An underwater acoustic telemetry modem for eco-sensing

An underwater acoustic telemetry modem is presented for ecological research (eco-sensing) applications. The modem is intended for the physical layer (PHY) in an ad hoc network of "AquaNodes". Each AquaNode will incorporate an acoustic modem, router and host processor with standardized interfaces to oceanographic sensors. The transmitted waveforms use a composite Walsh/m-sequence format in which each bit in an 8-bit long Walsh function is spread by a 7-chip m-sequence. The resulting waveform has a 5 kHz bandwidth for robustness to multipath and achieves a 133 bps data rate. An 11 msec, time-guard band for channel clearing eliminates the need for equalization. The receiver employs a generalized multiple hypothesis test with a matching pursuits (GMHT-MP) inner loop for symbol-by-symbol channel estimation. Symbol-error rate (SER) results demonstrate that the GMHT-MP is superior to the noncoherent RAKE receiver at usable SERs (<10^-2) and can accommodate up to 10 msec, multipath spreads with Doppler spreads on the order of 1 Hz