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Underwater acoustic (UWA) communication systems suffer from the frequency selectivity of the multipath channel, which causes inter-symbol interference (ISI). Conventionally, orthogonal frequency division multiplexing (OFDM) and equalization techniques are used to compensate the ISI at the receiver. We focus here on single carrier (SC) transmissions and consider frequency-domain equalization (SC-FDE).

Equalization in the frequency domain (FDE) is an interesting alternative to the time-domain equalization (TDE) and they both have the same theoretical performance. The gain of the FDE compared to the TDE is twofold: reduce the computational complexity and simplify the equalizer parameters setting. In many applications, the SC-FDE system includes the insertion of a cyclic prefix (CP) at the beginning of each block of N symbols to deal with inter-block interference at the expense of a spectral efficiency loss. Another solution, in which the insertion of CP is not necessary, is to use the overlapping methods at the receiver, such as “overlap and save” (OS) which avoids the overhead due to the transmission of CP symbols.

The UWA channel is also time-variant, yielding a Doppler spread. We propose an adaptive block processing based on a decision directed algorithm to track the time-variation in the channel and to avoid the transmission of preambles, used re-estimate channel whenever it varies.

We also propose a multiple input receiver using multiple hydrophones, in order to benefit from the diversity combining gain to achieve a higher equivalent signal to noise ratio (SNR). In order to deal with the residual frequency offsets within each input, second order phase locked loops (PLL) are introduced and optimized jointly with the equalizers.

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