849 research outputs found
Cancellation of IR Divergences in 3d Abelian Gauge Theories
Three dimensional abelian gauge theories classically in a Coulomb phase are affected by IR divergences even when the matter fields are all massive. Using generalizations of Ward-Takahashi identities, we show that correlation functions of gauge-invariant operators are IR finite to all orders in perturbation theory. Gauge invariance is sufficient but not necessary for IR finiteness. In particular we show that specific gauge-variant correlators, including the two-point function of matter fields, are also IR finite to all orders in perturbation theory. Possible applications of these results are briefly discussed
Neuroprotection in Parkinson’s disease : a realistic goal?
The current issue of CNS Neuroscience & Therapeutics contains an interesting review by Kinecses and Vecsei on the progress in our knowledge related to the pathophysiological mechanisms of Parkinson’s disease (PD) and on the development of putative neuroprotective molecules. Since the seminal discovery by Oleh Hornykiewicz that degeneration of DA neurons within the substantia nigra pars compacta (SNc) and the consequential dopamine depletion in the striatum was the cause of neurological symptoms in PD, thousands of reviews have been written on the subject, some of them possibly superfluous. Nevertheless, we found this last work enjoyable in terms of readability and in the way the authors decided to tackle such a difficult enterprise. This brief literature review is obviously far from comprehensive or exhaustive, as it would be impossible to summarize 50 years of fruitful research in the PD field in a few pages. The main contribution of this review is the general overview of the pathomechanism field and a survey of the literature that it provides on the hot topic of neuroprotection. Indeed, molecules able to slow and halt dopaminergic neuronal loss represent the highest ambition of PD research, drug companies and not least, patients. In recent years, research has advanced to the point that halting the progression of PD, restoring lost function, and even preventing the disease might be considered realistic goals. Nevertheless the ultimate goal of preventing PD may take years to achieve, and no strong experimental confirmation hitherto is available for any of the compounds described by Kinecses and Vecsei and others that the authors have not cited.peer-reviewe
Symmetries and topological operators, on average
We study Ward identities and selection rules for local correlators in
disordered theories where a 0-form global symmetry of a QFT is explicitly
broken by a random coupling but it re-emerges after quenched average. We
consider space-dependent or constant. In both cases we construct the
symmetry operator implementing the group action, topological after average. In
the first case, relevant in statistical systems with random impurities, such
symmetries can be coupled to external backgrounds and can be gauged, like
ordinary symmetries in QFTs. We also determine exotic selection rules arising
when symmetries emerge after average in the IR, explaining the origin of
LogCFTs from symmetry considerations. In the second case, relevant in AdS/CFT
to describe the dual boundary theory of certain bulk gravitational theories,
the charge operator is not purely codimension-1, it can be defined only on
homologically trivial cycles and on connected spaces. Selection rules for
average correlators exist, yet such symmetries cannot be coupled to background
gauge fields in ordinary ways and cannot be gauged. When the space is
disconnected, in each connected component charge violation occurs, as expected
from Euclidean wormholes in the bulk theory. Our findings show the obstruction
to interpret symmetries emergent after average as gauged in the bulk
Enhancing Coexistence in the Unlicensed Band with Massive MIMO
We consider cellular base stations (BSs) equipped with a large number of
antennas and operating in the unlicensed band. We denote such system as massive
MIMO unlicensed (mMIMO-U). We design the key procedures required to guarantee
coexistence between a cellular BS and nearby Wi-Fi devices. These include:
neighboring Wi-Fi channel covariance estimation, allocation of spatial degrees
of freedom for interference suppression, and enhanced channel sensing and data
transmission phases. We evaluate the performance of the so-designed mMIMO-U,
showing that it allows simultaneous cellular and Wi-Fi transmissions by keeping
their mutual interference below the regulatory threshold. The same is not true
for conventional listen-before-talk (LBT) operations. As a result, mMIMO-U
boosts the aggregate cellular-plus-Wi-Fi data rate in the unlicensed band with
respect to conventional LBT, exhibiting increasing gains as the number of BS
antennas grows.Comment: To appear in Proc. IEEE ICC 201
Indoor Massive MIMO Deployments for Uniformly High Wireless Capacity
Providing consistently high wireless capacity is becoming increasingly
important to support the applications required by future digital enterprises.
In this paper, we propose Eigen-direction-aware ZF (EDA-ZF) with partial
coordination among base stations (BSs) and distributed interference suppression
as a practical approach to achieve this objective. We compare our solution with
Zero Forcing (ZF), entailing neither BS coordination or inter-cell interference
mitigation, and Network MIMO (NeMIMO), where full BS coordination enables
centralized inter-cell interference management. We also evaluate the
performance of said schemes for three sub-6 GHz deployments with varying BS
densities -- sparse, intermediate, and dense -- all with fixed total number of
antennas and radiated power. Extensive simulations show that: (i) indoor
massive MIMO implementing the proposed EDA-ZF provides uniformly good rates for
all users; (ii) indoor network densification is detrimental unless full
coordination is implemented; (iii) deploying NeMIMO pays off under strong
outdoor interference, especially for cell-edge users
Surface Roughness Characterisation and Analysis of the Electron Beam Melting (EBM) Process
Electron Beam Melting (EBM) is a metal powder bed fusion (PBF) process in which the heat source is an electron beam. Differently from other metal PBF processes, today, EBM is used for mass production. As-built EBM parts are clearly recognisable by their surface roughness, which is, in some cases, one of the major limitations of the EBM process. The aim of this work is to investigate the effects of the orientation and the slope of the EBM surfaces on the surface roughness. Additionally, the machine repeatability is studied by measuring the roughness of surfaces built at different positions on the start plate. To these aims, a specific artefact was designed. Replicas of the artefact were produced using an Arcam A2X machine and Ti6Al4V powder. Descriptive and inferential statistical methods were applied to investigate whether the surface morphology was affected by process factors. The results show significant differences between the upward and downward surfaces. The upward surfaces appear less rough than the downward ones, for which a lower standard deviation was obtained in the results. The roughness of the upward surfaces is linearly influenced by the sloping angle, while the heat distribution on the cross-section was found to be a key factor in explaining the roughness of the downward surfaces
Uplink Sounding Reference Signal Coordination to Combat Pilot Contamination in 5G Massive MIMO
To guarantee the success of massive multiple-input multiple-output (MIMO),
one of the main challenges to solve is the efficient management of pilot
contamination. Allocation of fully orthogonal pilot sequences across the
network would provide a solution to the problem, but the associated overhead
would make this approach infeasible in practical systems. Ongoing
fifth-generation (5G) standardisation activities are debating the amount of
resources to be dedicated to the transmission of pilot sequences, focussing on
uplink sounding reference signals (UL SRSs) design. In this paper, we
extensively evaluate the performance of various UL SRS allocation strategies in
practical deployments, shedding light on their strengths and weaknesses.
Furthermore, we introduce a novel UL SRS fractional reuse (FR) scheme, denoted
neighbour-aware FR (FR-NA). The proposed FR-NA generalizes the fixed reuse
paradigm, and entails a tradeoff between i) aggressively sharing some UL SRS
resources, and ii) protecting other UL SRS resources with the aim of relieving
neighbouring BSs from pilot contamination. Said features result in a cell
throughput improvement over both fixed reuse and state-of-the-art FR based on a
cell-centric perspective
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