Underwater acoustic modem using multicarrier modulation

An underwater digital acoustic communications system based on multicarrier modulation technique is presented. This technique is relatively robust in a strong multipath fading environment and has been successfully tested in a shallow water channel at a data rate up to 10 kbps over 1 km. The test system using 48 carrier frequencies for transmitting 48 parallel bits of data in each packet. The data stream is encoded into a sequence of packets. These packets are encapsulated by headers, synchronisation and training signals. In order to reduce the multipath fading problem, an adaptive channel equalisation scheme with a LSM algorithm is used. The multipath fading problem can be further improved by a redundant carrier assignment and wavelet based MFM scheme. The advantage of this scheme is that the fading is reduced without affecting the channel capacity.