## 4 Simulation Results Analysis
The system that adopts the adaptive transmit diversity strategy has better BER performance, which is significantly improved compared to the conventional strategy because the adaptive transmit diversity strategy selects and combines high interference LED beams and uses transmit diversity technology to convert interference signals into signal gain. The OFDM-NOMA scheme is used within the beam to pair users two by two based on their channel gains. NOMA superimposed signals from different user groups are sent through different subcarriers. This can reduce the number of multiplexing users on the same carrier and reduce interference between NOMA users within the beam. This proves the superiority of the proposed strategy in reducing interference. Figure 8: The BER comparison of both the conventional and proposed strategy at different interference scenarios. Figure 7: BER performance for NOMA users with different \(a_{1}\) values. Figure 9 gives the constellation diagrams of user 1 and user 2 with SNR = 25 dB. Figure 9a,b show the constellation diagrams of the signals after power allocation for user 1 and user 2, respectively. Figure 9c,d show the signal constellation diagrams for two users and four users superimposed, respectively. In the superimposed constellation, the constellation of the second user is replicated for each symbol of the first user. Each replica represents one symbol for the first user. Thus, the superimposed symbol constellation consists of \(M_{x}=M_{1}\times M_{2}\times\cdots\times M_{Kn}\) symbols [26], where \(M\) is the modulation order of each user signal. Figure 9e,f show the signal constellations received by user 1 and user 2 using the conventional scheme under high-interference conditions, respectively. Due to the influence of inter-beam interference and NOMA superimposed inter-user interference, the constellation distribution of signals received by these users is chaotic. Figure 9g,h show the signal constellations of the signals received by user 1 and user 2 using the conventional scheme for the low-interference case, respectively. Due to the reduction of the inter-beam interference, the constellation points of the received signals from the users are more regularly distributed, but there is still interference. As a comparison, Figure 9i,j show the signal constellation diagrams of the two users employing the adaptive transmit diversity strategy after SIC detection. The constellation point distribution pattern shows that the interference can be effectively reduced after adaptively selecting the merging scheme and OFDM-NOMA scheme. Figure 10 depicts the BER curves of the users with the worst BER performance for the case of SNR = 20 dB with different numbers of users set in the multi-beam NOMA system. As the number of users served by the system increases, the BER of the worst user in the system also increases. Since the total transmit power of the system is limited, the more interference is generated by the increase in the number of users served. However, the BER performance of users who adopt an adaptive transmit diversity strategy is better than that of users who use conventional schemes. When the communication distance is 5 m, the BER of the worst user is lower than the FEC threshold, which verifies the feasibility of the strategy designed in this research to support multi-user communication in multi-beam NOMA systems. Figure 9: Constellation of (a) user 1 symbols at the transmitting end, (b) user 2 symbols at the transmitting end, (c) two users superimpose symbols, (d) four users superimpose symbols, (e) user 1 symbols using conventional scheme under high interference, (f) user 2 symbols using conventional scheme under high interference, (g) user 1 symbols using conventional scheme under low interference, (h) user 2 symbols using conventional scheme under low interference, (i) user 1 symbols using the proposed strategy under high interference, and (j) user 2 symbols using the proposed strategy under high interference.