FSK and PSK performance of the utility acoustic modem

The utility acoustic modem (UAM) is a high performance and compact digital signal processing system for acoustic communications, designed at Woods Hole Oceanographic Institution. The UAM is fully integrated, containing 4 hydrophone input channels, a switching power amplifier, and non-volatile memory. The device consumes 3 W when receiving and up to 30 W when transmitting with a source level of about 185 dB re microPascal. At the heart of the UAM is a 30MFLOP (million floating point operations per second) DSP chip capable of implementing a wide range of acoustic communication signalling and receiving algorithms in real-time. To date, two communications links, have been implemented on the UAM and evaluated. The first is a low-rate incoherent system using frequency-hopped FSK signalling and strong error-correction coding to provide robust communications in channels with rapidly-varying multipath. The second link is a high-rate coherent method using QPSK signalling and a Doppler-tolerant multi-channel adaptive equalizer. The two communications methods have been tested individually, and against each other, in a variety of shallow water channels. The paper reviews the design and capabilities of the UAM and describes in detail the FSK and QPSK implementations. Performance results for both schemes from at-sea experiments are then presented. The ultimate aim of the UAM development is to produce a robust communications link able to adapt modulation type and rate to the channel conditions. Some preliminary steps towards this are discussed in the light of the at-sea results