# Optimized Deployment of Generalized OCDM in Deep-Sea Shadow-Zone Underwater Acoustic Channels
## 5 Deep-Sea Experiment  
To validate the performance advantages of the GOCDM underwater acoustic communication system, a deep-sea experiment was conducted in the waters adjacent to China at a location with an average depth of approximately 5000 m. Utilizing the deep-sea channel propagation characteristics introduced in Section 2 and carefully controlling the transmission-reception distance ensured that the receiving hydrophone remained within the shadow zone. Both the transmitting and receiving vessels traveled at speeds ranging from 3.8 to 5.0 knots, introducing a significant Doppler effect. The transducer depth varied between 80 and 94 m, while the hydrophone depth ranged from 307 to 360 m. Based on these conditions, the channel in this experiment can be classified as a deep-sea shadow zone channel. Communication was conducted using OFDM, OCDM, and GOCDM, with all other parameters kept consistent. The number of subcarriers was set to 1024, and the sampling rate was 50 kHz. Table 1 provides a summary of the transmitted signal parameters and experimental result. Figure 8: The BER of OFDM, OCDM, and GOCDM when the SNR is 30 dB. Figure 7: The BER of OFDM, OCDM, and GOCDM when the normalized CFO is 0.1. A portion of the received signal spectrum is shown in Figure 9. The first segment consists of an OCDM signal with a single carrier, which can be regarded as a linear frequency modulation signal and serves as a pilot for channel estimation. The latter part contains GOCDM-modulated data signals. The spectrum indicates that the received signal exhibits a low SNR due to the shadow zone reception, consistent with the theoretical analysis presented earlier in this paper. The multipath delay characteristics for the three propagation paths considered in this experiment are shown in Figure 10, with results summarized in Table 2. Figure 10a illustrates the A-B channel multipath response, showing that the acoustic energy arriving via scattering, refraction, and multiple reflections is almost entirely attenuated, leaving only a single cluster of seabed reflection paths. This results in the lowest received signal SNR. Figure 10b presents the C-D channel multipath response, where the initial cluster is presumed to correspond to paths formed by scattering and diffraction. The channel exhibits a longer overall multipath delay and a more complex multipath structure, with a relatively higher received SNR. Figure 10c shows the E-F channel multipath response, where most of the acoustic energy arriving via scattering and diffraction is significantly attenuated. The remaining two propagation paths likely correspond to seabed reflection and combined seabed-surface reflection. \begin{table}
\begin{tabular}{c c} \hline
**System Parameters** & **Value** \\ \hline Sampling rate & 64 kHz \\ Bandwidth & 2 kHz \\ Number of subcarriers & 1024 \\ Constellation mapping & QPSK \\ Channel estimation & MMSE \\ Coding rate & 1/2 \\ \hline \end{tabular}
\end{table} 
Table 1: Transmitted signal parameters. Figure 9: Received signal time-frequency spectrum. According to the simulation results in Section 4, the Doppler effect impacts all three modulation schemes similarly. Therefore, despite the high Doppler factor in this experiment, it does not significantly affect the comparative performance of the modulation schemes. As shown in Table 2, when the received SNR is 4.2 dB and 6.8 dB, only GOCDM achieves a BER of zero, while OCDM shows a lower BER than OFDM due to its superior multipath resistance. GOCDM inherits the multipath resistance of OCDM while significantly reducing the PAPR compared to both OFDM and OCDM, making it particularly advantageous for low-SNR environments such as deep-sea shadow-zone communication. It is worth noting that, although SC-FDE achieves the lowest PAPR, its resistance to inter-symbol interference is relatively low as a single-carrier modulation scheme, resulting in a less effective performance in high-delay shadow-zone communications. At an SNR of 0.016 dB, the BER of OFDM is lower than that of OCDM.