Abstract:

Undersea acoustic signaling typically exploits the slowly varying, sparse impulse response of the physical channel. This paper addresses channels where the range is much greater than the water depth and where propagation ducts are absent. Here the existence of stable, discrete multipaths cannot be assumed. Transmission in these channels relies on forward-scattering from the rough seafloor and sea-surface boundaries. The Rayleigh and Ricean fading channel models are used to statistically describe the unpredictable transmission as a complex-valued Gaussian random process. Motion of the faceted scatterers at the time-variant sea surface further affects the transmitted signal by smearing the spectral content with Doppler spread. The prolonged signal duration and broadened signal bandwidth characterize a doubly spread transmission channel. An additional serious impediment is the dynamic and nonwhite channel noise. Coherent demodulation, channel-adaptive equalization, and matched-filter processing are inhibited in such doubly-spread, noisy channels. Robust 1- to 1000-bit/s undersea acoustic telemetry and ranging (telesonar) techniques are being developed by the U.S. Navy using digital coding, spread-spectrum modulation, diversity signaling, and noncoherent demodulation. Incidental benefits include CDMA networking, transmission security, and low power. [Work supported by ONR-321 and the SPAWAR SBIR Program.]