Covert Underwater Acoustic Communication based on Ocean Ambient Noise

To achieve covert underwater acoustic communication (UAC) at energies with a low probability of detection (LPD), a system should work at a low signal-to-noise ratio (SNR) with no repeating patterns in the transmitted signal. Not all covert UAC schemes can satisfy both requirements. The present paper studies a truly covert UAC scheme. The transmitted signal consists of both the initial signal (the previously recorded ambient noise of the ocean) and an information-bearing signal generated by multi-step operations on the initial signal (time-domain symmetrical division, phase rotation, time reversal, and counterpart exchange). As the two signals are transmitted together, the receiver does not require pre-knowledge of the initial signal for demodulation. Hence, each symbol can be assigned a different initial signal, thereby avoiding repeating patterns in the signal and improving the covertness of the system. After optimizing the rotation phase condition, the performance of the UAC system was analyzed in numerical simulations. The scheme achieved covert UAC and exhibited good performance at low SNR.