## 4 Experiment Results
### Experiment Parameter Settings  
In this experiment, Doppler frequency shifts disrupt the orthogonality of subcarriers in shallow water acoustic channels, and OCDM performs well in shallow water acoustic channels due to its insensitivity to Doppler frequency shifts. However, in deep water acoustic channels where there is no significant Doppler spread, so using OCDM offers no advantage and may instead increase complexity. Therefore, the application scenario is targeted towards shallow water acoustic channels with significant Doppler spread. Shallow water acoustic channel usually has a depth below 100 m and the length from 100 m to 3000 m. The parameters of the simulation are listed in Table 1. The pilot is OFDM signal and uses block-type pilots. The simulator considers the influence of environmental factors such as sound velocity profile, sea surface, and seabed depth. As shown in Figure 10, a SISO communication scheme is applied in the simulation. Mathematical representation of the UWA multipath channel is shown as Equation (13),  
\[h(t)=\sum_{i=0}^{m-1}h_{i}\delta(t-\tau_{i}), \tag{13}\]  
where \(h(t)\) is impulse response function of the channel, \(m\) is the total number of multipaths, \(h_{i}\) is discrete complex gain, and \(\delta(t-\tau_{i})\) is impulse response delayed by \(\tau_{i}\). The depth of the sea is 100 m. The transmitter is positioned 50 m above the sea bottom, the height of the receiver from the bottom is 30 m, and the distance between the two is 300 m, 800 m, and 3000 m. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Number of subcarriers & 128 \\ OCDM FT size & 128 \\ OFDM FFT size & 128 \\ Modulation & QPSK \\ Type of guard interval & CP \\ Length of guard interval & 32 \\ Channel bandwidth & 1280 Hz \\ Symbol duration & 100 ms \\ Noisy model & White gaussian \\ Channel model & UWA multipath channels \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of simulator. Figure 9: Frame structure. The channel delay plots at distances of 300 m, 800 m, and 3000 m are shown in Figure 11, Figure 12, and Figure 13, respectively. For example, Figure 12 shows a clear multipath structure of the channel, primarily manifested in the five arrival peaks of the signal. The channel remains overall stable with slow temporal variations, consistent with the characteristics of shallow water channels. The number of arrival peaks at 300 m is fewer than at 800 m, while the number of arrival peaks at 3000 m is greater than at 800 m.  
The normalized energy from both BCH1 and simulation at a distance of 800 m are shown in Figure 14. BCH1 is a SIMO channel from the commercial harbor of Brest, France. A single probe transmission over a range of 800 m resulted in a 59.4 s channel estimate, simultaneously recorded on the receive hydrophones [23]. In Figure 14, it can be seen that the simulation channel shares the same multipath features with real shallow water acoustic channel. In the simulation, 20,000 CFR matrices are generated for each channel model, which are randomly split into three subsets: 80% are designated as the training set, 10% as the validation set, and the final 10% are reserved for the test set. The size of the channel matrix is 128 \(\times\) 10--128 corresponds to the number of subcarriers, 10 represents the number of symbols per frame--and the third and seventh symbols are used as pilot symbols of the frame. In addition, the initial learning rate is 0.005, and then the learning rate decays to the previous 0.1 every 20 rounds, and the maximum number of training rounds is 100. To  
Figure 11: Delay plot of channel modem at 300 m distance. Figure 10: Model of shallow UWA multipath channel. mitigate the risk of overfitting, the training process will utilize early stopping, halting if the value of the loss function fails to decrease further over five consecutive epochs.