Study of the effects produced by time modulation applied to an antenna array in digital transmission systems

Resumo

An ever-increasing demand for higher mobility, capacity and reliability, together with a definitive compromise with sustainability, are the hallmarks of mobile and wireless communications systems nowadays. Under these premises, smart antenna devices –capable of sensing the electromagnetic environment and suitably adapting its radiation features– are correspondingly called to play a crucial role. In this sense, today’s wireless standards consider multiple-antenna techniques in order to exploit space diversity, spatial multiplexing and beamforming to achieve better levels of reliability and capacity. Such advantages, however, are obtained at the expense of increased system complexity which may be unaffordable in terms of size and energy efficiency. Consequently, some technical challenges remain to develop the adequate antenna technologies capable of supporting the aforementioned features in a limited physical space that the mobility demand dictates. The concept of time-modulated array (TMA) is a feasible multi-antenna technique that provides a significant hardware simplification: its radiated power pattern is controlled by the simple application of variable-width periodical pulses to the individual array excitations. The nonlinear nature of such an array operation causes the appearance of radiation patterns at the harmonic frequencies of the applied periodic pulses. The technique can be used for improving the side-lobe level topology of the radiation pattern at the central frequency and/or to profitably exploit the harmonic patterns in order to supply smart antenna capabilities. This thesis is the result of an investigation of TMAs from an interdisciplinary perspective, i.e., not only under a radiation pattern or an antenna array outlook but also from a signal processing point of view. More specifically, the thesis deals with an in-depth analysis of the application of TMAs in digital communications developed in four stages: 1) mathematical analysis of the feasibility of transmission of digital signals over TMAs, identifying the restrictions to safeguard the integrity of the signal and quantifying the radiated power, 2) characterization of the bit error rate of a digital communication system that incorporates a receive-TMA exploiting its fundamental mode and considering additive white Gaussian noise channels, 3) study of the performance of TMAs –exploiting their harmonics– for the angle diversity reception of digital communication signals over multipath fading channels, 4) an approach to the characterization of beamforming TMAs which use sum of weighted cosines pulses instead of rectangular ones, leading to the so-called enhanced time-modulated arrays, which endows them with a better response in terms of flexibility and efficiency.