## 1 Introduction  
Increasing system capacity and optimizing spectral efficiency in free space optical communication (FSO) is a major challenge. To address these challenges, we must adopt a new degree of flexibility in our approach in order to improve transmission capacity and spectrum efficiency [1]. Orbital angular momentum (OAM) technology, which uses twisted light beams with spiral spatial structure and an infinite number of natural orthogonal states, has received much attention in free space communication in recent years. By choosing suitable OAMmode multiplexing, a high-capacity and spectrally efficient FSO system can be developed using these intrinsic orthogonal states [2]. When this OAM technology is integrated with orthogonal frequency division multiplexing (OFDM), along with a mode diversity scheme, the capacity of the FSO link can be further improved. However, the transmission of OAM-OFDM multiplexed spatial beams through the atmospheric turbulence (AT) channel has rarely been investigated and studied in the literature. Hence, it is a key issue to analyze the impact of AT on the capacity of the transmitted OAM-OFDM multiplexed signal during propagation [3]. OAM multiplexing has been successfully implemented with a spectral efficiency of 95.7 bps/Hz and a system capacity of 2.56 Tbps by [PERSON] et al. [4] at 193.4 THz in the optical domain. A channel capacity of 100.8 Tb/s has been achieved in [PERSON] et al. [5] by incorporating OAM and polarization with wavelength division multiplexing (WDM) techniques. To achieve this high transmission capacity, the proposed system has successfully transmitted a total of 12 OAM channels, with each channel carrying out 100 Gbps of QPSK-modulated data at two different polarizations over 42 different wavelengths. A new approach to OAM beams using spatial modulation (SM) based millimeter wave communication systems, which employ spatial distribution characteristics, has been proposed to achieve a 341.6% improvement in energy efficiency over MIMO systems. The system performance has been analytically evaluated using performance metrics such as capacity, average bit error probability, and energy efficiency and has been identified as robust against path loss attenuation and suitable for long range transmission [6]. [PERSON] et al. [7] have proposed and analytically evaluated the performance of the capacity model of an OAM based MIMO wireless communication system. The simulation results showed that the OAM-MIMO wireless model outperformed conventional OAM and MIMO transmission systems in terms of transmission capacity over a long transmission range. The simultaneous transmission of spatially orthogonal and OAM multiplexed signals through a single aperture pair has evolved as a novel method to increase the transmission capacity by multiple folds with minimal crosstalk [8]. The proposed system has been designed to investigate the power loss and channel crosstalk of the link in the case of limited-size receiver apertures, as well as pointing and lateral errors between the transmitter and receiver [8]. [PERSON], [PERSON], et al. [9] have demonstrated a single aperture based, eight-OAM multiplexed FSO system. At a BER threshold of \(3.8\times 10^{-3}\), 32 Gbps of multiple coaxial information was transmitted over a 2.5-m link distance to achieve a spectral efficiency of 16 bit/s/Hz with crosstalk less than \(-12.5\) dB. Ref. [10], which has been investigated for crosstalk mitigation issues, is a 4 \(\times\) 4 OAM-MIMO FSO system using adaptive equalization techniques employing heterodyne detection. Each OAM channel has transmitted a 20 Gbps QPSK signal under weak turbulence conditions. The numerical results demonstrate that the signal quality can be improved by using MIMO processing techniques, which help in reducing the power penalty by greater than 4 dB at a threshold BER of \(3.8\times 10^{-3}\). In the article [11], the advantages of MIMO based spatial multiplexing technique have been incorporated in OAM multiplexing to increase the FSO system capacity.