## 2 Proposed RS-Based MIMO-OCC Scheme with Grouped LED Tx
### Rolling-Shatter Based MIMO Rx in OCC  
The CMOS IS, along with the imaging lens, is composed of a large number of pixels, with each pixel acting as an independent PD. Unlike a conventional PD-based Rx, which cannot be used to separate the mixed signals, CMOS-based IS can capture lights coming from different directions, and project them onto different sections of the IS [13]. Therefore, spatial separation of incoming light signals and their intensities can be measured by obtaining the pixel value for each light source image in the received frame. Image processing can be applied to the received image frames to extract the data from pixelated images [13]. Therefore, CMOS IS can be used as the MIMO-OCC Rx without the need for extended hardware. In [34], the analysis of the MIMO-OCC Rx using Bayer-pattern filters, which can differentiate the incoming signals being transmitted from the MIMO Tx, was reported.  
In an OCC-VLC system, the RS effect of a CMOS IS can be used to achieve flicker-free transmission and increased data rate [16; 17; 18; 19]. In this mode, the camera sequentially integrates light on all pixels at the exposure time \(t_{\text{row-exp}}\), similar to the scanning function, as illustrated in Figure 4a. In RS, the sensor scans the entire image row-by-row (line-wise) and generates a sequential readout. This scan process is governed by the system clock and is limited by the sampling rate of the analog to digital converter module. In GS-based IS, all pixels are exposed to the light simultaneously, i.e., ON or OFF states of the LED in a single frame [35], as illustrated in Figure 4b; while in RS, each row of pixels is exposed to  
Figure 3: The \(L_{\text{lux}}\)-\(I\)-\(V\) curves of the multiple-input multiple-output-optical camera communications (MIMO-OCC) Tx unit: (**a**) with the diffuser attached; and (**b**) without the diffuser.  
light at a given exposure time \(t_{\text{row-exp}}\), sequentially similar to the scanning function. In RS cameras, the readout time \(t_{\text{read-out}}\) ensures that there is no overlapping of the rows of pixels, and allows multiple exposures in a single captured image. The latter enables multiple LED states to be achieved at the same time in a single frame, as each row is exposed once to the light. Therefore, the captured image of the switched LED is composed of a set of black and white stripes. The proposed MIMO-OCC scheme differs from the RS-based OCC links (see Figure 4c), as in [16; 17; 18; 19]. In traditional RS-based OCC, only a single bit is captured within one exposure time \(t_{\text{row-exp}}\) (see Figure 4a), in contrast to the proposed work, where 8-bit (1-bit per channel) are captured in a single row with time \(t_{\text{row-exp}}\), as depicted in Figure 4c. In addition, the proposed scheme can be used for flicker-free and high data rates transmission by allocating multiple bits per \(t_{\text{row-exp}}\). The widths and the number of strips depend on the data rate (i.e., modulation frequencies) and the camera-LED distance, respectively [16; 18].  
Note that, in the RS capturing mode, each row starts with a certain delay, which results in the row shift, \(t_{\text{row-shift}}\). The frame time is given as:  
\[t_{\text{frame}}\leq N_{\text{row}}\times t_{\text{row-shift}}+t_{\text{row- exp}}, \tag{4}\]  
where \(N_{\text{row}}\) is the pixel rows, which are based on the camera resolution. Note, \(t_{\text{row-exp}}\) is the exposure time of the last row per frame (very small value).  
Figure 4: The camera capturing modes in OCC: (**a**) traditional rolling-shutter (RS) mechanism, (**b**) global-shutter (GS); and (**c**) RS-based capturing of the proposed multi-channel Tx.