# 6G Network Architecture Using FSO-PDM/PV-OCDMA System with Weather Performance Analysis
## 1 Introduction  
Free space optical communication (FSO) is a wireless optical communication that can be used as an alternative to the existing radio frequency (RF) infrastructure [1, 2]. FSO communication is a wireless data transmission method that uses a modulated optical beam directed through the atmosphere or vacuum as a communication channel between transceivers with the line of sight (LOS) [3, 4]. It possesses license-free, high-security, easy-to-employ in places where implementation of optical fibers is difficult due to geographical location, the transmission of excessive information with high data rates, and is immune to electromagnetic (EM) interference [5, 6, 7, 8]. These advantages make FSO transmission be used in 6G applications such as transmission of information between drones and buildings, vehicle to vehicle, hospitals, and drones, and inter-satellite. On the other hand, external and different weather conditions such as fog, dust storms, and snow become challenges during the transmission of data as they cause attenuation, leading to degradation of the received signal [8, 9, 10]. Recently, the optical code division multiple access (OCDMA) method gains more attention in either wired or wireless communication. It is based on the existence of light: binary \"1\" means there is light and wavelength while binary \"0\" means the absence of light [11]. In OCDMA, multiple users can share the same channel at the same time while transmitting their information with a high level of cardinality [12; 13]. Mostly, spectral amplitude coding (SAC) is used among various techniques with OCDMA. Each channel is assigned a unique code in SAC-OCDMA. Codes such as random diagonal (RD) [9], enhanced double weight (EDW) [14], and permutation vector (PV) [15] are used with SAC-OCDMA. Nowadays, to increase the capacity, researchers integrate different multiplexing techniques with SAC-OCDMA such as orbital angular momentum (OAM) [16], and orthogonal frequency division multiplexing (OFDM) [17], and wavelength division multiplexing (WDM) [18]. Polarization division multiplexing (PDM) is introduced to FSO communication by researchers [19]. In PDM, a single wavelength is used for carrying distinct signals and using orthogonal polarization states, which further leads to capacity enhancement [20]. In [17], the authors used OFDM multiplexing techniques in FSO communication. The results show successful transmission of 10 Gbps only under CA and fog conditions. Moreover, one channel is used. The authors in [21] suggested a hybrid FSO model formed by combining PDM with OCDMA using the RD code. Although the simulation results assure successful transmission of 100 Gbps carried by ten channels, the long code length of RD code makes it require complex components. Moreover, the results were obtained under fog conditions. In [14], the authors used PDM in FSO communication with OCDMA, using EDW code. As the EDW code has unity cross-correlation, it requires a suitable detection technique at the receiver to cancel the interference from other undesired channels, which makes its implementation expensive. Additionally, the results indicate successful transmission of overall 60 Gbps of information among six channels under CA, haze, rain, and fog conditions. In this paper, we propose a novel model of FSO-PDM/PV-OCDMA that is formed by integrating OCDMA with PDM and used in the FSO communication system. Four different channels, each carrying 20 Gbps information, are used with the proposed model for transmitting their information using two orthogonal polarization states (x and y polarizations). These channels are assigned with an OCDMA code which is a PV code. In addition, the effects of four different weather conditions on the FSO channel are considered in our study of the performance of the FSO-PDM/PV-OCDMA system: clear air (CA), different levels of fog (light fog (LF), medium fog (MF), and heavy fog (HF)), dust storms (low dust (LD), medium dust (MD), and heavy dust (HD)), and snowfall (dry snow (DS) and wet snow (WS)).