MFSK--The Basis for Robust Acoustical Communications

Subsea acoustic control and communications systems based on FSK modulation have been used in the offshore industry for the past few years. Prerequisites for reliable link operation included frequency and spatial diversity, signal coding, and channel equalization. These prerequisites limited previous systems to very low data rates. In this paper, an acoustic modem capable of an order of magnitude increase in data rates is discussed. This improved acoustic modem achieves higher data rates by simultaneous transmission of multiple tones using FSK modulation. While not new, until now this technique has been impractical because of the hardware constraints imposed by multitone signal processing. Rapid advancements in the areas of charge transfer devices and Fast Fourier Transform processors removed these constraints. Tradeoffs discussed in this paper include the various signal-processing alternatives available and the use of error correcting or detecting codes to reduce the system undetected-error rate to acceptable values.