underwater acoustic propogation channels

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


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).


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 SYSTEM


BANDWIDTH LIMITATIONS

Absorption coefficient increases rapidly with

frequency: fundamental bandwidth limitation.

Only very low frequencies propagate over long distances


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


• 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



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.


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


SCATTERINGWater surface

Fish shoaling

Bubbles



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


• Bandwidth-efficient modulation (PSK, QAM)

• Phase-coherent detection

• Synchronization

• Equalization

• Multichannel combining

Signal processing


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.


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


THANK YOU


underwater acoustic propogation channels

Engineering

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