## 4 Numerical Results and Discussions  
This section illustrates the impact of aperture selection on the error performance of an MISO-FSO system, transmitting OOK signals from \(N\) user transmitters, and under attack from a jammer; the legitimate signals and jamming signal suffer from channel fading following negative exponential statistics with PE. The derived expressions in the preceding sections are utilized to arrive at the analytical results; simulation results are realized by Monte-Carlo simulation after modeling the defined channel fading conditions. At the receiver, we take \(\kappa\) to be 10 in all the plotted graphs. The PE parameter is taken to be 4.5856 and 1.7196 for user transmitters and jammers, respectively. The PE parameter of the user transmitter is higher to represent the condition that user transmitters are designed to be more aligned to the receiver. In all figures, the x-axis denotes the average SJNR per bit. In Figure 4, we plot the analytical expression for ABER under the condition of an always-on jammer (\(\rho=1\)) with a varying number of user transmitters deployed from (39). The results from the simulation are also plotted on the same graph. The match between the analytical and simulation plots proves the accuracy of the derived expression. It can be observed that as the number of user transmitters increases, the ABER performance of the system improves significantly. At \(\gamma=20\) dB, we can see that the ABER drops drastically as \(1.668\times 10^{-2},3.587\times 10^{-3},1.132\times 10^{-3},2.91\times 10^{-4},1.8 \times 10^{-4}\) for \(N=2\), \(3\), \(5\), \(7\), and \(8\), respectively. For comparison purposes, we also plot the simulation result of a single-input single-output (SISO)-FSO system. It is noted that for \(\gamma=30\) dB, transmit aperture selection reduces the bit error from \(2.355\times 10^{-2}\) to \(1.664\times 10^{-4}\), by allowing the system to choose the maximum of three-channel gains. Moreover, we observe that with an increase in \(\gamma\), the improvement in bit error escalates. The diversity order of the plots for \(N=2\) and \(3\) are \(0.9970\) and \(1.494\), respectively, and thus in agreement with the derived results. **Remark 3**. : _The aforementioned discussion confirms the efficiency of the transmit aperture selection technique to improve the error performance of a jammer-attacked FSO system, while also significantly reducing the system's power requirements to operate multiple transmitters._  
Figure 5 plots the error performance of a \(2\times 1\) FSO system for different degrees of jammer activity; analytical results are obtained from (44). The figure verifies that as the jammer activity increases, the system's error performance degrades. When the value of \(\rho\) widens from \(0.05\) to \(1\) at \(\gamma=10\) dB, the bit error also jumps up from \(0.02916\) to \(0.08141\). This demonstrates the grievous effect of a jammer as compared to the Gaussian noise. Moreover, as the \(\rho\) increases from \(0.1\) to \(0.3\), the ABER inflates from \(0.03704\) to \(0.05567\), and an increase in \(\rho\) from \(0.3\) to \(1\) results in an ABER hike from \(0.05567\) to \(0.08141\); this shows that as the jammer's activity increases, the effect of Gaussian noise on ABER diminishes. Furthermore, it can be noted that the difference in the error performance reduces as the \(\gamma\) increases. This is evident from the gap of \(9.108\times 10^{-2}\) and \(9.29\times 10^{-4}\) at \(\gamma=5\) dB and \(27.5\) dB when the \(\rho\) rises from \(0.05\) to \(1\). This is because as the legitimate signals are transmitted with higher power, the effect of the varying degree of the jammer's activity reduces on the bit error performance. Figure 4: ABER versus SJNR (dB) curves for FSO system with varying number of user transmitters in presence of jammer (\(\rho=1\)). **Remark 4**. : _The prior analysis shows that the jammer has a more pronounced effect on the ABER of the FSO system than the Gaussian noise.