Multicarrier Transmission Schemes for Underwater ...

Multicarrier Transmission Schemes for Underwater Communication and Hybrid Networks Vinay Kumar Trivedi Indian Institute of Technology Patna Under the Supervision of Dr. Preetam Kumar

January 11, 2016

Vinay K. Trivedi (IIT Patna)

MC Schemes for UWA and Hybrid Networks

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Overview

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Underwater Acoustic Communication

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Underwater Acoustic Channel

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Hybrid Satellite/Underwater Acoustic Network

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MC-CDMA for hybrid Satellite/UWA network

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SC-FDMA for Next Generation Uplink Underwater Acoustic Communications

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Conclusion and Future Work



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Underwater Acoustic Communication

In past decades, underwater acoustic (UWA) communications have been attracting growing research interest. Underwater acoustic networks are formed by acoustically connected undersea sensor nodes, autonomous underwater vehicles (AUVs), and surface buoys. Underwater acoustic communications suffers from long propagation delays, limited bandwidth, frequency dependent attenuation and severe multipath fading. A key area of research in underwater acoustics is development of efficient modulation and detection schemes for improved quality of service.



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Application Areas and Factors Affecting

Underwater acoustic communication is essential in applications like Remote control in the offshore oil industry Pollution monitoring in environmental systems Collection of scientific data Disaster detection and early warning Underwater surveillance etc. Acoustic propagation is characterized by three major factors Frequency dependent attenuation Multipath propagation Low speed of sound (1500 m/s)



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Underwater Acoustic Channel Attenuation When a certain amount of energy is radiated from a source it spreads over a certain volume (Geometrical Spreading). For Underwater Acoustic Channel this attenuation is dependent on fre

k

quency A(d, f ) = ddr a(f )d−dr where dr is the reference distance, k is the propagation constant and a(f ) is absorption coefficient. It is seen than a(f ) is dependent on frequency. Multipath Mainly consisting of reflections in the surface, in the bottom and in possible objects that are in the scene. These reflections are the responsible for causing multiple arrivals to the receiver. P A general form of Channel impulse response is h(t) = p=P p=0 hp δ(t −τp ) where hp are the paths amplitudes Vinay K. Trivedi (IIT Patna)


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Underwater Acoustic Channel Noise Noise observed in the ocean exhibits strong frequency dependence as well as site dependence. Inshore environments such as marine work sites are much noisier than deep ocean due to the man made noise. Most of the ambient noise sources can be described as having a continuous spectrum and Gaussian statistics. Low speed of Sound The sound speed in the sea is dependent on ambient conditions, especially the water temperature, salinity and the surrounding pressure or depth of the water. A simple formula that is sufficiently accurate has been developed c = 1448.6 + 4.618T − 0.0523T 2 − 1.25(S − 35) + 0.017D

(1)

where c is the speed of sound, T is the temperature, S is the salinity and D is the depth in meters. Vinay K. Trivedi (IIT Patna)


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Underwater Acoustic Communication Doppler Effect When either the transmitter or/and the receiver in a communication system is in motion the signal will experience a frequency shift called a Doppler shift. Doppler spread is a measure of the spectral broadening caused by these Doppler shifts in the communication channel. The frequency shift, exactly as in mobile radio channels, is proportional to ∆f ∝ vcr which means that if c, the wave propagation speed, is low then the shift will be higher. The low speed of sound, which is about c = 1500m/s and varying slightly with the speed profile, is the principal cause of this effect. When B.W