## 3 Analysis of the Time Fluctuation of Underwater Acoustic Communication Signals
### Analysis of the Time Distribution Characteristics of Noise Fields  
The time distribution characteristics of the noise fields were mainly targeted at the commonly used high-speed underwater acoustic communication frequency bands of 5 kHz-20 kHz. According to the foregoing, the vertical elements of the noise distribution were weakly correlated to each other. Therefore, the array elements at 5 m, 15 m, and 25 m were selected as research objects to study the noise time distribution in the surface, seabed, and water column situations. In the ExQD_1701 experiment, the environmental noise data in the experimental stage was randomly taken for 600 s, and a 1 s Hanning window was applied to intercept the data at an overlap rate of 0.66. Therefore, a total of 1762 sample points were obtained. Statistics of narrow-band noise distribution at different frequencies are shown in Figure 16. In the high-frequency range, the environmental high-frequency noise was mainly distributed in the 60 dB to 70 dB range. As the frequency increased, the noise intensity gradually decreased; the noise intensity also gradually decreased as the depth increased. This was found to be in accordance with the theoretical model. In order to study the time distribution characteristics of noise further, the probability distribution of the single-frequency noise intensity at a depth of 15 m (Figure 16) was selected to explore the time distribution characteristics of noise. Figure 14: Temporal and spatial distribution at different horizontal distances (12 kHz): (**a**) spatiotemporal distribution map at 1 km, (**b**) spatiotemporal distribution map at 4 km, and (**c**) spatiotemporal distribution map at 8 km. Figure 15: Temporal and spatial distribution at different horizontal distances (20 kHz): (**a**) spatiotemporal distribution map at 1 km, (**b**) spatiotemporal distribution map at 4 km, and (**c**) spatiotemporal distribution map at 8 km. In Figure 17, subfigure (a) shows the time probability distribution of the 5 kHz narrowband noise with a mean of 68.59 dB and a standard deviation of 5.08 dB, subfigure (b) shows the time probability distribution of the 10 kHz narrowband noise with a mean of 66.16 dB and a standard deviation of 5.01 dB, subfigure (c) shows the time probability distribution of the 15 kHz narrowband noise with a mean of 65.30 dB and a standard deviation of 5.10 dB, and subfigure (d) shows the 20 kHz narrowband noise time probability distribution with a mean of 64.39 dB and a standard deviation of 4.97 dB. A comparison of these figures demonstrated that the probability of noise intensity basically followed the Gaussian distribution with a relativity small fluctuation on the time scale. As shown in Figure 17, the mean decreased with increasing frequency while the standard deviation remained stable. Figure 16: Noise distribution at different frequencies and different depths. Figure 17: Statistical distribution of the noise over time: (**a**) 5 kHz, (**b**) 10 kHz, (**c**) 15 kHz, and (**d**) 20 kHz. According to the analyses of the above experimental results, it can be concluded that the time distribution of the high-frequency noise field was relatively stable. The analysis results can provide a theoretical guide and data support for the underwater acoustic communication quality evaluation model. Analysis of the Spatiotemporal Variation Range of the Signal-to-Noise Ratio in Underwater Acoustic Communication  
Based on the spatial-temporal distribution characteristics of the sound field obtained in the previous section, the spatial-temporal variation range of the SNR was analyzed from the experimental data. The data was selected from the Yellow Sea Acoustic Communication Experiment ExDQ_1702. The external field experimental parameters were: water with a depth of 15 m, communication signal frequency band of 8 kHz-16 kHz, transducer placement with a depth of 5 m, and five arrays for each receiving array.