Publications by Year: 2013

Y. M. Lu, “A Framework for Adaptive Parameter Estimation with Finite Memory,” in Proc. IEEE Global Conference on Signal and Information Processing, Austin, TX, 2013.Abstract
We consider the problem of estimating an unknown parameter from a finite collection of different statistical experiments. The measurements are taken sequentially. Based on the observations made so far, we adaptively select the next experiment that provides the most information about the parameter. Summarizing past information with finite memory, we present a general framework for efficient adaptive estimation, with the sensing schemes fully characterized by finite-state parametric Markov chains. We establish an analytic formula linking the asymptotic performance of adaptive estimation schemes to the steady-state distributions of the associated Markov chains. Consequently, finding optimal adaptive strategies can be reformulated as the problem of designing a (continuous) family of Markov chains with prescribed steady-state distributions.We also propose a quantitative design criterion for optimal sensing policies based on minimax ratio regret.
A. Agaskar and Y. M. Lu, “ALARM: A Logistic Auto-Regressive Model for binary processes on networks,” in Proc. IEEE Global Conference on Signal and Information Processing, Austin, TX, 2013.Abstract

We introduce the ALARM model, a logistic autoregressive model for discrete-time binary processes on networks, and describe a technique for learning the graph structure underlying the model from observations. Using only a small number of parameters, the proposed ALARM can describe a wide range of dynamic behavior on graphs, such as the contact process, voter process, and even some epidemic processes. Under ALARM, at each time step, the probability of a node having value 1 is determined by the values taken by its neighbors in the past; specifically, its probability is given by the logistic function evaluated at a linear combination of its neighbors' past values (within a fixed time window) plus a bias term. We examine the behavior of this model for 1D and 2D lattice graphs, and observe a phase transition in the steady state for 2D lattices. We then study the problem of learning a graph from ALARM observations. We show how a regularizer promoting group sparsity can be used to efficiently learn the parameters of the model from a realization, and demonstrate the resulting ability to reconstruct the underlying network from the data.

Z. Sadeghipoor, Y. M. Lu, and S. Susstrunk, “A novel compressive sensing approach to simultaneously acquire color and near-infrared images on a single sensor,” in Proc. IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), Vancouver, Canada, 2013.Abstract

Sensors of most digital cameras are made of silicon that is inherently sensitive to both the visible and near-infrared parts of the electromagnetic spectrum. In this paper, we address the problem of color and NIR joint acquisition. We propose a framework for the joint acquisition that uses only a single silicon sensor and a slightly modified version of the Bayer color-filter array that is already mounted in most color cameras. Implementing such a design for an RGB and NIR joint acquisition system requires minor changes to the hardware of commercial color cameras. One of the important differences between this design and the conventional color camera is the post-processing applied to the captured values to reconstruct full resolution images. By using a CFA similar to Bayer, the sensor records a mixture of NIR and one color channel in each pixel. In this case, separating NIR and color channels in different pixels is equivalent to solving an under-determined system of linear equations. To solve this problem, we propose a novel algorithm that uses the tools developed in the field of compressive sensing. Our method results in high-quality RGB and NIR images (the average PSNR of more than 30 dB for the reconstructed images) and shows a promising path towards RGB and NIR cameras. 

S. H. Chan, T. Zickler, and Y. M. Lu, “Fast non-local filtering by random sampling: It works, especially for large images,” in Proc. IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), Vancouver, Canada, 2013.Abstract

Non-local means (NLM) is a popular denoising scheme. Conceptually simple, the algorithm is computationally intensive for large images. We propose to speed up NLM by using random sampling. Our algorithm picks, uniformly at random, a small number of columns of the weight matrix, and uses these ``representatives'' to compute an approximate result. It also incorporates an extra column-normalization of the sampled columns, a form of symmetrization that often boosts the denoising performance on real images. Using statistical large deviation theory, we analyze the proposed algorithm and provide guarantees on its performance. We show that the probability of having a large approximation error decays exponentially as the image size increases. Thus, for large images, the random estimates generated by the algorithm are tightly concentrated around their limit values, even if the sampling ratio is small. Numerical results confirm our theoretical analysis: the proposed algorithm reduces the run time of NLM, and thanks to the symmetrization step, actually provides some improvement in peak signal-to-noise ratios.

A. Agaskar and Y. M. Lu, “Detecting random walks hidden in noise: Phase transition on large graphs,” in Proc. IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), Vancouver, Canada, 2013.Abstract

We consider the problem of distinguishing between two hypotheses: that a sequence of signals on a large graph consists entirely of noise, or that it contains a realization of a random walk buried in the noise. The problem of computing the error exponent of the optimal detector is simple to formulate, but exhibits deep connections to problems known to be difficult, such as computing Lyapunov exponents of products of random matrices and the free entropy density of statistical mechanical systems. We describe these connections, and define an algorithm that efficiently computes the error exponent of the Neyman-Pearson detector. We also derive a closed-form formula, derived from a statistical mechanics-based approximation, for the error exponent on an arbitrary graph of large size. The derivation of this formula is not entirely rigorous, but it closely matches the empirical results in all our experiments. This formula explains a phase transition phenomenon in the error exponent: below a threshold SNR, the error exponent is nearly constant and near zero, indicating poor performance; above the threshold, there is rapid improvement in performance as the SNR increases. The location of the phase transition depends on the entropy rate of the random walk.

I. Dokmanic, R. Parhizkar, A. Walther, Y. M. Lu, and M. Vetterli, “Acoustic Echoes Reveal Room Shape,” Proceedings of the National Academy of Sciences (PNAS), vol. 110, no. 30, pp. 12186-12191, 2013. Full Text (PDF + Supplementary Info)Abstract
Imagine that you are blindfolded inside an unknown room. You snap your fingers and listen to the room’s response. Can you hear the shape of the room? Some people can do it naturally, but can we design computer algorithms that hear rooms? We show how to compute the shape of a convex polyhedral room from its re- sponse to a known sound, recorded by a few microphones. Geo- metric relationships between the arrival times of echoes enable us to “blindfoldedly” estimate the room geometry. This is achieved by exploiting the properties of Euclidean distance matrices. Fur- thermore, we show that under mild conditions, first-order echoes provide a unique description of convex polyhedral rooms. Our algorithm starts from the recorded impulse responses and pro- ceeds by learning the correct assignment of echoes to walls. In contrast to earlier methods, the proposed algorithm reconstructs the full 3D geometry of the room from a single sound emission, and with an arbitrary geometry of the microphone array. As long as the microphones can hear the echoes, we can position them as we want. Besides answering a basic question about the inverse problem of room acoustics, our results find applications in areas such as architectural acoustics, indoor localization, virtual reality, and audio forensics.
Y. M. Lu, “Adaptive sensing and inference for single-photon imaging,” in Proc. 47th Annual Conference on Information Sciences and Systems (CISS), Baltimore, MD, 2013.Abstract

In recent years, there have been increasing efforts to develop solid-state sensors with single-photon sensitivity, with applications ranging from bio-imaging to 3D computer vision. In this paper, we present adaptive sensing models, theory and algorithms for these single-photon sensors, aiming to improve their dynamic ranges. Mapping different sensor configurations onto a finite set of states, we represent adaptive sensing schemes as finite-state parametric Markov chains. After deriving an asymptotic expression for the Fisher information rate of these Markovian systems, we propose a design criterion for sensing policies based on minimax ratio regret. We also present a suboptimal yet effective sensing policy based on random walks. Numerical experiments demonstrate the strong performance of the proposed scheme, which expands the sensor dynamic ranges of existing nonadaptive approaches by several orders of magnitude.

C. Hu, L. Cheng, J. Sepulcre, G. E. Fakhri, Y. M. Lu, and Q. Li, “Matched signal detection on graphs: Theory and application to brain network classification,” in Proc. 23rd International Conference on Information Processing in Medical Imaging (IPMI 2013), Asilomar, CA, 2013.Abstract
We develop a matched signal detection (MSD) theory for signals with an intrinsic structure described by a weighted graph. Hypothesis tests are formulated under different signal models. In the simplest scenario, we assume that the signal is deterministic with noise in a subspace spanned by a subset of eigenvectors of the graph Laplacian. The conventional matched subspace detection can be easily extended to this case. Furthermore, we study signals with certain level of smoothness. The test turns out to be a weighted energy detector, when the noise variance is negligible. More generally, we presume that the signal follows a prior distribution, which could be learnt from training data. The test statistic is then the difference of signal variations on associated graph structures, if an Ising model is adopted. Effectiveness of the MSD on graph is evaluated both by simulation and real data. We apply it to the network classification problem of Alzheimer’s disease (AD) particularly. The preliminary results demonstrate that our approach is able to exploit the sub-manifold structure of the data, and therefore achieve a better performance than the traditional principle component analysis (PCA).
C. Hu, L. Cheng, J. Sepulcre, G. E. Fakhri, Y. M. Lu, and Q. Li, “A graph theoretical regression model for brain connectivity learning of Alzheimer's disease,” in Proc. International Symposium on Biomedical Imaging (ISBI), San Francisco, CA, 2013.Abstract

Learning functional brain connectivity is essential to the understanding of neurodegenerative diseases. In this paper, we introduce a novel graph regression model (GRM) which regards the imaging data as signals defined on a graph and optimizes the fitness between the graph and the data, with a sparsity level regularization. The proposed framework features a nice interpretation in terms of low-pass signals on graphs, and is more generic compared with previous statistical models. Results based on the data illustrates that our approach can obtain a very close reconstruction of the true network. We then apply the GRM to learn the brain connectivity of Alzheimer’s disease (AD). Evaluations performed upon PET imaging data of 30 AD patients demonstrate that the connectivity patterns discovered are easy to interpret and consistent with known pathology.

A. Agaskar and Y. M. Lu, “A Spectral Graph Uncertainty Principle,” IEEE Transactions on Information Theory, vol. 59, no. 7, pp. 4338-4356, 2013.Abstract

The spectral theory of graphs provides a bridge between classical signal processing and the nascent field of graph signal processing. In this paper, a spectral graph analogy to Heisenberg's celebrated uncertainty principle is developed. Just as the classical result provides a tradeoff between signal localization in time and frequency, this result provides a fundamental tradeoff between a signal's localization on a graph and in its spectral domain. Using the eigenvectors of the graph Laplacian as a surrogate Fourier basis, quantitative definitions of graph and spectral ``spreads'' are given, and a complete characterization of the feasibility region of these two quantities is developed. In particular, the lower boundary of the region, referred to as the uncertainty curve, is shown to be achieved by eigenvectors associated with the smallest eigenvalues of an affine family of matrices. The convexity of the uncertainty curve allows it to be found to within $\varepsilon$ by a fast approximation algorithm requiring $\mathcal{O}(\varepsilon^{-1/2})$ typically sparse eigenvalue evaluations. Closed-form expressions for the uncertainty curves for some special classes of graphs are derived, and an accurate analytical approximation for the expected uncertainty curve of Erdos-Renyi random graphs is developed. These theoretical results are validated by numerical experiments, which also reveal an intriguing connection between diffusion processes on graphs and the uncertainty bounds.