## 5 Conclusions  
In this research study, an ultra-high capacity OSC system was presented over a 22,000 km link distance between satellites, and we have also manifested the potential of OAM beams by incorporating LG00, LG140, and LG400 transverse mode profiles and PDM-256-QAM. OAM beams were selected at an optimal one number difference to reduce the effects of mode coupling and to avoid generating very higher order modes, which in turn reduce RAP at the receiver. A total of 15 channels were generated from only 5 wavelengths using LG modes, which reduced the cost of the system by one third. Results revealed that all the OAM beams had different output BER at diverse OSC link lengths such that the fundamental mode LG0,0 had the least BER due to the narrow beam mode spot size and LG40,0 had the highest BER because of the power losses at the receiver due to the higher order mode. The EVM% value for LG0,0 was 15.01, for LG14,0 was 16.99, and for LG40,0 was 19 at 0.5 \(\upmu\)rad, and for increased pointing errors such as 4 \(\upmu\)rad, EVM% increased to 32.98, 35.02, and 37.99 for LG0,0, LG14,0, and LG40,0, respectively. Higher-order modes have very wide spot sizes of light profiles, and it becomes tedious for the receiver antenna to detect the entire mode power. Therefore, it is interesting to observe the effects of different TADs. It is perceived that until the TAD of 20 cm, LG40,0 and LG14,0 OAM beams experienced more log BER as compared to LG00, but at TAD values greater than 20 cm, LG40,0 and LG14,0 OAM beams surpassed the performance of LG0,0 due to higher RAP at the receiver. Moreover, LG40,0 provided the least log BER (\(-\)17.23) followed by LG14,0 (\(-\)16.47) at 30 cm TAD at the receiver satellite, and the maximum log BER was observed in the case of LG0,0 (\(-\)15). Whereas LG0,0 encountered 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. We considered LG mode profiles with larger differences such as LG00, LG140, and LG400 to realize OAM multiplexing, and it was observed that mode coupling becomes insignificant in these distant mode profiles as compared to the reported research where LG/HG modes with lesser gaps are considered [1; 43; 44; 58]. We performed an extensive literature survey and found that the highest capacity that has been reported so far in OSC is 4 [44], 3.84 [61], 1.6 Tbps [62], and 320 Gbps [63]. Therefore, an ultra-high capacity 4.8 Tbps OSC-OAM system was presented for the first time in this work. Further, we explored the effects of DSP on the OAM-based OSC system and it was observed that DSP had significant effects on the performance of the systems due to the compensation of NL, dispersion, phase errors, etc.. In the end, a state-of-the-art comparison revealed that the proposed OAM-OSC system is better than reported OSC systems, as per the authors' best knowledge. In the near future, more OAM beams can be incorporated into the system.  
Conceptualization, S.S.; data curation, R.A. and M.S.; formal analysis, R.A., M.S. and K.A.; funding acquisition, G.P.J. and I.C.D.; methodology, S.S. and S.K.; project administration, G.P.J. and I.C.D.; resources, W.C. and G.P.J.; software, S.K.; Supervision, W.C. and I.C.D.; validation, R.A. and K.A.; visualization, K.A.; writing--original draft, S.S.; writing--review and editing, G.P.J. All authors have read and agreed to the published version of the manuscript. This work was supported by Hankuk University of Foreign Studies Research Fund (of 2023). Institutional Review Board Statement: Not applicable.  
Data Availability Statement: No new data were created or analyzed in this study. Data sharing is not applicable to this article.  
The authors declare no conflict of interest.