24,436 research outputs found
Fast solution of Cahn-Hilliard variational inequalities using implicit time discretization and finite elements
We consider the e�cient solution of the Cahn-Hilliard variational inequality using an implicit time discretization, which is formulated as an optimal control problem with pointwise constraints on the control. By applying a semi-smooth Newton method combined with a Moreau-Yosida regularization technique for handling the control constraints we show superlinear convergence in function space. At the heart of this method lies the solution of large and sparse linear systems for which we propose the use of preconditioned Krylov subspace solvers using an e�ective Schur complement approximation. Numerical results illustrate the competitiveness of this approach
The Effect of Macrodiversity on the Performance of Maximal Ratio Combining in Flat Rayleigh Fading
The performance of maximal ratio combining (MRC) in Rayleigh channels with
co-channel interference (CCI) is well-known for receive arrays which are
co-located. Recent work in network MIMO, edge-excited cells and base station
collaboration is increasing interest in macrodiversity systems. Hence, in this
paper we consider the effect of macrodiversity on MRC performance in Rayleigh
fading channels with CCI. We consider the uncoded symbol error rate (SER) as
our performance measure of interest and investigate how different
macrodiversity power profiles affect SER performance. This is the first
analytical work in this area. We derive approximate and exact symbol error rate
results for M-QAM/BPSK modulations and use the analysis to provide a simple
power metric. Numerical results, verified by simulations, are used in
conjunction with the analysis to gain insight into the effects of the link
powers on performance.Comment: 10 pages, 5 figures; IEEE Transaction of Communication, 2012
Corrected typo
Performance Analysis of Dual-User Macrodiversity MIMO Systems with Linear Receivers in Flat Rayleigh Fading
The performance of linear receivers in the presence of co-channel
interference in Rayleigh channels is a fundamental problem in wireless
communications. Performance evaluation for these systems is well-known for
receive arrays where the antennas are close enough to experience equal average
SNRs from a source. In contrast, almost no analytical results are available for
macrodiversity systems where both the sources and receive antennas are widely
separated. Here, receive antennas experience unequal average SNRs from a source
and a single receive antenna receives a different average SNR from each source.
Although this is an extremely difficult problem, progress is possible for the
two-user scenario. In this paper, we derive closed form results for the
probability density function (pdf) and cumulative distribution function (cdf)
of the output signal to interference plus noise ratio (SINR) and signal to
noise ratio (SNR) of minimum mean squared error (MMSE) and zero forcing (ZF)
receivers in independent Rayleigh channels with arbitrary numbers of receive
antennas. The results are verified by Monte Carlo simulations and high SNR
approximations are also derived. The results enable further system analysis
such as the evaluation of outage probability, bit error rate (BER) and
capacity.Comment: 24 pages, 7 figures; IEEE Transaction of Wireless Communication 2012
Corrected typo
Extremely low frequency based communication link
The paper discusses the literature review and the possibility of using the ground itself as transmission medium for various users’ transceivers and an administrator transceiver using Multi-Carrier-Direct Sequence-Code Division Multiple Access (MC-DS-CDMA), Orthogonal Frequency Division Multiplexing (OFDM),16-Quadrature Amplitude Modulation (16-QAM), Frequency Division Duplex (FDD) and Extremely Low Frequency (ELF) band for the applications of Oil Well Telemetry, remote control of power substations or any system that its responding time is not critical
Shocked by GRB 970228: the afterglow of a cosmological fireball
The location accuracy of the BeppoSAX Wide Field Cameras and acute
ground-based followup have led to the detection of a decaying afterglow in X
rays and optical light following the classical gamma-ray burst GRB 970228. The
afterglow in X rays and optical light fades as a power law at all wavelengths.
This behaviour was predicted for a relativistic blast wave that radiates its
energy when it decelerates by ploughing into the surrounding medium. Because
the afterglow has continued with unchanged behaviour for more than a month, its
total energy must be of order 10**51 erg, placing it firmly at a redshift of
order 1. Further tests of the model are discussed, some of which can be done
with available data, and implications for future observing strategies are
pointed out. We discuss how the afterglow can provide a probe for the nature of
the burst sources.Comment: 6 pages LaTeX, 1 postscript figure; minor edits, slightly more data
on light curve, MNRAS, IN PRESS (mid June/early July
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