# The Long-Range Biomimetic Covert Communication Method Mimicking Large Whale
## 5 Evaluation of Communication Performance  
### Simulation  
The BER performance of the proposed biomimicking method was compared with that of conventional methods (i.e., FSK, CV-CFM, and TFSK) in the long-range underwater environment. The modulation order and parameter were selected for the fair BER comparison to yield a similar data rate. These values are given in Table 2. M is the modulation order. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Modulation Scheme** & **Proposed** & **FSK** & **CV-CFM and TFSK** \\ \hline Recognition & 95\% & 10\% & 60\% \\ probability & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The result of the DoM evaluation. Figure 8: Mimicking whistles for the DoM evaluation: (**a**) the proposed method, (**b**) FSK, and (**c**) CVC-FM and TFSK. In Table 2, since all models with six window lengths were utilized for the proposed method, the data rate is an average value. In Table 2, let \(N\) be the number of transmitted whistles for simulation. \(b_{n}\) is the number of bits transmitted by the \(n\)-th whistle and \(T_{n}\) is the time length of this whistle. The data rate is obtained as \((\sum_{n=1}^{N}b_{n}/T_{n})/N\). The data rate of the conventional methods is about 12-15 bps, except for TFSK. The simulations were performed with the bandwidth used by the whale, and the data rate of the TFSK is limited. There are many simulators for modeling underwater acoustic sound propagation. However, we referred to the measured channel from the experiments of long-range (i.e., longer than 50 km) underwater acoustic communication for realistic simulation. Table 3 summarizes the cases of the long-range underwater acoustic communication experiment conducted in the bandwidth of the RHW. The depiction of the channel measured in the experiments is also described in Table 3.  
In Table 3, the maximum channel delay is 2.5 s and the maximum number of the multipath is 17. Based on these facts, three channels were generated by the bellhop according to maximum delay (i.e., 30 ms, 1 s, and 2 s), and these three channels were utilized for simulation. The three channels used for simulation and the BER results are displayed in Figure 9. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & \multirow{2}{*}{**Bandwidth**} & \multirow{2}{*}{**Range**} & **Number of Paths** & **Maximum Delay** \\ \hline
[16] & Fc: 250 Hz & & & \\  & BW: 100 Hz & & & \\ \hline
[17] & Fc: 250 Hz & 500 km & 1–2 & 600 ms \\  & BW: 50 Hz & 700 km & & \\ \hline
[18] & Fc: 500 Hz & & & \\  & BW: 100 Hz & & & \\ \hline
[19] & Fc: 400 Hz & & & \\  & BW: 100 Hz & & & \\ \hline
[20] & Fc: 500 Hz & & & \\  & BW: 100 Hz & & & \\ \hline
[21] & Fc: 500 Hz & & & \\  & BW: 100 Hz & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of long-range acoustic communication experiment and channel. In Figure 9, the orange line denotes the BERs of the proposed method. The blue line is the BER of the proposed method without multipath gain. The yellow, purple, and green indicate the BERs of the conventional biomimicking communication methods (i.e., CV-CFM, FSK, TFSK, respectively). In Figure 9, the proposed method showed better BER performances than the conventional algorithms. The conventional methods had error floors at 10\({}^{-2}\)\(\sim\)10\({}^{-1}\). When SNR was lower than \(-\)10 dB, TFSK showed the lowest BER. However, the data rate of the TFSK was also half that of the others. When proposed without multipath gain, the BER performance of the proposed method was similar to that of TFSK. Please note that, as shown in Table 2, the data rate of the proposed method was higher than that of TFSK. Therefore, the BER performance of the proposed method was better even without multipath combining. In addition, increasing SNR did not decrease the BER of the TFSK. The proposed method decreased BER by taking multipath gain. These results show that it is difficult for conventional methods to overcome the large multipath delay in the long-range underwater communication environment.