1,542 research outputs found
Error Rate Analysis for Coded Multicarrier Systems over Quasi-Static Fading Channels
This paper presents two methods for approximating the performance of coded
multicarrier systems operating over frequency-selective, quasi-static fading
channels with non-ideal interleaving. The first method is based on
approximating the performance of the system over each realization of the
channel, and is suitable for obtaining the outage performance of this type of
system. The second method is based on knowledge of the correlation matrix of
the frequency-domain channel gains and can be used to directly obtain the
average performance. Both of the methods are applicable for
convolutionally-coded interleaved systems employing Quadrature Amplitude
Modulation (QAM). As examples, both methods are used to study the performance
of the Multiband Orthogonal Frequency Division Multiplexing (OFDM) proposal for
high data-rate Ultra-Wideband (UWB) communication.Comment: 5 pages, 3 figures, 2 tables. Submitted to Globecom 200
Performance Analysis and Enhancement of Multiband OFDM for UWB Communications
In this paper, we analyze the frequency-hopping orthogonal frequency-division
multiplexing (OFDM) system known as Multiband OFDM for high-rate wireless
personal area networks (WPANs) based on ultra-wideband (UWB) transmission.
Besides considering the standard, we also propose and study system performance
enhancements through the application of Turbo and Repeat-Accumulate (RA) codes,
as well as OFDM bit-loading. Our methodology consists of (a) a study of the
channel model developed under IEEE 802.15 for UWB from a frequency-domain
perspective suited for OFDM transmission, (b) development and quantification of
appropriate information-theoretic performance measures, (c) comparison of these
measures with simulation results for the Multiband OFDM standard proposal as
well as our proposed extensions, and (d) the consideration of the influence of
practical, imperfect channel estimation on the performance. We find that the
current Multiband OFDM standard sufficiently exploits the frequency selectivity
of the UWB channel, and that the system performs in the vicinity of the channel
cutoff rate. Turbo codes and a reduced-complexity clustered bit-loading
algorithm improve the system power efficiency by over 6 dB at a data rate of
480 Mbps.Comment: 32 pages, 10 figures, 1 table. Submitted to the IEEE Transactions on
Wireless Communications (Sep. 28, 2005). Minor revisions based on reviewers'
comments (June 23, 2006
Cosmological Implications of the Tetron Model of Elementary Particles
Based on a possible solution to the tetron spin problem, a modification of
the standard Big Bang scenario is suggested, where the advent of a spacetime
manifold is connected to the appearance of tetronic bound states. The metric
tensor is constructed from tetron constituents and the reason for cosmic
inflation is elucidated. Furthermore, there are natural dark matter candidates
in the tetron model. The ratio of ordinary to dark matter in the universe is
calculated to be 1:5.Comment: 23 page
Structure functions for the three nucleon system
The spectral functions and light-cone momentum distributions of protons and
neutrons in 3He and 3H are given in terms of the three-nucleon wave function
for realistic nucleon-nucleon interactions. To reduce computational complexity,
separable expansions are employed for the nucleon-nucleon potentials. The
results for the light-cone momentum distributions suggest that they are not
very sensitive to the details of the two-body interaction, as long as it has
reasonable short-range repulsion. The unpolarised and polarised structure
functions are examined for both 3He and 3H in order to test the usefulness of
3He as a neutron target. It is found that the measurement of the spin structure
function of polarised 3H would provide a very clear test of the predicted
change in the polarised parton distributions of a bound proton.Comment: 30 pages, REVTeX, 11 figure
Self-reported difficulties with everyday function, cognitive symptoms, and cognitive function in people with HIV
BACKGROUND: We determined factors associated with self-reported decline in activities of daily living (ADLs) and symptoms of cognitive impairment in HIV positive (HIV+) adults in five European clinics. METHODS: HIV+ adults underwent computerized and pen-and-paper neuropsychological tests and questionnaires of cognitive symptoms and ADLs. We considered cognitive function in five domains, psychosocial factors and clinical parameters as potentially associated with symptoms. Separate regression analyses were used to determine factors associated with decline in ADL (defined as self-reported decline affecting ≥2 ADLs and attributed to cognitive difficulties) and self-reported frequency of symptoms of cognitive impairment. We also estimated the diagnostic accuracy of both questionnaires as tests for cognitive impairment. RESULTS: 448 patients completed the assessments (mean age 45.8 years, 84% male, 87% white, median CD4 count 550 cells/mm, median time since HIV diagnosis 9.9 years, 81% virologically suppressed [HIV-1 plasma RNA <50 copies/mL]). Ninety-six (21.4%) reported decline in ADLs and attributed this to cognitive difficulties. Self-reported decline in ADLs and increased symptoms of cognitive impairment were both associated with worse performance on some cognitive tests. There were also strong associations with financial difficulties, depressive and anxiety symptoms, unemployment, and longer time since HIV diagnosis. Both questionnaires performed poorly as diagnostic tests for cognitive impairment. CONCLUSION: Patients' own assessments of everyday function and symptoms were associated with objectively-measured cognitive function. However, there were strong associations with other psychosocial issues including mood and anxiety disorders and socioeconomic hardship. This should be considered when assessing HIV-associated cognitive impairment in clinical care or research studies
Radio-frequency discharges in Oxygen. Part 1: Modeling
In this series of three papers we present results from a combined
experimental and theoretical effort to quantitatively describe capacitively
coupled radio-frequency discharges in oxygen. The particle-in-cell Monte-Carlo
model on which the theoretical description is based will be described in the
present paper. It treats space charge fields and transport processes on an
equal footing with the most important plasma-chemical reactions. For given
external voltage and pressure, the model determines the electric potential
within the discharge and the distribution functions for electrons, negatively
charged atomic oxygen, and positively charged molecular oxygen. Previously used
scattering and reaction cross section data are critically assessed and in some
cases modified. To validate our model, we compare the densities in the bulk of
the discharge with experimental data and find good agreement, indicating that
essential aspects of an oxygen discharge are captured.Comment: 11 pages, 10 figure
Symptoms are highly prevalent among HIV outpatients and associated with poor adherence and unprotected sexual intercourse
"It All Ended in an Unsporting Way": Serbian Football and the Disintegration of Yugoslavia, 1989-2006
Part of a wider examination into football during the collapse of Eastern European Communism between 1989 and 1991, this article studies the interplay between Serbian football and politics during the period of Yugoslavia's demise. Research utilizing interviews with individuals directly involved in the Serbian game, in conjunction with contemporary Yugoslav media sources, indicates that football played an important proactive role in the revival of Serbian nationalism. At the same time the Yugoslav conflict, twinned with a complex transition to a market economy, had disastrous consequences for football throughout the territories of the former Yugoslavia. In the years following the hostilities the Serbian game has suffered decline, major financial hardship and continuing terrace violence, resulting in widespread nostalgia for the pre-conflict era
Different iron storage strategies among bloom-forming diatoms
Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 115(52), (2018): E12275-E12284. doi: 10.1073/pnas.1805243115.Diatoms are prominent eukaryotic phytoplankton despite being limited by the micronutrient iron in vast expanses of the ocean. As iron inputs are often sporadic, diatoms have evolved mechanisms such as the ability to store iron that enable them to bloom when iron is resupplied and then persist when low iron levels are reinstated. Two iron storage mechanisms have been previously described: the protein ferritin and vacuolar storage. To investigate the ecological role of these mechanisms among diatoms, iron addition and removal incubations were conducted using natural phytoplankton communities from varying iron environments. We show that among the predominant diatoms, Pseudo-nitzschia were favored by iron removal and displayed unique ferritin expression consistent with a long-term storage function. Meanwhile, Chaetoceros and Thalassiosira gene expression aligned with vacuolar storage mechanisms. Pseudo-nitzschia also showed exceptionally high iron storage under steady-state high and low iron conditions, as well as following iron resupply to iron-limited cells. We propose that bloom-forming diatoms use different iron storage mechanisms and that ferritin utilization may provide an advantage in areas of prolonged iron limitation with pulsed iron inputs. As iron distributions and availability change, this speculated ferritin-linked advantage may result in shifts in diatom community composition that can alter marine ecosystems and biogeochemical cycles.We thank the captain and crew of the R/V Melville and the CCGS J. P. Tully as well as the participants of the IRNBRU (MV1405) cruise for the California-based data, particularly K. Ellis [University of North Carolina (UNC)], T. Coale (University of California, San Diego), F. Kuzminov (Rutgers), H. McNair [University of California, Santa Barbara (UCSB)], and J. Jones (UCSB). W. Burns (UNC), S. Haines (UNC), and S. Bargu (Louisiana State University) assisted with sample processing and analysis. This work was funded by the National Science Foundation Grants OCE-1334935 (to A.M.), OCE-1334632 (to B.S.T.), OCE-1333929 (to K.T.), OCE-1334387 (to M.A.B.), OCE-1259776 (to K.W.B), and DGE-1650116 (Graduate Research Fellowship to R.H.L).2019-06-1
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