## 4 Experimental Results and Discussion
The measurement rate of the key frame was higher, the reconstructed image was clearer, and the PSNR value was larger. As can be seen in Figures 11 and 12, the \"foreman\" and \"coastguard\" were reconstructed under different BER. In the following figures, the abscissa is the AMR and the ordinate represents the average PSNR value of the reconstructed image. Experimental results demonstrate that the PSNR increases with average measurement rate. Figure 10: Comparison of the first three frames of “foreman” at different AMR: (**a**) AMR = 0.4667; (**b**) AMR = 0.5667; (**c**) AMR = 0.6667; and (**d**) AMR = 0.7667. Figure 9: Comparison of the first three frames of “coastguard “at different BER: (**a**) BER = 0.0062; (**b**) BER = 0.0081; and (**c**) BER = 0.0107. The first line of pictures are the original video frames, the second line of pictures are the result of direct transmission of video frames, and the third line of pictures is the result of transmission of video frames after distributed compressive video sensing coding. The optical signal became extremely weak when it passed through the underwater channel to the single photon detector. Since the communication distance was short and the rate was low, very few photons belonging to the slot of symbol \"1\" traverses to the slot of symbol \"0\" due to scattering. The bit error we observed was mainly because the number of photons in the time slot of the symbol \"1\" was not enough, causing the symbol \"1\" to be misjudged as \"0\". It was somewhat advantageous to identify the symbol \"1\" if the scattered photons run into the time slot of the symbol \"1\". The time slot of the symbol \"0\" had very few scattered photons. However, the distance between the dark count and the background noise photon was much larger than the threshold we set, thus the symbol \"0\" was not misjudged as \"1\". Figures 13 and 14 show the average PSNR of the reconstructed video sequences at different BER under five average measurement rates. The results show that the larger was the BER, the smaller was the PSNR. Even when the BER was \(\geq\)0.1, the video frame could still be restored clearly as long as the measurement rate was large enough. Figure 11: The AMR-PSNR performances for the “foreman”. Figure 12: The AMR-PSNR performances for the “coastguard”. The wavelength in our experiment system was 500 nm. The quantum efficiency of single photon detector was \(\eta=35\%\). Effective photosensitive area was a circle of diameter \(\diameter=500\)\(\upmu\)m. To increase the communication distance, a focusing lens with a diameter of 10 cm was placed before the single photon detector. According to the experiments, when the baud rate was 100 kbps and the average number of photon pulses per bit was 20, the bit error rate was 0.0421, and the video frame could still be restored clearly. We calculated the optical power corresponding to 20 photon pulses per bit as \(1.7\times 10^{-12}\) W, according to Equation (4). As shown in Table 1, we calculated the theoretical longest communication distance of three water types. The attenuation coefficients of three water types were taken from the work done by [PERSON] (1998). Farther communication distance could be achieved by increasing LED power or using multiple LEDs.