## 5 Conclusions  
In this paper, we proposed a monitoring system based on the 2D OOK-MIMO scheme for the OCC technique. By using an LED matrix based on a special spatial frame format, the proposed scheme facilitates the rotation effect, which is essential with two-dimensional (2D) code. The SN was inserted into each packet, which helps in supporting the frame rate variation effect and recovering large data packets from different images. The SNR measurement results were obtained using various communication distances and exposure times to verify the relationship between three parameters: communication distance, exposure time, and SNR value. The SNR increases with exposure time, but the system bandwidth decreases, so controlling the exposure time should be carefully considered. The sub-packet was merged to create a large packet, which was then transmitted via a signal by adding the SN to overcome oversampling and undersampling. With suitable SN length, the lost, redundant packet can be detected based on the non-sequential nature of SNs, leading to an improved BER. However, the SN does not carry data and decreases the data rate, leading to the trade-off with the BER. Finally, the different distances were considered to evaluate the bit error rate of the proposed scheme compared with other similar systems.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Color Intensity** & **Rolling-Shutter-Based** & **Proposed Scheme** \\  & **Modulation-MIMO Scheme**[23] & **MIMO Scheme**[24] & **Proposed Scheme** \\ \hline Modulation & Color intensity & On-Off keying & On-Off keying \\ Type of camera & Just global shutter camera & Just rolling shutter camera & Both of cameras \\ Distance & Short distance (1.4 m) & Short distance (1.4 m) & Long distance (20 m) \\ Number of links & Single link & Single links & Multiple links \\ Matched filter & No & No & Yes \\ Bit error rate (1.4 m) & \(>\)10\({}^{-2}\) & Not mentioned & \(<\)10\({}^{-4}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of comparison of MIMO-OCC methods.  
Figure 17: BER performance of the proposed scheme for an 8 \(\times\) 8 LED matrix with different exposure times.  
Supplementary Materials can be seen at: [[https://susy.mdpi.com/user/manuscripts/displayFile/eb6](https://susy.mdpi.com/user/manuscripts/displayFile/eb6) beed557f2 ade98b9 ceb127a38713a/supplementary]([https://susy.mdpi.com/user/manuscripts/displayFile/eb6](https://susy.mdpi.com/user/manuscripts/displayFile/eb6) beed557f2 ade98b9 ceb127a38713a/supplementary) (25 April 2021). The supplementary material video shows the implementation of our proposal scheme at the distance of 2 m.  
All authors contributed to this paper: H.N. proposed the idea and implementation methodology; H.N., V.N. and V.B. wrote the paper, verified the experiment process and results, and edited the paper; C.N. reviewed the paper and Y.J. supervised the work and provided funding support. All authors have read and agreed to the published version of the manuscript.  
This research was financially supported by the Ministry of Trade, Industry and Energy(MOTIE) and Korea Institute for Advancement of Technology(KIAT) through the International Cooperative R&D program and the MSIT(Ministry of Science and ICT), Korea, under the ITRC(Information Technology Research Center) support program(IITP-2018-0-01396) supervised by the0 IIITP(Institute for Information & communications Technology Promotion).  
Not applicable.