## 4 Experiments  
In order to verify the feasibility and robustness of TRM-SBDC communication method proposed in the paper, an UWA communication experiment was conducted in the Songhua River in Harbin, in May 2018. The water depth in the experimental area was about 2 m, the sound velocity was about 1487 m/s, the maximum water flow rate was 1.8 m/s, and it was windy during the experiment. A single sound pressure sensor was deployed off the receiving boat which was always anchored, while the sending boat which has a sound source was moored or drifting with the current. Both the receiving \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Hydrophone Channels of KAU2**} & \multicolumn{2}{c}{**BER (\%)**} \\ \cline{2-3}  & **TRM** & **TRM-SBDC** \\ \hline Hydrophone channel 4 & 34.000 & 0 \\ Hydrophone channel 8 & 30.893 & 0.321 \\ Hydrophone channel 12 & 34.036 & 1.429 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The decoding results of different hydrophone channels. Figure 8: The Doppler time delay error. (**a**) TRM; (**b**) TRM-SBDC. Figure 9: The decoding results. (**a**) TRM; (**b**) TRM-SBDC. sensor and the sound source were deployed to 1 m depth. During the experiment, we measured the speed using a handheld GPS device. In case of drifting with the current, the relative radial velocity between the two boats was about 1.0-1.8 knots. For the communication signal, the detailed parameters were the same as the signal in the simulations and the data length was 4000 bits. Table 3 shows the experimental scenes. A probe pulse signal is usually sent first to estimate the UWA channel. However, this experiment uses the sync signal instead of the probe pulse signal to estimate CIR, thereby improving the communication efficiency. Figure 10 illustrates the CIR corresponding to a certain frame data. It can be seen that the channel is characterized by serious multipath spreads, which is easy to cause ISI in communication signal. Figure 11 illustrates the copy-correlation outputs of the corresponding communication data, wherein Figure 10(a) without using TRM and Figure 10(b) using TRM. For the convenience of observation, only partial waveforms of the copy-correlation outputs are displayed. Obviously, without TRM processing, there are strong pseudo correlation peaks with a delay of about 5 ms, which will confuse the judgment of the real correlation peaks and lead to decoding errors. On the contrary, the signal energy is focused after TRM processing, and the positions of the true correlation peaks are complete clear, thus ensuring the correctness of decoding. Figure 11 shows the corresponding decoding results, including Figure 11(a) without using TRM and Figure 11(b) using TRM. As can be seen from Figure 11(a), the time delay errors of the error codes are all about 5 ms, which is consistent with the CIR in Figure 10 and the copy-correlation outputs in Figure 10(a). However, the decoding results shown in Figure 11(b) are greatly improved, indicating that the single element TRM used in this paper is simple and effective. In experimental scene 1, the sending boat is moored. Figures 13 and 14 show the communication results of a certain frame data in this scene. Figure 12(a) depicts the Doppler time delay errors calculated using only TRM, while Figure 12(b) depicts the Doppler time delay errors calculated using TRM-SBDC. It can be seen that even if the sending boat and the receiving boat are anchored, the ups and downs of the boats caused by the wind and waves will still induce weak Doppler. However, since the time delay errors caused by Doppler are within the error tolerance range, no error occurs in the decoding results, as shown in Figure 14. Wherein Figure 14(a) shows the decoding results using only TRM, while Figure 14(b) shows the decoding result using TRM-SBDC.