High-Frequency FH-FSK Underwater Acoustic Communications: The Environmental Effect and Signal Processing

This paper analyzes the environmental effect and signal processing approaches for frequency-hopped frequency-shift-keyed (FH FSK) underwater acoustic communications based upon data collected in two experiments: the RDS4 experiment near Halifax, Canada and MREA03 experiment near Elba, Italy. The FH FSK signals have a bandwidth of 4 kHz centered at 17 and 20 kHz. The source was towed at ~4 knots. The signals were received on a vertical line array anchored to the bottom. The acoustic environments at both sites have the same downward refractive sound speed profiles but very different bottom properties. The multipath spread last ~20 msec in the MREA03 experiment and is > 1 sec in the RDS4 experiment. The different lengths of multipath delays have a significant effect on the bit error rate (BER) and the appropriate signal processing needed to reduce the BER. We present the data analysis results, the signal processing approaches, including multi-channel beamforming and spatial diversity combining, and discuss the implications for the use of FH-FSK for multi-user communications.