Optoacoustic communication enables an airborne unit to directly reach nodes deep underwater. To achieve high data rates in optoacoustic communications, implementing a multilevel modulation scheme is necessary where distinct acoustic signals can convey multiple symbols. However, demodulating these signals proves challenging amidst the complexities of underwater environments characterized by multipath propagation and resultant inter-symbol interferences. To overcome these challenges, this paper presents a novel demodulation scheme using a Spiking Neural Network (SNN). Our SNN model has undergone training using a laboratory-constructed dataset, comprising eight levels of optoacoustic signals recorded from three different underwater positions. Validation is conducted with a dataset deliberately designed to include severe interference from multipath-generated echoes and reverberations. The results indicate that our SNN-based demodulation scheme achieves an impressive accuracy of 90.16%, surpassing the 65.30% accuracy obtained through conventional peak detection-based techniques.

The optoacoustic process can solve the longstanding challenge of wireless information transmission from an airborne unit to an underwater node (UWN). The nonlinear optoacoustic signal generated by proper laser parameters can propagate long distances in water. However, forming such a signal requires a high-power laser, and the buildup of a vapor cloud precludes the subsequent acoustic signal generation. Therefore, pursuing the traditional on-off keying (OOK) modulation technique will limit the data rate and power efficiency. In this paper, we analyze different modulation techniques and propose a vapor cloud delayed-differential pulse position modulation (VCD-DPPM) technique to improve the data rate and achieve high power efficiency for a single stationary laser transmitter. The symbol rate of VCD-DPPM is approximately 6.9 times and 1.69 times higher than OOK in our text communication simulation using a laser repetition rate of 10 kHz and 40 Hz, respectively. Furthermore, VCD-DPPM is 137% more power efficient than the OOK technique for both cases. We have generated different acoustic signal levels in laboratory conditions and simulated the bit error rate (BER) for different depths and positions of the UWN, while considering ambient underwater noises. Our results indicate that VCD-DPPM enables efficient data transmission.

Localization of underwater networks is important in many military and civil applications. Because GPS receivers do not work below the water surface, traditional localization methods form a relative topology of underwater nodes (UWNs) and utilize either anchor nodes or floating gateways with dual transceivers in order to determine global coordinates. However, these methods introduce logistical complications and security risks in deploying the anchor and/or surface gateways. This paper tackles such an issue by proposing new localization techniques which can remotely localize UWNs using optoacoustic signals. In our approach, GPS coordinates are transmitted from air to the UWN via creating an underwater temporary isotropic acoustic transmitter with the optoacoustic process. We analyze the process of controlling the shape and size of the plasma to create the isotropic acoustic transmitter and experimentally validate the generation of isotropic acoustic signals. Then two methods of localization are proposed for static and dynamic UWNs. Finally, the simulation results with experimental values show the effectiveness of our approach. Comparing to the traditional techniques, our approach achieves the same accuracy without using any surface or underwater anchor nodes.

The differential pulse position modulation (DPPM) is one of the popular power-efficient schemes for supporting visible light communication in underwater environments and across the air-water interface. Despite such an advantage, DPPM does not efficiently utilize the available channel capacity. This paper aspires to tackle such shortcomings by striking a better balance between power and bandwidth efficiency. Particularly, L-DPPM is considered where a block of M input data bits is mapped into one of the L distinct waveforms containing only one ‘on’ chip. A novel encoding algorithm and frame structure are proposed in order to shorten the time between consecutive symbols and consequently improve the bit rate of L-DPPM. The idea is based on avoiding bit patterns that contribute the most to bandwidth inefficiency. The proposed algorithm explores a number of bit patterns remapping through simple complement and shifting operations. A detailed frame structure with all necessary control bits is provided. Overall, boosting the bandwidth efficacy comes at the expense of a slight increase in control bit count and transmission power. The simulation results demonstrate the effectiveness of the proposed encoding algorithm and provide guidelines for determining M for best performance.