## 1 Introduction  
The rapid evolution of today's telecommunications networks is driven by users' need to stay connected anytime, anywhere. From the intelligent telephone network to today's high-speed wide area networks, the rapid evolution of communication networks has been accompanied by the people's social needs, from the growing demand for users to new applications. Even with the arrival of the fifth-generation networks (5G), an excessive increase in the number of subscribers using smartphones and social networking applications makes the demand for high-speed data rates rise exponentially, which leads to a telecommunication bottleneck. As a result, the need to investigate new alternatives to the existing radio frequency (RF) infrastructure has become urgent. Communication research has already shifted from 5G to the beyond-5G (or sixth-generation, 6G) networks, labeled the B5G/6G era. The RF regime cannot afford the evolution of data-hungry applications requiring high-speed data transmission and receiving [1; 2; 3]. Instead, the optical domain, with frequencies a thousand times higher, is an envisaged solution to improve the network's throughput. Therefore, the upcoming networks are expected to use higher-frequency bands than are currently in use. Free space optics (FSO) is a communication system that uses free space as a channel for sending signals carrying information [4]. It is technologically very close to optical fiber communication (OFC) in terms of optical modulated signal transmission. It supports high data transmission, unlicensed spectrum, as it uses light, and a high level of secure transmission [5; 6]. Additionally, it has the good advantage that it can be implemented in places where the geographical locations make the installation of optical fiber difficult [7]. Figure 1 depicts the various areas where FSO can be implemented, such as hospitals, submarines, business buildings, and agriculture farms. As clear from Figure 1, a high-speed FSO link can help patients living in remote areas to be supervised by doctors in central hospitals. In addition, people who live in the regions that contain mountains can use the FSO link to transmit and receive their information, as in these areas, the installation of cables will be complicated. Additionally, in agricultural fields, people who live in smart households can monitor their work on their farms using the FSO link. Although FSO communication has several advantages, other external challenges such as weather conditions affect its performance. For example, the different weather conditions cause attenuation to the information signal, leading to a higher bit error rate value (BER) [8]. According to climate condition, the value of attenuation differs, as for clear weather, its value is less than 1 dB/km, while for heavy dust (HD) storms, its value becomes hundreds of dB/km [9]. Nowadays, researchers use different multiplexing techniques in FSO communication in order to increase the capacity and allow multiple users to transmit their data over the FSO channel. Some of these techniques are wavelength division multiplexing [10; 11], orthogonal frequency division multiplexing [12; 13], and polarization division multiplexing [14]. Recently, optical code division multiple access (OCDMA) has gained attention in FSO communication. It is based on the presence of light, as a binary \"1\" indicates the presence of light and wavelength, whereas a binary \"0\" indicates the absence of light. Multiple users  
Figure 1: Different areas where FSO link can be applied for 5G and beyond network. in OCDMA can use the same channel at the same time while transmitting data with a high level of security [6; 7]. In [12], the authors proposed an FSO system by integrating OFDM with mode division multiplexing (MDM) using a 4-QAM scheme. The results showed a successful transmission capacity of 40 Gbps under the effect of different dust storms.