Spread spectrum underwater acoustic telemetry

Spread spectrum methods are used in communication systems to provide low probability of intercept in hostile environments, to provide multiple access capability in systems shared by many users, and to provide processing gain in channels where the transmitted signal is distorted by multipath propagation. All three of these scenarios are of interest to underwater acoustic communication systems. In this paper we consider spread spectrum modulation and demodulation methods for low rate transmission in underwater acoustic communication channels. Two types of spread spectrum systems are considered: direct-sequence spread spectrum (DS-SS) systems that employ digital phase modulation with coding, and frequency-hopping spread spectrum (FHSS) systems that employ coded MFSK modulation and noncoherent demodulation. These methods are investigated for application in fading multipath channels typical of underwater propagation. System design examples are given and performance estimates are made.