## 4 Experiment Result  
A lake trial was run in Munkyeong city, Korea, in March 2021. The goal was to demonstrate the performance of EC-BER algorithms in controlled underwater acoustic environmental conditions. Table 2 lists parameters of the underwater test as described in Section 2. Five trials of the same packet were iteratively tested, and four bands with 4-FSK modulation and turbo codes with a rate of one-third was tested by changing several chips with 8 and 32. The center frequencies of 4 bands were 14 kHz, 18 kHz, 22 kHz, and 26 kHz. The sampling frequency was 192 kHz, and the data rate was 20 bps. The iteration of the turbo equalizer was set to five times. A cross-section of the deployment in lake is shown in Figure 6. The transceiver hydrophone was deployed at 20 m and 5 m in a maximum water depth of 50 m. We collected data at the receiver anchored to the lakebed. Then, we transmitted data by moving the boat in the range of 300 m to 500 m. To transmit the communication waveforms, wave files were generated and saved in a database. The files were played using the transmit laptop sound file, amplified to the appropriate level. Figure 5: BER performance of weighted multi-band: (**a**) number of errors for multi-band and (**b**) performance comparison between weighting and non-weighting. A chirp sequence, called linear frequency modulation (LFM), is transmitted to characterize the channel impulse response (CIR) in the presence of Doppler. It shows the channel is affected by multi-path propagation caused by reflections from the surface and the bottom. After processing, the CIR was obtained using an LFM, as shown in Figure 7a. The multi-path delay spread is measured as a function of time and is approximately 0.3 s. It can be seen that there are multi-path echoes following the end of transmission. The frequency shift is also measured as a function of time, shown in Figure 7b. The frequency shift can be caused by the relative motion from one platform to the other, and it can either increase or decrease the frequency of the acoustic signal. Doppler spread occurs when multiple signals are received with different frequency shifts. The effect of Doppler spread is about 4 Hz. Specific objectives of the lake trial are to confirm the weighted multi-band underwater acoustic communication configuration. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline FEC & Turbo code with rate of 1/3 \\  & (\(K=112\), \(N=336\)) \\ \hline Preamble bit (\(n\)) & 255 \\ \hline Number of chips (\(N_{c}\)) & 8, 32 \\ \hline Number of multi-bands (\(N_{b}\)) & 4 \\ \hline Modulation & 4-ary FSK \\ \hline Equalizer & LMS-DFE \\ \hline Number of turbo equalization iteration & 5 \\ \hline  & \(f_{1}\)= 14 kHz \\ Center frequencies of multi-band & \(f_{2}\)= 18 kHz \\  & \(f_{3}\)= 22 kHz \\  & \(f_{4}\)= 26 kHz \\ \hline Sampling frequency & 192 kHz \\ \hline Data rate & 20 bps \\ \hline Distance & 300–500 m \\ \hline  & Water depth: 50 m \\ Depth & Transmitter: 5 m \\  & Receiver: 20 m \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Experimental parameters. Figure 6: Deployment of the lake trial. Fixed on \(N_{b}=4\), Tables 3 and 4 show lake trial results for different numbers of chips by applying a turbo equalization algorithm. We knew that two or three packets were succeeded among five packets. The errors of the succeeded packets are close to zero as the number of iterations increases. As shown in Tables 3 and 4, some packets still failed to decode. In the case of \(N_{c}=32\), the third and fifth packets still failed, even though the number of turbo equalization iterations was 5. We applied optimized weighting values of EC-BER to the error occurrence packets. After measuring the EC-BER, we assigned different weighting values to each band based on Table 1. As a result, we successfully decoded the third and fifth packets, as shown in Table 5. Regardless of applying the weighting algorithm to \(N_{c}=8\), errors were not corrected. This means that the more spreading there is, the better the performance is.