Improved chirp FSK modem for high reliability communications in shallow water

Reliable underwater acoustic communications devices are critical for the transmission of data between AUVs and remote sensors. This paper discusses a high reliability shallow water acoustic modem developed for communication between AUV and general oceanographic use. The modem uses 56 narrow band chirp FM pulses, each centered at a unique frequency located in the range of 16 kHz to 32 kHz. Data rates vary from 221 data bits per second to 1172 data bits per second, depending on the mode of transmission. Packets of information are synchronized using an adjustable number of chirp pulses in a known frequency hop pattern, followed by transmission format information and data. An “auto-baud” mode uses information garnered from previous transmissions adaptation of the bit rate to the acoustic environment. At the lowest rate, a four time slot frequency hop pattern is used to provide maximum immunity to multipath interference. The modem is capable of using both half rate convolutional and BCH encoding in order to maximize error resilience. Performance analysis based on numerous field trials is presented,