There are various methods for transmission which includes a connection between earths to satellite then to floating device or ship etc It is also sophisticated way 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 in underwater system are easy to set up in the forms of AUV s and wireless system The entire above mention scenario illustrated 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 Radio Frequency Communication Radio frequency RF is a measurement representing the oscillation rate of electromagnetic radio waves from frequencies ranging from 300 GHz to as low as 30Hz 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 To Wavelength distance time frequency Frequency and distance are directly proportional to each other the higher the frequency the more distance for the sound to dissipate the lower the frequency the shorter the distance for the sound to dissipate 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 effects 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 low frequency attenuation of signal propagation is less due to lower conductivity so achieved communication over several kilometres 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 low frequency ranges ELF or VLF are suffering low data throughput 5 Multipath effect is a positive approach of RF signals in sea water 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 low attenuation as compared to propagate only underwater Figure 2 Possible multipath propagation of an 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 sea water is high enough as compared to fresh water i e 4 s m and 0 01 s m respectively it is because of alternaes 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 sea water as compared to acoustic modem In addition the effect of suspended particles underwater has little impact on electromagnetic waves
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 choose the frequency up to 2 4 GHz the propagation distance is relatively in few centimeters Noise in RF In contrast with terrestrial wireless radio communications the communication channels in underwater wireless networks can be seriously affected by the marine environment by noise and by limited bandwidth and power resources and by the harsh underwater ambient conditions RF Transceivers The mechanization adopted for transceivers is alike as the one used for wireless communications in free space i e antennas For frequency range i e ELF and VLF there is need for large receiving antennas which may prevent the application of RF technology in some domains Moreover all the devices should be properly enclosed like 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 Moreover there are many problems inherent 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 loss 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 behavior 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 ocean bottom This technique is used to control the coastal erosion by deployment of sensors that can transmit information by RF signals
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