Underwater optical-acoustic hybrid networking can complement the advantages of both underwater wireless optical and acoustic communications, which holds great development prospects. This paper designs a novel and feasible underwater optical-acoustic hybrid sensor network topology, which fully considers the differences between underwater sound and light communication systems. Following this network topology, an adaptive and green power control aided routing protocol is proposed, mainly including two stages: the power control and the packet forwarding. These two procedures employ potential game theory and ${Q}$ -learning technique respectively, making each node adjust its transmitted power and forward packets in an adaptive and distributed manner with energy efficiency, packet advancement, and network connectivity as the optimization goals. According to the simulation results, the proposed power control and packet forwarding algorithms are of reasonable complexity and convergence speed, and adding the power control before the packet forwarding can achieve better routing performance. Compared with relevant routing protocols, the designed protocol has significant advantages in energy efficiency, end-to-end delay, and network lifetime.

Non-orthogonal multiple access (NOMA) is a promising technology to improve spectrum utilization effectively for underwater optical wireless communications (UOWC). To exploit the benefits of NOMA in a turbulent environment, cooperative transmission has been introduced in the NOMA–UOWC network. The existing studies on NOMA suggest that relay selection and power optimization are the main factors affecting system performance. In this paper, a general NOMA node pairing method and two power optimization schemes for NOMA–UOWC are proposed, and both schemes are proven to be strictly quasi-convex. The two optimization schemes are solved by the BFGS algorithm and the particle swarm algorithm, respectively. The effectiveness of the proposed schemes are evaluated by our simulations, and the main factors affecting the relay-aided NOMA performance are derived.

Both the long-term beam spreading caused by ocean turbulence and the pointing errors induced by the jitter of transmitters and receivers degrade the performance of underwater wireless optical communication (UWOC) links. To effectively alleviate their effects, an in-depth study was carried out over the Málaga turbulence channel with pointing errors and beam spreading in multiple-input and multiple-output (MIMO) UWOC. First, we analyzed the long-term beam spreading and the received light power for the finite receiving aperture in the presence of pointing error displacements. Based on this, the relationship between beam spreading, pointing errors, and signal power was established. Second, the approximate expressions of the average bit error rate (BER) and the communication outage probability were derived theoretically for this MIMO system using maximal-ratio combining (MRC) diversity. Third, the effects of the pointing errors on the coding and the diversity gains were explored for the MIMO links. Finally, using the observed ocean data from the Global Ocean Argo gridded dataset, we numerically verified the combined effects of ocean turbulence strength, beam spreading, and pointing errors on the average BER and outage probability of this system. These results also proved that adjusting the size of the receiving aperture or the order of the multiple quadrature amplitude modulation (mQAM) could effectively mitigate their effects.

Non-orthogonal multiple access (NOMA) is a multiple access technology of great potential and it has superior performance in underwater wireless optical communications (UWOC). In this paper, we investigate the self-interference problem caused by the superimposed transmission of NOMA-based wireless optical communication system in the underwater turbulent environment, and then propose an alternating composite phase shift (ACPS) coding scheme based on constellation optimization, which can effectively eliminate the decoding error caused by the self-interference within the NOMA paired group. To enhance the optimization efficiency of ACPS in the medium- and strong-turbulence environments, we design a golden-section-approach-based power allocation (GSPA) algorithm that minimizes the bit error rate (BER). The simulation results show that the BER of the low-power user within the NOMA paired group can be reduced by up to 3 orders of magnitude in the ACPS-based UWOC-NOMA system compared to the conventional UWOC-NOMA system. In addition, the proposed GSPA algorithm fully takes into account the light-intensity scintillation caused by the ocean turbulence, which not only significantly reduces the average BER of the NOMA system, but also accelerates the convergence rate.