# Deep Learning-Based Cyclic Shift Keying Spread Spectrum Underwater Acoustic Communication
## 4 Analysis of Experimental Results
### Analysis of Experimental Results  
In the water tank experiment, the performance of the conventional system and the DL-based system are compared. Moreover, in CSK-SS modulation, three schemes with communication rates of 180.45 bps (each symbol carries 4-bit), 146.34 bps (each symbol carries 3-bit), and 106.19 bps (each symbol carries 2-bit) are analyzed. In addition, in experimental data processing, noise interference is added artificially to obtain the condition of a low SNR. In the experiment, the training data samples of the neural network are received by hydrophones at positions B1, B2, B3, and B4, respectively. The data used to test the performance of the DL-based system are received by the hydrophone at position C. Due to the different locations of the hydrophone, the UWA channel structures in the training and test stage are also different. Due to the change of the position of the receiving hydrophone, the different CIRs is shown in Figure 16. \begin{table}
\begin{tabular}{c c} \hline \hline
**Transmission Signal Parameters** & **Parameter Setting** \\ \hline Spreading sequence length & m-sequence \\ Modulation mode & 31 \\ Number of bits carried by each symbol & 2/3/4 \\ Chip length (ms) & 0.5 \\ Sampling Rate (kHz) & 48 \\ Center frequency (kHz) & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameter setting of the transmitted signal. \begin{table}
\begin{tabular}{c c} \hline \hline
**Transmission Signal Parameters in Each Frame** & **Parameter Setting** \\ \hline Number of bits & 120 \\ Communication signal duration (s) & 0.93/0.62/0.465 \\ HFM signal duration (s) & 0.1 \\ Gap duration (s) & 0.1 \\ Communication rate (bps) & 106.19/146.34/180.45 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameter setting of the transmission signal frame structure. The performance of the conventional and the DL-based systems without preprocessing are compared experimentally. The scatter diagram of BER under three CSK-SS modulation schemes under different SNRs is given in Figure 17. The BER curve is given by calculating the mean of scatter points under each SNR. A more intuitive representation is given in Figure 18. Through the analysis of the data, it can be found that under the three CSK-SS modulation schemes, the performance of the DL-based system is better than that of the conventional system, and the anti-noise performance is improved by 1 dB to 3 dB. Figure 16: Training stage and test stage: CIRs at different positions of hydrophone. Figure 17: The scatter diagram of the conventional and DL-based systems without preprocessing. The experiment also compares the performance of the conventional and the DL-based systems after preprocessing. Figure 19 shows the scatter diagram of BER under three CSK-SS modulation schemes under different SNRs. Similarly, the BER curve is given through Figure 20. The data analysis shows that the preprocessing improves the performance of the DL-based CSK-SS UWA communication system. Under the three CSK-SS modulation schemes, compared with the preprocessed conventional system, the preprocessed DL-based system improves the anti-noise ability by 9 dB to 10 dB. When the SNR is greater than \(-8\) dB, each symbol in CSK-SS modulation carries 4 bits, 3 bits, and 2 bits, the BER of the DL-based system is less than \(2.5\times 10^{-3}\), \(1.7\times 10^{-3}\), and \(0.8\times 10^{-3}\), respectively. Under the condition of SNR = \(-14\) dB, when each symbol in CSK-SS modulation carries 4 bit, 3 bit, and 2 bit information, the BER is \(8.6\times 10^{-2}\), \(7.6\times 10^{-2}\), and \(4.5\times 10^{-2}\), respectively. Figure 19: The scatter diagram of the conventional and DL-based systems after preprocessing. Figure 18: The BER curve of the conventional and DL-based systems without preprocessing. under the condition of a low SNR. The memory and analysis of the received signal are completed through the BiLSTM network model, which solves the problems in the signal demodulation process of the conventional CSK-SS UWA communication system affected by multipath fading and noise.