Deep transfer learning-based variable Doppler underwater acoustic communications.

This paper proposes a deep transfer learning (DTL)-based variable Doppler frequency-hopping binary frequency-shift keying underwater acoustic communication system. The system uses a convolutional neural network (CNN) as the demodulation module of the receiver. This approach directly demodulates the received signal without estimating the Doppler. The DTL first uses the simulated communication signal data to complete the CNN training. It then copies a part of the convolution layers from the pre-trained CNN to the target CNN. After randomly initializing the remaining layers for the target CNN, it is trained by the data samples from the specific communication scenarios. During the training process, the CNN learns the corresponding frequency from each symbol in the selected frequency-hopping group through the Mel-spectrograms. Simulation and experimental data processing results show that the performance of the proposed system is better than conventional systems, especially when the transmitter and receiver of the communication system are in variable speed motion in shallow water acoustic channels.

Effects of receiving position and shell material on in-band full-duplex underwater communications' self-interference signal.

Despite the recent intensive research on adaptive algorithms for self-interference (SI) cancellation (SIC) in in-band full-duplex (IBFD) underwater acoustic communication (UWAC), there has been relatively little exploration of how the IBFD-UWAC modem shell affects the SI signal. This paper analyzes the effects of the shell material and the near-end receiver position on the SI signal. The analysis is done with a two-dimensional finite-element model in a free-field simulation environment, which combines the differential equation of motion and the time-dependent solver. The SI signal strength around the modem shell in the far-field conditions is obtained. The simulation and pool experiment results both show that (i) the strength of the received SI signal is lowest when the near-end receiver is on a line extending from the shell's geometric center perpendicularly to its central axis and (ii) a shell material with a high elastic coefficient is more conducive to suppressing the SI signal. A pool experiment showed that changing the spatial position of the near-end receiver and the shell material from aluminum to stainless steel enhanced the SIC performance of the IBFD-UWAC system by at least 12 and 4 dB, respectively.

Doppler scale estimation for varied speed mobile frequency-hopped binary frequency-shift keying underwater acoustic communication.

An optimized Doppler scale factor estimation method based on wavelet de-noising is proposed in this paper to improve the performance of frequency-hopped binary frequency-shift keying mobile underwater acoustic communication. The method is with regard to the Doppler scale factors of multiple symbols as a whole and focuses on the continuous variation overall trends, rather than the association between two adjacent symbols. The proposed method was demonstrated in a shallow water experiment conducted in November 2018 at Songhua River in Heilongjiang, China. Experimental data collected showed a bit error rate less than 10-4 with a data rate of 65.41 bps at a moving speed of 2.0-6.2 m/s over the horizontal range of 0.68-1.6 km.

Dolphin Sounds-Inspired Covert Underwater Acoustic Communication and Micro-Modem.

A novel portable underwater acoustic modem is proposed in this paper for covert communication between divers or underwater unmanned vehicles (UUVs) and divers at a short distance. For the first time, real dolphin calls are used in the modem to realize biologically inspired Covert Underwater Acoustic Communication (CUAC). A variety of dolphin whistles and clicks stored in an SD card inside the modem helps to realize different biomimetic CUAC algorithms based on the specified covert scenario. In this paper, the information is conveyed during the time interval between dolphin clicks. TMS320C6748 and TLV320AIC3106 are the core processors used in our unique modem for fast digital processing and interconnection with other terminals or sensors. Simulation results show that the bit error rate (BER) of the CUAC algorithm is less than 10 - 5 when the signal to noise ratio is over ‒5 dB. The modem was tested in an underwater pool, and a data rate of 27.1 bits per second at a distance of 10 m was achieved.