A new MFSK acoustic modem for operation in adverse underwater channels

With US Navy SBIR funding, Datasonics has teamed with Delphi Communication Systems and Naval Command, Control & Ocean Surveillance Center, RDT&E Division (NRaD) to develop a new acoustic telemetry and ranging (telesonar) modem. Periodic transmission of digital message packets satisfies most telesonar applications, including data reporting from oceanographic sensors, controlling autonomous devices, monitoring offshore-oil equipment, and communicating with undersea vehicles. These applications are well served by robust, low-data-rate, noncoherent, multiple frequency shift keying (MFSK) signaling offering greater affordability, networkability, and energy conservation than coherent modems. Well designed MFSK signaling furthermore provides superior reliability in noisy, doubly spread, forward-scattered, horizontal channels such as those found in littoral ocean environments. This paper describes the design of the new telesonar type-A MFSK acoustic modem providing communication rates of 100 to 2400 bit/s. Improved performance in adverse transmission channels is obtained from increased signaling diversity and stronger coding techniques based on symbol sets derived from Hadamard matrices. The new product also preserves the 1-of-4 MFSK modulation mode used in the Datasonics ATM-850 modem. The parallel goal of reduced cost is achieved by the new modem electronics design and packaging.