$L^2$-stability analysis for Gabor phase retrieval

TL;DR

This paper proves that stable phase retrieval from Gabor spectrograms can be achieved using only the $L^2$-norm, enabling practical discrete sampling methods for signal reconstruction.

Contribution

It demonstrates that the stability of Gabor phase retrieval can be characterized using the $L^2$-norm instead of Sobolev norms, facilitating practical sampling approaches.

Findings

Stability characterized by $L^2$-norm instead of Sobolev norm.

Discrete sampling sets suffice for stable reconstruction.

Improves practical applicability of Gabor phase retrieval.

Abstract

We consider the problem of reconstructing the missing phase information from spectrogram data $|\mathcal{G} f|,$ with $$ \mathcal{G}f(x,y)=\int_\mathbb{R} f(t) e^{-\pi(t-x)^2}e^{-2\pi i t y}dt, $$ the Gabor transform of a signal $f\in L^2(\mathbb{R})$. More specifically, we are interested in domains $\Omega\subseteq \mathbb{R}^2$, which allow for stable local reconstruction, that is $$ |\mathcal{G}g| \approx |\mathcal{G}f| \quad \text{in} ~\Omega \quad\Longrightarrow \quad \exists \tau\in\mathbb{T}:\quad \mathcal{G}g \approx \tau\mathcal{G}f \quad \text{in} ~\Omega. $$ In recent work [P. Grohs and M. Rathmair. Stable Gabor Phase Retrieval and Spectral Clustering. Comm. Pure Appl. Math. (2019)] and [P. Grohs and M. Rathmair. Stable Gabor phase retrieval for multivariate functions. J. Eur. Math. Soc. (2021)] we established a characterization of the stability of this phase retrieval problem in terms of the connectedness of the observed measurements. The main downside of the aforementioned results is that the similarity of two spectrograms is measured w.r.t. a first order weighted Sobolev norm. In this article we remove this flaw and essentially show that the Sobolev norm may be replaced by the $L^2-$norm. Using this result allows us to show that it suffices to sample the spectrogram on suitable discrete sampling sets -- a property of crucial importance for practical applications.