|M.Sc Student||Kleiman Avital|
|Subject||Optimal Design of Constant Beamwidth Beamformers|
with Concentric Ring Arrays
|Department||Department of Electrical and Computer Engineering||Supervisors||PROF. Israel Cohen|
|Full Thesis text|
Numerous applications require dealing with broadband signals. Acquiring acoustic data of high quality is a challenging task as the acoustic medium introduces artifacts like noise and reverberations. Thus, a wide variety of tasks, such as source separation, noise reduction, and signal enhancement, utilize wideband beamforming algorithms to mitigate possible artifacts. Implementing the desired beampattern is commonly done by changing the weights of each sensor and by choosing an adequate array geometry. Choosing the array geometry has a major impact on the performance. In three-dimensional problems, the planar geometry is preferred compared to linear geometry.
A main challenge in designing a wideband beamformer is maintaining a constant beamwidth over a wide range of frequencies. In common beamforming methods, the mainbeam becomes narrower as the frequency increases. To avoid non-uniform attenuation and distortion caused by the beamformer, several approaches implementing frequency invariant beamformers were presented in the literature. However, existing methods mostly focus on the azimuth-beamwidth. In practical 3D applications, the elevation angle is not restricted to the array plane. The assumption of arbitrary elevation angle directly affects the directivity factor and the white-noise-gain. In this thesis, we present several approaches in designing constant-elevation beamwidth beamformers on concentric ring arrays (CRAs). In the proposed configuration, all sensors on each ring share the same weight value. This constraint significantly simplifies the beamformers and reduces the resources required in a physical setup.
Based on similar approaches in one-dimensional beamformers design, a window-based constant beamwidth beamformer for CRAs is presented. By analyzing the properties of a continuous ring beampattern, we derive the relation between the beamwidth and the radius of the array. Following, the exact weight value is calculated to attain the desired beamwidth while gradually eliminating sensors from outer rings. We introduce a design method in the low-frequency range that modifies the filters applied to each array element from lowpass filters to bandpass filters. This method exploits the circular geometry and results in better performance compared to beamformers that are designed for linear arrays with an equivalent number of channels (i.e., beamformer order). A directivity index improvement and a time-domain implementation of the theoretical filters are also incorporated in the beamformer design. We demonstrate the advantages of the proposed beamformers compared to the one-dimensional configuration in terms of different performance measures.
Following, we propose a new approach for designing constant elevation beamformers on nonuniform CRA. Opposed to common solutions that consider uniformly spaced circular arrays, the suggested methodology simultaneously selects the ring placements and designs the beamformer weights that achieve optimal performance. The the considered beamformers is designed by solving a dedicated quadric optimization problem. Hence, the suggested beamformer aims to maximize the array directivity factor while maintaining fixed beamwidth. We present the problem constraints and cost function, resulting in a sparse beamformer design. In addition, a uniformly spaced beamformer is incorporated in the optimization cost function to attain improved performance. Time-domain implementation of the ideal beamformer is also presented. Experimental results demonstrate the advantages of the nonuniform beamformer compared to a uniform beamformer.