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所需积分：1Publications |
Signal & System Theory Group | University of Paderborn, Germany
Signal & System Theory Group
Professor Peter Schreier
1Peter J. Schreier and Louis L. ScharfStatistical signal processing of complex-valued dataThe theory of improper and noncircular signals330 pages,&Cambridge University Press,&2010.@book{SchreierScharf:2010:Statistical-Signal-Processing-of-Complex,
abstract = {Complex-valued random signals are embedded in the very fabric of science and engineering, yet the usual assumptions made about their statistical behavior are often a poor representation of the underlying physics. This book deals with improper and noncircular complex signals, which do not conform to classical assumptions, and it demonstrates how correct treatment of these signals can have significant payoffs. The book begins with detailed coverage of the fundamental theory and presents a variety of tools and algorithms for dealing with improper and noncircular signals. It provides a comprehensive account of the main applications, covering detection, estimation, and signal analysis of stationary, nonstationary, and cyclostationary processes. Providing a systematic development from the origin of complex signals to their probabilistic description makes the theory accessible to newcomers. This book is ideal for graduate students and researchers working with complex data in a range of research areas from communications to oceanography.},
author = {Schreier, Peter J. and Scharf, Louis L.},
title = {Statistical signal processing of complex-valued data: the theory of improper and noncircular signals},
year = {2010},
publisher = {{C}ambridge {U}niversity {P}ress},
Journal articles
2Rate region boundary of the SISO Z-interference channel with improper signaling(Christian Lameiro, Ignacio Santamar&a and Peter J. Schreier)IEEE Trans. Comm.,&2016, accepted for publication.@article{Lameiro:2016ac,
author = {Lameiro, Christian and Santamar{\'i}a, Ignacio and Schreier, Peter J.},
title = {Rate region boundary of the {SISO Z}-interference channel with improper signaling},
year = {2016, accepted for publication},
journal = {{IEEE} {T}rans.\ {C}omm.},
}3Spatial interference shaping for underlay MIMO cognitive networks(Christian Lameiro, Wolfgang Utschick and Ignacio Santamar&a)Signal Process.,&2016, accepted for publication.@article{Lameiro:2016ab,
author = {Lameiro, Christian and Utschick, Wolfgang and Santamar{\'i}a, Ignacio},
title = {Spatial interference shaping for underlay {MIMO} cognitive networks},
year = {2016, accepted for publication},
journal = {Signal Process.},
}4A pessimistic approximation for the Fisher information measure(Manuel S. Stein and Josef A. Nossek)IEEE Trans. Signal Process.,&2016, accepted for publication.@article{Stein:SPT:2016:1,
author = {Stein, Manuel S. and Nossek, Josef A.},
title = {A pessimistic approximation for the {F}isher information measure},
year = {2016, accepted for publication},
journal = {{{IEEE}} {{T}}rans.\ {{S}}ignal\ {{P}}rocess.},
}5Fisher information bounds with applications in nonlinear learning, compression and inference(Manuel S. Stein, Josef A. Nossek and Kurt Barb&)submitted,&2016.@article{Stein:SPT:2016:2,
author = {Stein, Manuel S. and Nossek, Josef A. and Barb{\'e}, Kurt},
title = {Fisher information bounds with applications in nonlinear learning, compression and inference},
year = {2016},
journal = {submitted},
}6Canonical correlation analysis of high-dimensional data with very small sample support(Yang Song, Peter J. Schreier, David Ram&rez and Tanuj Hasija)Signal Process.&128,& pp. 449&458,&November&2016.@article{Song:2016aa,
author = {Song, Yang and Schreier, Peter J. and Ram{\'i}rez, David and Hasija, Tanuj},
month = {{N}ovember},
title = {Canonical correlation analysis of high-dimensional data with very small sample support},
year = {2016},
journal = {{S}ignal {P}rocess.},
pages = {449&458},
volume = {128},
}7Sample-poor estimation of order and common signal subspace with application to fusion of medical imaging data(Y. Levin-Schwartz, Yang Song, Peter J. Schreier, V.D. Calhoun and T&lay Adali)NeuroImage&134,& pp. 486&493,&July&2016.@article{Levin-Schwartz:2016aa,
author = {Levin-Schwartz, Y. and Song, Yang and Schreier, Peter J. and Calhoun, V.D. and Adali, T{\"u}lay},
month = {{J}uly},
title = {Sample-poor estimation of order and common signal subspace with application to fusion of medical imaging data},
year = {2016},
journal = {{N}euro{I}mage},
pages = {486&493},
volume = {134},
}8Shrinkage of covariance matrices for linear signal estimation using cross-validation(Jun Tong, Peter J. Schreier, Qinghua Guo, S. Tong, Jiangtao Xi and Y. Yu)IEEE Trans. Signal Process.&64&(11),& pp. ,&June&2016.@article{Tong:2016aa,
author = {Tong, Jun and Schreier, Peter J. and Guo, Qinghua and Tong, S. and Xi, Jiangtao and Yu, Y.},
month = {{J}une},
title = {Shrinkage of covariance matrices for linear signal estimation using cross-validation},
year = {2016},
journal = {{{IEEE}} {{T}}rans.\ {{S}}ignal\ {{P}}rocess.},
number = {11},
pages = {},
volume = {64},
}9A unified scheme to achieve the degrees-of-freedom region of the MIMO interference channel with delayed channel state information(Mohsen Rezaee, Peter J. Schreier, M. Guillaud and B. Clerckx)IEEE Trans. Comm.&64&(3),& pp. ,&March&2016.@article{Rezaee:2016aa,
author = {Rezaee, Mohsen and Schreier, Peter J. and Guillaud, M. and Clerckx, B.},
month = {{M}arch},
title = {A unified scheme to achieve the degrees-of-freedom region of the {MIMO} interference channel with delayed channel state information},
year = {2016},
journal = {{IEEE} {T}rans.\ {C}omm.},
number = {3},
pages = {},
volume = {64},
}10Experimental evaluation of interference alignment for broadband WLAN systems(C. Lameiro, &O. Gonz&lez, J.A. Garc&a-Naya, I. Santamar&a and L. Castedo)EURASIP Journal on Wireless Communications and Networking,&2015.DOI:.@article{LameiroGonzalezGarcia-Naya:2015:Experimental-evaluation-of-interference-alignment,
abstract = {In this paper, we present an experimental study on the performance of spatial interference alignment (IA) in indoor wireless local area network scenarios that use orthogonal frequency division multiplexing (OFDM) according to the physical-layer specifications of the IEEE 802.11a standard. Experiments have been carried out using a wireless network testbed capable of implementing a 3-user MIMO interference channel. We have implemented IA decoding schemes that can be designed according to distinct criteria (e.g., zero-forcing or MaxSINR). The measurement methodology has been validated considering practical issues like the number of OFDM training symbols used for channel estimation or feedback time. In case of asynchronous users, a time-domain IA decoding filter is also compared to its frequency-domain counterpart. We also evaluated the performance of IA from bit error ratio measurement-based results in comparison to different time-division multiple access transmission schemes. The comparison includes single- and multiple-antenna systems transmitting over the dominant mode of the MIMO channel. Our results indicate that spatial IA is suitable for practical indoor scenarios in which wireless channels often exhibit relatively large coherence times.},
author = {C. Lameiro and {\'O}. Gonz{\'a}lez and J.A. Garc{\'i}a-Naya and I. Santamar{\'i}a and L. Castedo},
title = {Experimental evaluation of interference alignment for broadband {WLAN} systems},
year = {2015},
journal = {EURASIP Journal on Wireless Communications and Networking},
doi = {10.-015-0409-z},
}In this paper, we present an experimental study on the performance of spatial interference alignment (IA) in indoor wireless local area network scenarios that use orthogonal frequency division multiplexing (OFDM) according to the physical-layer specifications of the IEEE 802.11a standard. Experiments have been carried out using a wireless network testbed capable of implementing a 3-user MIMO interference channel. We have implemented IA decoding schemes that can be designed according to distinct criteria (e.g., zero-forcing or MaxSINR). The measurement methodology has been validated considering practical issues like the number of OFDM training symbols used for channel estimation or feedback time. In case of asynchronous users, a time-domain IA decoding filter is also compared to its frequency-domain counterpart. We also evaluated the performance of IA from bit error ratio measurement-based results in comparison to different time-division multiple access transmission schemes. The comparison includes single- and multiple-antenna systems transmitting over the dominant mode of the MIMO channel. Our results indicate that spatial IA is suitable for practical indoor scenarios in which wireless channels often exhibit relatively large coherence times.11Asymptotic parameter tracking performance with measurement data of 1-bit resolution(Manuel Stein, Alexander K&rzl, Amine Mezghani and Josef A. Nossek)IEEE Trans. Signal Process.&63&(22),& pp. ,&November&2015.DOI:.@article{Stein:SPT:2016,
author = {Stein, Manuel and K{\"u}rzl, Alexander and Mezghani, Amine and Nossek, Josef A.},
month = {{N}ovember},
title = {Asymptotic parameter tracking performance with measurement data of 1-bit resolution},
year = {2015},
journal = {{{IEEE}} {{T}}rans.\ {{S}}ignal\ {{P}}rocess.},
number = {22},
pages = {},
volume = {63},
doi = {10.1109/TSP.},
}12Detection of multivariate cyclostationarity(D. Ram&rez, P. J. Schreier, J. Via, I. Santamaria and L. L. Scharf)IEEE Trans. Signal Processing&63&(20),& pp. ,&October&2015.@article{RamirezSchreierVia:2015:Detection-of-multivariate-cyclostationarity,
author = {D. Ram{\'i}rez and P. J. Schreier and J. Via and I. Santamaria and L. L. Scharf},
month = {October},
title = {Detection of multivariate cyclostationarity},
year = {2015},
journal = {IEEE Trans. Signal Processing},
number = {20},
pages = {},
volume = {63},
}13Overdemodulation for high-performance receivers with low-resolution ADC(Manuel Stein, Theiler Sebastian and Josef A. Nossek)IEEE Wireless Comm. Lett.&4&(2),& pp. 169&172,&April&2015.DOI:.@article{Stein:WCL:2015,
author = {Stein, Manuel and Sebastian, Theiler and Nossek, Josef A.},
month = {{A}pril},
title = {Overdemodulation for high-performance receivers with low-resolution {ADC}},
year = {2015},
journal = {{IEEE} {W}ireless {C}omm. {L}ett.},
number = {2},
pages = {169&172},
volume = {4},
doi = {10.1109/LWC.},
}14Benefits of improper signaling for underlay cognitive radio(Christian Lameiro, Ignacio Santamar&a and Peter J. Schreier)IEEE Wireless Comm. Lett.&4,& pp. 22&25,&February&2015.DOI:.@article{LameiroSantamariaSchreier:2015:Benefits-of-Improper-Signaling-for-Underlay,
abstract = {In this letter we study the potential benefits of improper signaling for a secondary user (SU) in underlay cognitive radio networks. We consider a basic yet illustrative scenario in which the primary user (PU) always transmit proper Gaussian signals and has a minimum rate constraint. After parameterizing the SU transmit signal in terms of its power and circularity coefficient (which measures the degree of impropriety), we prove that the SU improves its rate by transmitting improper signals only when the ratio of the squared modulus between the SU-PU interference link and the SU direct link exceeds a given threshold. As a by-product of this analysis, we obtain the optimal circularity coefficient that must be used by the SU depending on its power budget. Some simulation results show that the SU benefits from the transmission of improper signals especially when the PU is not highly loaded.},
author = {Lameiro, Christian and Santamar{\'i}a, Ignacio and Schreier, Peter J.},
month = {{F}ebruary},
title = {Benefits of improper signaling for underlay cognitive radio},
year = {2015},
journal = {IEEE Wireless Comm. Lett.},
pages = {22&25},
volume = {4},
doi = {10.1109/LWC.},
}In this letter we study the potential benefits of improper signaling for a secondary user (SU) in underlay cognitive radio networks. We consider a basic yet illustrative scenario in which the primary user (PU) always transmit proper Gaussian signals and has a minimum rate constraint. After parameterizing the SU transmit signal in terms of its power and circularity coefficient (which measures the degree of impropriety), we prove that the SU improves its rate by transmitting improper signals only when the ratio of the squared modulus between the SU-PU interference link and the SU direct link exceeds a given threshold. As a by-product of this analysis, we obtain the optimal circularity coefficient that must be used by the SU depending on its power budget. Some simulation results show that the SU benefits from the transmission of improper signals especially when the PU is not highly loaded.15A flag representation for finite collections of subspaces of mixed dimensions(Bruce Draper, Michael Kirby, Justin Marks, Tim Marrinan and Chris Peterson)Lin. Alg. Appl.&451,& pp. 15&32,&Elsevier,&2014.DOI:.@article{draperLAA,
author = {Draper, Bruce and Kirby, Michael and Marks, Justin and Marrinan, Tim and Peterson, Chris},
title = {A flag representation for finite collections of subspaces of mixed dimensions},
year = {2014},
journal = {{L}in.\ {A}lg.\ {A}ppl.},
pages = {15&32},
volume = {451},
doi = {10.1016/j.laa.},
publisher = {Elsevier},
}16Multi-antenna spectrum sensing by exploiting spatio-temporal correlation(S. Ali, D. Ram&rez, M. Jansson, G. Seco-Granados and J. A. L&pez-Salcedo)Eurasip J. Applied Signal Process.&160,&2014.DOI:.@article{AliRamirezJansson:2014:Multi-antenna-spectrum-sensing-by-exploiting,
abstract = {In this paper, we propose a novel mechanism for spectrum sensing that leads us to exploit the spatio-temporal correlation present in the received signal at a multi-antenna receiver. For the proposed mechanism, we formulate the spectrum sensing scheme by adopting the generalized likelihood ratio test (GLRT). However, the GLRT degenerates in the case of limited sample support. To circumvent this problem, several extensions are proposed that bring robustness to the GLRT in the case of high dimensionality and small sample size. In order to achieve these sample-efficient detection schemes, we modify the GLRT-based detector by exploiting the covariance structure and factoring the large spatio-temporal covariance matrix into spatial and temporal covariance matrices. The performance of the proposed detectors is evaluated by means of numerical simulations, showing important advantages over existing detectors.},
author = {S. Ali and D. Ram{\'i}rez and M. Jansson and G. Seco-Granados and J. A. L{\'o}pez-Salcedo},
title = {Multi-antenna spectrum sensing by exploiting spatio-temporal correlation},
year = {2014},
journal = {{E}urasip\ {J}.\ {A}pplied {S}ignal {P}rocess.},
volume = {160},
doi = {10.80-},
}In this paper, we propose a novel mechanism for spectrum sensing that leads us to exploit the spatio-temporal correlation present in the received signal at a multi-antenna receiver. For the proposed mechanism, we formulate the spectrum sensing scheme by adopting the generalized likelihood ratio test (GLRT). However, the GLRT degenerates in the case of limited sample support. To circumvent this problem, several extensions are proposed that bring robustness to the GLRT in the case of high dimensionality and small sample size. In order to achieve these sample-efficient detection schemes, we modify the GLRT-based detector by exploiting the covariance structure and factoring the large spatio-temporal covariance matrix into spatial and temporal covariance matrices. The performance of the proposed detectors is evaluated by means of numerical simulations, showing important advantages over existing detectors.17Finding brain oscillations with power dependencies in neuroimaging data(S. D&hne, V. V. Nikulin, D. Ram&rez, P. J. Schreier, K.-R. M&ller and S. Haufe)NeuroImage&96,& pp. 334&348,&2014.DOI:.@article{DahneNikulinRamirez:2014:Finding-brain-oscillations-with,
abstract = {Phase synchronization among neuronal oscillations within the same frequency band has been hypothesized to be a major mechanism for communication between different brain areas. On the other hand, cross-frequency com- munications are more flexible allowing interactions between oscillations with different frequencies. Among such cross-frequency interactions amplitude-to-amplitude interactions are of a special interest as they show how the strength of spatial synchronization in different neuronal populations relates to each other during a given task. While, previously, amplitude-to-amplitude correlations were studied primarily on the sensor level, we present a source separation approach using spatial filters which maximize the correlation between the envelopes of brain oscillations recorded with electro-/magnetoencephalography (EEG/MEG) or intracranial multichannel re- cordings. Our approach, which is called canonical source power correlation analysis (cSPoC), is thereby capable of extracting genuine brain oscillations solely based on their assumed coupling behavior even when the signal-to- noise ratio of the signals is low. In addition to using cSPoC for the analysis of cross-frequency interactions in the same subject, we show that it can also be utilized for studying amplitude dynamics of neuronal oscillations across subjects. We assess the performance of cSPoC in simulations as well as in three distinctively different analysis sce- narios of real EEG data, each involving several subjects. In the simulations, cSPoC outperforms unsupervised state-of-the-art approaches. In the analysis of real EEG recordings, we demonstrate excellent unsupervised dis- covery of meaningful power-to-power couplings, within as well as across subjects and frequency bands.},
author = {S. D{\"a}hne and V. V. Nikulin and D. Ram{\'i}rez and P. J. Schreier and K.-R. M{\"u}ller and S. Haufe},
title = {Finding brain oscillations with power dependencies in neuroimaging data},
year = {2014},
journal = {NeuroImage},
pages = {334&348},
volume = {96},
doi = {10.1016/j.neuroimage.},
}Phase synchronization among neuronal oscillations within the same frequency band has been hypothesized to be a major mechanism for communication between different brain areas. On the other hand, cross-frequency com- munications are more flexible allowing interactions between oscillations with different frequencies. Among such cross-frequency interactions amplitude-to-amplitude interactions are of a special interest as they show how the strength of spatial synchronization in different neuronal populations relates to each other during a given task. While, previously, amplitude-to-amplitude correlations were studied primarily on the sensor level, we present a source separation approach using spatial filters which maximize the correlation between the envelopes of brain oscillations recorded with electro-/magnetoencephalography (EEG/MEG) or intracranial multichannel re- cordings. Our approach, which is called canonical source power correlation analysis (cSPoC), is thereby capable of extracting genuine brain oscillations solely based on their assumed coupling behavior even when the signal-to- noise ratio of the signals is low. In addition to using cSPoC for the analysis of cross-frequency interactions in the same subject, we show that it can also be utilized for studying amplitude dynamics of neuronal oscillations across subjects. We assess the performance of cSPoC in simulations as well as in three distinctively different analysis sce- narios of real EEG data, each involving several subjects. In the simulations, cSPoC outperforms unsupervised state-of-the-art approaches. In the analysis of real EEG recordings, we demonstrate excellent unsupervised dis- covery of meaningful power-to-power couplings, within as well as across subjects and frequency bands.18A quadratically convergent method for interference alignment in MIMO interference channels(&Oscar Gonz&lez, Christian Lameiro and Ignacio Santamar&a)IEEE Signal Process. Lett.&21,& pp. ,&November&2014.DOI:.@article{GonzalezLameiroSantamaria:2014:A-Quadratically-Convergent-Method-for-Interference,
abstract = {Alternating minimization and steepest descent are commonly used strategies to obtain interference alignment (IA) solutions in the $K$-user multiple-input multiple-output (MIMO) interference channel (IC). Although these algorithms are shown to converge monotonically, they experience a poor convergence rate, requiring an enormous amount of iterations which substantially increases with the size of the scenario. To alleviate this drawback, in this letter we resort to the Gauss-Newton (GN) method, which is well-known to experience quadratic convergence when the iterates are sufficiently close to the optimum. We discuss the convergence properties of the proposed GN algorithm and provide several numerical examples showing that it always converges to the optimum with quadratic rate, reducing dramatically the required computation time in comparison to other algorithms, hence paving a new way for the design of IA algorithms.},
author = {Gonz{\'a}lez, {\'O}scar and Lameiro, Christian and Santamar{\'i}a, Ignacio},
month = {{N}ovember},
title = {A quadratically convergent method for interference alignment in {MIMO} interference channels},
year = {2014},
journal = {IEEE Signal Process. Lett.},
pages = {},
volume = {21},
doi = {10.1109/LSP.},
}Alternating minimization and steepest descent are commonly used strategies to obtain interference alignment (IA) solutions in the K-user multiple-input multiple-output (MIMO) interference channel (IC). Although these algorithms are shown to converge monotonically, they experience a poor convergence rate, requiring an enormous amount of iterations which substantially increases with the size of the scenario. To alleviate this drawback, in this letter we resort to the Gauss-Newton (GN) method, which is well-known to experience quadratic convergence when the iterates are sufficiently close to the optimum. We discuss the convergence properties of the proposed GN algorithm and provide several numerical examples showing that it always converges to the optimum with quadratic rate, reducing dramatically the required computation time in comparison to other algorithms, hence paving a new way for the design of IA algorithms.19Detecting directionality in random fields using the monogenic signal(S. C. Olhede, D. Ram&rez and P. J. Schreier)IEEE Trans. Inform. Theory&60&(10),& pp. ,&October&2014.DOI:.@article{OlhedeRamirezSchreier:2014:Detecting-Directionality-in-Random-Fields,
abstract = {Detecting and analyzing directional structures in images is important in many applications since one-dimensional patterns often correspond to important features such as object contours or trajectories. Classifying a structure as directional or non-directional requires a measure to quantify the degree of directionality and a threshold, which needs to be chosen based on the statistics of the image. In order to do this, we model the image as a random field. So far, little research has been performed on analyzing directionality in random fields. In this paper, we propose a measure to quantify the degree of directionality based on the random monogenic signal, which enables a unique decomposition of a 2D signal into local amplitude, local orientation, and local phase. We investigate the second-order statistical properties of the monogenic signal for isotropic, anisotropic, and unidirectional random fields. We analyze our measure of directionality for finite-size sample images, and determine a threshold to distinguish between unidirectional and non-unidirectional random fields, which allows the automatic classification of images.},
author = {S. C. Olhede and D. Ram{\'i}rez and P. J. Schreier},
month = {{O}ctober},
title = {Detecting directionality in random fields using the monogenic signal},
year = {2014},
journal = {{IEEE} {T}rans.\ {I}nform.\ {T}heory},
number = {10},
pages = {},
volume = {60},
doi = {10.1109/TIT.},
}Detecting and analyzing directional structures in images is important in many applications since one-dimensional patterns often correspond to important features such as object contours or trajectories. Classifying a structure as directional or non-directional requires a measure to quantify the degree of directionality and a threshold, which needs to be chosen based on the statistics of the image. In order to do this, we model the image as a random field. So far, little research has been performed on analyzing directionality in random fields. In this paper, we propose a measure to quantify the degree of directionality based on the random monogenic signal, which enables a unique decomposition of a 2D signal into local amplitude, local orientation, and local phase. We investigate the second-order statistical properties of the monogenic signal for isotropic, anisotropic, and unidirectional random fields. We analyze our measure of directionality for finite-size sample images, and determine a threshold to distinguish between unidirectional and non-unidirectional random fields, which allows the automatic classification of images.20Optimization and estimation of complex-valued signals(Tulay Adali and Peter J. Schreier)IEEE Signal Processing Magazine&31&(5),& pp. 112&128,&September&2014.DOI:.@article{AdaliSchreier:2014:Optimization-and-estimation-of-complex-valued-signals,
abstract = {Complex-valued signals occur in many areas of science and engineering and are thus of fundamental interest. When developing signal processing methods in the complex domain, there are two key issues: making use of the full statistical information and optimization. In this article, we review the necessary tools to address these two key issues and provide examples in filtering and blind source separation (BSS) that utilize these tools.},
author = {Adali, Tulay and Schreier, Peter J.},
month = {{S}eptember},
title = {Optimization and estimation of complex-valued signals},
year = {2014},
journal = {{IEEE} {S}ignal {P}rocessing {M}agazine},
number = {5},
pages = {112&128},
volume = {31},
doi = {10.1109/MSP.},
}Complex-valued signals occur in many areas of science and engineering and are thus of fundamental interest. When developing signal processing methods in the complex domain, there are two key issues: making use of the full statistical information and optimization. In this article, we review the necessary tools to address these two key issues and provide examples in filtering and blind source separation (BSS) that utilize these tools.21Information-preserving transformations for signal parameter estimation(Manuel Stein, Mario Casta&eda, Amine Mezghani and Josef A. Nossek)IEEE Signal Process. Lett.&21&(7),& pp. 866&870,&July&2014.DOI:.@article{Stein:SPL:2014:02,
author = {Stein, Manuel and Casta{\~n}eda, Mario and Mezghani, Amine and Nossek, Josef A.},
month = {{J}uly},
title = {Information-preserving transformations for signal parameter estimation},
year = {2014},
journal = {{IEEE} {S}ignal {P}rocess.\ {L}ett.},
number = {7},
pages = {866&870},
volume = {21},
doi = {10.1109/LSP.},
}22A lower bound for the Fisher information measure(Manuel Stein, Amine Mezghani and Josef A. Nossek)IEEE Signal Process. Lett.&21&(7),& pp. 796&799,&July&2014.DOI:.@article{Stein:SPL:2014:01,
author = {Stein, Manuel and Mezghani, Amine and Nossek, Josef A.},
month = {{J}uly},
title = {A lower bound for the {F}isher information measure},
year = {2014},
journal = {{IEEE} {S}ignal {P}rocess.\ {L}ett.},
number = {7},
pages = {796&799},
volume = {21},
doi = {10.1109/LSP.},
}23A Bayesian approach for adaptive multiantenna sensing in cognitive radio networks(J. Manco-V&squez, M. L&zaro-Gredilla, D. Ram&rez, J. V&a and I. Santamar&a)Signal Process.&96, Part B,& pp. 228&240,&March&2014.DOI:.@article{Manco-VasquezLazaro-GredillaRamirez:2014:A-Bayesian-approach-for-adaptive-multiantenna,
abstract = {Recent work on multiantenna spectrum sensing in cognitive radio (CR) networks has been based on generalized likelihood ratio test (GLRT) detectors, which lack the ability to learn from past decisions and to adapt to the continuously changing environment. To overcome this limitation, in this paper we propose a Bayesian detector capable of learning in an efficient way the posterior distributions under both hypotheses. Our Bayesian model places priors directly on the spatial covariance matrices under both hypotheses, as well as on the probability of channel occupancy. Specifically, we use inverse-gamma and complex inverse-Wishart distributions as conjugate priors for the null and alternative hypotheses, and a binomial distribution as the prior for channel occupancy. At each sensing period, Bayesian inference is applied and the posterior for the channel occupancy is thresholded for detection. After a suitable approximation, the posteriors are employed as priors for the next sensing frame, which forms the basis of the proposed Bayesian learning procedure. The performance of the Bayesian detector is evaluated by simulations and by means of a CR testbed composed of universal radio peripheral (USRP) nodes. Both the simulations and experimental measurements show that the Bayesian detector outperforms the GLRT in a variety of scenarios.},
author = {J. Manco-V{\'a}squez and M. L{\'a}zaro-Gredilla and D. Ram{\'i}rez and J. V{\'i}a and I. Santamar{\'i}a},
month = {{M}arch},
title = {A {B}ayesian approach for adaptive multiantenna sensing in cognitive radio networks},
year = {2014},
journal = {{S}ignal {P}rocess.},
pages = {228&240},
volume = {96, Part B},
doi = {10.1016/j.sigpro.},
}Recent work on multiantenna spectrum sensing in cognitive radio (CR) networks has been based on generalized likelihood ratio test (GLRT) detectors, which lack the ability to learn from past decisions and to adapt to the continuously changing environment. To overcome this limitation, in this paper we propose a Bayesian detector capable of learning in an efficient way the posterior distributions under both hypotheses. Our Bayesian model places priors directly on the spatial covariance matrices under both hypotheses, as well as on the probability of channel occupancy. Specifically, we use inverse-gamma and complex inverse-Wishart distributions as conjugate priors for the null and alternative hypotheses, and a binomial distribution as the prior for channel occupancy. At each sensing period, Bayesian inference is applied and the posterior for the channel occupancy is thresholded for detection. After a suitable approximation, the posteriors are employed as priors for the next sensing frame, which forms the basis of the proposed Bayesian learning procedure. The performance of the Bayesian detector is evaluated by simulations and by means of a CR testbed composed of universal radio peripheral (USRP) nodes. Both the simulations and experimental measurements show that the Bayesian detector outperforms the GLRT in a variety of scenarios.24Testing blind separability of complex Gaussian mixtures(D. Ram&rez, P. J. Schreier, J. V&a and I. Santamar&a)Signal Process.&95,& pp. 49&57,&February&2014.DOI:.@article{RamirezSchreierVia:2014:Testing-blind-separability-of-complex,
abstract = {The separation of a complex mixture based solely on second-order statistics can be achieved using the Strong Uncorrelating Transform (SUT) if and only if all sources have distinct circularity coefficients. However, in most problems we do not know the circularity coefficients, and they must be estimated from observed data. In this work, we propose a detector, based on the generalized likelihood ratio test (GLRT), to test the separability of a complex Gaussian mixture using the SUT. For the separable case (distinct circularity coefficients), the maximum likelihood (ML) estimates are straightforward. On the other hand, for the non-separable case (at least one circularity coefficient has multiplicity greater than one), the ML estimates are much more difficult to obtain. To set the threshold, we exploit Wilks' theorem, which gives the asymptotic distribution of the GLRT under the null hypothesis. Finally, numerical simulations show the good performance of the proposed detector and the accuracy of Wilks' approximation.},
author = {D. Ram{\'i}rez and P. J. Schreier and J. V{\'i}a and I. Santamar{\'i}a},
month = {{F}ebruary},
title = {Testing blind separability of complex {G}aussian mixtures},
year = {2014},
journal = {{S}ignal {P}rocess.},
pages = {49&57},
volume = {95},
doi = {10.1016/j.sigpro.},
}The separation of a complex mixture based solely on second-order statistics can be achieved using the Strong Uncorrelating Transform (SUT) if and only if all sources have distinct circularity coefficients. However, in most problems we do not know the circularity coefficients, and they must be estimated from observed data. In this work, we propose a detector, based on the generalized likelihood ratio test (GLRT), to test the separability of a complex Gaussian mixture using the SUT. For the separable case (distinct circularity coefficients), the maximum likelihood (ML) estimates are straightforward. On the other hand, for the non-separable case (at least one circularity coefficient has multiplicity greater than one), the ML estimates are much more difficult to obtain. To set the threshold, we exploit Wilks' theorem, which gives the asymptotic distribution of the GLRT under the null hypothesis. Finally, numerical simulations show the good performance of the proposed detector and the accuracy of Wilks' approximation.25Neue Anwendungsgebiete f&r Computer Assisted Surgery (CAS)(Peter J. Schreier)ForschungsForum Paderborn&17,& pp. 24&30,&February&2014.@article{Schreier:2014:ForschungsForumPaderborn,
author = {Schreier, Peter J.},
month = {{F}ebruary},
title = {Neue {A}nwendungsgebiete f{\"u}r {C}omputer {A}ssisted {S}urgery {(CAS)}},
year = {2014},
journal = {{ForschungsForum} {Paderborn}},
pages = {24&30},
volume = {17},
}26Optical coherence tomography assessment of vessel wall degradation in thoracic aortic aneurysms(Eusebio Real, Alma Eguizabal, Alejandro Pontón, Marta Calvo Díez, José Fernando Val-Bernal, Marta Mayorga, José M. Revuelta, José M. López-Higuera and Olga M. Conde)J. Biomed. Opt.&18&(12),& pp. 003,&December&2013.DOI:.@article{RealEguizabalPonton:2013:Optical-coherence-tomography-assessment-of-vessel,
abstract = {Optical coherence tomography images of human thoracic aorta from aneurysms reveal elastin disorders and smooth muscle cell alterations when visualizing the media layer of the aortic wall. These disorders can be employed as indicators for wall degradation and, therefore, become a hallmark for diagnosis of risk of aneurysm under intraoperative conditions. Two approaches are followed to evaluate this risk: the analysis of the reflectivity decay along the penetration depth and the textural analysis of a two-dimensional spatial distribution of the aortic wall backscattering. Both techniques require preprocessing stages for the identification of the air–sample interface and for the segmentation of the media layer. Results show that the alterations in the media layer of the aortic wall are better highlighted when the textural approach is considered and also agree with a semiquantitative histopathological grading that assesses the degree of wall degradation. The correlation of the co-occurrence matrix attains a sensitivity of 0.906 and specificity of 0.864 when aneurysm automatic diagnosis is evaluated with a receiver operating characteristic curve.},
author = {Eusebio Real and Alma Eguizabal and Alejandro Pontón and Marta Calvo Díez and José Fernando Val-Bernal and Marta Mayorga and José M. Revuelta and José M. López-Higuera and Olga M. Conde},
month = {{D}ecember},
title = {Optical coherence tomography assessment of vessel wall degradation in thoracic aortic aneurysms},
year = {2013},
journal = {J. Biomed. Opt.},
number = {12},
pages = {003},
volume = {18},
doi = {10.1117/1.JBO.18.12.126003},
}Optical coherence tomography images of human thoracic aorta from aneurysms reveal elastin disorders and smooth muscle cell alterations when visualizing the media layer of the aortic wall. These disorders can be employed as indicators for wall degradation and, therefore, become a hallmark for diagnosis of risk of aneurysm under intraoperative conditions. Two approaches are followed to evaluate this risk: the analysis of the reflectivity decay along the penetration depth and the textural analysis of a two-dimensional spatial distribution of the aortic wall backscattering. Both techniques require preprocessing stages for the identification of the air–sample interface and for the segmentation of the media layer. Results show that the alterations in the media layer of the aortic wall are better highlighted when the textural approach is considered and also agree with a semiquantitative histopathological grading that assesses the degree of wall degradation. The correlation of the co-occurrence matrix attains a sensitivity of 0.906 and specificity of 0.864 when aneurysm automatic diagnosis is evaluated with a receiver operating characteristic curve.27Convergent systems vs. incremental stability(Bj&rn S. R&ffer, Nathan van de Wouw and Markus Mueller)Systems Control Lett.&62,& pp. 277&285,&2013.DOI:.@article{rufferwouwmueller2013-convergent-systems-vs.-incremental-stability,
abstract = {Two similar stability no 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 = {R{\"u}ffer, Bj{\"o}rn S. and van de Wouw, Nathan and Mueller, Markus},
title = {Convergent systems vs. incremental stability},
year = {2013},
journal = {{S}ystems {C}ontrol {L}ett.},
pages = {277&285},
volume = {62},
doi = {10.1016/j.sysconle.},
}Two similar stability no 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.28A unified framework for regularized linear estimation in communication systems(Jun Tong and Peter J. Schreier)Signal Process.&93&(9),& pp. ,&September&2013.DOI:.@article{TongSchreier:2013:A-unified-framework,
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.},
month = {{S}eptember},
title = {A unified framework for regularized linear estimation in communication systems},
year = {2013},
journal = {{S}ignal {P}rocess.},
number = {9},
pages = {},
volume = {93},
doi = {10.1016/j.sigpro.},
}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.29Regularized preconditioning for Krylov Subspace Equalization of OFDM systems over doubly selective channels(Jun Tong and Peter J. Schreier)IEEE Wireless Comm. Lett.&2&(4),& pp. 367&370,&August&2013.DOI:.@article{TongSchreier:2013:Regularized-Preconditioning-for-Krylov-Subspace,
author = {Tong, Jun and Schreier, Peter J.},
month = {{A}ugust},
title = {Regularized preconditioning for {K}rylov Subspace Equalization of {OFDM} systems over doubly selective channels},
year = {2013},
journal = {{IEEE} {W}ireless {C}omm. {L}ett.},
number = {4},
pages = {367&370},
volume = {2},
doi = {10.1109/WCL..130096},
}30Direct identification of breast cancer pathologies using blind separation of label-free localized reflectance measurements(Alma Eguizabal, Ashley M. Laughney, P. Beatriz Garcia-Allende, Venkataramanan Krishnaswamy, Wendy A. Wells, Keith D. Paulsen, Brian W. Pogue, Jose M. Lopez-Higuera and Olga M. Conde)Biomed. Opt. Express&4&(7),& pp. ,&June&2013.DOI:.@article{EguizabalLaughneyGarciaallende:2013:Direct-identification-of-breast-cancer-pathologies,
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 = {Alma Eguizabal and Ashley M. Laughney and P. Beatriz Garcia-Allende and Venkataramanan Krishnaswamy and Wendy A. Wells and Keith D. Paulsen and Brian W. Pogue and Jose M. Lopez-Higuera and Olga M. Conde},
month = {{J}une},
title = {Direct identification of breast cancer pathologies using blind separation of label-free localized reflectance measurements},
year = {2013},
journal = {Biomed. Opt. Express},
number = {7},
pages = {},
volume = {4},
doi = {10.1364/BOE.4.001104},
}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.31Robust stability of networks of iISS systems: Construction of sum-type Lyapunov functions(Hiroshi Ito, Zhong-Ping Jiang, Sergey Dashkovskiy and Bj&rn S. R&ffer)IEEE Trans. Autom. Control&58&(5),& pp. ,&May&2013.DOI:.@article{itojiangdashkovskiyruffer2013-robust-stability-of-networks-of-iiss-systems:-construction-of-sum-type-lyapunov-functions,
abstract = {This paper gives a solution to the problem of verifying stability of networks consisting of integral input-to-state stable (iISS) subsystems. The iISS small-gain theorem developed recently has been restricted to interconnections of two subsystems. For large-scale systems, stability criteria relying only on gain-type information that were previously developed address only input-to-state stable (ISS) subsystems. To address the stability problem involving iISS subsystems interconnected in general structure, this paper shows how to construct Lyapunov functions of the network by means of a sum of nonlinearly rescaled individual Lyapunov functions of subsystems under an appropriate small-gain condition.},
author = {Hiroshi Ito and Zhong-Ping Jiang and Sergey Dashkovskiy and Bj{\"o}rn S. R{\"u}ffer},
month = {{M}ay},
title = {Robust stability of networks of {iISS} systems: Construction of sum-type {L}yapunov functions},
year = {2013},
journal = {{{IEEE}} {T}rans. {A}utom. {C}ontrol},
number = {5},
pages = {},
volume = {58},
doi = {10.1109/TAC.},
}This paper gives a solution to the problem of verifying stability of networks consisting of integral input-to-state stable (iISS) subsystems. The iISS small-gain theorem developed recently has been restricted to interconnections of two subsystems. For large-scale systems, stability criteria relying only on gain-type information that were previously developed address only input-to-state stable (ISS) subsystems. To address the stability problem involving iISS subsystems interconnected in general structure, this paper shows how to construct Lyapunov functions of the network by means of a sum of nonlinearly rescaled individual Lyapunov functions of subsystems under an appropriate small-gain condition.32Locally most powerful invariant tests for correlation and sphericity of Gaussian vectors(D. Ram&rez, J. V&a, I. Santamar&a and L. L. Scharf)IEEE Trans. Inform. Theory&59&(4),& pp. ,&April&2013.DOI:.@article{RamirezViaSantamaria:2013:Locally-Most-Powerful-Invariant,
abstract = {In this paper we study the existence of locally most powerful invariant tests (LMPIT) for the problem of testing the covariance structure of a set of Gaussian random vectors. The LMPIT is the optimal test for the case of close hypotheses, among those satisfying the invariances of the problem, and in practical scenarios can provide better performance than the typically used generalized likelihood ratio test (GLRT). The derivation of the LMPIT usually requires one to find the maximal invariant statistic for the detection problem and then derive its distribution under both hypotheses, which in general is a rather involved procedure. As an alternative, Wijsman's theorem provides the ratio of the maximal invariant densities without even finding an explicit expression for the maximal invariant. We first consider the problem of testing whether a set of N-dimensional Gaussian random vectors are uncorrelated or not, and show that the LMPIT is given by the Frobenius norm of the sample coherence matrix. Second, we study the case in which the vectors under the null hypothesis are uncorrelated and identically distributed, that is, the sphericity test for Gaussian vectors, for which we show that the LMPIT is given by the Frobenius norm of a normalized version of the sample covariance matrix. Finally, some numerical examples illustrate the performance of the proposed tests, which provide better results than their GLRT counterparts.},
author = {D. Ram{\'i}rez and J. V{\'i}a and I. Santamar{\'i}a and L. L. Scharf},
month = {{A}pril},
title = {Locally most powerful invariant tests for correlation and sphericity of {G}aussian vectors},
year = {2013},
journal = {{IEEE} {T}rans.\ {I}nform.\ {T}heory},
number = {4},
pages = {},
volume = {59},
doi = {10.1109/TIT.},
}In this paper we study the existence of locally most powerful invariant tests (LMPIT) for the problem of testing the covariance structure of a set of Gaussian random vectors. The LMPIT is the optimal test for the case of close hypotheses, among those satisfying the invariances of the problem, and in practical scenarios can provide better performance than the typically used generalized likelihood ratio test (GLRT). The derivation of the LMPIT usually requires one to find the maximal invariant statistic for the detection problem and then derive its distribution under both hypotheses, which in general is a rather involved procedure. As an alternative, Wijsman's theorem provides the ratio of the maximal invariant densities without even finding an explicit expression for the maximal invariant. We first consider the problem of testing whether a set of N-dimensional Gaussian random vectors are uncorrelated or not, and show that the LMPIT is given by the Frobenius norm of the sample coherence matrix. Second, we study the case in which the vectors under the null hypothesis are uncorrelated and identically distributed, that is, the sphericity test for Gaussian vectors, for which we show that the LMPIT is given by the Frobenius norm of a normalized version of the sample covariance matrix. Finally, some numerical examples illustrate the performance of the proposed tests, which provide better results than their GLRT counterparts.33Amplify-and-forward strategies in the two-way relay channel with analog Tx-Rx beamforming(Christian Lameiro, Javier V&a and Ignacio Santamar&a)IEEE Trans. Veh. Techn.&62,& pp. 642&654,&February&2013.DOI:.@article{LameiroViaSantamaria:2013:Amplify-and-Forward-Strategies-in-the-Two-Way-Relay,
abstract = {In this paper, we study the multiple-input multiple-output two-way relay channel (MIMO-TWRC) when the nodes and relay use analog beamforming. Following the amplify-andforward (AF) strategy, the problem consists of finding the transmit and receive beamformers of the nodes and relay (as well as the power allocated to each one) that achieve the boundary of the optimal rate region. To solve it, we first express the optimal node beamformers in terms of relay beamformers and then show that the optimal rate region can efficiently be characterized using convex optimization techniques. We also extend our study to the multiple-relay scenario when the source nodes are single antenna and propose a distributed algorithm to compute the relay beamforming matrices. The proposed algorithm consists of solving two different subproblems. First, each individual TWRC is optimized independently. Next, a distributed beamforming is applied to make the signals from the relays add up coherently at the source nodes. Numerical examples are provided to illustrate the performance of the proposed techniques and to compare the performance of analog beamforming architectures against conventional MIMO schemes that operate at the baseband.},
author = {Lameiro, Christian and V{\'i}a, Javier and Santamar{\'i}a, Ignacio},
month = {{F}ebruary},
title = {Amplify-and-forward strategies in the two-way relay channel with analog {Tx-Rx} beamforming},
year = {2013},
journal = {IEEE Trans. Veh. Techn.},
pages = {642&654},
volume = {62},
doi = {10.1109/TVT.},
}In this paper, we study the multiple-input multiple-output two-way relay channel (MIMO-TWRC) when the nodes and relay use analog beamforming. Following the amplify-andforward (AF) strategy, the problem consists of finding the transmit and receive beamformers of the nodes and relay (as well as the power allocated to each one) that achieve the boundary of the optimal rate region. To solve it, we first express the optimal node beamformers in terms of relay beamformers and then show that the optimal rate region can efficiently be characterized using convex optimization techniques. We also extend our study to the multiple-relay scenario when the source nodes are single antenna and propose a distributed algorithm to compute the relay beamforming matrices. The proposed algorithm consists of solving two different subproblems. First, each individual TWRC is optimized independently. Next, a distributed beamforming is applied to make the signals from the relays add up coherently at the source nodes. Numerical examples are provided to illustrate the performance of the proposed techniques and to compare the performance of analog beamforming architectures against conventional MIMO schemes that operate at the baseband.34On the instantaneous frequency of Gaussian stochastic processes(Patrik Wahlberg and Peter J. Schreier)Probab. Math. Statist.&32,& pp. 69&92,&2012.@article{WahlbergSchreier:2012:On-the-instantaneous-frequency-of-Gaussi,
abstract = {We study the instantaneous frequency (IF) of continuous-time, complex-valued, zero-mean, proper, mean-square differentiable, nonstationary Gaussian stochastic processes. We compute the probability density function for the IF for fixed time, which generalizes a result known for wide-sense stationary processes to nonstationary processes. For a fixed point in time, the IF has either zero or infinite variance. For harmonizable processes, we obtain as a consequence the result that the mean of the IF, for fixed time, is the normalized first-order frequency moment of the Wigner spectrum.},
author = {Wahlberg, Patrik and Schreier, Peter J.},
title = {On the instantaneous frequency of {Gaussian} stochastic processes},
year = {2012},
journal = {{P}robab.\ {M}ath.\ {S}tatist.},
pages = {69&92},
volume = {32},
}We study the instantaneous frequency (IF) of continuous-time, complex-valued, zero-mean, proper, mean-square differentiable, nonstationary Gaussian stochastic processes. We compute the probability density function for the IF for fixed time, which generalizes a result known for wide-sense stationary processes to nonstationary processes. For a fixed point in time, the IF has either zero or infinite variance. For harmonizable processes, we obtain as a consequence the result that the mean of the IF, for fixed time, is the normalized first-order frequency moment of the Wigner spectrum.35Development and integration of xerogel polymeric absorbance micro-filters into lab-onchip systems(Ester Carregal-Romero, Cesar Fernandez-Sanchez, Alma Eguizabal, Stefanie Demming, Stephanus B&ttgenbach and Andreu Llobera)Opt. Express&20&(21),& pp. ,&October&2012.DOI:.@article{CarregalromeroFernandessanchezEguizabal:2012:Development-and-integration-of-xerogel-polymeric,
abstract = {This work reports on the implementation of different absorption micro-filters based on a dye-doped hybrid organic-inorganic xerogel polymeric material synthesized by the sol-gel process. Microstructures containing eight different filter widths were fabricated in polydimethylsiloxane (PDMS), bonded to glass substrates and filled with the corresponding dye doped polymeric material by a soft lithography approach. The filtering capacity as a function of dye concentration and filter width was studied and revealed a linear dependence with both parameters, as expected according to the Beer-Lambert law. Zero passband transmittance values and relatively sharp stopband regions were achieved with all the filters, also showing rejection levels between ?6 dB and ?55 dB. Finally, such filters were monolithically integrated into a disposable fluorescence-based photonic lab-on-a-chip (PhLoC) approach. Calibration curves carried out with a model fluorophore target analyte showed an over two-fold increase in sensitivity and a thirty-fold decrease of the limit of detection (LOD) compared with the values recorded using the same PhLoC system but without the polymeric filter structure. The results presented herein clearly indicate the feasibility of these xerogel-based absorbance filtering structures for being applied as low-cost optical components that can be easily incorporated into disposable fluorescence-based photonic lab on a chip systems},
author = {Ester Carregal-Romero and Cesar Fernandez-Sanchez and Alma Eguizabal and Stefanie Demming and Stephanus B{\"u}ttgenbach and Andreu Llobera},
month = {{O}ctober},
title = {Development and integration of xerogel polymeric absorbance micro-filters into lab-onchip systems},
year = {2012},
journal = {Opt. Express},
number = {21},
pages = {},
volume = {20},
doi = {10.1364/OE.20.023700},
}This work reports on the implementation of different absorption micro-filters based on a dye-doped hybrid organic-inorganic xerogel polymeric material synthesized by the sol-gel process. Microstructures containing eight different filter widths were fabricated in polydimethylsiloxane (PDMS), bonded to glass substrates and filled with the corresponding dye doped polymeric material by a soft lithography approach. The filtering capacity as a function of dye concentration and filter width was studied and revealed a linear dependence with both parameters, as expected according to the Beer-Lambert law. Zero passband transmittance values and relatively sharp stopband regions were achieved with all the filters, also showing rejection levels between ?6 dB and ?55 dB. Finally, such filters were monolithically integrated into a disposable fluorescence-based photonic lab-on-a-chip (PhLoC) approach. Calibration curves carried out with a model fluorophore target analyte showed an over two-fold increase in sensitivity and a thirty-fold decrease of the limit of detection (LOD) compared with the values recorded using the same PhLoC system but without the polymeric filter structure. The results presented herein clearly indicate the feasibility of these xerogel-based absorbance filtering structures for being applied as low-cost optical components that can be easily incorporated into disposable fluorescence-based photonic lab on a chip systems36Linear precoding for MIMO systems with low-complexity receivers(Jun Tong, Peter J. Schreier and Steven R. Weller)IEEE Trans. Wireless Comm.&11&(8),& pp. ,&August&2012.DOI:.@article{TongSchreierWeller:2012:Linear-precoding-for-MIMO-systems-with-l,
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.},
month = {{A}ugust},
title = {Linear precoding for {MIMO} systems with low-complexity receivers},
year = {2012},
journal = {{IEEE} {T}rans.\ {W}ireless {C}omm.},
number = {8},
pages = {},
volume = {11},
doi = {10.1109/TWC..110877},
}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.37Variability of moisture in coarse woody debris from several ecologically important tree species of the temperate zone of Europe(V. Pichler, M. Homol&k, W. Skierucha, M. Pichlerov&, D. Ram&rez, J. Gregor and P. Jaloviar)Ecohydrology&5&(4),& pp. 424&434,&July&2012.DOI:.@article{PichlerHomolakSkierucha:2012:Variability-of-moisture-in-coarse-woody,
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 \emph{Quercus} 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 = {V. Pichler and M. Homol{\'a}k and W. Skierucha and M. Pichlerov{\'a} and D. Ram{\'i}rez and J. Gregor and P. Jaloviar},
month = {{J}uly},
title = {Variability of moisture in coarse woody debris from several ecologically important tree species of the temperate zone of {E}urope},
year = {2012},
journal = {{E}cohydrology},
number = {4},
pages = {424&434},
volume = {5},
doi = {10.1002/eco.235},
}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 Quercus spp., Abies alba Mill., Fagus sylvatica L., and 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 A. alba and F. sylvatica, and an intense water reabsorpt