Chirp FSK modem for high reliability communication in shallow water

A high reliability shallow water acoustic modem has been developed for communication between AUVs and for general oceanographic use. The modem uses 56 narrowband chirp FM pulses, each centered at a unique frequency located in the range of 20 kHz to 30 kHz. Communication rates vary from 300 bps to 2400 bps, depending on the transmission mode. Packets of information are synchronized using two dedicated chirp pulses, followed by transmission format information. In the auto-baud mode, the modem uses information gathered from previous transmissions for adaptation of baud rate to the acoustic environment. At the lowest rate (300 bps), frequency hopping in combination with a one-of-eight mode of transmission is used to provide maximum SNR and immunity to multipath interference. Performance analysis is presented.