Using multi-frequency modulation in a modem for the transmission of near-realtime video in an underwater environment

The system described in this paper uses multicarrier modulation, and has been successfully tested at a data rate up to 10 kbps over 1 km. The system algorithm generates 48 frequencies for transmitting 48 parallel bits of data in each packet. A long transmitted signal sequence is combined with synchronisation, zero gap and information packets. The long multi-frequency signal packets have been implemented to minimise the effect of multipath fading, which is a particular problem in shallow, open water environments. To acquire the starting point of the transmitting sequence, a linear frequency modulation (LFM) signal is used for synchronisation. In order to reduce noise, adaptive threshold packets are used to set up a suitable signal. Experimental results from sea-trials have shown that the system can cope with multipath fading environments.