This research presents an innovative stacked autoencoder-based receiver design to address the challenges of efficient signal recovery for low probability of detection (LPD) constrained covert underwater acoustic communication (CUAC), thereby promoting secure and eco-friendly communications in underwater environments. Communicating through underwater channels presents significant challenges due to the long propagation of acoustic waves, signal attenuation, and ambient noise. These challenges are further intensified in LPD-constrained CUAC systems due to severely degraded SNR of the received signal at the desired range. The proposed system employs a data-driven approach for receiver design to effectively recover the signal in a low SNR environment by learning intrinsic channel features and replaces traditional physical-layer operations of de-spreading, demodulation, and decoding with a unified, joint feature learning framework, creating an efficient processing pipeline and enhancing the BER performance of the receiver. The performance of the stacked autoencoder-based receiver is validated through a lake experiment conducted at a range of 1.22 km, along with simulations. The experiment demonstrates that the proposed receiver achieves reliable performance with SNR levels as low as -15 dB, yielding a significant 3 dB improvement at a BER of 10-2compared to conventional receivers.

Abstract Multi‐user communication traditionally uses either code division multiple access (CDMA) or time‐division multiple access schemes. The chirp signals resilience to multipath and the Doppler effect makes it suitable for underwater acoustic communication. This paper presents a binary phase‐coded hyperbolic frequency‐modulated (BPC‐HFM) for multi‐user underwater acoustic communication. The proposed scheme has the advantage of both linear frequency‐modulated signal, which is immune to multipath and Doppler offset, while PN sequence is encoded in such a manner that a waveform possesses either a phase shift of 0 or 180 depending upon the code which results in a unique orthogonal waveform for every user. As a result, it helps to reduce multiple access interference; in addition, the time‐reversal mirror technique is used to enhance the performance of the system in the high multipath environment due to the virtue of hydro‐acoustic channel. Furthermore, it uses a relatively simple matched filter‐based receiver. To substantiate the robustness of the proposed multi‐user waveforms three types of hydro‐acoustic channels using the different numbers of multiple users are chosen for simulation and the bit error rate (BER) of the system is calculated. Furthermore, the results are compared to binary phase‐coded linear frequency‐modulated signal and conventional Direct sequence spread spectrum CDMA.