hai.. friends.I think it may helpful to understanding the concept of acoustic communication using OFDM

1. High-Speed Underwater
Acoustic Communication
Based on OFDM
By
G.RAVITEJA
P.V.MANIKANTA

2. Contents:
 What is acoustic communication?
 Why acoustic communication?
 Difficulties of under water acoustic communication channel
 Parameter variations in different underwater layers
 Uwa channel technique using OFDM
 About OSDM
Advantage of OFDM over UWA Channel
Characteristics of Acoustic channel using OFDM
1.Band width
2.Multipath
3.Doppler
Conclusion

3. What is Acoustic communication?
Underwater acoustic
communication is a technique of
sending and receiving message below
water either wired or wireless .

4. Why Acoustic communication ?
Oceans, which cover over 70% of the earth’s surface, are
being looked at in a new light as they become a source of
future resource potential as well as a source of
information about the earth’s carbon ecosystem and it
affect on climate change. To unlock the oceans secrets
there are major projects and research being undertaken
around the world to develop technologies required
to operate within the oceans and development of Marian
technology.

5. The difficulties of the underwater acoustic
communications channel are well-known:
1. overall bandwidth is limited due to acoustic
absorption that increases with frequency
2. Typical shallow water regions where
communications are desired have high
degrees of spatial and temporal variability and
different losses like propagation loss, absorption loss,
transmission loss, noise etc,

6. Parameters variations in different underwater layers

7. Underwater environment for data transmission using a single
transmitter-receiver pair.

8. Uwa channel TECHNIQUES:
There are many techniques to reach the better Under Water
Communication.
OFDM is an attractive technique beginning used in
UWA communication with three obvious advantages:
 Good performance against multipath interference
 Ability to combat the frequency selective fading
 The high frequency band efficiency.

9. OFDM
Orthogonal frequency division multiplexing (OFDM ),one
kind of multi-carrier modulation, as one charming technique
in present wireless results has high bandwidth efficiency. The
advantages of combating frequency select fading and
multipath interference.
Data is transmitted in parallel on multiple carriers that
overlap in frequency

10. Advantages of OFDM over UWA
channel :
In OFDM system, intersymbol interference (ISI) introduced
by multipath propagation, which is very popular in
underwater acoustic (UWA) channel, can be removed by
using guard time. For these reasons, OFDM has been used
in high-speed UWA.
Specifications
1.medium range communication about 5km-10 km
2.The data rate is 9.8kbps-2.8kbps.
3.Bit error rate( BER) is below 0.0001.

11. CHARACTERSTICS OF UWAC
1.BANDWIDTH : the bandwidth of this system ranging about 10 km is
only few kHz,it depends on transmission loss,
ambient noise.
2.MULTI PATH: With in the limited bandwidth ,the signal is subjected
to multi path propagation through under water
channel.Due to this multi path causes inter symbol
interference ISI which restricts the data throughputs
and degrades the system performance.

12. Multipath interference Under water communication

13. ofdm transmitter in uwa channel:

14. In the transmitter,
 The input data is a serial bit-stream that is to be mapped.
After that, the data streams are converted from serial to
N- paralled data streams
 Then a pilot signal is inserted.
 The modulation of OFDM is conducted by using IFFT.
The cyclic prefix signal is used to reduce the effect of ISI
and ensure the orthogonality of sub carriers.
 Then followed by P/S and D/A conversion

15. OFDM RECIEVER FOR UWA CHANNEL:
HYDRO
PHONE
A/D
DOPPLER
COMPEN
SATE
S/P
REMOVE
CP
CHANNEL
EQUILIZATI
ON
P/S
DEMODUL
ATION
FFT
RECE
IVED
SIGN
AL

16. 3.Doppler:
There are two aspects to cause the Doppler.
1. The motion of sea surfaces and currents
2. The relative motion between transmitter and receiver
Doppler frequency shift is estimated by using cyclic
prefix and compensated by resampling the
received signal.

17. In Receiver,
 Doppler shift is estimated and compensated
effectively by resampling the receiver signal
Resampling signal is followed by S/P and FFT.
The channel is estimated by using the pilot signal,
and the phase variety must be tracked in OFDM system.
Equalization is then implemented in time domain.

18. Effect of Doppler shift Relation between range and frequency

19. Conclusion:
OFDM has been succeeded in the RF
applications in wireless communication .
Recently OFDM has been used in UWA
communication channel . OFDM system is
more efficient for high rate UWA
communication not only at short range, but also
at medium range.