Dear Mir

A communication link consists of a transmitter , a receiver and a communication medium. Every medium has the suitable signal energy form to propagate easily in it.

Good electricity conductors such as metals are conveyor of electricity.Therefore transmission lines are made from metallic conductors like copper and aluminium.It is not practical to connect the transmitters and receivers in underwater by transmission line.

Therefore one uses the suitable wireless system for under ware communication.

Basically there are two basic wave types, the acoustic waves and the electromagnetic waves. Sea water is salty and therefore good conducting as Dr Garry mentioned. Consequently it attenuates the electromagnetic waves and this attenuation increase as the frequency increases. Only low frequency electromagnetic waves are capable to propagate an appreciable distance before it gets undetectable.Therefore the radio wave under sea water communication is restricted to very low frequency waves with very small band width and consequently very low data rates. Newly, short distance under water optical communication links are developed operating in the wavelength range from 0.4 to 0.55 micrometers.They can be considered also wireless electromagnetic wave communication.

As a medium the sea water as a fluid is suitable for acoustic wave propagation. Therefore, the under water communication systems are normally

based on acoustic wave transmission. Here piezo electric transducers are used to convert the electrical signal into acoustic waves and vice verse. The rest of the system remains the same.

As a communication medium its carrier frequencies are limited and consequently the

channel bandwidth. The power is limited because it is mostly battery operated system. The propagation speed is relatively small and hence larger delay time and delay time spread. Therefore, the link data rate is relatively small and limited.

From the principle point of view, OFDM can be used as a modulation technique to overcome multipass fading. occurring also in acoustic wave communication.

For more details please see the paper in the link:http://edge.rit.edu/edge/P14251/public/WorkingDocuments/EfficientCommInUWAcousticNetworks.pdf