## 7 Conclusions and Future Work
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Type** & **Scenarios** & **BER** & **MSE** & **PSNR** & **MAE** & **SSIM** \\ \hline \multirow{4}{*}{AISRT} & \multirow{4}{*}{Empty pipe} & & 17305 & 5.8742 & 130.89 & 0.50145 \\ \cline{3-6}  & & 17548 & 4.5832 & 141.71 & 0.47197 \\ \cline{3-6}  & & 16173 & 4.6725 & 140.48 & 0.48723 \\ \cline{2-6}  & \multirow{4}{*}{Water pipe} & & 22400 & 4.6284 & 141.25 & 0.47386 \\ \cline{3-6}  & & 22829 & 4.5459 & 143.05 & 0.41676 \\ \cline{3-6}  & & 21992 & 4.7082 & 139.92 & 0.45027 \\ \hline \multirow{4}{*}{DIFRT} & \multirow{4}{*}{Empty pipe} & 0.0061 & 158.47 & 26.131 & 0.77844 & 0.94118 \\ \cline{3-6}  & & 0.0037 & 76.156 & 29.314 & 0.42062 & 0.96146 \\ \cline{1-1} \cline{3-6}  & & 0.0074 & 162.54 & 26.021 & 0.82405 & 0.92654 \\ \cline{1-1} \cline{2-6}  & & 0.0098 & 302.23 & 23.327 & 1.5842 & 0.88439 \\ \cline{1-1} \cline{3-6}  & & 0.0083 & 276.87 & 23.709 & 1.3531 & 0.89202 \\ \cline{1-1} \cline{3-6}  & & 0.0079 & 232.13 & 24.473 & 1.0764 & 0.90926 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comprehensive performance comparison between DIFRT and AISRT for the final receiver. would become much longer. Besides, because of the relay node, the VLC system could be used in the non-line-of-sight (NLOS) transmission environment. Secondly, considering the image reconstruction ability, the image could be received by the final receiver and processed by MATLAB with less noise, more clear structure, and higher brightness. During the experiments, these three elements for evaluating in-pipe image quality could show a good image transmission performance when the DIFRT was adopted. Thirdly, in terms of transmission speed, due to the limitations of the transmitter hardware and pulse width modulation (PWM) method, the system transmission rate was set only 50 kbps in this research. This speed could basically satisfy the image transmission in the pipe. The transmission rate could be further improved by using other modulation methods or transmitters. Finally, in different mediums, the system was well verified. That means the system could be used in water or open air. Although convincing results have been realized, this method still possesses some deficiencies that should be overcome in future work. These deficiencies are highlighted below:  
* At present, the biggest and most difficult problem is the light alignment technology. Although visible light communication does not require high alignment requirements like laser communication, the alignment technology can severely affect the intensity of received optical power and finally influence the communication efficiency of the whole system. Currently, due to the difficulty in the light alignment, this system is unable to apply in the mobile robot system. * During the transmission tests, the selection of the DIFRT method in this research has not been appropriately justified. Since the BER still exists during the transmission, some other modulation/demodulation should be also sufficiently considered. In addition, currently, the offline process of image compression and decompression make the system unable to be used in the small pipe. * Currently, this DIFRT system cannot transmit and receive RGB images. There are mainly two important reasons for this: (1) The calculation of MSE, MAE, SSIM, and especially PSNR is based on 8 bit depth grayscale image. In order to compare transmission performance between the DIFRT and AISRT methods based on these four parameters, a standard grayscale image is required. (2) RGB images are considered as 24 bit depth images, which means that each pixel from the image has a maximum 24 value, whereas each pixel of a grayscale image has a 28 value. This big data scale of RGB pixels will increase the transmission task even if these data are compressed and transformed into a binary data. In the future, the effective transmission of RGB images using VLRC technique requires more in-depth research. * To realize efficient image transmission within a pipe over a much longer distance is still a difficult task.