## 3 Results  
### Simulation Results  
To investigate the available optical power and channel loss for communication in the proposed smart-farming architecture, an analysis with a series of simulations is executed. In this investigation, a farming room of 3 m \(\times\) 2 m \(\times\) 3 m is assumed. Figure 2 demonstrates the geometrical representation of the room.  
\begin{table}
\begin{tabular}{c c c c} \hline  & **Red LED** & **Green LED** & **Blue LED** \\ \hline lm/W & 217.3 & 543.9 & 58.7 \\ \hline \end{tabular}
\end{table}
Table 2: Luminous efficacy of tested RGB LEDs.  
0.09 W/m\({}^{2}\), respectively. This means there is an optical channel loss of ~11 dB per unit area on average. When a photodetector with an active area of 1 cm\({}^{2}\) is used, an optical channel loss of ~31 dB on average is obtained. The standard deviation across the receiver plane is just 0.01 W/m\({}^{2}\) (~13% of the average), enabling a reasonably even downlink performance at any receiver location.  
Next, the uplink performance is evaluated. Vegetables have their own reflectance in each growing stage. In the maturity stage, the mean value of reflectance for red, green, and blue are 0.121, 0.22 and 0.067, respectively [30]. Due to its relatively high reflectance, the green light can be a good candidate for uplink communication. In this simulation, the modulated green LED light is reflected by the leafy vegetables, and it is scattered upwards. It is assumed that the vegetables act as Lambertian sources. The modulated and reflected light is then detected by an uplink receiver at 1.35 m above. Figure 4a,b show the intensity distribution. The simulation results show that the minimum, average, and maximum intensity achievable are 0.0077 W/m\({}^{2}\), 0.0137 W/m\({}^{2}\), 0.017 W/m\({}^{2}\), respectively. It means there is an optical channel loss of ~18.6 dB per unit area on average. Also, the standard deviation across the uplink receiver plane is 0.0016 W/m\({}^{2}\) (~12% of the average). Although the evenness is slightly worse than the case of downlink, the standard deviation is still considered reasonably small, allowing a reasonably similar uplink performance at any uplink receiver location except for the corners.  
Unlike the downlink, however, the uplink channel can be blocked by the grow light units as they are located right above the vegetables. The impact of this line-of-sight (LOS) blockage is simulated by placing ray-stopping objects with a size of 0.2 m \(\times\) 0.2 m at 0.3 m  
Figure 4: (**a**) Optical channel loss per unit area for uplink without blockage from grow light fixture; (**b**) its contour plot.  
Figure 3: (**a**) Optical channel loss per unit area for the downlink; (**b**) its contour plot.  
above the vegetables. The size and height are chosen from the practical dimensions of RGB LED units and recommended distances [26]. In the simulation, each ray passing through the locations of ray-stopping objects is terminated. Figure 5a,b present the estimated optical channel loss per unit area in this case. The results are similar to the case without the blockage except that there are clear shadows under the fixed grow LED units. The minimum (at the shadow areas), average and maximum values are 0.0058 W/m\({}^{2}\), 0.0129 W/m\({}^{2}\) and 0.017 W/m\({}^{2}\), respectively. The average optical channel loss is \(\sim\)19 dB. The standard deviation across the receiver plane is still 0.0017 W/m\({}^{2}\) (\(\sim\)13% of the average), which is slightly worse than the case without the blockage. The shadow areas still have \(\sim\)34% of the maximum value due to the contributions of the uplink lights from the other smart-farming units. When only a single, smart farming unit is considered in the room, the shadow region is completely blocked. Therefore, choosing the right location for the uplink receiver becomes essential.