Abstract
This paper describes the design and sea trail of an underwater acoustic speech transmission system for the continuously increasing need for diver communications. The input speech signal is compressed down to 2.4k bit/s using a MELP (mixed excited linear prediction) coder. The bit rate is 6k bit/s after channel coding and frame synchronization. A QPSK modulation with differential encoding was chosen to transmit the useful signal. To overcome the phase fluctuation and multi-path effects caused underwater acoustic channel we employed a scheme where phase synchronization and fractional spaced Decision Feedback Equalization (DFE) were jointly optimized by RLS algorithm in the receiver. Two kinds of error correcting schemes were used including convolutional codes (CC) and Reed Solomon (RS) block codes. The whole system has been conducted successfully in very shallow water environments and the short range horizontal acoustic voice communication link performances are evaluated.
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