## 2 Related Work  
In recent years, wavelength division multiplexing (WDM) and OAM have been integrated using multi-mode fibers (MMFs) with a range of 1000 m and a 10 Gbps throughput [4]. In [8], the WDM and OAM techniques were integrated at a 1.6 Tbps data rate. A hybrid OAM, WDM, and orthogonal frequency division multiplexing passive optical network (PON) was designed over 40 km MMF at 40 Gbps throughput in another study [9]. OAM\({}_{31}\) and OAM\({}_{41}\) modes are used in [10] to realize an OAM-MDM system over a ring-core fiber range of 300 km and at a 20 Gbps data rate. According to [11], 100 Gbps OAM links incorporating quadrature phase-shift keying multiplexing can provide a 1.5 dB optical signal-to-noise ratio (OSNR) under turbulent conditions. The authors of [12], explored an OAM-based system that offers a 450 Gbps data rate over a 1.5 km range. Ref. [13] also proposed and investigated an OAM-PON system over 0.4 m free space links at 10 Gbps. An MDM system utilizing OAM modes over 1.4 km ring-core fibers at 32 Gbaud is presented in [6]. A 40 Gbps fiber-free space optics (FSOs) system using OAM modes over a 50 km fiber with a 2.5 km FSO range is described in [14].  
The authors of [15] further investigated the effect of vehicle motion on a turbulence model by using experimental measurements at a 40 C temperature gradient. The UWOC system in [16] transmits information over a 1.8 m distance in a sea water tank at 3.4 Gbps. Orthogonal frequency division multiplexing over 10 m distance in shallow water is demonstrated in [17]. There is a passive optical network-visible light communication (VLC) integrated system with over 100 km of fiber and a 5 m at 10 Gbps throughput for land-underwater scenarios in [18]. Ref. [3] presents a UWOC system with wavelength division multiplexing over 500 m of free space with a 5 m clear-ocean communication at 100 Gbps. Moreover, ref. [19] presents an integrated FSO and VLC system which can transfer signals over 430 m at 0.96 Gbps and over 1 m at 450 Mbps.  
Based on these existing works, this work presents an OAM-based satellite-ground-underwater OWC system that can operate under diverse atmospheric conditions. For satellite-ground-underwater scenarios, four different OAM modes are incorporated. According to our knowledge, this is the first time that higher-order OAM modes have been used for satellite-ground-underwater communication with combined OWC and UWOC links. We investigate the system under the influence of weak-to-strong turbulence, pointing error, geometric loss, and different types of water. These contributions are summarized as follows:  
* A high-speed, high-capacity, and long-reach satellite-ground-underwater OAM-based OWC communication system is designed.
* System performance is analyzed for satellite-to-ground and ground-to-underwater communication under diverse climate conditions and different OAM modes.
* System performance is verified w.r.t. another recent works.  
The paper is organized as follows: Section 3 depicts the system's design, demonstrating the system through a block diagram, the generated OAM modes, and the system's parameters. Section 4 describes the performance evaluation of the system. A conclusion and the scope of future research are presented in Section 5.