# Fault Tolerant Spectral/Spatial Optical Code Division Multiple Access Passive Optical Network
## 3 Performance Analysis  
This section analyzes the performance of the proposed architecture in terms of bit error rate (BER) and eye diagrams. The analysis is performed for a total of 128 subscribers in Optisystem. The proposed model is implemented with 32 \(X^{th}\) code sequences and 4 \(Y^{th}\) code sequences. The given number of subscribers is implemented with 64 LDs that are centered at 191.5 THz, 191.6 THz, and onwards until 197.9 THz. It is pertinent to mention that the utilization of 32 \(X^{\text{th}}\) code sequences and 4 \(Y^{\text{th}}\) code sequences enables the implementation of the proposed setup's broader wavelength spectrum, which also increases the overall spectral efficiency of the system. Moreover, a wider wavelength spectrum minimizes crosstalk and interference between channels. If the wavelength range is too close, it can cause overlap in the frequency domain, leading to increased noise and potential errors in data transmission. Furthermore, four remote nodes are employed at the feeder level, and each remote node is assigned a total of 32 subscribers. For instance, \(RN_{1}\) is assigned the subscribers with spectral code sequences from \(X_{0}\) to \(X_{31}\) and a spatial code sequence of \(Y_{0}^{T}\). Similarly, \(RN_{2}\) is assigned the subscribers with spectral code sequences from \(X_{0}\) to \(X_{31}\) and spatial code sequences ranging from \(Y_{1}^{T}\). Moreover, four time delay instances, from \(TDU_{0}\) to \(TDU_{3}\), are employed for simulation implementation, with reference of the four \(Y^{th}\) code sequences. The performance parameters that have been utilized to perform the given analysis are presented in Table 2.  
For the preliminary analysis, a back-to-back (B2B) model is utilized, and the BER is measured against different data rates for simultaneously accessing subscribers. It can be observed from Figure 5 that the BER, which is given as a log of the BER, increases with an increase in the amount of data that is being transmitted over the shared medium. As the data rate increases, the bit durations become shorter, which can cause pulse spreading \begin{table}
\begin{tabular}{l c} \hline \hline \multicolumn{1}{c}{**System Parameter**} & **Value** \\ \hline Signal power at CO & 0 dBm \\ \hline OF attenuation & 0.25 dB/km \\ \hline OF dispersion & 18 ps/nm/km \\ \hline Receiver’s responsitivity & 0.85 A/W \\ \hline User data & 10 Cbits/s \\ \hline Data format & NRZ \\ \hline Thermal noise & \(7.5\times 10^{-15}\) \\ \hline Distribution of noise & Gaussian \\ \hline OF length (feeder level) & 10 km \\ \hline OF length (between RNs) & Variable \\ \hline \hline \end{tabular}
\end{table} 
Table 2: System performance parameters for simulation analysis. and result in signal overlap, leading to inter-symbol interference. Additionally, the signal power is spread over a larger bandwidth at higher data rates, which can reduce the power spectral density [28]. However, the robust correlation properties of the DW-ZCC code and utilization of the spectral direct detection technique make sure that only the intended spectrum is recovered with maximum auto-correlation and minimum cross-correlation. Consequently, the proposed setup is able to support a data rate of up to 10 Gbps. The proposed system's performance is further analyzed by observing its reach in terms of the BER values through varying optical fiber (OF) lengths. The analysis is performed at randomly selected ONT nodes at each RN while 128 users access the medium simultaneously at 10 Gbps of data per subscriber. Moreover, the length of the OF medium is increased by a factor of 1 km between the adjacent RNs for each analysis. The system performance parameters in Table 2 are utilized for this study. It can be observed from Figure 6 that the proposed system can provide relatively lower values of BER, in the order of \(10^{-18}\), at 10 km of the deployed OF media between the transmitter and \(RN_{1}\).