131 research outputs found
Clustering of matter in waves and currents
The growth rate of small-scale density inhomogeneities (the entropy
production rate) is given by the sum of the Lyapunov exponents in a random
flow. We derive an analytic formula for the rate in a flow of weakly
interacting waves and show that in most cases it is zero up to the fourth order
in the wave amplitude. We then derive an analytic formula for the rate in a
flow of potential waves and solenoidal currents. Estimates of the rate and the
fractal dimension of the density distribution show that the interplay between
waves and currents is a realistic mechanism for providing patchiness of
pollutant distribution on the ocean surface.Comment: 4 pages, 1 figur
Statistical Studies of Fading in Underwater Wireless Optical Channels in the Presence of Air Bubble, Temperature, and Salinity Random Variations (Long Version)
Optical signal propagation through underwater channels is affected by three
main degrading phenomena, namely absorption, scattering, and fading. In this
paper, we experimentally study the statistical distribution of intensity
fluctuations in underwater wireless optical channels with random temperature
and salinity variations as well as the presence of air bubbles. In particular,
we define different scenarios to produce random fluctuations on the water
refractive index across the propagation path, and then examine the accuracy of
various statistical distributions in terms of their goodness of fit to the
experimental data. We also obtain the channel coherence time to address the
average period of fading temporal variations. The scenarios under consideration
cover a wide range of scintillation index from weak to strong turbulence.
Moreover, the effects of beam-collimator at the transmitter side and aperture
averaging lens at the receiver side are experimentally investigated. We show
that the use of a transmitter beam-collimator and/or a receiver aperture
averaging lens suits single-lobe distributions such that the generalized Gamma
and exponential Weibull distributions can excellently match the histograms of
the acquired data. Our experimental results further reveal that the channel
coherence time is on the order of seconds and larger which implies to
the slow fading turbulent channels
Weak compressibility of surface wave turbulence
We study the growth of small-scale inhomogeneities of the density of
particles floating in weakly nonlinear, small-amplitude, surface waves. Despite
the amplitude smallness, the accumulated effect of the long-time evolution may
produce strongly inhomogeneous distribution of the floaters: density
fluctuations grow exponentially with a small but finite exponent. We show that
the exponent is of sixth or higher order in wave amplitude. As a result, the
inhomogeneities do not form within typical time-scales of the natural
environment. We conclude that turbulence of surface waves is weakly
compressible and alone it cannot be a realistic mechanism of the clustering of
matter on liquid surfaces
MIMO Underwater Visible Light Communications: Comprehensive Channel Study, Performance Analysis, and Multiple-Symbol Detection
In this paper, we analytically study the bit error rate (BER) performance of
underwater visible light communication (UVLC) systems with binary pulse
position modulation (BPPM). We simulate the channel fading-free impulse
response (FFIR) based on Monte Carlo numerical method to take into account the
absorption and scattering effects. Additionally, to characterize turbulence
effects, we multiply the aforementioned FFIR by a fading coefficient which for
weak oceanic turbulence can be modeled as a lognormal random variable (RV).
Moreover, to mitigate turbulence effects, we employ multiple transmitters
and/or receivers, i.e., spatial diversity technique over UVLC links.
Closed-form expressions for the system BER are provided, when equal gain
combiner (EGC) is employed at the receiver side, thanks to Gauss-Hermite
quadrature formula and approximation to the sum of lognormal RVs. We further
apply saddle-point approximation, an accurate photon-counting-based method, to
evaluate the system BER in the presence of shot noise. Both laser-based
collimated and light emitting diode (LED)-based diffusive links are
investigated. Since multiple-scattering effect of UVLC channels on the
propagating photons causes considerable inter-symbol interference (ISI),
especially for diffusive channels, we also obtain the optimum multiple-symbol
detection (MSD) algorithm to significantly alleviate ISI effects and improve
the system performance. Our numerical analysis indicates good matches between
the analytical and photon-counting results implying the negligibility of
signal-dependent shot noise, and also between analytical results and numerical
simulations confirming the accuracy of our derived closed-form expressions for
the system BER. Besides, our results show that spatial diversity significantly
mitigates fading impairments while MSD considerably alleviates ISI
deteriorations
Robust Free-Space Optical Communication Utilizing Polarization
Free-space optical (FSO) communication can be subject to various types of
distortion and loss as the signal propagates through non-uniform media. In
experiment and simulation, we demonstrate that the state of polarization and
degree of polarization of light passed though underwater bubbles, causing
turbulence, is preserved. Our experimental setup serves as an efficient, low
cost alternative approach to long distance atmospheric or underwater testing.
We compare our experimental results with those of simulations, in which we
model underwater bubbles, and separately, atmospheric turbulence. Our findings
suggest potential improvements in polarization based FSO communication schemes.Comment: 13 pages, 5 figure
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