high speed underwater acoustic modems is shown in this paper. high speed digital communication and broadband underwater acoustics technologies have been combined to achieve this . increased data rate and decreased power consumption is achieved by Broadband Spread Spectrum Technology. The underwater communication has to deal with noises such as offshore drilling noise, ambient noise, ship noise etc. no communication errors has been seen to appear with these noises.
Broadband Spread Spectrum Technology us implemented using DSP which is suitable for the various ocean environments. The bit error rate was less than 10^-7 which is comparable to the wireline communication.

WORKING PRINCIPLE
Broadband Spread Spectrum Technology is utilized in these modems. advanced modulation scheme and channel equalization is used for tackling the multipaths and providing improved signal to noise ratio. error detection and correction coding have also been chosen based on the performance. The electronics of the modem is divided into three major parts:
-DSP board: It serves the functions of signal processing module and a microcontroller. The functions like System timing recovery, modulation, equalization, framing and serial interfacing coding/decoding are done by it.
-Analog Front End (AFE) board
It is interfaced with DSP board and transducer on either side.
-DC/DC converter board: The function of converting a wide range of input voltage to the operating voltage of the system and the transducer is done by the DC/DC converter board.

[attachment=5191]
http://aquanode.ece.ucsb.edu/Modem/index.htm
http://slideworldforframe.aspx?id=188153