## 1 Introduction
The performance of hybrid PDM with SAC-OCDMA was also evaluated in [14], and the obtained results showed successful transmission of 10 channels with 50 Gbps of information capacity. Recently, space division multiplexing (SDM) has been introduced into FSO communication that is based on the spatial distribution of electromagnetic (EM) waves. Mode division multiplexing (MDM) is one type of SDM in which multiple independent information data can be transmitted using two or more overlapping orthogonal modes, leading to transmission capacity enhancement [15]. Orbital angular momentum (OAM) modes are considered suitable for carrying the information data in FSO communication systems, among the other orthogonal modes employed. Its orthogonal nature makes it easier for multiplexing or de-multiplexing with low inter-channel crosstalk [16]. Additionally, the intensity of OAM beams has a donut ring pattern, while as these beams propagate, their phase rotates in a helical pattern. The generation of the OAM beams through the diffractive optics can be achieved by three methods. These methods are diffractive optical elements, computed-generated holograms, and spiral phase plates [17]. As for detecting the OAM beams, researchers proposed several methods to detect them. The far-field diffraction patterns of triangular apertures, wedged optical flats, twisted phases, Fresnel bi prisms, and tilted converging cylindrical or spherical lenses are used in these methods [18]. Furthermore, to increase the overall FSO system capacity, researchers integrate other multiplexing techniques with OAM such as QPSK [19], WDM and PDM [20], and OFDM [21]. The performance of OFDM in FSO using 16-quadrature amplitude modulation (QAM) under fog conditions was studied for two Indian cities in [22]. The conducted results show improvements in the system performance. Still, the authors did not consider the separate compensation mechanism to solve the effects of weather conditions of the channel on the received signal. This is clear from the constellation plots of the received signal that have unwanted phase shifts. In [23], the authors used spectrum slicing with WDM in the FSO system and studied the system's performance for the weather conditions of southern India. They used 16 channels; each carried 1.56 Gbps information. In [24], the performance of WDM in the FSO system was studied for desert areas. The proposed model achieved low data rates ranging from 0.3 Gbps to 0.7 Gbps. However, the authors used laser power of 50 dBm, which is not safe for human eyes [25]. The performance of an FSO system that uses QAM and quadrature phase shift keying (QPSK) modulation was studied by authors in [26]. As multiple symbols are transmitted simultaneously in these modulation techniques, information may be lost due to inter-symbol interference. In [27], the authors studied the performance of four independent channels transporting 10 Gbps of data transmitted on four distinct OAM beams in an FSO system under various weather conditions. The obtained results revealed successful transmission of 40 Gbps. QPSK modulated signals transported using two different OAM beams were demonstrated experimentally in [28]. The authors concluded a successful transmission of 80 Gbps unmanned aerial vehicle (UAV) to ground terminal FSO. This paper proposes a novel FSO communication system that integrates two OAM beams with spectral amplitude coding optical code division multiplexing (SAC-OCDMA) using fixed right shift code (FRS) for capacity improvement and secure transmission of data. The Main Contributions of This Paper  
* Enhance capacity through proposing a new FSO system based on integrating OAM with OCDMA using FRS code. * Assign FRS code with OCDMA and use single photodiode detection (SPD) technique. * Study the performance of the proposed FSO model under different foggy conditions. * As there is lack of study of the effect of dust storms, we study the performance of our model under different dust storms.