## 3 Measurement Results
### seawater Salinity and Water Turbulence Combination  
A motor pump with 1200 L/h speed was placed inside the water tank to simulate water flow in the water tank. The water turbulence (flow) caused small particles to float around the entire water tank. In oceanic water flow, temperature and salinity fluctuations are the main factors influencing a laser beam's propagating properties [21; 22]. Underwater water flow can be defined as random refractive index variations that affect the beam propagation through the transmission path. Based on [23], when the water pump is turned on, the bubble breaking increases due to high flow rates. The quicker the water flow is moving, the more particles are visible in the signal light path, and these impurities soon float throughout the entire water tank. The difference in the \(-9\) dBm transmitter power was needed to gain the same received power when the pump was turned off. The UWOC system design could be significantly changed depending on the salinity and size of air bubbles in the optical link. The water flow was measured in different hydrometer scales of 1.01, 1.02, and 1.03 to compare with the UWOC system performance. Some undissolved sea salt and dust made the UWOC system worse. The laser beam became unclear as the tiny particles floated around the water. This phenomenon caused attenuation in the UWOC transmission system. The data rate was generated using NRZ OOK modulation. This measurement aimed to compare the difference between data rates in general.  
As shown in Figure 8, in 22.78 g/L saline water, the best BER of 8 \(\times\) 10\({}^{-6}\) was obtained when the motor pump was turned on, and the best BER of 3 \(\times\) 10\({}^{-6}\) was obtained when turning off the motor pump. In 37.96 g/L saline water, the BER reached 3 \(\times\) 10\({}^{-5}\) and 4 \(\times\) 10\({}^{-6}\), respectively, when turning the motor pump on and off. Moreover, as the hydrometer scale increased to 1.03, or in the 45.56 g/L saline water, smaller floating particles caused different refractive indexes. When turning it on and off, the motor pump drifted into 4 \(\times\) 10\({}^{-4}\) and 5 \(\times\) 10\({}^{-6}\), respectively. The seawater salinity contributed to the UWOC system attenuation and degraded the signal. Figure 9 shows the statistical distribution of the received optical power due to the water flow changes in various water densities. Figure 10 shows the time (bit period) against the received power amplitude due to water flow changes in various water densities of 1.01, 1.02, and 1.03. As water salinity increased simultaneously with the water flow, the channel coherence time of the amplitude signal had more fluctuations in the 0.2 to 1-bit period. As the salinity increased, the signal distribution became large, indicating the signal fading condition. Figure 11 shows the channel estimation for each water flow measurement.  
\[\mathrm{P_{rx}}=\mathrm{P_{tx}}\times  
Figure 8: The water flow measurements for different types of water: (**a**) 22.78 g/L; (**b**) 37.96 g/L; and (**c**) 45.56 g/L.  
Figure 10: Time (bit period) against received power amplitude due to water flow changes in various water densities: (**a**) 1.01 density, (**b**) 1.02 density, and (**c**) 1.03 density.  
Figure 9: Statistical distribution of received signal amplitude due to water flow changes in various water densities: (**a**) 1.01 density, (**b**) 1.02 density, and (**c**) 1.03 density (R4Q2).  
The channel estimation was obtained by Equation (2), where P\({}_{\rm rx}\) is the received power, P\({}_{\rm rx}\) is the transmitted power, and H is the number of the channel.