# Modeling of Satellite-to-Underwater Integrated FSO-PON System Using NOMA-VLC
## 2 System Design
### Downstream Design  
Figure 2a illustrates the block diagram illustrating the suggested design employing four subsystems, viz. satellite, TWDM-PON, WDM-OCDMA as well as VLC. Figure 2b illustrates the satellite communication system using FSO links employing on-off keying modulation scheme. It indicates that the FSO links that carry the data from an earth station to the satellite in orbit. Here, at the earth station (Tx side), the incoming optical signal from a continuous wave (CW) laser diode (LD) is modulated with a pseudo-random bit sequence non-return to zero (PRBS-NRZ) digital signals through Mach-Zehnder modulator (MZM). Then an adequate signal is passed through modulated GG FSO turbulence channel towards the satellite and vice versa [21; 22; 23]. After this, at the satellite (Rx side), the optical signals are converted into electrical signals using a photodetector (PD). Then the low-pass Bessel filter (LPF) is used to filter out the noises from the original information [24; 25; 26]. A 3R regenerator is used to regenerate the distorted signal followed by the BER analyzer to analyze the system performance w.r.t. quality-factor (QF), BER, eye diagrams and eye height [26; 27; 28; 29; 30; 31]. Figure 2c shows that, after receiving the satellite data from the satellite using FSO links from the communication system at an OLT/central office (CO) onshore data center at TWDM-PON design, four optical wavelengths (1596-1598.4 nm with 0.8 spectral widths) originated from CW LDs sources at an input power (P\({}_{\rm in}\)) of 10 dB. Then these optical wavelengths were modulated with PRBS-NRZ digital signals utilizing MZM and multiplexed by 4 \(\times\) 1 array waveguide grading multiplexer (AWG MUX) at an aggregate data rate of 40 Gbps (=4 \(\times\) 10 Gbps), as shown in Figure 2a. After this, these multiplexed signals passed through a series of bidirectional components viz. optical distribution network (ODN) viz. an optical circulator (OC), an ideal MUX, a bidirectional single-mode fiber (SMF) and a NOMA-VLC channel in UOWC to provide the data signals to the different underwater devices. To distribute these inputs signals, the 1 \(\times\) 2 bidirectional power splitter (PS) followed by an AWG de-multiplexer (DEMUX) were utilized. It offered the TWDM design incorporating four sets of 1 \(\times\) 8 power splitters (PSs) to support 32 underwater sea devices  
Figure 1: Schematic diagram of integrated FSO-PON using VLC system for satellite-to-under-water communication. which are also known optical network units (ONUs), as shown in Figure 2d. Downlink Rx section consists of PIN PD, LPF, 3R regenerator and BER analyzer [21; 22; 23; 24; 25]. Figure 2e illustrates the WDM-OCDMA consisting of CO and ONUs with six pairs of Txs/Rxs as per MNZCC the code for three end users. The MNZCC code design for three downstream and upstream users is tabulated in Table 1.  
Figure 2: _Cont._  
Figure 2: _Cont_. WDM-OCDMA design incorporating MNZCC code can assist a maximum of 18 ONUs. Six wavelengths (1575-1577 nm with spectral width 0.4 nm) produced from six CW LDs were initially passed via 3 \(\times\) 6 ONUs MUX, succeeded by 6 \(\times\) 1 AWG MUX then sent to a 1 \(\times\) 3 fork corresponding to three OCDMA coded groups for three subsystems as illustrated in Figure 2e. Each coded group incorporated a 1 \(\times\) 6 AWG DEMUX, six subsystems as well as one encoder (E) for an underwater device. Subsystems 1 to 6 were utilized for modulating the multi-wavelength signal of OCDMA channels. A subsystem comprising PRBS, NRZ and MZM modulator components is shown in Figure 2h [25; 32]. Then, the coded signals are multiplexed through a 6 \(\times\) 1 AWG MUX. The signals spread to the MNZCC encoder, which incorporated the code design using a 1 \(\times\) 2 WDM DEMUX along with 2 \(\times\) 1 WDM MUX.