## 3 Simulation Design of Proposed PDM-256-QAM Modulated OAM-OSC System  
For OSC systems, single polarization QAM [57] and dual polarization QAM with different variants such as 4-QAM [37], 16-QAM [58], 64-QAM [59], and 128-QAM [60] have been reported several times and revealed that bandwidth efficiency of higher QAMs is better than conventional QPSK, DPSK, etc. modulations. Therefore, we employed dual polarization 256-QAM in our system. Figure 3 represents the block diagram of the proposed 4.8 Tbps OSC system using PDM-256-QAM and hybrid WDM-OAM multiplexing. A data rate of 320 Gbps is transmitted over each channel, and there are a total of 5 wavelengths, but a total of 15 channels are generated such that each wavelength is used thrice in the system. Three different LG modes are considered for each wavelength and distributed in a manner that three channels of the same wavelength receive different LG modes, e.g., LG0,0 for the first channel, LG14,0 for the second channel, and LG40,0 for the third channel in the case of all 5 wavelengths. In this way, the cost of multiple lasers is reduced by 1/3 rd using different LG modes that enable us to realize 15 channels from 5 wavelengths only. All the channels are modulated with PDM-256-QAM multilevel modulation due to its narrow spectrum, high tolerance against dispersion, and minimum level noise. As shown in Figure 4, first and foremost, a binary data stream at the rate of 320 Gbps is generated from the BER test set, followed by a serial to parallel converter (S/P). The major function of S/P data conversion is to provide bandwidth efficiency such that the bandwidth required for the transmission of serial data is higher than the parallel data bits. Figure 2: Different LG mode profiles at (**a**) \(l=0,\ m=0\) and (**b**) \(l=10,\ m=0\) in 2D. these horizontal and vertical SOP-based laser signals are given to the upper and lower Mach-Zehndar modulators, respectively. Further, data modulated on these two SOPs are combined with the help of a polarization controller (PC) and a particular LG mode is assigned to each channel. To accommodate all the channels, a WDM multiplexer is employed for different wavelengths, and an MDM multiplexer is incorporated to realize OAM multiplexing from different LG modes. WDM-OAM multiplexed signals are communicated over an OSC loop having an OSC channel, and pre and post-erbium-doped fiber amplifier (EDFA). Simulation specifications are listed in Table 1.  
Signals after traveling through the OSC channel reached the receiver section placed in the destination satellite. A de-multiplexer 1 \(\times\) 5 is placed to route all five wavelengths to the specific port and further, each wavelength is divided into three sub-channels. OAM beams are filtered at each sub-wavelength and followed by a PDM-256-QAM receiver as shown in Figure 5. Balanced detection employing PIN photodetectors, and phase synchronizations using local oscillators (LOs) are performed followed by electrical signal amplification. DSP module in the next section compensates for the performance deteriorating effects such as nonlinear effects, chromatic dispersion (CD), in-phase quadrature-phase (IQ) compensation, phase errors, etc., as illustrated in Figure 6. BPS, VPE, and CMA are the three important algorithms operating in DSP. A total of 256 symbols at different amplitudes and phases are then demodulated with a 256-QAM decoder, followed by the extraction of binary bits stored in the symbols. Binary data is in parallel form and then converted into a serial data stream. The BER test set in the end component compares the received and transmitted binary data bits and provides results in terms of BER at each polarization.