## 2 Proposed System Schematic Model  
Figure 1 depicts the concept and working principle of the OAM based spatial multiplexed FSO system model, while Figure 2 illustrates the detailed experimental setup of the proposed OAM-OFDM-SMM-MIMO spatial multiplexed FSO system, which incorporates OFDM and adaptive SMM and SMD multiplexing schemes. Figure 1: Schematic concept of OAM based spatial multiplexing in the FSO system. In Figure 1, the collimated Gaussian laser beams are produced to carry independent and orthogonal data streams at a wavelength of 1550 nm. Spiral phase mask (SPM) converts the high-speed 120 Gbps binary data into OAM mode division multiplexed (OMDM) signal. These OAM spatially multiplexed (SMM) beams are transmitted through a pair of transmitter lenses having \(f_{1}\) and \(f_{2}\) focal lengths, respectively, before transmitting over the turbulent transmission link. The equivalent focal length of this pair of lenses is given by  
\[\frac{1}{f_{0}}=\frac{1}{f_{1}}+\frac{1}{f_{2}}-\frac{d_{f}}{f_{1}f_{2}} \tag{1}\]  
where, \(\Delta=f_{1}+f_{2}-d_{f}\) is the lens offset and \(d_{f}\) is the center-to-center lens spacing. Figure 2: Schematic of the OAM-OFDM-SMM-MIMO spatially multiplexed FSO system. Through the careful design and adjustment of this lens pair offset, we can not only focus the transmitted OAM beams at the receiver end, but also help to minimize the transmitted signal power losses. This also makes the OAM-multiplexed FSO link robust against pointing error [15]. As shown in Figure 2, the OAM-OFDM-SMM-MIMO FSO optical transmitter consists of \(N\) independent OFDM subsystems. A 16-QAM-modulated real-valued discrete multi-tone (DMT) electrical signal is produced by a random waveform generator (AWG) at a sample rate of 25 G samples per second. These signals are further modulated in the OFDM modulation subsystems and multiplexed with different orthogonal OAM states. These M = 65,615 subcarriers, are used to transmit the data, with the effective bandwidth of 9.72 GHz of the DMT signal. A cyclic prefix of 40 samples is padded in front of each DMT signal. The \(j_{th}\) OAM states are chosen from the set S = \(\{-K,\)\(-K+1\)\(\ldots\)\(0\)\(\ldots\)\(K,\)\(K+1\) with the \(K\) as the maximum OAM state and the \(j_{th}\) of the OFDM modules is associated with this OAM states set. Total number of orthogonal OAM states generated are given by \(L=2K+1\) corresponding to OFDM symbols and, hence, share the same frequency spectrum in the OAM-OFDM-SMM-MIMO FSO system. These frequency channels are further multiplexed to enhance the spectrum efficiency of the proposed system. The frequency \(f_{m}\) of the \(m_{th}\) OFDM subcarrier in each OFDM module is chosen from \(F=\{f_{0},f_{2},\)\(\ldots\)\(f_{m},\)\(\ldots\)\(f_{M-1}\}\) and given by  
\[f_{m}=\frac{m}{T_{s}}+f_{sc}\text{ for }0\leq m\leq M-1 \tag{2}\]  
where, \(T_{s}\) and \(f_{sc}\) are the symbol period and minimum subcarrier frequency, respectively. In the \(j_{th}\) OFDM subsystem, S/P block converts a set of serial symbols into low data rate \(M\) parallel symbols. Further, the \(m_{th}\) parallel symbol is modulated in frequency domain signal \(X_{m}^{j}\) and later this symbol is again transformed to a time domain signal \(S_{m}^{j}\) in an inverse. Fourier transform (IFFT) block.