## 3 Measurement Results
### Seawater Salinity and Temperature Combination  
In seawater, different areas have different temperatures. Seawater temperature is affected by factors such as sunlight, atmospheric heat transfer, turbidity, and water depth. It also changes from place to place and from one season to another [18]. The temperature effect makes it possible for researchers to simulate the temperature of the actual ocean inside a laboratory. Ocean water with a median salinity of 35 psu freezes at \(-1.94\) degrees Celsius (28.5 degrees Fahrenheit). Therefore, it follows that sea ice can form at high latitudes. The average temperature of water at the ocean's surface is about 17 degrees Celsius (62.6 degrees Fahrenheit) [19]. As the temperature increases, the salinity of water decreases. Therefore, we measured the impact of temperature changes on seawater density. A heater was turned on to reach a specific temperature, as shown in the digital thermometer, for higher temperature measurements. The lower temperature measurement used a cooler fan to generate a wind breeze and an ice back inside the water. The temperature within the water tank changed automatically based on the temperature measurement tool. We conducted the experiment while the temperature automatically increased to the desired level. The heater and cooler fan made the temperature stable; however, the water temperature was non-uniform. The non-uniform water temperature caused the water to move in a turbulent manner, which was similar to realistic conditions in the ocean. The water tank could have had a temperature variation up to \(\pm 2\)\({}^{\circ}\)C; therefore, the optical attenuation in the water was increased due to non-uniform water refractive indices and non-uniform water temperatures. Each measurement was taken under the same environmental factors, and different data rates resulted in different BERs. Therefore, the data rate caused signal distortion as higher speeds led to faster modulation. In this experiment, we used data rates of 155 Mbps, 622 Mbps, and 1.25 Gbps. The graphs in Figure 5, for 155 Mbps and 622 Mbps, were nearly identical. The 1.25 Gbps drifted to a higher and worse BER even though it was still receiving higher optical power. In Figure 5, the 1.25 Gbps BER floor slightly changed as the temperature increased. The modulation speed of 155 Mbps and 622 Mbps was slightly different since the range was still in the \(\sim\)Mbps. In the \(\sim\)Gbps order of modulation speed, the noise of the information signal increased, which could have impacted the attenuation in the UWOC system. When  
Figure 4: (**a**) BER measurements in different types of water with 1.25 Gbps modulation speed; and (**b**) the BER comparison of UWOC system in the simulation and experiment. the modulation speed increased, the optical power also needed to be higher. The BER floor at a lower temperature of 15 \({}^{\circ}\)C was 1.07 \(\times\) 10\({}^{-5}\), while at a higher temperature of 30 \({}^{\circ}\)C, the BER floor reached up to 5.12 \(\times\) 10\({}^{-6}\). As we know, ocean temperature changes and is non-uniform, along with the depth of the ocean. Temperature decreases while depth increases. Based on the Thermocline (a layer within a body of water or air where the temperature changes rapidly along with depth), a temperature of 15 \({}^{\circ}\)C is around 600 m in depth, 26 \({}^{\circ}\)C is around 20-80 m depth in the summertime, and the highest ocean temperature is 30 \({}^{\circ}\)C on the surface. A higher temperature affects a lower salinity which can lower the density of seawater. In this measurement, the optical power slightly differed in various temperatures. Therefore, the UWOC system could still reach the information in seawater salinity with different temperatures. The main cause of optical signal fading in a typical UWOC channel is temperature or salinity-induced turbulence [20]. Therefore, we provide the statistical distribution of received optical power due to temperature changes in Figure 6. The received optical signal faded as the temperature increased.