Introduction:

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.

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 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 2.

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 3 .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.

 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 a proper waves and with the speed of light.3. 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

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. 4.

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 2.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 5.

 The multipath effect is a positive approach in RF signals in seawater 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 6. 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

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 7.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. 

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