nas principais cidades e tem-se mostrado atrativa no que concerne aos acessos ponto multiponto e ponto-a-ponto. Neste trabalho s?o elaborados, uma an?lise comparativa de predi??o de campo, e analisados modelos de predi??o das perdas de propaga??o, Modelo Okumura-Hata, Lee, Modelo de Ikegami-Walfisch, para uma rede de computadores sem fio, no bairro de Cajupiranga e no Centro, no munic?pio de Parnamirim, no Rio Grande do Norte. S?o usados equipamentos de redes sem fio do padr?o 802.11b, analisador de espectro, onde os n?veis de sinais s?o medidos, al?m das alturas das antenas e das dist?ncias em rela??o ao transmissor. Os dados de pot?ncia versus dist?ncia s?o adicionados nos gr?ficos gerados e comparados com os resultados obtidos