Using Continuous Additive Training Sequences for High Rate, Doppler Tolerant Communications

An underwater acoustic communications system designed to transmit 20 kbit/s over ranges from 100 m up to 5 km between a semi-autonomous underwater vehicle and a surface vessel moving with up to 20 knots relative velocity is presented. Under these conditions, attention must be given not only to the steady-state equalisation capability of the receiver, but also to its ability to respond to sudden changes in channel profile due to sudden movements of the remote vehicle. In addition to conventional packet-based transmissions, a novel continuous transmission system was also implemented during the experimental stages. Instead of using distinct channel probe, training and data blocks, a continuous training sequence was added on top of a continuous data transmission. Advantages of continuous additive training sequences (CATS) such as an improved tolerance to burst errors, greater data throughput and flexible re-synchronisation ability will be highlighted. These are balanced against the disadvantages of increased receiver complexity and compromised ambient noise performance. Results of several seatrials will be presented to verify the performance of the system at short and long ranges using both static and dynamic platforms. [Work sponsored by DERA Winfrith, UK.]