28-06-2013, 02:17 PM
UNDER WATER COMMUNICATION
UNDER WATER.pdf (Size: 641.97 KB / Downloads: 35)
INTRODUCTION
The future tactical ocean environment will be increasingly
complicated. In addition to traditional communication links there will be a
proliferation of unmanned vehicles in space, in the air, on the surface, and
underwater. Above the air/water interface wireless radio frequency
communications will continue to provide the majority of communication
channels.Underwater, where radio waves do not propagate, acoustic methods
will continue to be used.However, while there have been substantial advances
in acoustic underwater communications, acoustics will be hard pressed to
provide sufficient bandwidth to multiple platforms at the same time. Acoustic
methods will also continue to have difficulty penetrating the water/air
interface. This suggests that high bandwidth, short range underwater optical
communications have high potential to augment acoustic communication
methods.
The variations in the optical properties of ocean water lead to
interesting problems when considering the feasibility and reliability of
underwater optical links. Radio waves do not propagate underwater, however
with the proliferation of unmanned autonomous vehicles the need to
communicate large amounts of data is quickly increasing. Making physical
connections underwater to transfer data is often impractical operationally or
technically hard to do. Traditionally most underwater communication systems
have been acoustic and relatively low bandwidth. However, the development
of high brightness blue/green LED sources, and laser diodes suggest that high
speed optical links can be viable for short range applications. Underwater
systems also have severe power, and size constraints compared to land or air
based systems. Underwater vehicles also encounter a wide range of optical
environments.
Appling FSO Concepts to Seawater
The transmitter and receiver for an underwater link can be very similar
to a FSO link in air, the major difference being the wavelength of operation.
However, ocean water has widely varying optical properties depending on
location, time of day, organic and inorganic content, as well as temporal
variations such as turbulence. To construct an optical link it is important to
understand these properties. The loss of optical energy while traversing the
link arises from both absorption and scattering. Scattering also adversely
impacts the link by introducing multipath dispersion.
Absorption by Pure Seawater
Seawater is composed of primarily H20, which absorbs heavily towards
the red spectrum. It also has dissolved salts like NaCl, MgCl2, Na2SO4, CaCl2,
and KCl that absorb light at specific wavelengths10. As seen below, pure
seawater is absorptive except around a 400nm-500nm window, the blue-green
region of the visible light spectrum.
The absorption coefficient for pure seawater is the amount of
absorption per meter of sea water. However, the majority of the attenuation is
due to other mechanisms such as absorption by chlorophylls and humic acids,
and scattering from particulate.
4Phytoplankton – Chlorophyll-a
Phytoplankton, derived from phyto, meaning plant and planktos,
meaning wandering, is one of the most influential factors in light transmission
through ocean waters. Phytoplankton live in the euphotic zone, which is the
region from the surface to where only 1% of the sunlight reaches. Depending
on the geographical location, time of day and season, the zone ranges in depth
from 50m to about 200m in open ocean; typically it’s around 100m7.
Phytoplankton use chlorophyll-a, which absorbs mostly in the blue and red
region and scatters green light to produce “food” through the process of
photosynthesis10. As the concentration of chlorophyll-a increases, more blue
and red light are absorbed, leaving the water a greenish tint.
Rise time Budget
The purpose of a rise-time budget is to ensure that the complete system
is able to operate at the intended bit-rate. The rise-time characteristics of the
transmitter and receiver are usually known. The allocated rise time will
depend on the format used by the system, i.e. Return to Zero (RZ) or
NonReturn to Zero (NRZ). With NRZ format able to accommodate Where B
is the bit rate and r,max T is the quadratic sum of the following: transmitter rise
time, the receiver rise time, the rise-time that is induced by intermodal
dispersion, and group
26 velocity dispersion caused by the fiber. The 0.35 comes from the
assumption that a RC-low frequency band pass model can be used to describe
the response of the system to an impulse. The dispersion, the spread of the
optical pulse in time is expressed in ps/nm-km for chromatic dispersion or
ps/km for modal or multipath dispersion. In single mode fibers, chromatic
dispersion dominates while for multimode fiber systems the modal dispersion
dominates1.
Geometric Effects in Link Budget
In non-fiber based optical communications systems, such as free space
optical communications in air, radio frequency wireless, or acoustic
communication systems, the medium of the channel is complex with
environmental changes that are unpredictable. To decouple the real
environment and variability of the problem, it is useful to first consider the
medium to be isotropic and infinite so the performance bounds due to the
fundamental geometric effects introduced by the transmitting and receiving
apertures of the systems can be computed this way the effects of
environmental fluctuations on the channel can then be added to the model.
Environmental Consideration in Link Budgets
Significant efforts have been made in trying to understand what is
required for a reliable link for free space optical communications in air and
space. As mentioned above, the geometric effects are fairly straightforward to
compute. However, transmission through the atmosphere poses special
problems. In addition to water vapor and CO2 absorption, and scattering by
aerosol particles, the atmosphere has a structure that varies with altitude.
Visibility a useful way to evaluate the suitability of the atmosphere and
can range from a couple of hundred meters in fog or snow to tens of
kilometers in the upper atmosphere on a clear day.
UNDER WATER.pdf (Size: 641.97 KB / Downloads: 35)
INTRODUCTION
The future tactical ocean environment will be increasingly
complicated. In addition to traditional communication links there will be a
proliferation of unmanned vehicles in space, in the air, on the surface, and
underwater. Above the air/water interface wireless radio frequency
communications will continue to provide the majority of communication
channels.Underwater, where radio waves do not propagate, acoustic methods
will continue to be used.However, while there have been substantial advances
in acoustic underwater communications, acoustics will be hard pressed to
provide sufficient bandwidth to multiple platforms at the same time. Acoustic
methods will also continue to have difficulty penetrating the water/air
interface. This suggests that high bandwidth, short range underwater optical
communications have high potential to augment acoustic communication
methods.
The variations in the optical properties of ocean water lead to
interesting problems when considering the feasibility and reliability of
underwater optical links. Radio waves do not propagate underwater, however
with the proliferation of unmanned autonomous vehicles the need to
communicate large amounts of data is quickly increasing. Making physical
connections underwater to transfer data is often impractical operationally or
technically hard to do. Traditionally most underwater communication systems
have been acoustic and relatively low bandwidth. However, the development
of high brightness blue/green LED sources, and laser diodes suggest that high
speed optical links can be viable for short range applications. Underwater
systems also have severe power, and size constraints compared to land or air
based systems. Underwater vehicles also encounter a wide range of optical
environments.
Appling FSO Concepts to Seawater
The transmitter and receiver for an underwater link can be very similar
to a FSO link in air, the major difference being the wavelength of operation.
However, ocean water has widely varying optical properties depending on
location, time of day, organic and inorganic content, as well as temporal
variations such as turbulence. To construct an optical link it is important to
understand these properties. The loss of optical energy while traversing the
link arises from both absorption and scattering. Scattering also adversely
impacts the link by introducing multipath dispersion.
Absorption by Pure Seawater
Seawater is composed of primarily H20, which absorbs heavily towards
the red spectrum. It also has dissolved salts like NaCl, MgCl2, Na2SO4, CaCl2,
and KCl that absorb light at specific wavelengths10. As seen below, pure
seawater is absorptive except around a 400nm-500nm window, the blue-green
region of the visible light spectrum.
The absorption coefficient for pure seawater is the amount of
absorption per meter of sea water. However, the majority of the attenuation is
due to other mechanisms such as absorption by chlorophylls and humic acids,
and scattering from particulate.
4Phytoplankton – Chlorophyll-a
Phytoplankton, derived from phyto, meaning plant and planktos,
meaning wandering, is one of the most influential factors in light transmission
through ocean waters. Phytoplankton live in the euphotic zone, which is the
region from the surface to where only 1% of the sunlight reaches. Depending
on the geographical location, time of day and season, the zone ranges in depth
from 50m to about 200m in open ocean; typically it’s around 100m7.
Phytoplankton use chlorophyll-a, which absorbs mostly in the blue and red
region and scatters green light to produce “food” through the process of
photosynthesis10. As the concentration of chlorophyll-a increases, more blue
and red light are absorbed, leaving the water a greenish tint.
Rise time Budget
The purpose of a rise-time budget is to ensure that the complete system
is able to operate at the intended bit-rate. The rise-time characteristics of the
transmitter and receiver are usually known. The allocated rise time will
depend on the format used by the system, i.e. Return to Zero (RZ) or
NonReturn to Zero (NRZ). With NRZ format able to accommodate Where B
is the bit rate and r,max T is the quadratic sum of the following: transmitter rise
time, the receiver rise time, the rise-time that is induced by intermodal
dispersion, and group
26 velocity dispersion caused by the fiber. The 0.35 comes from the
assumption that a RC-low frequency band pass model can be used to describe
the response of the system to an impulse. The dispersion, the spread of the
optical pulse in time is expressed in ps/nm-km for chromatic dispersion or
ps/km for modal or multipath dispersion. In single mode fibers, chromatic
dispersion dominates while for multimode fiber systems the modal dispersion
dominates1.
Geometric Effects in Link Budget
In non-fiber based optical communications systems, such as free space
optical communications in air, radio frequency wireless, or acoustic
communication systems, the medium of the channel is complex with
environmental changes that are unpredictable. To decouple the real
environment and variability of the problem, it is useful to first consider the
medium to be isotropic and infinite so the performance bounds due to the
fundamental geometric effects introduced by the transmitting and receiving
apertures of the systems can be computed this way the effects of
environmental fluctuations on the channel can then be added to the model.
Environmental Consideration in Link Budgets
Significant efforts have been made in trying to understand what is
required for a reliable link for free space optical communications in air and
space. As mentioned above, the geometric effects are fairly straightforward to
compute. However, transmission through the atmosphere poses special
problems. In addition to water vapor and CO2 absorption, and scattering by
aerosol particles, the atmosphere has a structure that varies with altitude.
Visibility a useful way to evaluate the suitability of the atmosphere and
can range from a couple of hundred meters in fog or snow to tens of
kilometers in the upper atmosphere on a clear day.