@article{40785, abstract = {{Two concerns often arise simultaneously when applying linear estimation in communication systems: the computational complexity can be prohibitively high when the system size is large, and the performance may degrade dramatically when the presumed model is mismatched with the actual system. In this paper, we introduce a subspace expansion framework to jointly address these concerns, in which the observation is first projected onto a lower-dimensional subspace and then the solution of the projected problem is regularized. We discuss two projection methods based on eigensubspace and Krylov subspace expansions. We show that the Krylov subspace projection provides an economical solution to regularized linear estimation. We also compare different regularization methods, such as principal components and diagonal loading. We show that diagonal loading generally outperforms other alternatives and that Krylov subspace rank reduction can yield a regularization effect close to diagonal loading. Finally, we investigate the impact of preconditioning on the performance and complexity for mismatched modeling and propose a loaded preconditioner, which can reduce complexity as well as preserve the regularization effect. Under the proposed framework, various regularization schemes are studied and some guidelines for choosing the right scheme are provided.}}, author = {{Tong, Jun and Schreier, Peter J.}}, journal = {{Signal Process.}}, number = {{9}}, pages = {{2671–2686}}, title = {{{A unified framework for regularized linear estimation in communication systems}}}, doi = {{10.1016/j.sigpro.2013.03.002}}, volume = {{93}}, year = {{2013}}, } @inproceedings{40781, abstract = {{This article is concerned with global asymptotic stability (GAS) of dynamical networks. The case when subsystems are integral input-to-state stable (iISS) has been recognized as notoriously difficult to deal with in the literature. In fact, the lack of energy dissipation for large input denies direct application of the small-gain argument for input-to-state stable (ISS) subsystems. Here for networks consisting of iISS subsystems it is demonstrated that a two-phase approach allows us to make use of the ISS small-gain argument by separating a trajectory into a transient and a subsequent convergence. In contrast to the previous iISS results, the two-phase approach immediately leads to a sufficient criterion for GAS of general nonlinear networks in a matrix-like form (order condition).}}, author = {{Ito, Hiroshi and Rüffer, Björn S.}}, booktitle = {{Proc. IEEE American Contr. Conf.}}, pages = {{4838–4843}}, title = {{{A Two-Phase Approach to Stability of Networks Given in iISS Framework: Utilization of a Matrix-Like Criterion}}}, year = {{2013}}, } @article{40782, abstract = {{Two similar stability notions are considered; one is the long established notion of convergent systems, the other is the younger notion of incremental stability. Both notions require that any two solutions of a system converge to each other. Yet these stability concepts are different, in the sense that none implies the other, as is shown in this paper using two examples. It is shown under what additional assumptions one property indeed implies the other. Furthermore, this paper contains necessary and sufficient characterizations of both properties in terms of Lyapunov functions.}}, author = {{Rüffer, Björn S. and van de Wouw, Nathan and Mueller, Markus}}, journal = {{Systems Control Lett.}}, pages = {{277–285}}, title = {{{Convergent Systems vs. Incremental Stability}}}, doi = {{10.1016/j.sysconle.2012.11.015}}, volume = {{62}}, year = {{2013}}, } @inproceedings{40779, abstract = {{This paper provides sufficient conditions for the incremental stability of time-delayed nonlinear systems. It relies on the Razumikhin-Lyapunov approach, which consists in invoking small-gain arguments by treating the delayed state as a feedback perturbation. The results are valid for multiple delays, as well as bounded time-varying delays. We provide conditions under which the limit solution of a time-delayed nonlinear system to a periodic (resp. constant) input is itself periodic and of the same period (resp. constant). As an illustration, a specific focus is given on a class of delayed Lur’e systems.}}, author = {{Chaillet, Antoine and Yu. Pogromsky, Alexander and Rüffer, Björn S.}}, booktitle = {{Proc. 52nd IEEE Conf. Decis. Control}}, pages = {{1596–1601}}, title = {{{A Razumikhin approach for the incremental stability of delayed nonlinear systems}}}, year = {{2013}}, } @article{40780, author = {{Real, Eusebio and Eguizabal, Alma and Pontón, Alejandro and Díez, Marta Calvo and Val-Bernal, José Fernando and Mayorga, Marta and Revuelta, José M. and López-Higuera, José M. and Conde, Olga M.}}, journal = {{J. Biomed. Opt.}}, number = {{12}}, pages = {{126003–126003}}, title = {{{Optical coherence tomography assessment of vessel wall degradation in thoracic aortic aneurysms}}}, doi = {{10.1117/1.JBO.18.12.126003}}, volume = {{18}}, year = {{2013}}, } @inproceedings{40778, abstract = {{Separable Lyapunov functions play vital roles, for example, in stability analysis of large-scale systems. A Lyapunov function is called max-separable if it can be decomposed into a maximum of functions with one-dimensional arguments. Similarly, it is called sum-separable if it is a sum of such functions. In this paper it is shown that for a monotone system on a compact state space, asymptotic stability implies existence of a max-separable Lyapunov function. We also construct two systems on a non-compact state space, for which a max-separable Lyapunov function does not exist. One of them has a sum-separable Lyapunov function. The other does not.}}, author = {{Rantzer, Anders and Rüffer, Björn S. and Dirr, Gunther}}, booktitle = {{Proc. 52nd IEEE Conf. Decis. Control}}, pages = {{4590–4594}}, title = {{{Separable Lyapunov functions for monotone systems}}}, year = {{2013}}, } @article{40786, author = {{Tong, Jun and Schreier, Peter J.}}, journal = {{IEEE Wireless Comm. Lett.}}, number = {{4}}, pages = {{367–370}}, title = {{{Regularized Preconditioning for Krylov Subspace Equalization of OFDM Systems over Doubly Selective Channels}}}, doi = {{10.1109/WCL.2013.042313.130096}}, volume = {{2}}, year = {{2013}}, } @inproceedings{40784, author = {{Castañeda, Mario and Stein, Manuel and Antreich, Felix and Tasdemir, Emrah and Kurz, Lothar and Noll, Tobias G. and Nossek, Josef A.}}, booktitle = {{ION GNSS+}}, title = {{{Joint space-time interference mitigation for embedded multi-antenna GNSS receivers}}}, year = {{2013}}, } @article{40787, abstract = {{Breast tumors are blindly identified using Principal (PCA) and Independent Component Analysis (ICA) of localized reflectance measurements. No assumption of a particular theoretical model for the reflectance needs to be made, while the resulting features are proven to have discriminative power of breast pathologies. Normal, benign and malignant breast tissue types in lumpectomy specimens were imaged ex vivo and a surgeon-guided calibration of the system is proposed to overcome the limitations of the blind analysis. A simple, fast and linear classifier has been proposed where no training information is required for the diagnosis. A set of 29 breast tissue specimens have been diagnosed with a sensitivity of 96% and specificity of 95% when discriminating benign from malignant pathologies. The proposed hybrid combination PCA-ICA enhanced diagnostic discrimination, providing tumor probability maps, and intermediate PCA parameters reflected tissue optical properties.}}, author = {{Eguizabal, Alma and Laughney, Ashley M. and Garcia-Allende, P. Beatriz and Krishnaswamy, Venkataramanan and Wells, Wendy A. and Paulsen, Keith D. and Pogue, Brian W. and Lopez-Higuera, Jose M. and Conde, Olga M.}}, journal = {{Biomed. Opt. Express}}, number = {{7}}, pages = {{1104–1118}}, title = {{{Direct identification of breast cancer pathologies using blind separation of label-free localized reflectance measurements}}}, doi = {{10.1364/BOE.4.001104}}, volume = {{4}}, year = {{2013}}, } @article{40811, abstract = {{Deadwood moisture affects multiple functions of downed logs in forest ecosystems. They include provision of habitats for xylobionts, additional water stores and organic carbon stocks. In contrast to Northern American forests, little is known about moisture variability in downed logs of important tree species within the Temperate Zone of Europe. Therefore, our study aimed at elucidating this variability according to species, site and decay class (DC). Measurements were taken by TDR during two vegetation periods in eight Carpathian natural forests representing distinct site conditions, ranging from xerothermophilous to subalpine. Downed logs of \emphQuercus spp., \emph{Abies alba} Mill., \emph{Fagus sylvatica} L., and \emph{Picea abies} L., belonging to various DCs, were selected and instrumented with TDR probes. Species and DC-specific TDR calibration showed the importance of intrinsic wood porosity. The course of deadwood moisture consisted of drying during the early decay stage, except for \emph{A. alba} and \emph{F. sylvatica}, and an intense water reabsorption at later decay stages. Average moisture for all species and sites displayed seasonal trends, reflecting the occurrence of precipitation clusters and dry periods, as well as short-term fluctuations. Cross-spectral analysis revealed that both sapwood and heartwood participated in wetting and drying processes, but only after reaching an advanced stage of decay. New findings can be applied in interpreting, modelling and predicting deadwood water stores, habitat properties and respiration.}}, author = {{Pichler, V. and Homolák, M. and Skierucha, W. and Pichlerová, M. and Ramírez, D. and Gregor, J. and Jaloviar, P.}}, journal = {{Ecohydrology}}, number = {{4}}, pages = {{424–434}}, title = {{{Variability of moisture in coarse woody debris from several ecologically important tree species of the Temperate Zone of Europe}}}, doi = {{10.1002/eco.235}}, volume = {{5}}, year = {{2012}}, } @inproceedings{40812, abstract = {{Spectrum sensing has become one of the main components of a cognitive transmitter. Conventional detectors suffer from noise power uncertainties and multiantenna detectors have been proposed to overcome this difficulty, and to improve the detection performance. However, most of the proposed multiantenna detectors are based on non-optimal techniques, such as the generalized likelihood ratio test (GLRT), or even heuristic approaches that are not based on first principles. In this work, we derive the locally most powerful invariant test (LMPIT), that is, the optimal invariant detector for close hypotheses, or equivalently, for a low signal-to-noise ratio (SNR). The traditional approach, based on the distributions of the maximal invariant statistic, is avoided thanks to Wijsman’s theorem, which does not need these distributions. Our findings show that, in the low SNR regime, and in contrast to the GLRT, the additional spatial structure imposed by the signal model is irrelevant for optimal detection. Finally, we use Monte Carlo simulations to illustrate the good performance of the LMPIT.}}, author = {{Ramírez, D. and Iscar, J. and Vía, J. and Santamaría, I. and Scharf, L. L.}}, booktitle = {{Proc.\ IEEE Sensor Array and Multichannel Signal Process. Work.}}, title = {{{The Locally Most Powerful Invariant Test for Detecting a rank-$P$ Gaussian Signal in White Noise}}}, doi = {{10.1109/SAM.2012.6250547}}, year = {{2012}}, } @inproceedings{40820, abstract = {{An alternating optimization algorithm was recently proposed for the K-user multiple-input multiple-output (MIMO) interference channel. For flat-fading channels and feasible problems, this algorithm successfully aligns the interfering signals exploiting the spatial dimensions. In this paper, we consider the case in which all pairwise MIMO channels are frequency-selective (convolutive), and the users transmit broadband signals using a single-carrier scheme. Unlike the flat-fading case, for frequency-selective channels it is necessary to add a spectral mask in the frequency response of the precoders and decoders to avoid trivial solutions. We show in the paper that each step of the alternating minimization algorithm can be reformulated as a convex optimization problem in which the autocorrelation function of the precoders or decoders is obtained. Upon convergence, a final spectral factorization stage must be applied to obtain the precoders and decoders from their autocorrelation functions. Simulation results are provided to illustrate the performance of the proposed algorithm.}}, author = {{Lameiro, Christian and Vía, Javier and Santamaría, Ignacio and Heath Jr., Robert W.}}, booktitle = {{Proc.\ IEEE Int.\ Conf.\ Acoustics, Speech and Signal Process.}}, title = {{{Interference Leakage Minimization for Convolutive MIMO Interference Channels}}}, doi = {{10.1109/ICASSP.2012.6288506}}, year = {{2012}}, } @article{40817, abstract = {{This paper studies large multiple-input multiple-output (MIMO) communication systems with linear precoding and reduced-rank Krylov subspace receivers. We design precoders and analyze their performance by exploiting large-dimensional random matrix theory. We first devise low-complexity precoding schemes that can improve performance of low-rank Krylov subspace receivers in the regime of high signal-to-noise ratio (SNR). We then introduce a potential theory-based method for analyzing the convergence behavior of the mean-squared error (MSE) for various transmission schemes. This method can be applied to a broader range of problems compared to previous analytical tools. The analysis reveals that the MSE decreases super exponentially with the rank of the receiver. Numerical examples show that the proposed precoders can outperform conventional precoders when low-rank Krylov subspace receivers are used, and that the performance of such receivers can be accurately predicted.}}, author = {{Tong, Jun and Schreier, Peter J. and Weller, Steven R.}}, journal = {{{IEEE} {T}rans.\ {S}ignal\ {P}rocess.}}, number = {{5}}, pages = {{2482–2493}}, title = {{{Design and analysis of large MIMO systems with Krylov subspace receivers}}}, doi = {{10.1109/TSP.2012.2187287}}, volume = {{60}}, year = {{2012}}, } @inproceedings{40819, abstract = {{Spectrum sensing is a key component of the cognitive radio (CR) paradigm. Among CR detectors, multiantenna detectors are gaining popularity since they improve the detection performance and are robust to noise uncertainties. Traditional approaches to multiantenna spectrum sensing are based on the generalized likelihood ratio test (GLRT) or other heuristic detectors, which are not optimal in the Neyman-Pearson sense. In this work, we derive the locally most powerful invariant test (LMPIT), which is the optimal detector, among those preserving the problem invariances, in the low SNR regime. In particular, we apply Wijsman’s theorem, which provides us an alternative way to derive the ratio of the distributions of the maximal invariant statistic. Finally, numerical simulations illustrate the performance of the proposed detector.}}, author = {{Ramírez, D. and Vía, J. and Santamaría, I.}}, booktitle = {{Proc.\ IEEE Int.\ Conf.\ Acoustics, Speech and Signal Process.}}, title = {{{The Locally Most Powerful Test For Multiantenna Spectrum Sensing With Uncalibrated Receivers}}}, doi = {{10.1109/ICASSP.2012.6288655}}, year = {{2012}}, } @inproceedings{40821, abstract = {{This paper deals with different techniques for linear equalization (LE) of multipath channels with imperfect channel estimation (CE). We develop a unified framework based on Krylov subspace expansion, which allows us to compare the performance of the conjugate gradient (CG) method, diagonal loading (DL), and a hybrid scheme. Our analysis shows that the DL method generally outperforms its alternatives, but at the cost of higher complexity. However, we also demonstrate that a proper implementation of the low-complexity CG method can also approach the performance of DL. Finally, we show that preconditioning degrades performance when the CE is poor.}}, author = {{Tong, Jun and Schreier, Peter J.}}, booktitle = {{Proc.\ IEEE Int.\ Conf.\ Acoustics, Speech and Signal Process.}}, pages = {{3009–3012}}, title = {{{Regularized linear equalization for multipath channels with imperfect channel estimation}}}, doi = {{10.1109/ICASSP.2012.6288548}}, year = {{2012}}, } @inproceedings{40823, author = {{Eguizabal, Alma and Laughney, Ashley M. and Garcia-Allende, P. Beatriz and Krishnaswamy, Venkataramanan and Wells, Wendy A. and Paulsen, Keith D. and Pogue, Brian W. and Lopez-Higuera, Jose and Conde, Olga M.}}, booktitle = {{SPIE Photonics West 2012-BIOS, VI Biomedical Applications of Light Scattering}}, title = {{{Enhanced tumor contrast during breast lumpectomy provided by independent component analysis of localized reflectance measures}}}, year = {{2012}}, } @inproceedings{40822, author = {{Eguizabal, Alma and Laughney, Ashley M. and Garcia-Allende, P. Beatriz and Krishnaswamy, Venkataramanan and Wells, Wendy A. and Paulsen, Keith D. and Pogue, Brian W. and Lopez-Higuera, Jose and Conde, Olga M.}}, booktitle = {{SPIE Photonics West BIOS, VI Biomedical Applications of Light Scattering}}, title = {{{Blind breast tissue diagnosis using independent component analysis of localized backscattering response}}}, year = {{2012}}, } @inproceedings{40818, author = {{Enneking, Christoph and Stein, Manuel and Castañeda, Mario and Antreich, Felix and Nossek, Josef A.}}, booktitle = {{Proceedings of IEEE/ION PLANS 2012}}, title = {{{Multi-Satellite Time-Delay Estimation for Reliable High-Resolution GNSS Receivers}}}, year = {{2012}}, } @article{40810, abstract = {{This paper considers large multiple-input multiple-output (MIMO) communication systems with linear precoding and linear minimum mean-squared error (LMMSE) equalization based on the iterative conjugate gradient (CG) algorithm. Convergence of the CG algorithm is fast when the eigenvalues of the received signal’s covariance matrix are clustered, suggesting that mean-squared error and receiver complexity can be managed with judicious precoder design. In order to accelerate convergence of an iterative CG receiver, we incorporate constraints on two measures of eigenvalue clustering into the precoder design. Closed-form solutions to the optimal precoders are derived using majorization theory and convex optimization techniques. We show that if there are constraints on receiver complexity, the proposed precoders can improve performance for large MIMO systems operating over slowly time-varying fading channels.}}, author = {{Tong, Jun and Schreier, Peter J. and Weller, Steven R.}}, journal = {{IEEE Trans.\ Wireless Comm.}}, number = {{8}}, pages = {{2828–2837}}, title = {{{Linear precoding for MIMO systems with low-complexity receivers}}}, doi = {{10.1109/TWC.2012.070912.110877}}, volume = {{11}}, year = {{2012}}, } @inproceedings{40807, abstract = {{The Strong Uncorrelating Transform (SUT) allows blind separation of a mixture of complex independent sources if and only if all sources have distinct circularity coefficients. In practice, the circularity coefficients need to be estimated from observed data. We propose a generalized likelihood ratio test (GLRT) for separability of a complex mixture using the SUT, based on estimated circularity coefficients. For distinct circularity coefficients (separable case), the maximum likelihood (ML) estimates, required for the GLRT, are straightforward. However, for circularity coefficients with multiplicity larger than one (non-separable case), the ML estimates are much more difficult to find. Numerical simulations show the good performance of the proposed detector.}}, author = {{Ramírez, D. and Schreier, P. J. and Vía, J. and Santamaría, I.}}, booktitle = {{Proc.\ IEEE Int.\ Work. Machine Learning for Signal Process.}}, title = {{{GLRT For Testing Separability Of A Complex-Valued Mixture Based On The Strong Uncorrelating Transform}}}, doi = {{10.1109/MLSP.2012.6349785}}, year = {{2012}}, }