## 4 Simulation Results Analysis  
In this section, we verify the performance of the proposed adaptive transmit diversity strategy through simulation. Therefore, based on the system and channel models in Section 2, we simulated the UWOC multi-beam NOMA system using ACO-OFDM modulation and analyzed the BER performance of the multi-beam NOMA system with adaptive transmit diversity strategy in different scenarios, and the key parameters of the simulation work are listed in Table 2. For simplicity, we assume that the number of LEDs at the transmitting end is 17, the users are set up at different locations within the communication distance, and each user adopts one PD to receive the signal. Figure 6 investigates the relationship between the BER of two NOMA users and the parameter \(\rho\) under the signal-to-noise ratio (SNR) of 30 dB. Weak users have poorer channel quality and need more power allocated to maintain the quality of service (QoS). Strong users with better channel quality are allocated less power. Thus, weak users have better BER performance than strong users. The outcomes reveal that as the value of \(\rho\) progressively increases, the interference from other beam signals on the NOMA user signals becomes more pronounced, leading to a gradual degradation in the system's BER performance. The BER of the two users is located near the FEC threshold for \(\rho=0.27\). Hence, for the subsequent experimental simulations, the inter-beam interference tolerance threshold is set to \(\varepsilon=0.27\). This indicates that the inter-beam interference is serious for \(\rho>\varepsilon\), and Algorithm 1 needs to be used for selective merging. Figure 7 analyzes the BER performance of two superimposed users for different values of power allocation coefficients with SNR = 20 dB. Weak users, characterized by poorer channel quality, require more allocated power to ensure communication quality, leading to a corresponding reduction in power allocated to strong users. At the same SNR, weak users exhibit lower BERs compared to strong users. When the power allocation coefficient \(a_{1}\) is around 0.8, the BER of the strong user is minimized. Therefore, in this section using the OFDM-NOMA scheme for superposition coding of two users, consider setting \(a_{1}=0.8\) \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Modulation order & 4-QAM \\ Number of subcarriers & 256 \\ Number of OFDM symbol & 1000 \\ Imaging lens diameter & 50 mm \\ Total attenuation factor for seawater & 0.15 \\ Communication distance & 5 m \\ Emission LED conversion efficiency & 0.1289 \\ Detector conversion efficiency & 0.95 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation parameters of the multi-beam NOMA-UWOC system. Figure 6: BER performance for NOMA users with different \(\rho\) values. As shown in Figure 8, the BER performance of users with the adaptive transmit diversity strategy and the conventional strategy is compared under different inter-beam interference scenarios. The conventional scheme superimposes four users' signals on the power domain with the same time-frequency resources, and the user signals with low SIC decoding order are used as the priority decoding users' interference signals. In the case of low inter-beam interference, i.e., \(\rho<\varepsilon\), interference management is not required after judgment, and users with higher allocation power still have good BER performance at high SNR, while users with low decoding order have worse BER performance due to their low allocated power. In the case of high inter-beam interference, i.e., \(\rho<\varepsilon\), due to the intensification of inter-beam interference, the user's received signal cannot be demodulated correctly, and its BER performance deteriorates sharply.