## 4 Investigation of Proposed OAM-OSC System at Different Input Parameters
Figure 11 illustrates the effects of the receiver satellite incorporated telescope aperture diameter (TAD) on the log BER of different OAM beams. The results revealed that the higher the diameter of the receiver satellite antenna, the more power is accumulated at the receiver and the better the log BER. It is interesting to observe that until the TAD of 20 cm, LG40,0, and LG14,0 OAM beams experienced more log BER as compared to LG0,0, but at TAD values greater than 20 cm, LG40,0 and LG14,0 OAM beams surpassed the performance of LG0,0 due to more RAP at the receiver. Moreover, LG40,0 provides the least log BER (\(-\)17.23) followed by LG14,0 (\(-\)16.47) at 30 cm TAD at the receiver satellite, and maximum log BER is observed in the case of LG0,0 (\(-\)15). whereas LG0,0 was encountering the least BER (\(-\)2.4) at 5 cm TAD and LG40,0 had the highest (\(-\)1.5) bit errors. It is suggested to use higher TADs for the higher-order OAM beams. Figure 9: Variation of BER for OSC link length for LG0,0, LG14,0, and LG40,0 OAM beams. The proposed OSC systems have PDM-256-QAM modulation that has 8 bits per symbol and in turn, generates 256 different symbols in the data transmission. These 256 symbols have a fixed phase and amplitude at the transmitter, and due to noise and other performance-limiting effects, their phase and amplitude become distorted as they travel toward the receiver. At the receiver, optical signals are converted into electrical data with balanced detection, and these 256 symbols are represented by a constellation analyzer. A constellation analyzer is a two-dimensional (2D) plot with an I and Q axis for the symbols received at the receiver. EVM can also be calculated from the constellation analyzer as the displacement of symbols from their ideal positions can be seen from the 2D plot. Figure 12a depicts the constellation diagram for PDM-256-QAM at 22,000 km OSC link distance using DSP and it is evident that all the symbols have little effects of noises but, despite that are at their respective positions. On the other hand, at the OSC link length of 24,000 km, there are high distortions in the symbols on the I and Q axis as shown in Figure 12b. Figure 11: Log BER variation with different TADs at the receiver satellite. Figure 10: Performance of OAM beams in OSC link having different pointing errors at 10,000 km. Further, we explored the effects of DSP on the OAM-based OSC system and it was observed that DSP had a significant impact on the performance of the systems due to the compensation of NL, dispersion, phase errors, etc. Figure 13 illustrates the constellation diagram of the proposed OAM-OSC system at 22,000 km without using DSP. Results are completely distorted without DSP, and if DSP is incorporated, then the constellation gets improved as depicted in Figure 12a at the same OSC distance. Figure 14 represents the performance of the proposed system at OSC link lengths for different OAM beams and it is observed that SNR reduces as the link length prolongs. The performance of LG00 OAM beams was found to be best performing and LG400 exhibited the least SNR. Figure 12: A 2D plot of 256-QAM symbols on the I and Q axis at OSC link length of (**a**) 22,000 and (**b**) 24,000 km. Figure 13: Highly distorted constellation of 256-QAM symbols at 22,000 km without DSP.