# 6G Network Architecture Using FSO-PDM/PV-OCDMA System with Weather Performance Analysis
## 3 FSO-PDM/PV-OCDMA System Description  
The schematic diagram of the proposed FSO-PDM/PV-OCDMA system is illustrated in Figure 1. Like any communication system, it consists of three parts (transmitter, channel, and receiver). At the transmitter, eight channels are assigned a PV code according to Table 1, and their wavelengths are generated from a continuous wave (CW) laser source. channels 1, 2, 3, and 4 are transmitted using x-polarization, which is delivered from a CW source at 0\({}^{\circ}\) azimuthal angle and the other channels (channels 5, 6, 7, and 8) are transmitted using y-polarization, which is delivered from a CW source at 90\({}^{\circ}\) azimuthal angle. Each of these channels carries 20 Gbps of information that is generated from a pseudo-random bit generator (PRBG) and a non-return to zero (NRZ) modulator. To modulate the information signal into the channels that are generated from optical signals coming from a CW source, a Mach-Zehnder modulator (MZM) is used. Further, the transmitters of the eight channels that come from different polarization signals (channels 1, 2, 3, and 4 on x-polarization and channels 5, 6, 7, and 8 on y-polarization) are combined through a PDM combiner, which further transmits to the FSO channel.  
Figure 1: Schematic diagram of FSO-PDM/PV-OCDMA system.  
The signal during its transmission in the FSO channel is affected by various weather conditions such as clear air (CA), fog conditions (low fog, LF, medium fog, MF, and heavy fog, HF), dust storms (low dust storm, LD, medium dust storm, MD, and heavy dust storm, HD), and snow (wet snow, WS, and dry snow, DS). These weather conditions cause attenuation which degrades the signal strength during transmission between transmitter and receiver. CA weather condition has the least attenuation that is 0.14 dB/km [16]. While the attenuation under fog conditions differs according to the dense of the fog and it can be expressed as [21]  
\[\alpha_{F}=\frac{3.912}{D}\bigg{(}\frac{\lambda}{550 nm}\bigg{)}^{-p} \tag{11}\]  
where \(\alpha_{F}\), \(D\) and \(\lambda\), respectively, are the fog attenuation in dB/km, the visibility in km, and the wavelength in nm while \(p\) is the size distribution of the scattering particle that can be determined according to the Kim model as [22]  
\[p=\begin{cases}\begin{array}{c}1.6D>50\\ 1.36<D<50\\ 0.16D+0.341<D<6\\ D-0.50.5<D<1\\ 0D<0.5\end{array}\end{cases} \tag{12}\]  
However, the large attenuation is caused under dust storms, which is expressed in dB/km as [23]  
\[\alpha_{D}=52\ \times\ D^{-1.05} \tag{13}\]  
where \(\alpha_{D}\) is the attenuation of the dust storm in dB/km. In the case of snow, the attenuation differs according to whether it is dry snow or wet snow, as dry snow refers to a low rate of snowfall while wet snow refers to a high rate of snowfall. The attenuation of snow, \(\alpha_{S}\), in dB/km can be expressed as [24]  
\[\alpha_{S}=\text{a}F^{b} \tag{14}\]  
where \(F\) is the rate of snowfall in mm/h. While a and b are parameters which in the case of dry snow and wet snow given as [24]  
\[\begin{array}{c}\text{In case of dry snow, }\text{b}=1.38,\text{ and }\text{a}=5.42\ \times\ 10^{-5}\lambda+5.49\\ \text{In case of wet snow, }\text{b}=0.72,\text{ and }\text{a}=1.02\ \times\ 10^{-4} \lambda+3.78\end{array} \tag{15}\]  
Finally, at the receiver, the received signal is divided by the PDM splitter into the two polarization signals (x and y polarizations) and the desired channel is detected by the decoder that has the same spectral as the encoder. The output signal then enters the photodiode (PD) detector for electrical/optical conversion which is further passed by a low-pass filter (LPF) for blocking the unwanted signals and a BER analyzer is used for checking the performance of the eight channels.