Combined equalisation and beamforming to achieve 20 kbits/s acoustic telemetry for ROVs

For many ROV applications it is desirable to receive real time video and sonar images from the vehicle, without the need for an umbilical cable. With a required operating region of 2 km range and 100 m depth, acoustic telemetry is the only realistic option. The major problems in acoustic telemetry through such a channel are limited bandwidth, time-varying multipath due to surface and bottom reflections and Doppler effects caused by relative motion of the ROV and surface vessel. To achieve the data rates required for real time image transmission, bandwidth efficient modulation schemes such as QPSK must be employed. It was found that a combination of adaptive beamforming and equalisation was required to combat the effects of multipath and achieve an acceptable bit error rate. Experimental results are presented which demonstrate data rates of 20 kbits/s (10 ksym/s, QPSK) achieved over ranges up to 2 km, with bit error rates <10^-4