## 5 Results and Discussion  
The WDM-FSO architectures in Figures 1 and 2 were designed and analyzed using the Optiwave OptiSystem simulation tool for optical system design [41]. The parameters used in the system simulation are listed in Table 2. Figure 3 depicts the spectrum of the proposed FSO system after the gain flattening EDFA stage of amplification. It is clear that the proposed 32\(-\)channel FSO link has achieved gain flatness and that the power levels on all channels have been stabilized. Firstly, we evaluated the general WDM-FSO architecture shown in Figure 1. The performance of the FSO signal was investigated by measuring the bit error rate (BER) versus the optical signal-to-noise ratio (OSNR). The BER results after 500 m FSO transmission for five selected WDM signals: 1552.52 nm (\(\lambda_{1}\)), 1549.72 nm (\(\lambda_{8}\)), 1546.52 nm (\(\lambda_{16}\)), 1542.94 nm (\(\lambda_{24}\)), and 1540.16 nm (\(\lambda_{32}\)) are shown in Figure 4. The results indicate that, as would be expected, the BER improves as the OSNR increases. In addition, the BER deteriorates gradually when the FSO signal changes to a longer wavelength. For instance, at OSNR = 32 dB, the BER rises from \(2.5\times 10^{-4}\) for \(\lambda_{1}\) to \(4.1\times 10^{-5}\) for \(\lambda_{32}\). We also indicate in Figure 4 the forward error correction (FEC) limit [42], which represents the maximum BER level at which FEC could be used to bring the FEC to an acceptable level (\(\sim\)10\({}^{-9}\) for the level \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Value** \\ \hline Laser power & 10 dBm \\ \hline Length of FSO link & 500 m \\ \hline FSO attenuation & 3 dB  km\({}^{-1}\) \\ \hline Beam divergence (\(\theta\)) & 2 mrad \\ \hline Transmitter aperture diameter (\(d_{T}\)) & 5 cm \\ \hline Receiver aperture diameter (\(d_{R}\)) & 20 cm \\ \hline No. of WDM channels & 32 \\ \hline Spacing between WDM channels & 50 GHz \\ \hline Responsivity of PIN & 1 A W\({}^{-1}\) \\ \hline PIN thermal power density & \(15\times 10^{-24}\) WHz\({}^{-1}\) \\ \hline PIN dark current & 10 nA \\ \hline Gain of electrical amplifier & 10 dB \\ \hline Electrical amplifier power spectral density & \(-\)60 dBm  Hz\({}^{-1}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation parameters. Figure 3: Spectrum of proposed 32-channel WDM-FSO system. shown). The OSNR of the WDM signals changes slightly from 28 dB for \(\lambda_{1}\) to 27.5 dB for \(\lambda_{32}\), respectively, at this FEC limit of \(\sim\)\(3.8\times 10^{-3}(-2.4)\). Figure 5 depicts the BER performance of the FSO communication system for various link ranges. Again, in line with expectations, the BER performance of the system deteriorates as the transmission distance increases. It is clear from the results in Figures 4 and 5 that when the FSO signal moved to the longer wavelength position gradually, the BER performance slowly degraded. At the FEC limit, for instance, 1552.52 nm (\(\lambda_{1}\)) could reach \(\approx\)1200 m, whereas 1540.16 nm (\(\lambda_{32}\)) could reach \(\approx\)1400 m, respectively. Consequently, the aggregate FSO signal was capable of 1200 m free space transmission. Figure 4: BER versus OSNR plots for selected WDM signals. Figure 5: BER versus Link range plots for selected WDM signals. Secondly, we have evaluated the performance of the bidirectional WDM-FSO architecture in Figure 2. Figures 6 and 7 show the monitored BER performance of the OFDM downstream and OOK upstream FSO traffic, respectively, using wavelengths 1552.52 nm (\(\lambda_{1}\)), 1549.72 nm (\(\lambda_{8}\)), 1546.52 nm (\(\lambda_{16}\)), 1542.94 nm (\(\lambda_{24}\)), and 1540.16 nm  
However, for the uplink, it is plain from Figure 7 that there is a slight variation in BER with the increase in OSNR for each WDM signal, which is expected as the RSOA is operating in the gain saturation region. Furthermore, it is apparent that the BER performance worsens as the FSO signal gradually changes to a longer wavelength (from \(\lambda_{32}=1540.16\) nm to \(\lambda_{1}=1552.52\) nm).