25 research outputs found
Adaptive channel selection for DOA estimation in MIMO radar
We present adaptive strategies for antenna selection for Direction of Arrival
(DoA) estimation of a far-field source using TDM MIMO radar with linear arrays.
Our treatment is formulated within a general adaptive sensing framework that
uses one-step ahead predictions of the Bayesian MSE using a parametric family
of Weiss-Weinstein bounds that depend on previous measurements. We compare in
simulations our strategy with adaptive policies that optimize the Bobrovsky-
Zaka{\i} bound and the Expected Cram\'er-Rao bound, and show the performance
for different levels of measurement noise.Comment: Submitted to the 25th European Signal Processing Conference
(EUSIPCO), 201
Towards a holographic dual of large-N_c QCD
We study N_f D6-brane probes in the supergravity background dual to N_c
D4-branes compactified on a circle with supersymmetry-breaking boundary
conditions. In the limit in which the resulting Kaluza--Klein modes decouple,
the gauge theory reduces to non-supersymmetric, four-dimensional QCD with N_c
colours and N_f << N_c flavours. As expected, this decoupling is not fully
realised within the supergravity/Born--Infeld approximation. For N_f = 1 and
massless quarks, m_q = 0, we exhibit spontaneous chiral symmetry breaking by a
quark condensate, \neq 0, and find the associated massless
`pion' in the spectrum. The latter becomes massive for m_q > 0, obeying the
Gell-Mann--Oakes--Renner relation: M_pi^2= - m_q / \f_pi^2. In
the case N_f > 1 we provide a holographic version of the Vafa--Witten theorem,
which states that the U(N_f) flavour symmetry cannot be spontaneously broken.
Further we find N_f^2 - 1 unexpectedly light pseudo-scalar mesons in the
spectrum. We argue that these are not (pseudo) Goldstone bosons and speculate
on the string mechanism responsible for their lightness. We then study the
theory at finite temperature and exhibit a phase transition associated with a
discontinuity in the chiral condensate. D6/anti-D6 pairs are also briefly
discussed.Comment: 43 pages, LaTeX; v3: Scalar vs. pseudo-scalar nature of mesons
clarified, references added. v4: Small change in Acknowledgment
Thermodynamics and Instabilities of a Strongly Coupled Anisotropic Plasma
We extend our analysis of a IIB supergravity solution dual to a spatially
anisotropic finite-temperature N=4 super Yang-Mills plasma. The solution is
static, possesses an anisotropic horizon, and is completely regular. The full
geometry can be viewed as a renormalization group flow from an AdS geometry in
the ultraviolet to a Lifshitz-like geometry in the infrared. The anisotropy can
be equivalently understood as resulting from a position-dependent theta-term or
from a non-zero number density of dissolved D7-branes. The holographic stress
tensor is conserved and anisotropic. The presence of a conformal anomaly plays
an important role in the thermodynamics. The phase diagram exhibits homogeneous
and inhomogeneous (i.e. mixed) phases. In some regions the homogeneous phase
displays instabilities reminiscent of those of weakly coupled plasmas. We
comment on similarities with QCD at finite baryon density and with the
phenomenon of cavitation.Comment: 62 pages, 13 figures; v2: typos fixed, added reference
Thermodynamics, transport and relaxation in non-conformal theories
We study the equilibrium and near-equilibrium properties of a holographic
five-dimensional model consisting of Einstein gravity coupled to a scalar field
with a non-trivial potential. The dual four-dimensional gauge theory is not
conformal and, at zero temperature, exhibits a renormalisation group flow
between two different fixed points. We quantify the deviations from
conformality both in terms of thermodynamic observables and in terms of the
bulk viscosity of the theory. The ratio of bulk over shear viscosity violates
Buchel's bound. We study relaxation of small-amplitude, homogeneous
perturbations by computing the quasi-normal modes of the system at zero spatial
momentum. In this approximation we identify two different relaxation channels.
At high temperatures, the different pressures first become approximately equal
to one another, and subsequently this average pressure evolves towards the
equilibrium value dictated by the equation of state. At low temperatures, the
average pressure first evolves towards the equilibrium pressure, and only later
the different pressures become approximately equal to one another
Iron Behaving Badly: Inappropriate Iron Chelation as a Major Contributor to the Aetiology of Vascular and Other Progressive Inflammatory and Degenerative Diseases
The production of peroxide and superoxide is an inevitable consequence of
aerobic metabolism, and while these particular "reactive oxygen species" (ROSs)
can exhibit a number of biological effects, they are not of themselves
excessively reactive and thus they are not especially damaging at physiological
concentrations. However, their reactions with poorly liganded iron species can
lead to the catalytic production of the very reactive and dangerous hydroxyl
radical, which is exceptionally damaging, and a major cause of chronic
inflammation. We review the considerable and wide-ranging evidence for the
involvement of this combination of (su)peroxide and poorly liganded iron in a
large number of physiological and indeed pathological processes and
inflammatory disorders, especially those involving the progressive degradation
of cellular and organismal performance. These diseases share a great many
similarities and thus might be considered to have a common cause (i.e.
iron-catalysed free radical and especially hydroxyl radical generation). The
studies reviewed include those focused on a series of cardiovascular, metabolic
and neurological diseases, where iron can be found at the sites of plaques and
lesions, as well as studies showing the significance of iron to aging and
longevity. The effective chelation of iron by natural or synthetic ligands is
thus of major physiological (and potentially therapeutic) importance. As
systems properties, we need to recognise that physiological observables have
multiple molecular causes, and studying them in isolation leads to inconsistent
patterns of apparent causality when it is the simultaneous combination of
multiple factors that is responsible. This explains, for instance, the
decidedly mixed effects of antioxidants that have been observed, etc...Comment: 159 pages, including 9 Figs and 2184 reference