## 2 Proposed RS-Based MIMO-OCC Scheme with Grouped LED Tx  
### MIMO-OCC Tx Characterization  
A new, simple design of MIMO Tx unit is proposed in this paper, as illustrated in Figure 1. It is composed of a 64-neopixel array with an \(8\times 8\) small chip-LED, and a 1 cm-thick LED grouping grid, (see Figure 1a,b), which is attached over the Tx LED array. This LED grid is designed to divide a 64-neopixel chip-LED into eight different column-wise groups that are individual transmission channels with eight chip-LEDs per group to allow eight different data transmissions, using a single neopixel LED array. In addition, the LED grid (Figure 1c) is very effective in combating interference due to adjacent LEDs within the groups. The effect of interference with supporting analysis without using the grouping grid to capture the LED array with a GS-based camera Rx was initially reported by the authors in [27]. A 2 mm-thick opaline methacrylate LED diffuser, commonly used, is placed over the Tx [28]. The size of the LED array is \(7.2\times 7.2\) cm\({}^{2}\). Since the size of each chip-LED (i.e., \(5\times 5\) mm\({}^{2}\)) is much smaller than the distance between the adjacent chip-LED (i.e., 9 mm), light from each LED is captured as a discrete image using an IS. Since both optical and electrical characterization of neopixel RGB LEDs are yet to be investigated, we first provide a characterization of the proposed MIMO Tx in terms of its optical radiation pattern and output optical power-current-voltage (\(L_{\text{lux}}\)-\(I\)-\(V\)) curves. The optical radiation pattern of the neopixel LEDs was measured to obtain its spatial intensity distribution for use in analyzing the coverage and signal distribution in VLC and OCC links [8]. The light intensity of LEDs defined in terms of the angle of irradiance \(\theta\) is given by [8]:  
\[I(\theta)=\frac{m+1}{2\pi}I(0)\cos^{m}(\theta),\ \theta=[-\frac{\pi}{2},\frac{ \pi}{2}], \tag{1}\]  
where \(I(0)\) is the center luminous intensity of an LED and \(m\) is Lambertian order given as [8]:  
\[m=-\frac{\ln(2)}{\ln[\cos(\theta_{1/2})]}. \tag{2}\]  
A lux meter was used to measure the angular dependence of the luminance of the LED (i.e., single and \(8\times 8\) horizontal and vertical array). As expected, the profiles for (i) a single LED represent a complete hemisphere close to Lambertian emitter with \(m\) of 1 (see Figure 2a); (ii) the LED array with no diffuser is broader with \(m\) of 0.74, due to the 9 mm spacing between the adjacent LED chips; and (iii) the LED array with the diffuser has \(m\) of 0.75 (see Figure 2b). Note that the measured radiation patterns can be further used to study the proposed MIMO-OCC links with mobility, and in a multi-user scenario. For example, rotation compensation schemes based on different Tx configurations [29] and wide receiver orientations [30] can be employed to ensure the operability of the proposed MIMO-OCC system in IoT environments. The illuminance levels of the LEDs were measured using a Testo 545 lux meter. Each neopixel in the array draws up to 60 mA of current \(I\) to turn ON at the maximum brightness. In realistic  
Figure 1: The transmitter (Tx) unit: (**a**) assembled unit with the grid and a diffuser, (**b**) an LED array configuration and (**c**) an Arduino Uno controller board and the LED panel. Figure 2: Measured Tx radiation patterns of: (**a**) a single LED in an array and for full LED array; and (**b**) a full LED array with the grid and the diffuser. environments, when the LED panel is to be used as a lamp, luminance can be efficiently controlled by varying switching power supply output current at the LED drive circuit. Therefore, the implementation of dimming techniques can be included as a part of the further extension of the proposed scheme [31; 32]. However, increasing the power would induce crosstalk due to illumination from adjacent LEDs in the MIMO Tx [33]. In practical use, it is rare for all pixels to be turned ON at its maximum drawing current, due to the risk of overheating and damaging the LED panel.