Paper:
Learning without the Phase: Regularized PhaseMax Achieves Optimal Sample Complexity
The problem of estimating an unknown signal, x 0 ∈ R n , from a vector y ∈ R m consisting of m magnitude-only measurements of the form y i = |a i x 0 |, where a i 's are the rows of a known measurement matrix A, is a classical problem known as phase retrieval. This problem arises when measuring the phase is costly or altogether infeasible. In many applications in machine learning, signal processing, statistics, etc., the underlying signal has certain structure (sparse, low-rank, finite alphabet, etc.), opening of up the possibility of recovering x 0 from a number of measurements smaller than the ambient dimension, i.e., m < n. Ideally, one would like to recover the signal from a number of phaseless measurements that is on the order of the "degrees of freedom" of the structured x 0 . To this end, inspired by the PhaseMax algorithm, we formulate a convex optimization problem, where the objective function relies on an initial estimate of the true signal and also includes an additive regularization term to encourage structure. The new formulation is referred to as regularized PhaseMax. We analyze the performance of regularized PhaseMax to find the minimum number of phaseless measurements required for perfect signal recovery. The results are asymptotic and are in terms of the geometrical properties (such as the Gaussian width) of certain convex cones. When the measurement matrix has i.i.d. Gaussian entries, we show that our proposed method is indeed order-wise optimal, allowing perfect recovery from a number of phaseless measurements that is only a constant factor away from the optimal number of measurements required when phase information is available. We explicitly compute this constant factor, in terms of the quality of the initial estimate, by deriving the exact phase transition. The theory well matches empirical results from numerical simulations.

Review:
This paper presents a sharp asymptotic analysis of the regularized phase-max estimator for structured phase retrieval (PR) problems. The phase-max algorithm is a recently proposed Linear Programming based estimator for PR. The finite sample as well as the sharp asymptotic performance of phase-max for unstructured phase retrieval was well understood from prior work. The authors propose a natural modification  by penalizing the usual phase-max objective with a regularizer $f$. For structured PR, prior work showed that for a $k$-sparse signal vector, L1-penalized phase-max recovers the signal with $m = O(klog(n/k))$ samples given a good enough initialization. The primary contribution of this work is (i) extending the result to absolutely scalable regularizers which allows for other kinds of structural assumptions on the signal (ii) complementing the finite sample with a high dimensional asymptotic result giving an explicit formula for the asymptotic sampling  ratio $\alpha = m/n$ required to achieve exact recovery in terms of the quality of the initialization and the oversampling ratio $\delta = d/m$ where $d$ is the effective degrees of freedom of the signal vector. The main tool used to establish this result is the Convex Gaussian Min-Max (CGMT) framework which is a well-established technique from prior work.  The quality and clarity of the work can be improved. Several of the statements of the theorems and lemmas are not very precise and some of the arguments are non-rigorous. However, the results are definitely interesting and plausible (see detailed comments below). They are also supported by simulations. Section 4.5 is clearly incomplete and ends abruptly. Citations in paragraph 509-515 are informal notes. To the best of my knowledge, the formula for the phase transition presented by the authors was not known before and is a significant and original contribution. While CGMT is a well-established technique, applying it to this problem seems to require some original and interesting problem specific analysis.  Detailed Comments about proofs: Lines 409-412: The CGMT lemma requires additional assumptions which are omitted and not verified in the proof. The compactness assumption on $S_w$ and $S_u$ is stated but not verified when invoking this lemma in 545-547. This usually requires some technical arguments. Lemma 3, Eq 23: The claimed $n \rightarrow \infty$ limit depends on $n$.  Lines 434-436: Approximation of $f$ by the linear underestimator is justified non-rigorously. Likewise in lines 471-475.  Lines 480-482: Can the authors state which lemma/theorem of [Thrampoulidis et al, 2016] they are invoking to justify changing the order of min-max? I am unable to find such a result in that paper and verify their claim.  Pages 17-18: To apply the CGMT theorem, the authors need to verify that the AO optimal value and the norm of the AO optimal solution converge in probability. It appears to me this has not been done rigorously. In particular the AO and PO have a distinctive feature that they depend on additional randomness $q$ apart from the usual randomness $A, g, h$. The authors don’t state how they handle this. CGMT is a general purpose theorem and most of the work involved is in verifying its technical conditions and it would nice to have these details in the appendix.  Lines 198-201: I don’t think the citation [9] calculates the statistical dimension of the descent cone of this objective. I can see this was being done in the appendix section 4.5 which is incomplete.  Other minor comments: Lines 98-100: It should be clarified that the paper by Soltanolkotabi [33] breaks the \emph{local} sample complexity barrier (sample complexity required for recovery when initialized closed enough to the truth) and not the global complexity barrier.  Lines 133-138: The reference stated does not justify the optimality of the sample complexity. To my understanding, these references give the sample complexity required for certain estimators to succeed for linear measurements. It might be more appropriate to have a reference for an information theoretic lower bound/ identifiability lower bound in the specific context of phase retrieval since the title of the paper also claims optimality.  Line 213: orthogonal projection of sub-diff .. into the orthogonal… : Remove first orthogonal.  Line 216: Missing linebreak at Assumption 1.  Figure 1(b): missing legend. Lines 237-241: Use \begin{remark} Line 187: Missing linebreak at Example 1.  Lines 167-168: Has the claim about the function $R$ been proved anywhere in the paper? If it is a known result, it needs a citation.  Lines 169-171: Is the requirement on $R(\delta)$ for all $n$?  --------------------------------------------------------------------  POST-REBUTTAL:  My primary concern  regarding this paper was that some of statements and arguments appeared non-rigorous and imprecise to me. The authors addressed these issues in their rebuttal. Furthermore since I do think this result is interesting to the NIPS community, I am increasing my score to 7.