In this i tried to explain about under water communication. Introduction of underwater communication. Problem due to Multipath Propagation Techniques used for underwater communication 1. Single Carrier Systems 2. MCM Techniques 3. Space-Time Modulation Techniques Applications Limitations Conclusion

1. Presented by
P.PRAVEEN KUMAR REDDY 1602-14-CSP-005

2. Contents
 Introduction
 Multipath Propagation
 Single Carrier Systems
 MCM Techniques
 Space-Time Modulation Techniques
 Applications
 Limitations
 Conclusion

3.  Underwater acoustic communication is the wireless
communication in which acoustic signals carry digital
information through an underwater channel.
 In underwater communication there are low data rates
compared to terrestrial communication, since underwater
communication uses acoustic waves instead of electromagnetic
waves.
INTRODUCTION

4. Why Acoustic Communications
 Radio waves tend to fade rapidly in underwater environments.
To cover large distances, huge antennas and high transmission
power are required.
 Optical waves are affected by scattering. Moreover,
transmission of optical signals requires high precision in
pointing the narrow laser beams.

5. Onshore sinks
satellite
Surface
station
Surface sink
Cabled
seafloor
sensors
Acoustically connected
Sensors
Autonomous
Underwater
vehicle
Overview of Underwater Acoustic Communication

6. Propagation Speed
The propagation speed of acoustic signals in water is typically
1500 m/s.
Nominal: c=1500 m/s (compare to 3 x 108 m/s)
Two types of problems:
• Doppler distortion
• Long propagation delay.

7. Multipath Propagation
 Multipath is the propagation phenomenon that results in signals
reaching the receiving antenna by two or more paths.
 Multipath structure depends on the channel geometry, signal
frequency, sound speed profile.
 Multipath propagation can be seen in two regions.
 Deep water — ray bending
 Shallow water — reflections from bottom

8. • Deep water: a ray, launched at some angle, bends towards
the region of lower sound speed (Snell’s law).
tx
distancec
Rays bend repeatedly
towards the depth at which
the sound speed is minimal

9.  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.

10. Single Carrier Systems
 Underwater acoustic channel makes high data rate
transmission a very challenging task, due to the extended
multi-paths ,delay spread and large Doppler spread.
 To increase the data rate or spectral efficiency of
underwater acoustic communication, we resort to the
spatial structure of the oceans by employing multiple-
input, multiple-output (MIMO) technology. With MIMO
technology, very high data rates and spectral efficiencies,
which cannot be obtained by single transmitter systems,
can be achieved.

11.  To avoid Inter Symbol Interference ( ISI ) and to jointly
address equalization and synchronization, we use Decision
Feedback Equalizer ( DFE ) at the receiver.
 DFE receiver has feed forward filters, carrier phase
synchronizers, phase locked loop, DFE feedback and
decision loop.

12. DFE

13. Limitations
 Computational complexity is required is required to
operate bank of long adaptive filters.
 Equalization and Synchronization must be performed
adaptively

14. Multicarrier Modulation Techniques
Orthogonal Frequency Division Multiplexing:
• It transforms the frequency selective channel into several
narrower flat fading channels.
• Equalization can be performed by multiplying each flat
fading channel output by a single complex tap value,
thereby reducing complexity by eliminating the need for
long equalization filters to combat ISI.

15. OFDM Block Diagram

16. Limitations
 Effect of time variation in channel.
 Equalization depends on orthogonality of the carriers.

17. Space Time Modulation Techniques
Spatial Multiplexing :
• Spatial multiplexing is a transmission technique in MIMO
wireless communications to transmit independent and
separately encoded data signals, so-called streams, from each
of the multiple transmit antennas.
• To achieve higher efficiency over limited bandwidth.

18. Applications
 Can be used to provide early warnings of tsunamis generated
by undersea earthquakes.
 Weather and climate observation.
 Underwater navigation and tracking.
 Underwater data links can be combined with satellite data links
to provide data in real-time from instruments on the seafloor to
scientists ashore.

19. DART II surface Buoy
An acoustic link transmits data from the
bottom pressure sensor to the surface buoy.
Then satellite links relay the data to NOAA
tsunami warning centres.
Real-time data about tsunamis is given to
NOAA forecaster that could potentially
impact coastal areas.

20. Detecting Under Water Objects
A robot crawler carries a modem, a
camera, and a digital signal
processing unit.
Traversing the seafloor, searches
for an object.
When object found, sends an
acoustic signal to a ship or shore
based station.
Can then be commanded to take a
still frame photo, compress it and
transfer the image to an acoustic
signal that is sent back to the
investigator

21. Disadvantages
 Battery power is limited and usually batteries can not be
recharged because solar energy cannot be exploited.
 The available bandwidth is severely limited.
 Ocean’s depth.
 Channel characteristics including long and variable
propagation delays.

22. Conclusion
• Beside development of the underwater wireless
communication, there is an immense scope. Hence more
research of the ocean bottom still remains unexploded.
• The main objective about this emerging field is to overcome
the present limitations such as the environmental effects on
the noise performance of acoustic systems.
• Highly affected by heterogeneities of the water column,
variations of sound velocity versus depth, temperature and
salinity, multiple and random sea reflections and significant
scattering by fish, and plankton.

23. References
 M. Stojanovic, “Recent Advances in High-Speed Under-
symbol Communications,” IEEE J. Oceanic Eng., no. 2, 1996,
pp. 125–36.
 D. Kilfoyle and A. Baggeroer, “The State of the Art in
underwater Acoustic Telemetry,” IEEE J. Oceanic Eng.,
Jan. 2000, pp. 4–27.
• M. Johnson, L. Freitag, and M. Stojanovic, “Improved Doppler
Tracking and Correction for Underwater Acoustic
Communication,” ICASSP,1997,pp.575-78.
 W. Li and J. Preisig, “Estimation of Rapidly Time-Varying
Sparse Channels,” IEEE J. Oceanic Eng ., Oct. 2007, pp. 927–
39.