4,046 research outputs found
On the BER of Multiple-Input Multiple-Output Underwater Wireless Optical Communication Systems
In this paper we analyze and investigate the bit error rate (BER) performance
of multiple-input multiple-output underwater wireless optical communication
(MIMO-UWOC) systems. In addition to exact BER expressions, we also obtain an
upper bound on the system BER. To effectively estimate the BER expressions, we
use Gauss-Hermite quadrature formula as well as approximation to the sum of
log-normal random variables. We confirm the accuracy of our analytical
expressions by evaluating the BER through photon-counting approach. Our
simulation results show that MIMO technique can mitigate the channel
turbulence-induced fading and consequently, can partially extend the viable
communication range, especially for channels with stronger turbulence
Mechanism for a Decaying Cosmological Constant
A mechanism is introduced to reduce a large cosmological constant to a
sufficiently small value consistent with observational upper limit. The basic
ingradient in this mechanism is a distinction which has been made between the
two unit systems used on cosmology and particle physics. We have used a
conformal invariant gravitational model to define a particular conformal frame
in terms of the large scale properties of the universe. It is then argued that
the contributions of mass scales in particle physics to the vacuum energy
density should be considered in a different conformal frame. In this manner a
cancellation mechanism is presented in which the conformal factor plays a key
role to relax the large effective cosmological constant.Comment: 6 pages, no figur
Chameleon gravity on cosmological scales
In conventional approach to the chameleon mechanism, by assuming a static and
spherically symmetric solutions in which matter density and chameleon field are
given by and , it has been shown that mass of
chameleon field is matter density-dependent. In regions of high matter density
such as earth, chameleon field is massive, in solar system it is low and in
cosmological scales it is very low. In this article we revisit the mechanism in
cosmological scales by assuming a redshift dependence of the matter density and
chameleon field, i.e. , . To support our analysis,
we best fit the model parameters with the observational data. The result shows
that in cosmological scales, the mass of chameleon field increases with the
redshift, i.e. more massive in higher redshifts. We also find that in both
cases of power-law and exponential potential function, the current universe
acceleration can be explained by the low mass chameleon field. In comparison
with the high redshift observational data, we also find that the model with
power-law potential function is in better agreement with the observational
data.Comment: 7 pages, 11 figure
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