Field testing of a spread spectrum acoustic modem with sparse channel estimation

This paper describes the field testing of an underwater acoustic (UWA) modem in three distinct underwater environments: in the 5-7 m depth waters off the Goleta Pier in California, in the shallow 2-5 m depth waters of Viapahu Lagoon of Moorea, French Polynesia, and in a coral bommies field in Viapahu Lagoon. The modem employs M-ary direct-sequence spread-spectrum (DSSS) signaling, with joint detection and channel estimation performed by the matching pursuit (MP) algorithm. Our programmable DSP implementation of MP is shown to effectively estimate sparse multipath in real time. The current design employs a 24 kHz carrier and 7.8 kHz bandwidth to achieve a 161 bps data rate. Field tests show bit error-rates (BERs) les 10^-2 are achievable in varied underwater environments at up to 440 m range with multipath spreads on the order of 9 msec.