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UNDERWATER COMMUNICATION

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INTRODUCTION

While wireless communication technology today has become part of our daily life, the idea of wireless undersea communications may still seem far-fetched. However, research has been active for over a decade on designing the methods for wireless information transmission underwater. Human knowledge and understanding of the world’s oceans, which constitute the major part of our planet, rests on our ability to collect information from remote undersea locations.
The major discoveries of the past decades, such as the remains of Titanic, or the hydro-thermal vents at bottom of deep ocean, were made using cabled submersibles. Although such systems remain indispensable if high-speed communication link is to exists between the remote end and the surface, it is natural to wonder what one could accomplish without the burden (and cost) of heavy cables
UNDERWATER WIRELESS COMMUNICATION is a flourishing research field of wireless communications. Applications like oceanographic data collection, AUVs(autonomous underwater vehicles),underwater radio, transmission of video and audio signals by real time monitoring emphasized to overcome the present limitations.
Wireless is a used to describe the telecommunication with which the electromagnetic waves carry the signal over the communication path. The signals are not radio signals as electromagnetic waves propagate at a short distance so acoustic waves are used that propagate at a long distance. To overcome some impediments, here is an existence of underwater wireless communication. Acoustic modems employ advanced modulation scheme and channel equalization to combat multiple paths to have improved signal to noise ratio. A high performance error detection and correction coding scheme is employed which reduces the bit error rate to less than 10-7.
Underwater networks consist of a number of sensors and vehicles which are deployed to do collaborative monitoring tasks on a given area. There is a traditional approach for ocean bottom monitoring is to deploy underwater sensors which record data and then recover the instruments. With this method, real time monitoring is not possible and failures happen. It can overcome by connecting underwater instruments with the help of wireless links.


Underwater acoustic communication

It is a technique of sending and receiving message below water. There are several ways of employing such communication but the most common is using hydrophones. Under water communication is difficult due to factors like multi-path propagation, time variations of the channel, small available bandwidth and strong signal attenuation, especially over long ranges. In underwater communication there are low data rates compared to terrestrial communication, since underwater communication uses acoustic waves instead of electromagnetic waves
Figure1 Example of multipath propagation


UNDERWATER ACOUSTICS

Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water and its boundaries. The water may be in the ocean, a lake or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and 1 MHz . The propagation of sound in the ocean at frequencies lower than 10 Hz is usually not possible without penetrating deep into the seabed, whereas frequencies above 1 MHz are rarely used because they are absorbed very quickly. Underwater acoustics is sometimes known as hydro acoustics .
The field of underwater acoustics is closely related to a number of other fields of acoustic study, including sonar, transduction, acoustic signal processing, acoustical oceanography, bioacoustics, and physical acoustics.

THEORY
Sound waves in water

A sound wave propagating underwater consists of alternating compressions and rarefactions of the water. These compressions and rarefactions are detected by a receiver, such as the human ear or a hydrophone, as changes in pressure. These waves may be man-made or naturally generated.
Speed of sound, density and impedance
The speed of sound (i.e., the longitudinal motion of wavefronts) is related to frequency and wavelength of a wave by .
This is different from the particle velocity , which refers to the motion of molecules in the medium due to the sound, and relates the plane wave the pressure to the fluid density and sound speed by .
The product of and from the above formula is known as the characteristic acoustic impedance. The acoustic power (energy per second) crossing unit area is known as the intensity of the wave and for a plane wave the average intensity is given by , where is the root mean square acoustic pressure.
At 1 kHz, the wavelength in water is about 1.5 m. Sometimes the term "sound velocity" is used but this is incorrect as the quantity is a scalar.
The large impedance contrast between air and water (the ratio is about 3600) and the scale of surface roughness means that the sea surface behaves as an almost perfect reflector of sound at frequencies below 1 kHz. Sound speed in water exceeds that in air by a factor of 4.4 and the density ratio is about 820.

Absorption of sound

Absorption of low frequency sound is weak. (see Technical Guides - Calculation of absorption of sound in seawater for an on-line calculator). The main cause of sound attenuation in fresh water, and at high frequency in sea water (above 100 kHz) is viscosity. Important additional contributions at lower frequency in seawater are associated with the ionic relaxation of boric acid (up to c. 10 kHz) and magnesium sulfate (c. 10 kHz-500 kHz).
Sound may be absorbed by losses at the fluid boundaries. Near the surface of the sea losses can occur in a bubble layer or in ice, while at the bottom sound can penetrate into the sediment and be absorbed.

UNDER WATER COMMUNICATION

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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.