# Testbed for Experimental Characterization of Indoor Visible Light Communication Channels
## 4 Simulation Results
VLC systems based on affordable COTS devices are mainly limited to a few tens of MHz by LEDs and optical receiver bandwidths. Therefore, the temporal dispersion of the impulses responses due to the optical channel can be neglected for practical band rates with communication bandwidths below 100 MHz. Figure 4: Received power and illuminance at receiver plane for room described in Table 1: (**a**) Power balance. (**b**) Illuminance. (**c**) Contour map of LOS component percentage in the total received power. (**d**) Contour map of illumination level. Figure 5: Simulated impulse responses at three locations: (**a**) Impulse response under lamp 1 (close to a window). (**b**) Zoom of impulse response under lamp 1. (**c**) Impulse response at the center of the room. (**d**) Zoom of impulse response at the center of the room. (**e**) Impulse response under lamp 4 (close to a corner). (**f**) Zoom of impulse response under lamp 4. When several lamps collaborate to send information to a certain user, the orientation of the detector can be optimized to maximize the total received power. Figure 7 represents an illustrative case when there are two lamps serving a certain user and it moves along the x-direction. Four configurations of receiver orientation are considered: '_max_' indicates that the receiver is oriented to find the maximum received power as possible (this is the more complex case as the receiver has to look for all the possible orientations trying to maximize received power); '_near_' stands for the case when the receiver is pointing to its nearest lamp; '_vert_' symbolizes vertical orientation upward and '_comb_' is referred to an orientation that is the combination of receiver position \(\vec{r}_{\text{rec}}\) with respect to each \(l\)th lamp weighted by the reciprocal of the square of its distance \(d_{l}\) to the lamp, that is, the receiver orientation is:  
\[\vec{o}_{\text{rec}}=\sum_{l\in\mathcal{L}}(\vec{r}_{l}-\vec{r}_{\text{rec}}) /d_{l}^{2}, \tag{14}\]  
where \(\mathcal{L}\) represents the set of lamps serving to this specific user and \(\vec{r}_{l}\) is the position of the \(l\)th lamp. The results of Figure 7 are presented for lamps with Lambertian order of \(n=1\) (\(\phi_{1/2}=60^{\circ}\)) and \(n=2\) (\(\phi_{1/2}=45^{\circ}\)), range of values where typical WLED radiation patterns are. Figure 6: Simulated temporal dispersion of the impulse responses throughout the room: (**a**) Mean delay. (**b**) Delay spread. Figure 7: Received power from two lamps for different orientations of the receiver when it moves along the x-direction: (**a**) Results for a distance between lamps of 3 m. (**b**) Results for a distance between lamps of 2 m.  
We can see how the method based on a weighted combination of the position of receiver with respect to emitting lamps almost approximates the impractical best case, the former requiring just the information about the location of the lamps which could be provided by a centralized positioning system. Pointing towards the nearest lamp also requires knowledge of lamps and receiver position but its behaviour is the worst. Just pointing upward constitutes a simple but almost quasi-optimum solution, specially if the distributions of lamps is good, which only requires gyroscope information of the hand-held device where the optical receiver is included. As we can see, for a distance of 3 m between both lamps, the received power fluctuates leading to an appreciable penalization at the central area. On the contrary, when this separation is reduced to 2 m, the received power is more uniform along x-axis with gradual decrease at the edge areas and vertical orientation works quite well. This is why we have defined a 2-m neighbor lamp separation arrangement for the simulated room (see Table 1), which serves as guide for our mockup design. Figure 8 shows the power balance when the orientation method defined in Equation (14) is used so as to maximize the total received power from the four lamps.