## 4 Experimental Results
### Lake Trial  
We performed a lake trial after the theoretical demonstration via the simulation. The parameters for the lake trial are presented in Table 2, and the lake experiment was constructed as shown in Figure 7. The transmitter was the Neptune Sonar D/17/BB model, and the receiver was the Teledyne Reson TC4032. At the time of the experiment, the water depth at the receiver side was approximately 45.5 m, and the receiver was located 20 m below the water surface. The transmitter was located 300-400 m away from the receiver, and the transmitter was located 8 m below the surface. The bottom topography between the transmitter and receiver was irregular. The receiver was fixed at the center of the lake, the transmitter was moved around in a barge and continuously traveled by wind and engine, and the distance between the transceivers was kept at 300-400 m.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Sampling frequency & 192 kHz \\ Carrier frequency & 16 kHz \\ Data rate & 100 bps \\ Bandwidth & 2.5 kHz \\ Range between projector and hydrophone & 300–400 m \\ Projector depth & 8 m \\ Hydrophone depth & 20 m \\ Water depth at projector side & 30–40 m \\ Water depth at hydrophone side & 45.5 m \\ Projector & Neptune Sonar D/17/BB \\ Hydrophone & Teledyne Reson TC4032 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameters for the lake trial.  
Figure 7: Configuration for the lake trial.  
To analyze the received signals, an estimation of the channel environment is necessary. For this reason, the underwater channel characteristics were estimated prior to this experiment. The signal used was a 128 ms LFM pulse train with a 2.5 kHz bandwidth, and it was repeated 200 times.  
Figure 8a shows the measured channel impulse response, which represents a Doppler shift and multipath. The sloped shape in the figure means that the main path signal shifts slightly with time delay over time, demonstrating a Doppler shift. This figure shows that the transceivers moved away from each other over time. In Figure 8b, we can see the scattering function. Figure 8c represents the power delay profile. Figure 8d shows the Doppler spectrum and indicates that the Doppler shift was approximately \(-\)2.21 Hz.  
Figure 9 presents the packet structure used in the experiments. Both sides of the LFM waveform, which have a bandwidth of 2.5 kHz and a length of 1 s, are used to determine whether a signal exists, and the preamble, which was 511 bits modulated by binary PSK modulation for 1 kbps, is used to synchronize the signal frame accurately. The data has 336 bits of information. The lake trial was repeated 6 times.  
The results of the lake trial are shown in Table 3, which represents the estimated Doppler shift frequency and uncoded BER of the proposed method and the conventional CSS method. As a result of the experiment, the average uncoded BER of the proposed  
Figure 8: Underwater channel characteristics in the lake trial: (**a**) channel impulse response, (**b**) scattering function, (**c**) power delay profile, and (**d**) Doppler power spectrum.  
Figure 9: The Packet structure in the lake trial.  
method is \(3.52\times 10^{-2}\) and that of the conventional method is \(3.52\times 10^{-1}\), which shows that the proposed method is superior to the conventional method.