508 research outputs found
Information-theoretic analysis of MIMO channel sounding
The large majority of commercially available multiple-input multiple-output
(MIMO) radio channel measurement devices (sounders) is based on time-division
multiplexed switching (TDMS) of a single transmit/receive radio-frequency chain
into the elements of a transmit/receive antenna array. While being
cost-effective, such a solution can cause significant measurement errors due to
phase noise and frequency offset in the local oscillators. In this paper, we
systematically analyze the resulting errors and show that, in practice,
overestimation of channel capacity by several hundred percent can occur.
Overestimation is caused by phase noise (and to a lesser extent frequency
offset) leading to an increase of the MIMO channel rank. Our analysis
furthermore reveals that the impact of phase errors is, in general, most
pronounced if the physical channel has low rank (typical for line-of-sight or
poor scattering scenarios). The extreme case of a rank-1 physical channel is
analyzed in detail. Finally, we present measurement results obtained from a
commercially employed TDMS-based MIMO channel sounder. In the light of the
findings of this paper, the results obtained through MIMO channel measurement
campaigns using TDMS-based channel sounders should be interpreted with great
care.Comment: 99 pages, 14 figures, submitted to IEEE Transactions on Information
Theor
Twisted equivariant K-theory with complex coefficients
Using a global version of the equivariant Chern character, we describe the
complexified twisted equivariant K-theory of a space with a compact Lie group
action in terms of fixed-point data. We apply this to the case of a compact Lie
group acting on itself by conjugation, and relate the result to the Verlinde
algebra and to the Kac numerator at q=1. Verlinde's formula is also discussed
in this context.Comment: Final version, to appear in Topology. Exposition improved, rational
homotopy calculation completely rewritte
Rigid upper bounds for the angular momentum and centre of mass of non-singular asymptotically anti-de Sitter space-times
We prove upper bounds on angular momentum and centre of mass in terms of the
Hamiltonian mass and cosmological constant for non-singular asymptotically
anti-de Sitter initial data sets satisfying the dominant energy condition. We
work in all space-dimensions larger than or equal to three, and allow a large
class of asymptotic backgrounds, with spherical and non-spherical conformal
infinities; in the latter case, a spin-structure compatibility condition is
imposed. We give a large class of non-trivial examples saturating the
inequality. We analyse exhaustively the borderline case in space-time dimension
four: for spherical cross-sections of Scri, equality together with completeness
occurs only in anti-de Sitter space-time. On the other hand, in the toroidal
case, regular non-trivial initial data sets saturating the bound exist.Comment: improvements in the presentation; some statements correcte
Quantum critical fluctuations in an Fe-based superconductor
Quantum critical fluctuations may prove to play an instrumental role in the formation of unconventional superconductivity. Here, we show that the characteristic scaling of a marginal Fermi liquid is present in inelastic light scattering data of an Fe-based superconductor tuned through a quantum critical point (QCP) by chemical substitution or doping. From the doping dependence of the imaginary time dynamics we are able to distinguish regions dominated by quantum critical behavior from those having classical critical responses. This dichotomy reveals a connection between the marginal Fermi liquid behavior and quantum criticality. In particular, the overlap between regions of high superconducting transition temperatures and quantum critical scaling suggests a contribution from quantum fluctuations to the formation of superconductivity
Zinc deficiency and advanced liver fibrosis among HIV and hepatitis C co-infected anti-retroviral naĂŻve persons with alcohol use in Russia
Background and aims
Liver disease in people living with HIV co-infected with hepatitis C virus is a source of morbidity and mortality in Russia. HIV accelerates liver fibrosis in the setting of HCV co-infection and alcohol use. Zinc deficiency is common among people living with HIV and may be a factor that facilitates the underlying mechanisms of liver fibrosis. We investigated the association between zinc deficiency and advanced liver fibrosis in a cohort of HIV/HCV co-infected persons reporting heavy drinking in Russia. Methods
This is a secondary data analysis of baseline data from 204 anti-retroviral treatment naĂŻve HIV/HCV co-infected Russians with heavy drinking that were recruited into a clinical trial of zinc supplementation. The primary outcome of interest in this cross-sectional study was advanced liver fibrosis. Zinc deficiency, the main independent variable, was defined as plasma zinc \u3c0.75 mg/L. Exploratory analyses were performed examining continuous zinc levels and fibrosis scores. Analyses were conducted using multivariable regression models adjusted for potential confounders. Results
The prevalence of advanced liver fibrosis was similar for those with zinc deficiency compared to those with normal zinc levels, (27.7% vs. 23.0%, respectively). We did not detect an association between zinc deficiency and advanced liver fibrosis in the adjusted regression model (aOR: 1.28, 95% CI: 0.62–2.61, p = 0.51) nor in exploratory analyses. Conclusions
In this cohort of Russians with HIV/HCV co-infection, who are anti-retroviral treatment naĂŻve and have heavy alcohol use, we did not detect an association between zinc deficiency or zinc levels and advanced liver fibrosis
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Quantum critical fluctuations in an Fe-based superconductor
Quantum critical fluctuations may prove to play an instrumental role in the formation of unconventional superconductivity. Here, we show that the characteristic scaling of a marginal Fermi liquid is present in inelastic light scattering data of an Fe-based superconductor tuned through a quantum critical point (QCP) by chemical substitution or doping. From the doping dependence of the imaginary time dynamics we are able to distinguish regions dominated by quantum critical behavior from those having classical critical responses. This dichotomy reveals a connection between the marginal Fermi liquid behavior and quantum criticality. In particular, the overlap between regions of high superconducting transition temperatures and quantum critical scaling suggests a contribution from quantum fluctuations to the formation of superconductivity
Attitudes and Performance: An Analysis of Russian Workers
This paper investigates the relationship between locus of control and performance among Russian employees, using survey data collected at 28 workplaces in 2002 in Taganrog and at 47 workplaces in 2003 in Ekaterinburg. We develop a measure that allows us to categorize the Russian employees participating in our survey as exhibiting an internal or external locus of control. We then assess the extent to which there are significant differences between “internals” and “externals” in work-related attitudes that may affect performance. In particular, we focus on (1) attitudes about outcomes associated with hard work, (2) level of job satisfaction, (3) expectation of receiving a desired reward, and (4) loyalty to and involvement with one’s organization. In each case we identify where gender and generational differences emerge. Our main objective is to determine whether Russian employees who exhibit an internal locus of control perform better than employees with an external locus of control. Our performance measures include earnings, expected promotions, and assessments of the quantity and quality of work in comparison to others at the same organization doing a similar job. Controlling for a variety of worker characteristics, we find that (1) individuals who exhibit an internal locus of control perform better, but this result is not always statistically significant; (2) even among “internals,” women earn significantly less than men and have a much lower expectation of promotion; (3) even among “internals,” experience with unemployment has a negative influence on performance.http://deepblue.lib.umich.edu/bitstream/2027.42/40144/3/wp758.pd
Higher pre-infection vitamin E levels are associated with higher mortality in HIV-1-infected Kenyan women: a prospective study
Background: Low vitamin E levels are often found in HIV-1 infection, and studies have suggested that higher levels may decrease the risk of disease progression. However, vitamin E supplementation has also been reported to increase CCR5 expression, which could increase HIV-
1 replication. We hypothesized that vitamin E levels at HIV-1 acquisition may influence disease progression.
Methods: Vitamin E status was measured in stored samples from the last pre-infection visit for 67 Kenyan women with reliably estimated dates of HIV-1 acquisition. Regression analyses were used
to estimate associations between pre-infection vitamin E and plasma viral load, time to CD4 count less than 200 cells/[micro]L, and mortality.
Results: After controlling for potential confounding factors, each 1 mg/L increase in pre-infection vitamin E was associated with 0.08 log[sub]10 copies/mL (95% CI -0.01 to +0.17) higher set point viral load and 1.58-fold higher risk of mortality (95% CI 1.15�2.16). The association between higher preinfection vitamin E and mortality persisted after adjustment for set point viral load (HR 1.55, 95% CI 1.13�2.13).
Conclusion: Higher pre-infection vitamin E levels were associated with increased mortality. Further research is needed to elucidate the role vitamin E plays in HIV-1 pathogenesis.This research was supported by National Institutes of Health grants AI-43844 and AI-38518 (all authors), and Fogarty International Center grant D43 TW000007 (SMG)
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