Utilização conjunta de técnicas de formtação de feixe, diversidade espacial e equalização na recepção de sinais

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia Elétrica.Nesta dissertação, investiga-se a utilização conjunta de técnicas de formatação de feixe, diversidade espacial e equalização de canal para recepção de sinais em um sistema de comunicação sem fio. Utiliza-se um conjunto de arranjos de antenas (dois arranjos lineares) para obter tanto diversidade espacial como formatação de feixe. As antenas em cada arranjo são espaçadas segundo o critério de filtragem espacial (formatação de feixe), enquanto que os arranjos são espaçados buscando satisfazer o critério de diversidade espacial. Com a diversidade espacial, busca-se combater o desvanecimento plano, enquanto que com a formatação de feixe o desvanecimento seletivo em frequência. Tais fenômenos estão sempre presentes nos sistemas de comunicação sem fio, devido à propagação por múltiplos percursos do sinal transmitido. Três técnicas de combinação dos sinais na saída dos conformadores são apresentadas: seleção - escolhendo o sinal com menor ISI; soma não ponderada; e soma ponderada - enfatizando o sinal de maior potência. Um equalizador DFE é empregado na saída da técnica de combinação para mitigar a ISI residual. A estrutura do receptor em estudo é apresentada para operar de forma adaptativa, sendo que o algoritmo LMS é escolhido para essa tarefa. Também é abordado o cálculo ótimo dos coeficientes nos conformadores de feixe e no equalizador DFE, para as diferentes técnicas de combinação. Resultados de simulação permitem avaliar o desempenho dos esquemas sugeridos e compará-los com os da literatura existente.This dissertation investigates the joint use of beamforming, spatial diversity, and channel equalization techniques for receiving signals in a wireless communication system. A set of antenna arrays (two linear antenna arrays) is used in order to obtain both spatial diversity and beamforming. The antennas in each array are spaced according to the spatial filtering criterion (beamforming), while the arrays are spaced for satisfying the criterion of spatial diversity. The flat fading is combated with spatial diversity, while the frequency selective fading with beamforming. Such phenomena are always present in the wireless communication systems, due to the multipath propagation of the transmitted signal. Three techniques for combining the output signals of the beamformers are presented: selection - chooses the signal with lower ISI; unweighted sum; and weighted sum - emphases the highest power signal. A DFE equalizer is used in the output of the combination techniques for mitigating the residual ISI. The receptor structure under study is presented to operate adaptively, and the LMS algorithm is chosen for this task. In addition, the optimum coefficients of both DFE equalizer and beamformers are obtained, considering the three techniques of combination. Simulation results allow to evaluate the performance of the suggested schemes and compare them to those in the existing literature

Three Dimensional Bistatic Tomography Using HDTV

The thesis begins with a review of the principles of diffraction and reflection tomography; starting with the analytic solution to the inhomogeneous Helmholtz equation, after linearization by the Born approximation (the weak scatterer solution), and arriving at the Filtered Back Projection (Propagation) method of reconstruction. This is followed by a heuristic derivation more directly couched in the radar imaging context, without the rigor of the general inverse problem solution and more closely resembling an imaging turntable or inverse synthetic aperture radar. The heuristic derivation leads into the concept of the line integral and projections (the Radon Transform), followed by more general geometries where the plane wave approximation is invalid. We proceed next to study of the dependency of reconstruction on the space-frequency trajectory, combining the spatial aperture and waveform. Two and three dimensional apertures, monostatic and bistatic, fully and sparsely sampled and including partial apertures, with controlled waveforms (CW and pulsed, with and without modulation) define the filling of k-space and concomitant reconstruction performance. Theoretical developments in the first half of the thesis are applied to the specific example of bistatic tomographic imaging using High Definition Television (HDTV); the United States version of DVB-T. Modeling of the HDTV waveform using pseudonoise modulation to represent the hybrid 8VSB HDTV scheme and the move-stop-move approximation established the imaging potential, employing an idealized, isotropic 18 scatterer. As the move-stop-move approximation places a limitation on integration time (in cross correlation/pulse compression) due to transmitter/receiver motion, an exact solution for compensation of Doppler distortion is derived. The concept is tested with the assembly and flight test of a bistatic radar system employing software-defined radios (SDR). A three dimensional, bistatic collection aperture, exploiting an elevated commercial HDTV transmitter, is focused to demonstrate the principle. This work, to the best of our knowledge, represents a first in the formation of three dimensional images using bistatically-exploited television transmitters