5,343 research outputs found
Performance of binary block codes at low signal-to-noise ratios
The performance of general binary block codes on an unquantized additive white Gaussian noise (AWGN) channel at low signal-to-noise ratios is considered. Expressions are derived for both the block error and the bit error probabilities near the point where the bit signal-to-noise ratio is zero. These expressions depend on the global geometric structure of the code, although the minimum distance still seems to play a crucial role. Examples of codes such as orthogonal codes, biorthogonal codes, the (24,12) extended Golay code, and the (15,6) expurgated BCH code are discussed. The asymptotic coding gain at low signal-to-noise ratios is also studied
Why Do Proteins Look Like Proteins?
Protein structures in nature often exhibit a high degree of regularity
(secondary structures, tertiary symmetries, etc.) absent in random compact
conformations. We demonstrate in a simple lattice model of protein folding that
structural regularities are related to high designability and evolutionary
stability. We measure the designability of each compact structure by the number
of sequences which can design the structure, i.e., which possess the structure
as their nondegenerate ground state. We find that compact structures are
drastically different in terms of their designability; highly designable
structures emerge with a number of associated sequences much larger than the
average. These structures are found to have ``protein like'' secondary
structure and even tertiary symmetries. In addition, they are also
thermodynamically more stable than ordinary structures. These results suggest
that protein structures are selected because they are easy to design and stable
against mutations, and that such a selection simutaneously leads to
thermodynamic stability.Comment: 5 pages, 4 figures, RevTex, some minor changes from the original
version, also available at http://www.neci.nj.nec.com/homepages/tang.htm
Review of the Proposed Reserve Markets in New England
ISO New England proposes reserve markets designed to improve the existing forward reserve market and improve pricing during real-time reserve shortages. We support all of the main elements of the proposal. For example, we agree that little is gained by allowing reserve availability bids in the day-ahead market. Doing so greatly increases the complexity of the market without the prospect of more efficient pricing. Rather, offline reserves are most efficiently priced and awarded well in advance, as is done by the improved forward reserve market.Auctions; Multiple Object Auctions; Electricity Auctions
Non-convex optimization for 3D point source localization using a rotating point spread function
We consider the high-resolution imaging problem of 3D point source image
recovery from 2D data using a method based on point spread function (PSF)
engineering. The method involves a new technique, recently proposed by
S.~Prasad, based on the use of a rotating PSF with a single lobe to obtain
depth from defocus. The amount of rotation of the PSF encodes the depth
position of the point source. Applications include high-resolution single
molecule localization microscopy as well as the problem addressed in this paper
on localization of space debris using a space-based telescope. The localization
problem is discretized on a cubical lattice where the coordinates of nonzero
entries represent the 3D locations and the values of these entries the fluxes
of the point sources. Finding the locations and fluxes of the point sources is
a large-scale sparse 3D inverse problem. A new nonconvex regularization method
with a data-fitting term based on Kullback-Leibler (KL) divergence is proposed
for 3D localization for the Poisson noise model. In addition, we propose a new
scheme of estimation of the source fluxes from the KL data-fitting term.
Numerical experiments illustrate the efficiency and stability of the algorithms
that are trained on a random subset of image data before being applied to other
images. Our 3D localization algorithms can be readily applied to other kinds of
depth-encoding PSFs as well.Comment: 28 page
Linear MIMO Precoding in Jointly-Correlated Fading Multiple Access Channels with Finite Alphabet Signaling
In this paper, we investigate the design of linear precoders for
multiple-input multiple-output (MIMO) multiple access channels (MAC). We assume
that statistical channel state information (CSI) is available at the
transmitters and consider the problem under the practical finite alphabet input
assumption. First, we derive an asymptotic (in the large-system limit) weighted
sum rate (WSR) expression for the MIMO MAC with finite alphabet inputs and
general jointly-correlated fading. Subsequently, we obtain necessary conditions
for linear precoders maximizing the asymptotic WSR and propose an iterative
algorithm for determining the precoders of all users. In the proposed
algorithm, the search space of each user for designing the precoding matrices
is its own modulation set. This significantly reduces the dimension of the
search space for finding the precoding matrices of all users compared to the
conventional precoding design for the MIMO MAC with finite alphabet inputs,
where the search space is the combination of the modulation sets of all users.
As a result, the proposed algorithm decreases the computational complexity for
MIMO MAC precoding design with finite alphabet inputs by several orders of
magnitude. Simulation results for finite alphabet signalling indicate that the
proposed iterative algorithm achieves significant performance gains over
existing precoder designs, including the precoder design based on the Gaussian
input assumption, in terms of both the sum rate and the coded bit error rate.Comment: 7 pages, 2 figures, accepted for ICC1
Star Formation in the LMC: Gravitational Instability and Dynamical Triggering
Evidence for triggered star formation is difficult to establish because
energy feedback from massive stars tend to erase the interstellar conditions
that led to the star formation. Young stellar objects (YSOs) mark sites of {\it
current} star formation whose ambient conditions have not been significantly
altered. Spitzer observations of the Large Magellanic Cloud (LMC) effectively
reveal massive YSOs. The inventory of massive YSOs, in conjunction with surveys
of interstellar medium, allows us to examine the conditions for star formation:
spontaneous or triggered. We examine the relationship between star formation
and gravitational instability on a global scale, and we present evidence of
triggered star formation on local scales in the LMC.Comment: 6 pages, 6 figures, IAU Symposium 237, Triggered Star Formation in a
Turbulent Medium, eds. Elmegreen and Palou
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