Neither receiver requires knowledge of the spreading code in order to despread the data. It is further shown that adaptive equalizers are capable of code conversion, i.e., the DSSS receiver can demodulate the MCSS waveform, and vice versa. The tracking potential can be realized to some extent by hypothesis-feedback equalization. DSSS has the best tracking potential for rapidly varying channels, where the challenge is to obtain reliable chip decisions before symbol despreading. In slowly varying channels, MCSS offers the best performance. Four channels are examined, from nearly static to overspread. The DSSS receiver equalizes on the chips prior to explicit symbol despreading, whereas MCSS features joint multiband equalization and despreading. The second scheme is multicarrier spread spectrum (MCSS) and accomplishes spreading by using seven subbands. The first scheme is direct-sequence spread spectrum (DSSS), using a 7-chip spreading code to achieve a raw data rate of 1000 bps on a single carrier. Two modulation schemes are compared in a 7-kHz frequency band on a 14-kHz center frequency. Underwater communication experiments have been conducted in the Norwegian Oslofjord.
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