## 7 Cost Analysis  
In this section, we analyze the CAPEX of the proposed architecture in comparison with the conventional 2D-spectral/spatial FSO system to determine the overall impact of introducing the time-delay units on the cost of implementation [49; 50; 51; 52]. For a fair analysis of the CAPEX, the following system parameters and components were considered for the proposed and conventional 2D-spectral/spatial FSO system:  
* Analysis was performed for sixteen nodes with \(K_{1}=4\) and \(K_{2}=4\), where \(K_{1}\) and \(K_{2}\) respectively represent the number of spectral and spatial codes. * Spectral/spatial encoders and decoders were the same for both architectures, as major modifications were made in the star coupler section through introduction of time-delay units for the proposed architecture. Therefore, analysis was carried out for the components between the power couplers and combiners. * The per-unit cost of the FSO transceiver module was considered in the analysis through a market survey. Figure 8a shows the block diagram of the conventional 2D-spectral/spatial FSO system, where each combiner and splitter pair represent a star coupler and relate to dedicated FSO links to avoid interference between repeated codes. Figure 8b shows the block diagram of the proposed architecture that employs time-delay units along with an optical combiner and optical splitter to replace multiple parallel FSO links with a single FSO path. Figure 6: (a) Plot of BER versus the receiver aperture diameter and (b) plot of the Q-factor versus the receiver aperture diameter. Figure 7: (a) BER versus beam divergence plot and (b) Q-factor versus beam divergence plot. Now, the total CAPEX required for development of both networks can be written as follows:  
\[CAPEX_{c}=OC_{K}+FSO_{K}+OS_{K} \tag{11}\]  
\[CAPEX_{p}=OC_{K}+TD_{K}+OC+FSO+OS+TD_{K}+OS_{K} \tag{12}\]  
where \(CAPEX_{c}\) represents the capital expenditure for the conventional 2D-spectral/spatial \(FSO\) system with \(K=4\) number of optical couplers (\(OC\)), \(FSO\) links, and optical splitters (\(OS\)). Similarly, \(CAPEX_{p}\) shows the total cost of implementation for the proposed model with \(K=4\) number of \(OCs\), time-delay units, and \(OSs\), along with installation of a single \(OC\), \(FSO\) link, and \(OS\) to facilitate the implementation of time-delay units to compensate for the use of multiple parallel \(FSO\) links. Table 6 represents the cost of each component shown in Figure 8[49,50]. The cost of each component for the proposed and conventional architectures are used in Equations (11) and (12) to determine the cost of implementation for each component in both architectures along with the total CAPEX for the entire architecture. Table 7 shows the CAPEX required for installation of each component in both architectures. The same analysis was performed for networks with a total of sixteen nodes and with \(K_{1}=4\) and \(K_{2}=4\). It can be observed that a major difference exists between the cost of implementing FSO links in the two architectures. This can be attributed to the proposed architecture having a reduced number of parallel FSO links, specifically, a single \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sr. No** & **Component** & **Cost (S)** \\ \hline
1 & Optical combiner (1:4) & 2.5 \\
2 & Optical splitter (1:4) & 2.5 \\
3 & FSO module & 15 \\
4 & Time delay units & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Cost of each component shown in Figure 8.  
Figure 8: Block diagrams of (**a**) the conventional 2D-spectral/spatial FSO system and (**b**) the proposed architecture with time-delay units. link, and the introduction of time-delay units with a relatively lower in cost in comparison to the additional FSO links. Similarly, the addition of extra components in the proposed architecture for implementation of time-delay units has only a minuscule effect on the cost of each component, as all of these components are relatively low in cost.