664 research outputs found
Adaptive matched field processing in an uncertain propagation environment
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1992Adaptive array processing algorithms have achieved widespread use because they are
very effective at rejecting unwanted signals (i.e., controlling sidelobe levels) and in
general have very good resolution (i.e., have narrow mainlobes). However, many
adaptive high-resolution array processing algorithms suffer a significant degradation
in performance in the presence of environmental mismatch. This sensitivity to environmental
mismatch is of particular concern in problems such as long-range acoustic
array processing in the ocean where the array processor's knowledge of the propagation
characteristics of the ocean is imperfect. An Adaptive Minmax Matched Field
Processor has been developed which combines adaptive matched field processing and
minmax approximation techniques to achieve the effective interference rejection characteristic
of adaptive processors while limiting the sensitivity of the processor to
environmental mismatch.
The derivation of the algorithm is carried out within the framework of minmax
signal processing. The optimal array weights are those which minimize the maximum
conditional mean squared estimation error at the output of a linear weight-and-sum
beamformer. The error is conditioned on the propagation characteristics of the environment
and the maximum is evaluated over the range of environmental conditions in
which the processor is expected to operate. The theorems developed using this framework
characterize the solutions to the minmax array weight problem, and relate the
optimal minmax array weights to the solution to a particular type of Wiener filtering
problem. This relationship makes possible the development of an efficient algorithm
for calculating the optimal minmax array weights and the associated estimate of the
signal power emitted by a source at the array focal point. An important feature of
this algorithm is that it is guarenteed to converge to an exact solution for the array
weights and estimated signal power in a finite number of iterations. The Adaptive Minmax Matched Field Processor can also be interpreted as a two-stage
Minimum Variance Distortionless Response (MVDR) Matched Field Processor.
The first stage of this processor generates an estimate of the replica vector of the signal
emitted by a source at the array focal point, and the second stage is a traditional
MVDR Matched Field Processor implemented using the estimate of the signal replica
vector.
Computer simulations using several environmental models and types of environmental
uncertainty have shown that the resolution and interference rejection capability
of the Adaptive Minmax Matched Field Processor is close to that of a traditional
MVDR Matched Field Processor which has perfect knowledge of the characteristics
of the propagation environment and far exceeds that of the Bartlett Matched Field
Processor. In addition, the simulations show that the Adaptive Minmax Matched
Field Processor is able to maintain it's accuracy, resolution and interference rejection
capability when it's knowledge of the environment is only approximate, and is therefore
much less sensitive to environmental mismatch than is the traditional MVDR
Matched Field Processor.The National
Science Foundation, the General Electric Foundation, the Office of Naval Research,
the Defense Advanced Research Projects Agency, and the Woods Hole Oceanographic
Institution
Sparse channel estimation for multicarrier underwater acoustic communication : from subspace methods to compressed sensing
Author Posting. © IEEE, 2009. This article is posted here by permission of IEEE for personal use, not for redistribution. The definitive version was published in IEEE Transactions on Signal Processing 58 (2010): 1708-1721, doi:10.1109/TSP.2009.2038424.In this paper, we investigate various channel estimators
that exploit channel sparsity in the time and/or Doppler
domain for a multicarrier underwater acoustic system. We use a
path-based channel model, where the channel is described by a
limited number of paths, each characterized by a delay, Doppler
scale, and attenuation factor, and derive the exact inter-carrierinterference
(ICI) pattern. For channels that have limited Doppler
spread we show that subspace algorithms from the array processing
literature, namely Root-MUSIC and ESPRIT, can be applied
for channel estimation. For channels with Doppler spread, we
adopt a compressed sensing approach, in form of Orthogonal
Matching Pursuit (OMP) and Basis Pursuit (BP) algorithms, and
utilize overcomplete dictionaries with an increased path delay
resolution. Numerical simulation and experimental data of an
OFDM block-by-block receiver are used to evaluate the proposed
algorithms in comparison to the conventional least-squares (LS)
channel estimator.We observe that subspace methods can tolerate
small to moderate Doppler effects, and outperform the LS
approach when the channel is indeed sparse. On the other hand,
compressed sensing algorithms uniformly outperform the LS and
subspace methods. Coupled with a channel equalizer mitigating
ICI, the compressed sensing algorithms can effectively handle
channels with significant Doppler spread.C. Berger, S. Zhou, and P. Willett are supported by ONR
grants N00014-09-10613, N00014-07-1-0805, and N00014-09-1-0704
Chronic Rejection and Extrahepatic Biliary Tract Obstruction 8 Years After Orthotopic Liver Transplantation Using the Gallbladder-Conduit Technique
A case of delayed biliary obstruction and cholangitis, occurring in the setting of chronic allograft
rejection, 8 years after liver transplantation using the gallbladder-conduit, is presented. Extrahepatic
biliary obstruction may be seen in the late follow-up of liver grafting and rejection phenomena may play
a significant role in the development of such obstruction
Psychiatric symptoms and response quality to self-rated personality tests: Evidence from the PsyCoLaus study.
Despite the fact that research has demonstrated consistent associations between self-rated measures of personality dimensions and mental disorders, little has been undertaken to investigate the relation between psychiatric symptoms and response patterns to self-rated tests. The aim of this study was to investigate the association between psychiatric symptoms and response quality using indices from our functional method. A sample of 1,784 participants from a Swiss population-based cohort completed a personality inventory (NEO-FFI) and a symptom checklist of 90 items (SCL-90-R). Different indices of response quality were calculated based on the responses given to the NEO-FFI. Associations among the responses to indices of response quality, sociodemographic characteristics and the SCL-90-R dimensions were then established. Psychiatric symptoms were associated with several important differences in response quality, questioning subjects' ability to provide valid information using self-rated instruments. As suggested by authors, psychiatric symptoms seem associated with differences in personality scores. Nonetheless, our study shows that symptoms are also related to differences in terms of response patterns as sources of differences in personality scores. This could constitute a bias for clinical assessment. Future studies could still determine whether certain subpopulations of subjects are more unable to provide valid information to self-rated questionnaires than others
Do bipolar subjects' responses to personality questionnaires lack reliability? Evidence from the PsyCoLaus study.
Differences in personality scores between subjects with and without mood disorders might result from response biases rather than specific personality traits per se. The aim of this study was to compare subjects with bipolar disorders (BPD) to non-bipolar subjects in terms of response quality to the NEO-FFI. Using data from the population-based cohort study PsyCoLaus, subjects were compared in terms of responses to the NEO-FFI, and indices of response quality were calculated. Hierarchical regression analyses were performed and controlled for sociodemographic factors, depressive episodes, dysthymia, anxiety disorders and substance use disorders. Consistent with the literature, subjects with BPD had higher scores in neuroticism and openness, and lower scores in conscientiousness. However, significant differences were measured for response reliability and validity. In particular, the indices of response quality including response reliability were lower in subjects with BPD suggesting that bipolar subjects might have more difficulty in providing consistent answers throughout questionnaires. However, regression models resulted in small associations between mania/hypomania and response quality, and showed that differences in response quality were mainly attributable to correlates of BPD instead of the presence of mania/hypomania itself. The current findings suggest that bipolar subjects' responses to questionnaires are biased, making them less reliable
Evidence for chronic low-grade systemic inflammation in individuals with agoraphobia from a population-based prospective study.
BACKGROUND: Anxiety disorders have been linked to an increased risk of incident coronary heart disease in which inflammation plays a key pathogenic role. To date, no studies have looked at the association between proinflammatory markers and agoraphobia.
METHODS: In a random Swiss population sample of 2890 persons (35-67 years, 53% women), we diagnosed a total of 124 individuals (4.3%) with agoraphobia using a validated semi-structured psychiatric interview. We also assessed socioeconomic status, traditional cardiovascular risk factors (i.e., body mass index, hypertension, blood glucose levels, total cholesterol/high-density lipoprotein-cholesterol ratio), and health behaviors (i.e., smoking, alcohol consumption, and physical activity), and other major psychiatric diseases (other anxiety disorders, major depressive disorder, drug dependence) which were treated as covariates in linear regression models. Circulating levels of inflammatory markers, statistically controlled for the baseline demographic and health-related measures, were determined at a mean follow-up of 5.5 ± 0.4 years (range 4.7 - 8.5).
RESULTS: Individuals with agoraphobia had significantly higher follow-up levels of C-reactive protein (p = 0.007) and tumor-necrosis-factor-α (p = 0.042) as well as lower levels of the cardioprotective marker adiponectin (p = 0.032) than their non-agoraphobic counterparts. Follow-up levels of interleukin (IL)-1β and IL-6 did not significantly differ between the two groups.
CONCLUSIONS: Our results suggest an increase in chronic low-grade inflammation in agoraphobia over time. Such a mechanism might link agoraphobia with an increased risk of atherosclerosis and coronary heart disease, and needs to be tested in longitudinal studies
Development of a pulsar-based timescale
Using observations of pulsars from the Parkes Pulsar Timing Array (PPTA)
project we develop the first pulsar-based timescale that has a precision
comparable to the uncertainties in international atomic timescales. Our
ensemble of pulsars provides an Ensemble Pulsar Scale (EPS) analogous to the
free atomic timescale Echelle Atomique Libre (EAL). The EPS can be used to
detect fluctuations in atomic timescales and therefore can lead to a new
realisation of Terrestrial Time, TT(PPTA11). We successfully follow features
known to affect the frequency of the International Atomic Timescale (TAI) and
we find marginally significant differences between TT(PPTA11) and TT(BIPM11).
We discuss the various phenomena that lead to a correlated signal in the pulsar
timing residuals and therefore limit the stability of the pulsar timescale.Comment: Accepted for publication in MNRA
Adipocytokines, Hepatic and Inflammatory Biomarkers and Incidence of Type 2 Diabetes. The CoLaus Study.
CONTEXT: There is contradictory information regarding the prognostic importance of adipocytokines, hepatic and inflammatory biomarkers on the incidence of type 2 diabetes. The objective was to assess the prognostic relevance of adipocytokine and inflammatory markers (C-reactive protein - CRP; interleukin-1beta - IL-1β; interleukin-6- IL-6; tumour necrosis factor-α - TNF-α; leptin and adiponectin) and gamma-glutamyl transpeptidase (γGT) on the incidence of type 2 diabetes.
METHODS: Prospective, population-based study including 3,842 non-diabetic participants (43.3% men, age range 35 to 75 years), followed for an average of 5.5 years (2003-2008). The endpoint was the occurrence of type 2 diabetes.
RESULTS: 208 participants (5.4%, 66 women) developed type 2 diabetes during follow-up. On univariate analysis, participants who developed type 2 diabetes had significantly higher baseline levels of IL-6, CRP, leptin and γGT, and lower levels of adiponectin than participants who remained free of type 2 diabetes. After adjusting for a validated type 2 diabetes risk score, only the associations with adiponectin: Odds Ratio and (95% confidence interval): 0.97 (0.64-1.47), 0.84 (0.55-1.30) and 0.64 (0.40-1.03) for the second, third and forth gender-specific quartiles respectively, remained significant (P-value for trend = 0.05). Adding each marker to a validated type 2 diabetes risk score (including age, family history of type 2 diabetes, height, waist circumference, resting heart rate, presence of hypertension, HDL cholesterol, triglycerides, fasting glucose and serum uric acid) did not improve the area under the ROC or the net reclassification index; similar findings were obtained when the markers were combined, when the markers were used as continuous (log-transformed) variables or when gender-specific quartiles were used.
CONCLUSION: Decreased adiponectin levels are associated with an increased risk for incident type 2 diabetes, but they seem to add little information regarding the risk of developing type 2 diabetes to a validated risk score
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