## 3 Experimental Results
### Results and Analysis  
We measure the \(S_{21}\) parameters of the proposed UOWC system with and without the pre-equalizer based on the commercial LED source, respectively. The measurement results are shown in Figure 8. The \(-3\) dB bandwidth is improved from 5.03 MHz to 50 MHz after the pre-equalization. However, due to the addition of the pre-equalizer, the forward transmission gain of the system is reduced by approximately 20 dB. Different waveforms of the original signal and the equalized signal at a data rate of 100 Mbps are observed via the OSC, as shown in Figure 9. The equalized signal produces an evident overshoot on each rising and falling edge. The root-mean-square voltage is reduced from 480 mV to 280 mV, which means a higher transmitting power is required due to the additional pre-equalization circuit.  
Figure 7: Experimental setup of the proposed MDOF UOWC system.  
Furthermore, we test the transmission performance of the proposed MDOF UOWC system. A pseudo-random binary sequence (PRBS) with a word length of \(2^{9}-1\) is generated by the BERT. The initial DC voltage is set to 3 V and the operating current of the 450 nm blue LED is 98 mA. A \(30^{\circ}\) condenser lens is fixed upon the LED to converge the transmitted signal. By adjusting the position of the light source, the received signal is incident vertically on the central plane of the Fresnel lens array. After being transmitted through the water tank, the quality of the received signal can be observed by an OSC. Figure 10 shows the eye diagrams at different data rates after being amplified by the TIA circuit. As the transmission data rate increases, the eye diagram begins to close gradually and the eye height decreases from 14.1 mV to 4.6 mV. At 100 Mbps, the peak-to-peak voltage of the received signal is only approximately 38 mV. Then, the eye diagrams processed by the DSR module are measured, as shown in Figure 11. It can be seen that the quality of the received electrical signal has been significantly optimized, and the amplitude of the output signal is amplified to 380 mV. Moreover, the total power consumption of the receiver is as low as approximately 0.55 W.  
Figure 8: \(S_{21}\) parameter of the proposed UOWC system. (**a**) Without pre-equalizer; (**b**) with pre-equalizer.  
Figure 9: 100 Mbps signal with and without pre-equalization.  
Finally, to verify the communication reliability of the proposed system under MDOF conditions, we fix the receiver position. To simulate the reception of signals with different incident angles, the light source at the transmitter is moved in parallel, on the other side of the water tank. The incident angle is defined as the angle to the normal direction of the receiver. According to the existing experimental conditions, we choose two incident angles of approximately \(\pm\) 45\({}^{\circ}\) from the right and left sides of the receiver, as shown in Figure 12. By adjusting the receiving angle on the side of the Fresnel lens array, the received signal can be converged on the effective detection area of the same PIN detector. The BER performance of each communication link is analyzed via the BERT. Figure 12 shows the BER curves versus the different DC voltages at the transmitter. It can be seen that when the DC bias voltage is 2.74 V, the BER performance of the system is lower than 3.8 \(\times\) 10\({}^{-3}\). As the DC voltage increases to 2.83 V, the BERs of all the data signals with different incident angles are lower than 10\({}^{-8}\). All the received signals with different incident angles are transmitted successfully. The Fresnel lens array structure used in the experiment can effectively focus the signals emitted from different directions through the water tank channel.  
Figure 11: Eye diagrams after DSR. (**a**) 60 Mbps; (**b**) 800 Mbps; (**c**) 100 Mbps.  
Figure 10: Eye diagrams of received signals at different data rates after TIA circuit. (**a**) 60 Mbps; (**b**) 80 Mbps; (**c**) 100 Mbps.