## 3 Sea Experiment  
A long-range UWA communication experiment using CCVTRM was conducted in the South China Sea. Figure 3 illustrates the experiment area and the vessels' locations. The vessel which has a single sound source for transmitting information moored at the point S, while the vessel that deployed the VSA anchored at the point R. Table 1 shows the coordinates of the point S and R.  
Figure 4 shows the sound speed profiles of the point S and R. It can be seen that the inflexion points of the sound speed profiles are about 40 m deep. Figure 5 is a schematic of the experiment. Considering the sound speed profiles, the sound source, operated at a 300 Hz bandwidth (500-800 Hz) \begin{table}
\begin{tabular}{c c c} \hline
**The Test Point** & **Latitude** & **Longitude** \\ \hline S & 18\({}^{\circ}\)12\({}^{\prime}\)48\({}^{\prime\prime}\) N & 110\({}^{\circ}\)25\({}^{\prime}\)16\({}^{\prime\prime}\) E \\ R & 17\({}^{\circ}\)55\({}^{\prime}\)25\({}^{\prime\prime}\) N & 109\({}^{\circ}\)41\({}^{\prime}\)58\({}^{\prime\prime}\) E \\ \hline \end{tabular}
\end{table} 
Table 1: The coordinates of the test points. Figure 3: Sea experiment area and the vessels’ locations. with a source level of 186 dB, was setup at 40 m deep. Likewise, the VSA located 80 km far away from the sound source was deployed with its acoustic center at the same depth. The VSA is composed of 12 sensors with an equal spacing of 1.5 m.  
The procedure started with sending a probe signal to estimate the composite channel of the VSA, followed by the data stream. Both the probe signal and the data stream were using LFM signal form with a bandwidth of 500-800 Hz. And the sampling rate was 48 kHz at both the transmitter end and the receiver end. Trials at different communication rates were conducted by adjusting the pattern pulse width and the length of the encoding time window. Table 2 indicates the detailed values of \(T_{p}\), \(T_{c}\) and the corresponding communication rate. A communication experiment with a communication rate of 50 bit/s is taken as an example to describe the signal processing. \begin{table}
\begin{tabular}{c c c} \hline \hline \(T_{p}\) **(ms)** & \(T_{c}\) **(ms)** & **Communication Rate (bit/s)** \\ \hline
8 & 12 & 200 \\
16 & 24 & 100 \\
24 & 36 & 66.7 \\
32 & 48 & 50 \\
40 & 60 & 40 \\
80 & 120 & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The values of \(T_{p}\), \(T_{c}\) and communication rate. Figure 4: Sound speed profiles of the test points. (**a**) The point S; (**b**) The point R.  
Figure 5: Schematic of the UWA communication experiment. Figure 6 shows the very different detection results of the probe pulse signal before and after ASP, wherein Figure 6a shows the detection result of a single sensor, while Figure 6b is the detection result after ASP. Because of the long propagation distance, the SNR of the signal received by a single sensor is so low that the probe pulse signal can not be detected as shown in Figure 6a, and thus the channel impulse response of the single sensor can not be estimated. However, ASP brings the probe pulse signal a higher SNR, so that the probe pulse signal after ASP can be easily detected as shown in Figure 6b. As can be seen in Figure 7, the composite channel estimation, measured by the probe pulse signal, has a very complex multipath structure and a wide time delay spread nearly 80 ms. As a result, the symbols in the data stream are broadened, leading to strong ISI from temporal overlap, which is bad for UWA communication. Thus, using CCVTRM to process the data stream after ASP is necessary. Figure 8 illustrates the waveforms of the data stream before and after CCVTRM. Comparing Figures 8a and 8b, it is clear that CCVTRM can compress the signal in the time domain, thus focusing the energy of the signal and mitigating the ISI. Figure 6: The detection results of the probe pulse signal. (**a**) Before ASP; (**b**) After ASP. Figure 7: The estimation of the composite channel. Figures 9 and 10 show the results of the experiment at communication rate of 50 bit/s.