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Due to the nature of underwater environment, digital
communications through UWA channels
tend to encounter severe signal degradation caused by multipath
propagation and high temporal and spatial variability of the
channel conditions.
Therefore, it results in that some mature radio
communications techniques can not fully apply to underwater
communications.
In this thesis, the technique of data transmission via UWA
channels is investigated.
To simplify receiver design, we adopted pi/4 DQPSK scheme which
can be demodulated noncoherently,
that is, the carrier phase recovery is
not required. In order to reduce the UWA channel effects,
the direct-sequence spread spectrum (DS/SS) technique is also
considered.
In the proposed system, several levels of synchronization are
need. First, the auto-
frequency control (AFC) technique is applied to get carrier
synchronization. Second, the frame and symbol synchronization
are achieved by detecting the
synchronization bits. Finally, the matched filter and preamble
sequence are adopted for PN code
synchronization.
In addition to simulation, the proposed techniques are further
implemented on a DSP module, a C4D based on
TI TMS320C44 chip. The real-time ASCII data
transmission experiments between two personal computers via UWA
acoustic channels are conducted in the
laboratory. The transmission rate is about 650 bps, that is
equivalent to 20 Kchips/s. However, BER will increase if the
transmission distance is increased.
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