The growth taking advantages of
natural reserves under water that is notably in the Ocean has provoked the advancement
of various technical boosts in the region of environmental disaster management,
scientific research, particularly survey of oil and gas and for defensive
strategy and many more.  The impact of
underwater wireless communication in all above mention regions has significant
part. On the other hands the technical approach trust on Radio-Frequency or
Electronics, Optical, and Acoustic transmissions. The purpose of this report is
to highlight the overview, challenges and application of Radio Frequency
Communication underwater.

 Nowadays, there is comprehensive growing
research activity relating to underwater communications and new and noticeable
challenges when compared to wireless communications through the air, needed
refined transmission tools over short propagation range to gain low data
through put. Underwater wireless communication has a unique feature that makes
it distinct from transmission in free space. In underwater communication particular
circumstances may impact like as conductivity, turbidity (affecting camera
optics), water temperature and behaviours of and tides and so on.

Although all challenges, No
doubt, wireless communications has a significant role in underwater systems
with all challenging tasks. Observing different substances various in
underwater environment is admissible in many distinctive applications, such as
natural resources investigation, maritime security, and navigation system, and marine
pollution control 1 .For all the above mention tasks in underwater wireless communication
a complete and comprehensive system is built with the help of buoys, Autonomous
underwater vehicles, sensors that attached on sea floor, submarines and
watercraft.

Typically
for wireless communication underwater three main techniques are present.

·        
Radio
Frequency(RF)

·        
Optical
communication

·        
Acoustic
communication.

Review of Underwater
wireless communication:

There are various methods for transmission
which includes a connection between earth to satellite, then to floating device
or ship etc. It is also attainable for duplex transmission via the Radio
Frequency (RF) antennas located at ships base stations and earth stations.  Meanwhile, transmitted or received data
between buoys and underwater base stations with the aid of transmission
appliances. The numerous amounts of distinct types of transmission nodes underwater
system are easy to deploy in the forms of AUV’s and wireless system. The entire
above mention scenario depict in Fig 1.

                     Figure 1: Multiple Communication Technologies.

Source: www.google.com/search?q=multiplication+communication+technologies

It is easy to develop network architecture
to control or programmed the network with the help of software applications in
this type of adjustable communication circumstances each one with its basic properties,
can transfer data.

Electromagnetic
Spectrum:

One aspect of electromagnetic
spectrum is that it is the distribution of electromagnetic radiation according
to energy, frequency, or wavelength. It consists of as a flow of photons atoms that
are moving like proper waves and with the speed of light 2. Electromagnetic
spectrum containing forms of electromagnetic radiation that varied from visible
light only in terms of wavelength and wave frequency. Frequency is referred as
the repeating of waves per unit time, and unit is Hertz. Wavelength defines the
distance between wave crests of the two consecutive waves that have same in
phase.

Waves properties of
Electromagnetic Radiation:

Electromagnetic waves are defined
as that they travel through a vacuum at the speed of light and forms as a
result of oscillation of electric and magnetic fields which are synchronized. The
electric and magnetic fields are perpendicular to each other and perpendicular
to the direction of energy and wave propagation, forming a transceiver wave.
Figure 2 shows the pattern of electromagnetic wave.

 

Figure 2:
Electromagnetic wave

Source: www.google.com/search?q=electromagnetic+wave+pattern

Wavelength and frequency are
inversely proportional: that is, the shorter the wavelength, the higher the
frequency, and vice versa. This relationship is given by the following
equation:

c=??

Where ? (the Greek lambda) is
the wavelength (in meters) and ? (the Greek nu) is the frequency (in Hertz,
Hz). Their product is the constant c, the speed of light, which is equal to
3.00×10^8 m/s

This relationship reflects an
important fact: all electromagnetic radiation, regardless of wavelength or
frequency, travels at the speed of light. 3.

Radio Frequency
Communication:

Radio frequency (RF) is a representation
of repetitive variation of electromagnetic radiation, from frequencies ranging
from 30 KHz to 300 GHz. With the use of antennas and transmitters, an RF field
can be used for various types of wireless broadcasting and communications.

                                                   

Speed=distance/time

                                               Speed=wavelength*frequency

                                               Distance/time=wavelength*frequency

So,

                                               Wavelength=distance/(time*frequency)

 

Frequency and wavelength are inversely
proportional to each other the higher the frequency the less distance covered
by wave and the lower the frequency the higher the distance covered by wave.

In free space the use of
frequency ranges for telecommunication, broadcasting and for satellite
transmission is not deployed underwater because of its conductivity means the
ability to induce electricity, it effect the attenuation of frequency ranges
based on electromagnetic waves. As a result, it is hard to transmit data by
using higher frequency ranges for more than 10 meters in underwater .On the
other hand, for lower frequency the attenuation of signal propagation is less
due to lower conductivity so achieved communication over several miles. Maximum
distance for several frequencies (approximately 6 m at 100 kHz, 16 m at 10 kHz
and 22 m at 1 kHz).Some researchers of the Swansea Metropolitan University,
U.K, performed their simulation at 3 kHz and distance between nodes of about 40
meters 4. Unfortunately, due to lower frequency ranges (ELF or VLF) are suffering
low data throughput.

 The multipath effect is a positive approach in
RF signals in Shallow Ocean in the sense of signal propagation. The signal can
propagate the water to air or via the deep sea bead as directed in Fig 2.  It is a possibility to enhance the distance of
propagating signal underwater. As a result, the transmission can be done between
the station operates underwater and coastal station. In this scenario, the
propagating signal has lower attenuation as compared to propagate only
underwater.

 

 

Figure 2: Possible
multipath propagation of RF signal in shallow water environment

Source: www.google.com/search?q=:+Possible+multipath+propagation+of+RF+signal

Environmental
factor has a great impact while traversing of Radio signals such as
Conductivity and Pressure and obviously frequency. For each type of water
conductivity has a unique value. The value of conductivity in seawater is high
enough as compared to fresh water i-e 4 s/m and 0.01 s/m respectively (it is
because of alternates values of salinity and physical characteristics of sea water).As a consequence, the main
condition to be considered to characterize the wireless channel for RF
transmission is the conductivity/salinity of the water.

Electromagnetic waves have less
precise towards the phenomena of reflection and refraction in seawater as
compared to acoustic waves. More ever, the effect of solid particles underwater
has negligible impact on electromagnetic radiation. Thus, the vital issue by using
electromagnetic waves in seawater is attenuation in signal traversing occur by
the conductivity. The relation between the attenuation and frequency is
directly proportional to each other. In case of seawater when chose the
frequency up to 2.4 GHz the propagation distance is relatively in few centimetre.

RF Transceivers:

The
mechanization adopted for transceivers is alike as the one used for wireless
communications in free space, i.e., antennas. For frequency ranges i-e ELF and
VLF, there are need for large receiving antennas, which may prevent the application
of RF technology in some domains. Moreover, all the devices should be properly
enclosed alike encapsulated for manipulation and tooling functions in the
underwater environment.

Main Concerns in
Underwater RF Communications:

One of the main problems in
underwater communications is the low data rate available due to the use of low frequencies.
Although, there are many implicit problems to the medium such as reflections,
refraction, energy dispersion, etc., that greatly degrade communication between
devices.

On the other hand, the
underwater RF communication is heavily suffered signal losses that should be determined
correctly in application perceptive operations, such as recovery and storage of
data, and link power budget necessary to operate the devices underwater. Meanwhile,
the RF propagating signal has to undergo frequency selective channels (If the
channel behaviour is different for selected frequency) and it is affected by marine
noise. For reliable communication the channel and noise estimations should be in
proper range to gain the channel capacity. To overcome this issue the solution
could be multicarrier transceivers with feasible channel state estimation
(description of a signal propagates from transmitter to receiver and effects of
scattering, fading and power delay with distance) and channel loading 5.However,
there are some applications such as observation of sediment presents at bottom
of Ocean. This technique use to control the coastal erosion by deployment of
sensors that can transmit information by RF signals.