168 research outputs found
Exploiting channel symmetry in two-way channels
Wireless communication is frequently employed for the bi-directional exchange of information between devices with disparate capabilities. Mobile devices such as cellular phones and laptops are subject to stringent size weight and power constraints relative to the base stations or access points they communicate with. Asymmetry of constraints results in asymmetry of the cost of computation. This disparity in cost motivates efforts to shift signal processing such as channel estimation and equalization from cost sensitive devices to those with more relaxed constraints. In this work we develop a two-way training channel estimation and precoding scheme that shifts all the computation involved in dealing with multiplicative channel gain as well as frequency selective channels to the base station. This work demonstrates the first tractable implementation of a precoding based transceiver scheme of this class on the Rice University Wireless Open Access Research Platform (WARP)
Boundary quantum criticality in models of magnetic impurities coupled to bosonic baths
We investigate quantum impurity problems, where a local magnetic moment is
coupled to the spin density of a bosonic environment, leading to bosonic
versions of the standard Kondo and Anderson impurity models. In a physical
situation, these bosonic environments can correspond either to deconfined
spinons in certain classes of Z_2 frustrated antiferromagnets, or to particles
in a multicomponent Bose gase (in which case the spin degree of freedom is
attributed to hyperfine levels). Using renormalization group techniques, we
establish that our impurity models, which feature an exchange interaction
analogous to Kondo impurities in Fermi liquids, allow the flow towards a stable
strong-coupling state. Since the low-energy bosons live around a single point
in momentum space, and there is no Fermi surface, an impurity quantum phase
transition occurs at intermediate coupling, separating screened and unscreened
phases. This behavior is qualitatively different from previously studied
spin-isotropic variants of the spin-boson model, which display stable
intermediate-coupling fixed points and no screening.Comment: 15 pages, 10 fig
Universal crossovers and critical dynamics of quantum phase transitions: A renormalization group study of the pseudogap Kondo problem
The pseudogap Kondo problem, describing a magnetic impurity embedded in an
electronic environment with a power-law density of states, displays continuous
quantum phase transitions between free and screened moment phases. In this
paper we employ renormalization group techniques to analytically calculate
universal crossover functions, associated to these transitions, for various
observables. Quantitative agreement with the results of Numerical
Renormalization Group (NRG) simulations is obtained for temperature-dependent
static and zero-temperature dynamic quantities, at and away from criticality.
In the notoriously difficult realm of finite-temperature low-frequency
dynamics, usually inaccessible to both NRG and perturbative methods, we show
that progress can be made by a suitable renormalization procedure in the
framework of the Callan-Symanzik equations. Our general strategy can be
extended to other zero-temperature phase transitions, both in quantum impurity
models and bulk systems.Comment: 19 pages, 18 figures; (v3) version as publishe
A Unified Account of the Moral Standing to Blame
Recently, philosophers have turned their attention to the question, not when a given agent is blameworthy for what she does, but when a further agent has the moral standing to blame her for what she does. Philosophers have proposed at least four conditions on having “moral standing”:
1. One’s blame would not be “hypocritical”.
2. One is not oneself “involved in” the target agent’s wrongdoing.
3. One must be warranted in believing that the target is indeed blameworthy for the wrongdoing.
4. The target’s wrongdoing must some of “one’s business”.
These conditions are often proposed as both conditions on one and the same thing, and as marking fundamentally different ways of “losing standing.” Here I call these claims into question. First, I claim that conditions (3) and (4) are simply conditions on different things than are conditions (1) and (2). Second, I argue that condition (2) reduces to condition (1): when “involvement” removes someone’s standing to blame, it does so only by indicating something further about that agent, viz., that he or she lacks commitment to the values that condemn the wrongdoer’s action. The result: after we clarify the nature of the non-hypocrisy condition, we will have a unified account of moral standing to blame. Issues also discussed: whether standing can ever be regained, the relationship between standing and our "moral fragility", the difference between mere inconsistency and hypocrisy, and whether a condition of standing might be derived from deeper facts about the "equality of persons"
Software-Defined Radio for Space-to-Space Communications
A paper describes the Space- to-Space Communications System (SSCS) Software- Defined Radio (SDR) research project to determine the most appropriate method for creating flexible and reconfigurable radios to implement wireless communications channels for space vehicles so that fewer radios are required, and commonality in hardware and software architecture can be leveraged for future missions. The ability to reconfigure the SDR through software enables one radio platform to be reconfigured to interoperate with many different waveforms. This means a reduction in the number of physical radio platforms necessary to support a space mission s communication requirements, thus decreasing the total size, weight, and power needed for a mission
Selective Attention Increases Both Gain and Feature Selectivity of the Human Auditory Cortex
Background. An experienced car mechanic can often deduce what’s wrong with a car by carefully listening to the sound of the ailing engine, despite the presence of multiple sources of noise. Indeed, the ability to select task-relevant sounds for awareness, whilst ignoring irrelevant ones, constitutes one of the most fundamental of human faculties, but the underlying neural mechanisms have remained elusive. While most of the literature explains the neural basis of selective attention by means of an increase in neural gain, a number of papers propose enhancement in neural selectivity as an alternative or a complementary mechanism. Methodology/Principal Findings. Here, to address the question whether pure gain increase alone can explain auditory selective attention in humans, we quantified the auditory cortex frequency selectivity in 20 healthy subjects by masking 1000-Hz tones by continuous noise masker with parametrically varying frequency notches around the tone frequency (i.e., a notched-noise masker). The task of the subjects was, in different conditions, to selectively attend to either occasionally occurring slight increments in tone frequency (1020 Hz), tones of slightly longer duration, or ignore the sounds. In line with previous studies, in the ignore condition, the global field power (GFP) of event-related brain responses at 100 ms from the stimulus onset to the 1000-Hz tones was suppressed as a function of the narrowing of the notch width. During the selective attention conditions, the suppressant effect of the noise notch width on GFP was decreased, but as a function significantly different from a multiplicative one expected on the basis of simple gain model of selective attention. Conclusions/Significance. Our results suggest that auditory selective attention in humans cannot be explained by a gai
Giant intrinsic photoresponse in pristine graphene
When the Fermi level matches the Dirac point in graphene, the reduced charge
screening can dramatically enhance electron-electron (e-e) scattering to
produce a strongly interacting Dirac liquid. While the dominance of e-e
scattering already leads to novel behaviors, such as electron hydrodynamic
flow, further exotic phenomena have been predicted to arise specifically from
the unique kinematics of e-e scattering in massless Dirac systems. Here, we use
optoelectronic probes, which are highly sensitive to the kinematics of electron
scattering, to uncover a giant intrinsic photocurrent response in pristine
graphene. This photocurrent emerges exclusively at the charge neutrality point
and vanishes abruptly at non-zero charge densities. Moreover, it is observed at
places with broken reflection symmetry, and it is selectively enhanced at free
graphene edges with sharp bends. Our findings reveal that the photocurrent
relaxation is strongly suppressed by a drastic change of fast photocarrier
kinematics in graphene when its Fermi level matches the Dirac point. The
emergence of robust photocurrents in neutral Dirac materials promises new
energy-harvesting functionalities and highlights intriguing electron dynamics
in the optoelectronic response of Dirac fluids.Comment: Originally submitted versio
The Time-resolved Atomic, Molecular and Optical Science Instrument at the Linac Coherent Light Source
The newly constructed Time-resolved atomic, Molecular and Optical science
instrument (TMO), is configured to take full advantage of both linear
accelerators at SLAC National Accelerator Laboratory, the copper accelerator
operating at a repetition rate of 120 Hz providing high per pulse energy, as
well as the superconducting accelerator operating at a repetition rate of about
1 MHz providing high average intensity. Both accelerators build a soft X-ray
free electron laser with the new variable gab undulator section. With this
flexible light sources, TMO supports many experimental techniques not
previously available at LCLS and will have two X-ray beam focus spots in line.
Thereby, TMO supports Atomic, Molecular and Optical (AMO), strong-field and
nonlinear science and will host a designated new dynamic reaction microscope
with a sub-micron X-ray focus spot. The flexible instrument design is optimized
for studying ultrafast electronic and molecular phenomena and can take full
advantage of the sub-femtosecond soft X-ray pulse generation program
Economic Analysis of Knowledge: The History of Thought and the Central Themes
Following the development of knowledge economies, there has been a rapid expansion of economic analysis of knowledge, both in the context of technological knowledge in particular and the decision theory in general. This paper surveys this literature by identifying the main themes and contributions and outlines the future prospects of the discipline. The wide scope of knowledge related questions in terms of applicability and alternative approaches has led to the fragmentation of research. Nevertheless, one can identify a continuing tradition which analyses various aspects of the generation, dissemination and use of knowledge in the economy
The Effects of Herbivory by a Mega- and Mesoherbivore on Tree Recruitment in Sand Forest, South Africa
Herbivory by megaherbivores on woody vegetation in general is well documented; however studies focusing on the individual browsing effects of both mega- and mesoherbivore species on recruitment are scarce. We determined these effects for elephant Loxodonta africana and nyala Tragelaphus angasii in the critically endangered Sand Forest, which is restricted to east southern Africa, and is conserved mainly in small reserves with high herbivore densities. Replicated experimental treatments (400 m2) in a single forest patch were used to exclude elephant, or both elephant and nyala. In each treatment, all woody individuals were identified to species and number of stems, diameter and height were recorded. Results of changes after two years are presented. Individual tree and stem densities had increased in absence of nyala and elephant. Seedling recruitment (based on height and diameter) was inhibited by nyala, and by elephant and nyala in combination, thereby preventing recruitment into the sapling stage. Neither nyala or elephant significantly reduced sapling densities. Excluding both elephant and nyala in combination enhanced recruitment of woody species, as seedling densities increased, indicating that forest regeneration is impacted by both mega- and mesoherbivores. The Sand Forest tree community approached an inverse J-shaped curve, with the highest abundance in the smaller size classes. However, the larger characteristic tree species in particular, such as Newtonia hildebrandtii, were missing cohorts in the middle size classes. When setting management goals to conserve habitats of key importance, conservation management plans need to consider the total herbivore assemblage present and the resulting browsing effects on vegetation. Especially in Africa, where the broadest suite of megaherbivores still persists, and which is currently dealing with the ‘elephant problem’, the individual effects of different herbivore species on recruitment and dynamics of forests and woodlands are important issues which need conclusive answers
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