## 7 Underwater Trials  
The above systems were first tested in the pool in Figure 7. The signals were transmitted with the carrier frequency \(F_{\mathrm{c}}=12\) kHz and the sampling frequency \(F_{\mathrm{s}}\) was 100 kHz. The parameters shown in Table 3 were taken to generate baseband signals. The block-type pilots were used for estimation of the time-invariant channels, and the length of cyclic prefixes was \(1/3\) of the symbol length. Table 7 lists the BERs of different receivers. It should be noted that not all the NN-based detectors were retrained, which was the same as the trained networks in the last section. The optimal \(N_{\mathrm{c}}\)s for the NN was found to be 2. This was because the hard wall of the pool caused strong reflections that resulted in a small coherence bandwidth. By adjusting the transmitting power, two groups of signals were tested with SNR = 30 dB and 5 dB. The ELM-based receiver still showed the worst performance. It could also be found that the proposed NN showed low BERs in all conditions, while the  
Figure 11: BERs of different detectors with the LS channel estimator. The BER curves of the ZF equalizer are almost overlapped by that of the MMSE equalizers, while the NN shows the lowest BER both with BPSK and QPSK. The ELM-based equalizer shows highest BER. Figure 10: BER of different detectors with given MSE of channel estimations. ZF and MMSE equalizers showed relatively high BERs. Compared with the simulations, the gap of the performance between the proposed NN and other equalizers became smaller. This was because the delays of the real channels were much longer than the simulated ones, which caused intersymbol interference (ISI) besides intra-symbol interference. Because the NN and traditional equalizers are only designed to eliminate the interference in each symbol, the influence of ISI could not be well equalized. Further, the receivers were tested in real sea. The spot chosen was Xiamen Bay near the location in Figure 7c and the communication distance was \(1\) km. In addition to the block-type pilots, the comb-type pilots were applied to make more accurate estimations, for the channels were time-variant in the shallow sea. In addition, the LS estimator was used for both pilot types. The SNR was controlled intentionally to 4.5 dB for comparison of the performances in hostile environments, leading to the received signals in Figure 12b. The impulse interference can be observed, which severely affects the performance of the receiver. Table 8 lists the BERs for block-type pilots, \(N_{\mathrm{c}}=16\), and for comb-type pilots, \(N_{\mathrm{c}}=4\). It can be seen that the received symbols with block-type pilots can hardly be detected with all detectors, although the proposed NN performs best. With comb-type pilots, the results are better. Since the ELM-based detector can only use the block-type pilots, the BERs with comb-type ones are ignored. Nevertheless, the BERs of the NN-based detector are the lowest among all equalizers. These results show the generation of the proposed NN-based detector in different UWA environments. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**SNR = 30 dB**} & \multicolumn{2}{c}{**SNR = 5 dB**} \\ \hline  & **BPSK** & **QPSK** & **BPSK** & **QPSK** \\ \hline Proposed NN & 0.00206 & 0.0073 & 0.0193 & 0.0276 \\ ZF & 0.00209 & 0.0074 & 0.0205 & 0.0295 \\ MMSE & 0.00208 & 0.0074 & 0.0206 & 0.0293 \\ ELM & 0.1385 & 0.4023 & 0.1431 & 0.4051 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: BER of systems tested in the pool. Figure 12: The original and received signals in time domain. Compared with (**a**), the signal in (**b**) suffers from impulses as well as fading. It can be seen from Table 8 that compared with the ZF and MMSE equalizers, the proposed NN-based detector shows less difference than Figure 11. This is because the signals transmitted in sea trials experience more complex interferences caused by the time-variant sound speed field and noise distributions.