Watermarking sonar waveforms using knowledge of channel coherence

Acoustic emissions found in underwater channels cover a broad spectrum of sources. Signals are often characterized by parameters such as amplitude, frequency, phase or modulation. However, once transmitted, and given that other sonars can also be simultaneously transmitting, it is generally difficult to identify the origin of a signal such as location and source of the transmission. A solution to this problem was proposed by watermarking the sonar prior to transmission [1]. Prior to transmission, a low power digital signature called a “watermark” is embedded in the sonar pulse using spread spectrum. In this work, we propose an approach whereby the watermark design and embedding is closely tied to propagation characteristics of the acoustic channel. We show that the watermark can be placed in time-frequency distribution cells of the sonar pulse in such a way as to be robust to slow/fast or flat/frequency selective fading. We show that only gross channel parameters in the form of coherence bandwidth and coherence time are needed to accomplish this task.