Looking beyond Pixels

Sparse recovery is a powerful tool that plays a central role in many applications, including source estimation in radio astronomy, direction of arrival estimation in acoustics or radar, super-resolution microscopy, and X-ray crystallography. Conventional approaches usually resort to discretization, where the sparse signals are estimated on a pre-defined grid. However, sparse signals do not line up conveniently on any grid in reality. While the discrete setup usually leads to a simple optimization problem that can be solved with standard tools, there are two noticeable drawbacks: (i) Because of the model mismatch, the effective noise level is increased; (ii) The minimum reachable resolution is limited by the grid step-size. Because of the limitations, it is essential to develop a technique that estimates sparse signals in the continuous-domain--in essence seeing beyond pixels.

The aims of this thesis are (i) to further develop a continuous-domain sparse recovery framework based on finite rate of innovation (FRI) sampling on both theoretical and algorithmic aspects; (ii) adapt the proposed technique to several applications, namely radio astronomy point source estimation, direction of arrival estimation in acoustics, and single image up-sampling; (iii) show that the continuous-domain sparse recovery approach can surpass the instrument resolution limit and achieve super-resolution.

We propose a continuous-domain sparse recovery technique by generalizing the FRI sampling framework to cases with non-uniform measurements. We achieve this by identifying a set of unknown uniform sinusoidal samples and the linear transformation that links the uniform samples of sinusoids to the measurements. The continuous-domain sparsity constraint can be equivalently enforced with a discrete convolution equation of these sinusoidal samples. The sparse signal is reconstructed by minimizing the fitting error between the given and the re-synthesized measurements subject to the sparsity constraint. Further, we develop a multi-dimensional sampling framework for Diracs in two or higher dimensions with linear sample complexity. This is a significant improvement over previous methods, which have a complexity that increases exponentially with dimension. An efficient algorithm has been proposed to find a valid solution to the continuous-domain sparse recovery problem such that the reconstruction (i) satisfies the sparsity constraint; and (ii) fits the measurements (up to the noise level).

We validate the flexibility and robustness of the FRI-based continuous-domain sparse recovery in both simulations and experiments with real data. We show that the proposed method surpasses the diffraction limit of radio telescopes with both realistic simulation and real data from the LOFAR radio telescope. In addition, FRI-based sparse reconstruction requires fewer measurements and smaller baselines to reach a similar reconstruction quality compared with conventional methods. Next, we apply the proposed approach to direction of arrival estimation in acoustics. We show that accurate off-grid source locations can be reliably estimated from microphone measurements with arbitrary array geometries. Finally, we demonstrate the effectiveness of the continuous-domain sparsity constraint in regularizing an otherwise ill-posed inverse problem, namely single-image super-resolution. By incorporating image edge models, the up-sampled image retains sharp edges and is free from ringing artifacts.