Thesis (Ph. D.)--University of Oklahoma. / Bibliography: leaves 93-110.

Wireless communications systems of all types must deliver reliable connections to the end users to be accepted by the public. The reliability of these systems is composed of two aspects. The first aspect is the reliability of the actual hardware and software composing the device and is completely under the control of the designers of the equipment. The second aspect of reliability or availability is the wireless propagation link connecting the users. This link is very difficult to model exactly and is composed of a fixed propagation loss plus random elements of propagation loss.This thesis focuses on the propagation links associated with satellite communications systems (Satcom). The fixed portion of the link loss in this case is the "spreading loss" or free space loss which occurs due to the large distance between the user and the satellite. The random portion of the link loss in these systems is due to many things such as rain, absorption, shadowing, multipath and cross polarization effects. However the major element associated with fades in Satcom systems is rain and the fades associated with rain. Rain becomes an even more dominate term in the situation as higher frequencies are used to obtain the increases in bandwidths required to accommodate the increases in use. Rain fades on the satellite links are modeled as random processes whose parameters are given in RECOMMENDATION ITU-R P.837-5 of the International Telecommunications Union (ITU). Over the years this recommendation has been revised and the current revision is 837-5 as indicated above. However, the data used to develop these models has always come from the European Centre for Medium-range Weather Forecasts based upon data measured at approximately 100 stations around the world. Since 1998 satellite sensory data is available for the tropics which directly measures rain data in this area. The Tropical Rainfall Measuring Mission (TRMM) is a join satellite project involving the USA and Japan. As a result of the availability of TRMM data it is possible to improve the statistical rain rate models for the tropical regions of the world. Recently a number of researchers (T.V, Omotosho, C.O. Oluwafemi, C Prabhakara et all) have begun to use TRMM data to improve the rain rate and rain fade estimates. The ITU has also begun to study using TRMM data in their recommendations. In this thesis the TRMM data is used to construct a rain rate and rain fade models for the Indian sub-continent as well as other parts of the tropics. This model is compared to the predictions based upon the ITU 837-5 models and substantial differences are found in the heavy rain fall areas. India is currently building a satellite (GSAT-4) to measure rain fades at 20/30 GHz and it is hoped that these results can be used to compare with the measured GSAT-4 data when it is available.

Les systèmes de communication par satellite d’aujourd’hui reposent principalement sur le multiplexage temporel pour optimiser leurs performances. Chaque utilisateur utilise le canal pendant une fraction de temps connu. Pendant cette période, la modulation et le taux de codage sont choisis de manière à transmettre le plus d’information possible. En pratique, ce schéma est facile à mettre en œuvre ce qui justifie sa popularité. Cependant, il est désormais bien connu que la répartition temporelle n’est pas optimale en termes d’efficacité spectrale offerte aux récepteurs. En effet, la stratégie qui consiste à superposer des données offre de meilleures performances que le multiplexage temporel. C’est dans ce contexte que s’inscrit la problématique de cette thèse. Le travail réalisé propose des applications du codage par superposition dans le domaine des communications par satellite. Tout d’abord, nous étudions la modulation hiérarchique qui est une implémentation du codage par superposition au niveau de la modulation. Les performances de ce type de modulation sont évaluées d’un point de vue théorique et pratique. Dans un deuxième temps, nous quantifions l’amélioration en termes d’efficacité spectrale que peut apporter la modulation hiérarchique pour les systèmes de communication par satellite. Les standards de diffusion par satellite DVB-SH et DVB-S2 fournissent un cadre pratique. Nous montrons que des gains non négligeables sont envisageables selon la configuration du système. Le dernier point abordé concerne un système où des utilisateurs communiquent entre eux à l’aide d’un satellite qui sert de relais. Nous proposons un schéma de communication où plusieurs utilisateurs émettent en même temps en coordonnant leur puissance de transmission. Ainsi, les signaux vont naturellement se superposer. Les récepteurs utilisent deux mécanismes pour le décodage des signaux : le codage réseau couche physique et la démodulation de constellations superposées. Finalement, les gains de performance obtenus dans les différents domaines par le codage par superposition ouvrent des perspectives pour des travaux futurs. / Modern satellite communication systems mainly rely on time sharing to optimize the throughput. Each receiver uses the channel during a given fraction of time. During this period, the transmission parameters (i.e., the modulation and the coding rate) are chosen in order totransmit as much information as possible. The scheme is easy to implement which explains its popularity. However, it is today well established that time sharing is not optimal in terms of spectrum efficiency offered to the receivers. Indeed, the scheme that consists in sending superposed data offers better performance than the time sharing. This thesis investigates the application of superposition coding in satellite communication systems. First of all, we study the performance of hierarchical modulation which is an implementation of superposition coding at the modulation level. We propose a performance evaluation method for such modulations. We also compare the performance of hierarchical and non hierarchical modulations in terms of spectrum efficiency and link unavailability. These two criteria are very important for broadcast system and we show that hierarchical modulations often offer better performance than non hierarchical modulations.Then, we study the performance improvement in terms of spectrum efficiency when using hierarchical modulation in satellite communication systems. Two issues are addressed. The first one is how to group the receivers in pairs in order to transmit data with a hierarchical modulation. The second issue is the computation of the spectrum efficiency. We show that significant gains are possible depending on the system configuration. The last part considers a system where multiple users communicate through a satellite. The satellite acts as a relay in our scenario. We propose a communication scheme where several users emit at the same time with appropriate transmitting power. Thus the signals naturally superpose and generate interference. The receivers use two mechanisms for decoding the signals: physical layer network coding and demodulation of superposed constellations. Finally, we explain how the performance improvements obtained by superposition coding in several scenarios open perspectives for future work.

[pt] O desenvolvimento de sistemas de comunicações via satélite requer a introdução de novas tecnologias e novas técnicas, e o uso de freqüências elevadas superiores a 10 GHZ. Para aplicações utilizando freqüências acima de 10 GHz, deve ser considerada predominantemente a atenuação causada pela chuva. Este trabalho apresenta resultados intermediários de uma campanha de medidas realizada no CETUC, na faixa de 12 GHz. Esta campanha foi iniciada em 1987 com a realização de medidas radiométricas em diferentes regiões climáticas do Brasil. Em 1995, foram adquiridos sistemas de recepção de sinais de beacon de satélite. O principal objetivo das medidas é a aquisição das distribuições estatísticas de longo termo da atenuação causadas pela chuva, a serem utilizadas no planejamento e dimensionamento de futuros sistemas de comunicações via satélite. Além das distribuições cumulativas de longo termo, são analisadas: a variabilidade das distribuições ano a ano, as estatísticas de pior mês, as características dinâmicas da atenuação por chuva, a dependência da ocorrência de atenuação com a hora do dia, a distribuição da taxa de precipitação e a previsão da atenuação a partir da taxa de precipitação. / [en] The development of satellite communication systems requires the introduction of new technologies and techniques as well as the use of high frequencies, above 10GHz. Above such frequencies, rainfall induced attenuation effects, which are strongly frequency dependent, become important as the main source of reliability impairment. Thid work presents the intermediate results from a measurements campaign at 12GHz. This campaign began in CETUC in 1987 with radiometric measurements on several different Brazilian climatic regions. In 1995, systems for receiving satellite beacon signal were acquired. The main objective of the satellite beacon measurements is to obtain the long term distributions of rainfall induced attenuation, which will be used to aid the planning and design of future satellite communication systems. Besides the long term distributions of attenuation, the following results will be sought out: year-to-year variability of the attenuation distributions,worst-month statistics, the dynamic characteristics of rainfall induced attenuation, the daily variation of the occurrence of attenuation events, rainfall rate distributions and the possibility of modelling the attenuation as a function of the rainfall rate.

[pt] Com o objetivo de prover acesso global ao mundo da informação, as comunicações via satélite desempenham um papel fundamental, possibilitando o acesso de serviços de faixa larga em áreas onde a infra-estrutura terrestre não permite. Este trabalho apresenta uma caracterização destas aplicações potenciais para redes ATM via satélite, descrevendo os problemas e as limitações que ainda precisam ser superados, assim como as soluções que já foram sugeridas. Entre estes problemas, está o desenvolvimento de protocolos de múltiplo acesso que possibilitem a utilização eficiente do segmento espacial e, ao mesmo tempo, garanta a qualidade do serviço. Os protocolos de múltiplo acesso tradicionais não são adequados para redes com múltiplos serviços, como se espera num ambiente de faixa larga, visto que foram desenvolvidos para redes com um determinado tipo de aplicação. Desta forma, é necessário realizar uma adaptação destas técnicas convencionais, para esta nova realidade, pois através de protocolos de múltiplos acesso eficiente, será possível obter ganhos consideráveis nos recursos de segmento espacial, cada vez mais escassos e, conseqüentemente, caros. Para verificar em mais detalhes este problema, descreveremos três protocolos propostos para múltiplo acesso em redes ATM via satélite, todas eles baseados na técnica de método de acesso por divisão do tempo (ADTMA). O desempenho destes protocolos é analisado, através de simulação, comparando-se as vantagens e desvantagens de cada um. / [en] In order to provide global Access to the world of information, the satellite communications play a key rolo, making possible the Access of broadband services to areas where the terrestrial infrastructure doesn´t allow. This work shows a characterization of the potencials applications to the satellite ATM networks, making a description of the problems and limitations that need to be overcome, as well as the solutions that have been suggested. Among thesse problems, we have the development of multiple access protocols that allow an efficient utilization of space segment and, at same time, guarantee the quality of service. The traditionals multiples access protocols are not well suitable to multiple service networks, as we hope in a broadband environment, since they were created to networks with a specific application. So, it´s necessary to make an adaptation in these conventionals technics to this new reality, because through efficients multiple access protocols, will be possible to have considerable gains in space segment`s resources, more and more rare and, consequantly, expensive. To verify in more details this problem, we describe three protocols suggested to provide multiple access in satellite ATM networks, all of them based on the technic of time division multiple access (TDMA). The performance analysis of these protocols is done, through simulations, confronting the advantages and disadvantages of each one.

[pt] Um problema importante em sistemas de comunicações por satélite utilizando múltiplo acesso por divisão no tempo (TDMA) é a consignação de bursts no espaço tempo-freqüência disponível para estabelecer a interconexão entre as estações da rede. Alguns algoritmos tem sido sugeridos para realizar tal consignação de bursts. Este trabalho examina um conjunto de algoritmos que, embora heurísticos, procuram minimizar custos relacionados aos segmentos espaciais e de Terra bem como utilizar eficientemente os transpoders. Os algoritmos propostos são simples e nas situações examinadas apresentaram resultados pelo menos tão bons quanto os daqueles propostos anteriormente na literatura. / [en] An important problem in time division multiple Access (TDMA) sattelite systems is the assignment of bursts in the time-frequency space, available to provide the interconnection among a given pumber of earth stations. Algorithms for burst assignment in such an environment have already been considered in the literature. A set of algorithms are suggested here, which take into account that the resulting assignment should minimize space and earth terminal costs as well as leasd to efficient use of transponder capacities. The proposed algorithms are very simple and led to sebeduling results at least as good as those examined in the literature.

[pt] Em comunicações Via-Satélite, o regime de chuvas brasileiro se apresenta como fator limitante na análise da viabilidade de redes VSAT (Very Small Aperture Terminal) nas bandas Ka e Ku. O objetivo do trabalho é analisar a indisponibilidade por chuvas de redes VSAT com topologia em estrela usadas no Brasil para determinados serviços, tais como telefonia e transmissão de dados e TV, utilizando-se cenários hipotéticos em termos da localização das estações terminais, de satélites com transponders nas bandas Ka e Ku e das características reais dos equipamentos e sistemas. Um dos objetivos dentro deste tema foi a análise da influência do modelo de atenuação. Para tal foram utilizadas as duas últimas versões de modelo de atenuação por chuvas que tem sido recomendado pela UIT-R para aplicações em Engenharia. Complementando este objetivo, analisou-se também a influência de dados de taxas de precipitação para o Brasil utilizado nestes modelos, tendo sido utilizadas duas fontes: a Recomendação P.837 do UIT-R e medidas regionais realizadas pelo CETUC. Outro objetivo foi à comparação entre os valores estimados de indisponibilidade com limites máximos recomendados o que permite obter conclusões sobre a viabilidade destes sistemas no Brasil. Complementado o estudo acima descrito, um segundo caso foi analisado correspondendo a um sistema DVBS. O cenário é composto de outro satélite (PAS-9), porém com as mesmas configuração (rede VSAT e topologia em estrela) e estações terrenas com mesmas localizações. A diferença em relação ao primeiro caso reside principalmente na maior taxa de transmissão e por ter sido utilizado na análise da indisponibilidade um modelo de atenuação por chuvas regional, desenvolvido pelo CETUC com base em dados de medidas no Brasil. Uma análise similar foi feita considerando o modelo atual da UIT-R para fins comparativos. / [en] In satellite communication, the Brazilians rains distributions are presented like limited factor in the viability analyze of VSAT networks (Very Small Aperture Terminal) in the bands Ka and Ku. The aim of the work is analyze the unavailable caused by VSAT rains with star topology used in Brazil to some kinds of service, like telephone and data transmission and TV, using conjectural scenes on the satellite s terminal stations with transponders in the bands Ka and Ku and the real characteristics of the equipments and systems. One of the objectives is analyze the influence of the attenuation system. For it were used the last two versions models rainy attenuation that has been recommended by UIT-R for applications in engineering. Completing this objective, were also analyzed the influence precipitation taxes data for Brazil using this models, have been used two fonts: the recommendation P.837 from UIT-R and regional measured realized by CETUC. Other objective was the comparison between unavailable esteem values with maxim limits recommended what allows having conclusions about the viability of these systems in Brazil. Concluding the study, a second case was analyzed corresponding a system. The scene is composed by from other satellite (PAS-9), by the way, with the same localizations. The difference between both are: the first one has a bigger tax of transmission for being used analyze of the unavailable one model of attenuation for regional rains, made by CETUC based on measure data from Brazil. One similar analyze was made considering an actual model of UTI-R for final comparisons.

Alternative means of satisfying the high bandwidth and protected communications requirements for New Construction Naval Surface Ships in the midst of conflicting reduced radar cross section (RCS) requirements were investigated using the systems engineering process. <p>Various antenna, ranging from parabolic dish antennas to Luneberg lens antennas to phased array antennas, and feed and amplifier combinations were considered to provide a dual-band Super High Frequency (SHF) and Extremely High Frequency (EHF) Satellite Communications (SATCOM) Terminal (SEST). <p>Through the design of this hypothetical system, the various stages of the systems engineering process are considered-- definition of need, conceptual design, preliminary system design, production and installation, and utilization and support. Sample tasks are performed at each stage in the process (e.g., a system performance specification is prepared in the advanced system planning stage). <p>The set of technical solutions that remained in the preliminary design phase are compared based on life cycle costs. Two approaches are recommended -- one assuming lowest life cycle cost has highest priority and one assuming that the ability to communicate simultaneously on SHF and EHF has highest priority. / Master of Science

As the need for more bandwidth increases, satellite communication (SatCom) terminals are forced to climb higher in frequency. Higher frequency means greater propagation losses, and so antenna gain and sensitivity have to increase. The higher the gain, the more difficult it is to point the antenna. To make matters even more challenging, consumers are requesting satellite links in harsher environments and on moving vehicle and planes. In order to meet today's challenges and improve on dish feeds, research is ongoing to replace fixed-beam feedhorns with smaller, cheaper, and lighter PCB based antennas and to develop low-cost electronically steered array feeds (ESAF). ESAFs will not only improve the signal link, but they will also aid in pointing the antenna and then tracking the satellite independent of movement. Here is presented some of the first planar antenna dish feeds developed by the Brigham Young University's SatCom Group. Included are the simulation and test procedures to determine if they are viable for SatCom use. The results show that these antennas make significant advancements in efficiencies and prove a path forward to a feedhorn replacement. Several planar designs are presented, each with a unique solution to meet all the requirements for a dish feed. Also presented is the first low-cost ESAFs developed to give commercial SatCom an electronically steerable dish. None of the designed hardware requires a redesign of current modems and receiver boxes. The research looks at keeping costs low by minimizing the required electronics. This further led to researching the limits on how simple the electronics could be. The ESAF doubled the visible area of the dish and successfully acquired and tracked a satellite as the dish moved. The ESAF also demonstrates a path forward to increase the steerable range and improve pointing and tracking.

No / Noise amplitude distribution measurements relevant to%satellite-mobile radio systems are reported. The rationale for the%measurements is outlined and the choice of measurement parameters%justified. The measurement equipment and measurement methodology are%described in detail. Results characterizing the elevation angle%distribution of impulsive noise are presented for rural, suburban and%urban environments and also for an arterial road (U.K. motorway)%carrying high density, fast moving traffic. Measurements of the levels%of impulsive noise to be expected in each environment for high- and%low-elevation satellite scenarios using appropriate antenna%configurations are also presented