## 4 Results and Discussions
To achieve the BER of \(10^{-8}\) spatial diversity, OOK needs 8.8 dB SNR, and this SNR falls significantly to 5.1 dB in the case of the \(2\times 2\) BPSK link. Therefore, from the analysis, it can be seen that the FSO links with spatial diversity deliver improved SNR gains in contrast to single-channel links. The BER analysis discussed in Figure 7 was extended to moderate and strong turbulence conditions in Figures 8 and 9, respectively. Table 3 outlines the performance of different modulation techniques under various turbulence regimes for FSO links with and without spatial diversity. It may be seen from Table 3 that under all possible cases discussed here, it is the BPSK modulation technique that requires minimum SNR (dB) at the receiver to deliver the target BER performance. Furthermore, decent SNR gains (dB) are observed across all modulation formats when single-channel links are complimented with SDRT. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Modulation** & **Weak** & **Moderate** & **Strong** \\
**Technique** & **Turbulence** & **Turbulence** & **Turbulence** \\ \hline OOK & 19.9 & 33 & 45.2 \\ DPSK & 10 & 15.3 & 21.5 \\ BPSK & 8 & 14.3 & 19.5 \\
256 QAM & 8.4 & 15.1 & 21 \\
64 QAM & 9.4 & 17.3 & 23.5 \\
16 QAM & 11 & 20.7 & 28 \\ GMSK & 9.2 & 16.8 & 23 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: SNR values (in dB) for modulation techniques without SDRT at BER \(10^{-8}\) for weak, moderate and strong regimes. Figure 6: BER performance of different modulation formats without SDRT in a strong turbulence channel. Figure 8: BER performance of different modulation formats with SDRT and without SDRT in a moderate turbulence channel. Figure 7: BER performance of different modulation formats with SDRT and without SDRT in a weak turbulence channel. Percentage \((\%)\text{gain}=\dfrac{(\text{Gain without SDRT in regime})-(\text{ Gain with SDRT in regime})}{\text{Gain without SDRT in regime}}\times 100\) (14)  
Figures 10-12 highlights the BER performance of DH-PIM with and without SDRT under weak, moderate, and strong turbulence channels. To achieve the target BER of \(10^{-8}\), DH-PIM modulated FSO link without SDRT will require 79 dB SNR at the receiver which falls to 32 dB in presence of spatial diversity thus yielding a significant gain of approximately 47 dB. Similarly, as seen from Table 4, under moderate and strong turbulence conditions, the SNR gain achieved with SDRT is 17 dB and 11 dB, respectively. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Weak Turbulence Regime**} & \multicolumn{3}{c}{**Moderate Turbulence Regime**} & \multicolumn{3}{c}{**Strong Turbulence Regime**} \\ \cline{2-10}
**Modulation** & \begin{tabular}{c} **Without** \\ **SDRT** \\ **(dB)** \\ \end{tabular} & \begin{tabular}{c} **With** \\ **SDRT** \\ **(dB)** \\ \end{tabular} & \begin{tabular}{c} **SNR** \\ **(dB)** \\ \end{tabular} & \begin{tabular}{c} **Without** \\ **(dB)** \\ \end{tabular} & \begin{tabular}{c} **With** \\ **SDRT** \\ **(dB)** \\ \end{tabular} & \begin{tabular}{c} **SNR** \\ **(dB)** \\ \end{tabular} & \begin{tabular}{c} **Without** \\ **SDRT** \\ **(dB)** \\ \end{tabular} & \
\begin{tabular}{c} **With** \\ **SDRT** \\ **(dB)** \\ \end{tabular} &
\begin{tabular}{c} **SNR** \\ **(dB)** \\ \end{tabular} \\ \hline OOK & 19.9 & 8.8 & 55.7 & 33 & 26 & 21.2 & 45.2 & 37.2 & 17.69 \\ \hline DPSK & 10 & 5.25 & 47.5 & 15.3 & 13 & 15 & 21.5 & 18 & 16.27 \\ \hline BPSK & 8 & 5.1 & 36.25 & 14.3 & 12.5 & 12.5 & 19.5 & 17 & 12.82 \\ \hline
256 QAM & 8.4 & 5 & 40.47 & 15.1 & 13.2 & 12.5 & 21 & 18.2 & 13.33 \\ \hline
64 QAM & 9.4 & 5.6 & 40.42 & 17.3 & 15 & 13.2 & 23.5 & 20.2 & 14.04 \\ \hline
16 QAM & 11 & 6.5 & 40.9 & 20.7 & 18 & 13 & 28 & 24.8 & 11.42 \\ \hline GMSK & 9.2 & 5.7 & 38 & 16.8 & 14.6 & 13 & 23 & 20 & 13.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: SNR values (in dB) for modulation techniques with SDRT and without SDRT at BER \(10^{-8}\) for Weak, moderate and strong regimes.