This ppt is about propogation and scattering effects in underwater acoustic communication channels.

1. Propagation and Scattering
Effects in Underwater Acoustic
Communication Channel
SEMINAR PRESENTATION ON
SUBMITTED BY
SHUDHANSHU
SINGH
1104331044
EC 6TH
SEMESTER
Underwater Acoustic Communication Channel 1

2. UNDER THE
GUIDANCE OF
PROF. J.P. SAINI
Underwater Acoustic Communication Channel 2

3. 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

4. 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

5. 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

6. UNDERWATER ACOUSTIC
COMMUNICATION SYSTEM
Underwater Acoustic Communication Channel 6

7. BANDWIDTH LIMITATIONS
Absorption coefficient increases rapidly with
frequency: fundamental bandwidth limitation.
Only very low frequencies propagate over long
distances
Underwater Acoustic Communication Channel 7

8. 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

9. • 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

10. 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.

11. 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.

12. NOISE
Ambient (open sea)
•turbulence
•shipping
•surface
•thermal
Site-specific:
•man-made
•biological (e.g., shrimp)
•ice cracking, rain
•seismic events
Underwater Acoustic Communication Channel
12

13. SCATTERING
Water surface
Fish shoaling
Bubbles
Underwater Acoustic Communication Channel 13

14. 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
tt(1 v/c)
ff(1±v/c)
DOPPLER EFFECT

15. Underwater Acoustic Communication Channel 15
• Bandwidth-efficient modulation (PSK, QAM)
• Phase-coherent detection
• Synchronization
• Equalization
• Multichannel combining
Signal processing

16. Underwater Acoustic Communication Channel 16
inp. K
com-
biner
forward
forward
+
_
decision
feedback
adaptation algorithm
inp.1
inp.2 data out
sync.
filter
coefficients
training data
data est.

17. 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

18. 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

19. 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
Underwater Acoustic Communication Channel 19

20. THANK YOU
Underwater Acoustic Communication Channel 20