165,380 research outputs found
Adaptively refined large eddy simulations of clusters
We present a numerical scheme for modelling unresolved turbulence in
cosmological adaptive mesh refinement codes. As a first application, we study
the evolution of turbulence in the intra-cluster medium and in the core of a
galaxy cluster. Simulations with and without subgrid scale model are compared
in detail. Since the flow in the ICM is subsonic, the global turbulent energy
contribution at the unresolved length scales is smaller than 1% of the internal
energy. We find that the production of turbulence is closely correlated with
merger events occurring in the cluster environment, and its dissipation locally
affects the cluster energy budget. Because of this additional source of
dissipation, the core temperature is larger and the density is smaller in the
presence of subgrid scale turbulence than in the standard adiabatic run,
resulting in a higher entropy core value.Comment: Submitted to ApJ, 14 pages, 14 figures, 3 table
Opportunistic Error Correction for WLAN Applications
The current error correction layer of IEEE 802.11a WLAN is designed\ud
for worst case scenarios, which often do not apply. In this paper,\ud
we propose a new opportunistic error correction layer based on\ud
Fountain codes and a resolution adaptive ADC. The key part in the\ud
new proposed system is that only packets are processed by the\ud
receiver chain which have encountered ``good'' channel conditions.\ud
Others are discarded. With this new approach, around \ud
of the energy consumption can be saved compared with the\ud
conventional IEEE 802.11a WLAN system under the same channel\ud
conditions and throughput
An Opportunistic Error Correction Layer for OFDM Systems
In this paper, we propose a novel cross layer scheme to lower power\ud
consumption of ADCs in OFDM systems, which is based on resolution\ud
adaptive ADCs and Fountain codes. The key part in the new proposed\ud
system is that the dynamic range of ADCs can be reduced by\ud
discarding the packets which are transmitted over 'bad' sub\ud
carriers. Correspondingly, the power consumption in ADCs can be\ud
reduced. Also, the new system does not process all the packets but\ud
only processes surviving packets. This new error correction layer\ud
does not require perfect channel knowledge, so it can be used in a\ud
realistic system where the channel is estimated. With this new\ud
approach, more than 70% of the energy consumption in the ADC can be\ud
saved compared with the conventional IEEE 802.11a WLAN system under\ud
the same channel conditions and throughput. The ADC in a receiver\ud
can consume up to 50% of the total baseband energy. Moreover, to\ud
reduce the overhead of Fountain codes, we apply message passing and\ud
Gaussian elimination in the decoder. In this way, the overhead is\ud
3% for a small block size (i.e. 500 packets). Using both methods\ud
results in an efficient system with low delay
2D/3D Simulation of macrosegregation: a comparison between codes on a small cavity and on a large ingot
International audienceThis paper presents the coupled resolution of momentum, energy and solute conservation equations, for binary alloys by three different codes. The microsegregation is governed by the lever rule and the liquid flow in the mushy zone is modeled by a Darcy law. A 2D FV code, SOLID, a 2D FE code, R2SOL and a 3D FE code, THERCAST, are compared on an academic case on which experimental measurements have been done by Hebditch and Hunt, and on a benchmark steel ingot for industrial application. An adaptive anisotropic remeshing technique is used in each FE codes. For both codes, this technique is shortly described
A blind projection receiver for coded CDMA systems
Journal ArticleABSTRACT This paper presents a blind adaptive CDMA receiver that requires no knowledge of the spreading codes, the delays, and the energy of the received signals associated with the interfering users. Our receiver is based on linear interference cancellation and adaptive interference signal subspace tracking. It has error control coding embedded in the detector structure and employs "branch processing" to detect the bit stream. Simulation results demonstrating that the receiver suffers negligible performance loss over systems with complete knowledge of the interfering users are presented in the paper
Variational Integrators for the Gravitational N-Body Problem
This paper describes a fourth-order integration algorithm for the
gravitational N-body problem based on discrete Lagrangian mechanics. When used
with shared timesteps, the algorithm is momentum conserving and symplectic. We
generalize the algorithm to handle individual time steps; this introduces
fifth-order errors in angular momentum conservation and symplecticity. We show
that using adaptive block power of two timesteps does not increase the error in
symplecticity. In contrast to other high-order, symplectic, individual
timestep, momentum-preserving algorithms, the algorithm takes only forward
timesteps. We compare a code integrating an N-body system using the algorithm
with a direct-summation force calculation to standard stellar cluster
simulation codes. We find that our algorithm has about 1.5 orders of magnitude
better symplecticity and momentum conservation errors than standard algorithms
for equivalent numbers of force evaluations and equivalent energy conservation
errors.Comment: 31 pages, 8 figures. v2: Revised individual-timestepping description,
expanded comparison with other methods, corrected error in predictor
equation. ApJ, in pres
Machine Learning Inspired Energy-Efficient Hybrid Precoding for MmWave Massive MIMO Systems
Hybrid precoding is a promising technique for mmWave massive MIMO systems, as
it can considerably reduce the number of required radio-frequency (RF) chains
without obvious performance loss. However, most of the existing hybrid
precoding schemes require a complicated phase shifter network, which still
involves high energy consumption. In this paper, we propose an energy-efficient
hybrid precoding architecture, where the analog part is realized by a small
number of switches and inverters instead of a large number of high-resolution
phase shifters. Our analysis proves that the performance gap between the
proposed hybrid precoding architecture and the traditional one is small and
keeps constant when the number of antennas goes to infinity. Then, inspired by
the cross-entropy (CE) optimization developed in machine learning, we propose
an adaptive CE (ACE)-based hybrid precoding scheme for this new architecture.
It aims to adaptively update the probability distributions of the elements in
hybrid precoder by minimizing the CE, which can generate a solution close to
the optimal one with a sufficiently high probability. Simulation results verify
that our scheme can achieve the near-optimal sum-rate performance and much
higher energy efficiency than traditional schemes.Comment: This paper has been accepted by IEEE ICC 2017. The simulation codes
are provided to reproduce the results in this paper at:
http://oa.ee.tsinghua.edu.cn/dailinglong/publications/publications.htm
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