# Modeling of Satellite-to-Underwater Integrated FSO-PON System Using NOMA-VLC
## 1 Introduction  
With current advancements in fifth-generation (5G) networks, recent conventional networks are undergoing tremendous growth in the number of wireless consumers for several services in numerous scenarios, such as rural, urban and remote areas. As a solution to achieve a heterogeneous global network, satellite systems have been incorporated into underwater communications systems. Due to the increasingly large footprint, satellite-to-underwater communication offers wide and extensive wireless connectivity for several fixed as well as mobile users. Presently, it is extensively acknowledged that independent ongoing communication systems cannot cope with the growing traffic demands of several services. In this respect, a new configuration of satellite-to-underwater network has lately been realized [1, 2]. Conversely, free space optics (FSO) communication has obtained huge consideration over the past twenty years. FSO have various benefits over existing microwave and radio frequency (RF) communication systems, such as huge bandwidth, unregulated spectrum, license-free operation, high transmission rate, long-reach transmission, re-usability with respect to wavelengths and equipment, exemption from electromagnetic interference (EMI) and high security. FSO are eminnently suitable for numerous contexts, such as high-speed trains, unmanned aerial vehicles, indoor/outdoor local area networks (LANs), as well as wide area networks (WANs), building-to-building, deep-space and satellite communications. They sustain a higher transmission rate from Gbps to Tbps [3; 4; 5]. FSO underwater applications, called underwater optical wireless communication (UOWC), have been researched extensively. They transmit the data to industry, scientific and military communities. They are also involved in areas such as environmental monitoring, pollution monitoring, live video streaming, offshore exploration, maritime archaeology, port security, oil control and maintenance, imaging, oceanography, tactical surveillance and disaster precautions in UOWC. Due to their high transmission rate, short/long-range wireless links, low latency as well as less power consumption, UOWC has become an elegant substitute technology for conventional acoustic and RF in underwater communications [3]. Moreover, because of pioneering space technology and elaborated space-based instruments, FSO usher in a modern era of satellite communications. Taking into account practical atmosphere factors, the viability of an optical satellite-underwater link was proposed and verified [1]. Furthermore, by 2023, it is expected that there will be more than 13.1 billion wireless inter-connected devices, driving a remarkable growth in worldwide connectivity. The current RF spectrum is inadequate for managing such huge traffic. Visible light communication (VLC) ranging from 430 THz to 790 THz has developed as a potential candidate for 5G and even beyond 5G networks. VLC offers various advantages, such as ease of use, compact size, high durability, high power efficiency and no EMI radiation. In addition to this, it has a wide range of uses, such as light-fidelity (Li-Fi), healthcare, vehicular communication, door localization and underwater communication. Apart from these benefits, the VLC system is restricted in operation because of the light emitting diode (LED) bandwidth constraint operated in the MHz range. For this reason, various multiple-access technologies, such as code division multiple access (CDMA), orthogonal division multiple access (OFDM) and interleave division multiple access (IDMA), have in recent years been utilized with VLC systems. However, these multiple access schemes have a complex receiver design and affect the clipping noise. To suppress the above restrictions, non-orthogonal multiple access (NOMA) is utilized to operate in a corresponding frequency band as well as a time block with a distinct power allocation among the end users.