Antennas and Propagation from a Signal Processing Perspective
Abstract: This thesis studies several topics within the area of antennas and propagation from a signal processing perspective. However, theory and methods from electromagnetics and communications have also been used, contributing to an interdisciplinary character of the thesis. Several physical models are derived in the thesis that describes the wireless communications channel and several novel antennas. On the basis of these models, it is investigated how multiple antennas may be employed to increasing the capacity of future wireless communication systems. Furthermore, several aspects of using antenna arrays for finding the directions of arrival of electromagnetic waves, with applications in radar and acoustic sonar, are studied.An often neglected issue when employing several antennas is that the elements affect each other through mutual coupling. By deriving expressions for the mutual coupling, the achievable direction finding performance is analyzed for this case. It is found that if the coupling is known, the effects on direction finding are small and can even increase performance in some cases. Similar results are also obtained when examining the capacity of communication systems employing multiple closely spaced antennas at both the transmitter and receiver. To evaluate the performance of communication systems, a spatio-temporal channel model is proposed that is based on electromagnetic scattering and fundamental physics. By using a dyad notation and concepts from rough surface scattering, a compact formulation of the channel model is obtained. System models are then derived that employs multiple antennas at both the transmitter and receiver, so called Multi-Input Multi-Output (MIMO) systems. The polarization properties of the channel as well as those of the antennas are also included in the model, allowing for studies of different antenna arrangements. Two novel antenna solutions are proposed that can be used as diversity receivers in MIMO systems or when performing high-resolution direction finding. By exciting higher order modes of biconical or microstrip antennas, several directionally dependent radiation patterns are obtained. Different patterns can also be obtained by employing parasitic elements. It is found that these antennas, the multimode and switched parasitic antenna, offer performance comparable to that of an antenna array.
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