The growth taking advantages ofnatural reserves under water that is notably in the Ocean has provoked the advancementof various technical boosts in the region of environmental disaster management,scientific research, particularly survey of oil and gas and for defensivestrategy and many more. The impact ofunderwater wireless communication in all above mention regions has significantpart. On the other hands the technical approach trust on Radio-Frequency orElectronics, Optical, and Acoustic transmissions. The purpose of this report isto highlight the overview, challenges and application of Radio FrequencyCommunication underwater. Nowadays, there is comprehensive growingresearch activity relating to underwater communications and new and noticeablechallenges when compared to wireless communications through the air, neededrefined transmission tools over short propagation range to gain low datathrough put. Underwater wireless communication has a unique feature that makesit distinct from transmission in free space. In underwater communication particularcircumstances may impact like as conductivity, turbidity (affecting cameraoptics), water temperature and behaviours of and tides and so on.
Although all challenges, Nodoubt, wireless communications has a significant role in underwater systemswith all challenging tasks. Observing different substances various inunderwater environment is admissible in many distinctive applications, such asnatural resources investigation, maritime security, and navigation system, and marinepollution control 1 .For all the above mention tasks in underwater wireless communicationa complete and comprehensive system is built with the help of buoys, Autonomousunderwater vehicles, sensors that attached on sea floor, submarines andwatercraft. Typicallyfor wireless communication underwater three main techniques are present.· RadioFrequency(RF) · Opticalcommunication· Acousticcommunication.
Review of Underwaterwireless communication:There are various methods for transmissionwhich includes a connection between earth to satellite, then to floating deviceor ship etc. It is also attainable for duplex transmission via the RadioFrequency (RF) antennas located at ships base stations and earth stations. Meanwhile, transmitted or received databetween buoys and underwater base stations with the aid of transmissionappliances. The numerous amounts of distinct types of transmission nodes underwatersystem are easy to deploy in the forms of AUV’s and wireless system. The entireabove mention scenario depict in Fig 1. Figure 1: Multiple Communication Technologies.
Source: www.google.com/search?q=multiplication+communication+technologiesIt is easy to develop network architectureto control or programmed the network with the help of software applications inthis type of adjustable communication circumstances each one with its basic properties,can transfer data.
ElectromagneticSpectrum:One aspect of electromagneticspectrum is that it is the distribution of electromagnetic radiation accordingto energy, frequency, or wavelength. It consists of as a flow of photons atoms thatare moving like proper waves and with the speed of light 2. Electromagneticspectrum containing forms of electromagnetic radiation that varied from visiblelight only in terms of wavelength and wave frequency. Frequency is referred asthe repeating of waves per unit time, and unit is Hertz. Wavelength defines thedistance between wave crests of the two consecutive waves that have same inphase.Waves properties ofElectromagnetic Radiation:Electromagnetic waves are definedas that they travel through a vacuum at the speed of light and forms as aresult of oscillation of electric and magnetic fields which are synchronized. Theelectric and magnetic fields are perpendicular to each other and perpendicularto the direction of energy and wave propagation, forming a transceiver wave.Figure 2 shows the pattern of electromagnetic wave.
Figure 2:Electromagnetic waveSource: www.google.com/search?q=electromagnetic+wave+patternWavelength and frequency areinversely proportional: that is, the shorter the wavelength, the higher thefrequency, and vice versa. This relationship is given by the followingequation:c=??Where ? (the Greek lambda) isthe 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 to3.
00×10^8 m/sThis relationship reflects animportant fact: all electromagnetic radiation, regardless of wavelength orfrequency, travels at the speed of light. 3.Radio FrequencyCommunication:Radio frequency (RF) is a representationof repetitive variation of electromagnetic radiation, from frequencies rangingfrom 30 KHz to 300 GHz. With the use of antennas and transmitters, an RF fieldcan 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 inverselyproportional to each other the higher the frequency the less distance coveredby wave and the lower the frequency the higher the distance covered by wave.In free space the use offrequency ranges for telecommunication, broadcasting and for satellitetransmission is not deployed underwater because of its conductivity means theability to induce electricity, it effect the attenuation of frequency rangesbased on electromagnetic waves. As a result, it is hard to transmit data byusing higher frequency ranges for more than 10 meters in underwater .
On theother hand, for lower frequency the attenuation of signal propagation is lessdue to lower conductivity so achieved communication over several miles. Maximumdistance for several frequencies (approximately 6 m at 100 kHz, 16 m at 10 kHzand 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 40meters 4. Unfortunately, due to lower frequency ranges (ELF or VLF) are sufferinglow data throughput. The multipath effect is a positive approach inRF signals in Shallow Ocean in the sense of signal propagation.
The signal canpropagate the water to air or via the deep sea bead as directed in Fig 2. It is a possibility to enhance the distance ofpropagating signal underwater. As a result, the transmission can be done betweenthe station operates underwater and coastal station. In this scenario, thepropagating signal has lower attenuation as compared to propagate onlyunderwater. Figure 2: Possiblemultipath propagation of RF signal in shallow water environmentSource: www.
google.com/search?q=:+Possible+multipath+propagation+of+RF+signalEnvironmentalfactor has a great impact while traversing of Radio signals such asConductivity and Pressure and obviously frequency. For each type of waterconductivity has a unique value. The value of conductivity in seawater is highenough as compared to fresh water i-e 4 s/m and 0.
01 s/m respectively (it isbecause of alternates values of salinity and physical characteristics of sea water).As a consequence, the maincondition to be considered to characterize the wireless channel for RFtransmission is the conductivity/salinity of the water.Electromagnetic waves have lessprecise towards the phenomena of reflection and refraction in seawater ascompared to acoustic waves. More ever, the effect of solid particles underwaterhas negligible impact on electromagnetic radiation.
Thus, the vital issue by usingelectromagnetic waves in seawater is attenuation in signal traversing occur bythe conductivity. The relation between the attenuation and frequency isdirectly proportional to each other. In case of seawater when chose thefrequency up to 2.4 GHz the propagation distance is relatively in few centimetre.
RF Transceivers:Themechanization adopted for transceivers is alike as the one used for wirelesscommunications in free space, i.e., antennas. For frequency ranges i-e ELF andVLF, there are need for large receiving antennas, which may prevent the applicationof RF technology in some domains. Moreover, all the devices should be properlyenclosed alike encapsulated for manipulation and tooling functions in theunderwater environment.Main Concerns inUnderwater RF Communications:One of the main problems inunderwater 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 betweendevices. On the other hand, theunderwater RF communication is heavily suffered signal losses that should be determinedcorrectly in application perceptive operations, such as recovery and storage ofdata, and link power budget necessary to operate the devices underwater. Meanwhile,the RF propagating signal has to undergo frequency selective channels (If thechannel behaviour is different for selected frequency) and it is affected by marinenoise. For reliable communication the channel and noise estimations should be inproper range to gain the channel capacity. To overcome this issue the solutioncould be multicarrier transceivers with feasible channel state estimation(description of a signal propagates from transmitter to receiver and effects ofscattering, fading and power delay with distance) and channel loading 5.
However,there are some applications such as observation of sediment presents at bottomof Ocean. This technique use to control the coastal erosion by deployment ofsensors that can transmit information by RF signals.