default Coded modulation development for covert underwater acoustic communications

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Coded modulation development for covert underwater acoustic communications. Pelekanakis, Konstantinos. CMRE-FR-2017-004. December 2017.

This report is an initial study on covert Underwater Acoustic (UWA) communications at the physical layer. The challenge in any covert system design is to operate in very low Signal-to-Noise Ratio (SNR) as well as to cope with the unique factors of UWA channels such as distance-dependent bandwidth due to absorption, long reverberation time due to slow speed of sound (about 1500 m/s) and rapid phase variations due to Doppler effects. Our research focuses on the design of a system for Low Probability of Detection (LPD)/Low Probability of Exploitation (LPE)/Low Probability of Intercept (LPI) communications. The transmitter is based on Direct Sequence Spread Spectrum (DSSS) modulation of Trellis-coded Modulation (TCM) signals and the receiver combines adaptive equalisation with carrier-phase synchronization. A preliminary analysis of signals recorded in a shallow water environment off the coast of Portugal is presented.