Channel in OWC
THE ATMOSPHERE
Absorption and scattering of light by particles present in the atmosphere is a complex process involving Mie
scattering and nonselective scattering by large particles (such as fog, haze, rain) and Rayleigh scattering by
smaller particles.
i. Absorption—This takes place when there is an interaction between the propagating photons and
molecules (present in the atmosphere) along its path. A photon is absorbed when the quantum state of a
molecule is excited to a higher energy level.
ii. Scattering—This results in angular redistribution of the optical field with and without wavelength
modification. The scattering effect depends on the radius rp of the particles (fog, aerosol) encountered
during propagation. One way of describing this is to consider the size parameter
the scattering process is classified as Rayleigh scattering[60];
scattering
Mie Scattering
Mie scattering occurs when the particle size (aerosol) is comparable to the beam size from sub-micrometer to
a few tens of micrometers; In the atmosphere, aerosols differ in distribution, components, and profile
concentration,therefore influencing the interactions with the propagating optical beam in the forms of
absorption and scattering
The largest concentration of aerosols is normally located at 1–2 km immediately above the earth’s
surface, and they are classified based on the
following models:
• Maritime model: in the proximity of or over the sea and ocean surfaces, and typically
consist of salt particles in aggregation with water droplets.
• Rural model: over land with aerosol compositions consisting of dust and other particles
mixed with water droplets. The aerosol composition, density, and its particle size distribution
will vary with land composition, vegetation, weather, and seasonal climate
variations.
• Urban model: man-made aerosols, produced by industry.
• Desert model: mainly airborne dust particles with concentration mainly depending
on the wind speed.
The diffraction-limited beam spreading/geometric
loss in dB is thus given by
Ar is the receiver effective aperture areas
To characterize the attenuation of optical signal propagating through a medium, a term called “specific attenuation” is used which
means attenuation per unit length expressed in dB/km and is given as
show schematically this effect as well as
the variations (amplitude, frequency,
etc.) of the received signal. The beam
deviates when the heterogeneities are
large compared to the beam cross
section and the beam is widened when
heterogeneities are small. A mixture of
heterogeneities results in scintillation
In general, the refractive index of the atmosphere at any point r in space can be expressed as sum of the
average and the fluctuating terms, i.e.,
where P’ the atmospheric pressure in mbar, and T’ the temperature of the atmosphere in Kelvin.
The structure function for refractive index
Depending on the size of turbulent eddy and transmitter beam size, three types of atmospheric
turbulence effects are observed:
The signal after demodulation can be written as
System model for optical wave propagation in atmospheric turbulence.
The Lognormal model is considered and found that it has weak turbulence condition if long propagation path is
considered6. There is another model, K-distribution7 which has strong turbulence condition. The Negative
exponential has been proposed and shown that it has very strong turbulence condition because its PDF
(Probability Density Function) gives appropriate results in negative region8. There are some other models based on
doubly stochastic theory8 like Gamma-gamma, Malaga distributionwhich works under weak to strong turbulence
conditions.
Impact of various atmospheric effects on optical wireless communication