18,523 research outputs found
On the Achievable Rates of Decentralized Equalization in Massive MU-MIMO Systems
Massive multi-user (MU) multiple-input multiple-output (MIMO) promises
significant gains in spectral efficiency compared to traditional, small-scale
MIMO technology. Linear equalization algorithms, such as zero forcing (ZF) or
minimum mean-square error (MMSE)-based methods, typically rely on centralized
processing at the base station (BS), which results in (i) excessively high
interconnect and chip input/output data rates, and (ii) high computational
complexity. In this paper, we investigate the achievable rates of decentralized
equalization that mitigates both of these issues. We consider two distinct BS
architectures that partition the antenna array into clusters, each associated
with independent radio-frequency chains and signal processing hardware, and the
results of each cluster are fused in a feedforward network. For both
architectures, we consider ZF, MMSE, and a novel, non-linear equalization
algorithm that builds upon approximate message passing (AMP), and we
theoretically analyze the achievable rates of these methods. Our results
demonstrate that decentralized equalization with our AMP-based methods incurs
no or only a negligible loss in terms of achievable rates compared to that of
centralized solutions.Comment: Will be presented at the 2017 IEEE International Symposium on
Information Theor
The Majorana spin in magnetic atomic chain systems
In this paper, we establish that Majorana zero modes emerging from a
topological band structure of a chain of magnetic atoms embedded in a
superconductor can be distinguished from trivial localized zero energy states
that may accidentally form in this system using spin resolved measurements. To
demonstrate this key Majorana diagnostics, we study the spin composition of
magnetic impurity induced in-gap Shiba states in a superconductor using a
quantum impurity model (at the mean-field level). By examining the spin and
spectral densities in the context of the Bogoliubov-de Gennes (BdG)
particle-hole symmetry, we derive a sum rule that relates the spin densities of
localized Shiba states with those in the normal state without
superconductivity. Extending our investigations to ferromagnetic chain of
magnetic impurities, we identify key features of the spin properties of the
extended Shiba state bands, as well as those associated with a localized
Majorana end mode when the effect of spin-orbit interaction is included. We
then formulate a phenomenological theory for the measurement of the local spin
densities with spin-polarized scanning tunneling microscopy (STM) techniques.
By combining the calculated spin densities and the measurement theory, we show
that spin-polarized STM measurements can reveal a sharp contrast in spin
polarization between an accidentally-zero-energy trivial Shiba state and a
Majorana zero mode in a topological superconducting phase in atomic chains. We
further confirm our results with numerical simulations that address generic
parameter settings.Comment: 22 pages, 12 figures (references updated
From Color Fields to Quark Gluon Plasma
We discuss a model for the energy distribution and the early space-time
evolution of a heavy ion collision. We estimate the gluon field generated in
the wake of hard processes and through primordial fluctuations of the color
charges in the nuclei. Without specifying the dynamical mechanism of
thermalization we calculate the energy momentum tensor of the following plasma
phase. The results of this model can be used as initial conditions for a
further hydrodynamic evolution.Comment: Contribution to Quark Matter 2005; 4 pages, 4 figure
Chemically etched ultrahigh-Q wedge-resonator on a silicon chip
Ultrahigh-Q optical resonators are being studied across a wide range of fields, including quantum information, nonlinear optics, cavity optomechanics and telecommunications. Here, we demonstrate a new resonator with a record Q-factor of 875 million for on-chip devices. The fabrication of our device avoids the requirement for a specialized processing step, which in microtoroid resonators8 has made it difficult to control their size and achieve millimetre- and centimetre-scale diameters. Attaining these sizes is important in applications such as microcombs and potentially also in rotation sensing. As an application of size control, stimulated Brillouin lasers incorporating our device are demonstrated. The resonators not only set a new benchmark for the Q-factor on a chip, but also provide, for the first time, full compatibility of this important device class with conventional semiconductor processing. This feature will greatly expand the range of possible ‘system on a chip’ functions enabled by ultrahigh-Q devices
The Recovery of Weak Impulsive Signals Based on Stochastic Resonance and Moving Least Squares Fitting
In this paper a stochastic resonance (SR)-based method for recovering weak impulsive signals is developed for quantitative diagnosis of faults in rotating machinery. It was shown in theory that weak impulsive signals follow the mechanism of SR, but the SR produces a nonlinear distortion of the shape of the impulsive signal. To eliminate the distortion a moving least squares fitting method is introduced to reconstruct the signal from the output of the SR process. This proposed method is verified by comparing its detection results with that of a morphological filter based on both simulated and experimental signals. The experimental results show that the background noise is suppressed effectively and the key features of impulsive signals are reconstructed with a good degree of accuracy, which leads to an accurate diagnosis of faults in roller bearings in a run-to failure test
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