In recent decades, wireless power transfer (WPT) has gained significant interest from both academic and industrial experts. It possesses natural electrical isolation between transmitter and receiver components, ensuring a secure charging mechanism in an underwater scenario. This ground-breaking technology has also enabled power transmission in the deep-sea environment. However, the stochastic nature of the ocean highly influences underwater wireless power transmission and transfer efficiency is not up to that of terrestrial WPT systems. Recently, the research fraternity has focused on WPT in the air medium, while underwater wireless power transfer (UWPT) is challenging and yet to be explored. The major concerns are ocean current disturbance, bio-fouling, extreme pressure and temperature, seawater conductivity and attenuation. Thus, it is essential to address these challenges, which cause a substantial energy loss in UWPT. This study presents a comparison between various WPT techniques and highlights the research contributions in UWPT in recent years. Research and engineering challenges, practical considerations, and applications are analyzed in this review. We have also addressed influencing factors such as coil orientation, coil misalignment and seawater effects in order to realize an efficient and flexible UWPT system. In addition, this study proposes multiple-input and multiple-output (MIMO) wireless power transmission, which can significantly improve the endurance of autonomous underwater vehicles (AUVS). This idea can be applied to the design of an underwater wireless power station for self-charging of AUVs.

Underwater Optical Wireless Communication (UOWC) has gained significant attraction in many underwater activities because of its high bandwidth as compared to radio frequency and acoustic technologies. Underwater Optical Wireless Communication (UOWC) has high stature in underwater observation, exploration and monitoring applications. However, due to complex nature of ocean water, several practical challenges exist in deployment of UOWC links. Qualitative and effective research has been carried out in UOWCs from last few decades. Ambition behind this research systematic study is to provide a comprehensive survey on latest research in UOWCs. Herein, we provide a brief discussion on major research challenges, limitations and development in UOWCs. We provide a periodical review on UOWC issues and potential challenges highlighted in previous studies. In this paper, we have also investigated research methods to gain attention of research fraternity towards future technologies and challenges on the basis of existing approaches. Thus, it is our foremost requirement to provide state-of-the-art analysis of existing UOWCs. Significant deliberation has been provided with recent bibliography.

Physical-layer network coding (PNC) can reduce the required number of transmission time slots and increase network throughput. This study combines decode-and-forward (DF) and denoise-and-forward (DNF) in half-duplex two-way relay network (TWRN) using orthogonal offset quadrature phase shift keying (O-OQPSK) modulation under additive white Gaussian noise (AWGN) channels. This approach improves wireless network throughput by two and four times compared to ordinary PNC and conventional network coding schemes, respectively. First, we design the relay decoder in the denoise and decode forward (DNDF)-TWR scheme, relay denoising mapping and decoding functions in the TWR scheme, and decoder using the maximum likelihood (ML) principle. Then we analyse the relay bit error rate (BER) and end-to-end BER of the DNDF-TWR scheme with O-OQPSK modulation. Simulation results show that the BER performance of the DNDF-TWR scheme is better than that of DNF-TWR and DF-TWR schemes.