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The ocean is a varied and reverberant environment. The acoustic
wave is an important medium for underwater data transmission.
Because the energy of radio wave, when it is transmitted in
saltwater, has the property that the attenuation increases
rapidly with a increasing frequency. A major problem in the
underwater acoustic data transmission is multipath
interference, that is due to the reflection from surface and
bottom of the sea or the scatterer. The effect of multipath
cause echo at the receiver. For multiple echoes, the received
data will be overlapped in time and become degraded severely.
The spread spectrum method has the advantages of antijamming,
antiinterference and antimultipath. The frequency-shift keying
(FSK) is especially suitable for undersea acoustic digital
communications and telemetry system over highly perturbed
channels. We use direct sequence spread spectrum method and FSK
modulation/demodulation techniques to implement a prototype
underwater acoustic data transmission system. In practical
results, we build up hardware interfaces of transmitter and
receiver with TTL ICs. We can transmit ASCII code pictures as
well as data files from one computer, and display them on
another one in real time. The experimental results in the
watertank demonstrate that the performance of the developed
system is quite acceptable.
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