Wireless communication is difficult in the ocean. The reason for that is water molecules absorbs radio waves efficiently. An acoustic signal travel well but reduces in quality due to echoes, ambient noise, swirling currents and water absorbing the signals.
The technique named by acoustic time reversal can cover up the above mentioned problems. It transmits and cleans up the underwater signals and thus increases the range and capacity of signals.
William Kuperman and his co-researchers (Scripps Institute of Oceanography, La Jolla, California, US) with the researchers of NATO Undersea Research Centre, La Spezia, Italy, are testing this technique in Mediterranean.
Timer reversal technique changes the way of acoustic signals to enter undersea as the sound is clouded by echoes and move with different speeds. For example the sound of “ping” travels and arrives with three different sounds. One travels directly, second that travels with surface and the third which travels with ocean floor.
If the receiver sends the same sound back to the sender the route will be the same from where it came. The sound with long route will be sent first, second-slowest will go after that and the fastest will go at the end, in this way the sound will reach the destination at the same time.
To use this technique for communication, the person wanting to receive message sends carrier signal first. Sender reverse the received message but before sending it back he make changes in it and adds up the message he wanted to send. The receiver gets the message which is so clear that he decodes easily the added message.
Kuperman and his colleague researches started using this technique to send message at the rate of 15-kilobites/second for 4 kilometer range and 5-kilobites/second for 20-kilometers. This can also work for 3500-km range which is the distance equivalent to in which whales communicate each other with song. However for that long distance rate will decrease and would be 100 bits per second. The conventional underwater acoustic modems receives rate of few kilobits per second for 5-km in the shallow water.
Geoffrey Edelmann, who is physicist in US Naval Research Laboratory in Washington, DC, said that this time reversal method is regarded as the best way to improve the acoustic communication. He further said that Kupermann with his fellow researchers have achieved splendid results.
In his words;
“Their work is the best. I think they are leading the charge at the moment,”
Previous tests could not attain this long distance and high bandwidth links as achieved this time. Kupermann and his colleagues will present this technique in the coming Acoustics 08 meeting which will held in Paris on the 1st of July.