High-Speed Underwater Acoustic Communication

Based on OFDM

By G.RAVITEJA P.V.MANIKANTA

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

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

What is Acoustic communication?

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.

The difficulties of the underwater acoustic communications channel are well-known:

1. overall bandwidth is limited due to acousticabsorption that increases with frequency

2. Typical shallow water regions where communications are desired have highdegrees of spatial and temporal variability and different losses like propagation loss, absorption loss, transmission loss, noise etc,

Parameters variations in different underwater layers

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

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.

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

OFDM

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.

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.

Multipath interference Under water communication

ofdm transmitter in uwa channel:

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

OFDM RECIEVER FOR UWA CHANNEL:

HYDRO PHONE A/D

DOPPLERCOMPEN

SATES/P REMOVE

CP

CHANNEL EQUILIZATI

ONP/S DEMODUL

ATION

FFT

RECEIVEDSIGNAL

3.Doppler:

There are two aspects to cause the Doppler.

1. The motion of sea surfaces and currents2. The relative motion between transmitter and receiver

Doppler frequency shift is estimated by using cyclic prefix and compensated by resampling the

received signal.

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.

Relation between range and frequencyEffect of Doppler shift

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.