underwater acoustic propogation channels
DESCRIPTION
This ppt is about propogation and scattering effects in underwater acoustic communication channels.TRANSCRIPT
Propagation and Scattering Effects in Underwater Acoustic
Communication Channel
SEMINAR PRESENTATION ON
SUBMITTED BY SHUDHANSHU SINGH1104331044EC 6TH
SEMESTERUnderwater Acoustic Communication Channel 1
UNDER THE GUIDANCE OF
PROF. J.P. SAINI
Underwater Acoustic Communication Channel 2
CONTENT Traditional approach for ocean bottom monitering Sound as a wireless medium BW limitations Variations in speed of sound Multipath Propogation Noise Scattering Propogation speed Signal Processing Underwater Applications Challenges
Underwater Acoustic Communication Channel 3
Traditional approach for ocean-bottom monitoring
Deploy underwater sensors to record data during the monitoring mission, and then recover the instruments.
Disadvantages :• Real time monitoring is not possible. • No interaction is possible between onshore control
systems and the monitoring instruments. • If failures or misconfigurations occur, it may not be
possible to detect them before the instruments are recovered.
• The amount of data that can be recorded during the monitoring mission by every sensor is limited by the capacity of the onboard storage devices (memories, hard disks, etc).
Underwater Acoustic Communication Channel 4
Use sound as the wireless communication medium
• Radio waves propagate at long distances through conductive sea water only at extra low frequencies (30-300 Hz), which require large antennae and high transmission power.
• Optical waves do not suffer from such high
attenuation but are affected by scattering. Moreover, transmission of optical signals requires high precision in pointing the narrow laser beams.
Underwater Acoustic Communication Channel 5
UNDERWATER ACOUSTIC COMMUNICATION SYSTEM
Underwater Acoustic Communication Channel 6
BANDWIDTH LIMITATIONS
Absorption coefficient increases rapidly with
frequency: fundamental bandwidth limitation.
Only very low frequencies propagate over long distances
Underwater Acoustic Communication Channel 7
depth
c
surface layer (mixing)
const. temperature (except under ice)
main thermocline
temperature decreases rapidly
deep ocean
constant temperature (4 deg. C)
pressure increases
Sound speed increases with temperature, pressure, salinity.
continental shelf (~100 m)
continental slice
continental rise
abyssal
plain
land sea
surf shallow deep
Variations in speed of sound
Underwater Acoustic Communication Channel 8
• Multipath structure depends on the channel geometry, signal frequency, sound speed profile.•Models are used to obtain a more accurate prediction of the signal strength.• Ray model provides insight into the mechanisms of multipath formation:
deep water — ray bending
shallow water — reflections from bottom.
Multipath propagation
Underwater Acoustic Communication Channel 9
Underwater Acoustic Communication Channel 10
Mechanisms of multipath formation• Deep water: a ray, launched at some angle, bends
towards the region of lower sound speed (Snell’s law).
• Continuous application of Snell’s law ray diagram (trace).
tx
distancec
Rays bend repeatedly towards the depth at which the sound speed is minimal.
Underwater Acoustic Communication Channel 11
Shallow water: reflections at surface have little loss;
reflection loss at bottom depends on the type
(sand,rock, etc.), angle of incidence, frequency.
tx rx
Multipath gets attenuated because of
repeated reflection loss, increased path length.
NOISEAmbient (open sea)•turbulence •shipping •surface•thermal
Site-specific:•man-made •biological (e.g., shrimp) •ice cracking, rain•seismic events
Underwater Acoustic Communication Channel 12
SCATTERINGWater surface
Fish shoaling
Bubbles
Underwater Acoustic Communication Channel 13
Underwater Acoustic Communication Channel 14
Nominal: c=1500 m/s (compare to 3 x 108 m/s)
Two types of problems:• Motion-induced Doppler distortion (v~ few m/s for
an AUV)• Long propagation delay.
Propagation speed
tt(1±v/c)ff(1±v/c)
DOPPLER EFFECT
Underwater Acoustic Communication Channel 15
• Bandwidth-efficient modulation (PSK, QAM)
• Phase-coherent detection
• Synchronization
• Equalization
• Multichannel combining
Signal processing
Underwater Acoustic Communication Channel 16
inp. K
com-biner
forward
forward
+_
decision
feedback
adaptation algorithm
inp.1
inp.2 data out
sync.
filtercoefficients
training data
data est.
Challenges
• Battery power is limited and usually batteries can not be recharged because solar energy cannot be exploited.
• The available bandwidth is severely limited.• Channel characteristics, including long and variable
propagation delays, multi-path and fading problems.• High bit error rates.• Underwater sensors are prone to failures because of
fouling, corrosion, etc.• A unique feature of underwater networks is that the
environment is constantly mobile, naturally causing the node passive mobility.
• The ocean can be as deep as 10 km.Underwater Acoustic Communication Channel 17
Underwater applications Seismic monitoring, Pollution monitoring, Ocean currents monitoring, Equipment monitoring and control, Autonomous Underwater Vehicles
(AUV).
To make these applications viable, there is a need to enable underwater communications among underwater devices.
Underwater Acoustic Communication Channel 18
REFRENCES
Paul A. van Walree, Member, IEEE, “Propagation and Scattering Effects in Underwater Acoustic Communication Channels,” IEEE JOURNAL OF OCEANIC ENGINEERING, VOL.38, NO.4, OCTOBER 2013
Kalangi Pullarao Prasanth, Modelling and Simulation of an Underwater Acoustic Communication Channel, THESIS, January 2013
Thomas J. Hayward and T. C. Yang, Underwater Acoustic Communication Channel Capacity: A Simulation Study, Naval Research Laboratory, Washington, DC 20375
Milica Stojanovic, Northeastern University, Underwater Acoustic Communication Channels: Propagation Models and Statistical Characterizatio, James Preisig, Woods Hole Oceanographic Institution
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THANK YOU
Underwater Acoustic Communication Channel 20