## 5 Results and Discussion  
This section presents the simulation results considering the system model presented in the Section 4. Figure 3 shows the 3D representation of the received power from a group of four LED units at the transmitter end. As it can be seen, the transmitted energy from each LED is directed towards the central axis of the LED. The respective SNR is shown in Figure 4 where the area between the LED units experiences low SNR, as detected by the photo-receiver. The channel response in the time domain is shown in Figure 5 for the LOS and a strong non-LOS signal that degrades the performance of the actual transmission. Receiver diversity or the use of an appropriate MISO algorithm at the receiver can benefit from the multipath reflected signal at the cost of some complexity. The BER results are shown in Figure 6 for three cases using SISO transmission from single LED unit. The results are shown for (i) a LOS signal which is only affected by the atmospheric attenuation, (ii) a LOS signal with a reflected version from a shiny surface (like a junction safety convex mirror along the road) with strong impact, and (iii) when the LOS signal and reflected signal are received along with an interference signal from a neighboring optical source (such as a traffic light). The results for SISO shows that the threshold of \(3.8\times 10^{-3}\) has only been achieved for the ideal case, when no interference signal is present. The transmission is limited to approximately 7 m for such case and the system is unable to provide an appropriate performance with the reflections or interference signal. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Module** & **Parameter** & **Symbol** & **Value** \\ \hline \multirow{5}{*}{**Transmitter**} & Semi-angle of LED & \(\psi\) & 50\({}^{\circ}\) \\  & Number of LED units for MISO & \(m\) & 4 \\  & Angle of irradiance & \(\theta\) & 35\({}^{\circ}\) \\  & Angle of incidence & \(\varphi\) & 25\({}^{\circ}\) \\  & LED Bandwidth & \(B\) & 20 MHz \\  & Noise bandwidth factor & \(I\) & 0.5 \\ \hline \multirow{5}{*}{**Channel**} & Diffused reflectivity & \(\mu\) & 0.4 \\  & Angle of diffused reflection & \(\Phi\) & 70\({}^{\circ}\) \\ \cline{1-1}  & Distance between transmitter & \(D\) & 0–15 m \\ \cline{1-1}  & and receiver & & \\ \hline \multirow{5}{*}{**Receiver**} & Responsivity & \(R\) & 0.6 A/W \\ \cline{1-1}  & FOV of the PD & \(\Delta\) & 20\({}^{\circ}\) \\ \cline{1-1}  & PD aperture diameter & \(A\) & 10 cm \\ \cline{1-1}  & RF Bessel filter order & \(n\) & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Set of parameters used for the analytical model and their values for the simulation analysis. Figure 4: Estimated SNR for the four LED units. Figure 3: Received power (represented in the form of a 3D mesh) for the set of four LED units within the head light area of \(4\times 4\) cm\({}^{2}\). Figure 7 shows the improvement in SNR due to the array gain at the receiver. Comparing the results with those previously shown Figure 4, the SNR is evenly distributed here after MISO processing, hence increasing the reliability of the transmission. Figure 8 shows the comparison by using \(4\times 1\) MISO configuration and it can be seen that a significant amount of improvement is obtained for all three cases. Even in the presence of strong reflection and interference signal, the BER is under the limit of \(3.8\times 10^{-3}\) using the proposed approach. However, the transmission distance of only up to 1 m has been achieved in the presence of a strong interference signal. To resolve this issue, the proposed receiver presented in Section 2 is used and the results for the optimized operation are shown in Figure 9. The transmission distance has been increased up to 5 m for the case where both reflections and interference signal are present. Thus it can be concluded that the proposed optimum MISO technique can provide transmission over suitable ITS distances in the presence of real-world interference and noise.