Técnicas de diversidade aplicadas a sistemas θ-QAM para canais com desvanecimento.

Abstract

Diversity techniques have been proposed in order to improve the performance of communication systems, with different characteristics in terms of complexity and performance. This thesis presents the use of diversity techniques combined with the adoption of the θ-QAM (Theta Quadrature Amplitude Modulation) scheme. The θ-QAM constellations represent a family of parametric quadrature modulation schemes, in which the adjacent symbols are vertices of isosceles triangles with the angle parameter θ. The change of the angle θ produces different signal constellations, a feature that can be exploited to improve the performance of different diversity techniques. In addition, θ-QAM constellations are used to increase the performance gain of the adaptive modulation schemes by the dynamic change of the constellation density and of its internal structure In this thesis, the modulation and the cooperative diversity techniques are analyzed and their performance is evaluated considering the use of the θ-QAM schemes. Additionally, the optimization of the internal constellation structure (angle θ) and the operating parameters of each technique is performed. Suitable approximations for the symbol and bit error rates of the proposed schemes are derived. Evaluations are conducted considering the Nakagami-m and κ-μ fading models for the communication channel. Among the main contributions obtained, the following are noteworthy: development of an expression to evaluate the optimal value of the angle θ as a function of SNR; development and evaluation of an adaptive modulation system based on the use of the θ-QAM schemes in different fading conditions, using the κ-μ fading model; performance evaluation of the modulation diversity technique and its operating parameters in Nakagami-m e κ-μ fading channels using different analytical expressions proposed in this thesis; combination of the adaptive modulation and modulation diversity schemes in order to improve system performance; performance evaluation of the modulation diversity technique in channels subject to temporal correlation and estimation errors; evaluation of the modulation diversity technique in κ-μ channels combined with a spatial diversity scheme based on a Maximum-Ratio Combining (MRC) receiver; development of an expression to evaluate the symbol error probability of θ-QAM systems with cooperative diversity subject to κ-μ fading channels; and optimization of cooperative diversity systems using the constellation rearrangement technique.