## 4 Investigation of Proposed OAM-OSC System at Different Input Parameters  
This section covers the detailed performance analysis of the proposed system at different input parameters such as distance between satellites, pointing errors, and receiver antenna diameters, and we further explore the performance of each OAM beam in the system in terms of BER, EVM, and constellations. For the investigation of the proposed OAM-OSC system, a prominently used simulation environment, Optisystem, is considered. Figure 7 illustrates the presence of five different wavelengths in the system after the WDM multiplexer in the c-band has a power of 26.7 \(\pm\) 0.05 dB. Checking of OCSs after regular intervals is required to see the signal availability and any unusual drop or significant power loss that indicates the link breakdown or any obstruction between the transmitter and receiver. Light profiles of different LG modes such as LG0,0, LG14,0, and LG40,0 are represented in Figure 8a-c followed by the OAM multiplexed modes in Figure 8d. We have considered an appropriate (optimal) difference in 1 so that only minimum mode coupling occurs and is restricted to a maximum value of 40 because detection becomes difficult at higher-order modes. The majority of satellite communications take place in space, where there is no atmosphere and only a vacuum exists, resulting in less power loss for the signal. However, the distance between satellites is the major factor that derives the final performance of received signals. Therefore, the OSC link length is increased from 10,000 to 24,000 km in the proposed investigation to find out the maximum supported distance by OAM-OSC. BER is validated for different OAM beams at different OSC links as shown in Figure 9.  
Figure 5: Components of the PDM-256-QAM receiver. Figure 6: DSP module with 8 functions, pre-processing, and different algorithms. Figure 8: Transverse mode profiles (**a**) LG0\(0\),_ (**b**) LG14\(0\),_ (**c**) LG40\(0\), and (**d**) OAM multiplexed beams. Figure 7: Representation of OCSs of different wavelengths. With the increase in the OSC link length, symbols at the receiver deteriorate and displace from their ideal positions, which in turn increases the BER. Furthermore, phase errors in the 256-QAM symbols, attenuation, scattering, and nonlinear effects at high power also introduce bit errors. It is discerned that all the OAM beams have different output BER such that fundamental mode LG0,0 has the least BER due to narrow beam mode and LG40,0 has the highest BER because of the power losses at the receiver due to higher order mode. In this case, the receiver antenna diameter is 15 cm, and therefore, small-size antennas are sometimes not wide enough to accumulate all the power of higher-order modes. However, all the OAM beams have covered 22,000 km successfully within the acceptable log BER condition i.e., \(-\)2.42 [40]. Relay communication in OSC systems plays an important part in covering ultra-long transmission distances between satellites, and LoS is the utmost condition to be satisfied. Slight misalignment in the LoS can cause severe bit errors and performance degradation. Moreover, in the case of OAM beams, the spot size of LG modes increases with the distance, and slight misalignment can cause high mode power loss due to the small diameter of the OSC receiver telescope. Therefore, in Figure 10, we investigated the effects of different pointing errors on OAM beams in terms of EVM%. Maximum and minimum pointing errors investigated are 0.5 to 4 urad and it is observed that the lowest spot size of LG0,0 experienced the lowest symbol deterioration due to the highest RAP and, in turn, provided the lowest EVM%. The value of different EVM% for LG0,0 is 15.01, for LG14,0 is 16.99, and for LG40,0 is 19 at 0.5 urad, and for increased pointing errors such as 4 urad, 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 is tedious for the receiver antenna to detect the entire mode power.