Conjugate phase retrieval in a complex shift-invariant space

Abstract: The conjugate phase retrieval problem concerns the determination of a complex-valued function, up to a unimodular constant and conjugation, from its magnitude observations. It can also be considered as a conjugate phaseless sampling and reconstruction problem in an infinite dimensional space. In this paper, we first characterize the conjugate phase retrieval from the point evaluations in a shift-invariant space $\mathcal S(\phi)$, where the generator $\phi$ is a compactly supported real-valued function. If the generator $\phi$ has some spanning property, we also show that a conjugate phase retrievable function in $\mathcal S(\phi)$ can be reconstructed from its phaseless samples taken on a discrete set with finite sampling density. With additional phaseless measurements on the function derivative, for the B-spline generator $B_N$ of order $N\ge 3$ which does not have the spanning property, we find sets $\Gamma$ and $\Gamma'\subset (0,1)$ of cardinalities $2N-1$ and $2N-5$ respectively, such that a conjugate phase retrievable function $f$ in the spline space $\mathcal B_N$ can be determined from its phaseless Hermite samples $|f(\gamma)|, \gamma\in\Gamma+\Z$, and $|f'(\gamma)|, \gamma'\in\Gamma'+\Z$. An algorithm is proposed for the conjugate phase retrieval of piecewise polynomials from the Hermite samples. Our results provide illustrative examples of real conjugate phase retrievable frames for the complex finite dimensional space $\C^N$.