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Title: A Data-Driven and Distributed Approach to Sparse Signal Representation and Recovery

Paper content:
In this paper, we focus on two challenges which offset the promise of sparse signal representation, sensing, and recovery.
First, real-world signals can seldom be described as perfectly sparse vectors in a known basis, and traditionally used random measurement schemes are seldom optimal for sensing them.
Second, existing signal recovery algorithms are usually not fast enough to make them applicable to real-time problems.
In this paper, we address these two challenges by presenting a novel framework based on deep learning.
For the first challenge, we cast the problem of finding informative measurements by using a maximum likelihood (ML) formulation and show how we can build a data-driven dimensionality reduction protocol for sensing signals using convolutional architectures.
For the second challenge, we discuss and analyze a novel parallelization scheme and show it significantly speeds-up the signal recovery process.
We demonstrate the significant improvement our method obtains over competing methods through a series of experiments.
High-dimensional inverse problems and low-dimensional embeddings play a key role in a wide range of applications in machine learning and signal processing.
In inverse problems, the goal is to recover a signal X ∈ R N from a set of measurements Y = Φ(X) ∈ R M , where Φ is a linear or non-linear sensing operator.
A special case of this problem is compressive sensing (CS) which is a technique for efficiently acquiring and reconstructing a sparse signal BID12 BID6 BID1 .
In CS Φ ∈ R M ×N (M N ) is typically chosen to be a random matrix resulting in a random low-dimensional embedding of signals.
In addition, X is assumed to be sparse in some basis Γ, i.e., X = ΓS, where S 0 = K N .While
sparse signal representation and recovery have made significant real-world impact in various fields over the past decade (Siemens, 2017) , arguably their promise has not been fully realized. The reasons
for this can be boiled down to two major challenges: First, real-world signals are only approximately sparse and hence, random/universal sensing matrices are sub-optimal measurement operators. Second, many
existing recovery algorithms, while provably statistically optimal, are slow to converge. In this paper
, we propose a new framework that simultaneously takes on both these challenges.To tackle the first challenge, we formulate the learning of the dimensionality reduction (i.e., signal sensing operator) as a likelihood maximization problem; this problem is related to the Infomax principle BID24 asymptotically. We then show
that the simultaneous learning of dimensionality reduction and reconstruction function using this formulation gives a lower-bound of the objective functions that needs to be optimized in learning the dimensionality reduction. This is similar
in spirit to what Vincent et al. show for denoising autoencoders in the non-asymptotic setting BID38 . Furthermore, we
show that our framework can learn dimensionality reductions that preserve specific geometric properties. As an example,
we demonstrate how we can construct a data-driven near-isometric low-dimensional embedding that outperforms competing embedding algorithms like NuMax BID18 . Towards tackling
the second challenge, we introduce a parallelization (i.e., rearrangement) scheme that significantly speeds up the signal sensing and recovery process. We show that our
framework can outperform state-of-the-art signal recovery methods such as DAMP BID26 and LDAMP BID25 both in terms of inference performance and computational efficiency.We now present a brief overview of prior work on embedding and signal recovery. Beyond random matrices
, there are other frameworks developed for deterministic construction of linear (or nonlinear) near-isometric embeddings BID18 BID16 BID0 BID35 BID39 BID5 BID37 BID32 . However, these approaches
are either computationally expensive, not generalizable to outof-sample data points, or perform poorly in terms of isometry. Our framework for low-dimensional
embedding shows outstanding performance on all these aspects with real datasets. Algorithms for recovering signals
from undersampled measurements can be categorized based on how they exploit prior knowledge of a signal distribution. They could use hand-designed priors
BID7 BID13 BID9 BID29 , combine hand-designed algorithms with data-driven priors BID25 BID3 BID20 BID8 BID17 , or take a purely data-driven approach BID28 BID22 BID41 . As one moves from hand-designed approaches
to data-driven approaches, models lose simplicity and generalizability while becoming more complex and more specifically tailored for a particular class of signals of interest.Our framework for sensing and recovering sparse signals can be considered as a variant of a convolutional autoencoder where the encoder is linear and the decoder is nonlinear and specifically designed for CS application. In addition, both encoder and decoder contain
rearrangement layers which significantly speed up the signal sensing and recovery process, as we discuss later. Convolutional autoencoder has been previously
used for image compression ; however, our work is mainly focused on the CS application rather than image compression. In CS, measurements are abstract and linear whereas
in the image compression application measurements are a compressed version of the original image and are nonlinear. Authors in have used bicubic interpolation for upscaling
images; however, our framework uses a data-driven approach for upscaling measurements. Finally, unlike the image compression application, when
we deploy our framework for CS and during the test phase, we do not have high-resolution images beforehand. In addition to image compression, there have been previous
works BID34 BID22 to jointly learn the signal sensing and reconstruction algorithm in CS using convolutional networks. However, the problem with these works is that they divide
images into small blocks and recover each block separately. This blocky reconstruction approach is unrealistic in applications
such as medical imaging (e.g. MRI) where the measurement operator is a Fourier matrix and hence we cannot have blocky reconstruction. Since both papers are designed for block-based recovery whereas our
method senses/recovers images without subdivision, we have not compared against them. Note that our method could be easily modified to learn near-optimal
frequency bands for medical imaging applications. In addition, BID34 and BID22 use an extra denoiser (e.g. BM3D, DCN)
for denoising the final reconstruction while our framework does not use any extra denoiser and yet outperforms state-of-the-art results as we show later.Beside using convolutional autoencoders, authors in BID40 have introduced the sparse recovery autoencoder (SRA). In SRA, the encoder is a fully-connected layer while in this work,
the encoder has a convolutional structure and is basically a circulant matrix. For large-scale problems, learning a fully-connected layer (as in
the SRA encoder) is significantly more challenging than learning convolutional layers (as in our encoder). In SRA, the decoder is a T -step projected subgradient. However,
in this work, the decoder is several convolutional layers
plus a rearranging layer. It should also be noted that the optimization in SRA is solely over
the measurement matrix and T (which is the number of layers in the decoder) scalar values. However, here, the optimization is performed over convolution weights
and biases that we have across different layers of our network.
In this paper we introduced DeepSSRR, a framework that can learn both near-optimal sensing schemes, and fast signal recovery procedures.
Our findings set the stage for several directions for future exploration including the incorporation of adversarial training and its comparison with other methods BID2 BID14 BID10 ).
Furthermore, a major question arising from our work is quantifying the generalizability of a DeepSSRR-learned model based on the richness of training data.
We leave the exploration of this for future research.

assistant: text: We use deep learning techniques to solve the sparse signal representation and recovery problem.