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Phase Retrieval via Wirtinger Flow: Theory and Algorithms

2015/02/03 by Emmanuel J. Candès, Xiaodong Li, Mahdi Soltanolkotabi · 55 citations
Physics and Astronomy · Biochemistry, Genetics and Molecular Biology · Computer Science · #Advanced X-ray Imaging Techniques #Advanced Electron Microscopy Techniques and Applications #Optical measurement and interference techniques

paper · doi:10.1109/tit.2015.2399924

Abstract

We study the problem of recovering the phase from magnitude measurements; specifically, we wish to reconstruct a complex-valued signal <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX"> \boldsymbol x∈ \mathbb Cn </tex-math></inline-formula> about which we have phaseless samples of the form <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">yr = |⟨ \boldsymbol ar, \boldsymbol x ⟩ |2 </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">r = 1,… , m </tex-math></inline-formula> (knowledge of the phase of these samples would yield a linear system). This paper develops a nonconvex formulation of the phase retrieval problem as well as a concrete solution algorithm. In a nutshell, this algorithm starts with a careful initialization obtained by means of a spectral method, and then refines this initial estimate by iteratively applying novel update rules, which have low computational complexity, much like in a gradient descent scheme. The main contribution is that this algorithm is shown to rigorously allow the exact retrieval of phase information from a nearly minimal number of random measurements. Indeed, the sequence of successive iterates provably converges to the solution at a geometric rate so that the proposed scheme is efficient both in terms of computational and data resources. In theory, a variation on this scheme leads to a near-linear time algorithm for a physically realizable model based on coded diffraction patterns. We illustrate the effectiveness of our methods with various experiments on image data. Underlying our analysis are insights for the analysis of nonconvex optimization schemes that may have implications for computational problems beyond phase retrieval.

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