# Performance Analysis of Hybrid PDM-SAC-OCDMA-Enabled FSO Transmission Using ZCC Codes
## 1 Introduction  
Optical code division multiple access (OCDMA) transmission has recently become very popular in the field of optical communication systems. It can allow multiple users to access the same channel using the same spectrum simultaneously, with a high level of security using a unique code [1, 2, 3, 4]. SAC is used with OCDMA as it can eliminate the existence of multiple access interference in OCDMA due to the cross-correlation property of OCDMA codes that affect system performance. SAC-OCDMA codes are classified into two types: codes that have zero cross-correlation where no MAI exists and codes that have cross-correlation equal to or less than one. Zero-cross correlation codes use direct detection techniques while others use suitable detection techniques such as AND-subtraction, complementary subtraction, and single photodiode (SPD) detection techniques [5, 6]. Nowadays, optical-wireless-communication (OWC), also known as free-space-optics (FSO) communication, is preferred over radio frequency (RF) communications [7, 8]. It provides high speed data transmission, secure communication, no electromagnetic waves exist, and it does not require a license [9]. Additionally, it can be installed everywhere as it uses the atmosphere as a medium to transmit the information signal between transmitter and receiver; as such, it requires a line of sight (LOS) link [10, 11]. FSO networks provide high bandwidth wireless communication links that are desirable for meeting the demand of bandwidth hungry applications including social networking websites, video-conferencingevents, high-definition television, cloud computing, and over-the-top platforms such as Amazon prime and NETFLIX, etc. Since FSO communication has many advantages, it can be applied in next generation applications that require high transmission speeds, such as the transmission of data between high altitudes platforms and optical ground stations (requires data transmission greater than 100 Gbps), fixed wing aircraft and hospitals (requires data transmission greater than 1 Gbps), and quadcopter drones and residential buildings, as shown in Figure 1.  
However, various external weather conditions (fog, dust, and snow) cause optical beam attenuation, further affecting the received signal and causing degradation in its performance [12; 13; 14]. Recently, the demand for high transmission capacity has become essential due to the expeditious rise in the number of internet, mobile, and online video gaming users. To convoy this growth, researchers use one or more multiplexing techniques integrated in FSO transmission systems, such as orthogonal frequency division multiplexing (OFDM) with SAC-OCDMA [14], polarization division multiplexing (PDM) with SAC-OCDMA [15; 16], and SAC-OCDMA with orbital angular momentum (OAM) [17] for capacity enhancement. In our present work, a novel SAC-OCDMA with PDM in FSO transmission using zero-cross-correlation code (ZCC) is proposed. This code has a simpler construction than the other codes that have been discussed for use in FSO systems. Six users, each carrying 20 Gbps binary information, are transported on two different orthogonal polarized laser signals (\(X_{Polar}\) and \(Y_{Polar}\)). Each user is assigned a different ZCC code sequence. The system performance under external weather conditions is considered. These conditions are: clear weather (CW), light-fog (LF), medium-fog (MF), heavy-fog (HF), and different snowfall conditions, including wet snow fall (WSF) and dry snow fall (DSF). Log(BER), Q-factor, and eye diagrams are used for evaluating the performance of the proposed model. The rest of the paper is ordered as follows. Section 2 shows previous studies conducted using different multiplexing techniques in FSO communication systems. Section 3 explains general FSO channels affected by attenuation due to various climate conditions and the code construction of ZCC, followed by a description for the proposed PDM-SAC  
Figure 1: Different areas where FSO link can be applied for 5G and Beyond Network.