  Considering the ambiguousness of the discrete-time phase retrieval problem to
recover a signal from its Fourier intensities, one can ask the question: what
additional information about the unknown signal do we need to select the
correct solution within the large solution set? Based on a characterization of
the occurring ambiguities, we investigate different a priori conditions in
order to reduce the number of ambiguities or even to receive a unique solution.
Particularly, if we have access to additional magnitudes of the unknown signal
in the time domain, we can show that almost all signals with finite support can
be uniquely recovered. Moreover, we prove that an analogous result can be
obtained by exploiting additional phase information.
