## 3 Proposed System Model  
Figure 2 presents the proposed UOWC/CDMA system. Figure 2a shows the OCDMA transmitter, which consists of an OCDMA encoder for encoding and spreading the data using the WMPC spreading sequence. The encoded data are modulated using the M-ary DPPM to control the channel dispersion by controlling the pulse width of the DPPM signal and consequently reducing the inter-symbol interference (ISI). A Mach-Zehnder interferometer (MZI) modulator is used to modulate the laser light intensity using the encoded modulated signal. The received optical signal is first demodulated by the MZI demodulator, then DPPM demodulated and decoded using the WMPC decoder, and finally, the balanced detector is used to convert the optical data to electrical data and eliminate noise. Furthermore, the use of DPPM gives the system an advantage in the case of multimedia applications. In this case, we can control and optimize the BER performance as in the case of [40]. The LOS-UOWC channel in Figure 2c can be modeled using the equation below, as in the case of [3; 43; 44; 45; 46]. The level of optical power received \(P_{r}\) depends on the optical power transmitted by the laser diode \(P_{t}\) ; the transmitter and receiver optical efficiencies \(\eta_{t}\) and \(\eta_{r}\), respectively; the angle of the trajectory perpendicular to the transmitter-receiver planes \(\theta\); the aperture area of photodetector \(A_{r}\); and the divergence angle of the laser beam \(\theta_{d}\)[13]. \(\alpha(\lambda)\) is the attenuation coefficient of the wireless underwater channel and is discussed in detail in Section 4. \[P_{r}=P_{t}\ \eta_{t}\eta_{r}\ exp\bigg{(}-\alpha(\lambda)\frac{d}{cos( \theta)}\bigg{)}\bigg{(}\frac{A_{r}cos(\theta)}{2\pi d^{2}(1-cos(\theta_{d}))} \bigg{)} \tag{1}\] \begin{table}
\begin{tabular}{c c c c c} \hline
**Ref.** & **Parameters** & **Optical** & **RF Waves** & **Acoustic Waves** \\ \hline \multirow{8}{*}{[8]} & \multirow{3}{*}{Frequency band} & \multirow{3}{*}{10–150 MHz} & & & Distance dependent \\  & & & & 1000 km and 1 kHz \\  & & & & 1–10 km and 10 kHz \\  & & & & Less than 100 m and \\  & & & & 100 kHz \\  & & & 30–300 Hz for direct & & \\  & & & underwater & & \\  & & & & communication system & 10–15 kHz \\  & & & & or MHz for buoyant & \\  & & & & communication system & \\  & & & & Few mW to hundreds & \\  & & & & of Watts & \\  & & Antenna size & 0.1 m & 0.5 m & 0.1 m \\  & & Efficiency & 30,000 bits/J & - & 100 bits/J \\  & & & Absorption and organic & Conductivity and & Temperature and \\  & & & matter & permittivity & pressure \\ \hline \multirow{3}{*}{[29; 30; 31]} & Code length & 121–289 Chip & 121–289 Chip & 121–289 Chip \\  & BER performance & Less than \(10^{-20}\) & Less than \(10^{-15}\) & Less than \(10^{-10}\) \\  & Number of users & 110–180 & 90–100 & 30–50 \\  & Throughput performance & 1 Tbps*user & 10 Gbps*user & Less than Mbps*user \\ \hline \multirow{3}{*}{[43; 44; 45]} & Number of orthogonal frequencies & Less than 32 & Less than 16 & \\  & Number of users & 20–32 & 1–16 & \\ \cline{1-1}  & BER performance & Less than \(10^{-12}\) & Less than \(10^{-9}\) & - \\ \cline{1-1}  & Throughput performance & Approximately 100 & Approximately 1 & \\ \cline{1-1}  & & Gbps*user & Gbps*user & \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._The scintillation phenomenon is important and must be considered in the UOWC channel modeling. This phenomenon characterizes the chaotic change in the water flow velocity and pressure, and this turbulence affects the water characteristics when laser light beams propagate underwater. This results in fluctuations in the optical power and degrades the system performance owing to variations in the refractive index of water [22; 46; 47; 48; 49; 50; 51]. Various UOWC system models are used to represent the effect of turbulence on the system performance; however, for weak turbulence, the gamma-gamma model is suitable.